Cathodic Protection

Engineering Encyclopedia  Saudi Aramco DeskTop Standards

Cathodic Protection Systems

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chap te ter : M at aterials A nd nd Corrosio n Co nt ntrol For ad di dition al al inform at ation on this sub je ject, con ta tact File Reference: COE101R4 O.S. Abdul Hamid on 874-2532 or A.M. Al-Ghamdi on 873-1290

Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

Saudi Aramco DeskTop Standards

Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

Saudi Aramco DeskTop Standards

Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

SAUDI ARAMCO

Saudi Aramco DeskTop Standards

Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

THE SOURCE OF THE TECHNICAL MATERIAL IN THIS VOLUME IS THE PROFESSIONAL ENGINEERING DEVELOPMENT PROGRAM (PEDP) OF ENGINEERING SERVICES.

N OTE:

WARNING. THE MATERIALS CONTAINED IN THIS MANUAL WERE DEVELOPED FOR THE SAUDI ARABIAN OIL COMPANY (SAUDI ARAMCO) AND ARE INTENDED FOR THE EXCLUSIVE USE OF SAUDI ARAMCO EMPLOYEES. A NY M ATERIAL CONTAIN ED IN THIS MANUA L WH ICH IS  NOT ALREADY IN TH E PUB LIC D OMAIN MAY NOT BE C OPIED, REPRODUCED, SOLD, GIVEN, OR  DISCLOSED TO THIRD PARTIES, OR OTHERWISE USED, IN WHOLE OR IN PART, WITHOUT THE PRIOR WRITTEN PERMISSION OF VICE PRESIDENT, ENGINEERING SERVICES, SAUDI ARAMCO.

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Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

CONTENTS

PAGES

INFORMATION Principles and Operation of Cathodic Protection Systems How Cathodic Protection Works Measuring Electrical Potential Operation of Galvanic Anode Systems

1 1 5 7

Galvanic Anodes

12

Anode Chemical Backfill

16

Advantages and Disadvantages of Galvanic Anode Systems

17

Operation of Impressed Current Systems

18

Impressed Current Anodes

20

Coke Breeze

23

Advantages and Disadvantages of Impressed Current Systems

23

Calculating Cathodic Protection Requirements for Buried Pipelines

24

Calculating Cathodic Protection Requirements for Tanks and Vessels

26

Calculating Cathodic Protection Requirements for Marine Structures

28

Offshore Platforms

28

Marine Pipelines

29

Piers and jetties

30

WORK AID Work Aid 1: Formula, Criteria, and Guideline for Calculating Cathodic Protection Requirements for Buried Pipelines

31

Work Aid 2: Formulas, Criteria, and Guidelines for Calculating Cathodic Protection Requirements for Tanks and Vessels

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Work Aid 3: Criteria, and Guidelines for Calculating Cathodic Protection Requirements for Marine Structures

33

GLOSSARY

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PRINCIPLES AND OPERATION OF CATHODIC PROTECTION SYSTEMS How Cathodic Protection Works Cathodic protection (CP) is an electrical method to control corrosion. It is based on the  principles of the electrochemical corrosion cell. Figure 1 shows two typical corrosion cells. The diagram on the left is a laboratory corrosion cell that contains two dissimilar metals. The diagram on the right is a corrosion cell that can occur on the surface of a metal. In both diagrams, the potential difference between the anode and cathode is the driving force for  corrosion. The direction of the corrosion current is shown for each diagram. Electric current flows in the opposite direction of the electrons. The direction of current flow is from positive “+” to negative “-” in the metal path. The metal corrodes where current leaves its surface. The metal does not corrode where electric current enters its surface.

A Laboratory Corrosion Cell (left) and a Corrosion Cell on a Metal Surface (right) Figure 1

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How Cathodic Protection Works, Cont’d To understand how cathodic protection works, consider the electrochemical reactions that occur in a corrosion cell on the surface of a metal (Figure 2). As corrosion occurs, the electrons that are released in anodic reactions are consumed in cathodic reactions.

