Rust, Part 3

Last time we looked at the various methods and products that can be used to prevent rust on our tools. This time we’ll look at how to remove it when it’s already there.

An excerpt from The Perfect Edge, the Ultimate Guide to Sharpening for Woodworkers:

I’m using an aquarium for visibility while de-rusting a #5 plane body. I have it only partially in the water/baking soda bath to best illustrate what’s going on. Notice the negative (black) lead from the battery charger is connected to the plane body (the cathode) and the positive (red) lead connects to a piece of stainless steel (the anode) roughly bent to the shape of the tank to provide maximal exposure of the cathode to the anode.

Rust removal

Once rust has occurred, the evil forces of oxidation have stolen some of your iron and you can never have it back — the iron atoms that have been oxidized are no longer available to you — you cannot undo the rust and put the iron atoms back into the tool’s surface from whence they came. So there will be a pit, at the very least, in the surface. If that pit is at or near a cutting edge, you’ll need to abrade the surrounding surface down to the level of the bottom of the pit and just a molecule beyond because you want to be sure to remove all of the rust or it may continue to eat your steel. Anytime you expose fresh metal, it too will need to be protected from further rusting.

Some oxidation is really not material. The blue-gray patina that develops on carbon-steel kitchen knives does little damage, and is just a fact of having and using fine knives (I think carbon-steel knives are vastly superior to so-called “stainless-steel” ones). Sometimes they get a little too motley for my taste and I’ll gently and carefully scrub them back to uniform gray with steel wool. But aggressive red rust is nasty and must be dealt with before too much damage is done. Light sanding and oiling will suffice for some tools — a hammer head or wrench — but cutting edges should be cared for preventatively by one or more of the methods described above.

It is possible to remove rust by chemical or electrolytic action. Citric acid is acclaimed for its ability to dissolve rust much faster than it dissolves the underlying steel and is often used to de-rust tools. Recipes or mixture recommendations vary from one to two ounces of citric acid crystals to 32 ounces of water. Disassemble the tool and immerse the rusty steel parts only. Check on the progress and brush off the loosened rust every few minutes. Treatment with fresh solution can take as little as twenty minutes depending on solution concentration, temperature and agitation. You should experiment to find the technique that works best for you. The acid will eventually attack the steel so don’t leave it in longer than necessary to dissolve the rust. Be sure to rinse the treated parts thoroughly and immediately apply new rust protection.

The electrolytic method is quite simple and if done properly will not harm the iron substrate at all. Here’s what works for me: start with a 5-gallon (20 liter) plastic bucket or similar container almost filled with water into which you’ve added a handful (1.2 metric handfuls) of baking soda to make the electrolyte. For the anode, I prefer something stainless steel — a vegetable steamer or cheese grater. Some stainless steel wires in the bottom of the container will suffice. Stainless lasts longer as an anode — you can use any iron or steel as an anode but it will be consumed by the process. Disassemble as much as you can (some “impossible” screws may loosen in the process) and remove anything that isn’t steel. Suspend the rusty thing, the workpiece, in the solution being very careful that it does not touch the anode at all. The action is “line-of-sight” between the rusty thing and the anode, so you may need to reposition the workpiece or wrap the anode in order to surround it for complete coverage.

The rust has been converted to a chalky black surface coating that should now be wire brushed or sanded off. The clean and rust-free surface will begin to oxidize immediately; don’t delay in applying rust prevention measures.

Connect a battery charger’s positive lead to the anode and the negative lead to the rusty workpiece. Let me say this again: Positive to Anode, Negative to Workpiece. Don’t mix them up because the anode is sacrificed in this reaction and you only want to dissolve the rust, not your rusty thing. You’ll see bubbles rising in the solution as soon as the battery charger is switched on. Through the magic of chemistry the iron oxide that you want to remove is converted back into metallic iron and falls to the bottom of the bucket. It will not re-deposit itself where it came from. The rust will be removed; however, the pitted surface will still be pitted. De-rusting can take a few minutes or a few days depending on the amount of
rust, the amperage available from the charger, etc. Be patient and check the piece occasionally. Bubbles will continue to rise from the piece even after all the rust is gone but don’t worry; at that point you’re just splitting the water into its component oxygen and hydrogen. The resulting de-rusted surface will have a black layer on it that you’ll probably want to wash/wipe off. Dry it thoroughly in an oven, with compressed air or a hair dryer. Rust prevention measures should be applied immediately upon drying the piece or it will rust again in minutes.

That’s it for this excerpt. I hope you’ve learned something about the enemy and how to vanquish it from your shop.

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About Ron Hock

Owner of HOCK TOOLS (.com) and author of "The Perfect Edge, the Ultimate Guide to Sharpening for Woodworkers"
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6 Responses to Rust, Part 3

  1. Carl S says:

    I just tried a product called Evapo-Rust. It seems to work quite well on lightly rusted metal. I’m wondering if you have any thoughts on it. Perhaps it’s similar to the citric acid; I haven’t tried that.

    • Ron Hock says:

      I hear good things about Evaporust and citric acid but I’ve always been reluctant to use either for fear of remove or etching the base metal. I know that electrolysis removes only rust (assuming you have the polarity correct — DAMHIKT) and that there is no risk of eating away the good metal. I’m confident either Evaporust or citric acid can be used safely, they both have enthusiastic followings, but I’d keep a close eye on the progress and be sure to remove the part when the rust is gone. With any rust removal process, be sure to apply rust-preventative measures immediately after use.

  2. Donald Robak says:

    Ron, this was an excellent series of posts. Even though I worked for many years as a chemist the chemistry of iron was something I never knew. I will pass this on.

  3. Chris Blaney says:

    Your post is very helpful, but I’m very concerned that you recommend the use of a STAINLESS anode.

    Yes – Stainless will last much longer BUT when you use stainless steel for electrolysis, Chromium is leached out of the metal and a highly toxic substance is formed called Hexavalent Chromium which is highly carcinogenic and very hazardous by contact or ingestion, so the water used MUST be disposed of properly. Please read this article for info:

    All the best, Chris.

    • cary says:

      Yes, hexavalent Cr is a serious and insidious carcinogen. The reaction described here is in a basic solution, not acidic, with ample free Fe. Therefore, I believe, from considering the chemistry of Cr6, if any present, it would combine with the Fe as follows, and become harmless Cr3.

      Cr6+ + 3Fe2+ → Cr3+ + 3Fe3+

      Cr3+ + 3 OH → Cr(OH)3

      Fe3+ + 3 OH → Fe(OH)3

      Making the spent solution even more basic (with soda lime)
      CaOH, would ensure no Cr6 is present; but perhaps this step isn’t necessary.
      Using Cr in the reverse polarity direction (Negative to stainless steel), in an acidic solution would INDEED produce the Cr6 ions that you are concerned about.

  4. cary says:

    Thanks for the well documented tip. Since I’m a cheapskate, I usually dry with a rag and immediately ‘oil’ the piece. Cheap spray Canola oil works best!

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