When the surface of a metal is treated chemically so that an inert chemical film is formed by the reaction, such films are known as conversion coatings. They may be applied to a variety of metals in many different colours, shades and thicknesses and all confer some degree of protection against corrosion. Since the subject is extremely wide, this discussion will be confined to ferrous alloys only; steel is the metal most widely used throughout industry and is perhaps the most susceptible to corrosion in its ugliest form.