Inhibition of corrosion-related reduction processes via chromium monolayer formation

The oxygen reduction reaction (ORR) was examined on copper, platinum, and glassy carbon electrodes, with regard to its inhibition by exposure of the electrode to chromate ion (Cr-VI) in NaCl solution. All three electrode materials exhibited a mass transport limited current for the ORR at sufficiently negative potentials, but this current was strongly inhibited in the presence of Cr-VI. Inhibition persisted in Cr-VI-free solution after the electrode was rinsed thoroughly, indicating that Cr-VI formed an irreversibly adsorbed inhibiting layer. A reduction peak observed in Cr-VI solution had an area of 1.1-2.5 mC/cm(2), and the area varied little with Cr-VI concentration, electrode material, and potential in the range of +0.2 to -0.6 V vs. Ag/AgCl. This reduction peak is attributed to Cr-III formation, and corresponds to formation of approximately a monolayer of Cr-III oxyhydroxide. Once formed, this Cr-III monolayer inhibits both O-2 reduction and further reduction of Cr-VI. The onset of monolayer formation at about +0.25 V vs. Ag/AgCl is the same as the potential of the onset of ORR inhbition in dilute Cr-VI. The monolayer also decreases the electron transfer rate to ferrocene and Ru(NH3)(6)(+3), which are known to be outer sphere redox systems that do not require adsorption to the electrode surface. The results indicate that the adsorbed Cr-III film formed by Cr-VI reduction is a powerful inhibitor of oxygen reduction, due both to occupation of active chemisorption sites and to inhibition of electron transfer. In the context of corrosion protection, Cr-VI acts as a "site-directed'' irreversible inhibitor which migrates to active sites for the ORR, then is reduced to Cr-III, and forms a permanent inhibiting monolayer. (C) 2002 The Electrochemical Society.