High Temperature Oxidation of Nickel at 1373 K in CO-CO2-O2 Atmospheres and Morphology Analysis on Scales

High temperature oxidat ion of nickel has been studied at 1373 K in CO-CO~-O2 atmospheres, at oxygen partial pres-sures between 6.1 x 10-' ~ and 2.1 x 104 Pa. The oxidat ion k inet ics have been fol lowed by cont inuous mass gain measure-ments us ing a thermoba lance at the ox idat ion temperature and by measurements of the scale th ickness in meta l lographic cross sections, after cooling. The grain growth of NiO dur ing ox idat ion has been also invest igated. A double- layered NiO scale is formed near p la t inum markers, a l though only a single scale is observed away from the markers. The outer layer is ident ical in th ickness to the scale formed away from markers, and the growth rate is contro l led by the outward di f fusion of 9 ' 1 /6 nickel ions. The dif fusivity of n ickel ions is proport ional to Po2 at oxygen partial pressures h igher than =10 Pa. The oxy-gen pressure dependence of the dif fusivity indicates that the predominant defect is a doub ly-charged nickel vacancy. At oxygen part ial pressures lower than ~10 Pa, the oxygen pressure dependence becomes smaller, suggest ing extr ins ic be-havior. The format ion of the inner layer is explained, assuming the inward oxygen t ranspor t a long grain boundar ies in the outer layer. The size of the oxide grains, away f rom markers, is a lmost the same as that of the nickel substrate, but the ox-ide near the markers is much smaller. Gra ins near the markers coarsen, and the cube of the grain size is proport iona l to t ime, and the rate constant is also proport iona l to Po2 '/". These results suggest hat grain growth proceeds through the pore drag mechan ism, and that the rate is determined by the lattice di f fusion of nickel ions