Ellis et al.^22 consider that staining, tarnishing, and rusting are all indications of corrosion. The thought is that light stain or tarnish represents the early stages of corrosion since, unfortunately these changes do not proceed very far before pitting starts. Unreactive gear oils, which have not been subjected to excessive high temperature oxidation, have no tendency to corrode metals but, under moist or humid conditions or in the presence of most salts or acids, do not offer proper protection against rusting of ferrous metal surfaces such as gears. However, additives can be included in gear compounds which will provide rust prevention. Where conditions of incipient rusting prevail, the gears and other metal parts even to the inside of the gear case may require protection, particularly when idle. In such cases not only will the presence of a rust inhibitor but also the viscosity of the base oil be factors. Thus, the higher the viscosity of the gear lubricant, the slower this will drain from the metal surfaces and consequently the greater the rust prevention. Rusting may occur in different environments and various theories are offered as to the mechanisms of corrosion, but normally moisture and oxygen are the offenders
Most rust preventives are polar substances, such as long chain fatty acids, fatty amines, metal sulfonates, certain esters, oxidized petroleum fractions, etc. Such materials wet a metal surface preferentially and displace any water which may come in contact with the steel. The coating of polar substance then acts as a barrier against water reaching the metal surface.
As previously mentioned, controlled corrosion due to EP additives is generally beneficial in that it corrodes away high spots on the gear teeth after which corrosion may decrease. With the proper selection of the chemical agents, these are not activated except under extreme conditions of load and /or temperature. Further, most of the EP additives which are used in gear lubricants will have little effect upon other metals such as bronze, copper, etc; at the bulk oil temperatures maintained in normal gear operation .
