High temperature sulfidation of pack-tantalized iron

Abstract
The tantalization of iron in a pack containing Ta, CrF₂ and alumina leads to a duplex coating containing TaFe + TaFe₂ in the internal part and nearly pure Ta containing iron in the external part. The kinetics of tantalization are parabolic due to the limitation by a gas phase diffusional mechanism. Tantalized iron samples were submitted to flowing Ar-H₂S mixtures at temperatures between 500 and 700 °C and the kinetics followed continuously with a magnetic suspension balance. Due to the very low sulfidation rates, great care was taken to the amount of residual H₂O in the sulfidizing gas. The formed product was always FeS, with no evidence of Ta sulfide(s). This compound was shown to grow by outward transport of Fe from the bulk through the Ta-rich external layer of the coating. The kinetics exhibit a first decreasing rate period followed by a constant rate regime. A mode1 of diffusion-reaction explains correctly the experimental results. The decreasing rate period corresponds to transitory effects leading to steady state boundary condition where the rate law becomes linear. Calculated curves fit correctly the experimental results and reaction and diffusion data are derived.