Sulfate-induced hot corrosion of nickel

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.