Cerium-based conversion coatings on cast aluminum 380 and 413 alloys

This investigation focused on the deposition and characterization of cerium-based conversion coatings (CeCCs) on cast aluminum alloys. Previous research has shown that CeCCs are viable alternatives to chromate conversion on high strength alloys such as 2024-T3 and 7075-T6. For the casting alloys such as 380 and 413, the presence of Si affects the composition and stability of the native oxide, which means that pretreatment plays an important role for the coating deposition. This thesis consists of two papers that describe the results of the study. The first paper reports the effect of the final rinsing temperature before coating deposition on coating morphology, thickness, and corrosion performance. The AA 380 panels were activated in 60⁰C sulfuric acid and rinsed at 25 C or 100⁰C in distilled water before immersion in the coating solution for 2, 5, and 8 minutes. The morphology and thickness data suggest that rinsing at 25⁰C resulted in a faster deposition rate, but less corrosion resistance due to cracking. However, rinsing at 100⁰C reduced the deposition rate, but increased corrosion resistance by producing a more homogeneous coating. The second paper focused a deposition of cerium-based conversion coatings on aluminum 413 and 380 alloys under the assistance of ultrasound. Electrochemical impedance spectroscopy and the potentiodynamic measurements showed that ultrasound increased impedance and reduced the corrosion current. The morphology of coatings deposited with ultrasound showed reduced cracking. The salt spray corrosion testing (ASTM Bll7) showed that CeCCs deposited with ultrasound wave had better corrosion resistance than coatings deposited using the conventional process --Abstract, page iv