Tarnish resistance, corrosion and stress corrosion cracking of gold alloys

Tammann's extensive studies (1) established that the resistance of gold to tarnish and corrosion is not greatly reduced by the addition to it of silver and base metals, so long as the gold content of the resulting alloy is not below 50 at.%. This corresponds to about 15.6 carat for Au-Ag alloys and to about 18 carat for Au-Cu alloys. Tammann's observations are of limited significance, however, when considering the resistance to tarnish and corrosion in the three areas in which this property is of particular significance. These areas are firstly, the tarnishing, corrosion and stress corrosion cracking (SCC) of gold jewelry; secondly, the tarnishing and corrosion of dental gold alloys; and thirdly, the tarnishing of gold alloy contacts in electronic circuitry. Tarnishing, corrosion and stress corrosion cracking (SCC) of gold alloys are related phenomena. They present problems when they occur in gold jewelry, dental gold alloys and electronic devices. They are exploited, however, in the depletion gilding and finishing of gold jewelry; and in the extraction, refining and fire assaying of gold. There is still much that is not known about these phenomena, but a coherent picture of their mechanisms is emerging as a result of studies of not only gold alloys themselves, but also of other alloys. Aspects of this development are discussed. 2 figs., 2 tabs., 32 refs.