Effect of Surface Condition on The Abnormal Oxidation of Fe-Cr-Al Alloys

Abstract

Studies were made to investigate the reason for occurrence of the abnormal oxidation on heat-resistant Fe-Cr-Al alloys. The results are summarized as follows.
(1) This anomalous oxidation was accompanied with nitrogen absorption and proceeded markedly at the temperature above 1000°C. The voluminous oxidation products comprising iron-base oxides (α-Fe₂O₃Fe₃O₄) were observed on the alloy surface and numerous massive blocks of aluminum nitrides (AIN) in the inner matrix.
(2) It was found that the initiation of the anomalous oxidation depended not only on the chemical composition but also on the surface condition of an alloy, as observed in alloy C (Fe-18.2%Cr-3.7%AI). In this case needle-shaped AIN were observed on the surface of the alloy C before the oxidation test. The nitride formation decreased the contents of aluminum in the matrix (to ca. 0.5%), which was considered to be related in the remarkable deterioration of the oxidation resistance.
(3) The anomalous oxidation of alloy C was able to be prevented by preliminary removal of the surface layer containing AIN or by preannealing at 900°C for the diffusion of Al and Cr.