An Electrochemical Corrosion Cell on the Surface of a Metal Figure 2 If additional electrons are supplied to the metal from an external source, more electrons are available for cathodic reactions (Figure 3). As a result, the cathodic reaction rate and the evolution of hydrogen gas increases. Consequently, electron demand from the anode decreases and the anodic reaction rate decreases to produce fewer electrons. Reducing electron demand from the anode by supplying additional electrons to the metal is the basic  principle of cathodic protection. Decreased anodic reactions

Electrons from external source

Electrons Introduced from an External Source Figure 3

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How Cathodic Protection Works, Cont’d These additional electrons are supplied by direct electric current. As direct current is applied to the metal, the potential of the cathodic areas shifts toward the potential of anodic areas. This is shown graphically in Figure 4.

Cathodic Polarization Caused by Direct Current Figure 4

If enough direct current is applied, the potential difference between an anode and cathode will  be eliminated and corrosion will cease. For example, Figure 5 shows corrosion cells on a  buried section of pipeline. Corrosion currents flow between local anodic areas (A) and cathodic areas (C) in the corrosion cells.

Before Cathodic Protection Figure 5

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How Cathodic Protection Works, Cont'd In Figure 6, an external current source is connected to the buried pipeline through a lead wire. Direct current flows from the external current source onto the buried pipeline. This current causes the potential of the cathodic areas to shift in a negative direction. If enough current flows onto the metal surface, the potential of the cathodic areas will equal the potential of the most anodic area on the pipeline. As a result, corrosion will stop. To complete the circuit, the lead wire returns current to the current source. It is important to note that CP does not eliminate corrosion. It transfers corrosion from the protected structure to the expendable external current source or anode.

After Cathodic Protection Current is Applied Figure 6

Cathodic protection current will only protect external surfaces on buried structures.  Above ground, structures cannot be protected by cathodic protection because the current  discharged from the current source will not travel through the atmosphere. Internal surfaces of pipelines can only be protected by either corrosion inhibitors or coatings, or by using a corrosion resistant alloy for the pipeline. NOTE: 

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Measuring Electrical Potential

When direct current is applied to a metal surface, its potential becomes more negative. This shift in electrical potential can be measured and used as a criterion of cathodic protection. Electrical potentials are always measured with respect to a reference electrode. The coppercopper sulfate (Cu-CuSO4) reference electrode is the most common reference electrode used for buried structures. The Cu-CuSO4  reference electrode is durable, easy to make, and easy to maintain. Also, the potential of a Cu-CuSO4  reference electrode changes very little with temperature. Figure 7 is a diagram of a Cu-CuSO4 reference electrode. The voltage of a Cu-CuSO4 reference electrode depends on the concentration of copper sulfate in the electrolyte solution. Saturated solutions are easiest to make and they provide consistent measurements. To make a saturated solution, copper sulfate crystals are added to water until some of the crystals do not dissolve. The solution remains stable as long as some copper sulfate crystals are undissolved. The plug at the bottom of the reference electrode is made of porous material. The porous plug  provides a conductive path between the reference electrode and the electrolyte around a structure.

Diagram of a Copper-Copper Sulfate Reference Electrode Figure 7

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Figure 8 shows how the potential difference between a pipeline and the Cu-CuSO4 reference electrode is measured. The reference electrode is connected to the positive lead of a high impedance voltmeter. The common (negative) terminal of the voltmeter is connected to the  pipeline test lead. The voltmeter reading is the sum of the potential between the reference electrode and the soil and the potential between the pipeline and the soil. The potential  between the reference electrode and the soil is constant. The potential between the pipeline and the soil can vary. When connected as shown, the potential reading will normally be  positive. By convention, voltage readings are often reported as negative numbers.

