(Invited) Investigation of High Temperature Oxide Scale Using Chemical Reactor
Authors: Fukumoto Michihisa , Nakajima Kano , Kawamori Yasumasa 
Published: December 2020

Introduction
Ni aluminide forms highly oxidation-resistant Al2O3 on its surface when it contains a certain concentration of aluminum. However, when the Al content is too high, the melting point of the alloy decreases. As a result, the high temperature strength decreases. Furthermore, the material becomes brittle and the workability deteriorates. Studies have reported that it is effective to add Cr to produce alumina even with a low Al concentration.
In this study, the oxidation behavior of Ni-Al and Ni-Al-Cr alloys in Ar-H2O was investigated using a gas sensor. Then, the effect of Cr concentration is clarified from the oxidation rate and cross-sectional structure.
Experimental
Fig.1 shows the oxidation test equipment used in this study. A hydrogen sensor and an oxygen pump sensor were installed downstream the electric furnace. The partial pressure of hydrogen and the oxidation current (hydrogen concentration) of hydrogen in the discharged gas were measured. A CaZr0.9In0.1O3 tube was used for the hydrogen sensor, and a yttria-stabilized zirconia tube (8 mol% Y2O3-ZrO2) was used for the oxygen ion pump and sensor.
The hydrogen sensor measures the electromotive force and substitutes it into the Nernst equation, shown in equation (1), is used to obtain the hydrogen partial pressure.

E=(RT/2F) ln (P
H2(ref)/P
H2(mea)) (1)
Where R: gas constant, T: temperature, F: Faraday constant, E: measured electromotive force, : hydrogen partial pressure in reference gas (Ar-1.01 vol.%H2), : measured hydrogen partial pressure.
Fig. 2 shows details of the oxygen pump-sensor. In an oxygen pump-sensor, composed of a pump section and a sensor section, the electromotive force is measured by the sensor part and the electromotive force is set so that the initial oxygen partial pressure is obtained. When a current is applied to the pump section, the YSZ tube acts an oxide ion conductor, and oxygen is supplied to the atmosphere inside the pump section tube by the reaction shown in the figure. Then, the oxygen partial pressure of the pump section is maintained at the initial oxygen partial pressure by potentiostat. The calculation of the oxidation rate using the oxygen pump-sensor is performed according to Faraday's law, as seen in equation (2).

J= I / 4F (2)
Where, F: Faraday constant, I: applied current, and J: amount of oxygen supplied to the measurement system by the oxygen pump. The amount of hydrogen was calculated from the supplied amount of oxygen J, and the oxidation rate was calculated.
Results
Ni-5 wt.% Al showed a sharp increase in oxidation rate at 900℃. Thereafter, a linear increase in the oxidation rate was observed during the heating process. On the other hand, the Ni-5 wt.% Al-5 wt.%Cr alloy showed the same sharp increase in oxidation rate at 900℃, but the value reached was much lower than that of Ni-5 wt.% Al. Additionally, during the heating process from 900℃ to 1250℃, the oxidation rate peaked briefly, then declined, before resuming a linear increase. In its cross-sectional structure, thick and thin oxides were formed. For the thin layers , NiAl2O4 was formed on the surface and Al2O3 below. The oxidation rate of Ni-5 wt.% Al-10 to 20 wt.% Cr, the oxidation rate when the temperature reached 900℃ was very low. Looking at its cross-sectional structure, Cr oxide was formed on the surface, and Al2O3 below. When Cr oxide formed on the surface, a sharp decrease in the oxidation rate was observed during the heating process. Al2O3 transforms from θ to α around 1100℃. Thus, the oxidation rate decreases with the transformation of Al2O3 from θ to α during the heating process. Furthermore, the rate of oxidation during the heating process decreased more when chromium oxide was formed on the surface than when Ni oxide was formed on the surface. This reveals the relationship between the oxidation rate and the effect of Cr on Al2O3.
Conclusions
(1) Ni-5Al showed a rapid increase in the oxidation rate at 900℃, and after that a linear increase in the oxidation rate was observed during the heating process.
(2) For the Ni-5Al-5Cr alloy, the oxidation rate during the heating process from 900℃ to 1250℃ slowed at first, then dipped, before resuming a linear increase. Its scale structure consisted of thick and thin oxides. In the thin layers, NiAl2O4 was formed on the surface and Al2O3 was formed below.

Figure 1