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.
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.