Measuring Structure-to-Reference Electrode Potential Figure 8

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Operation of Galvanic Anode Systems Galvanic anode systems are based on the galvanic corrosion cell. A galvanic corrosion cell is two dissimilar metals connected in a common electrolyte. Corrosion current flows from the metal with the more negative potential to the metal with the least negative (more positive)  potential. The metal with the least negative potential is protected from corrosion. For  example, the Practical Galvanic Series below shows the potentials of metals in soil with respect to a Cu-CuSO4  reference electrode. If two metals in the Series form a galvanic couple, the metal nearest the top will be anodic to the other metal.

Metal

 Normal Electrode Potential, volts vs. Cu-CuSO4

High potential magnesium alloy Magnesium alloy (contains Al, Zn, Mn) Zinc Aluminum alloy (Contains zinc) Mild steel Cast iron Brass, bronze, or copper High silicon cast iron Mill scale on steel Carbon, coke, graphite

More anodic -1.80 -1.55 -1.10 -1.05 -0.50 to -0.80 -0.50 -0.20 -0.20 -0.20 More cathodic +0.30

Practical Galvanic Series In Neutral Soil Figure 9

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Operation of Galvanic Anode Systems, Cont’d Figure 10 shows two galvanic couples in soil. One couple contains magnesium and steel and the other couple contains zinc and steel. In each couple, the more negative metal (magnesium or zinc) corrodes and the steel is protected from corrosion. The corrosion rate is greater in the magnesium-steel couple because the potential difference is greater.

Galvanic Couples in Soil Figure 10

Galvanic anode systems work like the galvanic couples in the previous figure. The components in a typical buried galvanic anode system include anodes, chemical backfill, lead wire, and a junction box (Figure 11). Lead wires from the anodes go to a junction box. Another lead wire connects the lead wires in the junction box to the structure. When galvanic anodes are connected to a buried structure such as the steel pipeline, a galvanic corrosion cell develops. Electric current flows from the anodes, through the soil, and onto the pipeline. The  pipeline becomes cathodically protected. To complete the circuit, current returns to the anodes through the lead wires.

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Operation of Galvanic Anode Systems, Cont’d

Typical Galvanic Anode System in Soil (arrows show the direction of electric current) Figure 11

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Operation of Galvanic Anode Systems, Cont’d Figures 12 and 13 show how a galvanic anode shifts the potential of a structure. Figure 12 shows a section of corroding pipeline and a buried magnesium anode. Voltmeter E1 measures the potential of the pipeline using a Cu-CuSO 4 reference electrode. Voltmeter E2 measures the potential of the anode. The pipeline and anode are not connected because there is an open switch between them. The switch is open so that current does not flow.

The Potential Differences of a Section of Buried Pipeline and an Anode Figure 12

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Operation of Galvanic Anode Systems, Cont’d In Figure 13, the switch is closed and current flows from the anode onto the pipeline. The  potential of the pipeline shifts toward the potential of the magnesium anode. If enough current flows onto the pipeline, it will overcome all the corrosion current trying to leave the  pipeline at the previously anodic sites. When this occurs, corrosion stops. The pipeline  becomes the cathode of the corrosion cell and the magnesium becomes the anode.

Potential Differences of a Section of Buried Pipeline and an Anode During Cathodic Protection Figure 13

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Galvanic Anodes

Galvanic anodes can protect small structures or parts of larger structures. A galvanic anode supplies a given amount of electric current. Several anodes may be grouped to provide enough current to shift the potential of a structure. Galvanic anodes are effective when (1) electric current requirements are low, (2) the structure is well coated, or (3) the electrolyte has relatively low resistivity. Galvanic anodes are usually made from materials near the top of the Galvanic Series (e.g., magnesium, zinc, and aluminum). Magnesium Galvanic Anodes  - Magnesium is the most widely used material for buried galvanic

anodes. Saudi Aramco normally uses magnesium anodes on pipelines at road and camel crossings. A typical 27.3 kg (60 lb) magnesium anode is shown in Figure 14.

