Precursor sites for localised corrosion on lacquered tinplates visualised by means of alternating current scanning electrochemical microscopy

In solutions of low conductivity and at high frequencies the impedance of a SECM tip-auxiliary electrode cell is dominated by the solution resistance between the tip and counter electrode. Alternating current scanning electrochemical microscopy (AC-SECM) utilises the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping domains of different conductivity/electrochemical activity on surfaces immersed into electrolytes. In the present study, we employed AC-SECM in aqueous solutions to evaluate the integrity of the solid/liquid interface of lacquered tinplates as commonly used in industry to manufacture, i.e. food cans. Significant differences were determined between the AC response and the phase shift measured with the SECM tip above the intact coating and above defects where the surface of the steel base is exposed. This allowed with high lateral resolution to detect and to visualise artificial micro cavities which we consider as an experimental model of microscopically small precursor sites for localised corrosion.