The Use of Pseudocapacitance Measurements for Estimating the Corroding Surface Area on Underground Steel Structures

The use of interfacial capacitance to determine the surface area of actively corroding underground steel structures has been investigated. Some estimate of the active area is necessary for quantification of the corrosion rate. The behavior of steel corroding in two different soils over a range of moisture levels has been investigated using a novel cell design. Values of the specific interfacial capacitance as a function of moisture level, soil type and exposure time have been determined. These specific capacitance values, obtained using electrochemical impedance spectroscopy techniques, were found to range from 171 to 525 μF/cm² for all the test conditions investigated. These capacitance values are attributed to the existence of an adsorption pseudocapacitance. A decrease in the oxygen concentration with time in the soil cells used in this study results in impedances that interfere with the interpretation of the pseudocapacitance. While discussed for steel/soil corrosion, the methodology presented should be applicable to a broad range of corrosion problems.