Effect of Humidity on Cerium-based Conversion Coatings on Al 7075-t6

Cerium-based conversion coatings and Al 7075-T6 were exposed to 85% relative humidity at 40 °C to simulate ASTM B117 corrosion testing without NaCl. As-deposited coatings with cracks \u3e0.5 µm wide showed a significant increase in impedance from ~5 kΩ-cm2 before exposure to ~95 kΩ-cm2 after 168 hours of exposure compared to phosphate post-treated coatings (from ~60 to 90 kΩ-cm2) with cracks \u3c0.5 µm. the impedance of bare Al 7075-T6 increased from ~10 kΩ-cm2 before exposure to ~275 kΩ-cm2 after 168 hours of exposure. the increase in impedance in all cases was due to an altered layer containing Al and O on exposed alloy surfaces. X-ray photoelectron spectroscopy indicated that exposure to humidity reduced Ce(IV) to Ce(III) on as-deposited coatings, whereas post-treated coatings did not show any change in Ce oxidation state. Despite the increase in impedance, only post-treated coatings were able to inhibit corrosion in salt spray testing