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 breakawayoxidation of chromium-containing iron and nickel alloys. One proposal is that thepresence of hydrogen or carbon promotes internal oxidation of chromium, leading tochromium depletion and breakaway oxidation. This hypothesis was tested by directmeasurement of oxygen permeability in the presence of hydrogen and carbon,compared with that in only oxygen, under the same partial pressure of oxygen. Fouralloys, Ni–Cr (1, 2, 3.5 and 5 wt%), were oxidised in H2/H2O, CO/CO2, and Ni/NiORhines packs at 1,000 and 1,100 C. The oxygen partial pressures in the mixedgases were equal to that of the Ni/NiO equilibrium. Internal oxidation occurred inall cases, according to parabolic kinetics, indicating a diffusion controlled process.Oxygen permeabilities deduced from Wagner’s diffusion model showed that anyvariation between the H2/H2O and CO/CO2 reactions is within the range of measurement error (B10 %). However, oxygen permeability was slightly higher in theRhines pack environment, a result attributed to the formation of more needle/platelikeinternal precipitates. It is concluded that oxygen permeability in nickel is notincreased by the presence of hydrogen and/or carbon