Abstract
The oxidation rate of liquid Fe-Ni and Fe-Cr alloys with pure oxygen gas was studied by a constant-volume Sieverts' technique. The oxidation proceeded in two stages, namely, the first stage of instantaneous oxide formation and the second stage of slow oxidation followed. The present work was focused on the second stage in which three phases of gas, oxide, and liquid metal coexisted. The oxidation rate was greatly dependent on whether the oxide phase was solid or liquid.
The oxidation rate of Fe-Ni alloys ([%Ni]=080) in which the oxide phase was liquid, decreased with increasing nickel content in the alloys and was proportional to the square root of oxygen pressure in the gas phase. Therefore, it is suggested that the rate-determining step should be the mass-transfer through the liquid oxide phase at the gas-oxide interface. The same behavior was observed in Fe-Cr alloys ([%Cr]=017) of which the oxide phase was liquid or liquid+solid.
In pure Ni and Fe-Cr alloys ([%Cr]>17), the oxide phase was solid and the experimental results could be fitted to the parabolic rate law. Therefore, diffusion through the solid oxide phase is suggested to be the rate-determining step.
