Artifacts induced by large-amplitude probe vibrations in localized corrosion measured by SVET

A key feature of the scanning vibrating electrode technique (SVET) in the study of localized corrosion is that it is nonperturbing and therefore represents a spatially resolved measure of the corrosion activity in the absence of external factors. SVET probe vibrational amplitudes of >25 mum are shown to increase the rate of diffusion-limited oxygen reduction at a model platinum point cathode with the probe at typical scan heights (70-250 mum). The vibrating probe causes electrolyte impingement, reducing the thickness of the diffusion layer at the point cathode. In practical corrosion examples this can lead to current imbalance between total anodic currents (TAC) and total cathodic currents (TCC). Such a case is the corrosion of the cut edges of galvanized steel (0.7 x 20 mm steel area with 20 mum zinc coating on each face) exposed to 5% NaCl where oxygen diffusion to the metal surface is the rate-determining step. While increasing the probe amplitude increases local cathodic currents beneath it, it does not affect the overall measured TAC because it does not significantly alter the rate of oxygen reduction across the entire sample. An artifact of overestimation of TCC by up to an order of magnitude results for large-amplitude vibrations (25-250 mum). An optimum small-amplitude vibration (<25 μm) is therefore critical to the quality of the data obtained using SVET. (C) 2003 The Electrochemical Society.