Rustproofing and Paint-Adherence Technique Analyzed 350125
EXTENDING the life of those parts most quickly destroyed by corrosion, an important problem in the automotive industry, for practical purposes resolves itself into preventing the failure of paint on metal surfaces.
The porosity of the paint films permits moisture to get through to the metal, and thus to induce electrolytic rusting. Chemically cleaning before painting retards paint failure. Coating the metal with phosphates gives better protection still, by permitting thicker coats of paint. This phosphate surface is not ductile, though, and breaks when the metal is bent, causing paint failure.
Best protection is obtained by first plating the steel with zinc and then converting this plated surface into a zinc phosphate, so that the paint will adhere to it. The next best method is plating steel with a continuous coating of zinc phosphate by means of alternating-current electrolysis.
A chromate treatment known as the Cromodine process has been developed, producing a chromate surface on the steel at a low cost. This coating is as elastic and as ductile as the steel itself; it forms a perfect bond with paint, and is very effective in increasing the life and durability of paint, lacquer, or enamel finishes.