The sulfidation kinetics of a Ni-20 w/o (weight per cent) Cr alloy exposed to hydrogen sulfide-hydrogen atmospheres at 700~ have been determined as a function of the sulfur pressure. The morphology and phase composition of the duplex sulfide scale formed as a corrosion product have been investigated. The kinetics appear to be controlled by diffusion of the reactants through the scale. No significant dependence of the parabolic rate constant on the sulfur pressure has been determined within the wide range 8 x 10 -8 to 2 x 10 -2 atm. A correlation is presented for the growth kinetics and the structural features of the nickel and chromium sulfide constituents of the scales with the ternary nickel-chromium-sulfur phase diagram.A recent review paper by Stratford (1) points out the increasing interest during the past few years on the role of sulfur in the high-temperature corrosion of metals and alloys. Due to the difficulty of purifying natural fuels from sulfur and sulfur compounds and the inherent aggressiveness of such reagents at high temperature, considerable problems may arise in plant operations and, in general, when metal parts have to work in such environments. Although several explanations have been proposed, the fundamental mechanism underlying sulfidation phenomena is not completely understood. Comparatively few data have been collected in this field with respect to the amount of work done in the field of oxidation with air, pure oxygen, or oxygen-supplying atmospheres.Although the sulfidation of pure metals may possibly be described in terms of comparatively simple models, the situation becomes most complicated when dealing with alloys. In an attempt to gain a better insight into sulfidation properties, we selected for the present study a nickeI-base type alloy. The strengh and the high-temperature corrosion resistance of these alloys make them suitable for special applications, like turbines, jet engines, heat-exchangers, etc. A control of sulfur corrosion is therefore of paramount importance for these alloys.A comprehensive investigation on the sulfidation behavior of nickel-chromium alloys has been carried out by Mrowec and co-workers (2) by using pure sulfur at the constant pressure of 1 atm. The whole range of alloy compositions was studied. We thought that it would be of interest to establish the dependence between rate of sulfidation of nickel-chromium alloys and sulfur pressure, the latter being controlled by means of hydrogen sulfide-hydrogen atmospheres. Some work with these atmospheres has already been done by Hancock (3), Davin et al. (4), and Seybolt (5), but to our knowledge no systematic study has been made to establish the above correlation. At the same time, we intended to determine the equations governing the kinetics in the chosen conditions and the structure and composition of the phases formed as corrosion products, for the purpose of assessing a detailed mechanism of scale formation.

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The separate phases present in a Ni-base superalloy may offer different resistance to hot corrosion attack. Ni-Cr, Ni-A1, and Ni-Cr-A1 alloys of composition corresponding, respectively, to the nominal ,y matrix, the ~' phase, and a system of both these two phases, were coated with Na2SO4 and exposed to oxygen or H2S-H2 gas mixtures in the temperature range 850~176 The corrosive attack was monitored with a recording microbalance. The results indicate that the degree and mechanism of corrosion of the above phases is markedly influenced by the nature of the environment and the composition of the oxide scales formed at the alloy surface, in general, the ~,' phase appears to be a weak link in the over-all hot corrosion process.

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