Metal Plating Bible

Dear Customer, I would like to take this moment to congratulate you on your purchase of the “Metal Plating Bible”. It is without a doubt, the most practical learning guide for the beginner or intermediate plating enthusiast. There’s a lot of information crammed into the next 120 or so pages. You may be tempted to skip around, but if you do, you may risk missing vital pieces of information that will substantially aid you in your metal plating efforts. So do yourself a favor and read this eBook from front to back. Also, due to the technical nature of Metal Plating, there will likely be many words that you might not understand. Because of this inevitable fact, we have included a glossary at the very end of this digital book. If you ever need to refer to it, just go to the last few pages where you can read it. Ok, there’s nothing more to say, go ahead get started. The following 120 pages contain all the information you’ll ever need to do your own high quality metal plating. Enjoy!

Table of Contents CHAPTER 1 (page 4) Chapter 2 (page 10) Chapter 3 (page 20) Chapter 4 (page 37) Chapter 5 (page 42) Chapter 6 (page 44) Chapter 7 (page 47) Chapter 8 (page 48) Chapter 9 (page 72) Chapter 10 (page 89) Chapter 11 (page 95) Chapter 12 (page 99) Chapter 13 (page 106) Chapter 14 (page 109) Chapter 15 (page 114) Chapter 16 (page 117)

CHAPTER 1 Electroplating is the process of coating a metal object with another metal, using electrical current passed through a chemical solution. It is the process that produces a thin, metallic coating on the surface of another metal. The purpose of Electroplating is to improve appearance of the material, protection against corrosion and in certain special processes like printing. The process of Electroplating involves placing the metal to be plated, (“Metal A”) in the solution of the metal (“Metal B”), with which it has to be plated. The metal to be coated (Metal A) is made the cathode in an electrolytic cell and the anode is made up of another conductor mostly the “Metal B” (metal with which Metal A will get coated). When electric current is applied, the electrode reaction occurring on the cathode is the reduction of the metal ions to metal. E.g., gold ions can be discharged form a gold solution to form a thin gold coating on a less expensive metal to produce "custom" jewelry. To further illustrate the Electroplating process, let us assume that an object made of one of the copper (Metal A) has to be plated with nickel (Metal B). The Setup: •

Step 1: o

•

Attach a wire to the copper object (Metal A) while the other end of the wire should be attached to the negative pole of a battery (or a power supply). To the positive pole of the battery (or power supply) we connect another wire with its one end connected to a rod made of nickel (Metal B).

Step 2: o Next we fill the electrolytic cell with a solution of the metal salt to be plated. In our present example the nickel chloride salt dissociates in water into positively charged nickel cations and negatively charged chloride anions. As the copper object to be plated is negatively charged it attracts the positively charged nickel cations, and electrons flow from the copper object to the cations to neutralize them (to reduce them) to metallic form. o

Meanwhile the negatively charged chloride anions are attracted to the positively charged nickel rod (known as the anode of the electrolytic cell). At the anode electrons are removed from the nickel metal, oxidizing it to the nickel cations. This illustrates that the nickel dissolves as ions into the solution. That is how replacement nickel is supplied to the solution for plating and we retain a solution of nickel chloride in the cell.

Fig. GRAPHICAL REPRESENTATIONS OF THE ELECTROPLATING PROCESS

Nickel chloride is used here to exemplify the process of electroplating as it is simple to understand. It is not recommended; however, that nickel is used for, say, school science demonstrations because some individuals are quite allergic to it. It is also not recommended that chloride salts be used because they are amenable to release chlorine gas. For school or amateur type demonstration it is recommended to plate copper coins with zinc or nickel coins with copper. Another common example is the shining work on cars -- bumpers, door handles and manufacturer logo. Much of this begins as a piece of zinc, steel or plastic. The manufacturer uses a copper electroplate, then a nickel electroplate and then chromium depositing over one another. The result is a surface brighter and more corrosion resistant than bare metal or plastic. Electroplates are applied by immersing the object to be coated in a tank containing the proper chemicals dissolved in water. If nickel is being applied, nickel metal is one of the components of that solution. Now imagine that the part to be plated is attached to a negative electrical lead (like that on your car battery). Once it is attached to the negative electrical lead it is called a cathode. The other electrical lead, the positive (+) is in the solution. When current is turned on, the negatively charged part to be plated attracts positively charged metal from the solution

(opposites attract). This continues as long as current is on, and the coating or deposit becomes thicker and thicker. But most electroplates are not very thick. One thousandth of an inch (0.001 inch) is regarded as pretty thick. Since metal is being taken from the solution, it must be replenished. Often this is done by hanging pieces of the metal nickel, if nickel is being plated, for example, in the solution. The chunks of metal are called ANODES, and the positive electrical lead is then attached to them. They dissolve in the solution as metal is taken away by plating. So at this point we have metal being removed from the anode and deposited on the cathode, which are the parts to be plated. Since a car battery is not a good source of power for this application, electroplaters use electrical current supplied by their power companies. But they must have DIRECT current (DC), while the power company supplies ALTERNATING current (AC). To convert AC to DC electroplaters use a RECTIFIER. Its function is to convert AC to DC. Purpose / Objective of doing Electroplating 1)

Protection from corrosion Copper, nickel, and chromium on steel and zinc die castings Zinc or cadmium on steel

2)

Appearance Copper, nickel, and chromium on steel Nickel and gold on brass Silver on brass

3)

Superior hardness and better wear resistance Chromium on steel Electroless nickel on steel (This hardness improvement is gained without sacrifice of ductility; the plating allows a hard surface while maintaining a softer ductile core.)

4)

Lower contact resistance and increased reliability of Electrical contacts Gold on Brass or copper

5)

Improved solderability and/or weldability Tin on Brass Electroless nickel on steel

6)

Better base for other finishes Nickel under Gold or chromium (The nickel inhibits the migration of the gold into the brass basis)

7)

Improved lubricity under pressure

Silver on Bronze 8)

To strengthen the base and render it more temperature resistant Copper, Nickel and chromium on plastics

9)

To act as a stop-off in Heat treating Copper on Steel for carburizing Bronze on steel for nitriding

Chapter 2 Process description Virtually all metals and some metal alloys can be electroplated to produce a coating on a substrate. The substrate is usually made from a metal, though selective non-metals may also be coated. Electroplating is also referred to as electro deposition, and both terms are in common usage. In aqueous solution, metallic salts ionize to form positively charged metal ions and negatively charged acid radical ions. For example, copper sulphate in solution ionizes as follows: CuSO4→Cu2+ + SO42The ions exist independently of one another in solution but balance out electronically, i.e. the number of negative and positive charges is equal. Application of a potential from a direct current source by the immersion of two electrodes into the solution causes the ions to migrate. Positively charged ions migrate to the cathode whilst negatively charged ions migrate to the anode. In the example of copper sulphate solution above, the copper ions migrate to the cathode and accept electrons from the cathode causing the copper atoms formed to adhere to the cathode. Cu2+ + 2e- → Cu(metal) If the anode is copper, the negatively charged sulphate ions give up electrons at the anode to produce copper sulphate, which then ionizes to restore the equilibrium. Most metal electro deposition occurs via this route. In theory the solutions are maintained at their optimum concentrations, though it is necessary to ensure that there is an adequate supply of the anode metal source. Some deposition solutions, such as those used for chromium, gold or other precious metals, use insoluble anodes. Since no metal source is present to maintain the solution in equilibrium, the solution becomes progressively depleted in metal salt and to maintain optimum solution concentration frequent additions of the salt must be made. It is possible to electro deposit metal from a single salt solution, though this is rarely used in practice. Most solutions consist of several salts, which have different functions. For instance, chlorides are added to nickel plating solutions to promote anode corrosion and boric acid is added to act as a buffer to maintain pH equilibrium. Most metal cyanides are insoluble in water and must be dissolved in sodium or potassium cyanide solution. Other salts are used to promote conductivity. In practice, these solutions are referred to as the basic solution. It is possible to deposit metal from basic solutions but the deposits produced are generally unacceptable to users, as they are dull, not very adhesive and crystal formation occurs. For example, in the early part of the century, nickel plated deposits were dull and required mechanical polishing prior to deposition of chromium to produce a bright reflective finish; silver deposits were similarly treated.

Basic solutions are generally made up by the user, usually from the necessary salts, in crystal or powder form, which are purchased from the chemical manufacturer/supplier. Some solutions, particularly those for gold and other precious metals may be purchased pre-made. Modern electroplating solutions contain many complex organic or metallic organic chemicals, referred to as brighteners or addition agents. The purposes of these agents are numerous and include faster plating speeds, higher tolerance of contaminants in plating solutions, production of mirror-bright deposits, increase/decrease of hardness, changes to crystal structure of the deposit, and decrease of internal stress. These addition agents are the results of the research and development efforts of supply houses. Therefore the chemical composition of the agents is usually confidential. These addition agents are co-deposited with the metal. The optimum concentration is maintained in the solution by frequent additions, often by dosing meters, based on an ampere/hour basis. Quite low concentrations may be present in the solution but fairly high maintenance additions are made frequently during production. For example the concentration in basic solution may be 3 ml/l but the rate used may be 200 ml/l per 1000-ampere hour. The solutions supplied generally contain between 3-25% of the chemical. Therefore in this example, the concentration in the actual working solution may be between 0.009-0.075 percent. Plant installations for electroplating consist of several tanks assembled together in sequence and the articles for processing are transferred from tank to tank on racks or in barrels, by manual or mechanical means, and in the case of the latter, often by computer control. Some plants are of the return automatic type where the tanks are arranged in a double row and are typically to be found in a manufacturing organization where high volume output of similar type articles is required. What to Avoid during the Electroplating Process 1) Sharp edges and right angles should be avoided. Every sharp, protecting edge will draw extra current and build up with extra plate. Conversely the part will receive very little plate in the acute angle. All sharp edges and right angles should be rounded to the greatest degree design allows. 2) Holes should be either counter sunk or counterbored, because build up on the sharp edges may exceed tolerance allowance. 3) Deep recesses should be avoided. The recess will receive the lesser thickness of plate then the adjoining area, and either require heavier average thickness on the overall part to meet the minimum specification or receive too little plate if the average area is used to compute thickness. 4) If the article is a threaded fastener or threaded screw machine part, special care should be taken to build up of at least four times the plating thickness on the pitch diameter. 5) Formed tubular articles will often trap and carry over solution if drainage holes are not provided in the design. 6) Larger parts to be rack plated must be provided with some way to rack the part in

hole, lug or rim. Since the contact part will be poorly plated, the electrical contact should be in a non-significant area. 7) Blind holes, rolled edges, seams and other crevices will trap solution unless special plating techniques are used. 8) Bolted assemblies should be avoided because of possible unplated areas subject to subsequent corrosion. 9) Dissimilar metals are difficult to plate because cleaning methods vary for different basis materials. Good Habits for Electroplating Process In the design to be plated, it is well to utilize all the advantages that may be incorporated to permit as uniform a distribution of plating thickness as possible and still retain the basic design desired. Each sharp corner (recessed or protruding) should be provided with as large a radius as possible. Figure 1 illustrates the distribution of nickel-plating thickness on a formed part. Note the lack of adequate radii at the ends of the central recessed section. The ratio of 9.0 illustrates the effect of these sharp corners. If reasonably good distribution of plating thickness is desired, every effort should be made to avoid recesses, to fillet all sharp corners, and to use convex instead of concave surfaces wherever possible. This will improve distribution of plating thickness and, by so doing; will provide a finish having better corrosion resistance at a reduced cost of plating. Platinum Electroplating Platinum is rare, scarce, and very costly and is considered one of the most precious metals. Platinum electroplating is used to coat electrodes that are used in the refining of oil, and in the manufacturing of fertilizers, acids, and explosives. The automotive industry uses platinum plated catalytic converters to treat automobile exhaust emission. In the medical industry, platinum plate is used on instruments such as catheters and connectors for surgical equipment. The electrical and electronics industries use platinum plating for low voltage and low energy contacts. In electroplating, platinum is often used to coat titanium, niobium, or stainless steel anodes. It is also used in the jewelry industry. Platinum is considered a premium protective finish over sterling silver and nickel base metal. Platinum’s luster is much purer than silver or gold, enhancing the brilliance of gemstones and diamonds. Platinum electroplate coatings typically range from 0.5 to 5 microns depending on the application. It is applied utilizing a rack fixture that is submerged in a chloroplatinic acid or a sulfate based platinum solution. The finished product can range in color from tin white to a matte gray finish depending on the base metal finish, activation process, and the thickness of the platinum coating. Platinum electroplating is accomplished by placing the electrode tips into a solution of platinum chloride and applying a small current such that the platinum in solution is reduced, causing platinum deposition at the tip of the metal electrode. We can plate the electrodes

using a solution of hydrogen hexa-chloro-platinate (8% PtCl4 by weight) with a multi-channel, constant-current plating device. Rhodium Plating Rhodium is white in color and a precious metal, of the platinum group. Rhodium is the hardest of all of the precious metals. It provides the most wear resistant finish possible for the most demanding environments. It is one of the most suited metals for plating of parts such as sliding electrical contacts that require protection from corrosion or galling. Rhodium provides a bright, attractive finish that is non-tarnishing. Under-plating of nickel should be used when parts are of corrosion or heat resistant steels. When under-plated with nickel it provides a mirror surface that is highly reflective. Surfaces other than nickel, silver, gold, or platinum should be either nickel-plated or nickel over copper plated. Rhodium plating is widely used on high voltage switch gear, silverware, silver models, medals, white gold jewelry and top end furniture fittings to prevent tarnishing / corrosion as well as due to its hardness it makes the surface scratch resistant. As rhodium is a relatively inert metal, it cannot be stripped from the more active base material without damaging the less active substrate in the case, if an item is damaged and require repairs. Also, rhodium is plated from an acid solution which has poor throwing power. It cannot generally be used on items with deep cavities without some consideration to masking, jigging, pumping and shielding. As it is an expensive process, the areas that do not require coating can be masked with special masking tapes and paints. Of the platinum group metals, rhodium has found wide acceptance in decorative precious metals applications. Rhodium has several desirable properties – it has a brilliant white color, high reflectivity, and hardness, which makes it very popular with the jewelry and faux jewelry industries. Rhodium can provide excellent tarnish protection for sterling silver and silver plated flatware and hollowware from quite thin deposits. Typically, rhodium electroplate is deposited on precious and faux jewelry, sterling and silver plate to a thickness of 0.05 to 0.125 microns (2 to 5 micro inches). This thickness of rhodium is produced in about 20 to 60 seconds from phosphate, sulfate or phosphate-sulfate baths. Rhodium on Steel For rhodium to get plated on steel, it is needed to activate in order for the rhodium to adhere to the surface. Rhodium plating on the steel does not conceal surface flaws or blemishes. Many electroplaters typically use either a woods nickel strike or an acid gold strike to cover the steel with a thin layer of metal to achieve adhesion. For best results it is advisable to choose a good pre-plate of bright nickel, 5-10 microns in thickness. This will help level out any minor surface waves that may exist in the material, and should not interfere with any build tolerances on the surface. This is followed by a final layer of rhodium and at least .50-1.0 microns of rhodium are recommended.

Silver Plating

Silver plating offers the highest electrical conductivity of all metals. It is a semi-precious metal that gets oxidize rapidly. Silver plating is best suited for engineering purposes, as for soldering, electrical contact characteristics, high electrical and thermal conductivity, thermo-compression bonding, provides wear resistance to load-bearing surfaces, and spectral reflectivity, good corrosion resistance, and other electrical applications. Silver Plating - Grades A. With supplementary tarnish-resistant treatment. B. Without supplementary tarnish-resistant treatment Silver Plating – a Useful tool for Corrosion Protection For applications where corrosion protection is important, the use of silver plating with an electrodeposited nickel undercoat is advantageous. Silver Plating – Under-plate Recommendations Silver plating on steel, zinc and zinc-based alloys should have an undercoat of nickel over copper. Silver plating on copper and copper alloys should have a nickel undercoat. Copper and copper alloy material on which a nickel undercoat is not used, and other base metals where a copper undercoat is employed, should not be used for continuous service at a temperature in excess of 300 degrees F (149 degrees C). Adhesion of the silver plating is adversely affected because of the formation of a weak silver and copper inter-metallic layer. Silver Deposit and Tarnish Tarnishing is a natural process that occurs on the surface of silver jewelry. Tarnish starts as a light yellow discoloration of silver; it then starts to change to darker shades of brown as the tarnish gets to be more severe. In extreme cases the tarnishing of silver could look very dark and almost black. Tarnishing occurs due to certain climatic conditions and also due to certain ingredients that are present in some materials. One such chemical that causes silver to tarnish is hydrogen sulfide (H2S) and things that contain this chemical will cause silver to tarnish quickly. Materials that stimulate silver tarnish are wood, felt, rubber bands, food items like eggs, onions etc. High humidity in the climate also hastens the silver tarnish process. The factors that can cause silver tarnish are wide and varied, and is evident that the process of silver tarnishing can hardly be avoided because silver tarnish is a natural process and occurs with silver of all purities. Silver jewelry is generally made from silver that is around 92.50% pure and this is done to increase the hardness of silver. Sterling silver as it is normally referred to (925=92.50% purity), is an alloy of silver and other metals.

The tarnishing of sterling silver has nothing to do with the percentage of silver in the alloy. It would be safe to assume that all silver will tarnish. It is possible to clean tarnished silver jewelry and the procedure followed will depend on the degree of tarnish that is present on the silver jewelry.

Brass Plating Brass plating is primarily used as a decorative finish. However, the process is also used for some engineering applications, such as brass-plated steel wire promotes adhesion to rubber in steel-belted tires and as an anti-galling coating. Brass is also plated on the surfaces of bearing materials. For bright decorative brass finishing, material is first plated with bright nickel, followed by a brass flash plate for 35-90 sec. Such finishes are used in wire goods, decorative lamps, furniture hardware and builder’s hardware. Heavy brass deposits (0.0003-0.0006 inch) are used for finishes that will be buffed, burnished, antiqued and/or oxidized. Some of the brass plating is removed with antiquing and oxidizing processes and, therefore, the minimum thickness for such processes is 0.0003 inch. Heavy brass deposits are not as bright as brass plated over bright nickel. To obtain bright finishes with heavy brass deposits, they must be buffed or burnished. Addition agents can refine the grain of the brass so that the amount of burnishing or buffing is greatly reduced. Gold Plating Gold is unique for its yellow color. It is a precious metal and does not oxidize in air, so its electrical conductivity stays uniform over long periods of time. It is ideally suited for electroplating applications. Gold plating offers good corrosion resistance, good solderability, and when alloyed with cobalt, it has very good wear resistance. Gold is commonly used in electrical switch contacts, connector pins and barrels, and other applications where intermittent electrical contact occurs. Gold Plating - gold electroplating specification Specification: Gold Plating, Electro-deposition •

Type I 99.7 % gold minimum; hardness grades A, B, or C. Gold plating used for generalpurpose, high-reliability electrical contacts, solderability, and wire wrap connections.

•

Type II 99.0 % gold minimum; hardness grade B, C, or D. A general-purpose, wearresistant gold. It will not withstand high-temperature applications because the hardening agents in the gold plating will oxidize.

•

Type III 99.9 % gold minimum; hardness grade “A” only. Gold plating for semiconductor components, nuclear engineering, thermo-compression bonding, and high-temperature application.

Gold Plating - purity and coating thickness Co-deposited impurities can make soldering more difficult, and for this reason gold plating with high purity is preferred. Soldering requirements are best achieved when gold coatings range

between

0.00005

and

0.0001

inch

(50

and

100

micro

inches)

of

thickness.

Gold Plating - hardness grades 1. 90 knoop, maximum 2. 91-129 knoop, inclusive 3. 130-200 knoop, inclusive 4. 201 knoop, minimum Gold over silver is not recommended for electronics hardware. Gold Plating - underplate recommendations When gold is applied to a copper rich surface such as brass, bronze, or beryllium, copper metal ions from these base metals will diffuse into the gold layer and degrade its hardness and nonoxidizing properties. An anti-diffusion under-plate such as nickel (electroless or sulfamate) should be applied to prevent this. We recommend electroless nickel under gold where part flexure of deformation is not expected and a bright finish is desirable. Where part flexure or deformation is expected, we recommend sulfamate nickel as the under-plate because of its higher ductility. Palladium Plating Palladium is white in color, harder than cobalt gold, and is precious, it also retains the nonoxidizing property so is used in electrical connector applications. Palladium electrodeposits have better ductility, which provides superior contact bending tolerance, lower porosity, and superior resistance to corrosion than hard gold. This makes palladium an excellent candidate for applications such as reed switches or relay contacts. However, palladium has a generally lower wear resistance in sliding contact, such as pin/socket interfaces, than gold. Palladium mated against pure Palladium has less wear resistance than palladium mated against gold or a palladium surface with a thin overlay of gold. When under-plated with a flash of soft gold, palladium also demonstrates excellent solderability. Electroless Nickel Plating Nickel is a silver white, hard metal with satin to bright luster. It can be plated uniformly in recesses, blind holes and cavities, does not build up on edges, and has very high wear endurance. Higher phosphorus variations provide superior corrosion resistance. Nickel is often applied as a base layer for its leveling, smoothing and barrier characteristics which provide resistance to attack of some base metals by electrolytic metals such as, cyanide copper or silver. Nickel is a hard metal with generally poor ductility that is not recommended for applications where a part of flexure is required.

