Alumina formers

B. A. Pint "Study of the Reactive Element Effect in ODS Iron-Base Alumina-Formers, Materials Science Forum, in press (1996). 

Table 1 shows the equilibrium oxygen partial pressure of several metal oxides, i. e. the minimum oxygen partial pressure which is required for oxide formation. A comparison with Table 2, in which typical oxygen partial pressures of industrial processes are listed, clearly demonstrates, that in coal gasification processes and petrochemical plants stable oxide scales can only be expected on alumina formers. Chromia becomes unstable at the low oxygen partial pressures encountered in such processes. 

Cueff, R., Buscail, H., Caudron, E., Issartel, C., and Riffard, F. "Influence of Yttrium-Aloying Addition on the Oxidation of Alumina Formers at 1173k." Oxidation of Metals 58, nos. 5/6 (December 2002) 439-55. 

High-temperature stainless steels, most polycrystalline superalloys, and chromized coatings rely on the formation of a surface layer of chromia for oxidation protection. The effects of reactive element additions are often more dramatic in the case of chromia-forming alloys than alumina formers in that, in addition to improving adherence (Figure 5.41), they decrease the amount of transient oxidation, reduce... 

J. Jedhnski and G. Borchardt, On the oxidation mechanism of alumina formers. Oxidation of Metals 36 317-337,1991. 

From Fig. 7-1 it can be seen that after silica forming materials, those materials with the next most protective oxide scale would be alumina formers. Since alumina scales are less sensitive to impurities, AIN and AI4C3 appear, at first glance, to be ideal high temperature oxidation resistant materials. However, the recession rates in this figure are based on those observed for alumina forming metal alloys. Oxidation of both of these ceramic materials results in the formation of gaseous products which can alter the protective qualities of the alumina scale. 

As structural ceramics find more applications in high temperature systems, oxidation and corrosion at high temperatures becomes an important field of study. In this chapter, the critical issues in this field have been surveyed. Ceramics have been classified according to the type of protective oxide they form. These include silica formers, alumina formers, boria formers, and transition metal oxide formers. Most of the literature covers silica formers since there are a number of near-term applications for these materials. Basic oxidation mechanisms, water vapor interactions, volatilization routes, and salt-induced corrosion were discussed for these materials. Less information is available on alumina-forming ceramics. However the rapid oxidation rate in water vapor appears to be a major problem. Boria formers show rapid oxidation rates due to the formation of a liquid oxide film and are volatile in the presence of water vapor due to highly stable Hx-By-Oz(g) species formation. Transition metal carbides and nitrides also show rapid oxidation rates due to rapid transport in the oxide scale and cracking of that scale. 

While the spatial resolution of AES, XPS and SIMS continues to improve, atomic scale analysis can only be obtained by transmission electron microscopy (TEM), combined with energy dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS). EDX detects X-rays characteristic of the elements present and EELS probes electrons which lose energy due to their interaction with the specimen. The energy losses are characteristic of both the elements present and their chemistry. Reflection high-energy electron diffraction (RHEED) provides information on surface slmcture and crystallinity. Further details of the principles of AES, XPS, SIMS and other techniques can be found in a recent publication [1]. This chapter includes the use of AES, XPS, SIMS, RHEED and TEM to study the composition of oxides on nickel, chromia and alumina formers, silicon, gallium arsenide, indium phosphide and indium aluminum phosphide. Details of the instrumentation can be found in previous reviews [2-4]. 

Total mass gain (specimen + spalled oxide) during lOOh cycles at 1200°C in laboratory air for various Ni-Cr-AI alloys. Generally, the mass gain decreases with Al content due to the formation of a protective alumina scale with NiAl + Hf having one of the slowest growth rates of any alumina-former. Two different specimens are shown for alloy 214 to indicate the reproducibility. 

---