Behavior of the Copper Electrode in Alkaline-Sulfide Solutions Under Natural Corrosion Conditions

Potential-time curves are constructed for the Cu electrode in aerated sodium hydroxide (NaOH) solutions devoid of and containing increasing concentrations of S^2– ions. In sulfide-free solutions, the steady-state potentials (E_st) are reached from negative values indicating oxide film thickening. Lower concentrations of S^2– ions enhance oxide film thickening and the potential moves more in the noble direction. Increasing the S^2– ion content causes the potential to fluctuate in the active and noble directions during a time span that depends on the S^2– ion content. This behavior is attributed to the preferential formation of copper oxide and/or copper films. Higher concentrations of S^2– ions, on the other hand, cause the potential to change markedly into the active direction, denoting the formation of Cu₂S. Within this range of S^2– ion concentration, the E_st varies with [S^2–] according to: E_st = E° – 0.059 log [S^2–] at 25°C, while E° varies with the pH of the solution to give a straight line with a slope of 29 mV/pH unit. A mechanism is proposed that involves the formation of CuHS as an intermediate step in the process of Cu₂S formation.