Electroless Nickel plating - corrosion inhibitor As a corrosion inhibitor, nickel is used to protect iron, copper, or zinc alloys against corrosive attack in rural, industrial or marine atmospheres depending upon the thickness of the nickel deposit. Nickel, with its leveling and pore-filling characteristic, is also an excellent undercoat for the precious metals by reducing the total amount of the precious metal required to achieve performance specifications. Electroless Nickel plating - engineering purposes Electroless nickel plating intended for engineering purposes is used for wear resistance, abrasion resistance and such corrosion protection of parts as the specified thickness of the nickel plating provides. Heavy deposits of the electroless nickel plating may be used for build up of worn or undersized parts, or for salvage purposes, and to provide protection against corrosive chemical environments. Watts Nickel per QQ-N-290A - Nickel plating Watts nickel is an electrolytic system that provides very bright, decorative finishes. It also provides corrosion resistance according to thickness, good abrasion resistance, and a low coefficient of thermal expansion. It has a relatively low tensile strength and hardness and relatively high internal stress and therefore is not recommended in engineering applications where part deformation or flexure may occur. Teflon Electroless Nickel Plating The co-deposit of electroless nickel & Teflon contains15% Teflon particles dispersed in an electroless nickel matrix. It is a hard, ductile finish, brown in color, which has superior antifriction characteristics. This is ideally suited to high cycle mechanical sliding applications. In addition this is a good electrical conductor and provides corrosion resistance. Sulfamate Nickel Plating Sulfamate nickel provides the lowest hardness, lowest internal stress and highest ductility of all the nickel plating systems. The finish is dull; and is used as an engineering finish and not a decorative finish. Sulfamate nickel has excellent solderability good corrosion resistance. The high ductility of sulfamate nickel makes this product an excellent candidate for applications where part flexure or deformation, such as crimping, will occur. Note: As nickel finishes become brighter, they become harder and less ductile. Bright nickel finishes are not recommended if parts are intended for flexure applications or will be bent or crimped in manufacturing operations subsequent to electroless nickel plating. Tin Plating & Tin Alloys Tin is a silver-colored, ductile metal whose major application is to impart solderability to otherwise unsolderable base metals. Tin has generally good covering characteristics over a wide range of shapes. It is an electrolytic process. Tin and its salts are reported to be non-toxic and non-carcinogenic and are approved for food container and food contact applications.

Tin plating - soft, ductile finish Tin does not tarnish easily and can serve as a low cost decorative finish, although care must be exercised in subsequent part handling as tin is a soft, ductile finish that can scratch or mar easily. Alloying tin with lead to reduce it’s melting point for soldering and to prevent "whiskering". See specific descriptions below for details on each of the various tin systems. Tin plating - corrosions protection & conductivity Tin is a good electrical conductor and has historically been utilized for its combined corrosion protection and conductivity in aerospace avionics radio frequency applications. •

Primarily used to facilitate solderability to base metals that have poor solderability.

•

A ductile, bright finish. Can serve as a low cost decorative finish.

•

Bright acid tin’s ductility will help prevent galling of base metals in friction contact applications.

•

It applies well to most base metals; will act as a stop-off barrier in nitriding high strength steels; has a bright appearance; and provides some corrosion resistance.

Tin plating - corrosion properties For indoor environments, tin provides anti-corrosion properties to copper and copper alloys, and ferrous metals. Note that tin is not an optimal choice for corrosion protection where outdoor environments are expected. Tin should be not less than 99.5% pure except where alloyed for special purposes. Tin plating –Alkaline brightening system Alkaline brightening system provides excellent solderability, corrosion resistance with a .02% bismuth content to stabilize the structure of the metal deposit and stop "whiskering" in extreme temperatures. Finish is dull in appearance, very soft and is easily marrable. 60/40 & 90/10 Tin Lead The co-deposition of lead with the tin reduces the risk of whisker growth. Metal filaments, or whiskers, sometimes grow spontaneously from the surface of electrodeposited metals such as tin, cadmium, and zinc within a period that may vary from weeks or months to years. These whiskers are about 0.0001 inch (2.5 µm) in diameter, can grow up to 3/8 inch (10 mm) long and can have a current carrying capacity of as much as 10 mA. Tin plating - melting points Tin is also alloyed with lead to reduce the melting point of tin. This provides flexibility in selecting soldering temperatures that will not impart too much thermal energy to delicate assemblies.

60/40 melting point range is 361°- 374°F 90/10 melting point range is 450°- 464°F Copper Plating Copper is the second most common metal plated, behind nickel. It provides a soft, red, ductile, solderable surface. Copper is an excellent electrical conductor. However, it is not often used as a final plate, because it tarnishes easily. As copper has excellent leveling properties and very high throwing efficiency, it makes an excellent undercoat for most other metals. In addition, because copper is ductile, it polishes easily to a high shine so that it supports a bright, shiny finishing metal above it. Copper is able to fill sharp corners and surface imperfections, allowing smooth and uniform coverage of the base metal. The throwing and leveling properties of copper insure that pinholes and subsequent blistering of finish metals will be avoided. Copper makes an excellent undercoat on aluminum, which is a base metal that most other electrodeposited metals will not attach to. Copper is the only metal that can be electroplated onto zinc die casts. Generally copper is applied as an under-plate in thickness between 100 and 200 micro inches.

Chapter 3 Parameters for Plating of different Metals

Nickel Plating From a rusted component to a Nickel plated lasting showpiece in three hours. For decorative work, the time required for Nickel plating will range from 1/2 hour up to 3 hours according to the thickness of the Nickel plate required. The plating solution is reusable. 1kg of the Nickel salts (as supplied in the workshop kit) will last for over 13,000 sq cm of .001 (25 micron) Nickel plating without the need to replenish or replace. The salts will store for many years in liquid or solid form. Nickel plating can be carried out at any temperature from 15 degree centigrade up to 35 degree centigrade, although slightly better results are obtained at 24 degree centigrade or higher. The comprehensive instructions supplied with each Nickel plating kit will guide you through all the stages of the plating process from the initial preparation of the parts to be Nickel plated through to the finished product with hints and tips to get the very best from your kit. The Nickel plating process can be used as a substitute for Zinc or Cadmium plating. The finish is dictated by the preparation. For a matt finish wire brushings is sufficient. If the surfaces are polished prior to being electroplated they can be buffed up to achieve the beautiful sheen of the original.

Kit Contents 1kg of Nickel plating salts 2 Nickel plating anodes 150g of degreasing salts 150g fine pumice powder 30m of copper wire 1 set of comparator papers 1 set of instructions 5 pairs of polythene gloves

Makes approx 6 liters gives 1,000 sq inches of plating makes 5 liters of degreaser to scour the parts before plating to suspend the objects to check the condition of the solution comprehensive instructions and tips to avoid contaminating the degreased parts

Extra items you will need 1. A plastic bowl or bucket or small fish tank 2. 12v battery or a 12vdc battery charger 3. 1 or more standard automotive type 12v bulbs Gold Plating 1. For articles made of karat gold, gold-filled, rolled-gold plate, nickel, copper, and brass. •

•

Preparation o

Buff and polish item to be plated.

o

Steam clean and boil out.

o

Rinse.

o

Electroclean.

o

Rinse again.

Plate o

•

Gold plate.

Completion o

Rinse.

o

Soda rub.

o

Rinse again.

o

Alcohol dip.

2. For articles made of white metals or contaminated with soft solder - including most costume jewelry. •

•

•

Preparation o

Buff and polish item to be plated

o

Steam clean and boil out.

o

Rinse

o

Electroclean

o

Rinse again

Plate o

Copper plate (an under plate that prepares the item for receiving gold plate)

o

Rinse

o

Gold plate

Completion o

Rinse

o

Soda rub

o

Rinse again

o

Alcohol dip

Rhodium Plating 1. On articles made of karat gold and platinum •

•

Preparation o

Buff and polish item to be plated.

o

Steam clean and boil out.

o

Rinse.

o

Electroclean In rhodium plating use 14K or platinum holding wire instead of copper)

o

Rinse again.

Plate o

Rhodium plate.

•

Completion o

Rinse.

o

Alcohol Dip.

2. On articles of silver, palladium, gold-filled, rolled-gold, white metal, or contaminated with soft solder - including most costume jewelry. •

Preparation- as above.

•

Plate

•

o

Gold plate (flash under plate) 10-15 seconds to prepare item for receiving rhodium plate.

o

Rinse.

o

Rhodium plate.

Completion - as above.

Silver Plating Bowls and hollowware can be plated on the inside by filling bowls with solution and suspending silver anode inside. Negative plating lead wire is attached to item being plated. Positive lead wire is attached to anode. Procedure •

Plating solution at room temperature.

•

Immerse silver anode connected to positive lead wire.

•

Attach negative lead wire alligator clamp to handling wire on item to be plated.

•

Dial 2V and turn switch to "On Position".

•

Immerse article 30 seconds or until completely covered with silver. Article will usually emerge with a dull milky color.

•

Brighten color by buffing with a bristle brush.

It is important to realize what type of silver plating finish is required on the part to be plated. If a highly polished surface is required, the part must be buffed to a high shine before plating. If a matt finish is required, then the part should be blasted or buffed with something like a Britex wheel. The steps to achieve a brilliant silver finish are: 1. Buff & Polish 2. Degrease & Rinse

3. Acid Activate (with battery acid, not supplied) 4. Plate with Flash Copper (Steel, Pot Metal & Pewter only) 5. Plate with Silver 6. Acid Activate 7. Treat with Silver Conditioner (prevents tarnishing) Potassium Hydroxide is used to raise the PH of the solution when required. The system must be maintained at a PH of 8.8-9.5.

Figure Representing Silver Electroplating

Cadmium Plating: Cadmium plating generally is performed in alkaline cyanide baths that are prepared by dissolving cadmium oxide in a sodium cyanide solution. However, because of the hazards associated with cyanide, noncyanide cadmium plating solutions are being used more widely. The primary noncyanide plating solutions are neutral sulfate, acid fluoborate, and acid sulfate. The cadmium 7/96 Metallurgical Industry 12.20-7 concentration in plating baths ranges from 3.7 to 94 g/L (0.5 to 12.6 oz/gal) depending on the type of solution. Current densities range from 22 to 970 A/m2 (2 to 90 A/ft2). Copper Plating Copper cyanide plating is widely used in many plating operations as a strike. However, its use for thick deposits is decreasing. For copper cyanide plating, cuprous cyanide must be mixed with either potassium or sodium to form soluble copper compounds in aqueous solutions. Copper cyanide plating baths typically contain 30 g/L (4.0 oz/gal) of copper cyanide and either 59 g/L (7.8 oz/gal) of potassium cyanide or 48 g/L (6.4 oz/gal) of sodium cyanide. Current densities range from 54 to 430 A/m2 (5 to 40 A/ft2). Cathode efficiencies range from 30 to 60 percent. Other types of baths used in copper plating include copper pyrophosphate and copper sulfate baths. Copper pyrophosphate plating, which is used for plating on plastics and printed circuits, requires more control and maintenance of the plating baths than copper cyanide plating does. However, copper pyrophosphate solutions are relatively nontoxic. Copper pyrophosphate plating baths typically contain 53 to 84 g/L (7.0 to 11.2 oz/gal) of copper pyrophosphate and 200 to 350 g/L (27 to 47 oz/gal) of potassium pyrophosphate. Current densities range from 110 to 860 A/m2 (10 to 80 A/ft2). Copper sulfate baths, which are more economical to prepare and operate than copper pyrophosphate baths, are used for plating printed circuits, electronics, rotogravure, and plastics, and for electroforming and decorative uses. In this type of bath copper and sulfate and sulfuric acid form the ionized species in solution. Copper sulphate plating baths typically contain 195 to 248 g/L (26 to 33 oz/gal) of copper sulphate and 11 to 75 g/L (1.5 to 10 oz/gal) of sulfuric acid. Current densities range from 215 to 1,080 A/m2 (20 to 100 A/ft2). Zinc Plating The value of zinc as a rust-proof finish for iron and steel has long been appreciated. Zinc plating is being used on an increasing scale, particularly for components which would formerly have been cadmium plated. ZINC FAST Zinc Fast is a complete system that has made it possible for the small operators to achieve fully professional results without any previous experience in electro-plating. "Zinc Fast XL" gives excellent value for money and, most important, allows the user full Control of the operation.

How fast is "Zincfast XL"? A 10 MicrQn (UN) Average coating is achieved in less than 20 minutes. Passivation takes appx.30 seconds. The "WORKSHOP XL" Kit contains all the necessary chemicals for 12 liters. of Plating Solution and 12 liters of Passivating Solution. The Degreasant, the Copper Wire for suspending the parts, the pH papers and high Purity Anodes are all included in the Kit. To electro-plate you will need a l2 Volt battery, Plastic containers for the Plating and Passivating tanks 'and l2 Volt light bulbs for regulating the Amperage as used with the Nickel Plating System.

Kit Contents •

1 x litre Zinc Fast concentrate

•

1 x2.4kg 625CDP

•

1 x2.4kg 625CDP

•

1 x300g 3AB

•

1 x500cc IB brightener

•

1 x250cc MB brightener

•

1 x250cc wetter

•

1 x250cc 3CRP passivation

•

1 x250cc nitric acid

•

1 x250cc hydrochloric acid

•

1 x150g activax degreasant

•

1 x200g sodium hydroxide

•

1 xcomparator papers

•

1 xcomparator papers (set2)

•

5 pairs of gloves

•

3 xpure zinc anodes

•

1 reel of copper wire

•

1 instruction booklet

You will also need plastic containers • 1 metal container • 12vdc car battery • 12vdc light bulbs The formulations of the more common electroplating solutions used for industrial purposes are given in Tables 2.1 to 2.14. The operating temperature, where above ambient, and the pH are also indicated. For reasons of commercial confidentiality, the brighteners/addition agents cannot be named and are indicated by their generic group only. They are only a small percentage of the materials, which are added to the bath in aqueous solutions. The brightener solutions typically contain between 3 to 25% of the brightener compound. Table 2.1

Copper electroplating solution formulation Acid bath

Copper sulphate

170-200 g/l

Sulphuric acid

45-50 g/l

Temperature

20-40°C

Addition agents (sulphur containing compounds) Low concentration and wetting agents. For example benzotriazole and thiourea. Cyanide bath Copper cyanide

15-75 g/l

Potassium cyanide

25-125 g/l

Potassium hydroxide

0-30 g/l

Temperature

50-75 °C

Brighteners/addition agents are not usually required, but traces of cobalt or nickel may be employed Rochelle bath Copper cyanide (71% copper)

20-30 g/l

Potassium cyanide

25-50 g/l

Potassium carbonate

15-25 g/l

Potassium hydrogen tartrate

30-40 g/l

Temperature

50-60°C

Pyrophosphate bath Copper pyrophosphate (42.3 % copper)

50-85 g/l

Potassium pyrophosphate

200-300 g/l

Ammonium hydroxide (sg 0.880)

3-10 g/l

PH

8.6-9.2

Temperature

50-55°C

Sulphur containing compounds may be used as addition agents. For example benzotriazole and thiourea.

Table 2.2

Cadmium electroplating solution formulations

Cyanide Bath Cadmium oxide

15-30 g/l

Sodium cyanide

40-90 g/l

Sodium hydroxide

5-15 g/l

Addition agents are available but usually 1 g/l prohibited Fluoroborate bath Cadmium fluoroborate

200-240 g/l

Ammonium fluoroborate

50-60 g/l

Boric acid

15-25 g/l

Liquorice (addition agent)

1 g/l

Primary brighteners, have a sulphonic acid (=CO-SO2) active group in the molecule. Below table gives some typical examples of primary brighteners. Usually the alkali salt, in particular sodium, of the acid, is used as a water soluble salt. Typical concentrations in the nickel solution vary between 0.5-4 g/l and are dependent upon the type used.

Secondary brighteners have various active groups in the molecule; below table gives some typical examples. The concentrations used in the formulations can vary.

CHAPTER 4

Types of Nickel Plating Solutions Sulfate Solutions. The most common nickel plating bath is the sulfate bath known as the Watts bath. Typical composition and operating conditions are shown in Table I. The large amount of nickel sulfate provides the necessary concentration of nickel ions. Nickel chloride improves anode corrosion and increases conductivity. Boric acid is used as a weak buffer to maintain pH. The Watts bath has four major advantages: 1) Simple and easy to use; 2) Easily available in high purity grades and relatively inexpensive; 3) Less aggressive to plant equipment than nickel chloride solutions; and 4) Deposits plated from these solutions are less brittle and show lower internal stress than those plated from nickel chloride electrolytes. High Chloride Solutions - Chloride baths have an advantage over sulfate baths in deposition speed; not necessarily in current density, but in improved current distribution. All-Chloride Solutions - The advantages of all-chloride nickel plating solutions include the following: 1) Low voltage; 2) Good polishing characteristics; 3) Heavy coatings can be deposited; 4) Low pitting; 5) Improved cathode efficiency; and 6) No need to cool the plating solution. See Table I for composition and operating parameters. However, there are disadvantages to this bath as well: 1) Highly corrosive; 2) Nickel chloride is sometimes less pure than nickel sulfate (particularly important in bright nickel plating); 3) Mechanical properties of the deposit are not as good as those from the Watts bath. Fluoborate Solutions - In nickel fluorborate baths, the electrolyte is maintained at a pH of 2.0-3.5 using fluoroboric acid. Metal content is maintained at up to 120 g/liter of nickel, which is much higher than in a Watt's bath. Because of this, higher current densities are necessary. Nickel coatings deposited from this type of bath have properties similar to those deposited from Watt's baths; however, these coatings are usually specified for heavy nickel applications and electroforming. Anode dissolution in a nickel fluoborate bath not containing chloride is better than in a nickel sulfate solution with nickel chloride. Disadvantages of fluoborate baths include the following: 1) High cost of chemicals; 2) Throwing power less than that of sulfate solutions. Sulfamate Solutions - This bath is based on the nickel salt of sulfamic acid, and the pH is adjusted using sulfamic acid, nickel oxide or carbonate. When intensive agitation is used in solutions with a high nickel concentration, current densities up to 500 asf can be achieved. Nickel coatings from this type of bath usually have very low stress values and high elongations.

Another advantage is that it is possible to operate the sulfamate bath without difficulties related to anode dissolution at low chloride levels or even without chloride. The principle advantage of this bath is that it can be operated at nickel concentrations of 180-200 g/liter. This allows for the use of high current densities without losing the properties of the coating. Current Fluctuations while plating with a nickel sulfamate solution This bath is based on the nickel salt of sulfamic acid, and the pH is adjusted using sulfamic acid, nickel oxide or carbonate. When intensive agitation is used in solutions with a high nickel concentration, current densities up to 500 asf can be achieved. Nickel coatings from this type of bath usually have very low stress values and high elongations. Another advantage is that it is possible to operate the sulfamate bath without difficulties related to anode dissolution at low chloride levels or even without chloride. The principle advantage of this bath is that it can be operated at nickel concentrations of 180-200 g/liter. This allows for the use of high current densities without losing the properties of the coating. There can be a lot of reasons, stopped barrel, bad power supply, current to the barrel, dirty barrels that do not allow solution transfer and barrel danglers that are not riding with the load. All these can occur intermittently. Also, check to see if the parts are not over etched before the going into the nickel bath. We doubt that there is really an effective practical way to reduce the throwing power significantly, but the theoretical factors are: •

Low solution concentrations increase throwing power (by starving the HCD areas), so high concentration should reduce it.

•

Low temperatures increase throwing power the same way, so high temperature should reduce it.

•

Good agitation reduces throwing power.

Types of Nickel Plating Bright Nickel - Bright nickel plating baths are used in the automotive, electrical, appliance, hardware and other industries. It’s most important function is as an undercoating for chromium plating, helping finishers achieve a smooth bright finish as well as a significant amount of corrosion protection. Bright nickel plating baths use combinations of organic agents to achieve bright nickel deposits. There are two classes of these organic additives. The first class is the aromatic sulfonic acids, sulfonamides and sulfonamides that contain the functional group =C-SO2. Saccharin is a widely used example of this type of brightener. Nickel deposits plated using these additives are mirror bright initially; however as the nickel builds, brightness diminishes. This first class of brighteners incorporates sulfur into the bright nickel, reducing corrosion resistance.

Brighteners in the second class, also called levelers, have inorganic metal ions and organic compounds. These may include butynediol, coumarin, ethylene cyanohydrin and formaldehyde. These are used as leveling agents because they increase surface smoothness, as the nickel deposit thickness increases. See More on Brighteners and Levelers in Chapter 11.

