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 degrees C to simulate ASTM B117 corrosion testing without NaCl. As-deposited coatings with cracks >0.5 mu m wide showed a significant increase in impedance from similar to 5 k Omega-cm(2) before exposure to similar to 95 k Omega-cm(2) after 168 hours of exposure compared to phosphate post-treated coatings (from similar to 60 to 90 k Omega-cm(2)) with cracks <0.5 mu m. The impedance of bare Al 7075-T6 increased from similar to 10 k Omega-cm(2) before exposure to similar to 275 k Omega-cm(2) 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.