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 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 does not 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. (authors)