Typical 27.3 kg (60 lb) Magnesium Galvanic Anode Figure 14

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Two types of magnesium anodes are available—standard alloy and high-potential alloy. Both have a theoretical energy content of about 2,200 ampere-hours per kg and an efficiency of  50%. Therefore, the actual energy content available is 2,200 ampere-hours per kg x 0.50, or  1,100 ampere-hours per kg. The standard magnesium anode has a solution potential  of -1.55 volts versus Cu-CuSO4. High-potential magnesium anodes have a solution potential of -1.80 volts. Saudi Aramco uses high-potential magnesium anodes almost exclusively. NOTE: Magnesium anodes are susceptible to polarization. For this reason, their solution potential is reduced by 0.10 volt for design purposes. Zinc Galvanic Anodes   - Zinc anodes are most often used in sea water or in soil resistivities

 below 700 ohm-cm. Occasionally they are used in soils up to 2,500 ohm-cm. Pure zinc has a theoretical energy content of 820 ampere-hours per kg. Zinc anodes typically operate at about 95% efficiency. Therefore, the actual energy content available is 820 ampere-hours per kg x 0.95, or 779 ampere-hours per kg. Zinc anodes have a potential of -1.10 volts versus a CuCuSO4 half-cell. Zinc galvanic anodes used in soil have long slender shapes to achieve low resistance to earth (Figure 15). Their shape also provides practical current output despite their low driving voltages. Zinc anodes are not subject to significant polarization when they are used in suitable backfill. CAUTION: Do not use zinc anodes when the temperature will exceed 49° C. This temperature may cause the polarity of zinc anodes to reverse and result in corrosion of the structure.

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Typical 13.6 kg (30 lb) Zinc Anode Figure 15

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Aluminum Galvanic Anodes   - Aluminum anodes are used in offshore applications.

The three

general types of aluminum anodes are as follows: • • •

Heat-treated aluminum zinc-tin alloy Aluminum-zinc-mercury alloy Aluminum-zinc-indium alloy

All of these alloys have slightly different theoretical energy contents, but they average 2,700 ampere-hours per kg in sea water service. Aluminum anodes are effective in electrolyte resistivities less than 700 ohm-cm. The efficiencies of aluminum anodes range from 85% to 95% in sea water. The potentials of aluminum anodes range from -1.05 to -1.15 volt versus Cu-CuSO4. Aluminum galvanic anodes are manufactured so they can attach directly to an offshore structure. Three types of core arrangements are shown in Figure 16.

Aluminum Anodes for Offshore Structures Figure 16

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Anode Chemical Backfill

Figure 17 shows a typical vertical galvanic anode installed in soil. Galvanic anodes installed in soil are surrounded with a special chemical backfill material. A typical backfill mixture for  magnesium anodes is 75% hydrated gypsum, 20% bentonite clay, and 5% sodium sulfate. Clays in the backfill adsorb water from the soil and keep the anode moist for maximum current output. The backfill materials also prevent passive corrosion films from forming on the anode. The chemical backfill also lowers the resistance of the anode to the soil. Galvanic anodes are frequently pre-packaged in backfill material. The entire cloth backfill package may  be buried as shown in Figure 17.

Typical Vertical Surface Anode in Pre-Packaged Backfill Figure 17

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Advantages and Disadvantages of Galvanic Anode Systems

The advantages of sacrificial anodes are as follows: • • • •

An external power source is not required. Installation costs are low for new structures. Maintenance costs are low. Sacrificial anodes seldom cause interference problems with other structures.

The disadvantages of sacrificial anodes are as follows: • • •

The driving potential is limited. The current output from individual anodes is low and limited. Sacrificial anodes are effective in a limited range of soil resistivities.

