Oxygen Permeability Measurements in Ni Using H2/H2O, CO/CO2 and Ni/NiO Rhines Pack Atmospheres

Water vapour and carbon dioxide are known to accelerate breakaway oxidation of chromium-containing iron and nickel alloys. One proposal is that the presence of hydrogen or carbon promotes internal oxidation of chromium, leading to chromium depletion and breakaway oxidation. This hypothesis was tested by direct measurement of oxygen permeability in the presence of hydrogen and carbon, compared with that in only oxygen, under the same partial pressure of oxygen. Four alloys, Ni-Cr (1, 2, 3.5 and 5 wt%), were oxidised in H-2/H2O, CO/CO2, and Ni/NiO Rhines packs at 1,000 and 1,100 A degrees C. The oxygen partial pressures in the mixed gases were equal to that of the Ni/NiO equilibrium. Internal oxidation occurred in all cases, according to parabolic kinetics, indicating a diffusion controlled process. Oxygen permeabilities deduced from Wagner's diffusion model showed that any variation between the H-2/H2O and CO/CO2 reactions is within the range of measurement error (a parts per thousand currency sign10 %). However, oxygen permeability was slightly higher in the Rhines pack environment, a result attributed to the formation of more needle/plate-like internal precipitates. It is concluded that oxygen permeability in nickel is not increased by the presence of hydrogen and/or carbon.