Bearing Failure

Fretting Corrosion Bearing Failure

Fretting corrosion occurs when two loaded mating surfaces undergo repeated micro-motion, leading to the formation of an oxidized flat spot at the contact points. This article explains how fretting corrosion can be identified, how the failure mode progresses, and how it can be detected.

Also referred to as:
False Brinelling, Friction Oxidation, Chafing, Fretting Wear, Vibration Corrosion

Fretting corrosion – bearing failure severity rating 2

An overview of fretting corrosion

Fretting corrosion (also known as false brinelling) is an oxidised flat spot that develops when two mating metal surfaces under load experience repeated micro-motion between the contacting surfaces.

Under these conditions, surface asperities can oxidise and then be rubbed from the surface. Fretting corrosion can be seen on a low-angle oscillatory motion for in-service bearings, for example, pitch bearings.

Although it is seen on both bearing raceways and the rolling elements, it is often more prominent on the raceways. Fretting corrosion should not be confused with common corrosion formed through moisture on free surfaces. It should also not be confused with true brinelling, which although looks similar, is a different failure mechanism (depression of the original surface rather than wearing away the surface).

Contributing factors to fretting corrosion

Fretting corrosion is caused either by machine vibration, or by the micro-motion that exists when the bearings in a stationary turbine rock back and forth. It can also be caused by low-angle oscillatory motion on an in-service bearing, particularly pitch bearings.

The bearing lubrication is squeezed out from between the contacting surfaces allowing metal-to-metal contact. This can cause the material to oxidise and eventually be removed from the surface. Serious forms of fretting corrosion are caused by alterations in the rolling profile (e.g. flat spots) that can act as stress concentration points.

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Appearance of fretting corrosion

Fretting Corrosion of a Wind Turbine Main Bearing

When fretting corrosion occurs, oxidation lines will be visible along the contact patch of the bearing. For a rolling bearing raceway, these will be along the width of the raceway. For a cylindrical rolling element, they will be along the length of the roller. Matte, non-directional (no original finishing marks) lines are common and representative of mild fretting corrosion (also known as false brinelling) caused by micro-motion.

In black oxide bearings (see polishing for black oxide wear), mild fretting corrosion marks will appear of a lighter colour than the reddish bearing surface. Although not as common as gears, some dark red oxides can appear around and over the lines which represents a more progressed form of fretting corrosion. Micropitting and macropitting may occur on the edges of progressed fretting corrosion. This is possible when the bearing surface has been significantly altered to cause a stress concentration.

Progression of the failure

Mild fretting corrosion (also known as false brinelling) caused by standstill is relatively common and generally non-progressive. However, more serious forms of corrosion can be progressive and lead to micropitting and eventually macropitting along the edges of the line.

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Detectability

Method

Detection Efficiency

Notes

Borescope inspection

Easy

Fretting corrosion is readily observed and distinguishable with a borescope.

Visual inspection

Medium

If accessible, the naked eye can detect some fretting corrosion due to the change in colour on the bearing surface, however more subtle appearances may need a borescope.

Vibration analysis

Medium

Mild cases of fretting corrosion will not be detectable using vibration data, however more severe cases of fretting corrosion can be detected.

SCADA data

Not applicable

SCADA data does not usually aid detection of fretting corrosion.

Oil debris sensor

Not applicable

Fretting corrosion does not shed much debris so cannot usually be detected using oil debris sensors.

Oil sample analysis

Not applicable

Fretting corrosion does not shed much debris so cannot usually be detected using oil sample analysis.

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Discussion

Mild fretting corrosion (i.e. false brinelling) is relatively common and is likely to be found in some gearbox bearings. This is not generally a cause for concern, unless signs of progression are observed. Signs of progression include a reddish appearance and pitting on the edges. Ensuring the bearing surfaces are lubricated as much as possible will help reduce the chance of fretting corrosion. This can be done by avoiding long periods of non-running of the turbine and taking care during transportation.

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Severity rating

Rank 1 False brinelling: Mild, superficial, markings with no depth. Thin and black/grey in colour.

Detection:

Visual or borescope

Recommended action:

None – Run turbine as normal.

Rank 2 Emergence of thicker wear markings, with some indication of depth. Fretted, matte appearance in direction of rolling. No signs of micropitting

Detection:

Visual or borescope

Recommended action:

Run turbine but increase inspection frequency and monitor for progression.

Rank 3 Fretting corrosion: Reddish brown and black marking from the oxidation with depth. Emergence of micropitting on or around the fretting corrosion. Possible emergence of macropitting, but no coalescence into spalling. Likely progressive.

Fretting corrosion – bearing failure severity rating 3

Detection:

Visual, borescope and vibration

Recommended action:

Run turbine but consider scheduling replacement of component. Monitor for progression to macropitting.

Rank 4 Progresses to other failure modes

Detection:

N/A

Recommended action:

N/A