Iron-nickel alloys of several compositions were oxidized in oxygen at 1050 C. Subscale formed preferentially at grain boundaries. Equilibration under an inert atmosphere occurred mainly by the dissolution and diffusion of oxygen in the alloy phase. Porosity developed in the subscale region during equilibration, then began to sinter out on longer heating.
Several features of the iron-rich corner of the ternary phase diagram were determined. The three phase triangle was shown to have apices near 55 weight percent Ni in the alloy phase, 52.6 atomic percent oxygen in the wüstite phase, and 10 weight percent Ni in the spinel phase. Ni is virtually absent from the wüstite phase. Consequently the wüstite in equilibrium with an iron-nickel alloy has a higher oxygen content (lower iron activity) than when in equilibrium with pure iron at the same temperature.
The reduction in the concentration difference between the extremes of the wüstite layer on an iron-nickel alloy as compared with one on pure iron (with an overlying layer of magnetite in each case) leads to a lower growth rate of the wüstite phase on the alloy. By reference to the relative rates of growth on pure iron, it is anticipated that the elimination of wüstite as a stable phase should lead to a decrease in the overall oxidation rate by a factor of about 20', the actual rate determined by the growth of the spinel phase. 3.2.3
