Fretting corrosion is caused either by machine vibration, or by the micro-motion that exists when the bearings in a stationary turbine rock back and forth. It can also be caused by low angle oscillatory motion on an in-service bearing, particularly pitch bearings. The bearing lubrication is squeezed out from between the contacting surfaces allowing metal-to-metal contact. This can cause the material to oxidise and eventually be removed from the surface. Serious forms of fretting corrosion are caused by alterations in the rolling profile (e.g. flat spots) that can act as stress concentration points.
Oxidation lines will be visible along the contact patch of the bearing. For a rolling bearing raceway, these will be along the width of the raceway. For a cylindrical rolling element, they will be along the length of the roller. Matt, non-directional (no original finishing marks) lines are common and representative of mild fretting corrosion (also known as false brinelling) caused by micro-motion. In black oxide bearings (see polishing for black oxide wear), mild fretting corrosion marks will appear of lighter colour than the reddish bearing surface. Although not as common as gears, some dark red oxides can appear around and over the lines which represents a more progressed form of fretting corrosion. Micropitting and macropitting may occur on the edges of progressed fretting corrosion. This is possible when the bearing surface has been significantly altered to cause a stress concentration.
Mild fretting corrosion (known as false brinelling) caused by standstill is relatively common and generally non-progressive. However, more serious forms can be progressive and lead to micropitting and eventually macropitting along the edges of the line.
|Visual inspection||✓✓||If accessible, the naked eye can detect some fretting corrosion due to the change in colour on the bearing surface, however more subtle appearances may need a borescope.|
|Borescope inspection||✓✓✓||Fretting corrosion is readily observed and distinguishable with a borescope.|
|Vibration analysis||✓✓||Mild cases of fretting corrosion will not be detectable using vibration data, however more severe cases of fretting corrosion can be detected.|
|SCADA data||✓||SCADA data does not usually aid detection of fretting corrosion.|
|Oil debris sensor||✓||Fretting corrosion does not shed much debris so cannot usually be detected using oil debris sensors.|
|Oil sample analysis||✓||Fretting corrosion does not shed much debris so cannot usually be detected using oil sample analysis.|
Mild fretting corrosion (i.e. false brinelling) is relatively common and is likely to be found in some gearbox bearings. This is not generally a cause for concern, unless signs of progression are observed. Signs of progression include a reddish appearance and pitting on the edges. Ensuring the bearing surfaces are lubricated as much as possible will help reduce the chance of fretting corrosion. This can be done by avoiding long periods of non-running of the turbine and taking care during transportation.