In situ environmental scanning electron microscope observations of catalytic processes encountered in metal dusting corrosion on iron and nickel

Abstract

High temperature corrosion of pure iron and nickel has been studied in situ with an environmental scanning electron microscope (ESEM). Metal samples were exposed to methanol vapour and hydrogen–carbon monoxide gas mixtures at temperatures from 773 to 1073 K. Carbon filaments were formed on the iron metal surface at 773 K, and the extent of corrosion was found to depend on the gaseous environment. The precipitation of surface carbides and the catalytic gasification of carbon filaments on iron were followed at high temperatures. Nickel was apparently catalytically inert at lower temperatures but showed heavy surface restructuring in the higher temperature range. Grain boundary grooving, grain growth, surface rearrangement, and the nucleation of embedded particles were observed in situ on nickel. The surface restructuring of nickel due to massive diffusional processes was seen to be affected by the adsorbed gaseous species. The formation of filamentous carbon deposits was found to be controlled by reaction kinetics rather than by thermodynamic restraints. The in situ observations show the metal surface fragmentation encountered in metal dusting corrosion which is caused by the precipitation of a secondary phase and accompanied by the formation of filamentous carbon.