Unveiling the effect of the electrodes area on the corrosion mechanism of a graphite - AA2024-T3 galvanic couple by localised electrochemistry

Abstract

The corrosion mechanism of two AA2024-T3/graphite galvanic couples, with different electrodes area ratio, was studied in 12.00 mM NaCl solution using the Scanning Vibrating Electrode Technique (SVET) and the Scanning Ion-Selective Electrode Technique (SIET). Both techniques were used, quasi-simultaneously, to monitor the current density distribution and the pH associated to the corrosion process occurring in the galvanic couples. The morphological features and the elemental composition of the electrodes' surfaces, after 20 h of immersion, were analysed by Scanning Electron microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDX). The results show that the area of the electrodes in the galvanic couple significantly influences the corrosion mechanism. For the couple, with the highest AA2024/graphite area ratio (M1), both anodic and cathodic processes developed on the alloy surface, resulting in localised corrosion (trenching of the matrix around intermetallics) at the cathodic zones as detected by SVET and SIET. A more generalised anodic dissolution of AA2024 (corrosion of the matrix and grain boundaries) was observed for the sample with the lowest area ratio (M2), however.