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Pros/Cons Of Oxalic Acid Rust Removal

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  • Pros/Cons Of Oxalic Acid Rust Removal

    Funny how one starts doing some research on the internet and ends up way off topic on something that is equally or more interesting than the topic you started with.

    Today's tangent involves Oxalic acid for rust removal. It does not seem to be as widely discussed as the other methods with machine restoration threads, and I am not sure why. There's my first question.

    It seems that the chemistry involves a "chelate" process the same as Evaporust, but much cheaper albeit slower ? Also, I am reading that all acids run the risk of hydrogen embrittlement - and this is something that is to be avoided with high tensile alloys .... just the type of object that I would be interested in derusting.

    SO.....any chemists or other such educated people on here that can explain the pros and cons of Oxalic Acid rust removal?

    Curiously, I have come across hydrogen embrittlement discussions with the old electrolysis process - but about 50% say it's a problem and the other 50% say there is no embrittlement with the process - including a chemistry website that I quickly got lost in (I'm an electronics person - not chemical).

    Thanks !
    Lewis

  • #2
    I have seen oxalic used to remove rust stains (due to nails) from wood shake siding. Never used it for iron/steel.

    As it is less commonly available than phosphoric, it may not be a good candidate.

    Phosphoric is easily obtainable (in the US, at least), cheap, effective, and can be stored without rusting everything within sight of it. Therefore it would seem to be a better candidate, although oxalic acid is known to work.

    Phosphoric, at least, is also quick acting and thus is far less likely to cause any serious issues of embrittlement. The longest I have ever had to leave anything in it is about 40 minutes, with a cold solution. It would be rather quick diffusion that saturated a piece of iron with hydrogen in that time, cold.

    One might easily suppose that the electrolytic process is more likely to produce embrittlement. The rusty material is put at the negative terminal, which will attract the positively charged hydrogen ions, the process requires an extended time, and the process results in at least some warming of the solution and parts.
    CNC machines only go through the motions.

    Ideas expressed may be mine, or from anyone else in the universe.
    Not responsible for clerical errors. Or those made by lay people either.
    Number formats and units may be chosen at random depending on what day it is.
    I reserve the right to use a number system with any integer base without prior notice.
    Generalizations are understood to be "often" true, but not true in every case.

    Comment


    • #3
      Originally posted by J Tiers View Post
      I have seen oxalic used to remove rust stains (due to nails) from wood shake siding. Never used it for iron/steel.

      As it is less commonly available than phosphoric, it may not be a good candidate.

      Phosphoric is easily obtainable (in the US, at least), cheap, effective, and can be stored without rusting everything within sight of it. Therefore it would seem to be a better candidate, although oxalic acid is known to work.

      Phosphoric, at least, is also quick acting and thus is far less likely to cause any serious issues of embrittlement. The longest I have ever had to leave anything in it is about 40 minutes, with a cold solution. It would be rather quick diffusion that saturated a piece of iron with hydrogen in that time, cold.

      One might easily suppose that the electrolytic process is more likely to produce embrittlement. The rusty material is put at the negative terminal, which will attract the positively charged hydrogen ions, the process requires an extended time, and the process results in at least some warming of the solution and parts.
      J Tiers - Thanks for the fast reply. Yes, I was a bit miffed that the chemistry site I breezed by seemed to suggest absolutely no embrittlement issues with electrolysis given that hydrogen is being produced in the reaction.

      On Phosphoric - I was under the understanding that this may leave a surface coating that is hard/impossible to remove - some iron phosphate. In my case I don't want this, so is there a way to prevent it, or subject the item to an additional flush/wash immediately upon removing it from the Phosphoric bath?

      I actually have some Phosphoric acid sitting around - it's Kleen Strip Prep and Etch. I think that stuff is about 40%.

      Do you have any guidelines on strength of the Phosphoric along with comments on if it can be used in a precision situation with wiping carefully (albeit for an extended length of time).

      I'm wondering if this diffusion process of hydrogen is a 2 way street - i.e. any diffusion, regardless how small - will it reverse over a period of time ? So much to learn

      Thanks
      Last edited by LHC; 12-30-2016, 11:22 AM.

      Comment


      • #4
        I’ve used Oxalic acid in the past, it works. Comes in powder form, mix with water. Vinegar also works (5% Acetic Acid).

        As to hydrogen embrittlement, heating works to remove the hydrogen, off the top of my head it was something like 400 deg F for 1 hour.
        Mike Hunter

        www.mikehunterrestorations.com

        Comment


        • #5
          The iron phosphate coating is not hard, it's just gray.

          Electrolysis tends to leave a hard, tightly adhering black coating, MUCH worse than the phosphate.

          And, if you monitor the process, you get minimal coating from phosphoric.

          What I do is degrease the parts, then put into the solution. I use Jasco or Kleenstrip, etc, diluting maybe 5:1.

