Investigating the oxidation mechanism of Ni-Cr Alloy in water vapour

For use in a High Temperature Gas-cooled Reactor, the oxidation behavior of Ni22Cr14W1Mo Alloy was investigated at 850 degrees C under helium with a partial pressure of water vapour of 20-80 Pa and a partial pressure of hydrogen of 20-4000 Pa. For chromia-former materials, it is accepted that the oxidation potential in the gas may affect the growth of the surface oxide scale. A parametric study using thermogravimetry showed that oxidation of Ni22Cr-14W-1Mo Alloy follows parabolic kinetics for 250 h. The rate constant doesn't directly depend on the oxygen partial pressure in the gas but it changes as a function of both the partial pressure of water vapour and the partial pressure of hydrogen. Post-test examination of the specimen surface by electronic microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, showed formation of scale made of chromium-oxide containing manganese. Further analyses using photo-electrochemistry and secondary ion mass spectrometry implied that this oxide may include two types of cationic defects. A mechanist analysis based on the simultaneous diffusion of these two defects theoretically accounts for the simultaneous effect of the partial pressures of water vapour and hydrogen on the oxidation rate.