Theoretical Considerations in the Measurements of Polarization and Corrosion Potential Curves

The polarization curve is defined as the current density potential relationship for a given electrode immersed in a given electrolyte. Based on this definition, the use of hemi-spherical or of cylindrical iso-potential test electrodes is proposed for its accurate determination. For these test electrodes, combined with counter electrodes of equal symmetry, drop of the potential in the electrolyte is shown to be: I_1ρr₁ (1-r₁/r_c) for hemi-spherical and I_1ρr₁ 1_n (r_c/r₁) for cylindrical symmetry, where I₁ = current density on the test electrode (amp/cm²), ρ = resistivity of the electrolyte (ohm x cm), and y₁, r_c = radii of the test and counter electrode (cm), respectively. By adding to either of these values, the potential drop between test electrode and electrolyte and by subtracting the potential drop between counter electrode and electrolyte, the total potential drop between these electrodes is obtained. The product I_1ρr₁ is found to be the parameter of deciding importance for an exact determination of the potential field which surrounds electrodes and of the potential of test electrodes. For test electrodes of conventional shape, the average potential drop in the electrolyte is estimated and a similar parameter defined. For sets of hemi-spherical or cylindrical test electrodes arranged in circular order with the reference electrode in the center, the potential field is expressed by an equation which provides a sound basis for the use of such multi-test electrode arrangements. Finally, the effect on the course of polarization curves of redox reagents such as originally contained in the electrolyte or produced by corrosion, is reviewed. From the relations derived, some conclusions are drawn and incorporated in the design of various types of experimental equipment for the determination of polarization and corrosion potential curves in laboratory and field studies, including the study of crevice corrosion.