Rust is caused by external moisture being brought into the bearing; or when the equipment is out of service, the bearing temperature is below the dew point, and the moisture in the air condenses into water droplets and adsorbs on the bearing surface; and the equipment is working in corrosive media and the bearing is not tightly sealed. This causes chemical corrosion. Rust spots caused by rust cause early spalling on the bearing surface and also aggravate wear.