Fretting damage in tin-plated aluminum and copper connectors

A number of bolted-type tin-plated aluminum and copper connectors commonly used for distribution transformers were examined using scanning electron microscopy, energy-dispersive X-ray analysis, Auger electron spectroscopy, X-ray analysis, and optical microscopy. In addition, the contact resistance of the connector contact zones was measured using a point probe. The connectors studied had been removed from service because of unsatisfactory performance under normal operating conditions as manifested either by overheating or instability. The results of the detailed examination show the presence of extensive fretting damage in the contacting surfaces. The fretting debris was composed mainly of tin oxide and oxidized base metal particles. Localized melting and wear of the tin plating down to the underlying substrate were also observed in some of the connectors examined.