Transport properties of Al or Cr-doped nickel oxide relevant to the thermal oxidation of dilute Ni-Al and Ni-Cr alloys

Abstract

In the present study we have analysed the thermal oxidation of Ni and dilute (<1wt%) Ni-Al and Ni-Cr alloys between 700° and 1200°C, taking into account the transport properties of pure and Al or Cr-doped Ni1−xO single crystals. In the temperature range 900°–1200°C, Al3+ or Cr3+ additions decrease the oxidation rates of Ni1−xO single crystals in their stability range, due to the kinetic demixing of cations. At T<1000°C, Ni-Al alloys also oxidize slower than high-purity nickel, but faster above 1000°C, while on Ni-Cr alloys the oxide scales grow faster from 700°C. The higher oxidation kinetics of the alloys is explained by the presence of oxygen molecules, which penetrate along cracks or fissures through the oxide scales. The mechanism whereby these short-circuit pathways have been formed is associated with the growth of nickel oxide particles within the scale, leading to local tensile strengths. The way in which these particles develop is explained by a blocking effect of outward diffusion of nickel ions, due both to an extensive depletion of Al3+ and Cr3+ in the outer layer and to the decrease of the mobility of Ni2+ when the amount of Cr3+ or Al3+ decreases in Ni1−xO.