Electrochemical behaviour of Ni3S2 and its spontaneous formation on metallic iron in ammoniacal-carbonate solutions

The oxidative dissolution of iron in ammoniacal-carbonate solutions containing nickel(II) and thiosulfate ions was found to result in the formation of a nickel sulfide layer which, based on Grazing Incidence X-Ray Diffraction (GIXRD) studies conducted on the iron surface following immersion, was found to contain Ni3S2. Consistent with this, linear sweep and rotating disk cyclic voltammetry measurements conducted with the iron electrode following immersion in these solutions resulted in the presence of an anodic peak thought to correspond to the oxidation of the nickel sulfide layer. Different concentrations and relative ratios of the ammonia and carbonate species were found to have a significant effect on both the size of this anodic peak, as well as the electrochemical behaviour of the iron during the immersion period under open circuit conditions. The results suggested that an increase in total carbonate concentration relative to the total ammonia concentration promoted the formation of the nickel sulfide layer which, in more dilute solutions than those typically encountered in industry, was in turn found to promote passivation of the iron. In order to evaluate the re-dissolution behaviour of the nickel sulfide layer following its potential formation on iron, further investigations were conducted using commercial Ni3S2, which was found to have a similar XRD pattern and electrochemical behaviour to the nickel sulfide layer formed on iron. Electrochemical studies indicated that the dissolution of Ni3S2 in ammoniacal-carbonate solutions is likely to be limited by the formation of a nickel-deficient, sulfur-rich species