Formation of Protective Alumina on Ni3Al By Oxygen Partial Pressure Control Using Oxygen Pump Sensor
Authors: Nakajima Kano , Fukumoto Michihisa , Murakami Hideyuki 
Published: December 2020

Introduction

The oxidation of Ni aluminide with a low Al concentration produces NiAl2O4 and Al2O3 on the surface. It has been reported that the formation of NiO during the early stage of oxidation leads to the formation of NiAl2O4, resulting in poor-oxidation resistance.
In this study, an oxygen pump-sensor was installed in front of the oxidation furnace to accurately control the oxygen partial pressure in the atmosphere. During the initial stage of oxidation, the partial pressure was reduced to a level at which NiO was not generated, and the partial pressure of oxygen was rapidly increased to the temperature at which α-Al2O3 was formed.

Oxygen Partial Pressure Control and Oxidation Kinetics Assessment

Fig.1 shows the oxidation test equipment used in this study. An oxygen sensor and oxygen pump sensor were installed at the latter stage of the electric furnace, and the oxygen partial pressure and hydrogen oxidation current (hydrogen concentration) of the discharged gas atmosphere were measured. Yttria-stabilized zirconia tubes (8 mol%Y2O3-ZrO2) were used for the oxygen sensor and oxygen pump sensor.
For the oxygen sensor, the electromotive force was measured and substituted into the Nernst equation (Eq. 1) to obtain the oxygen partial pressure.

E=(RT/4F)ln(P
O2(ref)/P
O2(mea)) (1)
where, R: gas constant, T: temperature, F: Faraday constant, E: measured electromotive force, : oxygen partial pressure in the reference gas (atmosphere 0.21 atm) and : measured oxygen partial pressure.
The oxygen pump sensor consists of a sensor part and a pump part. The electromotive force is measured by the sensor, and the electromotive force is set so that the initial oxygen partial pressure is obtained. Since it is an oxide ion conductor, when a current is applied to the pump section, oxygen is supplied from the inside wall of the tube in the pump section via reactions (2) (inside wall) and(3) (outside wall).
2O2-→O2+4e- (2)
O2+4e-→2O2- (3)
As a result, the oxygen partial pressure of the pump section is maintained at the initial oxygen partial pressure.
Furthermore, by applying a negative current at the pump section, oxygen in the tube can be discharged by reaction (2) on the outside and reaction (3) on the inside. The experiment was performed by controlling the oxygen partial pressure of the atmosphere in this manner.
The calculation of the oxidation rate using the oxygen pump sensor was performed according to Faraday's law (Eq. 4).

J=I/4F (4)
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

Fig.2 shows the results of a oxidation test in which the oxygen partial pressure P
O2 of the Ni-25at.%Al alloy was changed from 10-19 atm to 10-6 atm at 1050 °C. In the figure, change in oxygen partial pressure, the oxidation current, and the oxidation rate are shown in (a), (b), (c), respectively. The P
O2 change (a) shows that the initial P
O2 is 10-19 atm. Thereafter, when the temperature reached 1050 ° C, the P
O2 was increased to 10-6 atm. It can be seen from the change in the current value (b) that the current abruptly increased when the P
O2 was changed. In other words, it can be seen that oxidation rapidly progressed when the P
O2 was changed, which agrees with the oxidation rate result(c).The oxidation rate rapidly increased when P
O2 was changed. then showed a low oxidation rate at 1250 °C.
Fig.3 shows the cross-sectional microstructure and the corresponding element map by EPMA after the oxidation experiment when the P
O2 was changed from 10-19 atm to 10-6 atm at 1050 °C. During the initial stage of the oxidation, the P
O2 was lowered, and a thin Al2O3 film was formed on the surface by increasing the P
O2 at 1050 °C. When the P
O2 was changed, only Al2O3 was formed. Therefore, at the beginning of oxidation, the P
O2 is reduced to avoid formation of Ni oxide. It was clarified that when the P
O2 was increased at 1050 ° C at which the a-Al2O3 was formed, only Al2O3 with a high oxidation resistance was formed. The oxide formed in this manner is expected to have high oxidation resistance.

Conclusion

The accurate control of oxygen partial pressure in the furnace is successfully conducted by applying the oxygen sensor and oxygen pump sensor. Using the furnace, we the direct formation of Al2O3 on Ni3Al was confirmed.
Acknowledgements
Part of this study was supported by the NIMS Joint Research Hub Program.

Figure 1