Characterization and Optimization of Oxide Films Formed on Aluminum-Copper Alloys

The Al-Cu alloy AA2219 (∼6.3 wt.% Cu) is of great interest for use in aerospace applications, primarily due its high strength to weight ratio. However, AA2219 suffers from poor corrosion resistance because of the presence of Al2Cu intermetallics, which are cathodic to the Al-rich matrix, thus promoting Al dissolution. Unfortunately, conventional anodization in sulfuric and chromic acids significantly lowers the fatigue strength of the alloy due to the dissolution of the Cu-rich particles. Therefore, this research has been focused on improving the corrosion resistance of AA2219 by using AC/DC spark anodization in alkaline silicate solutions, while also retaining the Al2Cu intermetallics in the underlying alloy.;It has been found that AC/DC spark anodization of AA2219 occurs in three stages, with barrier Al oxide film growth occurring in Stage I, the onset of porous layer growth commencing in Stage II (sparking stage), and the deposition of a rough, thick, and porous Si-rich layer in Stage III (arcing stage). Under all anodizing conditions used in this work, the oxide film passivity in neutral solutions is at least 1000 times higher, and the corrosion resistance at least 100 times greater, than what is seen for bare AA2219. The oxide film becomes thicker, less porous, and more corrosion resistant by increasing the applied current density and anodization time and also by increasing the AC:DC voltage ratio. The highest corrosion resistance is seen when anodization is carried out in 0.1 - 0.15 M KOH and at ≥ 0.12 M silicate.;It has also been shown that the anodizing solutions are very stable with time of storage in air (for at least three months) and that they can be used repeatedly (at least 15 times) for AA2219 spark anodization. An interesting discovery is that the addition of CO2 (or carbonate) to the anodizing solution can greatly improve the corrosion resistance of AA2219. Finally, it was discovered that SiO2 nanoparticles can be easily infiltrated into the porous oxide film, also forming a thick silica over-layer, resulting in a dramatic improvement in the passivity and corrosion resistance of the AA2219 alloy.