The effect of phosphate source on the post-treatment of cerium based conversion coatings on Al 2024-T3 and its correlation to corrosion performance

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ECS Transactions, ISSN: 1938-5862, Vol: 16, Issue: 43, Page: 47-60

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Daimon K. Heller; William G. Fahrenholtz; Matthew J. O'Keefe
The Electrochemical Society; The Electrochemical Society (ECS)
Engineering; Corrosion; Passivation; Anodic Films; Corrosion; Passivation; Anodic Films; Materials Science and Engineering
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The surface morphology, electrochemical characteristics, and salt spray corrosion performance were studied for cerium-based conversion coatings (CeCCs) 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 CePO, which further improved corrosion protection. Electrochemical analyses showed that CeCCs that contained hydrated CePOafter post-treatment had the highest resistance (∼100 kΩ-cm), most noble pitting potentials (∼ -270 mV), and best corrosion protection of the post-treatments that were tested. While pyrophosphate and polyphosphate post-treatments reduced cracks in the film, they did not promote formation of CePO·HO. The results suggest that the corrosion protection of CeCCs on Al 2024-T3 is dependent on both the surface morphology and phase of the coating. ©The Electrochemical Society.