Electrochemical and Structural Changes in Cerium Based Conversion Coatings During Exposure to Salt Spray

摘要

Cerium-based conversion coatings on aluminum alloy 2024-T3 substrates were characterized after various exposure times in salt spray corrosion testing. Coatings post-treated in phosphate solutions exhibited no visible corrosion after seven days. Impedances doubled for these coatings after 12 hours of testing and a ～150 nm thick alumina layer developed after 24 hours of exposure, leading to the conclusion that post-treated coatings facilitated the formation of a protective alumina layer. As-deposited coatings exhibited significant corrosion after 24 hours of exposure and had impedances that were an order of magnitude lower than post-treated coatings. Subsurface voids present underneath coatings corroded into nodules of alumina on the surface of as-deposited coatings. In contrast, the voids corroded into small pits with little corrosion product present in post-treated coatings, which was attributed to a protective oxide present around the voids.