David Stevens IEng - Bearing failures

Bearing CORROSION and its Causes.

Corrosion

Rust will form if water or corrosive agents reach the inside of the bearing in such quantities that the lubricant cannot provide protection for the steel surfaces. This process will soon lead to deep seated rust. Another type of corrosion is fretting corrosion.

Deep seated rust.

A thin protective oxide film forms on clean steel surfaces exposed to air. However, this film is not impenetrable and if water or corrosive elements make contact with the steel surfaces, patches of etching will form. This development soon leads to deep seated rust.

Deep seated rust is a great danger to bearings since it can initiate flaking and cracks. Acid liquids corrode the steel quickly, while alkaline solutions are less dangerous. The salts that are present in fresh water constitute, together with the water, an electrolyte which causes galvanic corrosion, known as water etching. Salt water, such as sea water, is therefore highly dangerous to bearings.

Appearance	Greyish black streaks across the raceways, mostly coinciding with the rolling element spacing. At a later stage, pitting of raceways and other surfaces of the bearing.
Cause	Presence of water, moisture or corrosive substances in the bearing over a long period of time.
Action	Improve sealing. Use lubricant with better rust inhibiting properties.

Deep seated rust in the outer ring of a deep groove ball bearing.

Figure 1 - Deep seated rust in the outer ring of a deep groove ball bearing.

Fretting corrosion

If the thin oxide film is penetrated, oxidation will proceed deeper into the material. An instance of this is the corrosion that occurs when there is relative movement between bearing ring and shaft or housing, on account of the fit being too loose. This type of damage is called fretting corrosion and may be relatively deep in places. The relative movement may also cause small particles of material to become detached from the surface. These particles oxidise quickly when exposed to the oxygen in the atmosphere. As a result of the fretting corrosion, the bearing rings may not be evenly supported and this has a detrimental effect on the load distribution in the bearings. Rusted areas also act as fracture notches.

Appearance	Areas of rust on the outside surface of the outer ring or in the bore of the inner ring. Raceway path pattern heavily marked at corresponding positions.
Cause	Fit too loose. Shaft or housing seating with errors of form.
Action	Adjust seatings.

Fretting corrosion on the outer ring of a deep groove ball bearing.

Figure 2 - Extensive fretting corrosion on the outer race of a deep groove ball bearing.

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