Study On The Preparation And Corrosioninhibition Performance Of Cerium Oxidebased Coatings On 2024-T3 Aluminum Alloy Deposited By Magnetron Sputtering

Corrosion and corrosion protection of metal materials always be the research hotspots in aerospace and automobile industries. Numerous studies have been focused on improving the corrosion performance of Al alloy by surface treatments. Chromium based coating is the most widely used, with the best inhibition performance, and intensively studied coating technique for Al alloy surface treatment in the past 80 years. However, the use of chromium compound has been limited and forbidden due to the high toxicity and carcinogenicity of hexavalent chromium compounds, which provides an incentive for the development of environmentally friendly coating systems. Rare earth conversion coatings, especially cerium oxide based conversion coatings, have been developed as a possible alternative to chromium based coatings for the corrosion protection of high strength aluminum alloys. However, most of the existing techniques for the deposition of cerium oxide based coatings are chemical methods based on the liquid phase reactions. Firstly, these chemical deposition processes involve the use and discarding of cerium based compound and other acidic or alkaline solutions, which have significant influence on the environment. Then, there are uneven composition distribution, poor adhesion to substrate, and numerous cracks and defects in micrometers accompanied with cerium oxide based coatings deposited by chemical methods. Last but not least, the corrosion protection mechanism of cerium based conversion coatings has not been intensively investigated. Thus, it is of great importance for the applications of high strength alloy to adopt environmentally friendly technique to deposit cerium oxide coatings; improve the coating surface morphology and adhesion strength; study the relationship between the chemical state of cerium species, the microstructure and the corrosion performance of deposited cerium coatings.In this study, CeO₂ coatings were deposited onto Al 2024-T3 alloy substrates by magnetron sputtering. The surface morphology, crystallite size, inhomogeneous lattice strain, adhesion strength to Al alloy substrates and electrochemical corrosion properties have been studied for CeO₂ coated samples. Results indicated that sputter deposited crystalline CeO₂ coatings acted as physical barriers that provide good cathodic inhibition for Al alloys in saline solution. However, the CeO₂ coating in this study did not show the electrochemical activity associated with phosphate post-treated cerium conversion coatings.Then, the Ce-Al-O coatings were deposited on Al 2024-T3 alloy substrates by magnetron co-sputtering of CeO₂ and Al targets. The incorporation of Al into the CeO₂ film resulted in a transition from a columnar textured, cubic fluorite sputter CeO₂ coating with predominately Ce（IV） species to a compact, amorphous or nanocrystalline Ce-Al-O oxide coating with predominately Ce（III） species after Al incorporation. Electrochemical measurements indicated that the amorphous or nanocrystalline Ce-Al-O coating increased the corrosion resistance of bare Al 2024-T3 substrates by more than three orders of magnitude, and exhibited a very different cathodic and anodic protective mechanism compared with Al and CeO₂ coated and as-polished Al 2024-T3 samples. The oxygen reduction cathodic reactions on the Al alloy surface have been significantly suppressed, and the tendency for pitting corrosion of Al alloy substrate has been significantly reduced by the Ce-Al-O coating.To further investigate the corrosion response mechanism of Ce-Al-O coated Al 2024-T3 alloy sample, the Ce-Al-O coatings with various chemical composition and thickness have been studied. The chemical composition and thickness of Ce-Al-O coatings have been changed by adjusting the CeO₂ target power and deposition time, respectively. The improvement of the anodic pitting corrosion process is related to the amount of Ce（III） species in the Ce-Al-O coatings. The Ce-Al-O coating tends to behave as barrier with the increase of Ce（IV） species. On the other hand, the coating thickness would not affect the dynamic response nature of the Ce-Al-O coating with Ce（III） species, however, the microstructure of Ce-Al-O coatings is closely related to the variation of thickness.The corrosion protection mechanisms of different cerium oxide based coatings deposited by magnetron sputtering to Al 2024-T3 alloy substrate have been summarized according to the electrochemical corrosion, salt spray and long term salt bath test results. The corrosion protection mechanisms of cerium oxide based coatings are related to their microstructure and chemical composition, especially the chemical state of Ce species. When the Ce element is presented as Ce（IV） species, like in CeO₂ coating, the cerium oxide coating deposited by magnetron sputtering acts as barrier which can effectively restrain the charge exchange between the aluminum alloy substrate and the corrosive medium. The CeO₂ coating is helpful to decrease the cathodic corrosion current density, but has little effect on inhibiting the anodic pitting reaction. On the other hand, when the Ce elements are mainly showing as Ce（III） species inside the coating, the Ce-Al-O coating can react dynamically with corrosive medium by the chemical state change of Ce（III） species and Ce-Al-O compounds due to the electrochemical activity of Ce（III） species, the selective release of Ce（III） ion and the re-deposition of other cerium compounds on the corroded area. This makes the Ce-Al-O coating not only a barrier between the aluminum alloy substrate and the corrosive medium, but also a dynamically reacted coating which can effectively reduce the tendency of Al alloy to anodic pitting corrosion and provide certain self-repairing ability.