Oxidation and Hot Corrosion of Superalloys

Abstract

The oxidation, mixed gas corrosion and hot corrosion of nickel-, cobaltand iron-base superalloys at temperatures above about 6OOC are examined. It is shown that the superalloys develop resistance to corrosion by forming either alumina or chromia scales upon their surfaces. The times for which such oxide reaction product barriers are stable upon the surfaces of superalloys is discussed by first considering how these oxide scales are formed and then how they are destroyed in use. It is shown for oxidation environments that these oxide scales are degraded primarily via cracking and spalling. When mixed gas environments are encountered, the degradation is also affected by the other reactants in the gas phase. The most severe conditions are shown to be those inducing hot corrosion attack where the oxide scales are subjected to not only mixed gas conditions but also the fluxing action of molten deposits. The behavior of nickel-, cobalt-, and iron-base superalloys are compared and the effects of the various alloying elements are discussed.