Phosphoric Acid Anodizing Effect on Morphology and Corrosion Resistance of Nanostructured Anodic Oxide Layers on 6061 Aluminum Alloy

Abstract

Phosphoric acid anodizing (PAA) exhibits a weakness of low corrosion resistance of anodic oxide layers on aluminum, necessitating a systematic analysis to understand a relationship between the PAA process and corrosion behaviors. This study investigated PAA effects on the morphology and corrosion resistance of nanostructured anodic oxide layers by varying electrolyte temperature, compared with sulfuric (SAA) and oxalic (OAA) acid anodizing, followed by NiF₂ sealing. 6061 aluminum alloy was anodized in 10 wt% phosphoric acid at 100 V for 30 min at 273, 293, and 313 K. The pore diameter, porosity, and oxide layer thickness increased with increasing the electrolyte temperature. Thin and irregular layers appeared at 313 K due to accelerated dissolution, resulting in the lowest corrosion resistance. The PAA sample at 293 K showed a current density of \(-\)0.75 V in potentiodynamic polarization, comparable to the sealed SAA and OAA samples, despite a thinner oxide layer. The barrier layer resistance of the PAA sample at 293 K was 1.60 \(\times\) 10⁷ \(\Omega\) cm², similar to SAA (1.44 \(\times\) 10⁷ \(\Omega\) cm²) and OAA (1.27 \(\times\) 10⁷ \(\Omega\) cm²). The barrier layer thickness was estimated at 60.4 nm for the PAA sample at 293 K, while minimal thickness was found at 273 and 313 K. A uniform AlPO₄ formation in PAA provides an effective protective barrier to significantly improve corrosion resistance without a requirement of sealing. This first detailed study on PAA provides benchmark processes and data that can be utilized for the rapid production of corrosion-resistant aluminum-based engineering components.

Graphic Abstract