PER ASTM G 15, CORROSION IS "THE CHEMICAL OR ELECTROCHEMICAL REACTION between a material, usually a metal, and its environment that produces a deterioration of the material and its properties." Corrosion is one of the causes of premature failure of a lubricated system. Fuels and lubricant will be exposed to water, one of the necessary components for electrochemical corrosion to occur. This chapter will outline the nature of ferrous and non-ferrous corrosion and the common additive approach to inhibiting corrosion and will summarize the common tests employed to evaluate the effects and prevention of corrosion.