          Once in, I check back in 10 or 15 minutes. If still not clean, I lightly wire brush to knock off loose rust, and put back. Usually in a total of 30 min all is ready, and there is no particular coating.

          Never have needed more than 45 minutes total. If left in longer than required to remove rust, the coating will develop. I suppose you have your choice.... rusty parts, or the chance (not guarantee) of a slight surface discoloration.

          Before (HSS milling cutter)




          After

          CNC machines only go through the motions.

          Ideas expressed may be mine, or from anyone else in the universe.
          Not responsible for clerical errors. Or those made by lay people either.
          Number formats and units may be chosen at random depending on what day it is.
          I reserve the right to use a number system with any integer base without prior notice.
          Generalizations are understood to be "often" true, but not true in every case.

          Comment


          • #6
            I don't believe the acids use the chelation process. The home brew version of Evaporest is diluted molases from the farm store. Cheaper but a lot slower. Phosphoric converts iron oxide(rust) into iron phosphate. Rust is porus so the moisture gets in and goes ever deeper. Iron ohosphate is not porus so it leaves a protective barrier. It is also sold mixed with paint or primer (self etching). It is not that hard get iron phosphate off with abrasion of some,sort. I am a big fan of evaporust. It is water based so keep it covered so it does not evaporate. You can re use it. It is expensive compared to other chemical methods. There is always electrolysis. I have not tried it yet but I mean to.

            Sent from my SM-G900V using Tapatalk

            Comment


            • #7
              Beware of the toxicity of oxalic acid. Many of us use Barkeepers friend to clean pans and it works great. But the package avoids any clear warnings on the toxicity and could lead to the incorrect impression that it isn't dangerous. Reading suggests it is.

              Regarding electrolysis, I agree that the black stuff can be tough to remove. On smooth surfaces that you can get to, it isn't so bad. But anything with texture is a pain. Also, don't let the black stuff dry out, or it becomes much more difficult to remove. I like that the electrolysis is low toxicity, if done correctly, in regard to disposal concerns (I don't like sending phosphorus into the environment). I do like to use it for cast iron pans, with their large smooth surfaces. The other challenge is avoiding flash rust. I always do my final rinses in cold water and then wipe with towels. The cold helps avoid the evap that seems to encourage rust.

              Comment


              • #8
                I keep an old hair dryer around for that. Pull the parts out, rinse (or not) hit with hair dryer right away. No rust.
                CNC machines only go through the motions.

                Ideas expressed may be mine, or from anyone else in the universe.
                Not responsible for clerical errors. Or those made by lay people either.
                Number formats and units may be chosen at random depending on what day it is.
                I reserve the right to use a number system with any integer base without prior notice.
                Generalizations are understood to be "often" true, but not true in every case.

                Comment


                • #9
                  Another source for oxalic acid is wood bleach.
                  gvasale

                  Comment


                  • #10
                    Oxalic acid is a very mild and non aggressive acid. White crystals , water soluble. Oxalic acid is what the rubarb has in it. You always need to cook the rubarb, otherwise it complexes with calcium and plugs up your kidneys. Then you die .Oxalic is a fruit acid like citric.
                    Citric acid is also water soluble and is in lemons. Both oxalic and citric acid complex with iron making both rust removers. Both are painfully slow in action and you may find both tend to put iron alloys. Acetic acid weakened is vinegar and it does some rust removing but slowest of the above mentioned choices . The water in the fruit acids my also cause rusting after you remove the rust in the first place. The best is phosphoric acid. It's more consistent and leaves a iron phosphate coating that inhibits rust. Industrial treatment call this phosphotizing. Widely used as a under treatment for steel before painting. Don't use muriatic or hydrochloric acid or even keep any , any where close to stall or iron. The chloride will make rust in minutes. Folks who have swimming pools have learned this the hardest if the keep pool supplies in Thier car garage.
                    I have used phosphoric acid on vertical service by using a Terry cloth towel dampened with the acid. Let set and reapply repeatedly. The problem with citric and oxalic is that the residue left promotes rust due to the acids attracting water from the air. Hope this gives you some details that will help. I'm a pharmacist and amature machinist/wood worker. Enjoy the forum and the participants. Happy New year! If tomorrow night brings the barley corn out and too much consumed, the next morning DO NOT take Tylenol or acetominophen for the head pain. Alcohol inhibited the metabolism of the Tylenol causing kidney problems. Tom

                    Comment


                    • #11
                      Guys - thanks for all the information and food for thought. I am learning that there seems to be no one size fits all with the de-rusting of machine tool objects. What's good for one is not the best for another etc.

                      The chelation process does exist for the oxalic acid approach - but not for the other acids. I have come across that in many chemistry places on the web, however, I also understood it to be a slow approach compared to the others.

                      As I understand it, any of the acid approaches will diffuse hydrogen into the metal if left long enough. J Tiers' comments on the phosphoric approach for no more than say 30 minutes cold, does tend to sound like it would minimize the effect though.

