Corrosion – Bearing Failure

Causes of bearing corrosion

Corrosion occurs when bearing surfaces come into contact with moisture. This can happen in a variety of circumstances, some of which include:

• Improper storage: Static corrosion of components during storage, prior to installation, is a real concern. Unused components should be stored in a dry, low humidity area in the original packaging and according to the manufacturer’s recommendations.
• Maintenance: Moisture can be accidentally introduced to the gearbox during routine inspection, or up-tower replacement of the high speed stage bearings. Even excessive handling of bearings with moist hands can facilitate the development of corrosion. Therefore, unnecessary handling of drivetrain components should be kept to a minimum and suitable gloves worn. Care should be taken not to accidentally spill liquids into the gearbox during inspection.
• Moisture ingress: Gearboxes breathe as a result of changing temperatures during operation. This allows water vapour an opportunity to enter the gearbox through the breather or damaged seals. This is exasperated in humid environments. Salt water ingress is particularly damaging, so additional precautions should be taken for offshore wind farms. An industry best practice is to use a desiccant breather system to reduce water ingress.
• Inadequate lubricant formulation: Extreme pressure (EP) additives are used in heavily loaded, slow moving components (such as main bearings) to help reduce friction and wear. However, this is achieved through the formation of a mildly corrosive protective layer. As such, the additive concentration is critical: if too high, it can cause excessive corrosion. Moisture which enters the gearbox may mix with the lubricant and form an emulsion with drastically decreased load carrying capacity. Corrosion is critically related to water content, so monitoring this value is a very important indicator for corrosion development. Finally, chemical decomposition of the lubricant can lead to its corrosive acidification.

Appearance of bearing corrosion in wind turbines

Initially, corrosion will result in the formation of randomly dispersed yellow or reddish-brown stains of various shapes and sizes. These can occur on both loaded rollers and raceways, as well as roller end faces. Mild corrosion will result in light surface stains without any appreciable depth. In more serious cases, the corrosion will begin to eat into bearing surfaces. This will etch the surface and result in the detachment of flakes of rusted material. Such material removal results in the formation of corrosion pits.

These pits tend to be shallow and irregularly shaped, with a darker reddish-black appearance. Try to wipe away surface stains, if possible, to reveal any corrosion pits which may have developed.

How can bearing corrosion progress?

In cases of mild surface staining without any appreciable depth, corrosion may arrest provided moisture ingress is halted. However, corrosion is generally progressive and spreads quickly across already affected surfaces.

Flaking which leads to the formation of corrosion pits on contact surfaces is cause for serious concern. This alters the contact surface geometry, in turn causing stress concentrations around the damage which will promote the formation of macropitting and, eventually, failure. Debris particles generated by the flaking process could cause abrasive wear elsewhere in the gearbox.

Detectability of bearing corrosion

Conclusion

Bearing corrosion is a significant and progressive issue that can lead to serious failure if not properly addressed. Most environments, whether caused by poor sealing or other environmental factors, accelerates the oxidation of bearing surfaces, creating rust and pitting that can ultimately result in macropitting and other forms of damage.

To prevent such failures, it is crucial to identify early signs of corrosion, implement proper maintenance strategies, and enhance protection against moisture and vibratory wear. By taking proactive measures, wind turbine operators can extend the life of their bearings and avoid costly downtime and repairs​.

Stainless steel bearings can also be used to prevent corrosion to a certain extend, however if used in offshore wind turbines, exposure to sea water causing damp environments means additional anti-corrosion treatment such as a protective coating is still required. In extremely corrosive environments, a ceramic bearing or plastic bearing may be suggested.

Severity rating

Rank 1 N/A

Detection:

N/A

Recommended action:

N/A

Rank 2 Corrosion staining. Randomly distributed superficial surface stains without depth. May wipe away. No flaking or pits. Grinding lines still visible. May arrest if moisture ingress is halted.

Detection:

Borescope, oil debris sensor, oil sample analysis

Recommended action:

Run turbine. Seek to identify and stem source of moisture ingress. Consider fitting desiccant breather, if not already used. Increase inspection frequency.

Rank 3 Corrosion pitting. Corrosion has advanced to rust flaking, resulting in the formation of pits. Has depth and has removed grinding marks. Progressive.

Detection:

Borescope, oil debris sensor, oil sample analysis

Recommended action:

Run turbine. Seek to identify and stem source of moisture ingress. Consider fitting desiccant breather, if not already used. Increase inspection frequency and monitor for progression to macropitting.