Citric Acid Rust Removal Brochure 4 (1)

Citric Acid Rust Removal Many different acids can be used to remove rust from iron and steel. Citric acid has an advantage over some other acids, because it is less caustic, not as hazardous environmentally, while remaining effective, cheap (although not as cheap as old battery acid). The chemistry is fairly simple, the rust (iron oxide) is solubilised by the acid (ammonium ion is also acidic) and the citrate ion coordinates to the iron ion forming soluble iron citrate. How do you do it? 1. Make a solution containing at least 10% citric acid. Add a bit of alcohol (methylated spirits) to break down the surface tension and a squirt or two of detergent to help with any degreasing. Swirl around to mix in the acid powder. Using heated (the hotter the better) water speeds the process. Remember to put the powder into the water, not the other way around. Eye protection is important; you may want to wear gloves and to protect your arms etc. 2. The article to be cleaned must be largely free from grease or it will not work (or will work very slowly). Use thinners or engine degreaser or paint stripper before placing item into the acid bath; depending on whether the item is painted etc. Citric acid will not strip paint, so for example if a file has paint on it, you need to remove the paint first. You do not need to clean the tool however; the acid solution will not only de-rust, but also clean, the tool; a good way to clean up grotty files. Naturally the acid solution will work quicker on clean tools; for really grimy tools consider the first application as a clean and wash, and then the second dip as the de-rusting (for really crook items). 3. When you place the item into the solution you will notice bubbles, this means the acid is working. No bubbles, little action. Soak item for as long as there is still rust on it 1, and bubbles are rising; check process by rubbing tool with a scouring pad periodically. 4. You can help the process along by swirling the solution, and by brushing the item(s) to assist with penetration. Mixing the solution diffuses away reacted particles and exposes the rust surface to additional acid ions, enhancing the reaction. 5. De-rusting can take from a few minutes to a few hours depending on item condition and the concentration/temperature of the solution. As the reaction time increases, concentration decreases as ions are used up and by-products (i.e. gunk) formed. Remember the acid keeps on working after the rust is dissolved, so do not leave items in an acid solution for long periods. 6. When finished give the item a scrub with a scouring pad under running water. You may wish to neutralise with a Baking Soda solution, but this is rarely necessary. Remember to wash the item in water to remove the acid solution before using a Baking Soda solution. 7. Repeat as required. You will know when the solution is no longer working by the absence of bubbles. Remember to remove the gunk before putting item into the new solution. Some badly rusted or intricate items may need soaking overnight. 8. Oil the item straight away after rinsing and drying, this is important. 9. Do not load up too many tools in the container and preferably they should not be piled in on top of each other; this slows down the process, and where tools touch the acid solution may not work effectively. 1

See Timing notes below for timing issues.

10. Citric Acid not only removes rust, but (like all acids) etches iron and steel. Therefore if you leave the item in too long, the metal will be covered in microscopic etched pits, this is the dull and grey appearance. Heavily rusted items will usually be pitted anyway of course. The pitting will be more obvious after de-rusting because the acid will have dissolved the rust and oxidation in the pits, some of which will be very fine and not visible except under a microscope. To the naked eye the tiny pits appear as a dull and not completely smooth appearance. Timing and Concentration Notes: a) All iron and steel tools have an oxidised surface or skin, this is often called patina. Gunsmiths go to great lengths to achieve chemically the “right” patina. This skin is tough, and may be removed if an item is left in an acid solution too long – under this patina the oxidation will not be completely even. Hence when you remove the patina with an acid etch, the surface is dull and not completely smooth. b) Items will not be uniformly rusted, so the rust will be removed at a differential rate (you could leave items out in the rain for a few weeks to achieve a sort of uniform rusting I guess). Also different types of metal will be affected differently – if you place a laminated plane iron in the solution, when you clean it you will clearly see the lines between the metals. Disassemble planes before dipping. c) Acid rust removal is not a chuck in and forget2 process. Think about the outcome you want, as well as the state of the item. The clue is to adjust the concentration to the task – light concentration for lightly rusted items, heavy concentration for encrusted items; AND keep an eye on what is happening; AND take item(s) out regularly to clean off the gunk. Check frequently to see when 0000 wool will reveal the old steel, show a shiny surface, and leave some darkened patina. Patina will often show up mottled and looking ancient but perfectly preserved. This enables you to stop at the point the rust is just removed. A small amount of spot rust not removed on the first go can usually be scraped or sanded down before the last dipping. Your aim is uniform de-rusting for the minimum necessary time. d) The stronger and hotter the solution the quicker it operates, so you need to pay attention – often the de-rusting will only take minutes. So if you are leaving the item in for an extended periods, use a weaker solution. Note: • Advantages: environmentally friendly, non-hazardous, non-toxic, does not affect paint or japanning or wood, plating etc. • Disadvantages: may be slower than electrolysis (but much safer). Solution grows some slime when stored, this can be strained off. • The method can be used for things such as ebony/rosewood and brass squares etc without affecting the brass or wood. • Whilst the solution can be reused, if it has any oomph left; it is so cheap that it is not really worthwhile. If you do store it, after awhile (maybe a week, depending on temperature) the solution gets mouldy. You will see little white fluffy spots on the surface. The mould will 2

