Conduction through corrosion films on silver plated copper in power contacts

It is well known that the presence of a corrosion film affects the conduction through an electrical contact. This paper presents a study on the influence of corrosion films on contact resistance and contact deterioration of silver plated copper contacts for power applications. The model contact consisted of two crossed copper cylinders (0=10 mm) electroplated with silver. The normal force was varied from 10 to 60 N and the direct current from 10 to 200 A. Only stationary conditions were studied. Corrosion films were produced prior to testing by exposure to a mixed flowing gas atmosphere, i.e. Battelle class III. All tests were performed in room atmosphere. The deterioration mechanisms were identified by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The contact resistance, which generally decreased throughout the tests, was found to be strongly influenced by the corrosion film thickness, while the silver coating thickness was of minor importance. Furthermore, the current load was found to strongly affect the contact resistance and the size of the contact areas, i.e. the contact resistance was lowered with increasing current load. It was concluded that electrolytic silver ion transport from the anode to the cathode surface was the mechanism causing this self-healing. However, in some cases this ion transport was detrimental for the silver coating, which could be completely transferred from the anode to the cathode.