Thermal stability, oxidation resistance and temperature extremes must be considered when selecting a grease for high temperature applications. In non-relubrication applications, it is very important to choose refined mineral oil or stable synthetic oil as the base oil when the operating temperature is above 121C. Table 28. Grease Temperature Range Contaminants Abrasive Particles When rolling bearing types are operated in a clean environment, the main source of bearing damage is fatigue of the rolling contact surfaces. However, when particulate contamination enters the bearing system, it can cause damage such as galling, a phenomenon that shortens bearing life. Wear becomes a major cause of bearing damage when contaminants in the environment or metal burrs on certain components in the application contaminate the lubricant. If, due to particulate contamination of the lubricant, bearing wear becomes significant, critical bearing dimensions can change, which can affect machine operation. Bearings operating in contaminated lubricants have higher initial wear rates than non-contaminated lubricants. However, this rate of wear decreases quickly when there is no further intrusion of the lubricant, as the contaminants shrink in size as they pass through the bearing contact surfaces during normal operation. Moisture and humidity are important factors in bearing damage. Grease can provide a protection against such damage. Certain greases, such as calcium complex and aluminum complex greases, have extremely high water resistance. Sodium based greases are water soluble and therefore cannot be used in applications containing water. Both the dissolved water and the suspended water in the lubricating oil can have a fatal impact on the bearing fatigue life. Water can corrode bearings, and corrosion can reduce bearing fatigue life. The exact mechanism by which water reduces fatigue life is not fully understood. But it has been suggested that water can enter microcracks in the bearing raceways, which are caused by repeated cyclic stress. This leads to corrosion and hydrogen embrittlement of microcracks, greatly reducing the time required for these cracks to grow to unacceptable crack sizes. Water-based fluids such as water glycol and converted emulsions have also shown a reduction in bearing fatigue life. Although the water from which it is derived is not the same as contaminated water, the results support previous arguments about water contaminating lubricants.