Q. Define Oxidation Process?
Oxidation is defined as a loss of electrons, as when a metal goes from the metallic state to the corroded state. When a metal reacts with oxygen, it forms an oxide and the metal is said to be oxidized. Elevated temperatures can significantly increase the rate of oxidation.
Generally speaking, scale growth due to oxidation is proportional to the square root of the exposure time (i.e. it is a parabolic relationship). Three different forms of iron oxide may form, depending on the temperature and the concentration of oxygen available, FeO (wustite), Fe2O3 (hematite) and Fe3O4 (magnetite). The growth of the oxide scale is based on the diffusion rate of oxygen through this layer, which can then continue to react with the iron. In order for the scale to provide a protective layer, the oxide must be tightly adherent and occupy a greater volume than the metal from which it is formed.
The addition of alloying elements such as chromium (Cr), nickel (Ni), as well as small quantities of silicon (Si) and aluminum (Al) improves the oxidation resistance of iron-base alloys. The formation of a stable oxide layer on high temperature materials is a significant factor in providing an acceptable service life. Service conditions that are likely to disrupt the formation of such scales are detrimental (e.g. reducing atmospheres). Unexpected catastrophic failures from oxidation are very unlikely for most high temperature alloys.