Oxide Layer Formation on AA2024-T3 Aircraft Alloy via Anodization in Environmentally Conscious Organic Acid Electrolytes

The recent endeavor to establish a sustainable society, with respect to environmental protection and occupational health prevention, necessitates the development of environmentally friendly anodizing electrolytes. In addition, these electrolytes should be composed of biocompatible organic acids derived from renewable sources. In response to these challenges, there is a need to seek environmentally conscious alternatives to the widely used sulfuric acid anodization electrolyte. Accordingly, a comparative study was performed on the anodic polarization of AA2024-T3 aircraft alloy samples for 30 min at 0 or 20 °C. The respective electrolytes were composed of 0.5 M solutions of oxalic, citric, tartaric acids, or glycine. The comparative analysis included optical metallographic microscopy (OMM), scanning electron microscopy (SEM), color and wettability characterization, chemical composition analysis by X-ray photoelectron spectroscopy (XPS), and assessment of the corrosion-protective properties of the obtained layers. The latter were defined using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization scanning (PDS) after 24 h of exposure to a 0.5% NaCl solution. Among the most important conclusions is that the barrier properties of the layers obtained in citric and tartaric acid electrolytes remarkably exceed those of the film obtained in oxalic acid. The use of glycine does not result in film formation at all. The process temperature had a weaker effect than the electrolyte composition. The recent commitment to building a sustainable society, emphasizing environmental protection and occupational health, requires the development of eco-friendly anodization processes. These electrolytes should use biocompatible organic acids from renewable sources. Meeting these needs demands alternatives to the commonly used sulfuric acid anodization. Therefore, a comparative study was conducted on the anodic polarization of AA2024-T3 aircraft alloy samples for 30 min at 0 or 20 °C. The electrolytes consisted of 0.5 M solutions of oxalic, citric, tartaric acids, or glycine. Analytical methods included optical metallographic microscopy (OMM), scanning electron microscopy (SEM), color and wettability assessment, chemical composition analysis by X-ray photoelectron spectroscopy (XPS), and evaluation of corrosion resistance. The latter was measured using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization scanning (PDS) after 24 h in 0.5% NaCl solution. Key findings showed that barrier properties of the citric and tartaric acid layers substantially surpassed those of oxalic acid films. Glycine did not produce a film. The electrolyte composition had a greater impact than process temperature.