The effect of water vapor on the oxidation of alloys that develop alumina scales for protection
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I. INTRODUCTION
WATER vapor is present in virtually every atmosphere to which high-temperature materials are exposed. There are numerous examples that show water vapor affects the hightemperature corrosion of metallic alloys and ceramics. Some effects that water vapor has been reported to cause during the oxidation of alloys are as follows. The presence of water vapor permits a H2-H2O mixture to be formed in voids, which facilitates a rapid inward transport of oxygen across the pores via dissociative gas transport (an oxidation-reduction process).[1,2] Water vapor can affect the plasticity of oxide scales. Some investigators propose that the plasticity of oxide scales is decreased and, consequently, the spalling of scales is increased.[3] Other investigators have proposed that plasticity of oxide scales is increased[2,4] with increased oxidation rates[2] or improved adherence.[4] The presence of water vapor adversely affects the selective oxidation of aluminum and chromium in iron-base alloys[5] and in TiAl.[6] Water vapor can affect transport through oxides such as SiO2 by causing the network structure to be changed.[7] Water vapor can cause the vaporization of some oxides to be increased because of the formation of hydrated species with high vapor pressures.[8] Water vapor causes the concentration of proton defects to be increased, which can influence defect-dependent properties such as high-temperature creep and diffusion.[9,10] Based upon the data in the literature, it is evident that, depending upon the alloy and the oxidizing conditions, water vapor can affect oxidation in a number of different ways. Most alloys and coatings that are developed for use above 1000 8C rely upon the formation of an a-Al2O3 scale for oxidation protection. The influence of water vapor on the oxidation of alumina-forming alloys has not been extensively studied.[5,11] Kvernes et al.[5] did not observe any
RAMKUMAR JANAKIRAMAN, Metallurgical Manager, is with Donsco Incorporation, Wrightsville, PA 17368-0040. G.H. MEIER and F.S. PETTIT, Professors, are with the Materials Science and Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted July 13, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
effect of water vapor on the isothermal oxidation of an Fe13Cr-4Al* alloy at temperatures of 680 8C, 820 8C, and 980 *All compostions are presented in weight percent unless noted otherwise.
8C. At 980 8C, these authors showed that a continuous layer of a-Al2O3 was formed on this alloy. Buscail et al.[11] have found that water vapor decreases the isothermal oxidation rate of an Fe-21.5 Cr-5.6 Al alloy at 1000 8C. The effect was not large. For example, kp , values of 2.4 3 10213 and 2.7 3 10213 g2/cm4 s were obtained for water vapor pressures of 0.15 and 0.32 atm, respectively, vs 2.9 3 10213 g2/cm4 s for dry oxygen. These investigators proposed that the initial transient oxidation processes are affected by water vapor. Smialek[12] has shown that the scale adhesion of alumina is sensitive to water vapor and
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