The Transient Oxidation of Fe-Cr and Ni-Cr Alloys

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Abstract
The early stages of oxidation of Fe‐Cr and Ni‐Cr alloys, containing respectively nominally 5, 10, 15, and 30 a/o Cr, in 1 atm oxygen at 600°C have been studied by transmission electron microscopy of stripped films and by electron diffraction. Substantial amounts of iron and nickel oxides, respectively, are produced on all alloys before steady‐state, healing, chromium‐containing oxide layers are developed. This occurs more rapidly and more completely as the alloy chromium content is increased. For dilute Fe‐Cr and very dilute Ni‐Cr alloys, grain and subgrain boundaries and other substructural defects of the alloy are covered by thicker oxide. This is due either to their efficiency as cation vacancy sinks or to more local diffusional paths in the oxide above them. Conversely, for alloys richer in chromium (up to 30%), healing is most rapid in these locations due to rapid chromium diffusion to the alloy/oxide interface, leading to thin oxide above the defects. The tendency of rapidly growing iron oxides to overgrow the chromium‐containing oxides, and also to undermine them by encroaching on the alloy, is countered by the rapid development of a general healing layer of for the dilute Fe‐Cr alloys and for the concentrated alloys. probably does not form a healing layer for dilute Ni‐Cr alloys and the completion of a healing layer is slower than for Fe‐Cr alloys. This is largely due to the lower alloy interdiffusion coefficient. However, the slow growth of limits overgrowth and scale encroachment. healing is eventually approached by dense internal oxide formation, resulting partly from the relatively high solubility of oxygen in Ni‐Cr alloys.

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