Copper oxidation outside cells

Crevice corrosion of copper alloys is similar in principle to that of stainless steels, but a differential metal ion concentration cell (Figure 53.4(b)) is set up in place of the differential oxygen concentration cell. The copper in the crevice is corroded, forming Cu ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu ions. These are strongly oxidizing and constitute the cathodic agent, being reduced to Cu ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system. 

Electrons in this zinc-copper electrochemical cell flow from the zinc strip to the copper strip, causing an electric current that powers the light bulb. As the spontaneous reaction continues, much of the zinc strip is oxidized to zinc ions and the copper ions are reduced to copper metal, which is d osited on the copper strip. If an outside voltage source is applied to reverse the flow of electrons, the original conditions of the cell are restored. 

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