Investigation of cyclic oxidation in Ar-H2O for NiAl containing Hf or Zr by the combination of a hydrogen sensor and an oxygen pump-sensor

Abstract

The oxidation rate of the NiAl alloy and the NiAl alloy containing a small amount of Hf or Zr during the cycle oxidation experiment in an Ar-H2O atmosphere was determined by a hydrogen sensor and oxygen pump-sensor. It was found from the result of the cycle oxidation experiment at 900－1250℃that the temperature dependence of the oxidation rate measured by both sensors was similar. During the first cycle, a sharp decrease in the oxidation rate was observed around 1100℃during the temperature increase. This is considered to be a transformation from θ-Al2O3 to α-Al2O3 since the scale to be formed is composed of Al2O3. In addition, the oxidation rate before the transformation of the NiAl containing Hf or Zr was greater than that of the NiAl without Hf and Zr during the heating process of one cycle. On the contrary, after the transformation, the oxidation rate of NiAl containing Hf or Zr was lower than that of NiAl without the Hf and Zr. Based on this result, it was found that the addition of Hf and Zr increased the growth rate on the θ-Al2O3 scale, while it reduced the growth rate on the α-Al2O3 scale.