METALLIC COATINGS

The use of metallic coatings as a protection against corrosion is a subject of considerable antiquity. Cladding and inlaying with gold, silver, and electrum was practised during the Byzantine and early Egyptian civilisations—the tinning of cast iron and copper vessels was known to be established prior to the birth of Christ by the Romans, to whom must also be attributed the lead cladding of ferrous fittings intended for service in marine environments. Metallic coatings can be produced by a wide variety of methods, and on the technique selected depends certain of the factors which control the protective value of the coating, e.g. thickness, uniformity, porosity, adhesion, and purity. Since the type of process used is often restricted by the nature of the article or component to be treated, it is intended to precede a description of the properties of the various metal coatings by a general discussion of the more important methods of coating application.