April: Dr Jianqiang Zhang, " 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 H2 /H2 O, CO/CO2 , and Ni/NiO
Rhines packs at 1,000 and 1,100 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 H2 /H2 O and CO/CO2 reactions is within the range of measurement
error (B 10 %). However, oxygen permeability was slightly higher in the
Rhines pack environment, a result attributed to the formation of more needle/platelike
internal precipitates. It is concluded that oxygen permeability in nickel is not
increased by the presence of hydrogen and/or carbon.
To read the full article, as published in Oxidation of Metals, click here.