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Thread: Passivating stainless at home...

  1. #11
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    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

  2. #12
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    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):
    http://bbs.homeshopmachinist.net/thr...79#post1206279

  3. #13
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    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.

  4. #14
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    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:
    https://ntrs.nasa.gov/archive/nasa/c...0110001362.pdf

    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.

  5. #15
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    Just curious, if the chemicals are hard to come by, how well would coca cola work?

  6. #16
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    I should do the passivating after the work is complete including all welding. Also, citric acid may not be one of the strong ones, but eye protection and decent gloves would be sensible.
    Last edited by old mart; 07-09-2019 at 04:04 PM.

  7. #17
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    Quote Originally Posted by old mart View Post
    I should do the passivating after the work is complete including all welding. Also, citric acid may not be one of the strong ones, but eye protection and decent gloves would be sensible.
    Yeah i think i need to get some stuff together and do some tests on this.

  8. #18
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    Quote Originally Posted by old mart View Post
    I should do the passivating after the work is complete including all welding. Also, citric acid may not be one of the strong ones, but eye protection and decent gloves would be sensible.
    Citric acid should be freely available as a food additive....at least in my part of the world one can buy it in any grocery store. The pure substance is a solid powder and pretty much harmless. In fact one is pretty hard pressed trying to get hurt by handling citric acid. It would take a well premeditated attempt
    But gloves and glasses are always good practice.

  9. #19
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    I may be showing my ignorance here, but would you be opposed to abrasive methods to clean up the cut edges? I was wondering about a glass-bead blast cabinet. I know about cleaning the stainless dross, its a pain to deal with. Very nice looking part you have, but I wouldn't want file all those edges...

  10. #20
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    I do have a small bead-blast cabinet, i'll do a test, my feeling is that its not man-enough to strip the edges, I only use it for texturing aluminium.

    Will find out later today.

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