A THERMODYNAMIC PERSPECTIVE OF COPPER TARNISHING IN MOIST AIR-SO2

A thermodynamic perspective of the initial stages of copper tarnishing in moist air-SO2 is presented. Previous experimental work has shown that in such a system the initial tarnish film consists mostly of a mixture of copper(l) oxide and copper(l) sulfide. Equilibrium criteria are used to calculate the ionic composition of the adsorbed water layer on the surface of the untarnished and the tarnished metal at various gas precursor concentrations. Using the adlayer chemistry, the electrode potentials of the redox reactions in the system are determined. Thermodynamically stable corrosion products are then calculated. The ability of SO2 dissolved in water to act as a cathodic depolarizer in this system, thereby forming sulfide in the tarnish film, is demonstrated.