Relationship Between the Inductive Response Observed During Electrochemical Impedance Measurements on Aluminium and Local Corrosion Processes

In-situ real-time imaging and electrochemical impedance spectroscopy (EIS) measurements were performed on pure aluminium, aluminium-copper and copper electrodes, with the aim to elucidate the relationship between the shape of EIS spectra and the local processes occurring on the surface of corroding aluminium, with particular attention to the appearance of inductive behaviour. When a stable oxide was present on the aluminium surface, the impedance modulus increased with decreasing frequencies, indicating the absence of inductive behaviour. Conversely, the presence of depassivating condition coupled with the availability of some anodic current, generated localized corrosion and associated hydrogen evolution at the corrosion front (also known as superfluous hydrogen evolution). In these conditions, a clear inductive behaviour was observed, regardless on the source of the anodic current at the corrosion front, i.e. galvanic coupling or external anodic polarization. On the other hand, hydrogen evolution per-se did not produce an inductive behaviour, as confirmed by control experiment on copper electrodes maintained at the same potential of corroding aluminium. Thus, the presence of inductive behaviour during EIS measurement on corroding aluminium is correlated to the phenomenon of hydrogen evolution at the corrosion front.