CorrosionDOE-HDBK-1015/1-93CORROSION THEORYRev. 0CH-02Page 7Figure 3 A Galvanic CellAn active electrochemical cell (oxidation-reduction reaction) is an unstable chemical system.The potential associated with a galvanic cell, for example, steadily decreases as current flows andthe oxidation-reduction reaction proceeds. Eventually, the potential falls to zero, the cell nolonger supplies electrical energy, and no further net reaction takes place. At this point thesystem is at equilibrium. In electrochemical cells, the decrease in cell potential caused by theoperation of the cell (current flow) is calledpolarization.This change in cell potential can be determined. Consider the zinc-copper galvanic cell shownin Figure 3. As the reaction takes place, Zn ions (produced by the oxidation of zinc metal) pass+2into solution. The Cu ions in solution are reduced as the copper metal plates out. Thus, the+2concentration of Zn in solution increases and the concentration of Cu decreases according+2+2to the following overall reaction.(2-5)As Zn increases and Cu decreases, the electrical potential decreases. This decrease in cell+2 +2potential, which results from changes in concentrations, is one form of polarization calledconcentration polarization.
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