Corrosive wear in the presence of a gear lubricant may be due to the environment if air, water , or electrolytes are present. If gear cases are not tight and high humidity prevails, rusting may occur, not only on idle gears above the oil line, but also on the walls of the gear case. If necessary, rust preventive compounds can be added to gear oils to counteract the action of moisture. Such additives may be polar compounds, often containing long chains, which will be adsorbed at the metal oil interface to form hydrophobic films. Prevention of corrosion due to electrolytes may be more difficult than prevention of rusting. However, if the contaminant is salt, the same types of additives as mentioned above will aid in corrosion prevention. If water soluble acids entering the gear case cannot be prevented, ordinary gear oils will not serve to prevent corrosion. In this case it may be necessary to use gears of different composition. Stainless steel will resist most acids and some electrolytes. High silica irons, while somewhat brittle, also have this faculty.
The corrosive wear most apt to occur in gear operations is that due to chemical additives, known as EP agents. The secret of a satisfactory EP gear oil is to obtain controlled corrosion so that welding of the metal surfaces will not take place and yet asperities will be reduced. In the case of most EP gear oil compositions corrosive wear should not be excessive and is actually beneficial in that it extends the life of the gears under extreme operating conditions
