The corrosion of chromium and nickel-chromium alloys in oxygen-sulfur-carbon gases at 800°C

The simultaneous oxidation and sulfidation of Cr, Ni-10Cr, Ni-20Cr, and Ni-30Cr was studied at 800°C in three gases falling within the Cr₂O₃ stability field of the Cr-S-O system. The sulfur partial pressure remained constant at 1×10^?6 atm, whereas the oxygen partial pressure varied from 5×10²¹ to 5×10^?20 atm, and the carbon activity varied from 0.108 to 0.416. Reaction kinetics were measured, and the reaction products were characterized by X-ray diffraction, metallography, scanning electron microscopy, and X-ray energy dispersive analysis. Reaction rates decreased with increasing oxygen partial pressure and decreased with increasing chromium content of the alloys. Sulfides always formed along with Cr₂O₃, even though the gases fell within the oxide stability field. No carburization was observed even though carbon activities were sufficiently high to form carbides. The reaction mechanisms are discussed, and the absence of carburization is analyzed on the basis of a three-dimensional stability diagram.