Mathematical models for time-dependent impedance of passive electrodes

Three mathematical models are reported for the analysis of time-dependent e lectrode impedance data of passive electrodes. These models describe the co mplex relationships between the impedance, the components of the electrical equivalent circuit, the characteristics of the passive layer, the ac frequ ency, and the measurement time. Hence, they can be used to analyze time-dep endent electrode impedance data obtained from the anodic oxidation of metal s, alloys, conducting or semiconducting metal compounds in aqueous solution s. Both single and double passive layer models predict that the real impeda nce and negative imaginary impedance are approximately proportional to the square root of the resistivity of the passive layer and the measurement lim e. A solid-state transport model predicts that the impedance increases with measurement time, and with a decrease in either the concentration and/or t ransport coefficient of the electroactive ions. (C) 2001 The Electrochemica l Society. All rights reserved.