Passivation of stainless steel is typically done with nitric acid. The point is to remove any
bits of iron or steel that are providing local points of electrochemical difference which can
cause corrosion. Clean with a strong alkali cleaner like 409 or castrol purple, plug the muzzle
and pour in some 20-50% nitric acid and leave for about 30 minutes or more with the weaker
solutions. Actual treatments are often done at higher than room temp.
In addition to removing the contamination, it also creates an oxide film which protects
against further corrosion. I have seen cases of severe corrosion of stainless steel because
it was never passivated.
Bill