Phosphate post-treatment of cerium-based conversion coatings on Al 2024-T3

"Phosphate post-treatment of cerium-based conversion coatings (CeCCs) on high strength aluminum alloys can significantly improve corrosion resistance. As-deposited CeCCs exhibit corrosion pits and salt tails across the specimen surface after 3 days of exposure, but post-treated CeCCs have withstood 14 days of salt spray exposure without visibly corroding. The morphology, phase, and electrochemical properties of spray deposited CeCCs were affected by post-treatment parameters such as immersion time, solution temperature, and phosphate source. The best performing coatings were post-treated in aqueous orthophosphate solutions for at least 5 min at temperatures of at least 85⁰C. These conditions converted cerium hydroxy/peroxy species in the as-deposited CeCC to hydrated CePO₄ and minimized cracks in the coating. Despite demonstrating the kinetic dependence of processes active during post-treatment, these results suggested that the corrosion resistance of CeCCs was dependent on the coating phase and morphology. Using an aqueous precipitation technique, hydrated CePO₄ coatings were directly deposited onto Al 2024-T3 substrates and compared to as-deposited and post-treated CeCCs. After salt spray exposure, analysis revealed the formation of pits in the alloy where the substrate was exposed by cracks in the directly deposited CePO₄ coating. Post-treated CeCC specimens did not exhibit corrosion at crack/substrate interfaces, indicating that CeCCs can provide electrochemical protection. Post-treated CeCCs also formed an interfacial reaction layer at CeCC/substrate interfaces, a response not observed for directly deposited CePO₄ coatings or as-deposited CeCCs. These results demonstrate that post-treated CeCCs are not static barrier coatings, but respond actively to corrosion"--Abstract, page iv.

Name thesis:degree_name

Ph. D. in Materials Science and Engineering

Grantor

Missouri University of Science and Technology

Year dc:date.available

2016

Author dc:creator

• Heller, Daimon K.

• Materials Science and Engineering

Repository record dc:identifier

https://scholarsmine.mst.edu/doctoral_dissertations/1900

OAI identifier oai:identifier

oai:scholarsmine.mst.edu:doctoral_dissertations-2902