Utilization of the Specific Pseudocapacitance for Determination of the Area of Corroding Steel Surfaces Available to Purchase

Determination of the capacitance of a corroding metal-electrolyte interface offers the possibility of an independent measurement of the area of the corroding metal. This in turn allows for the evaluation of the corrosion rate from the current calculated from the measured polarization resistance (Δν/Δl). Low carbon steel in fully aerated 1 M sodium sulfate solution was investigated using electrochemical impedance spectroscopy (EIS) to evaluate the interfacial capacitance as well as the polarization and ohmic resistances. The value of the capacitance per surface area unit was determined with samples ranging from 0.5 to 40 cm² over periods of up to 1000 h. The capacitance values changed quite rapidly during the first 72 h while the local chemistry and corrosion potential were stabilizing and then slowly increased to a stable level in ~500 h. A specific capacitance of ~200 μF/cm² was determined and is being applied to polarization resistance values from buried gas transmission pipeline samples to determine the corrosion rates of coating holiday areas.