Abstract
Nickel specimens with layers of Na2SO4 deposited on the metal surface have been reacted in O2+4% SO2 in the temperature range 660–900°C. At temperatures from 671°C (the eutectic temperature of Na2SO4+NiSO4 liquid solutions) to 884°C (the melting point of Na2SO4), molten Na2SO4+NiSO4 is formed in the scales above critical pressures of SO3, and the molten sulfate causes accelerated hot corrosion of nickel. The rapid hot corrosion is preceded by an incubation period during which Na2SO4+NiSO4 solid solutions and eventually molten sulfate are formed. The critical SO3 pressures for formation of molten sulfate as a function of temperature have been delineated through experimental observations, and these are in agreement with theoretical estimates. When only solid solutions of Na2SO4+NiSO4 can be formed, the reactions are slower than specimens with no Na2SO4 layer. The reaction mechanism is concluded to involve inward transport of SO3/NiSO4 and of oxygen through the molten sulfate distributed as a network in the NiO layer of the outer part of the scale. Beneath the NiO/molten sulfate layer, the scale consists of NiO with a network of Ni3S2. Sulfur, present as (Ni-S)liq, is enriched at the metal/scale interface. Nickel diffuses outward through the Ni3S2 network in the inner layer to the boundary of the NiO/molten sulfate layer, where it reacts with the inwardly diffusing oxygen and SO3/NiSO4. The enrichment of sulfur next to the metal is concluded to be due to inward sulfur transport in the NiO+Ni-sulfide layer.
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Lillerud, K.P., Kofstad, P. Sulfate-induced hot corrosion of nickel. Oxid Met 21, 233–270 (1984). https://doi.org/10.1007/BF00656835
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DOI: https://doi.org/10.1007/BF00656835