Here are a few selected articles on the subject of Passivating Stainless Steel.
I found them on the internet as I was looking for a way to get the shine back on some stainless steel kitchen utensils that I had modified by forging. These articles are certainly not the whole story on passivating stainless steel.
From some of the message lists I visited, it almost seems that there is as much art as science to the process. But, if you have done or are considering working with stainless, you need to know that after forging or heating sufficiently, it will turn dark and sometimes get little black spots on the surface.
Lastly, not all stainless steels behave the same and you'll have to do more research on your own if you want more details.
To get the list of hundreds of Internet hits, through which I searched, I simply typed in the word 'passivate' and went from there.
Passivating Stainless Steel
A situation that often comes up is, "Hey, my stainless steel is rusting! Why? What can I do to fix it?"
Stainless steel is stainless because of the protective chromium oxides on the surface. If those oxides are removed by scouring, or by reaction with bleach, then the iron in the steel is exposed and can be rusted. Stainless steel is also vulnerable to contamination by plain carbon steel, the kind found in tools, food cans, and steel wool. This non-stainless steel tends to rub off on the surface (due to iron-to-iron affinity), and readily rusts. Once rust has breached the chromium oxides, the iron in the stainless steel can also rust. Fixing this condition calls for re-passivation.
Passivating stainless steel is normally accomplished in industry by dipping the part in a bath of nitric acid. Nitric acid dissolves any free iron or other contaminants from the surface, which cleans the metal, and it re-oxidizes the chromium; all in about 20 minutes. But you don't need a nitric acid bath to passivate. The key is to clean the stainless steel to bare metal. Once the metal is clean (and dry), the oxygen in the atmosphere will form the protective chromium oxides. The steel will be every bit as passivated as that which was dipped in acid. The only catch is that it takes longer-- about a week or two.
To passivate stainless steel at home without using a nitric acid bath, you need to clean the surface of all dirt, oils and oxides. The best way to do this is to use an oxalic acid based cleanser like those mentioned below, and a non-metallic green scrubby pad. Don't use steel wool, or any metal pad, even stainless steel, because this will actually promote rust. Scour the surface thoroughly and then rinse and dry it with a towel. Leave it alone for a week or two and it will re-passivate itself. You should not have to do this procedure more than once, but it can be repeated as often as necessary.
Oxalic Acid-based cleaners:
If you have a particularly tough stain liked burned malt extract then you may need something stronger. There are oxalic acid based cleansers available at the grocery store that are very effective for cleaning stains and deposits from stainless. They also work well for copper. One example is Revere Ware Copper and Stainless Cleanser, another is Bar Keeper's Friend, and another is Kleen King Stainless Steel Cleanser. Use according to the manufacturer's directions and rinse thoroughly with water afterwards.
Stainless Steel Passivation with Citric Acid
More than 99% of the citric acid sold in the US is used in food and beverage products. While this may seem a testimony to its safety, it is actually a testimonial of its effectiveness. When formulated properly, citric acid passivation will perform better than nitric acid - and the food industry requires the best, otherwise systems degrade and taste of food products suffers. Moreover, using citric acid process for passivation eliminates all risk to personnel, much of the risk to parts (generally save to expose surfaces longer than necessary) and on large systems where field services crew and large volumes of fluid are required, citric acid is a far more economical way of passivating stainless steel. Citric acid removes iron from the surface effectively in much lower concentrations than nitric acid does.
Typical solutions of citric are only 4-10% by weight; hence disposal requires only dilution that will occur naturally during the rinse cycle. The resulting effluent is very close to industrial wastewater.
Another widely recognized advantage of using citric acid is its ability to remove free iron and iron oxides without removing significant amounts of nickel, chromium or other heavy metals. Thanks to the pioneering work in Germany for beverage container passivation, the citric acid process has shown to be advantageous for fasteners, medical devices, semi-conductor, automotive and aerospace applications.
National Technical Systems chooses to employ the citric process in almost all its field applications in order to safe clients money, protect the environment (nitrogen oxides promote smog while citric acid does not harm the atmosphere), and speed up the process (faster processing, easier disposal).
Carbon Steel and Other Metals
Carbon Steel benefits from a process similar to citric acid passivation performed on stainless.
Properly formulated solution performed at the right temperature can greatly reduce corrosiveness of carbon steel. The process is particularly successful in piping systems where the first step - cleaning and descaling - is part of the cleaning requirement.
A last note by Pete: Sometimes heat treatment is also part of this passivation process.
Another search opportunity for the Google-inclined!