A new method for the spatially-resolved measurement of pH during corrosion processes with the scanning electrochemical microscope (SECM) is presented. Antimony tips are employed because the dual-function characteristics of this material allow the combined amperometric/potentiometric operation of the SECM. The applicability of this technique is illustrated by considering the galvanic corrosion of a model zinc-iron pair immersed in 0.1 M NaCl aqueous solution. Spatially resolved images of pH and oxygen concentration above the metal specimens could be obtained in the same experiment.

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The fabrication of a solid-contact, micropipette-based magnesium ion-selective micro-tipped electrode (ISME) suitable for scanning electrochemical microscopy is reported and compared against a conventional micro-tipped ISME having a conventional aqueous internal reference electrode. Measurements showed that the solid-contact ISME had a lower internal resistance and a faster response time than the one with a liquid-contact. These advantages increased the spatial distribution and improved 2D images depicting concentration distributions of Mg 2+ . The ability of the microelectrode to image local ionic concentration has been tested over magnesium surfaces freely corroding or galvanically coupled to iron in aqueous chloride-containing solution. Scans of magnesium ion distribution, in the absence of corrosion currents, were also made over a micro-pipette source containing a concentrated magnesium chloride gel as a source of Mg 2+ and over a current source in the absence of Mg 2+ . From these measurements it was concluded that the potentiometric measurements over corroding surfaces were dominated by the changes in Mg 2+ distributions with small electric potential contributions due to corrosion current.

The applicability of scanning electrochemical microscopy for the local detection and quantification of relevant species participating in the corrosion of magnesium-based materials is presented. The ironmagnesium galvanic pair exposed to aqueous NaCl solution was adopted as model system for this purpose. Mg 2+ ion concentration and pH profiles were investigated using ion selective electrodes, containing a liquid membrane and Sb/Sb 2 O 3 as sensor elements, respectively. Oxygen consumption at the substrate related to the cathodic reaction was also monitored with the antimony-based electrode though operated amperometrically. Data show a major production of hydroxyl anions at the cathodic sites as result of the oxygen reduction half-reaction, whereas in the vicinity of the magnesium surface pH is greatly affected by the anodic dissolution process instead. The later produces the release of metal cations accompanied by hydrogen evolution.