Measurements of the parabolic sulfidation kinetics were carried out at sulfur pressures ranging from i0 -Iz to 10 -2 arm and at temperatures in the range 600~176The effect of sulfur activity on these kinetics is most pronounced at low sulfur pressures. Temperature exhibits the largest influence on these kinetics at high sulfur pressures. An expression is derived for the parabolic rational rate constant in terms of the chemical diffusivity and nonstoichiometry of iron sulfide using the Wagner equation for scaling by iron diffusion and the Libowitz point defect model involving a strong repulsive interaction between iron vacancies to relate nonstoichiometry to sulfur pressure. The predicted values for these rate constants were in good agreement with those found experimentally. A comparison of available results on the parabolic scaling rates at sulfur pressures ranging up to 1 atm suggests that a transition in scale texture with sulfur pressure influences the sulfidation rates since iron diffusion is more rapid in the "c" than in the "a" crystallographic direction of iron sulfide.

Measurements of the sulfidation scaling kinetics of iron under transient and steady-state conditions and relaxation kinetics of different changes in the nonstoichiometry of FeS plates were carried out under sulfur pressures 10-11-10 -3 atm in the temperature range 600~176The scales and plates .exhibited a preferred texture in the "a" crystallographic direction. The chemical diffusion coefficient is independent of the degree of nonstoichiometry at a given temperature and its temperature dependence is given by RTThe sulfur pressure and temperature dependence of the nonstoichiometry of iron sulfide is interpreted by a point defect model involving a large repulsive interaction between iron vacancies. The resulting correlation yields an expression for evaluating the iron self-diffusion coefficient from the chemical diffusion coefficient. The predicted values for the iron self-diffusion coefficient, the sulfur pressure, and temperautre dependence are in good agreement with results for the iron tracer-diffusion coefficient in the "a" crystallographic direction of monocrystal iron sulfide.

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