Semi-Bright Nickel - At first, coumarin was used to obtain a high-leveling, ductile, semi-bright and sulfur-free nickel deposit from a Watts nickel bath. However, coumarin-free solutions are now available. A semi-bright nickel finish is semi-lustrous, as the name implies. However, it was specifically developed for its ease of polishing and buffing. Or, if subsequently bright nickel plated, buffing can be eliminated. Brightness and smoothness are dependent on operating conditions. The reason semi-bright nickel finishes are so easily buffed and/or polished is that the structure of the deposit is columnar, whereas the structure of a bright nickel finish is plate-like (lamellar). However, the structure can be changed with additives, a change in pH, current density or even an increase in solution agitation. This is not a problem unless it affects properties such as internal stress. Internal stress can be compressive or tensile. Compressive stress is where the deposit expands to relieve the stress. Tensile stress is where the deposit contracts. Highly compressed deposits can result in blisters, warping or cause the deposit to separate from the substrate. Deposits with high tensile stress can also cause warping in addition to cracking and reduction in fatigue strength. Watt baths and high-chloride type baths can produce high tensile stress. During bright-nickel plating, stress-reducing additives are used, but these co-deposit sulfur materials that affect the physical and/or engineering properties of the deposit. Saccharin is often used as a stress reducing agent. Nickel sulfamate baths can deposit pure low-stressed finishes without using additives. Other Types of Nickel - To obtain other types of finishes such as satin nickel, organic additives are used and deposition conditions are altered. Deposits from a Watts bath are usually 7-10 mm thick, with the appearance dependent on the temperature and/or pH. At higher temperatures and a pH of 4.5-5.0, nickel deposits are matte. At 122F and a pH of 2.5-3.5, deposits are bright. Black nickel plating is lustrous and has a black or dark gray color. Plating is done with little or no agitation. Occasionally it is necessary to remove hydrogen gas (bubbles) from the part's surface using wetting agents. The pH of the bath ranges from 5-6, and the temperature varies from ambient to 140F. Current density remains at approximately 0.5 A/dm2. The coatings average 2 mm thick and corrosion resistance is limited, therefore they are usually lacquered or coated with oil or grease. If the black nickel must have good corrosion resistance, an undercoating such as bright or dull nickel, zinc or cadmium is necessary. Barrel Nickel Plating Barrel plating solutions are relatively similar to rack plating solutions; however, operating conditions may differ, although not radically. The pH is usually maintained at about 4, unless plating zinc die-casting, in which case a pH higher than 4 may be necessary. However, anode corrosion is better at a lower pH, and anode area is limited. The anode area should be as large as possible to avoid the liberation of oxygen and chlorine. Temperatures can vary for barrel nickel plating from 86-104F for some solutions and 104-140F for others. Current density can also vary. For a typical barrel, approximately 24-32 inches long and 16 inches in diameter, the load is 300-600 amps per load or between 1-1.5 A/dm2. Other considerations are the barrel loading, surface area and coating thickness.

There are some special considerations for barrel plating: 1) Parts must be able to move about freely in the barrel; 2) Precise surface preparation is essential, including thorough rinsing; and 3) When the electrolytes are used to full capacity, low-current-density treatment should be used continuously. Properties of Nickel Deposits Thickness - Corrosion resistance is often intimately related to the thickness of the coating; however, the functional requirements of the coating are also important. Micrometer readings are used most often to determine coating thickness. Hardness - Certain addition agents, such as saccharin or napththalene sulfonic acid, can increase the hardness of a nickel deposit. Wetting agents may also increase hardness. Nickel deposits plated from Watts nickel baths, sulfamate or fluoborate baths can rise to 650 HV (HV is Vickers hardness). Heavy nickel baths produce deposits with hardness between 250-350 HV. Hardness is not only a result of addition agents but is also affected by the plating bath composition, temperature, current density and other operating conditions. Ductility - Ductility can be measured using a tensile testing machine; however this test is specific to measuring plated thin foils. Nickel Plating without use of Cyanide This can be done using the NICKELSOL process. The NICKELSOL process is a hydrogen peroxide-sulfuric acid formulation designed to strip nickel and copper from aluminum, plastic and stainless steel. The NICKELSOL process can replace nitric acid strippers, which cause the evolution of harmful NOx fumes. The NICKELSOL process does not contain cyanide or chelating agents and treatment of the subsequent rinse water is reduced to simple neutralization and precipitation. The NICKELSOL process offers the following advantages: • •

The bath can be regenerated indefinitely, eliminating frequent dumping and the related waste treatment cost The economical recovery of the dissolved nickel and copper is made possible by crystallization.

•

The system, in most cases, is readily adaptable to most existing automatic, semiautomatic and manual operations

•

Simple control and maintenance

The NICKELSOL Process may be used in almost any industrial application where the removal of nickel and copper from base surfaces of aluminum, plastic and stainless steel is required. The bath composition can be adjusted to meet the specific operating requirements.

Chapter 5 Process description The development of electrophoretic coating started in the USA for the painting of automotive bodies. Whilst still often referred to as "electrophoresis", it is now known that the deposition mechanism has more to do with the electrolysis of water and de-stabilization of polymer particles than with the simple movement of a polymer in an electrical field. Systems are available for both anodic and cathodic coating. In recent years, anodic coatings have given way to cathodic coatings. The solutions used are 70-90% water and the remainder consists of resin, pigments, additives and small quantities of organic solvent. The resin systems used may be acrylic, phenolic/acrylic, epoxy, epoxy/polyester or polybutadiene. It is important that whichever resin is used, it must possess a reactive chemical group which will form a salt with an acid or a base. The choice of resin system is therefore dependent upon the use for which the coating is required. The application of a potential to a solution causes the electrolytic breakdown of water at the anode and cathode. The secondary products developed begin the process of coagulation of the resin in solution. In anodic systems hydrogen ions are the secondary product whilst in cathodic systems hydroxyl ions are the secondary product. The reactions involved are: Anodic deposition: Reaction at anode: 2H2O → 4H+ + O2 + 4ePolymer-COO(soluble) + H+ → Polymer-COOH(insoluble) Cathode deposition: Reaction at cathode: 2H2O + 2e-→ H2 + 2OHPo1ymer-N+R2H(soluble) + OH- → Po1ymer-NR2(insoluble) + H2O The hydrogen and oxygen released produce foam on the wet film. This acts as an electrical resistance, hence limiting the film thickness. When a direct current potential is applied, the current seeks the path of least resistance and products nearest the electrode are coated first. As the electrical resistance increases, the current seeks paths of least resistance, thus virtually all areas can be coated, even internal sections. Clear lacquer coats (unpigmented) can be applied as top coat protection, for example to Silverware. Prior to application, the products must first be cleaned using an aqueous alkaline solution. It is

then usual to apply a phosphate coating (see Section 2.3.2). After treatment in the paint bath, the product is rinsed to remove the surface material and then heated to produce a continuous coating. The solution remaining on the surface of the product on removal from the bath, known as cream coat, is richer in resin than the basic solution. It is rinsed and the rinse water subject to ultrafiltration. This method of filtration uses membranes of various constructions. The material passing through the membrane (permeate) is used to rinse the product, preferably during its removal from the paint bath. This is followed by rinsing in further tanks of the permeate or by spraying, followed by a final rinse in de-ionized water. The material held back during filtration, which has a higher concentration of resin materials, is returned to the paint bath to avoid excess loss and assist in maintaining the optimum concentration. Solution formulation Materials for manufacture of solutions consist of a suitable resin, pigment, additive, and a small quantity of organic solvent in an aqueous solution. All formulations are proprietary and supplied in a concentrated form for dilution with de-ionized water. Typically, in use, they have a solids content between 8-14 %. Anodic systems Cathodic systems

Styrene-maleic anhydride copolymers or acrylic acid-acrylic ester copolymers. Resin based on aminoalkylesters of acrylic acid or systems with tertiary suiphonium ions, epoxides and secondary amines.

Releases It is claimed that discharges from electrocoating installations are very low with over 95% of solution being recycled by ultra-filtration. The filtration elements used in conventional filtration systems for the removal of foreign particles are disposed of by registered disposal contractors when no longer serviceable. \

Chapter 6 Process description Conventional paints are synthetic (organic) chemical materials in suitable solvents (organic or water), which dry by the evaporation of the solvent, generally by the application of heat. Lacquers are similar except they are generally unpigmented or slightly tinted. Methods of application are numerous and include spray, dip, flow coating and barreling. The solids content of the as-used material (that which remains on the product after drying) is usually less than 50% and as a consequence the remainder must be driven off by natural or forced evaporation methods to enable the coating to fulfil its decorative or protective requirements. Solution formulation Solvent and resin losses from the use of 1 litre of organic solvent/resin paint may be as follows: Total material at start of process

1000 ml (50% solids)

Amount of material lost to over spray Coating applied to article

(25%) 250 ml 750 ml

Solvent losses (50%): From overspray From article Total

125 ml 375 ml 500 ml

Resin loss: From overspray

125 ml

Chapter 7 Powder coatings are mixtures of resins and pigments blended together and supplied in fine powder form. The materials used are usually thermoplastics or thermoset powders, for example polythene, nylon, PVC, mixed epoxy polymers, polyesters, acrylics and polyurethanes. Electrostatic spray guns are generally used for the application of powders to components, although some use is made of fluidized bed principles. The component for coating is at earth potential, and is usually supported on some form of conveyor system. The powder is emitted from the gun. At the point of emission it is electrically charged and attracted to the component. The essential requirements for such a system are: •

Charged powder particle

•

An electrical field

The particles are charged by a phenomenon known as "Corona" discharge. If a voltage is applied to a needlepoint, the current flowing to the workpiece will be negligible at first. As soon as the high tension (HT) reaches about 20 kV, a current will commence to flow - this is the Corona discharge and is the voltage at which the air in the vicinity of the needle breaks down and becomes ionized. As the voltage is increased still further, the current flowing between the needle and the workpiece will rapidly increase. Thus, the area between the workpiece and the gun consists of an electric field, a cloud of particles and ionized air molecules. Particles are attracted to areas nearest to the gun first; as the covering builds up the covered area becomes insulating and so deposition occurs on more distant areas. The process is selflimiting in terms of the thickness of the coating. The powder particles are attracted to the workpiece and remain adherent, by electrostatic forces, for sufficient time to enable the workpiece to be transferred for heating, where the particles melt to form a continuous coating. Whilst some powder is lost during application, any overspray material is collected, using cyclone recovery systems for in-house re-application. Where re-use may not be economical, or color contamination is a problem, it is often sold for less critical applications (e.g. automotive chassis use). If not sold it is disposed of via landfill.

Chapter 8 Conversion coatings are produced by the chemical treatment of a metallic surface to produce a superficial layer of compound on the metal surface. Often these coatings are given their own Terminology: • • • •

Passivating or Chromating Phosphating Anodizing Galvanizing

Process description Passivating may be applied direct to a manufactured product for the following reasons: •

To extend its corrosion resistance, for example stainless steel.

•

To benefit the adhesion of a subsequent coating, for example prior to painting of aluminium or zinc-based die castings.

•

To previously applied coatings of metal, in particular cadmium and zinc electrodeposits and galvanizing. Passivated coatings increase the corrosion resistance of these coatings and prevent the oxidation of the coating (white rust formation).

Various degrees of passivation are available. These are usually designated by the color obtained: •

Bright Colorless to pale blue

•

Full

Yellow, iridescent color Olive drab (khaki) Black (Produced directly from a passivate solution or by dyeing of an olive drab film)

Generally, passivation solutions for the treatment of zinc and cadmium consist of an aqueous solution of inorganic chemicals, traditionally based on chromates or dichromates. Recently some organic based materials have been developed to comply with environmental legislation, and the use of trivalent chromium in place of hexavalent chromium salts is becoming more common. All solutions are proprietary developments of `supply companies' from whom the product is obtained in either liquid or solid form for use in an acidic media. In addition to

chromium salts, activators are also present in very low concentrations, such as acetate, formate, chloride, nitrate, phosphate and sulphamate ions. The solutions are used at room temperature and fume extraction is not normally required. Discharges are due to drag-out into the water rinses. Solutions are replaced periodically and the spent solution treated for reduction of hexavalent chromium prior to discharge. The amount discharged is dependant upon the method used, such as rack or barrel. Aluminium and its alloys may be passivated prior to painting, as an alternative to the more costly process of anodizing. The solutions used are similar to those used for zinc and are based on acidic hexavalent chromium. They are used at slightly elevated temperatures. Solution Formulation

Phosphate coatings consist of layers of crystalline, water insoluble metal phosphates of varying crystal size. The crystal size is dependent on the type of phosphate used and the surface condition of the product being treated. Most metal phosphates are insoluble in water but soluble in mineral acids. This forms the basis of the phosphate coating reaction. Commercial phosphating solutions are carefully balanced solutions of metal phosphates dissolved in phosphoric acid. When a reactive metal is immersed in the solution, light pickling takes place at the liquid/metal interface. When metal from the substrate is dissolved, hydrogen is evolved and the phosphate coating deposited. As the coating is formed in place at the metal surface, it incorporates metal ions dissolved from the surface of the product. The deposit formed is a conversion coating and differs from electrodeposited coatings, which are added to or superimposed on the metal. Phosphate coatings fall into three main types: • • •

Iron phosphate: An amorphous coating suitable where a coating film in the order of 300-700 mg/m2 is required. Zinc phosphate: Lightweight (1.0-4.5 g/m2), medium weight (4.5-10 g/m2) and heavy weight (10-3 0 g/m2) Manganese phosphate: For coatings of 10-30 g/m2.

Solutions for phosphating are based on the tribasic acid, ortho-phosphonic acid H3P04 and give rise, on neutralization, to three series of salts: • • •

Primary salt NaH2PO4.Zn(H2P04)2 Secondary salt Na2HPO4.ZnHPO4 Tertiary salt Na3PO4.Zn3(P04)2

An example of a phosphating reaction is: Fe + 3Zn(H2P04)2 →FeHPO4 + Zn3(P04)2 + 3H3P04 + H2 While, a very simple process, the theoretical equations, by which, it occurs are complex. Phosphating from a simple phosphoric bath is time consuming; hence other chemicals may be added to reduce process times. Referred to as accelerators, they may be divided into two classes: • •

Additions of heavy metals, particularly small quantities of copper and nickel in the form of a soluble salt at a concentration of 0.002-0.010%. Additions of oxidizing agents, particularly nitrates, nitrites, chlorates and some organic nitro compounds.

Modification of the coating crystal structure may be made by the deposition of a mixed element layer, such as calcium or manganese. The proprietary processes are usually stated, for example calcium modified zinc phosphate and nitrate accelerated zinc phosphate.

The product should be in a clean, rust free condition prior to treatment; therefore most installations include pre-treatment stages. Post treatment is advisable to impart the best corrosion resistance properties. Post treatments are based upon chromic acid or alkaline metal chromates or dichromates with a chromium concentration of between 0.10-0.5 g/l. A typical phosphating operation may be: • • • • • • • •

Cleaning 2-5 minutes Rinse Rinse Phosphating 2-30 minutes (Dependent on type and weight of coating) Rinse Rinse Chromate rinse 15-60 seconds Dry

The method of application may be either by immersion or spraying. There are some processes which both clean and phosphate in a single operation. Solution formulation All phosphate preparations are proprietary but consist of iron, zinc or manganese phosphate in phosphoric acid with low concentrations of other metals such as iron or copper and calcium. They are usually supplied as a liquid concentration and between 20-100 ml/l are used. The size of bath used varies depending upon the object to be processed; the smallest size in use is approximately 1000 liters. Table 2.19 gives the typical formulations of some phosphating solutions. Note that the concentrations in this table are those in the formulation, not those in the actual treatment bath (in contrast to most other tables in this document). Table 2.20 gives examples of concentrations of species in baths in use, from German industry.

Process description Anodizing is an electrolytic process designed to produce an oxide film integral with the surface of the metal. In theory anodizing can be applied to a number of metals such as zinc, magnesium and titanium, though its only commercial application at present is as a treatment for aluminium. Both of these processes are usually used prior to sulphuric acid anodizing. There is also a usage of phosphoric acid in electropolishing which is used in a variety of processes. There is also a growing volume of matt chemical polishing. This is carried out in a bath containing approximately the following formulation: • • •

Phosphoric acid 80% v/v Sulphuric acid 20% v/v Operating temperature 90-105°C

Aluminium coil coaters use an electropolishing solution of a similar formulation. The final stage for almost all anodizing processes is sealing, which is preceded by dyeing in many decorative applications. The dyestuffs are complex organic materials, usually at low concentrations of around 2g/l, with concentrations of 6 g/l for black dye. Sealing is usually accomplished with boiling water, but sealing effectiveness can be improved by rinsing with nickel acetate. Actual concentrations of nickel acetate used vary widely, but 5-15 g/l is common. At the cleaning stage, the chemicals used are alkaline in nature and sometimes pH adjustment may be sufficient before release. If an acid rinse is in the process line then virtually automatic pH control may be obtained. If the acid is used for pickling it may contain heavy metals, and so precipitation will be required followed by settlement or filtration prior to discharge. Similar treatment may also be needed if there is any significant release of metals from the substrate being cleaned during the process. The requirements for effluent treatment or discharges from the phosphating rinse are dependent upon the local water authority and may require one of the following: •

Simple neutralization

•

Neutralization and removal of suspended solids

•

Neutralization plus removal of phosphates

Where chromate treatment is used, no rinsing is generally undertaken; hence no waste treatment is necessary. Sludges from phosphating can be a problem during production. They can settle out at the bottom of the process tank and also coat the heating coils, immersion heaters etc. Periodically the solution must be pumped to a storage tank and the sludge removed

and disposed of by registered contractors. Anodizing Releases The major discharges from the anodizing industry are sodium sulphate and aluminium. There will be very minor discharges of other metals from a variety of low concentration sources. In addition there will be some discharge of oxides of nitrogen to air, and discharge of nitrates and phosphates to water from chemical brighteners. The following releases are based upon information provided by a supplier to the anodizing industry. The amount of phosphoric acid consumed and therefore ultimately going to effluent is 1500-2000 tones/annum, Nitric acid is removed from exhaust fumes by scrubbing and ultimately discharged to effluent, the total amount discharge is approximately 500 tones/annum. Approximately 100 tones/annum nitrous oxide fumes are produced, of this 25% will be scrubbed and discharged to water, the remainder being discharged to air. There is currently no significant volume of phosphoric acid recycling.

Mechanical plating Mechanically deposited coatings of cadmium, tin, tin/zinc and zinc can be cold welded onto ferrous metals, individually or in combination. Deposits are produced by impingement, cold welding and compaction of metal powder or granules onto cleaned and suitably activated ferrous substrates. The structure of the deposit is typified by the presence of agglomerated particles and voids in the coating. The protective and functional properties of the coatings are similar to electrodeposits of equivalent film thicknesses. The main advantage of this process over electroplating is that coated parts can be produced which are free from hydrogen embrittlement; the process is especially suited to coating severely cold-worked parts, heat-treated or surface-hardened components and components manufactured from high tensile steels. In common with barrel electroplating, limits exist with regard to component size, weight and shape. After degreasing and cleaning, components to be coated are loaded into a barrel with the appropriate quantity of glass beads, water and promoter chemicals in order to condition the surface of the components. The metal to be deposited is then added in powder form, the quantity being dependent on the surface area of the components and the coating thickness required. Rotation of the barrel at the appropriate speed results in the generation of impact forces by the glass beads or the components, with the subsequent cold welding of metal granules on the substrate. After the prescribed time, the components are separated from the glass beads and dried. A specialized version of mechanical plating operates under the proprietary name of Sherardizing. While not strictly a mechanical process, it can be compared to carburizing, with which it has similarities. In carburizing, heating with a carbon-bearing media causes the carbon to be absorbed into the surface. In sherardizing a similar phenomenon takes place but zinc is absorbed in the surface. In reality, the process could be called a mechanical diffusion process. After the necessary cleaning and pre-treatment, the articles are loaded into a container with the

pre-determined quantity of zinc dust, which is dependant upon the thickness required, and inert filler which prevents mechanical damage and ensures even distribution of the zinc dust. The sealed container is then loaded into a furnace and the temperature is raised to the required level. The temperature used is normally between 350-450°C, and is chosen so as to not affect the physical properties of the material being processed. When the operation is complete, the sealed container is removed and cooled. The parts are separated from the inert filler, which after screening can be re-cycled. The residual zinc dust is discarded and disposed of to landfill. Articles processed in this way are often given a post treatment to further increase their corrosion resistance and life span by such treatments as passivating, phosphating or blackening/oiling. Similar to galvanizing, this process gives very good uniformity of coating over contoured and recessed articles.

Process description Galvanizing is the most widely used of the major methods for the coating of iron and steel with zinc, particularly for corrosion resistance in the `as produced' state or as a pre-coating for paint finishes. The protection afforded to iron and steel is not due solely to the barrier effect of zinc forming a continuous coating over the whole area, but largely due to its behavior as the anode in electro chemical reactions. The result is that zinc corrodes in preference to the underlying substrate. In this context, the zinc coating acts as a sacrificial coating. Corrosion of zinc results in the development of a tenacious carbonate film, which resists further attack. Zinc/ aluminium alloys can also be used for coatings in a similar way. Zinc coated sheet is used for many presswork applications. The zinc coating has the ability to `roll over' the cut edges during the press operation, and often gives the required protection without further treatment. The usage of zinc is in the order of 100,000 tones per annum in the United Kingdom, of which some 45% is used for continuous strip and sheet, 15% for wire and tube and the remainder for general component processing. When a clean and fluxed component is dipped into molten zinc at a temperature of round 450CC, a series of zinc-iron alloys are formed by reaction of the zinc with the component surface. At the normal galvanizing temperature of 450°C, the reaction between the iron and zinc is usually parabolic with time i.e. the reaction is rapid at first then slows down. Hence the zinc layer reaches a certain thickness quite rapidly, after this there is no significant increase in the thickness. An exception is with high silicon steel where the reaction is linear with time and hence very high thicknesses can be produced. Cleaning and acid rinsing are essential treatments prior to a specific treatment known as Fluxing. Fluxing is categorized by three descriptions: • • •

Old-Dry - The components are rinsed in hydrochloric acid and dried without rinsing. The acid salts on the surface act as the flux when the components are treated in the molten zinc bath. Dry - After acid rinsing, the components are treated in a flux bath and dried prior to transfer to the zinc bath. Typically the flux could be based on zinc ammonium chloride of about 30% concentration. Wet - The components are transferred after rinsing directly to the zinc bath, which has a blanket of molten flux floating on the surface of the zinc. The blanket typically zinc ammonium chloride together with foaming agents to thicken the blanket and lower the surface tension.

