1. Field of the Invention
This invention relates to a heat and corrosion resistant multilayer plating used to protect the surfaces of plates, pipes, joints, clamps, bolts, nuts, and other parts used in motor vehicles, machines and apparatus, and made mainly of iron and steel.
2. Description of the Prior Art
It has been usual to plate the surfaces of plates, pipes, joints, clamps, bolts, nuts, and other parts used in motor vehicles, machines or apparatus and made mainly of iron and steel, with zinc or a zinc alloy to form a zinc-based plating for protecting them against corrosion. Alloys, such as of zinc and nickel and of zinc and tin, have beem used to form a plating of higher corrosion resistance. It has often been the case that a chromate film is formed on any such plating.
All of these platings have, however, been found satisfactory for corrosion resistance only at low ambient temperatures. In an environment having an elevated temperature, for example, in the engine room of an automobile in which a temperature of about 200.degree. C. prevails, the plating lowers its sacrificial corrosive action due to the oxidation of zinc, or its corrosion resistance due to the destruction of the chromate film.