Fundamental aspects of high-temperature corrosion

Abstract
Some recent considerations in three widely different aspects of high-temperature corrosion are summarized: 1) reactions at the metal/scale interface in support of scale growth; 2) mass transfer effects in the control of evaporation of volatile reaction products; and 3) the codeposition of multiple elements for diffusion coatings using halide-activated cementation packs. The climb of misfit edge dislocations from the metal/scale interface can achieve the annihilation of vacancies associated with scales grown by cation diffusion. For scale growth at the metal/scale interface by anion diffusion, the creation of anion vacancies occurs by the climb of misorientation dislocations. For cation-diffusing scales, a blocking of the vacancy annihilation reaction may be achieved by the specific adsorption of reactive elements at the metal/scale interface. The pinning of the interfacial misfit dislocations by segregated reactive elements can explain the changes in growth mechanism, in growth stresses, and in reaction kinetics associated with the reactive element effect for the growth of chromia scales on alloys. Although the vapor pressures of volatile species are readily calculated from available thermodynamic data, the use of these values to predict the kinetics of transport-limited evaporation has not been presented specifically for various substrate geometries and differing flow conditions. The empirical mass transfer relations are evaluated and plotted for the flat plat and tube geometries in the laminar and turbulent flow regimes. Acriterion for the stability of a flat interface in selective evaporation from an alloy is still lacking. Despite the inherent difficulty to codeposit two or more elements with greatly differing thermodynamic stabilities for their halide species, cementation coating packs can be designed with selected combinations of powdered masteralloys and halide activator salts whereby elements are codeposited to yield a desirable surface composition. Selected examples for effective aluminizing/chromizing of Ni-base superalloys and for chromizing/siliconizing of steels are presented.