THE POLARIZATION IMPEDANCE OF COMMON ELECTRODE METALS OPERATED AT LOW CURRENT-DENSITY

The objective of this study was to characterize the polarization impedance (resistance and capacitance) of several common metal/0.9% saline interfaces operated at low-current density and to thereby provide a useful reference for those wishing to calculate the impedance of such electrodes. The series-equivalent resistance (R) and capacitive reactance (X(c)) of stainless steel, platinum, silver, MP35N, palladium, aluminum, rhodium and cooper electrodes, all having a surface areas S = 0.005 cm2 and all in contact with 0.9% saline, were measured as a function of frequency (100 Hz to 20 kHz) at low-current density (0.025 mA/cm2). For all the metals tested, both R and X(c) decreased with increasing frequency and the relationships were linear on a log-log plot. That is, R and X(c) exhibited power-law behavior (R = A/f-alpha and X(c) = B/f-beta). However, it was not generally true that A = B and alpha = beta = 0.5 as stated in the Warburg low-current density model. Furthermore, the Fricke constant phase model in which alpha = beta and phi = 0.5-pi-beta was found not to be applicable in general. In particular, the constraint that alpha = beta was a good approximation for most of the metals tested in this study, but the constraint that phi = 0.5-pi-beta did not hold in general. Although the Warburg low-current density model provides a useful conceptual tool, it is not the most accurate representation of the electrode-electrolyte The Fricke constant phase model is a better representation of electrode behavior, but is also may not be valid in general. We have found that a better representation is provided by the general power-law model R = A/f-alpha and X(c) = B/f-beta, where A, B, alpha, and beta depend on the species of both the metal and electrolyte and A and B depend, in addition, on electrode area. Using this model and the data presented in this study, the impedance of an electrode-electrolyte interface operated at low-current density may be calculated as Z = (0.005/S) square-root (Af-alpha)2 + (Bf-beta)2, where S is the surface area of the electrode in cm2.