Analysis of impedance spectra of a pitted Inconel alloy 600 electrode in chloride ion-containing thiosulfate solution at temperatures of 298–573 K

In the present work, impedance spectra of a pitted Inconel alloy 600 electrode were analysed in aqueous 0.1 M Na₂S₂O₃+0.1 M NaCl solution at elevated solution temperatures of 298–573 K and at pressures of 0.1–8 MPa in terms of pit size distribution, fractal dimension and surface roughness in sequence as the solution temperature rose. From impedance spectra of the Nyquist plot obtained from the pitted specimen exposed to solutions at temperatures of 60–150°C, a constant phase element (CPE) was observed in the frequency range from 10³ to 1 Hz. Especially, it was found that the impedance spectra were divided into two sections, i.e. a first CPE with a smaller slope in the higher frequency range and a second CPE with a larger slope in the lower frequency range. The occurrence of the two kinds of CPE results from a transition of ion diffusion through the pit to ion accumulation at the pit bottom, which is caused by double-layer charging at the pit bottom and prior double-layer charging at the pit wall. Impedance spectra were simulated in terms of pit size distribution and values of the fractal dimension of the pits as a function of solution temperature to compare with those spectra measured experimentally. The spectra were simulated based upon the conventional transmission line model for a cylindrically shaped pit at 60 °C and on the basis of a modified transmission line model for the triadic Koch construction at 100 °C and for the quadratic Koch construction at 150 °C. The modified transmission line model takes into account the resistive and capacitive elements in the lateral direction as well as those in the downward direction of the three-dimensional Koch constructions. Above 200 °C the Nyquist plots were found to be depressed more noticeably from a perfect semicircular form with increasing solution temperature. This is due to the increase in the surface roughness of the specimen by the formation and growth of the pits.