Growth and Mass Transport in Ceramic Type Protective Scales on Metals

Abstract

A survey is presented of growth and mass transport in ceramic type protective scales on metals and alloys at high temperatures by lattice and short-circuit diffusion of metal and oxygen through low resistance diffusion paths in oxide boundaries. A methodology based on correlating phenomenological theory of oxidation with reactant short-circuit diffusion across reaction product layers is discussed in some detail for Cr2O3, Al2O3 and SiO2 scales on the pure metals and on iron and nickel alloys because independent measurements of diffusivities by isotopes are available to confirm the oxidation models. Isotope studies using sequential oxidation in O16 and O18 have been definitive in describing oxide growth within protective Cr2O3 and Al2O3 scales. Mechanisms relating loss of scale adherence to the metal substrate with the formation of new oxide within these protective scales are discussed and they are shown to remain open to speculation.