There are three types of process used in the galvanizing industry: • • •

General hot dip galvanizing Continuous hot dip galvanizing Continuous electroplating processes.

In General hot dip galvanizing, the components to be coated, after flux treatment are dipped into a bath of molten zinc. Larger items are lowered into the bath by crane; smaller items are immersed in perforated steel baskets. The duration of the immersion varies from a few minutes to 30 minutes. After treatment the items are removed from the bath and excess zinc is removed - this may be returned to the bath or may be sent for reclamation. Fumes can be generated during the treatment, so the baths either have an extraction system or are located in a ventilated enclosure. The ventilation air is cleaned by bag filters. The zinc used is generally of a good commercial standard (98.5%) and contains just over 1% lead, as lead is soluble to about 1% in molten zinc. Excess lead separates out at the bottom and is usefully employed to prevent the dross (a pasty zinc iron alloy of ratio 25:1) from sticking to the bottom of the bath, and hence aiding in its periodic removal. Aluminium is often present in very small quantities (0.005%) to prevent surface oxidation, improve the surface brightness and give a smoother coating. Following removal from the zinc bath the components are quenched to cool and remove any residual flux, where a blanket flux has been used, to prevent staining and to facilitate easier handling. In Continuous hot dip galvanizing, steel sheet/strip material is surface cleaned, then fed through a heat treatment furnace with a reducing atmosphere for cleaning and annealing. It is then fed directly into the galvanizing bath without contact with air to prevent re-oxidation, therefore eliminating the need for fluxing. Since the substrate is already at temperature, most of the heat required for the galvanizing bath is supplied from the substrate. Coils are automatically welded together before entering the system to give a fully continuous process. The speed at which the strip passes through the zinc bath means that the coating consists mainly of zinc metal rather than of zinc-iron alloys. After treatment, gas `knives' are used to remove excess zinc. The strip is then cooled gradually, quenched in water and dried. Any further finishing to give the desired surface properties and appearance is then carried out; the strip is cut to the required length and then recoiled. Coils of finished galvanized steel are very valuable and are always protected against oxidation by a chromate rinse layer. An oil film, plastic wrap or interleaved paper, or a combination of all. They are stored under cover, usually with controlled temperature and humidity. Releases The aqueous discharges from the pre-treatment sections are similar to those experienced with other metal finishing operations. The dross removed is collected and sent for reclamation, since it is rarely economical for processors to carry out reclamation themselves. Zinc ash is formed by the disturbance of the surface of the liquid during the operation; as a result the zinc oxidizes and particles of zinc are entrapped. The ash is therefore a mixture of zinc oxide and varying quantities of entrapped zinc which may be as high as 80%. Oxidation also occurs during idle periods and further increases the production of ash. The ash is periodically removed and subjected to various methods for zinc reclamation; these include the cylinder method, the static crucible method and the rotary crucible method. In practice it is possible to obtain a yield of about 50% by the above methods.

Both types of hot dip galvanizing involve the use of air extraction systems, with bag filtration of the ventilation air, and recovery of zinc from the bag filters. Run-off loss from continuous hot dip treated steel is considered to be negligible in view of the post-treatment handling and storage of these materials. More detailed consideration of the releases of zinc from the galvanizing processes can be found in the draft risk assessment report.

Vacuum deposition Physical vapor deposition In the physical vapor deposition (PVD) process material is vaporized and transmitted in the vapor phase through a vacuum or low-pressure environment to a substrate where it condenses. PVD processes are used to deposit films of compound materials by the reaction of the material with the ambient gas environment or with a co-deposited material. Film thicknesses can vary (11000 nm) and layers can be built up to form multilayer coatings and thick deposits. Vacuum evaporation is a PVD process in which material from a thermal vaporization source reaches the substrate without collision in the gas phase. As such there is no scattering and the process is by line of sight. Typically vacuum evaporation takes place in the pressure range of 10-3 to 10-7 Pa. Vacuum evaporation is widely used to form optical interference coatings, minor coatings, decorative coatings, barrier films and electrically conducting films as well as corrosion protection coatings. Examples of products processed include: minors, lamp reflectors, costume jewellery, and toys; examples of the coatings produced are anti reflective oxide coatings on spectacle lenses and sun glasses, barrier films on flexible packaging materials and abrasive and wear resistant coatings. Sputter deposition is the deposition of particles vaporized from a surface. It is a non-thermal process in which the surface atoms are physically ejected by an energetic bombarding particle, usually an ion accelerated from a plasma stream. It is performed in a vacuum or low-pressure gas (800°C) and generally involves the transport of volatile species to the surface of the component being coated. The volatile species then undergoes a chemical reaction and deposition can then occur. The following equations represent the common reactions that take place: 2MX(g) + H2(g) →M(g) + 2HX(g) 2MX(g)

→ M(g) + X2(g)

M(l)X + M(2) → M(1)(s) + M(2)X(g)

(1) (2) (3)

Notes: M and M(1) are the depositing material, M(2) is the substrate material and X is a halide such as chloride, iodide or fluoride. The MX species is generated in a maimer that is convenient with respect to its physical properties. For example where M is aluminium or titanium, then the vapor pressure of these compounds is sufficiently high to be able to generate these compounds external to the hermetically sealed retort and using only moderate heating to the line (up to 200°C) pass them into the retort along with any inert gas and/or hydrogen. MX compounds with a low vapor pressure (e.g. CrCl3) are generated in the coating reactor by the reaction of the metal with HX.X2 or a salt such as NH4X. Practical experience shows that reactions (1) and (3) commonly occur during CVD and the waste gases tend to be hydrogen, hydrogen halides and inert gases. Flow rates of the exhaust gas are not high (e.g. 10 liters/minute) and the high solubility of the acidic hydrogen halides in water means that a water scrubbing tower is a convenient and effective way of removing these compounds from the gas stream Passing exhaust gases up a tower countercurrent to a mist of alkaline water (e.g.. Water containing dissolved sodium bicarbonate) is adequate for this purpose. The resulting scrubbing solution is kept alkaline with extra additions of sodium bicarbonate as it reacts with the hydrogen halide as: HX + NaHCO3→ NaX + H20 + CO2 This produces easily disposable liquor. Any heavy metals that are produced during the process (or indeed that are not consumed) will normally condense in the cooler zone of the retort and be collected for disposal at the end of the run.

Vitreous enameling Process description Vitreous enamel is also known as porcelain enamel, especially in the USA. Vitreous enamel is the fusion of an inorganic coating (glass) to metal to produce a hard coating, which is permanently bonded to the metal substrate. It has all of the properties of glass - hardness, temperature, chemical and abrasion resistance, durability, and color stability. It is widely used where these properties are an advantage, for example in kitchen equipment and bathroom fittings. In these applications it is usually applied to steel or cast iron. The steel required for this application has specialist properties to make it suitable for the process. It is also used in architectural applications where its durability, fire resistance and graffiti resistance are finding increasing uses. Its chemical resistance makes it a suitable coating for agricultural and sewage storage tanks. A combination of its chemical and heat resistance properties make it suitable for use in elements for flue gas desulphurization plants and heat exchangers for power stations. High technology applications such as printed circuit boards, heating elements and aerospace equipment are growth areas. Other applications include in jewellery and ornamental goods. Vitreous enamel materials can be produced in a range of colors and decorated by screen-printing, transfers or painting. The process of vitreous enameling starts with the production of the glass, normally of the borosilicate type, which is smelted to form a `frit'. This is formed by quenching the glass rapidly in water forming a granular or flake form. This is ground in a ball mill with high density alumina media. For wet applications (by spraying or dipping) it is ground with water into a suspension with clays and salts to produce the appropriate rheology. It can also be applied electrostatically as a dry powder. In the wet process the enamel is dried to remove the majority of the water and then fired in a furnace at temperatures of about 800°C for steel substrates, and at lower temperatures for aluminium and copper substrates. The frit then fuses forming a metallurgical bond with the substrate. For jewellery applications onto copper and its alloys or precious metals, the enamel is often applied as a dry powder and held in place with a gum such as Gum Tragacanth. After firing it may be polished. Vitreous enamel may be applied as a single coat called the direct-on process, or by the prior application of a ground coat. Coloring pigments are complex metal-alumina-silicates formed by calcinating transition metal oxides with alumina and silica. For deep colors up to 8% pigment may be used. Prior to the enameling operation, it is important to ensure that the substrate is chemically clean and conditioned to promote the formation of the metallurgical bond and to achieve good adherence of the enamel to the substrate. The pre-treatment necessary will be dependant on whether a ground coat or direct-on process is used for enameling. If the ground coat process is used than a hot alkaline soak may be sufficient, particularly if the ground coat is highly reactive. However, it is more usual to use acid pickling followed by deposition of a thin layer of nickel applied by either electroless electrolytic plating. A typical

process sequence may be: • • • • • • • • •

Hot alkaline soak Rinse Sulphuric acid Rinse Nickel Deposition Rinse Rinse Enameling Application Dry and Fire

Solution Formation The frit may be purchased ready for use. Electroless plating (Autocatalytic plating) Process description A limited number of metals can be deposited by chemical reduction rather than by electrical reduction. The basic reaction is: M2+ + 2e -→ M The deposits produced by electroless plating are almost completely uniform in thickness compared to electrodeposits, which vary in thickness. It is also possible to plate onto nonmetallic surfaces, for example plastics and ceramics. Chemicals need to be added to the bath continually to replace materials as they are used up. This leads to a build up of breakdown products in the bath, which reduces its efficiency. Several metals can be deposited in this way, but in practice copper and nickel are the only two deposited on a large commercial scale. Copper is used in printed circuits and electroless plating is the major method used for depositing copper through the hole connections. Electroless copper is also used for the decorative plating of plastics, but has being largely replaced in this field by electroless nickel. The main use of electroless nickel is in engineering where it is applied as a hard, corrosion-resistant coating. Electroless gold is being developed for use in the electronics industry, though due to its high costs its use is likely to remain limited. Electroless silver is used in the electroforming industry, as a means of metallizing non-metallic mandrels. Electroless cobalt has a special application in computer memory discs, and possible applications in rocket technology. The other electroless deposits have no serious commercial applications at present. Formulations A common factor in all electroless formulations is the presence of complexing agents. These range from very strong chelators such as EDTA to acids such as citric and tartaric acids. For electroless nickel, carboxylic acids are used extensively. For instance the following compounds are in regular use: acetic, propionic, lactic, glycollic, maleic, succinic, citric, and tartaric acid.

Addition agents are used sparingly in electroless formulations. Sulphur compounds such as mercaptobenztriazole are used as stabilisers, and lead and cadmium salts can be used as brighteners, though their use is declining. Copper Table 2.24 gives the typical formulation of a electroless copper bath. Copper sulphate is usually used as the source of metal ions, though copper formate and copper nitrate may also be used. Complexing agents used included the tartrates and EDTA. The stabilizers used are sulphurcontaining compounds such as thiourea, thiodiglycollic acid and mercaptobenzthiazole. Sodium cyanide and vanadium oxide may also be used. Releases The losses due to dragout in normal use will be small and in many cases so small as to require little or no treatment. Waste disposal problems may occur at the end of the working life of the solution when the solution has to be discarded. The time interval between solution changes varies depending upon the size of the user. For large scale users changes may be required at 2-3 day intervals while for smaller scale users changes at intervals of 1-2 weeks may be required. The typical size of an electroless nickel tank is 200-1000 liters, though tanks up to 6000 liters are in use. The solution for disposal contains a number of breakdown products. The typical content of a spent electroless nickel solution is given in Table 2.30. Table 2.30 TypicaL content of spent electroless nickel solution

Nickel

Approximately 5 g/l

Sodium hypophosphite

Approximately 10 g/l

Other phosphates and phosphites

30-50 g/l

Mixed carboxylic acids

50-80 g/l

Lead

2 PPM

Sulphur compounds

1 PPM

Unlike electroplating solutions, electroless plating solutions have a finite life. This is usually expressed as the number of metal turnovers accomplished, and is commonly of the order of 3 to 8 metal turnovers with 6 metal turnovers being the mean. At the point of disposal the solution will contain about 3-5 g/l of Nickel, together with a mixture of phosphates and phosphites, a considerably quantity of sodium sulphate, and a quantity of the complexing acids, typically acetic acid, lactic acid and glycolic acid. The actual concentrations of

these acids can vary considerably, but will normally be higher at this stage than the original make up concentration, and could be as much as 50% higher. Methods of disposal vary widely. Very small operators will bleed the spent solution into the main effluent treatment system, where the nickel will be partially removed, but all other materials will pass directly to the waste stream. Some operators treat the solution first to remove the nickel. There are two main methods. Precipitation of the nickel as a fine powder by the addition of a powerful reducing agent such as hydrazine or sodium borohydride is in some use. The other method is to break the complex with sodium dithionite, then precipitate the nickel as hydroxide at high pH. The resultant waste stream will still contain the various complexing acids and phosphates. None of the above methods is truly satisfactory and so there is a growing tendency for spent solutions to be disposed of to landfill through licensed waste disposal contractors. There has been some investigation into nickel recovery of bulk solutions by specific ion exchange, but this has not proved financially viable. As a consequence when the material goes to landfill, it goes as a total spent solution. The nickel in the solution may be removed relatively easily by oxidation, precipitation, reduction, electrowinning or ion exchange. The carboxylic acids can be removed by biological degradation, though no viable system is in use at present. There is also no viable system for the removal of phosphates at present. Electroless copper solutions have a longer life. In this case the residual materials are copper and formate. The life is often extended by a system of bleed and feed, which means that small amounts are continually run to waste. As in the case of electroless nickel the final destination of these materials is landfill. In electroless copper solutions the copper metal is quite strongly complexed which has led to considerable problems. The best method of removing copper is by treatment with complexing ion exchange resins. This leaves a residue containing formaldehyde, formic acid, and assorted complexing agents such as EDTA and tartrates. Barrel Plating is used when the plating is done inside of a perforated barrel, and the barrel is rotated to even the plating. It is mainly used in the plating small diverse objects, devoid of sharp and long edges that tend to plate badly. Functions of barrel plating a. The primary function of barrel plating is to provide an economical means to electroplate manufactured parts that also meets the customer’s specific finishing requirements. b. The four most important requirements are: - Engineering applications, such as building up the thickness of metal to change the physical size of a part or to provide a good surface for some other treatment such as painting or screening. - Decorative coatings such as Bright Nickel, Brass, and Antiquing.

- Cosmetic uses such as Zinc plating to improve shelf life and selling ability. - But by far, the most important use of barrel plating is to extend the corrosion protection of the customers' parts. Barrel plating fundamentals and the production process a. Parts need only to be free-flowing enough to enter the mouth of the barrel. b. Loads should not exceed half the volume of the barrel or improper tumbling will occur and a loss of plating uniformity. c. The surface area of the plated parts should generally be about 25 sq. feet for every foot length of the barrel at a 14 inch diameter. d. Parts must be able to tumble freely to insure a good plating distribution. Such interior protrusions as breaker bars, dimples or ribbed sides should be used as necessary. e. The rotation of the barrel while in the plating tank is also very important. Typically a speed of 3 to 6 RPM is considered adequate but faster speeds facilitate a more uniform deposit even though there may be some physical wear on the barrel itself. As long as the parts themselves will not be harmed it is more desirable to maintain as fast a rotational rate as is practicable. f. Barrel sizes and hole perforations should be chosen with care depending on the size of the parts to be plated. Too small a hole will trap solution by capillary action and drag the chemicals all along the plating line. Too small a barrel and the parts will not tumble properly. Quality control a. Proper and on-going training is extremely important for successfully barrel plating any part. b. Records should be kept regarding all of the important parameters involved in each step along the plating cycle. These should include things such as part description, load size, voltage, time, thickness readings, chemical additions and also any problems which may have taken place during the cycle. c. All relevant data and notes should be routinely reviewed to assure that the product will remain at a consistent level of quality and that the process can be continuously improved. d. There are numerous quality systems which the customer may require the barrel electroplater to employ such as the ISO 9000 standard which is one of the more recent attempts to help barrel electroplaters achieve the highest level of customer satisfaction possible. The single, most important, factor to be considered when purchasing barrel plating equipment is to understand that the equipment you are buying is part of a system. Your plating line is a kind of an industrial ecosystem. Every component barrels, tanks, rinsing system, etc. affects the results generated by every other component. Any kind of slightest change in one piece of equipment can result you to pay the penalty further down the line.

This principle applies to both new equipment purchases or the repair and retrofitting of existing plating lines. COMMON PROBLEMS ASSOCIATED WITH BARREL PLATING How can one limit the amount of or recover the waste in the barrel plating process. Barrel plating has existed—in one form or another—since the close of the Civil War. And while the technology has seen some radical improvements in the last 140 years, modern day barrel plating is not without its challenges. Problem: Spikes in Cyanide Concentration During a plant visit, along with a careful analysis of their operations it showed that periodically, the conventional horizontal barrel line is over-loaded with work and the oscillating barrel line is then used to plate the over-load. Unfortunately, the over-load consists of cup-shaped parts that create a very high drag-out, as the oscillating barrel line carries these cups into the rinse system without emptying them over the plating tank prior to transfer. A normally rotating barrel would empty the cups over the tank (as is done on your other plating line) and would do a better job of rinsing these parts. After measuring the drag-out rate it was found to be about 1.5 gallons per barrel. As a result of operating the oscillating line on these parts, a large amount of cyanide entered the rinse system after plating. Even a well working waste treatment system can be over-loaded by a spike in cyanide concentration. The systems were designed around 100500ppm of cyanide, while the spikes were around 2000-2500ppm. The solution to the problem is to not use the oscillating barrel plating line on cup shaped parts. Problem: Damaged Parts Since tin is a soft metal, it can easily be abraded in a barrel plating operation. The factors to look at include the condition of the electrical contacts, barrel rotational speed and use of ballast. An examination of the parts under the microscope (see photo) indicated that some severe scraping is going on in at least some of the barrels you are using. The following corrective actions should be considered: 1. Change Method of Electrical Contact The barrels used employ conventional danglers, which can build up in metal to the point of being abrasion sources. Button contacts or rod contacts may be more gentle. 2. Maintenance of Electrical Contacts One of the frequently neglected tasks in barrel plating is maintenance of the dangler. As metal builds up on the electrical contacts within a barrel, they develop sharp edges that can cause damage to a moving load. If the electrical contacts have any heavy build-up of metal, this must be removed on a more frequent basis.

3. Change the Barrel Speed The barrel speed may cause too much friction between the dangler and parts and between the parts themselves. If possible change to rotational speed of the barrel. By lowering the speed, you may also need to lower the barrel loading. The best combination of barrel loading and speed will have to be determined by trial and error. 4. Use/Change Ballast Ballast can be used to keep parts separated during plating, reducing damage from contact with sharp features on the parts and also improving coverage. If you are not using ballast, it is recommended to try this. Common ballast is copper beads as which comes in a variety of sizes. But you need to experiment with both shape and size to arrive at the optimum combination. Problem: Plating Solution "Growth" This is the most common reported problem. In most cases it is a case of more drag-in than drag-out from the plating solution. The chloride zinc process typically contains a high concentration of wetting agent (surfactant), which lowers the surface tension of the plating solution and results in better drainage of the barrel as the barrel is removed from the plating tank. Since the rinse before the plating tank does not contain any wetter, the barrel does not drain as well before going into the plating tank. Over time, the difference in dragin volume vs. drag-out volume causes the plating solution to “grow.” If there is a drag-out rinse, try going into this rinse before you bring the barrel into the plating tank. Since the drag-out rinse will contain some of the wetter, this may solve the problem. Some wetter may need to be added to the drag-out rinse to bring the surface tension closer to that of the plating solution. Solution: If there is no drag-out tank, some wetter may be bled into the last rinse prior to plating. Heating the last rinse before plating may also help, as warm water drains better than cold. Brush Plating Brush plating is an electrochemical process that uses systems to electroplate, anodize, and electro polish localized areas on both OEM components and parts that need coatings for repair and dimensional restoration. Brush systems are portable. Unlike their tank counterparts, brush plating systems use very small volumes of solution (usually only one or two gallons) and hand-held tools to apply the deposits and coatings onto localized areas. These hand-held tools are covered with an absorbent material that is saturated with a solution and then brushed or rubbed against the part. Brush plating requires different hand-held tools for each different solution in the operation. A portable power pack (rectifier) provides the direct current required for all the processes. The power pack has at least two leads. One is connected to the tool and the other is connected to the part. The direct current supplied by the power pack is used in a circuit that is completed when the tool is touching the work surface.

