Anomalous currents determined by SVET due to composition gradients on corroding Zn surfaces in 0.1 M NaCl

Abstract

The uniform and localized corrosion of Zn in chloride solution were analyzed using the scanning vibrating electrode technique (SVET). Initially, shallow crystallographic pits nucleate, which then spread laterally assuming prismatic hexagonal orientations (100), and finally cover the whole surface. Anomalous cathodic currents are observed by SVET over the whole surface after the corrosion type changes from localized to uniform corrosion of the metal surface due to the lateral growth of pits. These anomalous residual cathodic currents are unreal and were assigned to pH and oxygen concentration variations near the electrode surface, changing the open circuit potential (OCP) of the vibrating Pt microelectrode of the SVET system in the negative direction toward the surface. The combined action of the /Zn2+ hydrolysis and oxygen reduction causes a pH increase toward the metal surface. A similar effect is expected in the case of oxygen depletion near the surface by the cathodic reaction. For a corrosion process under diffusion control, the concentration gradients will form in the region of the μ-electrode vibration, as confirmed by measuring the OCP of the μ-electrode versus the distance to the electrode surface.