A process designed to remove foreign metals from the surface of stainless and corrosion resistant steels and to promote natural tendency of surface to oxidize.Does not change the appearance of the basemetal. Process purifies surface and therefore improves corrosion resistance. Some corrosion resistant alloys (AM355, 350, 17-4PH, 17-7PH, 15-7MO) are not always stable as far as corrosion resistance is concerned (depending upon the heat treat condition).
It should also be noted that if heat treating is performed in an oxidizing atmosphere, the 400 series exhibits different characteristics when passivated, and can result in light rusting shortly after passivating. In many cases, fabricators have resorted to cadmium, nickel, or oiling of 400 series to prevent rusting.