Stainless steels are among those classes of alloys subject to preferential attack at grain boundaries by atmospheric oxygen at elevated temperatures. Boundary oxidation becomes important in high-temperature applications of thin sheet. The present investigation was undertaken in order to establish the effects of time and temperature on intergranular oxide penetration and to determine whether appreciable differences in composition exist between the intergranular oxides and the external scales. Intergranular penetration of oxides has been noted in 18-8 stainless steel by Baeyertz (1), who identified the intergranular material metallographically as consisting of a Cr2O3-rich phase and a glassy silicate phase. Other investigators, (2, 3, 4) have reported the presence of intergranular oxidation both in ferrous and non-ferrous systems. Rhines (5) has reported that in the case of copper-base binary alloys, intergranular segregation of solute oxides tended to disappear at higher temperatures, being replaced by a uniform oxide dispersion. It has been known for some time that the external scales formed on stainless steels are usually composed of two phases, one having a rhombohedral crystal structure and the other, a face-centered-cubic crystal structure (6). The rhombohedral phase has been shown to consist of solid solutions of Fe2O3 and Cr2O3 compounds which can perhaps best be denoted by FexCr2−xO3, where 0 ≤ x ≤ 2. The cubic phase is a spinel, which has been variously identified as NiCr2O4, FeCr2O4, Fe3O4, or MnCr2O4 (6, 7). These spinels in the pure state have lattice parameters ranging from 8.299 kX for NiCr2O4 to 8.428 kX for MnCr2O4 (see Table I). Recent work (8) shows that the system Fe(Fe2−xCrx)O, where 0 ≤ x ≤ 2, has lattice parameters which vary in an irregular fashion between 8.350 and 8.390 kX, a range which covers most of the reported lattice parameters for stainless steel scale spinels. In addition to this iron-chromium spinel phase, however, it has been shown (9) that MnCr2O4 exists as a separate phase in the chromium-nickel stainless steels. Thus, three separate phases, all of which were found in this investigation, are to be expected as being stainless steel scale components—FexCr2−xO3, Fe(Fe2−xCrx)O4, and MnCr2O4.
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