Investigation of High-Temperature Oxidation in Steam for Ni–Al Alloys Using the Combination of a Hydrogen Sensor and an Oxygen Pump–Sensor

The oxidation rates of Ni–Al alloys in an atmosphere of Ar–12 vol.% H2O were evaluated by a hydrogen sensor and oxygen pump–sensor installed in the latter stage of an oxidizing furnace. Steam oxidation behavior was also investigated. The oxygen pump–sensor was evaluated using Ar–1000 ppm H2 gas. It was found that the H2 gas concentration could be accurately measured as an oxidation current, such that the hydrogen concentration could be measured by the oxygen pump–sensor, and the oxidation rate could be calculated using this hydrogen concentration. From an isothermal oxidation experiment at 1523 K for 14.4 ks, the oxidation rates measured by the hydrogen sensor and the oxygen pump–sensor were almost the same values in each sample. In addition, the mass gain curve was obtained for each sample based on the result of the time change in the oxidation rate. Furthermore, from a cycle oxidation experiment in which the temperature was varied between 1173 and 1523 K it was found that for the Ni–11 at.% Cr–10 at.%Al and Ni–50 at.%Al alloys, during the temperature increase process of the first cycle, a sharp decrease in the oxidation rate was observed around 1373 K. This was considered to be due to the transformation from θ- to α-Al2O3 since the scales formed on both alloys consisted of Al2O3. However, during the temperature increase process of the second cycle, this decrease was not observed in both alloys. Both the hydrogen sensor and the oxygen pump–sensor method could measure small changes in the oxidation rate during the steam oxidation process, so it became clear that the change in the oxidation rate accompanying the phase transformation of the oxide scale could be determined.