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Dangers of using Phosphoric/Chromic Acid mix?

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  • Dangers of using Phosphoric/Chromic Acid mix?

    I have read in many places that a proper mixture of phosphoric and chromic acid is about the only chemical mixture that will safely remove an anodized layer without attacking the raw aluminum. I've also heard that there are some concerns with this mixture but nothing in detail. First of all if I was to use this mixture could I even buy the chemicals? Second what kind of safety precautions should be taken before and during use? For example is it flammable? Are the fumes poisonous? Also what kind of container would the mixture need to be put in? Stainless steel? Plastic? And finally, how do you dispose of this stuff when you are done? I appreciate any input from those that know. Thanks.

  • #2
    I'm always amazed that folks want to go off on a tangent, when there is a perfectly good system already in place.

    Our anodize stripper is a commercial product, proprietary, and used my many professional plating shops.
    We can give tech support on it, and have direct access to our suppliers labs and support dept.

    Why would you want to tinker?
    We certainly won't support this.
    Mike Caswell
    Caswell Inc
    Need Support? Visit our online support section at


    • #3
      Excuse me for asking!

      Isn't the only way to really learn to ask questions? I never said I was or wasn't going to use that approach. I was only curious to the dangers of it. In the time it took to get this question answered I found out that it's not something I want to mess with. I decided to order some Caswell Stripper last week. Thanks for the help.


      • #4
        i have to give it to caswell for once again shunning any other method than their own. i realize you're working very hard to provide a quality product, but there is always room for improvement, (*hint* LCD Anodizing *hint*).

        your stripper is an efficient way of removing the old ano layer, but it does, despite inhibitors, eat away at the aluminum at a dangerous rate if one is not careful. while a few microns here and there might not matter for most, those in the market of high precision paintball markers demand very precise internal bore measurements, something your stripper can degrade.

        so does ayone have any information about this mixture of phosphoric and chromic acid? i have heard (and it may have been posted by fibergeek) that it is so toxic and dangerous that it is impractical to use. just trying to help out.

        disclaimer: i apologize for any comments that may villify Caswell in any way. my intention was to provide an objective perspective on a matter of proprietary products and methods.

        read: please don't ban me


        • #5
          The concoction I was referring to was CONCENTRATED nitric acid with phosphoric (or chromic?) acid. You really don't want to mess with that.

          What's being asked about here is a mixture of phosphoric acid and chromic acid (aka chromium trioxide) diluted in water. This is from MIL-A-8625F, where it is used to dissolve the anodize before weighing the sample in order to ascertain the anodize thickness. The milspec kind of implies that it won't dissolve the aluminum but it doesn't say that. It is used according to the milspec, at 100 deg. C. Repeated treatments are specified until all of the anodize is gone. Both chemicals are poisonous and corrosive. Disposal? you are definitely in hazmat city.

          If this stuff was practical for stripping, I would have thought it would be available as such. I've never heard or read of this being used as a stripper other than in the milspec.

          Potsked, Why don't you download MIL-A-8625 from the web, get the chemicals, and try it? You can buy the chemicals at hazmat shipping charges apply. Let us know if it works. Who knows? it might.

          But watchout for the fumes! at elevated temperatures the fumes will be nasty.


          • #6
            Gees...anyone who would try anything else much be an idiot.



            • #7
              Hey, I'm not being sarcastic or condescending here.

              Its entirely possible that the diluted chromic/phosphoric stripping idea could work. If I had the time to investigate this myself I would. We would all welcome a concoction that truly didn't dissolve metallic aluminum. The fact that it hasn't been used for this purpose (as far as we know) doesn't prove that it won't work.

              Caswell's stripper is as good as they get for commercial anodizing strippers, but alas all will attack aluminum to some degree. Its understandable why Mike would not want to support this even if it did work. He won't have the legion of chemical company pros to support him. If we do this its at our own risk entirely, and that's fair.

              Since the milspec formula dilutes these chemicals to roughly 45/1000 parts in water, a mixture somewhat stronger may permit use below 100 deg. C. If this stuff isn't used so hot the fuming will be less objectional.

              I think this is worth some experimenting. You will have to be careful with these chemicals, but the plating guys use chromic acid in much larger quantity and stronger form and they aren't keeling over dead.


              • #8
                I tried using Lye at that is some NASTY stuff. The anodizing stripper from Caswell works really really nice. Easy to use, safe and seems to last a long time.