Effect of Thickness on the Structure and Corrosion Performance of Sputtered Ce-Al-O Coatings on Al 2024-T3 Alloy Substrates

Abstract

Ce-Al-O coatings with thicknesses between ca. 136 nm and 1094 nm were deposited on Al 2024-T3 alloy substrates using magnetron co-sputtering of independent Al and CeO2 targets. The Ce-Al-O coatings with thicknesses from ca. 136 nm to 490 nm were compact, dense, and did not have distinct X-ray diffraction peaks. The ca. 1094 nm thick Ce-Al-O coating exhibited a columnar-textured outer layer ∼700 nm thick and a dense, ∼390 nm thick inner layer. X-ray photoelectron spectroscopy analysis revealed that the Ce-Al-O coatings contained Ce(III) species and that the Al/Ce atomic ratio decreased with increasing coating thickness. Electrochemical corrosion data corresponded with neutral salt spray testing. The Ce-Al-O coating not only inhibits the cathodic reaction but also decreases pitting nucleation sensitivity by increasing the difference in values between pitting potential and corrosion potential. The ca. 490 nm thick Ce-Al-O coated panel exhibited the best performance, the highest corrosion resistance and the lowest corrosion current density in neutral salt spray test, EIS scans, and cyclic polarization, respectively. Suppression of ionic diffusion as Ce-Al-O coating thickness increased resulted in improved corrosion performance up to ca. 490 nm, but corrosion inhibition decreased for ca. 1094 nm thick coatings due to a columnar microstructure.