Decorative Electrodeposited Metallic Coatings

Abstract

Electrodeposition of metallic coatings for decorative purposes is such an extensive topic that it is intended to review in detail only the most important metals. Some indication of types of plating solutions and general formulations is given, but voluminous patent literature exists on this subject. The physical and mechanical properties of deposits will be considered only in so far as they are relevant to the use of coatings for decorative purposes. Though the coatings to be discussed are essentially employed to provide an attractive appearance, in most instances some degree of protection of the substrate is also inevitably implied. Decorative coatings would be of little value if they either deteriorated rapidly themselves or allowed corrosion of the substrate to occur. The severity of the environment to which a coating is subjected varies considerably. For example, motor-car components may be exposed for almost their entire life in an industrial atmosphere containing a high concentration of sulphur compounds, while expensive silver-plated tableware may remain in a dry centrally heated house. The coating may be more electronegative than the substrate; in which case the latter will be sacrificially protected but the coating will eventually lose its decorative appearance, e.g. bright zinc or cadmium on steel. Alternatively, the coating may be cathodic with respect to the substrate; in this instance, deterioration usually occurs in the form of pits that penetrate to the substrate. When this stage is reached, preferential attack of the substrate takes place and ultimately the coating is undermined, e.g. nickel + chromium on steel