Oxidation of Nickel and Ni-Cr and Ni-Na Alloys at High Temperatures

Oxidation kinetics of high purity nickel, as well as the nonstoichiometry and chemical diffusion in nickel oxide, have been studied as a function of temperature (1373-1673 K) and oxygen pressure (10-10 5 Pa) using modern microthermogravimetric techniques.In order to eliminate the possible participation of grain boundary diffusion in scale growth at lower temperatures, the oxidation rate measurements have always been started at highest temperature (1673 K) when coarse-grained scale was formed and the temperature and pressure dependence of the oxidation rate was determined by step-wise lowering the temperature of such pre-oxidized sample.Nonstoichiometry and chemical diffusion coefficient in Ni,.y O have also been determined on coarse-grained oxide samples, obtained by complete oxidation of nickel at highest temperature (1673 K).It has been found that under such conditions the parabolic rate constant of nickel oxidation is the following function of temperature and oxygen pressure: k p = 0.142 • p^ • exp| -239kJ I mol λThe results of RT Jnonstoichiometry and chemical diffusion measurements, in turn, may be described by the following relationships