As before, it is convenient to consider this system in order of increasing Cr content.At low Cr contents, no internal oxidation zone is seen. This is because the rateat which the external scale is formed is so rapid that the thickness of the internaloxidation zone is negligibly small. The system is best described in terms of theFe–Cr–O phase diagram shown schematically in Figure 5.18. The rhombohedraloxides Fe2O3 and Cr2O3 show a continuous series of solid solutions. The iron- andchromium-oxides react to form spinels which form solid solutions with Fe3O4.