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Operation of Impressed Current Systems When current requirements are high, Saudi Aramco cathodically protects structures with impressed current (IC) systems. The operation of a typical IC system is shown in Figure 18. An electrical grid supplies high-voltage alternating current to a rectifier. The rectifier reduces the voltage of the alternating current and converts it to pulsating direct current. The direct current goes from the positive terminal of the rectifier to a junction box. At the junction box, the current is distributed to a anode bed of impressed current anodes. The anodes drive, or  impress, the current into the earth. The current travels through the earth and is collected by the structure. The current returns to the negative terminal of the rectifier via a cable, which is connected to the structure.

A Typical Impressed Current System with a Surface Anode Bed Figure 18

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Operation of Impressed Current Systems, Cont'd

A Typical Deep Anode Bed Design Figure 19

Deep anode beds (Figure 19) are used when surface soil resistivity is too high for normal anode bed design. Deep anode beds are also used in congested areas such as pipeline corridors and in-plant areas to provide better current distribution. Saudi Aramco installs a PVC vent pipe to allow gases formed by anodic reactions to escape. A separate loading pipe runs to the bottom of the hole and pumps a water slurry of coke  breeze into the hole. The loading pipe is slowly withdrawn from the hole as it is filled with coke breeze. This procedure allows the slurry to be pumped upward from the bottom of the well until the anodes are completely surrounded.

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Operation of Impressed Current Systems, Cont'd Saudi Aramco uses solar powered systems to provide direct current where alternating current is not available and it is not economical to extend power lines. For example, solar systems are used to protect the external casings of many remote oil and gas wells. Solar power also  provides impressed current for remote sections of the East-West Pipeline, the QQ pipeline, and along various pipeline corridors. Saudi Aramco also uses engine driven generators to  provide power to remote impressed current anode beds along the East-West Pipeline and the QQ Pipeline. Impressed Current Anodes

Saudi Aramco normally uses five types of impressed current anodes. •

graphite (no longer used but still in service)

•

high silicon chromium cast iron

•

scrap steel

•

mixed metal oxide composite

•

platinized niobium

Other types of anodes offer little economic advantage over the above materials. Figure 20 shows a typical center-connected anode. Center connections reduce the consumption of  anode material at the ends of anodes compared to end-type connections.

Typical Center-Connected Impressed Current Anode Figure 20

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  High silicon chromium cast iron anodes   contain 14% to 15% silicon and about 5% chromium.

High silicon cast chromium iron loses weight at a rate of 1.0 kg /A-year. It forms a stable amorphous hydrated oxide film that protects the anode surface and conducts electricity. High silicon chromium cast iron anodes are rated at a maximum current density of 1.0 mA/cm2 in soils. Scrap steel is sometimes used as impressed current anode material because it is abundant and

inexpensive. Ordinary steel loses weight at the rate of 9.1 kg/A-year. However, the current discharge is seldom uniform. The area around the cable connection often corrodes rapidly; consequently, several cable connections are used. Scrap steel anodes perform erratically, and the anode bed life is fairly unpredictable. The suggested maximum current density of scrap steel is 0.5 mA/cm 2 in soil. Mixed Metal Oxide Composite Anodes   are used in soil and marine environments.

Their major  advantage is their small size and high current output compared to other impressed current anode materials. The consumption rate of mixed metal oxide composite anodes in soils is low (7 g/A-year). Their suggested current density is 10 mA/cm2 in soils and 12.5 mA/cm2 in sea water.

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Platinized niobium anodes   protect offshore structures with high current requirements. Niobium

alone is unsuitable as an anode material. Niobium forms very stable oxide films on its surface. These films have high chemical and electrical resistance. Therefore, niobium is  plated with platinum or platinum alloys, which form conductive oxide films. The niobium substrate maintains excellent chemical resistance while the platinum layer allows high current output with low consumption rates (8.63 x 10-6 kg/A-yr). The result is a more efficient anode.

