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Passivating stainless at home...

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  • Passivating stainless at home...

    I want to make some arty bits in 304 stainless to add to my garden products range, they will be plasma cut, some will be welded.

    I know i need to keep a set of tools strictly for stainless to stop iron contamination and also use stainless wire brushes etc.

    I have been told i need to passivate the edges to stop rust - i cant get a brush wheel or disc in to these edges due to the design, what is the best way to achieve this?

    All of my stuff so far is plain mild steel so this is a learning curve

    I have got stainless MIG wire and also know i need 98/2 gas at a minimum, these are basket brackets so not stairs or handrails etc.

    The surface finish desired is achieved by using a cup-brush in the grinder, a bright, swirly effect.

    Parts vary in size but generally 18" dia or so, 1/8" thick or less.

    Any suggestions

    If it does'nt fit, hit it.

  • #2
    Would you show a pic of the stuff?Scratches are always a problem compared to polished surfaces. Would you consider sending it for electro polishing.


    • #3
      I would try googling passivating SS. Citric acid can be used which is more environmentally friendly than other acids, but I don't know the strength, temperature or pretreatments required.


      • #4
        Its stuff like this...

        Can't really afford to add any treatment that i can't do myself.
        If it does'nt fit, hit it.


        • #5
          Originally posted by old mart View Post
          I would try googling passivating SS. Citric acid can be used which is more environmentally friendly than other acids, but I don't know the strength, temperature or pretreatments required.
          Yes found stuff about citric, its easy to get hold of too, seems 4-10% by weight as a guide. Not found out yet how to actually do it
          If it does'nt fit, hit it.


          • #6
            They make special passivating acids and coming from darkest third world S Africa we dont care about the environment.I soppose it would be safer to do that in 316 seeing that it has rough plasma cut edges and grind marks on the flat areas that would soak up salt if its in a seaside environment. Looks good.


            • #7
              That's a very nice piece, Dave.

              Citric acid in powder form is easy to find and easy to dispose of. It's used in cooking so no hazards involved. I get five pound bags for periodic cleaning of my dishwasher (keeps the spray holes and impeller from plugging up with mineral salts).

              There are no stupid questions. But there are lots of stupid answers. This is the internet.

              Location: SF Bay Area


              • #8
                I have used phosphoric acid but maybe that is not what you need.



                • #9
                  Thanks for the compliments guys,

                  I think it can be done with Nitric, Phosphoric and a couple of other acids but for a home setup i think the citric acid is looking best.

                  I have also been looking electro-passivation where you pass a current between a carbon fibre brush wet with acid and the work, it seems easy to set up and pretty fast too, might take that further and see what it does.

                  Anyone know if white vinegar/acetic acid would remove the plasma cut edge? I know it removes mill-scale pretty well but not tried much else with it
                  If it does'nt fit, hit it.


                  • #10
                    You could keep the acid in a polypropylene container with a lid, and it would probably work for six months or more. Passivating SS will not use up the acid fast as the actual amount of chemical reaction each time is tiny. A good soak in a detergent followed by a thorough rinse in running water should suffice before the passivating, then another rinse and a final dip in hot water to finish off.


                    • #11
                      Back in the 70s, I spent an afternoon at a major SS fabricator in Vancouver BC. I am a chemical engineer and the passivation process that they used made me shudder! They had a large timber tank with a thick Hypalon lining. This was filled with a strong solution, (don't recall the exact proportions,) of a mixture of nitric, sulfuric, and hydrofluoric acids. This solution was then heated to near boiling using a Heath-Robinson heater consisting of a large SS pipe through which a propane flame burned. It could not be left in the tank or it would dissolve-fairly quickly! Bundles of SS tubing would be suspended in the hot solution for a brief time, (sorry, details are a bit vague,) hauled out and rinsed with water. The object of this whole exercise was to dissolve the iron from a thin layer of the metal, leaving a surface layer consisting of chromium-nickel alloy.
                      This whole operation was VERY third-world, but back then, the world was a bit simpler! This is DEFINITELY one of those "do NOT try this at home" processes, but I very much doubt that you could even buy the chemicals. As an example, I struck out trying to buy nitric acid and I live in Gatineau-Ottawa, more than a million people!
                      Duffy, Gatineau, Quebec


                      • #12
                        When I was searching for info on passivating, nitric acid baths (heated) came up. To scary for me. I wound up using Oxalic acid (wood bleach):
                        Location: Long Island, N.Y.


                        • #13
                          Amazing, things were different back then I guess

                          I think citric acid is the way forwards, but i am still unsure of the process.

                          I know i need to use tools strictly for stainless, certain of that.

                          The parts are plasma cut, this leaves a dark edge, not black but grey, there seems to be no HAZ around the cuts.

                          I hit the back of the parts with a stainless cup-brush, this knocks off the small dross line that appears and then I hit the whole front and back to give a finish.

                          Now the test part i made (pictured) was done with my normal steel-working tools so is contaminated, but its been in the garden a couple of weeks now and has had rain on it although not much as summer time, we are not by the coast though.

                          I also made another scrap part up the same way and salt dipped it, it rusted badly in an hour or two, but that was a strong salt solution so maybe not a good test, and it was only the parts i worked with the steel tools that rusted, the untouched bits were ok.

                          Maybe i need to do some more tests with proper tools before diving into acids etc???

                          I know the welded joints will need treating but that will come later.
                          If it does'nt fit, hit it.


                          • #14
                            Stainless, as I understand, shall resist surface corrosion mostly by having a chromium oxide layer on it that "passivates" and seals the metal from outside influences. Much like titanium and aluminium alloys resist corrosion by forming a dense protective oxide on the surface. These oxide layers have a tendency to form on their own under the action of atmospheric oxygen and moisture. So in time the stainless surface repairs itself and regains the former properties. That is if the surface does not have much contaminants (iron from tools that were used to work it or porous oxide inclusion from welding).

                            The passivation processes are meant to clean the surfaces and speed up the formation of oxide coating (acid treatments for stainless and anodization for aluminium alloy e.g.)

                            The following link has a report regarding citric acid passivation of stainless steel:

                            From it one can conclude that presumably the most effective set of parameters for 304 SS passivation is to immerse it in a 4% citric acid solution at 140F for 30 minutes.
                            I suggest you try it by immersing a test piece that has been cut, brushed and treated like your final product would, but leave half of the part out of the passivation solution. That way you should see a contrast between the passivated and unpassivated area by subjecting the piece to corrosive conditions after the passivation. It should give a good imperssion wether the citric passivation method works and how much of an improvement in corrosion resistance can be hoped for using it.


                            • #15
                              Just curious, if the chemicals are hard to come by, how well would coca cola work?