Abstract
High-purity nickel has been reacted with 96% O2+4% SO2 at 700–900°C. The reaction has been studied at 700°C as a function of the total gas pressure (0.06–1 atm) and at 1 atm as a function of temperature (700–900°C). The reaction mechanism changes with the effective pressure of p(SO3) in the gas. When NiSO4 (NiO + SO3 = NiSO4) is formed on the scale surface, the scale consists of a two-phase mixture of NiO + Ni3S2; in addition, sulfur is enriched at the metal/scale interface. A main process in the reaction is rapid outward diffusion of nickel through the Ni3S2 phase in the scale; the nickel reacts with NiSO4 to yield NiO, Ni3S2, and possibly NiS as an intermediate product. When NiSO4 cannot be formed, the scale consists of NiO, and small amounts of sulfur accumulate at the metal/scale interface. It is proposed that the reaction under these conditions is primarily governed by outward grain boundary diffusion of nickel through the NiO scale, and in addition, small amounts of SO2 migrate inward through the scale—probably along microchannels.
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