Stainless will rust especially if it's exposed to harsh chemical fumes (acids, chlorine, etc.). Tanker trailers rust inside after carrying harsh materials, and the tank wash companies do a procedure called passification. They circulate a hydrofluoric acid based solution in the tank, and ,as was described to me, basically drive the carbon steel back in to the tank material, stopping the rust. I use hydrofluoric based aluminum brightener when I recondition a pressure washer to clean up the stainless.