Among the widely used antioxidants are phenolic types and zinc dithiophosphates. The former are considered to be of the chain-breaking variety, whereas the latter are believed to be peroxide destroyers. The corrosion of bearing metal is generally considered to be due largely to reaction of the acid with the oxides of the bearing metal. In engine operation, these acids either originate from products of incomplete fuel combustion that find their way into the lubricant as blowby gases or are produced from lubricant oxidation. Oxidation inhibitors can significantly reduce this tendency.
Detergents can reduce bearing corrosion by neutralizing the corrosive acids.
Other inhibitors such as zinc dithiophosphates and phosphosulfurized olefins not only inhibit oxidation but also form a protective film on the bearing surface, making it impervious to acid attack. The inhibitor efficiency of phenol is markedly increased by substituting alkyl groups in the two ortho and para positions. It is particularly enhanced when the ortho substituents are bulky groups such as tert-butyl and the para substituent is a primarily alkyl group. A variety of hindered phenols are produced commercially and employed as inhibitors in transformer, turbine, and engine oils. The methylenebis structure is more effective in high-temperature applications due to its lower volatility characteristics compared to the other molecule.