Effect of Sulfur Partial Pressures on Oxidation Behavior of Fe-Ni-Cr Alloys

Fe-Ni-Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050 A degrees C with varied sulfur partial pressures. The oxide-scale growth on Fe-Ni-Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCr2O4 spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe-Ni-Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed.