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Scanning electrochemical microscopy (SECM) and the Scanning vibrating electrode technique (SVET) were employed to identify the local reactivity and the cathodes and anodes on a cut edge of galvanized steel. The SECM was used in the amperometric feedback mode (for the oxidation of ferrocenemethanol as mediator) for sensing the conductivity of the surface and in the redox competition mode for locating the depletion of the cathodic reactant. Good agreement was observed in acidic solution between the location of the conductive regions of the surface and the depletion of oxygen estimated from amperometric lines across the cut edge. In neutral medium, non-uniform activity of the steel surface was observed and correlated well with the ionic current flows in solution and with the accumulation of zinc corrosion products on steel. In alkaline medium, the entire metallic surface operated as electron source for the regeneration cycle of the redox mediator. Irrespective of the pH of the solution, a maximum of the feedback observed over the cut edge is consentaneous with the thermodynamic stability of iron in the reduced form. SECM provided useful information regarding the steel conductivity, whereas the zinc anodes could only be resolved by the SVET.

A model electrode consisting of a narrow zinc anode and a split iron cathode was used for assessing galvanic corrosion at cut edges. This setup aimed at a better understanding of the mechanisms of galvanic corrosion at cut edges and of the effective resolution of microprobe electrochemical techniques, namely the scanning electrochemical microscope (SECM), the localized electrochemical impedance spectroscopy (LEIS) and the scanning vibrating electrode technique coupled with the scanning ion-selective electrode technique (SVET/SIET).Hydroxyl ions diffuse across the cathode, in the direction of the anode, until they reach the critical area of precipitation above the cathode, at a distance from the edge of the cathode. The zinc corrosion products ultimately precipitate on the iron cathode, once the critical pH and solubility limits for precipitation of zinc corrosion is reached, at a distance from the edge that is determined by the diffusion of Zn 2+ and counter-diffusion of OH -. These precipitates revealed no inhibition on the cathodic reaction.The local activity on the zinc was easily detected by the LEIS and the SVET, whereas the zinc corrosion products were detected by the SECM. The local a.c. admittance measured on the cathode closer to the zinc anode at the early stages was insensitive to the local activity despite the d.c. ionic current measured in solution, whereas for longer immersion times an increase of admittance is explained by an increase in the solution conductivity.