The work surface is prepared using the same types of tooling and equipment that are used for the final finishing operation. As with a tank plating process, brush plating requires good preparation of the work surface to produce an adherent deposit. Brush plating has come a long way from the early days of tank plating when it was a common practice to touch up bad spots on plated parts using solution saturated rags wrapped around pieces of pipe. Today, brush plating and anodizing systems are used to selectively apply engineered deposits and coatings in very precise thicknesses for both OEM and repair applications. Brush plating and anodizing are now completely divorced from their tank counterparts, although some of the equipment and terms still resemble those used in tank processes. Tools, equipment and solutions, however, cannot be used interchangeably between brush and tank systems. Since it is more difficult to control temperature and current density in portable finishing processes than in tank processes, it was necessary to develop complete, integrated portable finishing systems for commercial applications. These systems were developed for operators who are not familiar with tank finishing techniques. Today, brush plating systems are available for electroplating, anodizing, hard coating and electro-polishing. These systems vary in their sophistication and coating capabilities. Small pen-type systems apply only flash deposits on small areas. Larger, more sophisticated systems use power packs with outputs up to 500 amps and are capable of producing excellent quality finishes and high thicknesses on large surface areas.

Chapter 9 Avoiding Contamination & Corrosion Before anything is plated, the parts to be coated must be CLEANED. Electroplaters use CLEANERS for this. They are alkaline materials that remove oils, dirt and rust. In a typical plating line, the part is first immersed in a cleaning tank, then in an electro-cleaning tank (uses power from a rectifier to aid in cleaning), and then into the plating tank. A typical plating tank has three copper bars suspended over its top: One connected to the negative lead from the rectifier and two connected to the positive lead. The racks of parts to be plated hang from the bar that is connected to the negative lead, the anodes (metal to be plated) from the positive bars. The solution in the tank may have to be heated or cooled. For this, electroplaters use Immersion Heaters or Heat Exchangers. The solution becomes contaminated with dirt and other particles, which would cause rough plates. To prevent this, electroplaters use filters. In some cases the plated part is chromated. Zinc plated parts; for example, will become bluish or yellowish if they are chromated. You can see such appearances on nuts and bolts you buy in a hardware store. The chromate coating is applied by dipping the zinc plated part in a tank containing chromic acid and other chemicals. The acid reacts with the zinc plating to form a zinc chromate. This is called a conversion coating, because the chromic acid solution converts the surface to zinc chromate. This coating further improves corrosion resistance. There are also black and olive drab conversion coatings. Larger parts are usually plated on racks. But if you have a million nuts and bolts to plate, you don't want to hang each of them individually on a plating rack. For this reason a plating barrel is used. The parts are dumped into a plastic barrel with holes drilled into the plastic sides. Then the barrel load of parts is immersed into the plating solution. Inside the barrel is a dangler, a piece of flexible metal that reaches down into the load of nuts and bolts to carry current to them. The current is conducted from part to part by their electrical conductivity and the whole load begins to be plated. The barrel is rotated while current is applied. The nuts and bolts become plated with zinc or cadmium or whatever is desired. This is barrel plating. If you have lots of racks or lots of barrels and you don't want to hand carry them from tank to tank you can attach them to a conveyor that moves the racks or barrels from tank to tank, immersing them in each solution for a preset time. This is conveyor based plating, which may be done from an automatic line or from a hoist line. Understanding and Avoiding Corrosion There are 3 types of corrosion: • • •

Auto Corrosion Contact Corrosion External Corrosion

The most commonly occurring types of corrosion are Auto and Contact corrosion.

Auto corrosion occurs when a metal is in contact with an electrolyte but is not at the same time in contact with any other electrical conductor, neither metallic nor non-metallic. Simple case of Iron and Rust creation – In chemically pure iron, corrosion would proceed simply by the exertion of the solution pressure of the metal, in conjunction with the presence of hydrogen ions and the oxygen dissolved in the electrolyte, which depolarize the metal surface, oxidize and precipitate the primary products of solution as ferric hydrate or rust. In practical cases, auto corrosion proceeds by the galvanic action which is set up as a result of the heterogeneous structure of the metal or alloy. No commercial metal exists in which there is perfect homogeneity, there is always some characteristic of structure, some slight degree of segregation or the presence of embedded impurities which is sufficient to impart varying potentials or solution pressures to adjacent areas of the metal surface. For this reason auto electrolysis is set up by which the more electro-positive areas dissolve and, in the case of ferrous material, are eventually precipitated as rust. The pronounced heterogeneity of some alloys, such, for instance, as brass, is no ' doubt largely responsible for the rapidity with which they frequently corrode, and in the case of iron and iron alloys there is a large volume of evidence to show that heterogeneity, whether induced by structure or segregation, etc., is conducive to accelerated corrosion. Contact corrosion occurs when the metal is in contact with some other conducting material, which is also immersed either wholly or partially in the electrolyte: • •

If this other conductor is a metal, then the corrosion of the first metal will be either accelerated or retarded, according to the electro-chemical relationship between the two metals. If the second metal is electro-positive to the first, then it will protect the latter at its own expense by itself corroding or dissolving preferentially, but if it be electronegative to the first metal then the corrosion of this will be accelerated (or the second metal will receive protection at the expense of the first).

The practical recognition and application of this may be found in the practice of protecting boilers from corrosion by inserting slabs of the more electro-positive metal zinc and in the protective coatings of zinc which are applied to iron products by various processes. Other conditions being the same, the rate of the contact corrosion of a metal is usually greater than the rate of its auto corrosion. If the second conductor is nonmetallic in character, it may generally be assumed to be electro-negative to the metal, and contact between them will therefore result in an accelerated corrosion of the metal. External Corrosion is the result of the passage of a current, generated from some external source, through the metal whilst the latter is in contact with an electrolyte: •

If the current flows in that direction which necessitates the metal acting as anode, then corrosion results.

•

If the current flows in the opposite direction, i.e., from the electrolyte into the metal, the latter receives protection from corrosion which may be complete provided the E.M.F. of the current is sufficiently high.

Figure Demonstrating the chemical reaction of corrosion on Iron/Steel surface Surface preparation It is commonly accepted and often quoted by electroplaters that one can make a poor coating perform with excellent pretreatment, but one cannot make an excellent coating perform with poor pretreatment. Surface pre-treatment by chemical and/or mechanical means is important in the preparation for electroplating. Surface treatment and plating operations have three basic steps: 1. Surface cleaning or preparation. Usually this includes employing of solvents, alkaline

cleaners, acid cleaners, abrasive materials and/or water. 2. Surface modification. That includes change in surface attributes, such as application of

(metal) layer(s) and/or hardening. 3. Rinsing or other work-piece finishing operations to produce/obtain the final product.

Success of electroplating or surface conversion depends on removing contaminants and films from the substrate. Organic and nonmetallic films interfere with bonding by causing poor

adhesion and even preventing deposition. The surface contamination can be extrinsic, comprised of organic debris and mineral dust from the environment or preceding processes. It can also be intrinsic, one example being a native oxide layer. Cleaning methods are designed to minimize substrate damage while removing the film or debris. If the chemistry and processing history of a metal surface is known, one can anticipate cleaning needs and methods. In practice, extrinsic organic and inorganic soils originate with processing of the substrate before plating, as well as from the environment. Specific residues include lubricants, phosphate coating, quenching oils, rust proofing oils, drawing compounds, and stamping lubricants. In short, the mixture of potential contaminants to which a part is exposed is typically complex. Again in case of a metal substrate it must be remembered that all metals form oxide and inorganic films to a degree with environmental gases and chemicals. Some of these are protective against continuing attack such as the aluminum oxide formed on aluminum alloys That phenomenon is the reason of the usefulness of aluminum siding on some homes. On the other hand, some are non-protective, such as iron oxide on steel. Some of these films can even be plated directly with nickel over aluminum oxide over aluminum being an example. The cleaning and activation steps must account for the fact that surface oxide re-forms at different rates on different metals. Specifically, in case of iron or nickel the oxide re-forms slowly enough that the part can be transferred from a cleaning solution to a plating bath at a normal rate. In case of aluminum or magnesium the oxide re-forms very fast such that special processing steps are required to preserve the metal surface while it is being transferred to electroplating. Cleaning processes are based on two approaches – Physical Cleaning and Chemical Cleaning. In Physical Cleaning, mechanical energy is introduced to release both extrinsic and intrinsic contaminants from the (metal) surface. Examples are ultrasonic agitation and brush abrasion. In Chemical Cleaning contaminant films are removed by active materials, dissolved or emulsified in the cleaning solution. Extrinsic contaminants are removed with surface-active chemicals while the chemical energies involved are modest. Intrinsic films are removed with aggressive chemicals that dissolve the contaminant and often react with the surface (metal) itself. The energy involved in surface preparation is substantial. Pie-treatment Pre-treatment is a sequence of processes necessary to ensure that the product for subsequent coating or surface modification is in a suitable condition. For all metal finishing technologies some form of pre-treatment is an essential requirement. The three main pre-treatment methods are: • Cleaning Aqueous, solvent and mechanical (blasting) •

Activation

•

Brightening

Acid rinsing Bright dipping, chemical polishing and pickling

Cleaning Cleaning may be defined as the removal of soils from metal surfaces by employing chemical solutions or mechanical methods. Chemical cleaning can cope with a wide variety of soils including those from heavy oils and greases, light cutting oils and polishing compositions. Cleaning may be considered the most important process in metal finishing because the final appearance and acceptance depends upon the presentation of a clean and active substrate, irrespective of the final coating process. Aqueous cleaning •

Where products are to be treated by subsequent aqueous-based metal finishing technologies, such as electroplating, electroless plating, and electrocoating, the cleaners used are normally of an aqueous and alkaline nature. The type of cleaner used is dependent upon the nature of the soil for removal and the material of the base substrate. Where ferrous materials are to be cleaned then a highly alkaline solution may be employed, but for copper and copper alloys, zinc based alloys and aluminium, only mildly alkaline solutions are suitable.

•

Chemical cleaners act through solubilization, emulsification and saponification of the contamination.

•

Most cleaners are supplied as proprietary product in powder form, the ingredients being selected from sodium carbonate, sodium hydroxide, sodium metasilicates, trisodium phosphate and sodium borates, with complexing agents (EDTA, gluconates, heptonates, and polyphosphates) and organic surfactants to reduce the surface tension of water and to promote oil emulsification. Complex phosphates are included to chelate calcium and magnesium ions present in hard water, and to prevent their precipitation as insoluble salts. The traditional use of phosphates has been reduced in recent years due to environmental concerns.

•

The type of cleaner and strength used is dependent upon the metal being cleaned and the soil to be removed. Heavy-duty cleaners may have an alkalinity of 20-30% expressed as sodium hydroxide whilst light duty cleaners may have only 5-10% alkalinity. Complexing agents in the cleaner are of the order of 1-2% by volume.

•

Many cleaners are used as soaks in which the oils and greases are softened and released from the component surface. Alternatively electrolytic means (anodic or cathodic) are used in which the gas generated assists contamination removal by its scrubbing action. The efficiency of cleaners may be increased by air or mechanical agitation. In certain applications spray cleaning is preferable. To make more efficient use of aqueous cleaning solutions the use of ultrasonic vibration is sometimes advantageous.

•

Where possible none or low foaming surfactants are used, being an essential requirement for electrolytic cleaners and spray applications.

•

Thorough water rinsing after cleaning is essential. This is true particularly where high sodium hydroxide concentrations are in use. A dip in dilute acid is required after rinsing, due to the difficulty in rinsing caustic solutions from substrate.

•

Where a sequence of cleaners is in use e.g. soak, cathodic and anodic, cleaners may be selected which are compatible with each other thus eliminating the need for interstage rinsing and the consequent drag-out losses.

•

Some cleaners are of the emulsion type with the use of an organic hydrocarbon solvent in alkaline solution Suitable emulsifiers are used to form an oil-in-water or water-in-oil emulsion. Cleaners of this type were commonplace but have been superseded by more sophisticated conventional alkaline materials. Further, these types of cleaners cannot be rinsed 100% free of solvent and produce subsequent process problems.

•

The selection of the cleaning formulation is in some cases specific to the product substrate although there are some, which have a more universal application.

•

Cyanide containing formulations are still available where cold electrolytic cleaning applications are required, but their use has largely been curtailed due to the need to treat the discharge for cyanide destruction.

•

Discharges are determined by the type of articles being processed and drag out into water rinse systems (see Section 3). The outflow to effluent usually only requires pH adjustment, unless it contains cyanide.

•

Periodic replacement of the total cleaning solution is required, the frequency of which is dependent on the soil contamination removed from the articles processed, the plant throughput and the volume of cleaning media contained in the tank. As a guide solutions are disposed of after 4-8 weeks. The sludge produced is removed and disposed of to landfill. It is normal to discharge cleaning materials at the same time as the acids from the process line so that pH neutralization is nearly automatic. Following settlement of solids the solution can then be discharged to sewer.

•

Typically the concentration of a commercially available cleaner used is in the range of 25-75 g/l at 50-80°C. Periodically additions would be made to the solution after simple alkalinity analysis, to compensate for drag-out losses.

Metal Surface Preparation and Cleaning NATURE OF THE SURFACE Metal surface under normal circumstances are not atomically smooth. Crystal effects such as dislocation, twins and grain boundaries, emerging at the surface can give rise to steps and ledges that can be many atoms high. Surface treatments prior to electroplating can lead to further enhancement of this surface roughness. Atoms in ledges and steps are even more energetic than those in smooth and are sites of strong adsorption. Different crystal phases within the surface of alloys as well as impurities and non-metallic inclusions such as entrapped slag, create additional surface in homogeneities. DEPOSIT GROWTH MODE In the presence of large quantities of growth inhibitors it becomes impossible for even the initially depositing atoms to follow the basis metal crystal structure. Under these conditions epitaxial growth does not occur, growth being determined solely by plating conditions and bath composition, often resulting in a fine-grained, randomly oriented deposit structure. ADHESION Certain basis metals may contain non-conducting or poorly conducting phases. Where such phases are exposed on the surface, plating will not occur, although coverage may be produced by bridging of the deposit from neighboring areas that are conducting. The same effect is produced is produced by abrasive particles such as silicon carbide that become embedded in the surface during the polishing or grinding operation and are not subsequently removed by electro polishing or etching. POROSITY Many of the factors which cause poor adhesion also produce porosity in the electro deposit. Areas of the basis surface that have soils remaining or contain non conducting phases such as slag particles are not plated and pores are formed as the deposit bridges over them. Like-wise low hydrogen over-voltage phases produce hydrogen bubbles which may be occluded by the growing deposit or create channels through the deposit as the bubbles evolve from the surface. Aggressive mechanical treatment of the surface may produce fine surface cracks that are not plated, again leading to porosity. Some of these causes of porosity may be removed by treatments prior to plating such as adequate degreasing, chemical or electro polishing. However there is little that can be done about low conducting and low hydrogen over voltage phases except to avoid them where possible. BRIGHTNESS The basis metal effect on brightness is essentially that due to surface topography. This topography is very much dependent on surface treatments. Surface roughness greater than the wavelength of visible light-i.e., about 0.15 micrometers (6 micro inches) causes diffuse scattering of light and a dull appearance.

STRESS It was earlier mentioned that stresses could result from the atomic mismatch between an epitaxial deposit and the basis metal surface. If the interatomic spacing of the deposit is smaller than that of the basis metal, the crystal structure of the initially depositing atoms will be stretched and consequently be in the state of tensile stress. Compressive stress arises when the deposit has a larger spacing than the basis metal. These stresses can be significant in thin deposits, often leading to the formation of dislocations, which influence the mechanical properties, and corrosion resistance of the deposit. Apart from co-deposition during plating, atomic hydrogen may be introduced into the basis metal during pre-plating treatment processes such as acid pickling or cathodic cleaning. This hydrogen can collect as molecules as voids and produce considerable internal pressures, leading to brittle cracking of the basis metal under relatively low stresses. Heat treatment is the only way to remove the entrapped hydrogen, but this cure may itself cause problems if it reduces desired properties such as hardness. SOLVENT CLEANING Cleaning is the removal of undesirable material from the base material and is normally limited to the surface. The unwanted material may be rust or oxide films, metal fines, shop dirt such as dust or paper, rust preventatives, buffing or polishing compounds, wax oil, fingerprints, grease, asphalt, and even water in some cases. Earlier chapters of this section have described buffing, polishing, barrel finishing and electropolishing. These processes are valuable in removing rust or oxide films or metal oxide imperfections. Salts and other water-soluble soils, soaps and some buffing and polishing suspending agents can be cleaned from parts by alkali cleaning as described in section F. the presence of organic compounds such as oil or grease can foul these cleaning processes and add to their cost, maintenance and waste. These organic compounds are easily solubilized by solvent and removed from the work parts. In some cases solvent cleaning before other surface preparations can extend the life of those cleaning operations; reduce their costs. In other cases solvent cleaning provides work parts in a condition ready for the next operation such as assembly, painting, inspection, further machining or packaging. Prior to plating solvent cleaning is usually followed by an alkaline wash or other similar processes, which provide a hydrophilic surface. Further solvent cleaning can be employed to remove water from electroplated parts, as is common in jewelry industry. Solvent cleaning can be accomplished in room temperature baths or by employing vapor-degreasing techniques. Room temperature solvent cleaning is commonly referred to as cold cleaning. Vapor degreasing is the process of solvent cleaning parts by condensing solvent vapors of a non-flammable solvent on a work part(s). COLD CLEANING The prime properties of a solvent for simple cold cleaning are good solvency, minimum flammability, low toxicity and a moderately fast evaporation rate. Carbon tetrachloride was regarded as nearly ideal cold cleaning solvent before its highly toxic properties were recognized. Cold cleaning solvent properties are summarized in table 1.

DIPHASE COLD CLEANING Frequently, solvent cleaning is chosen to avoid exposing the processed work to water. In some instances, such as before plating, aqueous cleaning after solvent cleaning is desirable. In such situations, diphase cleaning can offer distinct advantages. In this cleaning operation, a water layer, which may contain surfactants, is placed on top of one of the chlorinated solvents or fluorocarbon 113. The chlorinated and fluorocarbon solvents are heavier than water and remain below the water surface. Generally, the petroleum solvents are not used in diphase cleaning because they are lighter than water and float. Control System for Cold Cleaning Control System A Control Equipment: 1. 2. 3.

Cover Facility for draining cleaned parts Permanent, conspicuous label, summarizing the operating requirement.

Operating Requirements: 1. 2. 3.

Do not dispose of waste solvent or transfer it to another party, such that greater than 20% of the waste (by weight) can evaporate into the atmosphere. *store waste solvent only in covered containers. Close degreaser cover whenever not handling parts in the cleaner. Drain cleaned parts for at least 15 sec or until dripping ceases.

Control System B Control Equipment: 1.

2.

3. 4. 5.

Cover: Same as in system A, except if (a) solvent volatility is greater than 2 kPa (15 mm Hg or 0.3 psi) measured at 38°C (100°F), ** (b) solvent is agitated, or (c) solvent is heated, then the cover must be designed so that it can be easily operated with one hand. (Covers for larger degreaser may require mechanical assistance, by spring loading, counter weighting or powered systems.) Drainage facility: Same as in system A, except that if solvent volatility is greater than about 4.3 kPa (32 mm Hg or 0.6 psi) measured at 38°C (100°F), then the drainage facility must be internal, so that parts are enclosed under the cover while draining. The drainage facility may be external for an application where an internal type cannot fit into the cleaning system. Label: Same as in system A. If used, the solvent spray must be a solid, fluid stream (not a fine, atomized or shower type spray) and at a pressure which does not cause excessive splashing. Major control device for highly volatile solvents: If the solvent volatility is > 4.3 kPa (33 mm Hg or 0.6 psi) measured at 38°C (100°F), or if solvent is heated above 50°C (120°F), than one of the following control devices must be used: a. Freeboard that gives a freeboard ratio*** = 0.7

b. c.

Water cover (solvent must be insoluble in and heavier than water) Other systems of equivalent control, such as refrigerated chiller or carbon adsorption.

Operating Requirements: Same as in system A CONTROL SYSTEMS FOR CONVEYORIZED DEGREASERS Control System A Control Equipment: None Operating Environment: 1. Exhaust ventilation should not exceed 20 m3/min/m2 (65 cfm/ft2) of degreaser opening, unless necessary to meet OSHA requirements. Workplace fans should not be used near the degreaser opening. 2. Minimize carry-out emissions by: a. Racking parts for best drainage. b. Maintaining conveyor speed at < 3.3 m/min (11ft/min). 3. Do not dispose of waste solvent or transfer it to another party, such that greater than 20% of the waste (by weight) can evaporate into the atmosphere. Store waste solvent only in covered containers. 4. Repair solvent leaks immediately, or shutdown the degreaser. 5. Water should not be visibly detectable in the solvent exiting the water separator. Control System B Control Equipment: 1. Major control devices; the degreaser must be controlled by either: a. Refrigerated chiller, b. Carbon adsorption system, with ventilation = 15 m2/min/m2 (15 cfm/ft2) of air /vapor area (when down time covers are open), and exhausting 10 cm (4 in). c. Vapor level control thermostat (shuts off sump heat when vapor level rises too high.) 4. Minimize openings: Entrances and exits should silhouette work loads so that the average clearance (between parts and the edge of the degreaser opening )is either < 10 cm (4 in) or < 10 % of the width of the opening.

