Influence of Sulfur and Water Vapor on High-Temperature Oxidation Resistance of an Alumina-Forming Austenitic Alloy

Abstract

The oxidation behavior of alumina-forming alloys was studied after oxidation treatments conducted at 900 °C for 24 h in air or in steam. Alloys with sulfur contents ranging from 1 to 82 ppm in wt% were used. The influence of trace sulfur as well as the presence of steam in the oxidizing atmosphere were investigated. Under oxidation conditions, higher sulfur contents led to higher mass gains and the trend was more pronounced in steam than in air. Oxides were identified by Raman spectroscopy: a thin and continuous α-Al2O3 layer was formed at the metal-oxide interface in all cases. The mass gain differences were caused by other oxides formed at the surface of the samples—mainly spinel and Cr2O3—and in the internal oxidation zone too— mainly α-Al2O3 and θ-Al2O3—indicating that the protectiveness of the alumina layer greatly depends on the sulfur content in the base material and the oxidizing atmosphere. In order to explain this phenomenon, oxide structures were analyzed at various scales using scanning electron microscopy, transmission electron microscopy and nanoscale secondary ion mass spectrometry. Sulfur was detected at metal-oxide interfaces and also in the alumina layer in regions enriched with chromium. In addition, we demonstrate that steam oxidation leads to finer alumina grains as compared to air oxidation. Finally, the relationship between oxidation conditions, nanoscaled structural features and oxidation kinetics is discussed.