Fretting — wear damage that is caused by movement at the contact surfaces — is another type of corrosion that must be designed for and protected against. If left unchecked, fretting can cause a material to fail completely.
Fretting, or fretting corrosion, is a type of wear damage that is caused by movement at the contact surfaces. This movement can be characterized by vibration, or simply just a load applied by one surface onto the other. The movement between the surfaces causes pitting and mechanical wear, which reduces the material’s resistance to fatigue. Generally the length of this motion is between a micron and a millimeter in amplitude, but fretting can be caused by vibrations as small as a few nanometers.
Once the material is sufficiently worn, oxidation follows quickly on both the exposed surfaces as well as the debris formed by wear. The oxidized debris is much harder than the exposed surfaces, which causes fretting, and a similar process known as false brinelling, to proceed even faster. False brinelling is a type of damage that is caused by fretting and characterized by imprints visually similar to brinelling (though not caused by the same mechanism).
False brinelling typically is found in bearings that lack a method to redistribute lubricant. Under vibration, the lubricant is slowly pushed out of the loaded region and wear on the bearing increases as the amount of lubricant in the loaded region decreases. Fretting can then occur as wear between the bearing and its contact surfaces creates debris which is then susceptible to oxidization and can cause further damage.
The aerospace industry in particular is developing research into fretting mechanisms and ways to avoid such wear as the spline coupling in gas turbine aero engines as well as the dovetail blade-root connection commonly experience fretting.
When objects under cyclic stress experience fretting, their resistance to fatigue is reduced, known as fretting fatigue. In this type of fatigue, fatigue cracks are initiated in the fretting zone (the pits caused by fretting serve as stress concentrators) and then are able to propagate through the material to failure.
Fretting Corrosion Prevention
The simplest way to avoid fretting is to ensure that contact surfaces will have no relative motion. However, since this is often difficult or downright impossible to achieve, fretting can also be reduced through smart material choices. Choosing materials with a lower surface roughness will reduce fretting as these materials experience less friction, and thus less mechanical wear. Additionally, lubricants can serve to reduce friction, inhibit oxidation, and therefore mitigate the effects of fretting.
The hardness of a material must also be taken into account when designing against fretting. As would be expected, softer materials are much more prone to mechanical wear and therefore are not ideal choices of material when designing against fretting. However, softer materials can increase fretting of the harder metal adjacent to them after capturing hard (possibly oxidized) debris which then becomes embedded in their surface. The polymer then becomes more abrasive and is able to wear down the harder metal in conjunction with the captured harder debris.
Fretting is yet another type of corrosion that must be designed for and protected against. If left unchecked, fretting can cause a material to fail completely. If the choice of material cannot be changed and/or the relative motion of the surfaces cannot be avoided, fretting can also be reduced by inserting a protective layer between the two moving surfaces, much like our composite wear pads and pipe sleeves.