Corrosion and dangers of engine bearings
Corrosion is a common failure mode in bearings. In the ISO standard failure analysis classification, corrosion is categorized as a type of chemical change.
Typical bearing corrosion is essentially an oxidative change triggered by moist conditions in the air reacting with the bearing steel.
If this oxidation occurs within the bearing race, during bearing operation, the rolling elements will pass over the corroded area, resulting in noise and vibrations.
The failure will then continue to spread from the corroded area, disrupting lubrication and forming a vicious cycle with the surrounding track.
If corrosion occurs outside the raceway, if oxidation continues to develop, it can affect the strength of mating bearing components and can potentially lead to fracture of the bearing ring under load.
Engine bearing corrosion diagnosis
Generally, bearing corrosion occurs on non-operating parts of the bearing or when the bearing is not in operation. The operating surfaces of bearings do not normally rust during operation.
Consequently, in bearings that operate frequently, corrosion occurs mainly on non-operating surfaces, while on operating surfaces, corrosion usually occurs when the bearing is idle.
From this we can derive the following:
If the raceway shows signs of corrosion and the traces of corrosion match the spacing between the rolling elements, bearing corrosion likely occurred while the machine was idle.
At this point, bearing corrosion may have started before the bearing was installed or may have occurred when the bearing was not in operation after installation.
Typically, bearings are coated with a layer of anti-rust oil before leaving the factory. This oil protects the bearings during storage. If the anti-rust oil fails and condensation occurs at the contact points between the rolling elements and the raceway, corrosion can easily occur.
Likewise, when engine bearings are put into use, they are usually filled with lubricating grease.
Many types of grease contain antioxidant components, and the grease also prevents the bearing steel from coming into contact with oxygen, providing a certain level of rust prevention.
Once installed, corrosion is unlikely to occur in the raceways between the rolling elements unless there is a problem with the grease application.
Another situation is when the bearing's working environment is very humid, the water content in the grease increases and corrosion may occur between the rolling elements when the bearing is idle.
On the surfaces of the bearing's outer and inner rings, which are contact surfaces with the shaft and bearing housing, corrosion should not occur under normal conditions.
However, if fretting corrosion appears, it needs to be carefully discerned, since both fretting corrosion and rust leave similar traces of iron oxidation on the surface.
The end faces of bearing rings do not normally participate in load bearing and may not necessarily be protected by grease. Therefore, in humid storage and operating environments, rust may occur.
Measures to prevent corrosion in bearings
Firstly, the bearing storage environment must be strictly managed. Generally, humidity should not be too high and specific humidity requirements can be obtained from the bearing manufacturer.
Furthermore, the packaging of bearings must generally be kept intact. Any damaged packaging must be covered immediately to prevent moisture from entering.
For bearings with anti-rust oil, it is generally not recommended to clean the oil. The anti-rust oil used by most manufacturers is compatible with various grease lubricants and can be directly refilled with lubricating grease.
Bearings should only be unpacked immediately before use, and care should be taken to manage condensation caused by changes in ambient temperature during bearing operation.
In humid working environments, in addition to ensuring adequate lubrication to protect bearings from the threat of corrosion, seal design must also be taken into consideration. For liquid contamination or vapor pollution, appropriate seals must be used for protection.
Protection against condensation caused by some specific processes must be provided.
For example, some engine factories use a cold installation method, cooling the bearings to a certain temperature before installation.
The most crucial aspect of this operation is to avoid condensation when the cold bearing meets air. Although condensation on the outer surface of the bearing is easy to deal with, internal condensation is difficult to clean.
