Theoretical Analysis of the Diffusion Processes Determining the Oxidation Rate of Alloys

The interplay of diffusion processes in metallic and oxide phases during the oxidation of alloys is analyzed theoretically for specified ideal conditions. The oxidation rate of alloys containing a noble metal (Au or Pt) and an oxidizable metal (Ni, Cu, or Zn) is calculated as a function of the alloy composition. The oxidation rate of Ni‐Pt alloys at 850° and 1100°C is essentially determined by the diffusion of nickel to the alloy‐ NiO interface if the mole fraction of nickel is less than 0.5. Observed oxidation rates are in accordance with values calculated from diffusion data. Alloys in which both constituents are oxidizable may form either a one‐phase or a two‐phase scale. A necessary but not sufficient condition for the formation of a scale consisting of only one oxide is stated. Results for Cu‐Ni and Cu‐Zn alloys are essentially in accordance with theoretical considerations. On the other hand, it is shown that two oxides may nucleate initially and continue to grow even under conditions under which exclusive formation of the oxide of the less noble metal is a possible process. This is illustrated by observations on Cu‐Al alloys.