The unwanted wastage of metallic materials due to reaction with the environment. The effect includes the loss of strength of material, a change in appearance, change in surface heat transfer and fluid flow properties, contamination, seizure, electrical contact failure, leakage, and general surface damage. Corrosion rates are determined to a large extent by the chemical nature of the process stream and its pressure and temperature; due account must be taken of the flow conditions, and how they interact with the ongoing chemical processes. The electrochemical corrosion process of rusting involves the oxidation of iron to form a hydrated iron oxide that occurs in the presence of both water and oxygen, and is particularly damaging to process equipment and support structures. Compare erosion. In some cases, the corrosive action of the environment can be reduced through the use of chemicals known as corrosion inhibitors. Cathodic corrosion inhibitors include oxygen scavengers such as sodium sulphite used in enclosed systems such as boilers, and ions such as Ca²⁺, Mg²⁺, HCO₃^-, and Zn²⁺, which form insoluble precipitates at the cathodic (alkaline) surface. Anodic corrosion inhibitors are used to encourage oxidized passive films of surfaces using an oxidizing agent such as chromates (Cr^VI).