5. Down Time Covers: Covers should be provided for closing of the entrance and exit during shutdown hours. Operating Requirements: 1 to 5. Same as for system A. 6. Down time cover must be placed over entrances and exits of conveyorized degreasers immediately after the conveyor and exhaust are shut down and removed just before they are started up. ALKALINE CLEANING The object of this pre-plating cycle is to remove those surface films which can be characterized as soils, and replace them with films which will be compatible with the solutions being used to apply the final finish. When the sequence is properly selected and operated, the parts will enter the final processing solution with a surface in an activated or receptive state for the finish to be applied. To accomplish this preparation, four basic steps are required: 1. Gross cleaning – the removal of heavy soil. 2. Fine cleaning- the removal of residues from gross cleaning, along with fine particulate matter. 3. Oxide removal – the removal of the thin layer of oxide, which covers every metallic surface. 4. pH adjustment- to bring the residual surface film close to the same pH as the processing solution. BUFFING COMPOUNDS Buffing Compounds are mixture of lubricating materials (usually fatty acids), abrasives (complex silicates, carbides or metal oxides) and materials to control the melting points (often high melt parafinnic compounds or waxes). Since the buffing process is a friction related process, very high temperatures may be generated at the point of contact, and all the ingredients can react with each other and the metal surface. These temperatures can vary widely with buffing conditions; the reaction can vary as well. RUST PROOFING COMPOUNDS Rust proofing compounds can roughly be placed in three categories: • •

Inorganic, water-soluble compounds for protection between operations or short term protected storage. These normally do not present any cleaning problems. Emulsifiable organic mixtures cut back with water to form the required emulsions. When the emulsion “breaks” due to a change of temperature or the evaporation of water the organic portion is left on the surface as a protective film. The formulation usually contains one or more volatile constituents, which evaporates with the water during drying so the protected film is no longer emulsifiable. Protection is adequate for long term protected storage, or interplant transfer. Cleaning problems are similar to the next category.

•

Solvent cut back organic mixtures provide a wide degree of protection of, depending on composition and degree of cut back. Protection may be adequate to permit out door storage for reasonably extended periods. They may be formulated with water displacing characteristics so parts to be protected may be immersed wet. The organic protective materials generally contain an oil base, a highly protective material such as a fatty acid, a metallic soap, or a polar material with an affinity for the substrate. If they are not fully dry to touch they become magnet for shop dirt. Dryness or lack of tack is usually imparted by incorporating a wax, a drying oil or a film forming resin. Since these materials are designed to protect by preventing the penetration of moisture to the metal surface, they are often difficult to clean in aqueous system. Solvent or vapor degreasing before aqueous cleaning is often helpful. A solvent dip to penetrate the film and reduce its viscosity also helps. If waxes are used for dryness, the temperature of the cleaning solution must be higher then the melting point of the wax.

Age of the film can be an important factor. Some of the polar materials may react with the metal surface. Unsaturated compounds may polymerize to form varnish like material. Evaporation of the solvent use for cutback will alter the viscosity. Coiled Stock is particularly is susceptible to these effect. Depending on the tightness of the coiling, these variations may occur at different rates in various areas of the coil. Hence differences in cleaning requirement from point to point on the coil are not unusual. MACHINING AND FORMING OILS. Increasingly often, these oils are being fortified with additives providing extreme pressure lubrications. Since these adhere strongly to the substrate, aggressive, high alkalinity cleaners may be required. SMUTS. A smut is defined as finely divided particulate matter strongly adherent to the metal surface. It may be conductive or non-conductive. The nonconductive smuts consists of inorganic residues including carbon from acid treatment of high carbon steels or from heat treating operations such as oil quenching or controlled atmosphere heat treatment; pigments from the use of pigmented drawing compounds; insoluble constituents of an alloy brought to the surface by previous chemical treatment; i.e. silicon in aluminum alloys beryllium in beryllium copper etc.; BASE METAL EFFECTS The nature of the base metal has a critical bearing on the type of cleaning system selected. Materials must be selected to provide the required cleaning action without undue or selective attack on the base metal. Since metals vary greatly in reactivity, allowable limits of pH, temperature and concentration and the type and concentration of inhibiting agents are dictated by the base metal. Cleaners for aluminum or zinc will generally be quite different from those for brass or steel.

BUFFING COMPOUND REMOVERS These are essentially highly specialized form of soak cleaners, designed for the effective removal of buffing compound residues. They fall into three basic categories: 1. Neutral detergent-usually liquids; mixtures of surfactants; pH close to neutral with buffering provided by the surfactants used. Concentrations in the range of 1 to 10% by volume. 2. Enhanced detergent-similar to neutral detergent but fortified with organic alkalis, which can react with the fatty acid in the buffing compound to form organic soaps. Concentrations 2 to 10% by volume 3. Modified soak cleaners-similar to soak cleaners (q.v.) but modified to be especially effective on buffing compounds. Concentration 45 to 120 g/1 (6 oz/gal) Type 1 and 2 often show poor performance on oily soils other than buffing compounds. Temperature of operation should be above the melting point of the buffing compound 60 to 80 C (140 to 180 F) Use of ultrasound. Or vigorous agitation will often permit operation at lower temperatures. ALKALINE CLEANERS Alkaline cleaners are blends of various inorganic alkaline salts with deflocculants, Inhibitors and surfactants as required providing the various cleaning mechanisms and functions discussed below. Saponification: The chemicals action by which a fatty acid, oil or other reactible soil is converted to a water-soluble compound such as soap. Elevated temperature, Concentration and pH promote the speed and completion of the reaction. The main advantage is that cleaning will proceed in the absence of surfactants, and that the reaction products may function as additional cleaning agents to improve the performance of the cleaner. Disadvantages include the fact that at least initially only reactible soils will be affected; the reaction products may build up tolevels that cause rinsing and drying on problems; incomplete rinsing may result in re-deposition of the soils in a subsequent acid treatment the solubilized soils unless separated will contribute heavily to hexane solubles in the effluent, and such separation is not always easy to attain. Emulsification: The chemical process by which surfactants penetrate oily soils and break them down into globules sufficiently small to allow dispersion and suspension in the solution. Advantages include the fact that the reaction is often independent of pH; temperatures and concentrations required can be somewhat lower than with saponification; all types of oily soil will be removed: and rinsing will generally be somewhat better than for saponified soils.

Disadvantages are similar to those for saponification except as noted, and with the added possibility that the surfactant concentration may be depleted at a rate different from the alkali depletion. The cleaner may therefore drift out of balance and fail to perform even when concentrations appear to be within limits. Deflocculation: The process by which special chemical compounds surround particles of solid soil, removing them from the surface and dispersing them in solution. The process is generally improved by mechanical action and/or the development of gas by electrolysis. Elevated temperatures may also be helpful. Different deflocculants may be specific to certain solids, so complex soils may require mixtures of several agents for effective action. Displacement The process by which surfactants lifts oily soils from the surface of the parts to be cleaned. A film of surfactant and solution is left on the part surface. The oily soil floats to the surface of the cleaning bath. Advantages include longer solution life and the possibility of operating at lower concentrations and temperatures. The main disadvantage is the need to continually skim the solution surface to remove the displaced oil. Failure to keep the solution surface properly cleaned my result in their deposition of the oily soil as the parts are removed from the solution. When properly operated. Hexanes soluble in the effluent are reduced, since the oil soil is constantly separated from the cleaning solution. SPRAY CLEANERS Cleaning solutions, which are sprayed on the parts, sometimes under considerable pressure. Any of the mechanisms previously discussed, including emulsified solvents may be used. Careful attention must be give to choosing materials with low foaming characteristics. The combination of chemical action and the mechanical action of the spray produces effective cleaning. Spray patterns must be designed to provide complete coverage of the parts, and the units given periodic maintenance to insure that nozzles are not plugged. Except for the foaming requirement, alkaline spray cleaners are similar to soak cleaners. Concentrations and temperatures, however, are generally much lower, in the range of 15 to 30 g/1 (2 to 4 oz/gal) and 35 to 60 C (100 to 140 F). The newer low temperature spray cleaners often operate at 4 to 15g/1(1/2 to 2 oz/gal) and 20 to 30 C (70 to 90 F) Liquid forms of the materials are sometimes available and operate at ½ to 2% by volume.

ULTRASONIC CLEANING • • • • •

Cavitation Electrical effects Transmission effects Cleaning materials Equipment considerations

OXIDE REMOVAL Oxide removal operations represent the greatest single use of chemicals in metalworking. During 1952 such operations consumed 5% of all sulfuric acid produced, 25% of the hydrochloric acid and most of the hydrofluoric acid. In addition large quantities of nitric acid, phosphoric acid and ferric sulfate are used. Oxide removals in the metal finishing trade are far over-shadowed by the large scale pickling operations necessary during the metal production. In general heavy oxides scales are produced and must be removed during metal production operations. Oxide removal prior to plating is usually necessary because heat-treating, welding or other similar operations have oxidized surfaces, which are to be plated. Occasionally work is allowed to rust and corrode between fabrication steps, necessitating oxide removal treatments. Reclaiming old machinery and fabricated metal parts require special oxide removal operation. Bright dipping and removal of superficial oxide films are an important part of metal finishing procedures. CHARACTER OF OXIDES AND SCALES ON METALS For instance heat-treated scales on steel consist of largely Fe2O3 on the outside Fe3O4 as an intermediate phase and approximately FeO next to the metal. The same general condition is sometimes noted for copper alloys with CuO on the outside and more or less Cu2O on the inside OXIDE REMOVAL FROM LOW AND MEDIUM ALLOY STEEL Oxide removal, pickling or de-scaling operations are practiced to a far greater extent on low and medium alloy steels than on all other classes of metallic materials combined. This is true for total tonnages as well as for the number of de-scaling installations. Most steel in fabricated articles has undergone at least one de-scaling operations and in many cases three or four at various stages of manufacture. Because of this commercial interest there is more detailed technical information available on oxide removal from steel then for most other metals ¾ PICKLING WITH SULFURIC ACID • Pickling process • Effect of acid concentration and temperature • Effect of dissolved iron • Effect of scale breaking • The Effect of inhibitors • Electrolytic Pickling ¾ PICKLING WITH HYDROCHLORIC OR PHOSPHORIC ACID • Hydrochloric Acid • Phosphoric Acid ¾ GAS-PHASE PICKLING ¾ BRIGHT DIPPING

Bright Dipping solutions are used for Ferrous and MonoFerrous alloys and usually involve mixtures of two or more of the acids, sulfuric, phosphoric, chromic, nitric and hydrochloric An Example of a typical bath is: • • • •

20% nitric acid 25% acitic Acid 55% phosphoric acid 0.5% hydrochloric Acid

The bath is operated at 190F, and the dipping time ranges up to 5 min. another example is: • 40% nitric acid • 30% phosphoric acid • 30% acitic Acid • 1.0% sodium chloride A typical bright dipping sequence for racked work is as follows: 1) 2) 3) 4) 5) 6) • • • • •

Dip in a scaling bath containing three parts concentrated nitric acid and one part concentrated sulfuric acid (by volume) Cold water rinse Dip in a bath containing one part concentrated nitric acid and three parts concentrated sulfuric acid (by volume) Double cold water rinse or spray rinse Rinse in cold 5% sodium cyanide solution Final cold and hot water rinses

3 part sulfuric acid 1 part nitric acid 1/20 part hydrochloric acid 6 parts water ½ lb chromic acid per gallon of solution

Chapter 10 Thickness of coating is the most important parameter, which decides the quality of coating and therefore it needs to be controlled. Controlling the thickness of the electroplated object is generally achieved by altering the time the object spends in the salt solution. The longer it remains inside the bath, the thicker the electroplated shell becomes but there must also be an adequate amount of metallic ions in the bath to continue coating the object. The shape of the object will also have an effect on the thickness of the coating. Sharp corners will be plated thicker than recessed areas. This is due to the electric current present in the bath since it flows more densely around corners. Before electroplating an object, it must be cleaned thoroughly and all blemishes and scratches should be polished. As mentioned, recessed areas will plate less than sharp corners, so a scratch will become more prominent, rather than being smoothed over by the plated material. Determining Coating Thickness There are five basic, non-destructive methods of determining coating thickness. Each method is devised to achieve cost-effective, accurate, and repeatable results. These methods are: •

X-Ray fluorescence

•

Eddy-current

•

Magnetic induction

•

Beta backscatter

•

Micro-resistance

Each method is particularly suited to a specific coating(s)/substrate combination. We'll discuss below each system and applications for each. X-Ray Fluorescence When a material is subjected to x-ray bombardment, some of its electrons will gain energy and leave the atom, creating a void in the vacated shell, thereby releasing a photon of x-ray energy known as x-ray fluorescence. The energy level or wavelength of fluorescent x-rays is proportional to the atomic number of the atom and is characteristic for a particular material. The quantity of energy released will be dependent upon the thickness of the material being measured. Basically, the x-ray fluorescence unit consists of an x-ray tube and a proportional counter. Emitted photons ionize the gas in the counter tube proportional to their energy, permitting spectrum analysis for determination of the material and thickness.

X-ray fluorescence is the most precise measurement method, especially for smalldiameter parts, or dual coatings such as gold and nickel over copper. Eddy-Current The eddy-current technique is used for measuring both non-magnetic, metallic coatings (zinc, cadmium, copper, etc.) over steel as well as non-conductive coatings over non-ferrous metals such as anodize or paint over aluminum. When a conductive material is subjected to an AC magnetic field from a probe, eddycurrents occur in the material in proportion to the frequency and resistance. The induced eddy currents generate an opposing magnetic field which alters the circuit reactance and the output voltage of the probe. The change in output voltage is used to calculate the coating thickness. Electrical conductivity between the coating and substrate should differ by a ratio of 2:1 for optimum accuracy. Non-conductive coatings introduce a gap (lift-off) between the probe and non-ferrous base material. This gap produces a loss in eddy current penetration which is compared to a measurement directly on the base material to determine coating thickness. With conductive coatings over steel, eddy currents are generated in both the coating and ferrous base material. Eddy current loss in both materials is proportional to the coating and substrate material thickness and will range somewhere between readings taken directly on pure samples of each material. The eddy current loss differential is used to calculate coating thickness. Magnetic Induction This principle is used for measuring the thickness of a non-magnetic coating (zinc, cadmium, paint, powder coating, etc.) over a steel substrate. The probe system is essentially the secondary of a transformer circuit that reacts to the presence of a magnetic material. The circuit efficiency and output voltage increase when the probe is brought near a magnetic surface, providing parameters which may be used to measure the distance (coating thickness) from the magnetic surface. Beta Backscatter Beta rays are electrons emitted from unstable radio isotopes. If a highly collimated beta source is directed at a plated sample (gold over nickel on a printed circuit board, for instance), the electrons will penetrate the plating material and be reflected back (back scattered) toward the source. They can be collected and counted with a GeigerMueller tube for subsequent conversion to coating thickness. The atomic number of the coating material must be sufficiently different (at least four atomic numbers) from the atomic number of the base material to achieve accurate readings of coating thickness. Micro-resistance The new micro-resistance method of determining plating thickness is ideally suited for printed circuit board plated-through-holes and for surface copper measurements.

This technique requires precise measurement of the resistance of the copper cylinder that forms the plated through hole. Once this parameter is known, it is combined with data on the board and hole aspect ratio to calculate the average copper plating thickness. Calculations are performed automatically by software associated with the measurement device. Specially designed, pyramidal, electrically isolated probe tips simultaneously inject current and take voltage-drop measurements. The resistance is then calculated by Ohm's Law. For electro coatings, powder coating and paint over a non-ferrous substrate, use the eddycurrent method. Use the magnetic induction method on a ferrous substrate. Portable Tools to Measure Thickness HELMUT FISCHER offers a family of hand-held instruments with features to meet the individual requirements of customers. The DUALSCOPE® MP0R/ MP0RH are designed for quick and easy measurements. This economic instrument accurately and conveniently displays the coating thickness readings on two LCD displays: on a large front panel display and a top panel display. It features an ergonomic design with an integrated constant pressure probe allowing easy one hand operation. The possibility of a statistical evaluation of the measurement series exists, with an integrated radio transmitter for wireless online or offline transmission of the measurements directly to a computer, up to 10 – 20 m (33 – 66 feet) away. The DUALSCOPE® MP0R/ MP0RH utilize both the eddy current test method according to DIN EN ISO 2360, ASTM B244 and magnetic induction test method according to DIN EN ISO 2178, ASTM B499. It is used to measure non-conductive coatings on non-ferrous metals as well as non-ferrous metal coatings and non-conductive coatings on iron and steel. It will automatically recognize the material to be measured and utilize the appropriate test method. The DUALSCOPE® MP0RH has a considerably larger measurement range. The DUALSCOPE® MP0R/ MP0RH is ideal for measuring: • Non-ferrous metal coatings (e.g. chromium, copper, zinc etc.) on steel and iron. • Paint, lacquer and synthetic coatings on steel and iron. •

Electrically non-conductive coatings on non-ferrous metals such as paint, lacquer, synthetics on aluminum, copper, brass, zinc and stainless steel.

•

Anodized coatings on aluminum.

Table 1: Suggested Norms @ Thickness, hours needed and type of objects to be plated Degree Exposure

of

Min.Thick (microm eter) (1)

Chromate Finish

MILD-Exposure too indoor atmospheres with rare condensation & subjected to minimum wear or abrasion MODERATEExposure mostly to dry indoor atmospheres but subjected to occasional condensation, wear or abrasion SEVEREExposure to condensation perspiration, infrequent wetting by rain, cleaners

5

None

VERY SEVERE (2) Exposure to bold atmospheric conditions, and subject to frequent exposure to moisture, cleaners and saline solutions plus likely damage by denting, scratching or abrasive wear

25

Clear

Salt Spray Hrs to White Corr. 12-24 24-72

iridescent yellow olive drab 8

None

12-24 24-72

iridescent yellow olive drab None iridescent yellow olive drab

25.4 Micrometers = 1.0 mil.

None

Tools, zipper, pulls, shelves, machine parts

72-100

Clear

Screws, nuts, and bolts, buttons, wire goods, fasteners

72-100

Clear

13

Typical Applications

12-24 24-72 72-100

Tubular furniture, insect screens, window fittings, builders’ hardware, military hardware, washing machine parts, bicycle parts Plumbing fixtures, pole line hardware

(1) (2)

Thickness of the coating after chromate treatment. Although there are some applications for heavy electrodeposited coatings for very severe service they are most usually satisfied by hot dipping or sprayed coatings.

