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Corrosion

Corrosion – Bearing failure

Corrosion, commonly known as rusting, is a natural chemical or electrochemical reaction between a bearing and its environment. It leads to the eventual decay of bearings through surface material being converted to oxides. While inevitable, the rate of such decay is drastically increased by moisture, which can be present within a gearbox for a variety of reasons (such as poor sealing). Due to the high degree of surface finish, bearing rollers and raceways are particularly susceptible to corrosion. However, corrosion can also occur on unloaded areas, such as roller end faces. It produces yellow or reddish-brown stains of various size and shape. If the operating conditions are not changed, corrosion will be progressive. It will etch contact surfaces, promoting flaking and the formation of corrosion pits. This will eventually lead to macropitting. Premature corrosion due to moisture should not be confused with fretting corrosion due to minute vibratory motion.

Also referred to as: Rusting, Oxidation, Etching, Water Damage

Click here for: Corrosion – Gear failure

Causes

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, unneccessary 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

Initially corrosion will result in the formation of randomly dispersed yellow or reddish-brown stains of various shape and size. 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.

Progression

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 accross 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

MethodDetection EfficiencyNotes
Visual inspectionDue to the location of bearings, a clear line of sight for inspection by naked eye is unlikely to be possible. Use endoscope instead.
Borescope inspection✓✓Corrosion marks will be identifiable using a borescope. In some instances it may be difficult to distinguish mild corrosion staining from oil on the bearing rollers or raceways.
Vibration analysis✓✓Moderate corrosion which has resulted in flaking and the formation of corrosion pits may be detectable by vibration.
SCADA dataSCADA data does not aid detection of corrosion.
Oil debris sensor✓✓Debris particles of substantial size produced from flaking and the formation of corrosion pits may be detected by an oil debris sensor.
Oil sample analysis✓✓✓Oil sample analysis will identify increased water content in the lubricant and evidence of debris particles produced from corrosion pitting.

Discussion

Regular oil and grease samples should be collected and the water content of the lubricant analysed. It is well known that elevated water content within the lubricant has an effect on bearing fatigue life.

While efforts can be made to remove excessive moisture from a gearbox, good design which limits moisture ingress in the first place is desirable. Rust and oxidation inhibiting (R&O) lubricant additives should be used. If there are signs of corrosion, it is recommended to retrofit a breather system that either actively removes moisture or prevents further moisture ingress. Figure 1 shows both a commonly used, simple breather and a typical desiccant breather which is best practice to reduce moisture ingress.

Figure 1: a) A simple type of breather commonly used on wind turbines; b) A typical desiccant breather used to remove water at the air inlet port.

Severity Rating

RankDescriptionDetectionRecommended Action
1N/AN/AN/A
2Corrosion 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.Borescope, oil debris sensor, oil sample analysisRun turbine. Seek to identify and stem source of moisture ingress. Consider fitting desiccant breather, if not already used. Increase inspection frequency.
3Corrosion pitting. Corrosion has advanced to rust flaking, resulting in the formation of pits. Has depth and has removed grinding marks. Progressive.Borescope, oil debris sensor, oil sample analysisRun 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.
4N/AN/AN/A
1
Not applicable
2
Example of rank 2 corrosion (a bearing failure)
3
Example of rank 3 corrosion (a bearing failure)
4
Progresses to other failure modes
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