Oxidation of dense iron sulfide

Oxidation of stoichiometric iron sulfide was investigated. Rectangular plates of dense FeS were oxidized in an Ar-O₂ gas mixture at 1023 to 1123 K. Oxygen partial pressure was varied between 1.01 × 10³ and 2.03 × 10⁴ Pa. During the initial five minutes of oxidation, a magnetite layer of about 10 µm in thickness was formed on the surface without the evolution of SO₂ gas. Diffusion of iron from the interior of the sulfide to the sulfide/magnetite interface controlled the oxidation rate. Mass transfer through the gaseous boundary layer at the sample surface also affects the oxidation rate at lower oxygen partial pressures. Following this rapid formation of magnetite, the magnetite layer continued to grow for several hours in accordance with the parabolic rate law. Diffusion of iron through the magnetite layer controlled the oxidation rate during this stage. A thin layer of hematite was also formed on the outer surface of magnetite. When the composition of the inner sulfide core reached Fe_0.9S, the oxidation proceeded irregularly into the interior of the remaining sulfide. Porous oxide was formed and SO₂ gas was evolved.