Potentiodynamic anodizing of aluminum alloys in Cr(VI)-free electrolytes

The aerospace industry progressively develops alternatives for chromic acid anodizing, because Cr(VI) is known to be toxic and carcinogenic. In this work, potentiodynamic anodizing of AA1050 and AA2024-T3 clad was performed in phosphoric-sulfuric acid (PSA) and sulfuric acid (SAA). All anodizing cycles started with a linear voltage sweep, followed by a constant voltage, or a dynamic voltage. Current density responses were recorded during each anodizing cycle and comprised different stages, which could be related to growth phases of the anodic oxide film. Interesting differences were found between cycles with an intermediate increase in anodizing voltage versus cycles with an intermediate decrease in voltage. Cycles including an increase in voltage resulted in higher anodic oxide formation efficiencies because of a temporary exceedance of the steady state current (recovery period) directly after the voltage step. Also, a sudden decrease in voltage led to distinct border between a fine and coarse region in the film morphology, while a sudden increase in voltage did not. For prolonged anodizing in PSA, coarsening of the upper film part was observed because of the high solubility of Al2O3 in phosphoric acid. Pore walls close to the outer surface did not only get thinner, but completely dissolved in the electrolyte. Consequently, anodic oxide formation efficiencies were higher for SAA than for PSA. Copyright (C) 2016 John Wiley & Sons, Ltd.