A microstructural study of the oxidation of Fe-Ni-Cr alloys II. 'Non-protective' oxide growth

Abstract

The scaling characteristics of a range of laboratory prepared Fe—10Cr—Ni alloys and a commercial Fe-9C r-iMo steel when oxidized in air and in 1 % f CO - CO 2 at 600 C for periods up to 1500 h are described. The results are compared with similar data obtained for the Fe-20Cr-Ni alloys discussed previously in I. The microstructure and microchemistry of the ‘non-protective scales formed on the low chromium alloys were investigated by ‘edge-on’ transmission electron microscopy although data obtained by standard investigative techniques are also given. The simultaneous outward and inward growth of chemically and structurally different oxides was shown to occur in both oxidation environments but in a number of different ways and occasionally with both lateral and temporal instability. For example, in air, an inward growing fine grained spinel was found to be important in determining the oxidation behaviour in more than one manner: it could migrate bodily inward by ion exchange allowing further epitaxial oxidation above it, or alternatively it could inhibit further inward oxidation beneath it apparently increasingly as its chromium content rose. Evidence is also presented for the different ways in which either inward oxidation or carburization can occur in CO -CO 2 , this apparently requiring CO 2 to pass through pore-free outward and inward growing scales.