Abstract
The corrosion behavior of pure Y and two Y–Al alloys containing 5 and 10 wt.% Al was studied over the temperature range 800–950°C in a H2/H2S/H2O gas mixture. Both alloys had the two-phase structure of Y+Y2Al. With the exception of Y–10Al, for which a kinetics inversion was observed between 800°C and higher temperatures (T≥ 850°C), the parabolic rate constants generally increased with increasing temperature, but decreased with increasing Al content. The scales formed on pure Y and the Y–Al alloys were single but heterophasic, consisting of mostly Y2O3 and minor Y2O2S. XRD results showed no evidence of Al2O3 and pure sulfides. The formation of Y2O3 and Y2O2S on Y–10Al at 800°C resulted in a subsurface phase transformation from Y+Y2Al to YAl2 and broke the structural integrity of the scale, being responsible for the fast corrosion rate.
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