Platinized Niobium Impressed Current Anode on an Offshore Platform Figure 21

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Coke Breeze

Except for scrap steel, impressed current anodes in soil are usually surrounded with carbonaceous backfill. The carbonaceous backfill is usually calcined petroleum coke (sometimes called coke breeze). Coke breeze serves the following purposes: 1. Coke breeze increases the effective size of the anode and lowers the resistance of  the anode to the earth. 2. Coke breeze extends the life of the anode because the coke breeze is consumed rather than the anode. Coke breeze consumption depends on good electrical contact between the anode and the  backfill. The backfill must be packed solidly around the anode so that little of the current will discharge directly from the anode to the soil. Advantages and Disadvantages of Impressed Current Systems

The advantages of impressed current systems are: • • •

greater driving voltages higher current outputs adjustable current output

The disadvantages of impressed current systems are: • • •

higher equipment and installation costs higher maintenance costs possible interference problems with foreign structures

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CALCULATING CATHODIC PROTECTION REQUIREMENTS  FOR BURIED PIPELINES To design and monitor CP systems, it is necessary to know the amount of current required to  protect structures. The CP current must overpower the corrosion currents on the structure. When CP current is applied to a structure, the potential of the cathodic areas shifts toward the  potential of the anodic areas. This shift in potential, or polarization, is directly related to the amount of CP current that flows onto the structure. This relationship between current and  potential is given by Ohm’s Law, E=IR. The most anodic areas on a corroding steel structure has a structure-to-soil potential of about 0.80 volts. The structure-to-soil potential is measured with a Cu-CuSO4  reference electrode  placed close to the anode. When the entire structure has a potential of -0.80 volts or more negative, no corrosion can occur because anodic and cathodic areas cease to exist. The reference electrode is usually placed on the ground directly over the buried structure (e.g., a  pipeline). CP experts have generally accepted a potential of -0.85 volt or more negative as a criterion for  adequate corrosion protection. For cross-country pipelines, Saudi Aramco has established a criterion of -1.20 volts. The stricter criterion compensates for special local conditions such as high reference cell contact resistance and large “IR” drops in dry soils. Enough CP current must be applied to a structure to reach these protective potentials. The amount of CP current needed can be calculated if the required current density is known. Saudi Aramco has determined the design current densities needed for most structures. The design current density is given in milliamperes per square meter, mA/m2. To calculate the amount of CP current needed to control corrosion, multiply the current density by the surface area of the structure in the electrolyte. For example, the amount of current needed to protect a coated structure with a surface area of 1,000 m2 and a required current density of 0.75 mA/m2 is 1,000 m2

x

0.75 mA/m2 =

750 mA

=

0.75 amperes

Coatings prevent direct contact between the metal surface and corrosive electrolyte. The coatings greatly reduce the amount of CP current required to control corrosion. Most of the CP current is needed at areas where the coating is damaged. For this reason, coatings and cathodic protection are often used together.

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CALCULATING CATHODIC PROTECTION REQUIREMENTS FOR BURIED PIPELINES, CONT'D Saudi Aramco uses CP to protect pipelines, well casings, exterior storage tank bottoms, and the interior of vessels that contain water. CP is also used for marine structures like offshore  production platforms, piers, pipelines, and ships. Aramco Engineering Standard SAES-X-400 specifies procedures and equipment for CP system design. The following information shows how to calculate the amount of CP current needed to protect various structures. Impressed current systems are always used except for short sections of buried pipelines. Rectifiers supply power where a-c power is available. Solar panels with lead storage batteries supply power where a-c power is not available. Impressed current systems should be designed to give a 20-year life for anode beds. The number of anodes in the anode bed is adjusted so that the anode bed will provide 120% of the rated current output of the d-c power  source. Galvanic anodes are used for short sections of buried pipelines such as paved road crossings or camel crossings. Saudi Aramco's current requirements and a guideline for calculating the amount of current needed for buried pipelines are shown in Work Aid 1.

