Effect of Phosphate Source on Post-Treatment of Cerium-Based Conversion Coatings on Al 2024-T3

The surface morphology, electrochemical characteristics, and salt spray corrosion performance were studied for cerium-based conversion coatings on Al 2024-T3 that were post-treated in heated aqueous solutions of orthophosphate, pyrophosphate, and polyphosphate compounds. Phosphate post-treatment reduced cracking in the coatings, which resulted in better corrosion protection compared to coatings that were highly cracked or contained other defects. In addition, post-treatment in orthophosphate solutions converted the as-deposited hydrated cerium oxide to hydrated , which further improved corrosion protection. Electrochemical analyses showed that cerium-based conversion coatings that contained hydrated cerium phosphate after post-treatment had the highest resistance , the most noble pitting potentials , and the best corrosion protection of the post-treatments that were tested. While pyrophosphate and polyphosphate post-treatments reduced cracks in the coatings, they did not promote the formation of hydrated cerium phosphate. The results suggest that the corrosion protection of cerium-based conversion coatings on Al 2024-T3 is dependent on both the surface morphology and the phase of the coating.