Stainless steel film of chromium oxide by reaction with the oxygen in air and even the small amount of dissolved oxygen in water. This passive film prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal. This film is self-repairing if it is scratched or temporarily disturbed by an upset condition in the environment that exceeds the inherent corrosion resistance of that grade. However, stainless steels may suffer uniform corrosion when exposed to acidic or basic solutions. Whether a stainless steel corrodes depends on the kind and concentration of
Stainless steel Hydrochloric acid will damage any kind of stainless steel, and should be avoided. All types of stainless steel resist attack from phosphoric acid and nitric acid at room temperature. At high concentration and elevated temperature attack will occur and higher alloy stainless steels are required. In general, organic acids are less corrosive than mineral acids such as hydrochloric and sulfuric acid. As the molecular weight of organic acids increase their corrosivity decreases. Formic acid has the lowest molecular weight and is a strong acid. Type 304 can be used with formic acid though it will tend to discolor the solution.