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

Abstract

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 Na 2CO 3 or NaOH. Scanning electron microscopy indicated that as-deposited coatings on Na 2CO 3 activated substrates had fewer surface cracks, fewer craters, and were more uniform than those activated with NaOH. Post-treatment of coatings in a NaH 2PO 4 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 Na 2CO 3 activated substrates compared to coatings on NaOH activated substrates. Electrochemical impedance spectroscopy testing showed that post-treated coatings on Na 2CO 3 activated panels had total impedance of ~ 200 kΩ cm 2 compared to ~ 50–100 kΩ cm 2 for coatings on NaOH activated panels. Alkaline activation promoted deposition of corrosion resistant coatings with Na 2CO 3 activation exhibiting the best corrosion resistance and the least amount of subsurface damage.