Assessment of the corrosion resistance of self-ordered anodic aluminum oxide (AAO) obtained in tartaric-sulfuric acid (TSA)

Abstract

The corrosion performance of self-ordered porous anodic aluminum oxide (AAO) has been studied and correlated to its structure. High purity aluminum alloy (> 99.999%) has been anodized using three different conditions: (a) in tartaric-sulfuric electrolyte employing common disordered conditions (TSA), (b) in sulfuric acid applying self-ordering conditions (SA), and (c) in tartaric-sulfuric electrolyte applying self-ordering conditions (TSA-SA). Some samples were post-treated in boiling water with the aim of sealing their pores. The morphology and order of the AAOs were determined by means of scanning electron microscopy and Fast Fourier Transform (FFT), while their corrosion behavior was assessed in 0.59M NaCl employing potentiodynamic polarization tests (LP) and electrochemical impedance spectroscopy (EIS). The evolution of impedance of samples SA and TSA was monitored during 1000 h of immersion in 0.59M NaCl. The three types of samples present similar pore diameter (Dpo) and certain degree of order, although TSA shows the lowest. However, TSA shows smaller interpore distance (Dint), thinner cell wall thickness (tw) and higher pore density (ρpo) and porosity (P) than SA and TSA-SA, which feature very similar values for these structural parameters. The LP curves do not allow to discriminate between samples. SA and TSA-SA have higher impedance than TSA among samples not sealed, probably because a thicker barrier layer forms in samples with self-ordering regime. When sealed, TSA samples show higher impedance than SA and TSA-SA in the medium frequency range because porous membranes with lower order are more easily sealed. All types of AAO, both unsealed and sealed, undergo self-sealing process during the immersion experiments. According to EIS interpretation, TSA porous oxide is self-sealed in a more efficient manner than SA and TSA-SA. These findings imply that self-ordering conditions can enhance the corrosion resistance of AAO.