Growth of the Iron Sulfide ( Fe1 − δ S ) Scale on Iron at Temperatures 600°–1000°C

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Abstract
Measurements of the parabolic sulfidation kinetics were carried out at sulfur pressures ranging from 10−11 to 10−2 atm and at temperatures in the range 600°–1000°C. The 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.
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