Rust Removal

There are a lot of simple ways  to soak off rust.  Here is a comparison of several of these.  All the items used in these tests were totally covered in rust before treating.

On the left one part molasses to six parts water for one week.  On the right citric acid in water for two days.

This is after one day in distilled white vinegar.

All of these items where too rusty to just rub the rust off with some turpentine or penetrating oil.

Vinegar makes little bubbles while it works.

Metal comes out with a black residue that rubs off easily.

This rubs off and you have a flat grey finish.

So now you have the basic idea.  Soaking in some mixtures will remove rust faster than steel is removed.  With just a little work, the rust can be rubbed right off.

Make sure that the metal you treat is fully submerged.  The borderline between wet and dry can be eaten into and pitted badly by most of these treatments.

So here are a few notes on the different methods.  Please be aware that while some of these materials start out being food safe, they can be used to produce materials that are not food safe.   Also be aware that some food safe materials can cause issues when skin is exposed to them.  I prefer to use food safe materials, but have learned, due to a very serious and painful injury caused by apple juice, that care must still be taken.

I am not kidding about the apple juice.  I have scars.

Vinegar is food safe and easy to get.  It will help clean and sharpen files.  Vinegar will leave a grey finish that is a type of rust crystal that adds a small degree of rust resistance.  A lot of old kitchen knives have this finish.    I usually leave stuff in vinegar overnight.

Citric Acid is also food safe and used to pickle.  I get mine in a 7 oz bag for $3  at an Indian or Pakistani Grocery.

I add enough water to the citric acid to let it fully dissolve.  Then I drop rusty metal in it.   Rust may take less than a day to be removed.

Molasses is slow, may take a week or so.  I mix one part to 6 parts of water.   Molasses won’t remove as much steel if you leave it in the bath too long.  It seems to be the most gentle on steel.

Phosphoric Acid is one of my favorites, and is considered one of the milder acids available for cleaning cement.  Some hardware and some office supplies will stock it.   When I use it, it performs a double service.  First I use it to remove zinc from steel that I plan to weld or use in my forge.   Zinc vapors can cause horrible organ damage so I remove it before I use scrap that is galvanized.

After Phosphoric Acid has removed zinc it becomes a very valuable rust remover.  As it removes rust it parkerizes the steel and lends it a touch of rust resistance and a flat grey finish.  After using Oxpho Blue on parkerized steel, the result is quite rust resistant and scratch resistant black steel.

The top and third from the top tools in the picture below were treated that way.

Phosphoric acid stops bubbling when the rust is gone and the steel is treated.

As a clarification, All of these recipes use materials that are not going to be bad environmentally, diluted and in the quantities a home user is likely to use. However, in some steels there are elements that can be less healthy when removed from a solid and impermeable matrix. Some toxic metals can become much easier to absorb after acidic interaction. As an example, nickel salts are carcinogenic.

Bob

 

 

13 comments to Rust Removal

  • John M.

    Thank you for the write-up.

    Of these three methods, which one leaves the surface finish of the treated part intact and “as-is” the most?

  • Bob Strawn

    If the metal is not completely submerged, the borderline at the surface will mangle horribly in every case, so when I treat, the metal is removed from any other materials and fully treated, rusty or not. Since rust can pit, the best way to judge excessive metal removal is to examine the unrusted areas after treatment. All of these acids are slower to eat bare metal than they are rust.

    I have not used molasses much, but it does seem to leave the unrusted areas mostly alone. All of these methods leave a dark layer on the surface that needs to be rubbed off. That would be a bit of corrosion in any case. I can’t say for the rest, but zinc coatings are definitely eaten up by phosphoric acid and citric acid.

    Bob

  • Joshua

    What an excellent study in rust removal! I was wondering about the metal after the zinc coating is eaten up, is there any way to age the metal to make it look antique or have a patina for an aging effect.

  • Bob Strawn

    As far as making a patina, I would advise experimentation. Citric is fairly fast, an you might be able to control patterns with wax or oil.

    Bob

  • angelo

    Is there a proper way to dispose of the phosphoric acid when removing the zinc?

  • Bob Strawn

    Before disposal, you should use the now quite valuable zinc laden phosphoric acid to parkerize with. If you now soak a nice tool in this, it will make it quite a bit more rust resistant. If you then treat it with Oxpho Blue, you can have a really nice dark surface with quite a bit of rust resistance added. As an example,
    Striking Knives

    I modified Bob Smalser’s original method by adding zinc into the phosphoric acid first.

    As far as disposal goes, phosphoric acid, zinc and iron are all used to grow things like tomatoes hydroponically. I prefer my food a bit more naturally grown, and the stuff we are using is probably not food grade, but with care and dilution should be healthy enough as a fertilizer for plants unless there is another chemical in your phosphoric acid that you don’t want the residue of in your yard. In the past I have poured it out on an ant bed, but it did not seem effective as an ant killer. The grass around the ant bed did not seem any worse afterwards.

    Bob

  • Skip J.

    Hi Bob;

    Thanks for the nice article by Bob Smalser! I have a bit of that Jasco in my shop too, along with a little bluing… that marking knife looks somewhat familiar…

    Skip

  • Bob Strawn

    Good to hear from you, Skip! Yes I think it would be familiar!

    Bob

  • Skip J.

    Good to be here Bob…. I usually don’t say much, but I lurk around here most days… That magical time of year is coming around again, I plan to finally work on my M&G parts this year.

  • DL

    If I want to put a rusty painted car jack in the molasses mixture will the paint bee moved too?

  • Bob Strawn

    It might. I have seen paint remove and I have seen paint remain. Only one way to know for sure.

    Bob

  • Rodger

    Hey Bob,
    I recently got an old block plane from a friend and I’m the process of cleaning it up. To remove some of the rust, I dunked the depth adjuster in a small container filled with water and powdered citric acid. I walked away and actually forgot about it for a couple of days. I checked on it today and all the nickel plating has come off. Are there any concerns I should have about disposing of or coming into contact with the solution or the part itself?

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