The anodic dissolution of iron in ammoniacal–carbonate–thiosulfate–copper solutions with formation of Cu2S and dendritic copper

• S2O32 − prevents the passivation of Fe by Cu(II) in aqueous NH3/NH4HCO3.
• Non-adherent surface layer containing Cu dendrites and Cu2S forms on Fe surface.
• Solid layer remains undissolved in solution following detachment.

The presence of thiosulfate ions in ammoniacal–carbonate solutions containing copper(II) ions was found to prevent the passivation of iron, even though iron passivation is observed in solutions with no thiosulfate at very low copper(II) concentrations. The prolonged anodic dissolution of iron resulted in the formation of a partly crystalline sulfide layer on its surface, which based on Grazing Incidence X-Ray Diffraction (GI-XRD) analysis is thought to consist mainly of Cu2S. The effect of the sulfide layer was investigated using rotating disk cyclic voltammetry. Unlike the formation of an amorphous CoSx layer, which took place in similar solutions containing cobalt ions, the cuprous sulfide layer was not found to promote passivation of the iron. A significant amount of solid precipitate also became detached from the iron surface, remaining undissolved in the solution. This was analysed by X-Ray Diffraction (XRD) and scanning electron microscopy (SEM)–energy dispersive X-ray spectroscopy (EDX). Dendritic copper was observed both in the solid separated from the solution and in the precipitate still attached to the iron surface. The absence of iron passivation is thought to be due to both to the non-adherent nature of the cuprous sulfide layer, and to a disrupting effect caused by the cementation of copper.