New Generation of Nanocrystalline Coating Materials Resistant to High Temperature Corrosion

Abstract

The effect of transport properties of transition metal sulphides on the kinetics and mechanism of high temperature sulphide corrosion of metals and alloys is discussed. It has been shown that, due to very high concentration of point defects in common metal sulphides, not only pure metals but also modern oxidation resistant alloys undergo rapid degradation in sulphur containing environments. Refractory metals, on the other hand, and in particular molybdenum and niobium, show excellent resistance to sulphur attack, because of very low defect concentration (MoS i ) or low defect mobility (NbS 2 ) in the Sulphides of these metals. It has been shown that alloying of common metals by molybdenum and even combined alloying by this metal and aluminum is insufficient to create a rational basis for designing new coating materials resistant to sulphide corrosion. The results obtained during the last 10 years in a joint research program between this Laboratory and the Institute for Materials Research, Tohoku University in Sendai have demonstrated that sputter-deposited, amorphous aluminum base-refractory metal alloys, which crystallize on heating to form a nanocrystalline mixture of intermetallics, may be considered as a starting point for a new generation of coating materials resistant to multi-component sulphidizing-oxidizing atmospheres, often encountered in many branches of modern technology. The main results obtained in the framework of this research program are briefly discussed and some general conclusions have been formulated.