Frequency-dependent alternating-current scanning electrochemical microscopy (4D AC-SECM) for local visualisation of corrosion sites

For a better understanding of the initiation of localised corrosion, there is a need for analytical tools that are capable of imaging corrosion pits and precursor sites with high spatial resolution and sensitivity. The lateral electrochemical contrast in alternating-current scanning electrochemical microscopy (AC-SECM) has been found to be highly dependent on the frequency of the applied alternating voltage. In order to be able to obtain data with optimum contrast and high resolution, the AC frequency is swept in a full spectrum at each point in space instead of performing spatially resolved measurements at one fixed perturbation frequency. In doing so, four-dimensional data sets are acquired (4D AC-SECM). Here, we describe the instrument set-up and modus operandi, along with the first results from the imaging of corroding surfaces. Corrosion precursor sites and local defects in protective organic coatings, as well as an actively corroding pit on 304 stainless steel, have been successfully visualised. Since the lateral electrochemical contrast in these images varies with the perturbation frequency, the proposed approach constitutes an indispensable tool for obtaining optimum electrochemical contrast.