The rate of iron sulfide formation in the solar nebula

The kinetics and mechanism of the reaction H₂S(g) + Fe(s) = FeS(s) + H₂(g) was studied at temperatures and compositions relevant to the solar nebula. Fe foils were heated at 558-1173 K in H₂S/H₂ gas mixtures (∼25 to ∼10,000 parts per million by volume (ppmv) H₂S) at atmospheric pressure. Optical microscopy and X-ray diffraction show that the microstructures and preferred growth orientations of the Fe sulfide scales vary with temperature and H₂S/H₂ ratio. Initially, compact, uniformly oriented scales grow on the Fe metal. As sulfidation proceeds, the scales crack and finer grained, randomly oriented crystals grow between the metal and the initial sulfide scale. The composition of the scales varies from Fe_0.90S to FeS with temperature and H₂S/H₂ ratio, in agreement with thermodynamic calculations. The weight gain and thickness change of the samples give nearly identical measures of the reaction progress. Sulfide layers formed in 25-100 ppmv H₂S grow linearly with time. Iron sulfides formed in ∼1000 ppmv H₂S originally grow linearly with time. Upon reaching a critical thickness growth follows parabolic kinetics. Iron sulfide formation in 10,000 ppmv H₂S also follows parabolic kinetics. The linear rate equation for sulfidation of Fe grains (≤20 μm diameter) in the solar nebula is d(FeS)/dt = k_fP_H2S - k_r P_H2(cm hour^-1). The forward and reverse rate constants are (cm hour^-1 atm^-1) k_f = 5.6(±1.3)exp(-27950(±7280)/RT) and k_r = 10.3(±1.0)exp(-92610(±350)/RT), respectively. The activation energies for the forward and reverse reactions are ∼28 kJ mole^-1 and ∼93 kJ mole^-1, respectively. FeS formation in the solar nebula is rapid (e.g., ∼200 years at 700 K and 10^-3 bars total pressure for 20 μm diameter Fe grains) as predicted by simple collision theory models of FeS formation.