Electrochemical Polarization

the variation in electrode potential E from the steady-state potential E_st, which the electrode exhibits in the absence of an external current. Electrochemical polarization is measured in volts (V) and millivolts (mV). If the variation is negative, that is, caused by a movement of electrons, which should be consumed in reactions, toward the cathode, then the electrochemical polarization is called cathode polarization. When the electrons move in the opposite direction, anode polarization occurs. Graphs of the functional relation between the electrochemical polarization and the current density i are called cathode and anode polarization curves, respectively, and are used extensively in the description and investigation of electrochemical and corrosive processes.

Generally, the relation between i and the electrochemical polarization is curvilinear, but in the range of deflections ±10–15 mV from E_st it as a rule is rectilinear. The slope of this segment, that is, the ratio of the increment in the electrochemical polarization to the increase in i has the dimension of resistance per unit surface area (ohms·cm²) and is called the polarization resistance of the electrode R_p. Electrodes with high R_p are said to be strongly polarizable, since even at very low i their potentials deviate markedly from E_st. Electrodes with low R_p are weakly polarizable. There exists an inverse relation between R_p and the rate of exchange of electric charges that occurs between the electrode and the electrolyte at E_st. At a corroding electrode this rate usually coincides with that of the electrolytic decomposition; the measurement of R_p is therefore sometimes used to determine the rate of electrochemical corrosion. If only one electrode reaction is possible at the electrode, then E_st coincides with the equilibrium potential E_e of the reaction, and the electrochemical polarization corresponds to the excess voltage of the reaction. The polarization resistance R_p turns out to be inversely proportional to the equilibrium exchange current.

The term “concentration polarization” is used to designate those variations in E that arise from the slow transfer of the initial or final components of a reaction occurring at the electrode. In the reaction zone, the concentration of the former (c_in decreases while that of the latter (c_fin) increases. This increases the tendency of the reaction to proceed in the reverse direction, which must be compensated for by the application of increased voltage. This tendency increases especially sharply when the reaction rate approaches the rate of the diffusion current. Here, either c_in decreases practically to zero or the products of the reaction crystallize, covering the electrode surface. The diffusion current can be increased by facilitating the transfer of matter through such means as vigorous agitation. The term “overvoltage” is often used instead of the term “concentration polarization,” since here the variation in E is calculated not from E_st but from the E_e of the reaction being considered.

The phenomenon of electrochemical polarization may be both harmful and beneficial. While electrochemical polarization causes a loss of electric power in electrolysis and reduces the efficiency of galvanic cells, it also has the capacity to inhibit certain undesirable corrosive processes.