Kinetics and Morphological Development of the Sulphide Scale on a Nickel - 20 w/o Chromium Alloy at 700°C and Low Sulphur Potentials

Abstract

The kinetics for sulphidation on a Ni-20 wt. % Cr alloy at 700ºC and sulphur potentials below 10⁻⁹ atmospheres have been studied using a thermogravimetric technique. The morphology of the reaction product was studied using optical microscopy, electron probe microanalysis and X-ray techniques. The kinetics were observed to be parabolic at sulphur potentials below 5 x 10⁻¹⁰ atmospheres with rates much slower than those reported for pure chromium under similar conditions. The parabolic kinetic reaction rate constant was found to vary as a semilogarithmic function of sulphur potential. The reaction product consisted of an external scale, identified as mainly Cr₃S₄, with dissolved nickel up to 5% and a subscale with morphological break-down of the alloy/external scale interface. Approximate determinations of the diffusivity of chromium in the alloy and the scale as well as sulphur diffusivity in the alloy have been made. Tentative mechanisms have been proposed to explain the diffusion control of the reaction and the stabilization of the external scale by dissolved nickel.