Fundamental Factors Determining the Mode of Scaling of Heat‐Resistant Alloys

The compact scale structures developed on heat‐resistant alloys are determined to a considerable degree by interplay of the following factors:– The free energies of formation of the various possible doped simple and complex oxides, sulphides, etc. The growth rates of these compounds. The bulk alloy composition. The alloy interdiffusion coefficient, taking into account transport both through the alloy grains and through grain boundaries. The solubility and diffusivity of oxygen, sulphur, etc., in the substrate. The influence of the alloy substructure. These structures may be modified by the consequences of loss of adhesion, spalling, breakaway, oxide interdiffusion, etc. The principles involved in transient and steady‐state oxidation are illustrated briefly in terms of the comparative scaling behaviour of Fe‐Cr. Ni‐Cr and Co‐Cr alloys, and of Fe‐Cr‐Al, Ni‐Cr‐A1 and Co‐Cr‐A1 and Fe‐Cr‐Si alloys, in oxygen. The aim of this discussion is to present a general background, in terms of which the behaviour of further addition or tramp elements may be assessed. Attention is then turned briefly to the theories that have been postulated for explaining the effectiveness of rare earth and similar elements in improving scale adhesion and/or reducing scale growth rate. A probable role of rare earths in diminishing hot corrosion is outlined. Possible contributions to the scaling pattern of other additives, such as manganese and carbon, are described.