Study of corrosion protection of the composite films on A356 aluminum alloy

Composite films were fabricated on A356 aluminum alloy by combined anodizing and rare earth deposition. The corrosion protection effect and corrosion behavior of the composite films in 3.5% NaCl solution were studied by electrochemical impedance spectroscopy (EIS). SEM observation indicated that the rare earth Ce film completely sealed the porous structure of the anodic film, and the composite films composed of anodic film and Ce film were compact and integrated. According to the characteristics of EIS, the EIS plots of the composite films at different immersion times were simulated using the equivalent circuits of Rsol(QceRce)(QaRa), Rsol(QceRce)(QpRp)(QbRb) and Rsol(QpRp)(QbRb) models, respectively. The test results showed that the Ce film at the outer layer of the composite films had good protection effect at the initial stage of the immersion corrosion. It effectively helped the anodic film at the inner layer to prevent chloride irons from penetrating the aluminum alloy matrix. After 18 days, the Ce film lost its anticorrosive property, and the anodic film took the leading role of the corrosion protection. When the corrosion time was up to 42 days, the aluminum matrix was not corroded yet. Thus, the higher protection degree of the composite films for A356 aluminum alloy was attributed to the synergism effects of anodic film and rare earth Ce film.