Home passivation of stainless?

I'd like to try passivating some stainless steel whose surface is
currently prone to light rust (see below if you want to know how it got
that way). Citric acid seems a lot easier to get and safer than nitric
acid for this purpose, particularly for home use. Can anyone suggest a
good "recipe" for the process - concentration of acid, temperature, and
time?
The steel is not marked as to composition, but it's an ordinary
stainless steel saucepan, so it's probably approximately 304 or 316
stainless.
Dave
The background: Someone left some food heating in the saucepan, went
away "briefly", and then forgot about it. The food boiled dry and then
charred, leaving hard chunks of black carbon in the pot. My wife tried
cleaning it with scouring pad and (mild abrasive) stainless cleaner,
but eventually gave up with some carbon still firmly attached to the
inside of the pot.
I tried a couple of applications of oven cleaner, but that didn't seem
to have any effect on the carbon at all. Then I used a razor blade
scraper to remove most of the carbon blobs on the flat inside bottom
surface of the pot. That worked well - the small amount of carbon that
remained came off with some stainless cleaner. But that left some
carbon lumps around the inside walls of the pot where the walls meet
the base. The compound curvature at this point means no straight-edge
scraper can get in there, so I couldn't use the razor blade.
So I got out the angle grinder, attached a wire cup wheel, and attacked
the remaining carbon. This worked really well - most of the carbon
vanished in seconds. The rest went away with some stainless cleaner
and scrubbing. I was happy - for a little while.
But the areas that were wire brushed now develop surface rust quickly
after they become wet. I suspect the problem is that the brushing
removed some of the surface layer of stainless steel, exposing the
iron-containing bulk steel which can rust. Some reading suggests that
I can fix the problem by redoing the passivation of the stainless
steel, which removes the iron or makes it non-reactive.
Reply to
Dave Martindale
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The wire brush embedded iron in the surface of the stainless.
I use a commercial citric acid paste for passivation. It is sold by Stellar Solutions
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You can use food grade citric acid powder, but it won't be as aggressive.
The trick is to add electricity. You need about 4 amps of 24 volts DC.
You could try just using Naval Jelly from the hardware store. It is a phosphoric acid gel.
Reply to
Ernie Leimkuhler
For future reference: there are several compounds made for cleaning guns that do a fine job of carbon removal. One such that is particularly good for carbon on stainless is
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Reply to
Don Foreman
Note:-
Have the pan positive, you're trying to remove the iron, leave the chrome and oxidise that chrome
Mark Rand RTFM
Reply to
Mark Rand
What is safe to use as the negative electrode? Bare copper (easiest, but will it contaminate the solution)? Iron? Sacrificial stainless? Carbon rod?
Dave
Reply to
Dave Martindale
You can electroclean with a conductive wand wrapped on the working end with an absorbent non-conductive material like fiberglass cloth or scotch-brite. Battery acid (sulfuric) will work for the electrolyte. Copper tube or pipe works well as the wand - if you get the polarity wrong the copper will plate on to the stainless. I used to use this method occasionally for cleaning welds on SS hardware that weren't accessible for mechanical polishing.
Mechanical polishing would also work to fix your pan.
Reply to
Ned Simmons
I suspect that the problem is a result of using a carbon steel brush on the stainless. This will have embeded small particles of steel in the surface, and these rust at every possible chance. Stainless steel wire brushes are availabel for the clean up of stainless welds. The passivation may or may not fix the problem.
Reply to
Grumpy
I'd scrub it with a Scotchbrite, then boil water in it. I have been modifying a stainless steel coffee pot to percolate faster, and leaving it on the back of the wood stove to simmer a while after each change. The shiny fresh surface turns visibly duller and slightly brown.
Reply to
Jim Wilkins
I use 5-10% citric acid by volume in water, with constant stirring or agitation, at 150-175F, and 10 minutes up to whenever I remember to take it out (:-)) for the time. I've read 10 minutes but I figure longer is better so I probably average 2-3 hours but I have no quantitative data to say if that is overkill or not enough. In your case I'd let it simmer in the pot for at least an hour. Just add water if needed to make up for any evaporation. Unfortunately, you may just have too much steel embedded for anything short of soaking in nitric acid. Hey, maybe the alum trick will work? Try the citric acid passivation first, and if it stops the rust then great, you are done. Otherwise, get some Alum from the spice aisle at the grocery store and put just enough water in the pan to cover the areas that rust, and then a tablespoon or two of alum. Heat over very low heat until warm to the touch, and let it sit overnight. This should dissolve the plain steel, and then repeat the passivation. I used the alum trick to dissolve a small drill bit broken off in 304 stainless steel, and made a posting about it a year or more ago. Try a google search on "ijames" and "alum".
-- Regards, Carl Ijames carl dott ijames aat verizon dott net (remove nospm or make the obvious changes before replying)
Reply to
Carl Ijames
The usual precautions about hexavalent Chromium if you choose an electrolytic method.... /mark
Reply to
Mark F
Almost anything conductive. You are effectively plating onto the negative electrode, so it isn't going to be harmed by the electrolysis, just the acid.
Mark Rand RTFM
Reply to
Mark Rand

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