Reducing subjectivity in EIS interpretation of corrosion and corrosion inhibition processes by in-situ optical analysis

An in-situ hyphenated optical and electrochemical method for the real-time study of corrosion and corrosion inhibition processes is presented and validated for the case of AA2024-T3 exposed to two NaCl concentrations and six inhibitors. During testing, 5 μm resolution optical images of the exposed surface are obtained in parallel to electrochemical impedance measurements using a home-made 3D printed electrochemical cell. This method allowed obtaining both optical and electrochemical information of the studied surface with high time correlation. A data treatment analysis of the optical images was established thereby allowing the identification and quantification of corrosion-features related to intermetallic corrosion (e.g. trenching and meta-stable pitting) and co-operative corrosion (e.g. corrosion-rings, domes and surface-oxides) on a spatiotemporal scale, generally only observed through the use of ex-situ methods such as SEM. In addition, the study of the long-term corrosion inhibition of six inhibitors at concentrations ranging from 10⁻³ M to 10⁻⁶ M allowed quantifying inhibition kinetics as well as identifying different inhibitor and concentration dependent mechanisms (e.g. Cerium and DEDTC vs. Lithium) and decreasing incongruences between impedance and inhibition behaviour (e.g. DMTD). As a result, the use of quantifiable in-situ optical analysis is confirmed as a powerful tool to better interpret electrochemical signals or monitor electrochemical-dependent surface phenomena.