Simultaneous sulfidation-oxidation of nickel at 603°c in argon-so2 atmospheres

The simultaneous oxidation-sulfidation rate of nickel has been measured as a function of SO₂ pressure (0.04 to 1 atm) in Ar-SO₂ gas mixtures at 603°C. The observed corrosion rates are about 10⁷ times faster than the oxidation rate of nickel in oxygen at 1 atm. The product scale consists of an inner Ni₃S₂ layer and an outer two-phase layer of NiO and Ni₃S₂. A linear rate law is observed during an initial time period, and the most probable rate-controlling step is dissociation of SO₂. An increase in the scale-gas interfacial area increases the corrosion rate during intermediate time periods. With increasing time, parabolic corrosion rates are measured for SO₂ pressures of 0.25 and 1 atm. Values of the nickel diffusivity in Ni₃S₂ calculated from our measured parabolic-rate constants are in good agreement with recently reported values. This agreement indicates that an interconnected Ni₃S₂ phase in the outer two-phase layer provides rapid transport paths for nickel diffusion through the scale. Formerly a Graduate Student in the Department of Metallurgy and Materials Science at the University of Pennsylvania, Philadelphia, PA