Malic-Sulfuric Acid Anodizing of AA 2024 Compared to Other Dicarboxilic-Sulfuric Acid Anodizing Processes

Aiming to develop hexavalent chromium (CrVI)-free anodizing baths for Al-alloys, the porous anodizing of AA2024 was studied in different mixtures of sulphuric acid-organic acid formulations with similar concentrations. Mixtures of sulphuric with dicarboxylic acids as malic, malonic, oxalic and tartaric and with the tricarboxylic citric acid were tested. The corrosion behavior of the unsealed anodized layers exposed to 0.1M NaCl were compared by voltammetry and chronoamperometry at polarizations slightly above the pitting potential of blank AA2024 with the behaviour of standard chromic acid anodizing. The obtained oxide layers were characterized by SEM, FTIR and micro-Raman spectroscopy, which confirmed the incorporation of organic acids in relative different amounts. Best corrosion resistances expressed in terms of pitting nucleation time were observed for malic-sulphuric, followed by tartaric-sulphuric anodizing. However, none of the formulations achieved the resistance conferred by the chromic acid anodizing. A direct correlation between mesoporosity of the anodizing layer and corrosion resistance was verified.