HIGH-TEMPERATURE SULFIDATION OF PACK-TANTALIZED IRON

The tantalization of iron in a pack containing Ta, CrF2 and alumina le ads to a duplex coating containing TaFe + TaFe2 in the internal part a nd nearly pure Ta containing iron in the external part. The kinetics o f tantalization are parabolic due to the limitation by a gas phase dif fusional mechanism. Tantalized iron samples were submitted to flowing Ar-H2S mixtures at temperatures between 500 and 700-degrees-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 gr ow by outward transport of Fe from the bulk through the Ta-rich extern al layer of the coating. The kinetics exhibit a first decreasing rate period followed by a constant rate regime. A model of diffusion-reacti on explains correctly the experimental results. The decreasing rate pe riod corresponds to transitory effects leading to steady state boundar y condition where the rate law becomes linear. Calculated curves fit c orrectly the experimental results and reaction and diffusion data are derived.