The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

Abstract The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373–1673 K) and oxygen activity (10−10 5  Pa O 2 ), using microthermogravimetric techniques. It has been shown that the oxidation of Ni–Cr and Ni–Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni 1− y O, it has been found that the oxidation rate of the Ni–Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni 1− y O–Cr 2 O 3 solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni–Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.