Temperature effects on the passivity breakdown of copper in chloride-containing borate buffer solution

Passivity and passivity breakdown of pure copper were studied in alkaline chloride-containing borate buffer solution from 25 degrees C to 80 degrees C by electrochemical measurements and scanning electron microscopy. The corresponding E-pH diagrams were also calculated. Increased temperatures make the open circuit potential and formation potential for cuprous oxide (Cu2O) shift in a negative direction. The mean breakdown potential shifts in a positive direction and the distribution of breakdown potential has a wider width, while the minimum breakdown potential decreases slightly at higher temperature. The anodic passive film formed at 0.4 V versus Ag/AgCl for 6 h in a chloride-free solution at T >= 40 degrees C can maintain passivity after the addition of 0.1 M NaCl, but cannot maintain passivity at 25 degrees C at the same concentration. High temperature benefits the growth of copper oxide over the formation of copper chloride species. Cu2O and CuO have excellent corrosion resistance to 0.1 M NaCl and passivity can be maintained well above the CuO/Cu2O equilibrium at pH > 8.