During finishing of stainless steel, the metal is typically rolled and then annealed to achieve the desired structure and material properties. Because annealing is carried out in the presence of air, a film of oxide consisting mainly of chromium oxide (Cr2O3), ferrous oxide (FeO), and manganous oxide (MnO) forms on the surface of the steel. The Cr2O3 content is normally between 55 and 65%. Normally, the film of oxide is fairly porous and contains small cracks. A zone that is depleted of chromium is normally formed under the oxide film.
A process called pickling is used to clean and condition the surface of the metal after annealing. The pickling of stainless steel requires three distinct processes. The first is removal of the thermally grown scale for appearance purposes and to facilitate cold working of the steel. The second process increases the corrosion resistance of the final product by dissolving the chromium-depleted zone that generally forms during annealing. During the third process, a minimum amount of bulk steel is dissolved, giving the desired brightening effect to the final product.
Recovery of stainless steel pickle liquors: Purification vs. Regeneration