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CALCULATING CATHODIC PROTECTION REQUIREMENTS  FOR TANKS AND VESSELS Moisture in the soil causes external tank bottoms to corrode. Saudi Aramco bonds all buried structures and external tank bottoms together into one large system. The entire system is usually cathodically protected with surface or deep anode beds (Figure 22). Impressed current or galvanic anode systems may also be installed near the tanks or along pipe runs. This provides extra current to local areas with low potentials and distributes the current more uniformly. Distributed anodes should be installed with a direct “line of sight” to all metal surfaces.

External Tank Bottom Impressed Current System Figure 22

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CALCULATING CATHODIC PROTECTION REQUIREMENTS FOR TANKS AND VESSELS, Cont’d Corrosion in water-wet areas inside tanks and vessels is also controlled by cathodic  protection. In many cases, protective current is furnished by galvanic anodes (Figure 23). Internal water-wet surfaces are often coated to reduce CP current requirements.

Tank Galvanic Anode System Figure 23 Saudi Aramco's current requirements and a guideline for calculating the amount of current needed for tanks and vessels are shown in Work Aid 2.

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CALCULATING CATHODIC PROTECTION REQUIREMENTS  FOR MARINE STRUCTURES Saudi Aramco cathodically protects offshore platforms, marine pipelines, breasting dolphins, and loading/mooring buoys. The following information shows how to calculate the amount of  CP current needed to protect various marine structures.

Offshore Platforms Saudi Aramco’s objective is to quickly polarize offshore platforms to a minimum of 0.90 volts versus a silver/silver chloride reference electrode. This has two advantages. First, little corrosion occurs. Second, chemical reactions at the cathode form a protective calcium carbonate scale. This scale reduces current requirements and allows current to reach metal surfaces further from the galvanic anode. Aluminum alloys are often used as the galvanic anode material for large offshore structures. Indium or zinc-tin are alloyed with aluminum to  prevent a passive aluminum oxide layer from forming on the anode. The galvanic anodes are sized to provide current for 25 years. Offshore structures have several thousand meters of  surface area and require many anodes. The anodes are distributed so they quickly polarize critical structural areas on the platform. Figure 24 shows a typical aluminum anode installation for an offshore platform. A galvanic cell forms when the structure and anode are immersed in sea water.

Typical Aluminum Anode Installation for an Offshore Platform Figure 24

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Offshore Platforms, Cont’d Impressed current systems provide flexible current output and weigh a lot less than galvanic anode systems. Impressed current systems cost less initially but they require a continuous  power supply and routine maintenance. These systems cannot be installed until power is available and they are frequently turned off during well workovers. Saudi Aramco requires a 15-year design life for impressed current systems. Marine impressed current systems use  platinized niobium or mixed metal oxide anodes.

Marine Pipelines Subsea pipelines are always coated before they are installed. The pipelines connect directly to the offshore platform and they usually receive some protective current from the platform’s CP system. Extra galvanic anodes, which are spaced along the length of each pipeline,  provide uniform current distribution along the pipeline and reduce current requirements from the platform’s CP system. Figure 25 shows a bracelet type anode used for subsea pipelines.

A Typical Anode for a Subsea Pipeline Figure 25

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Piers and Jetties Steel piers and jetties are cathodically protected like offshore platforms. Metal sheet piling may corrode on both the earth side and the water side of the pilings. Two separate CP systems may be needed to protect each side of the sheet piling. Galvanic anodes may be used to protect the water side of the pilings. The earth side of the pilings is usually protected with impressed current surface anode beds. To cathodically protect the entire structure, each section of sheet piling must be electrically bonded to adjacent sections of piling. Saudi Aramco Engineering Standard SAES-X-400 specifies procedures and equipment for CP system design. Saudi Aramco's current requirements and a guideline for calculating the amount of current needed for marine structures are shown in Work Aid 3. The following information shows how to calculate the amount of CP current needed to protect various structures.