THICKNESS TESTS In as much as corrosion resistance has often been shown to be intimately related to the thickness of the deposit, the stipulation of minimum thickness in the product specification is obvious. However, corrosion resistance is not the only criterion that makes thickness specification necessary; at least of equal importance is the functional requirements of the deposit itself. Many products are plated to achieve definite physical and chemical properties such as conductivity on printing wiring boards and other electronic devices, wear resistance in industrial chromium plating and, in some instances, electroless nickel plating, and silver plating on bearing retainers to impart lubricity at relatively high temperatures. Plating specific coatings greatly enhances the functions of a particular item, and there are minimums and maximums in plating thickness specifications, which must be adhered to in order for the item to perform as designed. • • • • • • • • • • Copper

Nickel

Chro miu m

Auto catalytic Nickel

Zinc

Cadmium

Gold

Palla dium

Rhodium

Silve r

Tin

Lead

TinLead Alloy

NonMeta l

CM

CMA

CM

CBMA

CM

BCM

BM

BM

BM

BC M

BCM

BCM

BCCC M

BM

CED

CMA

C

CB

C

BC

B

B

B

BCE

BC

BC

BCCC

BE

C only on

CMA

C

CB

C

BC

B

B

B

BC

BC

BC

BCCC

BE

Coatings Substrat es Magnetic Steel (includin gcorrosion resisting steel) Non magnetic Stainless steel Copper and

MICROSCOPIC-OPTICAL METHODS DOUBLE BEAM INTERFERENCE MICROSCOPE, INTERFEROMETRY CHEMICAL METHODS MAGNETIC METHODS EDDY CURRENT MASS PER UNIT AREA COULOMETRIC Factor = Standard calibration Unit Machine Reading X-RAY METHODS BETA BACKSCATTER (BBS) MICRORESISTANCE TECHNIQUE

D

alloys

Brass and CuBe C

MA

…

…

…

B

B

B

B

B

B

B

BC

BE

BC

BCMA

BC

BCBEA,

BC

BC

B

B

B

BC

BC

BC

BCCC

E

B

BMA

B

B

B

B

B

B

B

B

B

B

BC

E

C

…

C

…

C

BC

B

B

B

BC

BC

BC

BCCC

BE

Silver

B

BMA

B

B

B

…

B

…

…

…

…

BC

BC

BE

Glass Sealing NickelCobaltIron alloys UNS No. K94610 Non metals Titanium

M

CMA

M

CBMA

M

BM

BM

BM

BM

BM

BM

BCM

BACC M

BM

BCED

BCMA

BC

BCB

BC

BC

B

B

B

BC

BC

BC

BCCC

…

B

BMA

…

BEA, B

B

B

B

B

B

B

B

B

BC

BE

Zinc and Alloys Aluminu m and alloys Magnesi um and alloys Nickel

A

B

Method is sensitive to permeability variation of the coating. Method is sensitive to variation in the phosphorus content of the coating. C Method is sensitive to alloy composition. D Method is sensitive to conductivity variation of the coating. B

Chapter 11

Polytetrafluoroethylene (PTFE) is better known by the trade name Teflon®. It's used to make non-stick cooking pans and anything else that needs to be slippery or non-stick. PTFE is also used to treat carpets and fabrics to make them stain resistant. What's more, it's also very useful in medical applications. It can be used for making artificial human body parts, because human bodies rarely reject it. These materials can be used to make a variety of articles having a combination of mechanical, electrical, chemical, temperature and friction-resisting properties unmatched by articles made of any other material. Commercial use of these and other valuable properties combined in one material has established PTFE resins as outstanding engineering materials for use in many industrial and military applications. Table 9a shows data that an EN/PTFE process should be able to fulfill to meet today's environmental and economical needs. To achieve the "perfect coating" and the required lifetime, it is necessary to modify the electroless nickel electrolyte and the manufacturing of the dispersion to develop an improved proprietary process. A nice side effect of this was that the lifetime of the plating solutions was extended to five metal turnovers or more under normal job-shop conditions. Customers using EN/PTFE coatings of the newest generation report generally the same deposition quality for five metal turnovers or more. Also, the incorporation rate of the PTFE remains at about 25 to 30% volume during the entire bath life. To incorporate higher PTFE content does not appear practical because the plating rate is reduced and the nickel matrix becomes more fragile, increasing the wear rate. Also, agglomeration of PTFE particles is hardly a problem. TABLE 9a- EN/PTFE Coating Characteristics: PTFE incorporation - Variable in between 15 - 30 volume - % Particle size - 0.3 - 0.5 mm Bath lifetime - About 5 MTO Deposition Rate - 5 mm/h at 28 volume % up to 5 MTO Agitation during Plating - None Wear properties - A widely held prejudice is that only hard coatings can solve wear problems. That is correct for abrasive wear, but for nearly all other problems, a reduction of the coefficient of friction can handle it and is sometimes even better. The main reason for the success is that forces that could fatigue one or both partners are reduced, so less wear is transferred into the material. In the top view, the color is coordinated with height, and in the hardness map with hardness. Even on the surface, the PTFE is uniformly distributed so phenomena such as fretting and

galling are minimized. The comparison of a heat-treated electroless nickel and hard chromium layers shows why hardness is not the main factor for low wear. Both layers have an overall hardness of about 1000 Hv (Vickers hardness) 0.1. The EN coating is abraded more quickly than the chromium coatings, because EN has a higher coefficient of friction. Even though the EN/PTFE coating is much softer (320-400 Hv 0.1), the coefficient of friction is lower than that of hard chromium (about 0.07 0 0.1 for EN/PTFE and 0.12 to 0.25 for hard chromium). Therefore it wears more quickly. Because of the fatigue behavior of the coating, PTFE and EN particles are broken out of the surface. In the case of the very smooth surface, the small Ni-P particles can increase the wear on the next layer of the coating, and the PTFE balls are simply wiped away. Against that the PTFE particles remain longer in the area of wear and can be used like ball bearings on the rougher surface. The Ni-P particles are too small to increase the wear on the rough surface. Comparison to PVD/CVD coatings (PVD: physical vapor deposition. CVD: chemical vapor deposition) In comparison to other wear-protective coatings such as PVD and CVD, there are some other advantages of soft dispersion coatings: 1. Because the hardness of the deposited layer is similar to that of the substrate (in most cases), there is no need for a hardening process before plating the protective layer. This is necessary in a PVD or CVD system to achieve an appropriate adhesion. Similar to the PVD/CVD processes, the surface has to be purely metallic (no oxides). 2. Because of PTFE's softness, the coating can store and dissipate more energy in small volumes before it is plastically deformed. 3. The EN/PTFE coating can use the base material as a support structure because of the similar hardness. 4. Because of the porosity-free deposition, it is possible to get another degree of freedom for the deposit's lifetime (thickness). 5. The corrosion behavior is much better than that of PVD/CVD coatings (EN: 500 hrs at a plating thickness of 12mm). 6. The "soft dispersion coatings" cannot increase the wear at the tribological partner in the system. In most PVD/CVD coatings, corrosion resistance is a problem. To cope with that, some companies apply "sandwich coatings." Usually plating of 10mm of a high-phosphorus EN layer (d=10mm) below the thin ceramic layer can help with the problem.

Galling and corrosion problems are covered best by EN/PTFE dispersion layers. This is because of the anti-adhesive and hydrophobic properties of the surface and the thermal conductivity of the coating. While ceramic coatings are electrical and thermal insulators, EN/PTFE coatings have electrical and thermal conductivity. Different applications for EN/PTFE coatings Table 9b shows some of the most common applications for EN/PTFE coatings. New improvements in the coatings have helped in many industries: drilling equipment for paper; gardening tools; gas meters; and spiral pumps. In automotive industries there are new parts tested to avoid noise and wear or reduce weight and costs (Table 9c). Combining electroless nickel deposits with sub-micron PTFE particles has not yet reached its peak. That is due not only to a lack of information, but also to the quality of the solid layers. The consistency was not good enough, and the results differed too much, so that many electroplaters did not dare apply this coating in high-tech applications. Since improving the chemistry and the deposit, most applications can now be approached successfully. Also, the life of the electrolyte convinces some of the electroplating industry customers to use this trend-setting coating. Many problems can be solved where some years ago no one thought this could be done with a soft coating. Only some physical properties have to be changed and many applications could be managed easily.

Chapter 12 Plastics cannot be plated in the same way as metals because plastics are not electrically conductive. Thus one cannot immerse a plastic part connected to the negative lead and expect it to plate. Instead, electroless plating is applied first to get a conductive surface, and then the electroless plated parts are electroplated. Many automotive parts, including grilles and all manner of decorative trim have been plated plastic. The Term plated plastics generally reefers to plastic parts finished with bright chrome electroplate. Plated plastic parts are used in a variety of automotive, appliance, and hardware applications. This section will review the general procedures for electroplating plastics. SELECTION AND MOLDING OF THE PLASTIC At present, about 85% of plated plastic parts are injection molded of acrylonitrilebutadiene-styrene (ABS) terpolymer. Special plating grade abs molding compounds are generally preferred for better-quality plating. Also, for successful plating, basic design criteria should be observed avoiding blind holes, large flat surfaces, and sharp corners. During the molding special; parameters recommended by the resin manufacturer should be observed. Finally, silicone mold releases should be avoided in all cases. PLATING The typical plating cycle for ABS plastics is described in fig 1, other plastics based on polyphenylene oxide, nylon, polysulfone, fluoropolymers, and alloys of ABS and polycarbonate have been commercially plated. Special plating cycles are available for these materials, either from the manufacturer of the resin or from the suppliers of the plating chemicals. These cycles differ mainly by requiring special cleaning, solvent treatment, or etching procedures. The steps indicated in fig 1 can be carried out as one single plating line or divided into two lines, the first comprising all steps needed to render the plastic conductive and the second containing the electroplating operations. In either approach, automatic plating machines, in general programmed hoist units, are used for better economy and consistent quality. Adequate water rinses should be included after each step. The main characteristics of steps of fig 1 are as follows: Cleaning Cleaning of the plastics may require a separate treatment, particularly when molding and plating are being done at different locations. Extra care should be taken in the handling, packaging, and transportation of molded parts to eliminate such dirts as airborne dust, oils, and fingerprints. Some of these materials could absorb into the plastic and effect subsequent processing. The purpose of cleaning is to remove materials that might interfere with uniform chemical attack in the next steps. Alkaline detergent solutions are commonly used.

Etching The etching solutions are specifically designed to render the plastic surface hydrophilic, and to produce a micro-etch of the surface by selectively attacking components in the plastic. It is this selective micro-etch that supplies the required bond between the plastic and the conductive first coating. The etchants usually are strongly oxidizing solutions, often containing high concentrations of chromic acids and sulfuric acid. These etching solutions are used for ABS at 60 to 70 C (140 to 160 F), for 5 to 10 minutes. Neutralizing Neutralizers normally are used following the chromium containing etchants so as to eliminate as completely as possible the carryover of hexavalent chromium compounds which are detrimental to operation of all other treatments in the cycle. The elimination of chromium salts from the plastic surface also improves the absorption of the catalyst or activator in the next step.

Alkaline Cleaning Etching Neutralizing Catalyzing Accelerating Electroless Nickel

Electroless copper

Electrolytic Strike Bright acid copper Bright Chromium

Duplex Nickel Micro cracked Chromium

Fig 1: Typical electroplating process for ABS plastics. Rinses have been omitted

Catalyzing In this step a catalyst is adsorbed on the plastic to initiate the electroless deposition process. In general, catalysts or activators are strongly acidic mixtures of tin and palladium salts designed to be adsorbed onto the plastic surface in limited and controllable amounts. The tin compound is strongly attracted to the organic surface and bonds the palladium to produce a catalytic surface. These catalysts are generally supplied as proprietary concentrates which are suitably diluted (1 to 10% by volume) at 15 to 50C(60 to 120 F) for immersion times of 2 to 10 minutes. Accelerating An accelerator or post-activator is used in the next step to remove excess tin compounds to expose the palladium. Dilute acid or alkaline solutions are used at 20 to 50 C (70 to 120 F) for 1 to 2 minutes immersing times. Electroless Plating Electroless Nickel or electroless copper deposits without electric current on the catalyzed surface to produce the conductive coating required for electroplating. Most present operations utilize electroless copper because of specific outdoor performance requirements. Electroless nickel baths are more stable and are acceptable for many less severe applications. In general proprietary electroless solutions are used. These baths will deposit sufficient metal at room temperature in 5 to 10 minutes to render the parts conductive. Present proprietary baths are highly stabilized. Baths lives of several months for electroless copper and over a year for nickel can be expected. Since the usual thickness of these electroless deposits will only be 0.12 to 0.75 mm (0.005 to 0.030 mil), it is very important that the parts now be handled with extreme care. Electroplating The previous steps can be carried out having the parts racked or in bulk using barrels or baskets. Most of the electroplating is done on racks, and only small parts plated in specially designed barrels, since plastic parts float in the plating bath. For same reason, the racks should make strong positive contact with the parts to avoid floating and give adequate electric conductivity. Although the conductivity of the electroless deposit may be sufficient to carry the required electroplating current on small and medium sized parts, it often is insufficient for large parts such as automotive grilles when electric rack contacts may be quite far apart. In general, it is advisable to use a nickel or copper strike deposit from a high efficiency solution at low current density in order to avoid burn-off of the thin electroless layer. Thickness of the strike deposit usually is less than 2.5 mm (0.1mil). The next electrodeposit is bright, leveling ductile copper obtained from acid copper sulfate baths. This layer of copper serves both to improve surface appearance prior to other metal deposits and to act as a stress-absorbing layer both for stresses in the following nickel deposits and the stresses which may be set up because of the difference in the thermal expansion of plastic and the plated metals. Thickness of this deposit will

vary with the size and design of the plastic part and with its intended use. For example, 15 mm (0.6 mil) usually will be sufficient for decorative knobs a frame on radios and television sets and decorative parts of small appliances. A minimum 20 mm (0.8 mil) may be necessary on larger appliance parts subject to some temperature variation and on most interior automotive parts. Exterior automotive parts, which usually are fairly large and which must withstand extreme temperature changes, require thicker deposit with the average usually being 20 to 25 mm (0.8 to 1 mil). In all cases, greater thickness may be applied in order to improve final surface finish. Over the bright copper, any of the other plated finishes may be applied. However, the most common finish is bright nickel and chromium, usually to meet a specific specification. Most appliance and interior automotive specifications will call for only bright nickel and chromium with exterior automotive specifications require a deposit of sulfur-free, semi bright nickel, a full bright nickel, and a special micro cracked or micro porous chromium. Table 1 shows the minimum plate thickness normally considered advisable for various service conditions. Major manufacturers will specify thickness and specific performance tests. These may include thermo cycling, adhesion tests, and corrosion tests. Table 1. ELECTRODEPOSIT THICKNESSES FOR PLATING OF PLASTICS Minimum Thickness Type of service Copper µm Mild indoor 15 exposure; warm dry minimum wear Indoor exposure 15 with moderate temperature and moisture changes, medium wear Indoor or outdoor 15 exposure, periodic wetting, possible major temperature changes; some abrasive wear Very severe outdoor 15 exposure such as exterior automotive

Bright Nickel

mils 0.6

Semi-Bright Nickel µm mils 0 0

µm 7.5

mils 0.3

Special* Chromium Nickel µm mils µm mils 0 0 0.125 0.005

0.6

0

0

15

0.6

0

0

0.25

0.01

0.6

15

0.6

5

0.2

2.5

0.1

0.25

0.01

0.6

19

0.75

12.5

0.5

2.5

0.1

0.25

0.01

* Special deposit of non-metallic particles in nickel matrix. Used to produce micro porous chromium structure. Special micro cracked chrome may be an acceptable substitute for some applications.

Making Printed Circuits Boards The simplest printed circuit begins with a plastic board that has copper foil glued onto its surface. The circuit paths are made by printing onto the copper foil coatings that resist the etchants used. These coating or resists are applied in the shape of the circuits that must remain. Then the exposed copper is etched away and the circuit paths remain. The resist is removed and you have a rudimentary printed circuit. Some circuits or portions of them are plated with tin or tin lead to provide solder ability or with gold to provide contact reliability. So this is another market for plating chemicals and equipment. Many small electronic parts are also plated with tin or tin lead or gold or other metals, to provide various electrical properties. A printed circuit board consists of "etched conductors" attached to a sheet of insulator. The conductive "etched conductors" are called "traces" or "tracks". The insulator is called the “substrate”. The vast majority of 'printed circuit boards' are made by adhering a layer of copper over the entire substrate, sometimes on both sides, (creating a "blank PCB") then removing unwanted copper by etching in an acid or ferric chloride solution, leaving only the desired copper traces. A few PCBs are made by adding traces to the bare substrate usually by a complex process of multiple electroplating. Some PCBs have trace layers inside the PCB and are called multi layer PCBs. These are formed by bonding together separately etched thin boards. After the circuit board has been manufactured, components are connected to the traces by soldering (usually by passing their leads through holes pre-drilled in the board) There are three common methods used for the production of printed circuit boards: 1. Silk screen printing, using etch-resistant inks to protect the copper foil. Subsequent etching removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank (non-conductive) board. The latter technique is also used in the manufacture of hybrid circuits. 2. Photoengraving, the use of a photo-mask and chemical etching to remove the copper foil from the substrate. The photo-mask is usually prepared with a photo-plotter from data produced by a technician using computer-aided PCB design software. Laser-printed transparencies are sometimes employed for low-resolution photo-plots. 3. PCB Milling, the use of a 2 or 3 axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format. PCBs are rugged, inexpensive, and can be highly reliable. They require much more layout effort than either wire-wrapped or point-to-point constructed equipment. Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components' leads were then passed through the holes and soldered to the PCB trace.

This method of assembly is called through-hole construction. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wavesoldering machine. Through-hole mounting is still used. However, the wires and holes are wasteful. It costs money to drill the holes, and the protruding wires are merely cut off. Most PCBs have alignment marks and holes (called fiducials) to align layers and permit the PCB to be mounted in equipment that automatically places and solders components. Some designs place alignment and etch test-patterns on break-off tabs that can be removed before installation. Layers may be connected together through drilled holes called vias. Either the holes are electroplated or small rivets are inserted. High-density PCBs may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither, but these are expensive to build and difficult or impossible to inspect after manufacture.

Chapter 13 Tin-lead (SnPb) solder has been widely utilized for electrical connections because of its convenience, economy, and electrical and mechanical characteristics. As a result of recent environmental concerns regarding lead (Pb), the requirement for Pb-Free semiconductors has been receiving increasing attention within the semiconductor industry. Countries around the world continue to enact stricter bans on the content of hazardous materials in semiconductors. Such legislation has prompted the semiconductor industry to develop environmentally friendly products. The semiconductor industry is under pressure to develop environment friendly interconnection and packaging technologies. Lead and lead alloys have had a long history of being components in solder and in connector leads for a good reason: the lead/tin compound known as solder forms good electrical connections with other materials such as copper and silver at the relatively low temperature of 183°C. The resulting joints are reliable and the process is cost effective. So finding Pb-free replacements involves a combination of materials science and clever process control. Process control is, in fact, the key. "It is getting tougher to make a reliable joint," says Philips' van de Water. "The window for success (in the soldering process) is getting smaller." Semiconductor companies have four primary problems to solve when designing Pb-free packages: solderability, reliability, whiskers, and moisture sensitivity. Not surprisingly, the attributes are interrelated. Solderability relates largely to the ability to melt the Pb-free solder at temperatures close to those of lead-based solder and lead coatings. Compatibility with the equipment used in most of today's wave-soldering assembly lines is an imperative if it to be kept in service with a minimum of retrofitting. Reliability addresses the strength of the joint; and, moisture sensitivity determines how long the component may be kept in storage before it is attached to a board. Solderability Conventional Pb-based soldering takes place in a range of 215° and 240°C for lead frame devices. Due to higher melting temperatures, matte tin requires a range of 235° to 260°C and this potentially has an impact on reliability. Tests have shown that tightly managing the temperature profile during the soldering process provides an acceptable solution, says van der Water. Other package types have similar results. Reliability The reliability of a solder joint can be compromised by changes in temperature while the chip is in service and is known as thermo-mechanical solder fatigue. Joint failure follows a well known process that begins with diffusion and re-crystallization. Crack initiation and growth follow until the fracture can actually be observed. Testing the reliability of solder joints has been conducted by cycling the product through a range of temperatures from -40° to 125°C for 10,000 cycles. Solder fatigue failure is

visualized and analyzed using a technique called Weibull statistics. Many package types have been tested. Reliability has been comparable to conventional connectors and solder pastes. Whiskers As previously mentioned, when pure tin is used for plating a lead frame, the growth of tin filaments has been identified as a potential reliability problem. If these "whiskers" grow long enough they can conceivably short circuit two pins. Irregular intermetallic growth at the copper-tin interface causes stresses that extrude the tin whisker from the surface. Whisker growth is not immediately apparent. It occurs during storage at ambient temperatures—not during the soldering process—so countermeasures must be taken. One approach is to make the tin layer thicker. This dramatically reduces the length of whiskers because a thicker tin layer can absorb more stress. In test results reported by ST Microelectronics, maximum whisker length decreased 160 microns to less than 10 by increasing the thickness of the tin layer from 1.82 microns to 10 microns. Another approach is to post-bake the component at 150°C for an hour. It was found that the higher temperature created bulk diffusion in the material and regular intermetallics. No whisker growth has been observed under these circumstances. Still another approach involves chemically pre-treating the tin surface of the lead frame to create a matte—as opposed to shiny—finish. A matte finish has proven to be less susceptible to whisker formation that a shiny tin finish. There is no reason for choosing just one of these solutions because they are, in fact, compatible. Moisture Sensitivity If any component is stored outside a dry pack for a significant amount of time moisture can accumulate which will change its solderability and reliability characteristics. Pb-free components are more inclined to be susceptible to moisture but using a different soldering profile (245°C for packages greater than 350 mm² and 250°C for packages smaller than 350 mm²) helps alleviate the problem, says Freescale's Mike Thomas. Every Pb-free SMD package will have to be re-qualified according to JEDEC standards, however, and the Moisture Sensitivity (MSL) classification of some will drop, which means more care will have to be taken when they are stored and used in board manufacturing facilities. For lead-free products, there are different types of solder pastes available and would work in board reflow at 260° C and below:

Sn/Ag4.0/Cu.5 Sn/Ag2.5/Cu.8/Sb.5 Sn/Ag3.5 Sn/Cu.75 Sn/Bi3.0/Ag3.0

217° C 216° C 221° C 227° C 213° C

(240-255° C) (225-240° C) (245-255°C) (250-260° C) (225-244° C)

Chapter 14 CONTROLLING HAZARDS IN THE ELECTROPATING INDUSTRY The Occupational Safety and Health Regulations 1996 set down specific requirements for workplaces that use hazardous substances. These cover such things as: • • • • • •

Labeling of containers Material Safety Data Sheets (MSDS) Induction and safety training Record keeping Risk reduction; and Health surveillance

The Regulations say employers, main contractors and self employed persons must: • • •

Identify hazardous substances; Assess the risk of injury or harm; and Reduce the risk by: 1. Preventing exposure to the hazardous substance 2. Means other than personal protective equipment; and 3. Where 1 and 2 are not practicable, by the use of personal protective equipment.

The Act says employees must take reasonable care of their own safety and health and avoid adversely affecting the safety and health of others. They must comply within reason with safety instructions, use personal protective equipment provided and report hazards or injuries. • • •

Manufacturers of hazardous substances must prepare a material safety data sheet Suppliers of hazardous substances must ensure containers are adequately labeled. They must provide a current MSDS to the workplace when first supplying a hazardous substance, and thereafter when requested. Designers, manufacturers, importers and suppliers must ensure, as far as practicable, that people installing, maintaining or using their plant are not exposed to hazards.

