Sulfidation Properties of a Nickel-20 w/o Chromium Alloy at 700°C and Low Sulfur Pressures

Abstract

The sulfidation kinetics and the morphological development of the reaction product for a Ni-20 weight per cent Cr alloy have been determined at 700~ and sulfur pressures below 5 • 10 -10 atm. The kinetics were observed to be parabolic; the parabolic sulfidation rate constant was found to vary as a semilogarithmic function of the sulfur pressure. The reaction product consisted of an external scale identified mainly as Cr3S4 with dissolved nickel up to 5 atomic per cent, and a chromium sulfide subscale with morphological breakdown of the alloy/external scale interface. Approximate determinations for the diffusion rates of chromium in the scale and sulfur in the alloy were made. Tentative mechanisms are proposed to explain the diffusion-controlled reaction and the stabilization of the external scale by dissolved nickel.An increased interest in sulfur corrosion has developed in recent years due to the natural occurrence of sulfur and its compounds in fuels and the difficulty of purifying these fuels. The behavior of Ni-Cr alloys of various compositions in sulfidizing gaseous environments has been fundamentally investigated by several workers (1-7). Romeo et al. (5) studied the diffusional growth of the duplex scale containing chromium and nickel sulfides on a Ni-20 w/o (weight per cent) Cr alloy at 700~ in H~-HsS atmospheres with sulfur partial pressures ranging from 8 • 10 -lo to 2 • 10 .2 atrn. The present study is a continuation of this work using the same alloy composition and temperature, but under conditions of extremely low sulfur patentials (Ps~ ~ 5 • 10 -10 atm). It is known that under these conditions chromium sulfide is the only reaction product, both externally and internally (7). Our purpose, accordingly, was to precisely establish the morphology of the scale and to ascertain the dependence of the nature of the chromium sulfide and the reaction kinetics on sulfur pressure.

Discontinuous Precipitation as a Mode of Internal Oxidation

“…To rationalize these results we note that data are available for the diffusion coefficient of S in Ni (10) and in the sulfides. At 700~ Ds Ni ~_ 10 -11 cm 2 sec -1, and for the diffusion of S through the Mo sulfides (11), Ds N 10-18 cm 2 sec-1.…”