Fretting in hostile environments

Abstract

Metallic materials for use in corrosive environments are almost entirely of the type which derive their corrosion resistance from a protective oxide film which may withstand the effects of fatigue and creep, but which is completely disrupted by fretting. This may have serious consequences, for example, in a nuclear reactor where a stainless steel structure may be operating at a high temperature in an oxidising atmosphere. Not only will disruption of the normally protective oxide film allow corrosion to continue, it may also enhance corrosion since the protective film is rich in chromium and the metal surface is therefore depleted in chromium and has a lower corrosion resistance than the original surface. In certain designs of diesel engine lands on the outer surface of the cylinder liner bear on corresponding lands on the cylinder block forming a seal separating the region of the exhaust gases and the region of the cooling water which may contain antifreeze. Fretting due to vibration allows these environments to gain access to the seal surfaces resulting in accelerated fretting damage with rapid breakdown of the seal. A third example is that of the surgical implant. The importance of corrosion resistance here is that corrosion products can be toxic to surrounding tissue. Fretting between the surfaces of screwholes in the implant and the underside of the heads of screws used to fix it into the bone can result in toxic soluble corrosion products being released into the body fluid.