RISKS: Workers at electroplating workplaces may be exposed to hazardous substances. These substances are mainly in the form of: • • •

Fumes Vapors or mists Metal dusts

Other hazards in electroplating involve the use of: • •

Electricity Mechanical plant

•

Manual handling

What are the health risks? Workers exposed to electroplating chemicals can develop: • • •

Short term throat, lung, sinus, skin and eye irritation and burns Long term health problems such as asthma, heart, lung and nerve disorders and Cancer

The risk of developing health effects depends on how much chemical is absorbed into the body. In addition, electrolysis releases hydrogen bubbles which, unless safely contained or ventilated, can: • •

Become explosive Carry other chemicals in a toxic mist

What are the hazardous substances? Hazardous substances in electroplating include: • • • • • • •

Solvents such as methylene chloride, phenol, cresylic acid (a chemical similar to phenol) Gases such as hydrogen cyanide Acids such as chromic acid, sulphuric acid and hydrochloric acid Alkalis such as sodium hydroxide ( also known as caustic soda) Cyanides such as sodium and potassium cyanide Heavy metals such as nickel, chromium, cadmium and lead Toxic wastes

These substances are commonly used or produced in the: • • •

Preparation Coating Polishing of metal items.

CONTROLLING HAZARDS IN THE ELECTROPATING INDUSTRY When can chemical exposure occur? People working in electroplating industry can be harmed when: • • • • • •

Containers leak or spill during transport, storage, decanting or disposal Explosive or toxic gas or fumes build up during storage in confined areas Operators are splashed by items entering or leaving plating tanks Excessive bubbling or fuming occurs in acids, caustic or other chemicals Dust is breathed in during buffing or grinding of plated items Excessive hydrogen or oxygen is emitted during electrolysis or anodizing, causing an explosive or flammable atmosphere

• • • • • •

Local exhaust ventilation fails, or is inadequate to handle escaping gases, fumes and mists Overhead gantry cranes, hooks or slings fail when lowering or lifting items from dip tanks Residue liquid and sludge is removed from dip tanks Maintenance and repair work is done to tanks Chemical wastes are disposed of in sewers before being properly neutralized Chemical wastes are disposed of at tipping sites without proper authority approvals

How can hazardous substances enter the body? Hazardous substance can enter the body through: • • •

The skin or eyes, following contact with liquids or droplets The lungs and nasal passages, when fumes, droplets, gases or dusts are inhaled The mouth, when eating or smoking with contaminated hands

How can hazards be identified? Workplace hazards can be identified through: • • • • •

Checking packaging or container labels and material safety data sheets Regular communication between workers, supervisors and employers about likely hazards Regular inspection of workplaces, plant and equipment Regular review of tasks and procedures Checking of previous incident and injury records for recurring situations.

How can risk be assessed? General hazards: The risk of injury or harm from general workplace hazards can be assessed by: • • • •

Assessing the likelihood of the hazard causing injury or harm, e.g.. very likely or remotely possible Assessing the likely severity of injury or harm, e.g.. serious or minor injury Checking records of previous incidents and injuries where hazards have caused injury or harm Checking plant and equipment to make sure hazards are properly controlled

Hazardous substances: In addition, the risk of injury or harm from hazardous substances can be assessed by: • •

Obtaining information about the hazards Checking work processes to make sure hazards are adequately controlled;

• •

Conducting atmospheric monitoring to determine levels of exposure to chemicals such as chromic acid Conducting health surveillance to detect any adverse health effects from chemicals at an early stage.

How can risk be reduced? Risk can be reduced by using control methods, in the following order of priority: • • • • • •

Eliminate or remove the hazard – e.g. do not use a chemical or item in the plant if it is not required. Substitute or replace it with safer plant, equipment or substance. Isolate it from workers – e.g. enclosed systems for chemicals, relocation of employees or physical barriers. Introduce engineering controls – e.g. guarding or exhaust ventilation. Administrative controls – e.g. limiting workers’ time spent near the hazard. Personal protective equipment – e.g. safety goggles and respirators. While essential for some work procedures, these should be last in the list of priorities.

What information and training is required? • • • • • •

All workers must be informed of hazards from exposure to harmful substances. They must be given information, instruction, training and supervision in safe procedures, including personal protective equipment. Workers should know how to identify hazards, and to report them to a supervisor. Training on hazardous substances must include potential health effects of the substances used, control measures, correct use of protective equipment and the need for and details of health surveillance. Workers from non-English speaking backgrounds may have special needs and should be provided with information in their first language. Training should be ongoing, with regular revision of safe procedures.

Controlling plating tank hazards • • • • • • •

Substitute hazardous substances with less hazardous ones. Where possible, pump chemicals into plating tanks rather than pouring manually from containers. Pumps need to be cleaned before use with a different chemical. Use local exhaust ventilation along one or more sides of the tank to remove mists and vapors. Use a suppressant to minimize the amount of mist generated during electro plating. Minimize risk of items accidentally dropping into tanks, splashing operators. Ensure overhead cranes, hooks and slings are regularly maintained.

Controlling cyanide hazards •

Acids and cyanides are an explosive combination, and should be clearly labeled and stored in locked, dry places, well away from each other.

• •

Articles treated in acid baths should be thoroughly rinsed with water before being placed in plating tanks. Drainage should be designed so there is separation of acid spillage from cyanide spillage or effluent.

Buffing, grinding and polishing • • • • •

Newly electroplated surfaces on heavy machinery parts are usually finished with portable or fixed grinding machines. Finer finishes on personal, hobby or household items are achieved with buffing and polishing wheels, containing various polishes and waxes. These processes generate large amounts of metal dusts, some of which are hazardous if inhaled. Local exhaust ventilation should be fitted to grinding and buffing machines to remove dust as it is generated. Where substances that are known to be carcinogenic are used, exposure levels should be kept as low as possible

Chapter 15 Treating Metal Plating Effluents The field of electroplating or metal finishing today is moving away from large manufacturing operations and into smaller job shops. With increasingly strict regulations governing metals in wastewater and increasing costs for disposal of metal-contaminated waste, many metal finishers find that implementing pollution prevention measures such as filtering or treating process water to reduce or eliminate metal contamination and allow water reuse is the best option, both environmentally and economically. Plating processes in this sector include chrome, bright and electroless nickel, zinc, copper, tin, conversion coatings, and more. Because metal plating has many significant environmental aspects and is a highly polluting industry, there are many resources available to help the metal plating industry improve its environmental performance. Trade associations have websites with technical guidance, suppliers, and discussion forums where questions can be asked and answered, all for free. There are several major programs focused specifically on helping the sector with pollution prevention. Researchers looking for cost-effective ideas for improvement should examine the sites dedicated to environmental improvement, and the trade association sites. The tables below list low-cost or no-cost solutions to reduce waste and pollution in any metal plating company, including ones in developing countries. All of these ideas have been proven to help small companies, anywhere in the world, save money while protecting the environment.

Top Low-Cost Solutions to Increase Efficiency and Reduce Waste in Metal Plating Operations

Install spill containment

Spills can be contained and managed to prevent losses of valuable resources.

Allows more chemical to drip back to process tank, so reduces the amount of chemical introduced in rinse water. Post dragout times on signs at tanks to remind employees. A drain time of at Establish dragout timing least 10 seconds has been demonstrated to reduce dragout by 40+%, compared to the three-second industry average. Install drain boards or Boards and guards minimize spillage between tanks and are sloped away from drip guards rinse tanks so dragout fluids drain back to plating tanks. These sensors indicate the cleanliness of the rinsed water. They cost only a Use conductivity few hundred dollars to order, and can greatly improve plating quality and sensors prevent unnecessary dumping of rinsed water. Agitation promotes better rinsing. Agitate water or part manually or with Agitate rinse bath mechanical means (stirring or air bubbling). Reuse spent Spent acid can be used to neutralize an alkaline waste stream. Spent alkali acid/alkaline can be used to neutralize an acid waste stream. Concentrate rinsed water and captured dragout liquids for reuse; the water Evaporation condensate can also be reused. Mechanical evaporators or simple boilers can be used. Evaporates bath water so relatively clean waste rinsed water can be reused as Increase bath bath makeup water. Reduces solution viscosity so more chemical drains back temperature to process tank during dragout. Do Not Use On Cyanide or Hexavalent Chromium Baths. Run tests with successively lower bath concentrations to find the minimum level needed to achieve quality. This saves money by reducing overuse of Optimize bath chemicals and reduces contamination in wastewater. Remember that concentrations concentrations recommended by vendors are usually higher than the minimum needed for quality. Lengthen dragout time

How to restore old chrome plated parts When chrome plated finishes become scratched or marred there really isn't much you can do with them. Beyond cleaning it with metal polish and keeping it waxed, there isn't much you can do yourself. The items can certainly be re-chromed by a plating shop. Some people use chrome polish to help shine the surface, but if the surface is scratched, it will eventually corrode. Your best bet is to take them to a custom plating shop in your area. They will strip the chrome and hopefully any underlying nickel or copper. Conversely, some shops will only strip the chrome and polish the nickel and reapply another layer of nickel, perhaps acid copper, perhaps copper buff and more nickel and chrome. As you can see, the process can be quite extensive and therefore, rather expensive. Most people assume we just "dip it in a vat" and the part miraculously emerges with the shiny, reflective surface we all associate with nickel/chrome. Please be aware that there is a tremendous amount of manual labor involved in stripping the parts, buffing them, and replating them, so you should expect that you will pay considerably more than you probably expected. What are organic additives Organic additives (carriers, brighteners, levelers) work to increase the current density or plating rate that can be maintained with satisfactory throwing power. The additives fall into three main categories: • Carriers • Brighteners • Levelers Carriers increase the polarization resistance and are current suppressors. The suppression is a result of the carrier being adsorbed to the surface of the cathode; this results in increasing the effective thickness of the diffusion layer. The result is better organization. This gives rise to a deposit with a tighter grain structure. The carrier modified diffusion layer also improves plating distribution without burning the deposit. The brightener is a grain refiner. Its random adsorption may produce a film that will suppress crystallographic differences. Alternatively, brighteners may be adsorbed preferentially on particular active sites such as lattice kinks, growth steps, or tops of cones, or surface projections in general; growth at these locations is then blocked. Levelers or leveling agents are inhibitors present at low concentrations in the electrolyte as compared to the depositing metal. In case of a micro profile, the diffusion layer does not follow the profile contour, but is maximum at the valleys and minimum at the peaks. Consequently, in absence of a leveling agent, depositing ions diffuse more rapidly to the peaks than to the valleys, and deposits grow more rapidly on the peaks, resulting in an exaggerated profile. With good solution agitation, the leveler will accumulate more rapidly and readily at the peaks and it will inhibit growth or deposition. The valleys will allow faster deposit growth and allow the valleys to catch up to the peak, thus creating leveling.

CHAPTER 16 The purpose of this chapter is to present in brief information on plating baths, for the most part commonly used in production, essential for intelligent handling of the engineering problems that may arise in connection with their installation and operation. The three variables of temperature, current density, and agitation are interrelated in their effect on a plating operation. It is customary to control all three closely by standardization and instrumentation. Increasing temperature and agitation usually enables one to obtain higher current efficiencies and to use higher current densities. It is good practice to operate at the higher stvalues of temperature and current density consistent with the limitation imposed by the quality of deposit required, or by the equipment and materials of construction. One thus obtains the maximum production from the available facilities. The operating temperatures in the tables represent the best average, if a single value is given or the usual temperature range, if two values are given. The operating temperature is an important consideration in the selection of suitable linings and protective coating. It also determines what provision must be made for heating or cooling. Single values of current density in the tables are good average values for the conditions of temperature and agitation shown. A practical operating range of current density is indicated by a high and low value. It is well to allow for somewhat higher values when estimating current requirements. The various types of anodes to be used with the different plating baths are mentioned in the tables. Soluble and insoluble anodes of various types and compositions are discussed in Chapter 29. Most modern plating processes and specifications require the use of high purity soluble anodes. The ration anode-to cathode area is also important in determining the anode current density. Too high an anode current density may cause objectionable polarization when using soluble anodes with resultant loss in anode efficiency, formation of solid particles, or sludge rough deposits and adverse changes in the composition of the plating bath.

Key No. 1

Material Steel, low carbon

2 3 4 5 6 7 8

Cast Iron Stainless Steel High silicon cast iron (‘Duriron’) Lead usually 6% antimony alloy Copper Nickel Carbon (“Karbate”)

9 10

Glass (“Pyrex” or tempered) Chemical stoneware

11 12 13 14

“Haveg” Hard rubberb Rubber (approved compositions) b Plastics (approved compositions) b

15 16

Acid resisting bricks Wood

Uses Tanks, filters, pumps, pipes, fittings, heating coils Pumps, filters, valves, fittings Tanks, pumps, filters Pumps, pipe, fittings, heat exchanges Linings, pipe Heating coils Heating coils Heaters and heat exchangers primarily, pumps, air diffusers Heat exchangers, tanks, pumps Tanks, tower concentrators and tower packing Tanks, pipe Pipe, fitting, pumps Linings, hose Linings, hose, pipe, fittings, barrels, heating coils and exchangers Linings Tanks

EFFECT OF HYDROGEN Embrittlement of metals by hydrogen has been recognized for many years but, as two authors have pointed out124 some other defects in electrodeposits, such as blistering, cracking, gas pits, peeling poor adhesion may also be related to hydrogen in ways not yet identified. Their conclusions, quoted below, have added greatly to our understanding of these matters. “1. Many electroplating processes favor the absorption of large quantities of hydrogen by metals. Embrittlement of the metals may be due to hydride formation, to strain imposed by occlusion of molecular hydrogen in submicroscopic rifts and to deposition of hydrated ionsphenomena that are already recognized by electroplaters. “2. Less recognized, perhaps, are the functions of hydrogen occluded within the steel. The extremely low solubility of steel for hydrogen at ordinary temperature, combined with the extraordinary ability of this metal to absorb huge quantities of gas when presented atomically at the surface during pickling and cathodic electrolysis, causes important effusion of this gas from the steel when the atomic layer is removed by the presence of an applied coating or by cessation of the hydrogen-producing process. “3. When the effusion occurs beneath the coating of any material, including metals, whose permeability is unsuited to the quantity of the effusion, the coating may be (1) lifted from the steel or ruptured; or both, by pressure of the accumulating gas, or (2) blistered, depending upon the qualities of the coating. “4. If the plated ware is heated, the effusion of hydrogen is accelerated and the coating is weakened, so that the occurrence of the above defects is favored.

“5. Effusion of the gas from the steel base during electroplating leads to the formation of the gas pits in the coating. “6. Cathodic cleaning in either acid or alkaline solutions provides large quantities of hydrogen for absorption. “ A true bronze consisting of a plated copper tin alloys similarly used as a stop off for nitriding. These coatings can be removed by one of the proprietary strippers listed above or by one of the following methods. EXAMPLE 1. Treat anodically in a solution of 12.2 oz/gal sodium cyanide and 2.7 oz/gal sodium hydroxide at room temperature using 6 volts. 2. Immerse in solution of 13.4 oz/gal sodium hydroxide, 2.7 oz/gal sodium cyanide and 2.7 oz/gal sodium chlorite at a temperature of 80-90 C (175-195 F). Note that sodium chlorite is a hazardous chemical and instructions should be followed implicitly. EXAMPLE 1. Anodically treating at 20 to 40 asf and 82 C (180 F) in a solution consisting of sodium hydroxide 13 oz/gal, sodium metasilicate 10 oz/gal and Rochelle salts 6.7 oz/gal. 2. Anodically treating at 20 to 200 asf and 20 to82 C (70-180 F) in a sodium nitrate solution of 67 iz /gal at a pH of 6 to 10. 3. Immersion in a solution of acetic acid 10 to 85% by volume and hydrogen peroxide (100 vol) at 5% by volume. EXAMPLE 1. Reverse current in a sodium cyanide solution (4 to 8 oz/gal) at room temperature and an anode current density of 10 to 20asf. 2. Same as (1) but at high pH. I.e. with an addition of 4 to8 oz/gal of sodium hydroxide to the stripping bath. 3. Immersion in an acid mixture consisting of 95 and 5% by volume of concentrated sulfuric and nitric acids, respectively, and operated at about 80 C (175 F) This bath should be covered when not in use to prevent dilution by moisture from the air parts should be dry when immersed, otherwise the basis metal attack will be excessive.

Well, that’s about it! If you’ve read this far than you can now call yourself a metal plating expert. You should now have a solid grasp of exactly what is entailed in the metal or electroplating process, across a huge range of possible metal plating applications and situations. We hope that you have enjoyed reading the “Metal Plating Bible”, and most of all, we hope you can now feel confident doing your own metal plating, in your own specific plating situation. Metal plating of course, is a technical and oftentimes advanced topic by nature. This guide has been an extensive attempt to take what was normally an activity that only chemistry experts could partake in, and give beginners and other not so knowledgeable people a step-by-step guide that they could learn from to form their own metal plating action plan. On that note, I would like to wish you (my treasured customer) a pleasant evening, and the best of success in your future metal plating activities! And remember, I’m always an email a way if ever need assistance. Warm Regards, Craig Bellinger Owner and Co-Author

GLOSSARY Anion- A negatively charged ion or radical which is attracted to the anode because of the negative charge. Anode -The positively charged electrode at which oxidation or corrosion of some component occurs (opposite of cathode). Electrons flow away from the anode in the external circuit. Barrel plating (or cleaning) - Plating or cleaning in which the work is processed in bulk in a rotating container. Brass - An alloy consisting mainly of copper (over 50%) and zinc, to which smaller amounts of other elements may be added. Brightener - An agent or combination of agents added to an electroplating bath to produce a smooth, lustrous deposit. Cathode -The negatively charged electrode of an electrolytic cell at which reduction occurs. Cation - A positively charged ion that migrates through the electrolyte toward the cathode under the influence of a potential gradient. See also anion and ion. Cell - Electrochemical system consisting of an anode and a cathode immersed in an electrolyte. The anode and cathode may be separate metals or dissimilar areas on the same metal. The cell includes the external circuit, which permits the flow of electrons from the anode toward the cathode. See also electrochemical cell. Chromate coating (chromating) - A corrosion protection technique; can be applied to steel, aluminum, magnesium, and zinc. It results in the formation of metal oxide on the surface of the work piece which reacts to form metallic chromates. Chromating of aluminum and magnesium improves corrosion resistance considerably. With steel it is much less permanent. Conversion coating - A coating produced by chemical or electro-chemical treatment of a metallic surface that provides a superficial layer containing a compound of the metal; for example, chromate coatings on zinc and cadmium or oxide coatings on steel. Corrosion - The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. Electrochemical Cell - An electrochemical system consisting of an anode and a cathode in metallic contact and immersed in an electrolyte. The anode and cathode may be different metals or dissimilar areas on the same metal surface. Electro-deposition - The deposition of a substance on an electrode by passing electric current through an electrolyte. Electroless plating - A process in which metal ions in a dilute aqueous solution are plated out on a substrate by means of autocatalytic chemical reduction. Electroless plating uses a redox reaction to deposit metal on an object without the passage of an electric current.

Electrolysis - Production of chemical changes of the electrolyte by the passage of current through an electrochemical cell. Electrolyte - (1) A chemical substance or mixture, usually liquid, containing ions that migrate in an electric field. (2) A chemical compound or mixture of compounds which when molten or in solution will conduct an electric current. Electrolytic cells - An assembly, consisting of a vessel, electrodes, and an electrolyte, in which electrolysis can be carried out. Electro-cleaning - An electrochemical cleaning process by which a metal is first made the cathode in an electrolytic cell. When current is applied, electrolysis of water occurs at the surface of the metal. This results in generation of Hydrogen gas. This gas creates a highly efficient scrubbing action. Following initial treatment as a cathode the circuit is reversed so that the metal is the anode. Oxygen gas, which is generated at the surface, produces a final cleaning action. Electrolytic etch - A technique generally applied to steels which attack the surface to produce a clean, oxide free material. It is often used prior to electroplating, especially chromium plating. Since it preferentially attacks edges it will open us small cracks in the surface of the metal. Due to this, this process can be used to inspect finishes for flaws. Etching - Etching is sometimes used a surface preparation technique prior to electroplating or for removal of metal such as in the printed circuit industry where material not required on the finished product is removed by a chemical solution. It can also be used as an inspection technique due to its ability to accentuate surface cracks and defects. HCD - High Current Density - High amperes per surface area Indicator (pH) - A substance that changes color when the pH of the medium is changed; in the case of most useful indicators, the pH range within which the color changes is narrow. Leveling - Electrodeposited materials tend to be concentrated at sharp corners, peaks, and ridges, therefore, when a metal with a rough surface is electroplated, the rate of deposition will be faster on convex irregularities resulting in an accentuation of the item's original roughness. To counteract this effect, additives are added to the electrolyte solution to produce a polarization effect concentrated at the peaks and ridges. This polarization effect lowers the current density at the peaks and reduces deposition rates. The net result is to smooth or "level" the surface of the metal. Reducing agent - A compound that causes reduction, thereby itself becomes oxidized. Sensitizing - A relatively non-specific term used to cover a range of metal finishing processes that improve the treatment ability of a metal for subsequent processes. It often refers specifically to a part of electroless plating procedure on plastics or non-metal surfaces. After the surface has been etched it is reacted with solution that deposits a very thin film of a metal or metallic compound. The surface is then referred to as sensitized. Substrate - Surface material or electroplate upon which a subsequent electro-deposit or finish is made.

Solder plating - The term covers deposition of an alloy of 60% tin and 40% lead that is widely used in the electrical and electronics industries. It provides two valuable features, corrosion resistance and "solderability".

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