Alkaline activation of Al 7075-T6 for deposition of cerium-based conversion coatings

The effect of alkaline activation on the morphology and corrosion performance of cerium-based conversion coatings deposited on Al7075-T6 was studied. Prior to coating deposition, alloy panels were activated in aqueous solutions containing Na2CO3 or NaOH. Scanning electron microscopy indicated that as-deposited coatings on Na2CO3 activated substrates had fewer surface cracks, fewer craters, and were more uniform than those activated with NaOH. Post-treatment of coatings in a NaH2PO4 solution improved the corrosion resistance and reduced the number of visible cracks; however, some regions still exhibited large cracks (> 0.5 μm wide). Subsurface crevices were found under craters and regions with large cracks, which were fewer for coatings on Na2CO3 activated substrates compared to coatings on NaOH activated substrates. Electrochemical impedance spectroscopy testing showed that post-treated coatings on Na2CO3 activated panels had total impedance of ~ 200 kΩ cm2 compared to ~ 50–100 kΩ cm2 for coatings on NaOH activated panels. Alkaline activation promoted deposition of corrosion resistant coatings with Na2CO3 activation exhibiting the best corrosion resistance and the least amount of subsurface damage.