Corrosion of electrical components by their atmospheric environment

Mechanisms of atmospheric corrosion are first surveyed. Dry conditions are usually harmless except when hydrogen sulphide is present; above a certain critical humidity, electrochemical corrosion can occur, and is stimulated by gaseous and solid atmospheric contaminants such as sulphur dioxide and soot. Aggressive vapours may be evolved by organic insulating and packing materials. Corrosion may be enhanced at bimetallic contacts or where externally applied potential differences exist. Metallic `whisker¿ growth occasionally occurs. Next, possible effects of corrosion and whisker growth on electrical components are discussed. Failures may result from disconnections due to corrosion of wires, prevention of contact and clogging of moving parts by corrosion products, and from the making of unwanted contacts by metal whiskers, corrosion products and silver migration. A list of corrosion cases is then given, classified according to cause, illustrating some of the corrosion and fault mechanisms. The more interesting cases are commented on briefly. Finally, preventive measures are covered. The ideal environment is provided by a hermetically-sealed enclosure containing a dry uncontaminated inert gas. More practical steps include air purification, adequate ventilation of enclosed equipments, and avoidance of humid conditions and metal-surface contamination.