This paper is a brief review of the extensive literature relating to the formation of protective α—Al2O3 scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.

The formation of aluminum oxide scales on high-temperature alloys

The Na2SO4-induced accelerated oxidation of nickel-base alloys containing elements such as Cr, Al, Mo, W, and V has been studied in 1.0 atm O2 in the temperature range of 650° to 1000°C. It has been found that the hot corrosion behavior of these alloys can usually be characterized according to one of two types of attack: 1) Na2SO4-induced accelerated oxidation; 2) Na2SO4-induced catastrophic oxidation. In both types of hot corrosion, accelerated oxidation occurs as a result of the formation of a liquid flux based on Na2SC>4 which dissolves the normally protective oxide scales. Catastrophic, or self-sustaining rapid oxidation can occur in alloys which contain Mo, W, or V, because solutions of oxides of these elements with Na2SO4 decrease the oxide ion activity of the molten salts, producing melts which are acidic fluxes for oxide scales. The accelerated oxidation type of attack which was observed with most alloys which did not contain Mo, W, or V, was more severe than for normal oxidation, but much less severe than catastrophic oxidation. Na2SO4-induced accelerated oxidation occurs because the oxide ion activity of the Na2SO4 increases to the point where oxide scales can partially dissolve in the basic melt. Generally, this basic fluxing results from the diffusion of sulfur from the Na2SO4 into the alloy. In some alloys, the formation of sulfides during basic fluxing is a sufficient condition to cause accelerated oxidation. In other alloys, changes in the oxidation mechanism occur because of depletion of the alloy surface, concomitant with basic fluxing, of those elements needed for protective oxide scales, such as aluminum and chromium.

Mechanisms for the hot corrosion of nickel-base alloys

The transient oxidation of β-NiAl in air at 800 °C and 1100 °C has been studied using electron microscopy. The oxide scale consists predominatly of metastable Al2O3 phases. θ-Al2O3 is the major oxide phase within 10.0 hr of oxidation at 800 °C and 0.1 hr at 1100 °C. The scales form epitaxially on (001)β and (012)β specimens throughout the transient stage, whereas the degree of preferred oxide orientation decreases with oxidation time on (011)β and (111)β specimens. The orientation relationships reflect the small mismatch between parallel close-packed directions in the metal and in the cation sublattice of the oxides. The correlation of distinctive oxide surface morphologies with internal structural defects indicates the strong tendency of the Al2O3 scale to growvia short-circuit diffusion paths.

Transient oxidation of Single-Crystal β-NiAl

The development, growth, and adhesion of α-Al2O3 scales on platinum-aluminum alloys containing between 0.5 and 6 wt.% aluminum have been studied at temperatures in the interval between 1000 and 1450° C. The morphologies and microstructures of the α-Al2O3 scales were found to be influenced by the temperature, oxygen pressure, and the microstructures of the alloys. The oxidation rates of the alloys appeared to be controlled by transport of oxygen along grain boundaries in the α-Al2O3 scales. The α-Al2O3 scales adhered to the platinum-aluminum substrates even after extensive periods of cyclic oxidation. The good adhesion of the α-Al2O3 may result from mechanical keying of the oxide to the alloys due to the development of irregular oxide-alloy interfaces.

Development, Growth and Adhesion of Al2O3 on Platinum-Aluminum Alloys.

Some of the main principles which determine the protective nature of oxide scales on metals in oxidising environments at high temperature are considered and discussed with particular respect to NiO scales on nickel and Cr2O3 and Al2O3 scales on high-temperature alloys. Emphasis is given to the transport processes in the scales and the importance of short-circuit diffusion paths. The development of stresses during oxidation are discussed and related to the effectiveness of the scales in resisting mechanical breakdown. Relief of these stresses can occur by deformation of the oxide or metal substrate but, in certain circumstances, may result in fracture and/or loss of adhesion of the scale followed by spalling from the metal surface, sometimes with catastrophic consequences.

https://iopscience.iop.org/article/10.1088/0034-4885/50/7/002/pdf

The protective action of oxide scales in gaseous environments at high temperature

Stages in the development of a protective α-Al2O3 scale on a Ni-15Cr-6Al (wt.%) alloy have been examined. It is shown that prior to the formation of a continuous α-Al2O3 layer, a transient stage of oxidation occurs that consists of a rapid uptake of oxygen with conversion of a thin surface layer of alloy to predominantly spinel and the subsequent development of a discrete layer of Cr2O3. It is also shown that during the transient period of oxidation metastable phases of aluminum oxide are formed which transform to α-Al2O3 upon incorporation into the external oxide scale.

L. P. Lemaire

Papers

On the transient oxidation of a Ni-15Cr-6Al alloy

CO-PRECIPITATION OF $gamma$'(Ni$sub 3$Al) AND $alpha$-Cr IN A NICKEL-BASE ALLOY.

Structural Imperfections in thin Oxide Films Formed on Some Fe- and Ni-Base Alloys