Alumina scales kinetics

The oxidation of Ti3A1 alloys would not be expected, in light of the above thermodynamic considerations, to form continuous alumina scales. Instead they form mixed rutile-alumina scales [49].The oxidation kinetics of Ti3Al between 600 and 950°C are reported to be essentially those expected for rutile growth [50,51]. These kinetics result from the development of a complex oxide layer which contains continuous paths of Ti02 through which rapid transport occurs. Typical oxide scales are shown schematically in Figure 14. 

Fig. 22. Oxide map showing the regions of Cr and A1 contents which form external alumina scales in air at various temperatures (left) and cyclic oxidation kinetics forTi-Cr-Al alloys over a range of compositions for exposures in air at 1000°C (right).

In some cases a measure of the reaction kinetics is not as useful as a measure of the useful life under a given set of experimental conditions. Cyclic oxidation tests using weight-change versus time measurements and visual examination of exposed specimens can be useful to determine times beyond which protective scales, such as alumina, are no longer formed on alloy test coupons. 

Adsorption kinetics will be discussed first. Figure 11A shows the equilibration of the alumina system after the addition of HCl [16]. Figure IIB shows that the adsorption of Cu, with liberation of protons [47], takes about 24 h to reach equilibrium. Figure 11C gives kinetic data for the adsorption of Co, Fe, and Ni ions [54], evidently in equilibrium after about 1 h. On the other hand, pressure jump experiments of Cu adsorption used a time scale of 10 -10 s [55]. These results are mentioned to demonstrate that reactions taking place in the adsorption system may have rates that differ by several orders of magnitude. 

Catalytic hydrogenation of an unsaturated ketone was studied by high-pressure kinetic experiments to provide an accurate model for the scale-up of an industrial pilot reactor. The selective reduction of the conjugated double bond was performed in supercritical carbon dioxide as a solvent in the presence of an industrial Pd on alumina catalyst. ... 

D. M. Lipkin, H. Schaffer, F. Adar, and D. R. Clarke, Lateral growth kinetics of -alumina accompanying the formation of a protective scale on (111) NiAl during oxidation at 1100° C, Appl. Phys. Lett., 70,2550 (1997). 

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