The protective action of oxide scales in gaseous environments at high temperature

Some of the main principles which determine the protective nature of oxide scales on metals in oxidising environments at high temperature are considered and discussed with particular respect to NiO scales on nickel and Cr₂O₃ and Al₂O₃ scales on high-temperature alloys. Emphasis is given to the transport processes in the scales and the importance of short-circuit diffusion paths. The development of stresses during oxidation are discussed and related to the effectiveness of the scales in resisting mechanical breakdown. Relief of these stresses can occur by deformation of the oxide or metal substrate but, in certain circumstances, may result in fracture and/or loss of adhesion of the scale followed by spalling from the metal surface, sometimes with catastrophic consequences.