Fretting Corrosion – Gear Failure

Contributing factors to fretting corrosion

In a stationary wind turbine, even with the rotor lock engaged the gears can experience a rocking motion. This action generates repeated vibratory micro-movements, causing the metal surfaces of the gears to intermittently come into and out of contact with one another. This interaction, called fretting, effectively displaces the lubricant that is typically present between the gear surfaces, leading to direct metal-to-metal contact at localised high points referred to as asperities. The absence of lubricant facilitates this contact and results in the oxidation of these asperities. The wear patterns that emerge from this phenomenon are collectively known as fretting corrosion.

Appearance of fretting corrosion

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.

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.

Failure 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.

Detection of fretting corrosion damage

Discussion

Mild fretting corrosion caused by a 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 gently wipe it 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 the 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

Rank 1 Standstill marks. Long, light black markings with no depth, likely caused by prolonged periods of idling during commissioning. Generally arrests after run-in.

Detection:

Visual, borescope

Recommended action:

None – Run wind turbine as normal

Rank 2 Emergence of thicker wear markings, with some indication of depth. Signs of oxidation, with reddish-brown appearance.

Detection:

Visual, borescope

Recommended action:

None – Run wind turbine as normal

Rank 3 Substantial marking with depth, evidence of reddish-brown oxidation or emergence of micropitting. Likely progressive.

Detection:

Visual, borescope, vibration

Recommended action:

Run wind turbine as normal. Increase inspection frequency and monitor for progression to macropitting.

Rank 4 Progresses to other failure modes

Detection:

N/A

Recommended action:

N/A