Diffusion-limited current density of oxygen reduction on copper

There are many important processes in corrosion for which the diffusion-limited current density of oxygen reduction, i_L, plays a dominant role in terms of kinetic control. The conventionally accepted value of i_L (for the four-electron reduction mechanism) which can be found in many corrosion textbooks is in the range of 50-100 μA cm^-2, but the origins of this range of values are a bit mysterious. Previous research in our group aimed at ascertaining i_L (under stagnant conditions) on a planar Cu electrode and a Cu microelectrode array in a naturally aerated 0.1 M Na₂SO₄ electrolyte found that i_L was in the range of 20-30 μA cm^-2. In situ scanning tunneling microscopy was used to characterize the Cu surface at relevant potentials. Rotating disk electrode studies were used to measure i_L in a naturally aerated 0.1 M Na₂SO₄ electrolyte as a function of pH for both Pt and Cu electrodes. By comparing results for Pt and Cu we conclude that oxygen reduction occurs on a Cu surface via the four-electron mechanism. The oxygen diffusion-limited current density was found to be independent of pH (in the range 1.5-14). Finally, we conclude that our previous determination of i_L in a stagnant electrolyte reflects an accurate range of values of the oxygen diffusion-limited current density.