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WORK AID 1: FORMULA, CRITERIA, AND GUIDELINE FOR CALCULATING CATHODIC PROTECTION REQUIREMENTS FOR BURIED PIPELINES Formula

Surface Area of a Pipeline Figure 26

Criteria Saudi Aramco’s design current densities for coated and uncoated pipelines, shown in the table  below, are listed in Section 4.4 of SAES-X-400.

Pipeline Surface Uncoated Tape or P-2 wrap Coaltar epoxy Fusion bonded epoxy Polyethylene

Current Density (mA/m2) 20.00 1.25 0.75 0.10 0.10

Guideline 1.

Locate the required current density for the structure in the table above.

2.

If the surface area is not provided, calculate the exposed surface area of the section of   pipeline using the formula provided.

3.

Multiply the required current density by the surface area of the pipeline.

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Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

WORK AID 2: FORMULAS, CRITERIA, AND GUIDELINE FOR CALCULATING CATHODIC PROTECTION REQUIREMENTS FOR TANKS AND VESSELS Formulas

Surface Areas of Tank Interiors and Exteriors Figure 27

Criteria Saudi Aramco’s design current density requirements for tanks and vessels, shown in the table  below, are listed in Section 4.4 of SAES-X-500 and Section 4.4 of SAES-X-600. Surface

Current Density (mA/m2)

External tank bottoms (uncoated) Coated tanks (internal) Coated vessels (internal) Uncoated tanks & vessels (internal)

20.0 0.5 3.0 30.0

Guideline 1.

Locate the required current density for the structure in the table above.

2.

If the surface area is not provided, calculate the exposed surface area of the tank or  vessel using the formula provided.

3.

Multiply the required current density by the exposed surface area of the tank or vessel.

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Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

WORK AID 3: CRITERIA AND GUIDELINE FOR CALCULATING CATHODIC PROTECTION REQUIREMENTS FOR MARINE STRUCTURES Criteria

The design current density requirements for marine structures, shown in the table below, are listed in Section 4.4 of SAES-X-300.

Environment Sea water structures Structures in mud or soil Marine pipelines

Current Density (mA/m2) Coated Uncoated 10.0* 10.0 2.5

50.0* 20.0 —  

* Higher current density may be required depending on turbulence and/or velocity.

Guideline

1.

Locate the required current density for the structure in the table above.

2.

If the surface area is not provided, calculate the immersed surface area of the marine structure.

3.

Multiply the required current density by the total immersed surface area of the marine structure.

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Engineering Encyclopedia

Materials And Corrosion Control Cathodic Protection Systems

GLOSSARY activation polarization

The characteristics of the reaction that control the corrosion rate. Examples of characteristics are the type of metal, the hydrogen ion concentration, and the temperature of the system.

anode

The negative electrode or area where oxidation occurs.

anodic half-cell reaction

The chemical oxidation reaction that occurs at the anode.

area effect

The effect on corrosion rate of the ratio of the cathodic to anodic area in a galvanic couple.

cathode

The positive electrode or area where practically no corrosion occurs. Reduction reactions take place at the cathode.

cathodic half-cell reaction

The chemical reduction reaction that occurs at the cathode.

cation

A positively charged ion.

close electrode

An electrode placed on the surface directly above a coated structure.

concentration cell

A cell that consists of an electrolyte and two identical electrodes. The potential in the cell results from differences in the chemistry of the environment near the metal surface.

concentration polarization

Polarization of an electrode caused by concentration changes in the environment near the metal surface.

contact resistance

Resistance at the interface between two materials.

corrosion potential

The potential that a corroding metal exhibits under specific conditions of concentration, time, temperature, aeration, velocity, etc.

electrochemical reaction

A chemical reaction that involves the transfer of electrons and electric current.

electrolyte

An ionic conductor. Examples are soil and sea water.

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