Application of Admittance Analysis for Corrosion Rate Evaluation of Carbon Steel with Rust Layer in Electrochemical Impedance Spectroscopy

This paper describes an evaluation method for corrosion rate of carbon steel covered with rust layer by admittance analysis in electrochemical impedance spectroscopy. The polarization resistance, Rp, is often estimated from the impedance spectrum of carbon steel to determine the corrosion rate. The Rp can be determined from the impedance spectrum when the low frequency impedance is converted to the real axis on the Nyquist diagram. Because the impedance spectrum of carbon steel covered with rust layer often describes a part of loop in the low frequency range, it is difficult to determine the Rp by extrapolating the low frequency impedance to the real axis. In the present study, an admittance analysis was employed to determine the Rp from the admittance spectrum of carbon steel covered with rust layer. The admittance is plotted as the reciprocal of impedance on the complex plane. In this case, the Rp can be determined from the admittance spectrum when the low frequency admittance is converged to the real axis. The admittance spectrum of carbon steel with rust layer indicated that the low frequency admittance was converted to the real axis, namely, the Rp could be determined from the admittance spectrum. The corrosion rate of carbon steel with rust layer could be estimated from the Rp by admittance analysis, demonstrating that the value was corelated to that estimated from corrosion loss. The impedance and admittance simulations were performed using an equivalent circuit to discuss the time constant observed in the low frequency range.