A review of advances in high-temperature corrosion

Abstract

The current understanding of selective oxidation is discussed. The transition from internal to external oxidation of binary alloys and the effects of transient oxidation and third element additions are considered. It is shown that the classic approach for calculating the critical solute content to form an external scale must be modified when applied to systems in which the thermodynamic stability of the lowest oxide of the matrix element approaches that of the solute oxide and/or the oxygen permeability in the alloy becomes very small. These conditions are pertinent to the oxidation of many intermetallic compounds. The results of recent studies of stress generation and relief in oxide scales and on oxide adherence are briefly reviewed. Emphasis is placed on the effects of minor elements such as the noble metals and “oxygen-active elements”. The effects of second oxidants (e.g. sulfur, carbon, nitrogen etc.) on the selective oxidation process are discussed. It is shown that the presence of a second oxidant can retard the formation of a protective oxide film in certain alloy systems. Additionally, the influence of a second oxidant on the breakdown of previously formed protective oxide films is discussed. Finally, recent developments in the understanding of the hot corrosion of alloys, particularly alumina-forming coatings, are reviewed. The effects of deposit composition, atmosphere composition, and temperature on the initiation and propagation of hot corrosion degradation are included in this discussion.