Acid rust removal is not suitable for all situations. For instance saws are generally not suited to acid rust removal. Scraping and sanding work better; if a saw is heavily rusted it may be worthwhile to reduce the rust in an acid solution (if you have the right sized tank, or use soaked towels) – and then finish with scraping + lubricated (WD40) sanding.

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eventually eat up the citric acid, but it will stay active for de-rusting (slowly) for quite some time. Note - reuse is for skinflints with patience. If, after mixing for a few minutes, you see citric acid crystals forming on the bottom of the container, don’t add any more crystals. The solution is at or beyond the saturation point. You do not need it this strong, so dilute as appropriate to the task for use. Steel treated with acid seems to attract rust almost immediately it emerges into fresh air. Rinse thoroughly and dry (use a hairdryer or heat gun or put in front of a heater). Then immediately oil the surfaces or use G15 available from Jim Davey. Any rust removal method leaves the clean surface susceptible to re-rusting; in hot and humid conditions you can see the rust starting to return after a few minutes! In these situations dry each item immediately after you rinse and scrub it. For items that are too big to go into a container, such as saws, coat one side of the item, using towelling/rages, at a time with a concentrated solution, renewing when it starts to dry out. After a period wipe off the mess and rinse, and repeat if necessary. In most cases, seized up bolts and screws etc will now work free. Dirty chisel handles will clean up nicely, and are ready for oiling, shellac etc after a quick sanding. Labels will generally come adrift, such as on Berg chisel handles (don’t ask). Hot water speeds up the process significantly. Heating increases the velocity of the ions, resulting in a more violent collision with the rust particles. Remember to check progress regularly, in most cases every few minutes. You can leave tools in the solution for an extended period without actually damaging them (however there is a slight etching going on). Experience shows that the gunk tends to stick to the tool if left in for a long period (day, week or so), and needs some serious scrubbing or wet & dry sanding to get off. Flat surfaces such as plane soles clean up much quicker than knurled screws and the like. Therefore long soaking is not recommended; use of hot solution for a few minutes for easy to de-rust items to a few hours at a time for recalcitrant items, and then repeat application is recommended. You will notice that different steels and different levels of hardening will show up as clearly different levels of grey/black, and with defined lines. Citric acid is good for rusty old screws, they will come out like new; give them a brisk brushing to remove the gunk, oil them and reuse. Generally people tend to (a) use too weak a solution, (b) not use hot enough water, (c) leave the tool in the solution too long, and (d) not protect the cleaned steel/iron quickly enough. Really thick, hard rust on non-machined cast iron surfaces will tend to stay that way. In these cases you are better off: (a) using the citric acid to remove the loose and surface rust, (b) drying thoroughly using a hair dryer or heat gun, (c) applying a rust converter to the affected area, (d) and painting the surface (talking about machinery here mainly). Use plastic buckets/containers, not metal. Do your de-rusting outside; acid fumes are corrosive and may cause rust to develop on exposed metal surfaces. Whist this is a problem mainly with stronger acids, may also be caused by citric in some circumstances. There are suggestions that placing dissimilar metals, such as cast iron and steel, in the same solution may induce unpredictable and undesirable electrolytic effects. Not seen by the author though. Cast iron de-rusts and etches unevenly, so treat with care. Dipping cast iron pieces, such as machinery is useful to free up a steel pin or screw. Also the acid solution is great for intricate cast iron work, eg old sewing machine frames, and for rough castings which you can only

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attack otherwise with electrolysis. Flat surfaces are best partially de-rusted in the acid solution and finished off with sandpaper blocks. Be careful of spring steel (saws, lever cap spring, coil springs), and do not leave in too long. Also ensure the spring is not under tension.

These notes are for general guidance only, and you should seek professional advice if you have any concerns at all about using citric acid. In no way are these notes to be construed as providing advice of any kind. If you use citric acid for de-rusting ensure you take all necessary protective measures. Eye protection is highly indicated, as is skin protection. Do not breathe the fumes.

Notes compiled by Peter Evans from a number of sources and personal experience. The Traditional Tools Group Inc P.O. Box N240 Royal Exchange Sydney, NSW 1225

Website: tttg.org.au