The structure of the oxide scale formed on commercial chromium steels containing 5 to 26 percent Cr when oxidized in air or oxygen at temperatures from 700 C to 1160 C for times up to 100 hours were determined by X-ray diffraction methods, supplemented in some instances by chemical analysis. Two distinct types of scale were observed: A type scale occurs when the rate of metal loss is less than approximately 10 mg/cm2/day, and B type when the attack rate is in the excessive range. For exposures near the critical conditions an initial A type scale transforms to B type during oxidation.
The essential component of A type scale is Cr2O3. This is usually accompanied by αFe2O3 in an amount which increases with the iron content of the alloy (i.e., with rate of attack). At high temperatures or long oxidation times, dilute solid solutions of each of these phases in the other are formed. When the alloy contains a few tenths percent of Mn, the A type scale may also include copious amounts of MnCr2O4, especially for high chrome alloys and for low temperature oxidations in air.
B type scale is more complex than A scale and may be considered as two layers. The outer layer is duplex and is similar in all respects to the Fe3O4 and Fe2O3 layers of the scale on pure iron. The major constituent of the inner layer, corresponding to the FeO layer on pure iron, is a solid solution, FeFe(2-x)CrxO4, of the spinel type. Throughout most of the layer x varies slowly, but near the metal it rises rapidly to x=2 and near the Fe3O4 layer it falls rapidly to x = 0. The mean value of x lies in the range of about 0.5 to 1.5. The greater the chromium content of the alloy, the higher the temperature and the longer the time of oxidation, the larger is the value of x. In cases of very high attack the series of spinels is accompanied by a series of wüstites which are modified by chromium additions ranging downward to zero at the Fe3O4 layer. For 10 to 20 hour oxidations at 925 C and at 1000 C the chromium fraction of total metal in the inner layer is about 1.8 times its value in the alloy, whereas the excess is only about one percent in the scale as a whole.
The experimental data are compared and contrasted with existing information and are qualitatively interpreted by use of a tentatively proposed Fe-Cr-O phase diagram and a special theory of alloy oxidation. 3.2.3
