The structure of the scales formed on a series of typical Ni-Cr steels when oxidized for 100 hours in air at temperatures from 1600 F to 2200 F were investigated by X-ray diffraction methods. The scales are of two distinct types. The principal and definitive constituent of the more protective type is Cr2O3. A solid solution of Fe2O3—Cr2O3 also may be present; in such cases its Fe2O3 content tends to increase with attack rate at any given temperature. For very low attack rates considerable amounts of MnCr2O4 occur when the alloy contains a few tenths percent of Mn. As the attack rate increases this Mn-chromite spinel is replaced by a Ni-Cr-Fe spinel, the amount and the alloying element content of which increase with temperature and with the Ni level of the alloy. For an 80 Ni — 20 Cr alloy, NiO is also present.
When the attack rate becomes excessive the scale changes to a type containing no Cr2O3 phase but consisting of one or more Fe2O3 — Cr2O3 solutions and Ni-Cr-Fe spinels. The ferric-chromic oxide solution tends to concentrate in the outer layer and it then contains less than 10 mole percent Cr2O3; the proportion may increase when it occurs in an inner layer. Spinels are present in all layers but their content of the alloying elements increases in the inward direction.
These results are compared with existing data and interpreted using a theory of selective oxidation and depletion. 3.2.3
