Abstract
Experiments with a simple bridge for the determination of polarization resistance, electrode capacitance, and time constant of corroding electrodes are presented. The novelty is the use of low-frequency square waves (e.g., 10 mHz, 5 mV peak-to-peak) impressed on the specimens via capacitors in parallel with resistors. The instrumentation is very cheap and can be put together easily. The method is non-disturbing and reasonably fast. Data from eight different corroding systems have been compared with results from electrochemical impedance spectroscopy (EIS) and potentiokinetic sweep. The polarization current can alter performance of electrodes, so polarization resistance varied substantially with the type of measurements and the measuring parameters for seven out of the eight test cases. Therefore, the instrument was designed to handle modeling with two resistance-capacitance (RC) links in series. Since the lack of a well-defined polarization resistance is not easily observed with conventional methods with potentiostats and galvanostats, the bridge may have an advantage. The system with two RC links gives two sets of values for polarization resistance and electrode capacitance. The polarization resistance of the link with the shortest time constant can apply for nearly undisturbed, steady-state corrosion. In contrast to EIS are equivalent circuits with component values found by very simple calculations. The serious limitation is that many corroding systems (e.g., diffusion-limited electrode kinetics) cannot be modeled with RC circuits.
