Multilayer scale formation during the sulfidation of nickel

Abstract

The reaction between pure nickel and sulfur vapor has been studied over the temperature range 380–475° C. A scale consisting of four separate layers is formed and the growth of the three outer layers obeys parabolic kinetics. The innermost layer grows only in the very early stages, after which the thickness remains approximately constant. The layers have been identified as three different sulfides-Ni₃S₂, which forms the two inner layers, Ni₇S₆, and NiS. The three outer layers grow with preferred orientations; growth of the Ni₃S₂ and NiS layers is with the basal planes of their hexagonal structures parallel to the nickel substrate. The formation of the scale can be described in terms of a diffusion-controlled process and the same basic process continues after scale-metal separation occurs. The description of scale growth is combined with experimental data to calculate the diffusion coefficient of nickel ions in each phase.