High temperature oxidation behavior of Co-20Re-25Cr-2Si alloy at different oxygen pressures

The oxidation of the quaternary Co-20Re-25Cr-2Si alloy has been studied at 1000−1300 °C for 24 h. On the whole, at high oxygen pressure of 0.1 MPa the alloy shows mass loss while at low oxygen pressure of 3.0×10−5 Pa, however, the alloy shows mass gain. Compared with the irregular kinetics under the oxygen pressure of 0.1 MPa, the kinetics approximately conform to parabolic law under the oxygen pressure of 3.0×10−5 Pa, with the value of oxidation activation energy equal to 262.7 kJ/mol. The scales formed under the oxygen pressure of 0.1 MPa at 1000−1300 °C are very similar. The outermost layer is CoO, followed by a mixed oxide layer probably composed of CoO and CoCr2O4. The innermost layer is probably composed mainly of a thin discontinuous layer of Cr2O3, accounting for the continuous mass loss due to the evaporation of Re element in the form of ReO3. However, the sales formed under the oxygen pressure of 3.0×10−5 Pa are probably composed of an outer thin layer of CoCr2O4 spinel and an inner dense and continuous layer of Cr2O3, which provides the good oxidation resistance for the alloy. The combination of the addition of Si and low oxygen pressure has shown a synergistic effect on the formation of a protective chromia layer.