The anodic formation of sulfide and oxide films on copper in borate-buffered aqueous chloride solutions containing sulfide

The formation of sulfide and oxide films on copper has been studied in sulfide solutions containing chloride and buffered to pH = 9 with borate over the temperature range 20 °C to 80 °C. The primary goal was to investigate the susceptibility of copper to pitting corrosion over a range of temperatures expected in a deep geological repository. Films were formed electrochemically and characterized using scanning electron microscopy (SEM), energy dispersive X-ray and Raman spectroscopy and X-ray diffractometry (XRD). Chalcocite (Cu2S) formation was observed to occur under partially transport-controlled conditions in the potential range −0.9 V to −0.3 V vs SCE. At less negative potentials (≥ −0.3 V vs SCE), a transition from active dissolution to partial passivation by a copper oxide film was observed. Temperature had only a minor effect on the formation of the sulfide film, but the onset of active dissolution and oxide film formation shifted to lower potentials as the temperature increased.