No abstract

Model Fe-Cr and Fe-Cr-Ni alloys were exposed to Ar-CO 2 and Ar-CO 2-H 2 O gas mixtures at 650 • C. While all alloys initially formed protective Cr 2 O 3 scales, nucleation and growth of iron-rich oxide nodules resulted in some cases in breakaway oxidation. The conditions leading to departure from the protective stage are discussed in terms of Cr 2 O 3 thermodynamic and kinetic stability. The morphological and compositional evolutions accompanying nodule development were examined. The influence of carbide precipitation on alloy chromium diffusion and the ability of the alloy to form and maintain Cr-rich oxide layers was investigated.

Model Fe-Cr and Fe-Cr-Ni alloys were reacted in Ar-CO 2 and Ar-CO 2-H 2 O gas mixtures at 650 • C. Initially protective oxidation, due to Cr 2 O 3 formation, was usually followed by a transition to rapid oxidation, associated with Fe-rich oxide nodule development. Nodule surface coverage and overall weight gains are modelled on the basis of continued nucleation of nodules, and their diffusion-controlled growth. In general, water vapour accelerated nucleation. Alloying with nickel decreased average nucleation rates, but austenitic alloys showed large variations in nucleation susceptibility with grain orientation. Slower nodule growth induced by high chromium and nickel levels counteracted the effects of frequent nucleation.