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 S2- ions. In sulfide-free solutions, the steady-state potentials (E-st) are reached from negative values indicating oxide film thickening. Lower concentrations of S2- ions enhance oxide film thickening and the potential moves more in the noble direction. Increasing the S2- ion content causes the potential to fluctuate in the active and noble directions during a time span that depends on the S2- ion content. This behavior is attributed to the preferential formation of copper oxide and/or copper films. Higher concentrations of S2- ions, on the other hand, cause the potential to change markedly into the active direction, denoting the formation of Cu2S. Within this range of S2- ion concentration, the E-st varies with {[}S2-] according to: E-st = E degrees - 0.059 log {[}S2-] at 25 degrees C, while E degrees 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.