Gear Failure

Gear Corrosion

Corrosion, commonly known as rusting, is a natural chemical or electrochemical reaction between a gear and its environment. This article concerns oxidisation-based corrosion - not fretting corrosion.

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

Overview

Gear corrosion, or oxidisation, is a natural chemical or electrochemical reaction that leads to the eventual decay of the gears through surface materials 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. Gear corrosion is generally found on non-functional areas, but it may also occur on active gear teeth. Signs of this include yellow or reddish-brown stains of various size and shape.

If the operating conditions are not changed, corrosion will be progressive. Essentially. gear gear corrosion will etch contact surfaces, promoting flaking and the formation of corrosion pits. This will eventually lead to macropitting and fracture. Premature corrosion due to moisture should not be confused with fretting corrosion due to minute vibratory motion.

Causes of gear corrosion

Corrosion occurs when gear 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. Even excessive handling of components 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 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.

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Appearance

Gear corrosion stains and marks

Figure 1: Mild corrosion – randomly distributed superficial surface stains without depth.

Initially, gear corrosion will result in the formation of randomly dispersed yellow or reddish-brown rust stains of various shape and size. These can occur on both loaded gear flanks and inactive parts of the gear. Mild corrosion results in light surface stains without any appreciable depth, nor does it obscure the original grinding marks.

But in more serious cases, or if gear corrosion is left untreated, it will begin to eat into the gear surface. 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 to reveal any corrosion pits which may have developed.

Progression of gear corrosion

In cases of mild surface staining without any appreciable depth, corrosion may simply arrest (provided that moisture ingress is halted).

However, corrosion is more 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 macropmacroitting and, eventually, fracture.

Debris particles generated by the flaking process could also cause abrasive wear elsewhere in the gearbox.

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Detectability

Method

Detection Efficiency

Notes

Visual inspection

Easy

Corrosion marks should be visible on gears with a clear line of sight. Else use a borescope.

Borescope inspection

Easy

Gear corrosion marks will be identifiable using a borescope.

Vibration analysis

Medium

Moderate gear corrosion which has resulted in flaking and the formation of corrosion pits may be detectable by vibration.

SCADA data

Hard

SCADA data does not aid detection of corrosion.

Oil debris sensor

Medium

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

Easy

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 the fatigue life of gears.

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 system that either actively removes moisture or prevents further moisture ingress. Figure 2 shows both a commonly used, simple breather and a typical desiccant breather which is best practice to reduce moisture ingress.

Figure 2: 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.

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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.

Corrosion – gear failure severity ranking 2

Detection:

Visual, borescope

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.

Corrosion – gear failure severity ranking 3

Detection:

Visual, 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.

Rank 4 Progresses to other failure modes.

Detection:

N/A

Recommended action:

N/A