Robust Fitting Polarization Curves of X70 Steel Under MIC and AC Interference

Extracting reliable corrosion kinetic parameters from polarization curves is particularly challenging for X70 steel under the synergistic effect of microbiologically influenced corrosion (MIC) and alternating current (AC) interference, as both factors significantly distort the electrochemical response. To address this, a novel fitting strategy was developed. In this approach, the overall polarization curve is reconstructed by combining separately modeled anodic and cathodic branches. To preserve the shape of the polarization curve, a slope-consistency constraint was enforced through dynamic penalty factors that balance fitting accuracy with shape preservation during optimization, which was performed using an intelligent algorithm. The results demonstrate the robustness of the proposed strategy, demonstrating low sensitivity to initial guesses and algorithmic parameters. Evaluation confirms that accuracy is maintained even when the data density is reduced by half. Among the parameters extracted, the Tafel slopes and corrosion current density show higher reliability than others. This work provides a robust and effective tool for the kinetic analysis of complex corrosion systems involving MIC and AC.