Fretting Corrosion

Fretting corrosion occurs where two metals are in contact and there is a relatively small motion between the materials. It can be thought of as the combination of wear and a corrosive environment. This process usually presents itself in material interfaces not designed to be in motion with respect to each other. Typical applications that have produced fretting corrosion are motor shafts and electrical contacts. In the case of motor shafts, machinery vibration causes fretting and usually results in decreased fatigue life, known as fretting fatigue. Proper alignment of the rotating shafts is critical to reducing fretting fatigue failures. A second form of fretting corrosion appears in electrical contacts where thermal expansion and contraction cycles result in degradation of the contacting materials. Electrical contacts are most often coated with a noble metal, which are resistant to fretting corrosion. Cyclic motion, however, can cause wear and failure of the coating leaving the base metal vulnerable to fretting corrosion and other forms of attack. Once the base metal is exposed, the formation of highly resistive oxides occurs resulting in intermittent or open electrical circuits. Fretting corrosion is often undetected due to the nature of its existence in hidden material interfaces. The best way to mitigate fretting corrosion is to be knowledgeable of the typical material combinations and applications where it occurs and the methods used to combat it.

Factors contributing to fretting corrosion include contact conditions, environmental conditions, and materials properties. These factors all interact to produce fretting corrosion or fretting fatigue, as represented in Figure 21.

Managing Fretting Corrosion

Methods used to reduce fretting corrosion include the following:

• Soft metal against hard metal contacts
• Roughen surface to reduce slippage
• Increase load to reduce relative motion
• Low viscosity fluids in combination with phosphate treated surfaces
• Increase surface hardness of contacting metals
• Use one metal with a low coefficient of friction
• Use corrosion preventive compounds on electrical contacts