Sulfidation Properties of Ni-Al Alloys in H₂S-H₂ Atmospheres

Abstract

<p>The Sulfidation kinetics of Ni-Al Alloys containing up to 13 a/o Al exposed in H₂S-H₂ atmospheres were investigated using a thermogravimetric technique. The scale morphologies were studied using scanning electron microscopy and eptical microscopy. The scales were analyzed using electron phone microanalyses, λ-ray and differential thermal analyses.</p> <p>It was observed that the sulfidation kinetics of these alloys are linear, that the rates decrease with increasing amounts of Al at constant PS₂ in the atmosphere and at a given alloy composition the rates increase with increasing PS₂. The scale consists of three regions. The outer scale consists of NiS when PS₂ is greater than 10ˉ⁴ atm and it consists of Ni₃S₂ otherwise. The external scale subscale boundary corresponds to the original alloy-gas interface. The subscale grows by inward diffusion of S whereas the external scale grows outward diffusion of Ni. The subscale consists of two layers having different morphologies, and compositions. The layer next to the external scale contains alternate dark and bright lamellae. The bright lamellae consist of Ni₃-S₂ whereas the dark lamellae consist of an Al rich sulfide phase, presumably Al₂S₃. The second layer of the subscale is liquid at the reaction temperature of 700°C, which contains Ni, Al and S.</p> <p>A model is advanced to account for the linear kinetics, assuming that a reaction at the outer scale-gas interface can be used to describe the sulfidation properties. In order to explain the formation mechanism of the Lamellar morphology of the subscale isothermally at the reaction temperature, a model is advanced based upon concepts of eutectic decomposition. A ternary Ni-Al-S phase diagram is constructed which is consistent with the mechanism and the observations on the Ni Al-S system reported in the literature and in the present work. The functional relationship between the growth velocity of the Lamellar front and the resulting Lamellar spacing is derived and verified. According to this model, segregation at the Lamellar front is not the rate controlling proceeds because the linear rate constant would be proportional to the sulfur partial pre-pressure in the atmosphere, which contradicts the observation.</p>