SULFIDATION-OXIDATION BEHAVIOR OF A NI-CO-CR-SI ALLOY IN AN SO2+O2 ATMOSPHERE

The high-temperature-corrosion behavior of a Ni-Co-Cr-Si superalloy (HAYNES Alloy HR-160) in an SO2+O2 (SO2:O2=2:1) has been studied at 625-degrees-C and 750-degrees-C. Alloy samples were corroded for 72 hr, followed by XRD, SEM, EDS, and XPS analysis on the surface and/or cross-section of the samples, It was found that the simultaneous nucleation of nickel and cobalt oxides and chromia at the initial stage lead to the subsequent formation of a sulphate layer on reaction with the gas phase. This eventually lead to the formation of a thick sulphide layer above the chromia scale. The stable phases for Cr, Ni, and Co in such an atmosphere are chromia as well as Ni and Co sulphates. The existence of a thin sulphate layer was suggested some time ago but could not be detected and was disputed. The XPS analysis in this work now provides evidence for the existence of this sulphate layer. The transport of sulphur through the oxide scale into the alloy substrate is discussed It was concluded that sulphur advanced into the substrate by the oxidation of sulphides, thus liberating the sulphur. The possibility of sulphur transport in the form of SO2 molecules was also discussed. Based on the results and discussion, a schematic corrosion mechanism is presented.