Corrosion behavior of Al2O3in static state water vapor environment at high temperature

Abstract

Water vapor corrosion test at 1500 ◦C for single crystal and polycrystalline Al2O3 was performed in static state water vapor environment. In both cases, there was an increase in weight. The lattice parameter for polycrystalline bulk decreased by the corrosion test. The grain surface was corroded by water vapor and terrace fields were generated. No anisotropic corrosion behavior was observed. Alumina is a hopeful material for high temperature application due to its high stability at elevated temperatures [1]. Because the oxygen diffusion in alumina at high temperatures is low enough [2], it is considered that alumina phase is a promising material for oxidation protection layer for metal or non-oxide ceramics in high temperature applications. Non-oxide ceramics, such as silicon nitride and silicon carbide, oxidize at high temperatures and form a silica layer on their surface. It is known that the silica phase easily corrodes in water vapor environment at high temperatures [3, 4]. Recently, Yuri and Hisamatsu reported the recession rate for pure Al2O3 at high temperature and high water vapor partial pressure in high velocity gas flow [5]. In that report, the recession rate of Al2O3 is high enough when compared to that of silicon nitride, i.e., Al2O3 is easily corroded in actual gas turbine environment. A detailed recession rate expression for polycrystalline Al2O3 in actual combustion gas flow was derived previously [5]. However, Yue et al. reported that no weight change was observed by the water vapor corrosion test in static state water vapor environment at 1700–2000 ◦C [6]. On the other hand, Tai et al. suggested that the weight of Al2O3 phase increased in water vapor environment at high temperatures, even if the boundary phase was corroded and removed [7]. In that report, the weight is decreased up to 1500 ◦C and is increased above 1600 ◦C in Ar-H2O static state environment [7]. More detailed consideration of water vapor corrosion mechanism has not been established yet. Al2O3 (99.99% purity, High Purity Chemicals Co. Ltd.) powder was used as the starting material for preparing single crystalline and polycrystalline Al2O3. The high temperature water vapor corrosion test was performed for single crystal and polycrystalline Al2O3 using a Corrosion Testing Machine (Japan Ultra-high Temperature Materials Research Center). The sample was placed on a high purity Al2O3 plate and then heated under the following conditions; temperature: 1500 ◦C, time: 100 hrs, gas flow: 30 wt% water (air:H2O = 70:30 (wt %)), gas flow rate: 175 ml/min that correspond to gas velocity 4.6 × 10−4 m/s. To exclude the water vapor corrosion occurring at low temperatures, the corrosive gas was introduced when the temperature reached 1500 ◦C and the gas flow was stopped after the 100 hrs testing period. The density of the sintered polycrystalline Al2O3 bulk was 3.581 g/cm3 that is 90.0% of ideal value. No remarkable changes in external view for polycrystalline bulk were observed after the corrosion test. The weights of the samples (single crystalline and polycrystalline Al2O3) were increased during the corrosion test. The weight gain for single crystal and