A review of principles governing alloy oxidation is presented together with the means by which practical oxidation-resistance alloys have been developed. The reactions involved are complex and involve acquiring data from many fields. Wagner developed the basic diffusional models for the formation of oxide scales, but these need to be expanded using detailed physical and metallurgical studies of the alloy phases, oxides and their defects, as well as the mode of scale formation and the structural relationships between alloy and oxide scale. At this time the engineering solutions have outstripped fundamental understanding, but an attempt is made to show how the development of oxidation theory has contributed to this effort by a term now referred to as 'informed empiricism'.

https://iopscience.iop.org/article/10.1088/0034-4885/39/5/001/pdf

The oxidation of alloys

The rates and modes of oxidation of a wide range of pure Fe-Cr, Ni-Cr and Co-Cr alloys in 1 atm oxygen, and in 0.1 atm water vapour/0.9 atm argon contaminated with traces of air, at 1000°C are compared, using data originating largely from a single laboratory. The main features of the mechanisms of oxidation for specific groups of alloys are considered in the light of existing knowledge on the alloy and oxide Properties determining these mechanisms.



Vergleich der Oxydation von Fe-Cr, Ni-Cr und Co-Cr in Sauerstoff und Wasserdampf



Die Geschwindigkeit und der Verlauf der Oxydation zahlreicher Legierungen aus Fe-Cr, Ni-Cr und Co-Cr in Sauerstoff (1 at) und Wasserdampf (0,1 at) + Argon (0,9 at) + Spuren von Lu# als Verunreinigung bei 1000°C werden unter Verwendung von weitgehend aus einem einzigen Laboratorium stammenden Daten verglichen. Die kennzeichnenden Eigenschaften der Oxydationsmechanismen fur spezifische Legierungsgruppen werden im Lichte der verfugbaren Kenntnisse uber die Legierungs- und Oxid- eigenschaften betrachtet, welche die Oxydationsmechanismen bestimmen.

A Comparison of the Oxidation of Fe&bCr, Ni ? Cr and Co ? Cr alloys in oxygen and water vapour

The role of grain boundaries in the initial oxidation behavior of austenitic stainless steel containing alloyed Cu at 700 °C for advanced thermal power plant applications

The assumptions involved in Wagner's original treatment of alloy depletion profiles are examined and found to be acceptable for many situations. Finite difference analyses do not result in profiles which are significantly different from those obtained by the much simpler analytical solution once steady-state parabolic growth is established. Consequently an analytical solution is preferred and its combination with the classical Wagner expression for scale growth leads to a unified description of alloy oxidation when only the least noble metal is oxidized. The description is tested for an Fe-27.4wt.% Cr alloy oxidized at 1273°K and agreement between theoretical and experimental results is satisfactory. Alternative treatments of alloy oxidation which require that there be no recession of the alloy-scale interface are discussed and it is concluded that this assumption is unnecessarily restrictive in many cases. Suggestions that the oxidation of austenitic steels is controlled by diffusion in the alloy and that an interfacial transfer step is of importance in determining the oxidation rate in some cases are shown to be based on invalid assumptions. An analytical solution to the diffusion equation is developed for the case when a phase change occurs in the alloy because of less noble metal depletion and an expression is also presented for the profile developed in the limiting case where depletion is determined by scale evaporation.

Alloy depletion profiles resulting from the preferential removal of the less noble metal during alloy oxidation

The identification of thin healing layers at the base of oxide scales on FeCr base alloys

, containing small quantities of dissolved iron, is the only oxide formed in the oxidation of iron‐chromium alloys in the range 22–68% Cr in oxygen at 800°–1200°C; it is also formed, at least initially, on alloys containing 14 and 18% Cr. The oxide growth is diffusion‐controlled, with an apparent activation energy of 36 kcal mole−1. The increase in oxidation rate with alloy chromium content is slight up to 37% Cr, but becomes more marked with the chromium‐rich alloys. Iron only enters the scale during the very early stages of oxidation, so the weight fraction of iron in the oxide decreases with time. Alloy interdiffusion coefficients have been determined from chromium concentration profiles in the underlying alloy. The activation energies for inter‐ diffusion are 51 and 59 kcal mole−1 for Fe‐27% Cr and Fe‐59% Cr, respectively, but interdiffusion is much slower for the chromium‐rich alloy. The chromium concentration at the alloy/oxide interface, which is determined largely by the ratio of oxidation rate to alloy interdiffusion rate, governs the type of oxide formed if lifting and cracking of the stressed scale occurs. Factors affecting the tendency for scale failure in this way are discussed.

G. C. Wood

Papers

Chromium Oxide Scale Growth on Iron‐Chromium Alloys II . Influence of Alloy Composition