Sulfide Film Formation on Copper Under Electrochemical and Natural Corrosion Conditions

The mechanism and kinetics of Cu corrosion in anoxic aqueous chloride solutions containing sulfide (10⁻³ mol/L) have been investigated electrochemically and under natural corrosion conditions. Under these conditions Cu is thermodynamically unstable in anoxic water, and the anodic growth of a chalcocite (Cu₂S)/digenite (Cu_1.8S) film is supported by the cathodic reduction of water. Electrochemical experiments at rotating disc electrodes and impedance spectroscopy show that the film growth occurs under SH⁻ transport control as stagnant conditions are approached. At this concentration, film growth can follow two distinct pathways. The initially formed film grows rapidly via an ion (or associated defect) transport process. If this film remains coherent, subsequent film growth/corrosion is extremely slow. If the development of interfacial stresses leads to film fracture, then growth continues and a much thicker nodular deposit is formed. The primary goal of this research is to develop a mixed potential model, which can be used to assess the performance of copper nuclear waste containers in granitic nuclear waste repositories.