Combination effect of fretting and corrosion on bolt-type power connectors

Analysis of failed bolt-type power connectors showed serious contamination and low contact force (Luo et al, 1999). Further studies show that combined fretting and corrosion effects greatly aggravate connection performance. Electrochemical corrosion occurs and white corrosion products are observed between Al connector and steel flat washer. XPS shows that the product is mainly a compound of Al/sub 2/O/sub 3/ and its hydrate. Crevice corrosion between nut, lock washer and flat washer is also found. Corrosion not only decreases mechanical strength, but also reduces the lock washer load-bearing area. Several lock washers and connectors have cracks at their maximum tension stress position, which is evidence of stress corrosion. This corrosion damage may develop soon after a period of latency, quickly causes loss of contact force, and is much more dangerous than other kinds of corrosion. Reduced compressive force allows fretting to take place at contact interfaces. Analysis shows various kinds of micro-motion at contact interfaces, with one-third of samples showing fretting tracks. A tarnish film of oxide debris and dust accumulates on the contact surface. Connection resistance increases, body temperature increases and corrosion is thus accelerated. This cycle may finally cause connector failure. Therefore, appropriate protective measures are needed for harsh environments. Surface analysis devices such as SEM/EDS and XPS are used to observe electric contact interface morphology and analyze corrosion product compound formation. Metallographic examination is also used to analyze corrosion products and metallurgical structure.