                      I actually emailed one of the Evaporust type companies earlier to round out my understanding of the tradeoffs involved and got a response from the president himself (maybe it's a one man company and I should not be so impressed - lol)- he sounds like a chemist -
                      -----------------
                      .....has not performed any hydrogen embrittlement testing on any steels, etc. Although, we have had customers that are also concerned about this. Our product does not have any extra reactive hydrogens to contribute to hydrogen embrittlement, other than the available hydrogens found in the water used in diluting our solution.

                      Our product also has a neutral pH and does not harm plastics, paints and rubbers.

                      Our customers have been successful with using our products for these applications. Our suggestion is to test a small area or part to make your final determination. You could also post treat your parts with heat (heat treating soak) to relieve the hydrogens from the base metals if your concerned.

                      -------------------

                      I've got several parts that I have been avoiding derusting on an old lathe I have been picking at for years now - mainly due to the fear I would destroy them. However, the more I educate myself the closer I get to an approach that will at least minimize the damage.

                      One part in particular is the spindle with an L00 nose and tapered roller bearing cone that I am not going to remove. The locking ring on the nose is rusted up and surprisingly very little else is rusted on the spindle. I would really like to derust it, but not being able to remove it from the assembly means that a portion of the spindle has to be submerged along with the collar. The spindle is high tensile alloy and the bearing, like the spindle, is obsolete, and it's in pretty decent shape. I'd be swearing loud and long if I was to bung it up.

                      There is always the option of a nicely restored lathe and a butt-ugly spindle nose I guess - haha

                      Comment


                      • #12
                        I de-rusted a couple of spindles in place with evapo rust. With some difficulty I stood the lathe on end, plugged the spindle rear and filled it with ER. Used wicking paper towel to spread the ER to the outside - worked like a champ. You could leave it horizontal and built a wax dam around the nose.


                        Among many products/methods I've used ER since it came out. I won't use anythings else mixed metals or fine surfaces BUT there is one thing I learned early on. You MUST submerge the hardened steel parts in ER or you will get erosion along the air/fluid interface (rapid rusting/de-rusting action). Use a small coolant or aquarium pump to coat with ER fluid if submersion isn't possible.
                        Last edited by lakeside53; 12-30-2016, 03:20 PM.

                        Comment


                        • #13
                          Originally posted by lakeside53 View Post
                          I de-rusted a couple of spindles in place with evapo rust. With some difficulty I stood the lathe on end, plugged the spindle rear and filled it with ER. Used wicking paper towel to spread the ER to the outside - worked like a champ. You could leave it horizontal and built a wax dam around the nose.


                          Among many products/methods I've used ER since it came out. I won't use anythings else mixed metals or fine surfaces BUT there is one thing I learned early on. You MUST submerge the hardened steel parts in ER or you will get erosion along the air/fluid interface (rapid rusting/de-rusting action). Use a small coolant or aquarium pump to coat with ER fluid if submersion isn't possible.
                          Lakeside - you've given me some more ideas - and many thanks for them ! The spindle is out of the machine so I don't need to hike the entire lathe on end. I'm not above that sort of an approach though if it was needed. We're not on the clock here and many hours to fabricate jigs and fixtures has often been happily undertaken.

                          However, the wax dam concept is the trigger for further thought. I know full well about the ER etched line - I had it happen to some sheet metal parts I was derusting a few years back. Scrub as I could - those damned lines would not come off. It was not until I inspected them under a loupe I discovered they were etched and not just discoloration. No big deal, I'm painting those parts anyway.

                          Now then - back to this wax dam - as long as the wax seals the surface well, and the ER does not leak - this would avoid the air/liquid etching process to take place yes ?

                          Where does one get this magic wax or do I raid the wife's candle drawer ?

                          Edit - given that the spindle is round I might be able to use a large o ring to seal against the shaft, stand it on end, and eliminate the air/liquid interface from happening as well. As I understand it, there's some sort of galvanic reactions going on when that air/liquid interface is present and it's not strictly limited to hardened steel.
                          Last edited by LHC; 12-30-2016, 03:58 PM.

                          Comment


                          • #14
                            Originally posted by LHC View Post
                            ... ?

                            Where does one get this magic wax or do I raid the wife's candle drawer ?

                            ....
                            Wax toilet rings are a pretty good source of workable wax. Sticky enough to seal and stay sealed while the material does its stuff, cheap at the store.
                            CNC machines only go through the motions.

                            Ideas expressed may be mine, or from anyone else in the universe.
                            Not responsible for clerical errors. Or those made by lay people either.
                            Number formats and units may be chosen at random depending on what day it is.
                            I reserve the right to use a number system with any integer base without prior notice.
                            Generalizations are understood to be "often" true, but not true in every case.

                            Comment


                            • #15
                              Another thought crossed my mind this morning - for parts that fit - has anyone used ultrasonic cleaning to remove rust and can expound on the medium / liquid used and success/failures?

                              Some of these units are heated as well which would assist in speeding up any chemical process.

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