Degradation may result in oil thickening, increased acidity, sludge formation, or a combination of these. Corrosion usually occurs from organic acids that develop in the oil and from contaminants picked up and carried by the oil. Detergents, dispersants, and corrosion inhibitors help neutralize the acids and keep the system clean. Rust inhibitors can be used in most types of oils, but the selection must be made carefully to avoid corrosion of nonferrous metals or the formation of emulsions with water. Because rust inhibitors are adsorbed on metal surfaces, they are depleted with time, particularly in hot systems.
Detergents chemically neutralize deposits that form under high temperature conditions.
Dispersants are chemical compounds that disperse or suspend potential sludge-forming material in the oil. There is no sharp line of demarcation between detergents and dispersants. They help to prevent the deposition of lacquer and varnish resulting from oil oxidation. These materials are miscible in oil, and the correct choice of polymer size is critical if settling during long-term storage is to be avoided. Excessive use of defoamants may increase air entrainment in the oil.