A metal in a gas atmosphere at high temperature is a complex chemical system. The metal usually is not pure but contains in addition to metallic impurities O, N, H, C, S, etc. The gas atmosphere also is usually complex, containing (in addition to O2) N3, H2, CO3, H3O, etc. A wide variety of chemical and physical phenomena occur in and on the metal surface in a typical gaseous environment at high temperature. The various phenomena are systematized into five zones of activity: metal, metal-oxide interface, oxide, oxide-gas interface, and gas.
The general concepts are considered by setting down classification schemes of diverse observations and phenomena. Eight classification systems are given, then discussed. These are (1) type of reaction, (2) sign and magnitude of free energy change, (3) extent of reaction, (4) crystal structure, (5) empirical and theoretical rate laws, (6) physical and chemical properties of metal and oxide, (7) topology of metal and reaction product and (8) effect of reaction on metal or environment.
