High temperature corrosion in energy systems

Abstract

At high temperatures, particularly in response to the unique environments associated with the conversion or combustion of fossil fuels, further fundamental studies of alloy reactions with mixed gaseous oxidants are required. Thermodynamic, phase equilibria and diffusion data are lacking in part, and isotope and tracer studies have not been forthcoming. Corrosive thin films of salts and slags on the hardware of gas turbines, heat exchangers, fuel cells and batteries cause an accelerated “hot corrosion”. Thin film electrochemical studies for simple salts and alloys, and supporting thermodynamic studies (solubilities of solids and gases in salts), are required to understand the corrosion mechanism. The effects of several trace gaseous impurities (particularly chlorine) both on the growth and damaging of protective oxide scales and on the degradation of alloy mechanical properties should be studied. High resolution in situ scanning electron microscopy studies could prove fruitful in clarifying uncertain scale growth mechanisms. New protective coating compositions must be found for specific corrosive environments, and more reliable but less expensive coating methods are required. Factors critical to the adhesion of oxide scales (e.g. α-Al 2O 3 and Cr 2O 3) on alloys, and the effects of trace alloying elements or dispersed second phases on scale adherence, deserve further attention. The effect on gas-alloy attack of solid deposits, either reactive or relatively inert, should be examined. Electrochemical studies should be made of alloy corrosion in deep salt melts or slags, where the gaseous environment is remote from the alloy surface. The role of grain boundaries in corrosion product scales as short-circuit transport paths for the outward diffusion of metal and the inward ingress of oxygen, sulfur and carbon needs to be clarified. Erosion-corrosion interactions should be studied, with attempts to define the types of coatings that are most resistant to such conditions. Particularly in solar applications, the role of thermal cycling and cyclic stressing on high temperature scaling (corrosion fatigue) needs to be studied. New methods for the monitoring of the concentrations of corrosive components, particularly sulfur and chlorine, in gaseous and fused salt environments require development. The influences of temperature gradients and heat fluxes on material compatibility, redistribution of chemical components and properties of corrosion product layers need further study. High temperature corrosion-resistant alloys excluding the strategic metals chromium and cobalt need to be developed.