Surface Characterization of 7075-T73 Aluminum Exposed to Anodizing Pretreatment Solutions

Localized corrosion damage in Type 7075-T73 aluminum alloy was investigated for various anodizing pretreatment solutions. The postexposure surface corrosion was characterized by use of scanning electron microscopy (SEM) examination. In addition, SEM and energy dispersive spectroscopy (EDS) were used for second-phase (constituent) particle identification for those found to induce pitting corrosion during solution exposure. The pitting mechanisms were identified as circumferential where the particles are noble with respect to the matrix phase and by selective dissolution where they are anodic. The designated category-1 degreasing and category-2 inhibited alkaline solutions did not initiate localized corrosion after 1200 s exposures. However, the category-3 high-pH NaOH and category-4 low-pH HNO₃ based solutions were found to initiate pitting attack, with the NaOH being significantly more aggressive. It was hypothesized that if the pits initiating during the pretreatment exposures were beyond a threshold size, on the order of 10-20 μm, a higher current density existed at these locations during subsequent electrochemical processes, thus resulting in larger and deeper pit structures. These surface defects are of primary concern with respect to accelerated fatigue crack nucleation. For smaller pits, on the order of 1-5 μm, the anodic process had a smoothing affect where the film growth tended to passivate the pits.