A Comparison of the Corrosion Behavior and Protective Value of Electrodeposited Zinc and Cadmium Coatings on Steel

Abstract

Electrodeposited coatings of both zinc and cadmium are commonly used to prevent the rusting of steel parts exposed to the atmosphere. During the period of World War II the armed services and, particularly, the Navy Department were inclined to favor the use of cadmium for this purpose, as judged from their general specifications for that time. Because of the acute shortage of cadmium and cadmium plating facilities during World War II it was necessary to give serious consideration to the substitution of zinc wherever possible. This resulted in considerable controversy as to the relative merits of the two metals, particularly their protective value when applied in equal thicknesses. Previous work (1−3) had shown zinc coatings to be superior to cadmium coatings when exposed to industrial atmospheres. The tests of Blum, Strausser, and Brenner also showed zinc coatings to have somewhat longer protective life than cadmium coatings in mild, rural locations such as State College, Pa., and urban sites such as Washington, D. C. While only comparatively thin coatings were involved, it was concluded from this same work that the difference in protective value between the two kinds of coatings in marine locations was not significant. On the other hand, the results of many comparative salt spray tests had shown cadmium coatings to be markedly superior to zinc coatings under the particular conditions prevailing in a salt spray cabinet. Undoubtedly the results of these accelerated tests contributed in a large measure to the generally prevalent attitude favoring cadmium coatings in marine atmospheres. The work reported herein was undertaken to determine the relative protective value of zinc and cadmium coatings in a wide variety of environments including accelerated corrosion tests.