SIMULTANEOUS SULFIDATION-OXIDATION OF NICKEL AT 603-DEGREES-C IN SO2-O2-SO3 ATMOSPHERES

The corrosion rates of nickel in SO2-O2-SO3 atmospheres have been measured to determine corrosion mechanisms under conditions of simultaneous sulfidation-oxidation. The corrosion rates are observed to be independent of P q2 (0.1 to 0.75 atm) and PSO2 (0.2 to 0.85 atm) but decrease with increasing -PSO3 (0 to 0.17 atm). Although thermodynamic calculations predict the formation of NiSO4, the product scale consists of an inner Ni3S2 layer and an outer two-phase layer of NiO and Ni3S2. The reaction rates are believed to be influenced by both a surface reaction and diffusion through the outer two-phase layer. It is demonstrated that the observed rapid corrosion rates are due to Ni3S2 providing a rapid transport path for nickel diffusion across the outer two-phase layer. When SO3 is present, the corrosion rates decrease due to an extremely irregular distribution of Ni3S2 and a decrease in its fractional area in the two-phase layer. The precise mechanism by which SO3 changes the Ni3S2 distribution is unknown, but it is possible that SO3 reoxidizes part of the chemisorbed sulfur on the scale surface. © 1979 American Society for Metals and the Metallurgical Society of AIME.