Gears in a stationary wind turbine will rock back and forth, even when the rotor-lock is on. This results in repeating vibratory micro-motion, making the gears rub into and out of contact with each other. This type of contact is known as fretting and squeezes out the lubricant from between the contacting gear surfaces, resulting in metal-to-metal contact between asperities. The lack of lubricant leads to the metal-to-metal contact and oxidisation of localised high points on the gears surfaces (known as asperities). The resultant wear marks produced by this process are known as fretting corrosion.
Although identically located, the appearance of fretting corrosion varies with severity:
- Mild fretting corrosion: Commonly referred to as standstill marks or parking lines. It appears as long, thin black lines.
- Moderate fretting corrosion: This is the generic form of fretting corrosion. It is broader and includes the emergence of reddish-brown oxides around and over the initial black lines. The wear can result in the formation of flattened patches. It arises after longer periods of fretting.
- Severe fretting corrosion: Emergence of micropitting or small, isolated macropits, around the edges of the fretting corrosion. It occurs when fretting corrosion progresses enough to cause significant surface distress and resultant stress concentrations.
Mild fretting corrosion is generally non-progressive, being accepted as relatively normal run-in wear which will arrest. However, in some circumstances the standstill period may be long enough for moderate fretting corrosion to develop. The dark, red-brown oxides are abrasive, and the metal-to-metal contact will lead to increased surface roughness and stress concentrations. Such moderate fretting corrosion may or may not progress further. If it does progress, it will be identifiable by the formation of micropitting around the edges of the fretting corrosion. Nonetheless, fretting corrosion itself will not cause failure and is not a cause for serious concern. If it does progress to micropitting, the only recommended action will be to continue to monitor for further progression to macropitting.
|Visual inspection||✓✓||If accessible, the naked eye can detect some fretting corrosion due to the change in colour on the gear face, however more subtle appearances may need a borescope.|
|Borescope inspection||✓✓✓||Fretting corrosion is readily observed and distinguishable with a borescope for mild cases.|
|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 caused by standstill during commissioning is very common and frequently found on parallel-stage gears in wind turbines. It is not generally a cause for concern, even in the early stages of progression. If moderate fretting corrosion develops, it may be possible to gentyly wipe away to reddish-brown oxide abrasives.
Ensuring adequate lubrication of the gearbox will help reduce the likelihood of fretting corrosion progression. Other preventative measures include additional packaging and care during transportation to site. Minimising downtime with the rotor-lock on, and frequently rotating the turbine during commissioning is also beneficial.
Despite the name, fretting corrosion should not be confused with conventional corrosion due to the presence of moisture in the gearbox.