| A rate model for the growth of multi sulfide layers during the sulfidation of a metal was proposed by considering the partitioning of cations between sulfides, their relative stabilities, rate of growth and molar volume of each sulfide. From this model, the theoretical ratios of thicknesses of sulfide layers formed were estimated as a function of temperature. Theoretical values obtained from the proposed model were compared with experimental results for the sulfidation of pure nickel by Bastow in order to verify excellence of this model. Sulfides formed during the sulfidation of pure nickel were three phases, such as Ni₃S₂, Ni_7S_6 and NiS, and the equilibrium sulfur pressures at the interfaces between sulfides were calculated by thermodynamic data. Good agreement was found between the theoretical and experimental values of the ratio of sulfide thicknesses. The parabolic rate constants for the growth of the entire sulfide layers could be theoretically obtained from the parabolic rate constants for the growth of only Ni₃S₂and also Ni_7S_6 layer, but values obtained were only in good agreement with experimental values at high temperatures. This may be due to the fact that the thicknesses of NiS are extremely small at high temperatures. |
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