The reversibility of organic inhibitors and their synergistic effect with lithium salt technology for the corrosion protection of AA2024-T3 aluminium alloys

Due to environmental and carcinogenic effects of hexavalent chromium (Cr (VI)), finding an alternative corrosion inhibitor for structural aluminum alloys such as AA2024-T3 is of utmost importance. For this purpose, various corrosion organic inhibitors (benzotriazole, 2-mercaptobenzothiazole, sodium diethyldithiocarbamate, 2,5 dimercapto–1,3,4 thiadiazole and a proprietary inhibitor) were evaluated for their performance on AA2024-T3 in chloride environment. Electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to investigate the corrosion inhibiting properties of the organic inhibitors. The results confirmed that the selected organic inhibitors are effective corrosion inhibitors for AA2024-T3 when they are present in the chloride solution as soluble species. The electrochemical studies showed that organic inhibitors retard both cathodic and anodic processes through the inhibitor adsorption on the metal surface and blocking the active corrosion sites. In addition, it was found that the inhibition performance could be enhanced by the increased inhibitor concentrations and longer immersion times. Further analysis showed that the corrosion inhibition of AA2024-T3 by these organic inhibitors was reversible after replacing the inhibitor containing solution with the blank chloride solution. In the second part of this work, the synergism between organic inhibitors and lithium salts for the corrosion protection of AA2024-T3 has been investigated. The combination of the organic inhibitor and lithium carbonate salt resulted in a synergistic effect for the corrosion inhibition as confirmed by the results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Then, the Scanning Vibrating Electrode Technique (SVET) was used to study the galvanic coupling behavior between the aluminium matrix and the copper containing IM particles. Measurements on model system (Al/Cu/AA2024-T3) showed the effect of galvanic coupling in the presence of uninhibited chloride solution. Finally, the performance of the selected organic inhibitor was investigated when incorporated in an organic model coating. After application, the samples were scribed and exposed to the neutral salt spray test (168h). Optical Microscopy, EIS and White Light Interferometry (WLI) investigations were performed to characterize the corrosion protection behavior after the neutral salt spray test (ASTM B-117). Ultraviolet-visible spectroscopy (UV-VIS) confirmed that the selected organic inhibitor leached out of the coating.