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Fretting corrosion

Fretting corrosion – Gear failure

Fretting corrosion refers to wear marks developed when gears rub against each other due to repeated vibratory micro-motion. The fretting process displaces lubricant, leading to the oxidization and rubbing away of asperities. Depending on the severity, fretting corrosion has a black, or reddish-brown appearance. It occurs in a long, straight line across gear faces. Mild fretting corrosion, known as standstill marking, is common in wind turbine gearboxes. It is caused by extended periods of standstill – when the turbine is left stationary with the rotor-lock on – during commissioning. It can also result from vibratory motion during transportation of the gearbox. Mild fretting corrosion is not a cause for concern and is generally non-progressive. However, it does increase surface roughness, and may progress into micropitting. Fretting corrosion is distinct from – and should not be confused with – conventional moisture corrosion.

Also referred to as: Standstill Marks, False Brinelling, Parking Lines, Friction Oxidation, Chafing, Fretting Wear, Vibration Corrosion

Click here for: Fretting corrosion – Bearing failure

Causes

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.

Appearance

Fretting corrosion occurs along the contact line of gear teeth. For the helical gears typically used in parallel stage gearing, fretting corrosion appears as a diagonal line along tooth flanks. Whereas on spur gears, fretting corrosion will appear in a horizontal line along the tooth flanks.

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.

Progression

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.

Detectability

MethodDetection EfficiencyNotes
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 dataSCADA data does not usually aid detection of fretting corrosion.
Oil debris sensorFretting corrosion does not shed much debris so cannot usually be detected using oil debris sensors.
Oil sample analysisFretting corrosion does not shed much debris so cannot usually be detected using oil sample analysis.

Discussion

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.

Severity Rating

RankDescriptionDetectionRecommended Action
1Standstill marks. Long, light black markings with no depth, likely caused by prolonged periods of idling during commissioning. Generally arrests after run-in.Visual, borescopeNone – Run turbine as normal
2Emergence of thicker wear markings, with some indication of depth. Signs of oxidation, with reddish-brown appearance.Visual, borescopeNone – Run turbine as normal
3Substantial marking with depth, evidence of reddish-brown oxidation or emergence of micropitting. Likely progressive.Visual, borescopeRun turbine as normal. Increase inspection frequency and monitor for progression to macropitting.
4N/AN/AN/A
1
Example of rank 1 fretting corrosion (a gear failure)
2
Example of rank 2 fretting corrosion (a gear failure)
3
Example of rank 3 fretting corrosion (a gear failure)
4
Progresses to other failure modes
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