The Susceptibility of Copper to Pitting Corrosion in Borate-Buffered Aqueous Solutions Containing Chloride and Sulfide

Abstract

The formation of sulfide and oxide films on copper (Cu) was studied in chloride solutions with and without added borate and sulfide. Electrochemical experiments demonstrated that Cu sulfide (Cu2S) films formed at considerably lower potentials than oxides (Cu2O/CuO) films. The growth of Cu2S was controlled by a combination of sulfide transport to the reacting Cu surface and the competition between SH− and Cl− ions for adsorption sites on the Cu surface. Scanning electron microscopy on focused ion-beam cut cross sections demonstrated that the films were porous and not passive. At more positive potentials, Raman spectroscopy and X-ray photoelectron spectroscopy demonstrated that the formation of oxide films, likely with the formation of a passive oxide film, dominated over anodic dissolution of Cu as CuI chloride complexes when the borate to chloride concentration ratio was sufficiently high. If pitting is to occur on Cu, it will require oxide not sulfide film growth.