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Thread: Passivating stainless at home...

  1. #21
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    Bead-blast does a pretty good job - fast too, only needs a quick pass.



    I'll give this part a salt-dip today see what happens.

  2. #22
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    Interesting...

    The blasted part is showing no signs at all of rusting in the salt-dip

    However, the surface of the blasted part is unable to be brought back to the desired finish with the wire cup-brush so it looks like chemicals are the way forwards.

  3. #23
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    Quote Originally Posted by Davek0974 View Post
    Interesting...

    The blasted part is showing no signs at all of rusting in the salt-dip

    However, the surface of the blasted part is unable to be brought back to the desired finish with the wire cup-brush so it looks like chemicals are the way forwards.
    Check out "pickled and passivated" google

  4. #24
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    Yep,

    just got to find the minimum process to provide good results, it can get pretty involved but these parts do not need that much work i feel, they are ornamental not food processing etc

    I think bead blasting the welds, bead blasting the plasma cut edges where possible and then citric passivating may work.

    Waiting for acid to arrive.

    One thing i have found is the stainless cup brush is far more aggressive on stainless than the plain steel brush - it tends to tear the metal, I think its the same as stainless fasteners galling easily - the two alike metals tend to tear rather than brush, really makes a mess out of the surface.

    I might try just using the brush to kill the plasma dross then get the finish with discs in a little angle grinder - easy stop contamination that way.

  5. #25
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    Quote Originally Posted by Davek0974 View Post
    Yep,

    just got to find the minimum process to provide good results, it can get pretty involved but these parts do not need that much work i feel, they are ornamental not food processing etc

    I think bead blasting the welds, bead blasting the plasma cut edges where possible and then citric passivating may work.

    Waiting for acid to arrive.

    One thing i have found is the stainless cup brush is far more aggressive on stainless than the plain steel brush - it tends to tear the metal, I think its the same as stainless fasteners galling easily - the two alike metals tend to tear rather than brush, really makes a mess out of the surface.

    I might try just using the brush to kill the plasma dross then get the finish with discs in a little angle grinder - easy stop contamination that way.
    Be mindful of using carbon steel brushes as they leave behind iron which will rust.
    If you passivate after brushing, that will solve the problem.

    We used to use a electro polishing process on stainless tube. I was going to try to remember the process but found this resource instead... it's likely to be more accurate than my memory form 25 years ago :>)
    see:
    https://www.ableelectropolishing.com...ctropolishing/
    paul
    ARS W9PCS

    Esto Vigilans

    Remember, just because you can doesn't mean you should...
    but you may have to

  6. #26
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    Quote Originally Posted by ironmonger View Post
    Be mindful of using carbon steel brushes as they leave behind iron which will rust.
    If you passivate after brushing, that will solve the problem.

    We used to use a electro polishing process on stainless tube. I was going to try to remember the process but found this resource instead... it's likely to be more accurate than my memory form 25 years ago :>)
    see:
    https://www.ableelectropolishing.com...ctropolishing/
    Yes, i got a softer bristle stainless brush today, will try that.

    I did look at electropolishing but this needs to be able to be done in a home-shop setup, not sure the electrolyte would be friendly

  7. #27
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    Hmm, anyone know about citric acid passivating?

    I just got the stuff and mixed up a little test solution, chucked a bit of scrap plasma-cut stainless in and ......

    well, nothing, not yet, been in about 30 minutes at 25c.

    Seems 10% is the norm, I found that out after I started and the mix I have is probably about 25% - 30%

    Is it fast, slow, should i see anything happen?

  8. #28
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    Industry standard is phosphoric acid, as it works well, but is much less dangerous than nitric or hydrochloric or sulfuric.
    I have a lot of work electropolished- they use about a 100 degree heated phosphoric mix- its some special stuff they buy from a plating supply, not 100% phosphoric, but it works great. Then, they hook up a 1000 amp 100volt power supply electrode negative. And that piece of yours would probably be in that tank for maybe a half hour.

    No electricity, and citric, I would figure 12 to 24 hours would be a good test.

    I use citric for site electropolishing- I use a small battery charger style power supply- mine is 40 volts- and an insulated wand, and a ground clamp. It will clean off the weld discoloration, but its not enough juice or a strong enough acid to do what you want.

    Try a day or two with the citric, and, if that doesnt work, find some phosphoric. Its available, and its not too expensive or dangerous. It will probably only work on a couple of batches, though, before it loses its bite. Over here, we can buy it on Amazon, or cheaper at restaurant or industrial cleaning suppliers.

  9. #29
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    Ok, thanks,

    I'll do another test and leave 24hrs, see what happens.

    I did read up on electropolishing and they certainly do use some heavy currents.

  10. #30
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    Quote Originally Posted by Davek0974 View Post
    Hmm, anyone know about citric acid passivating?

    I just got the stuff and mixed up a little test solution, chucked a bit of scrap plasma-cut stainless in and ......

    well, nothing, not yet, been in about 30 minutes at 25c.

    Seems 10% is the norm, I found that out after I started and the mix I have is probably about 25% - 30%

    Is it fast, slow, should i see anything happen?
    You shall not see a visible reaction between the stainless and citric acid. If you have some fine active ferrous dust or particles on the surface then you might see the citric solution discolor a bit into yellowish. It happens because iron salts usually have a brown or yellow coloration when dissolved. Can also be green-blue when Fe(II) ions are mostly present. But these are unstable and will react with oxygen from air forming Fe(III) which are stable and have a brown to yellow coloration depending upon concentration. The more concentrated the solution, the darker the brown tint.
    All that the citric acid does is to enforce the formation of chromium oxide layer on the stainless and make it unifrom and perhaps a tiny bit thicker. The actual mechanism is somewhat of a mystery to be honest. The chromium oxide is transparent and you can not really see it or assess the thickness or uniformity visually. Judging by eye the passivated piece will look exactly the same as the untreated piece of material. Only the corrosion test shall reveal wether the treated vs. untreated stainless surfaces act differently with respect to susceptibility to the formation of corrosion.

    Also it is important to adhere to a well established procedure regarding the passivation e.g. : 4% citric acid solution, 140F temperature of the solution, 30 minutes residence time in solution.

    Just mixing up an unknown concentration of citric acid, not controlling temperature and dipping stainless into it for extended periods of time shall likely worsen the situation and at best do nothing to improve the corrosion resistance. Even if it should work then you shall have a hard time replicating the results on actual products that you want to passivate.

    Please look through the reference document:
    https://ntrs.nasa.gov/archive/nasa/c...0110001362.pdf

    It can be seen that all factors (acid concentration, temperature and dip time) have an influence on the result and if chosen improperly the passivation is likely not going to yield strongly positive results. Also the type of alloy has an effect and a set of parameters that works for one stainless alloy may not work or even worsen the situation for another type.

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