Effect of cerium (III) on the corrosion protection properties of the film formed on the AA2524-T3 alloy by hydrothermal treatments

Abstract

Many industries use chromium-based anodizing processes with a subsequent sealing post-treatment containing hexavalent chromium ions to protect Al alloys. However, this method of protection causes large environmental and health related problems. This has encouraged the search for new surface treatments for metals with the requirements of effective protection against corrosion and generation of non-toxic residues to the environment. Recent studies are pointing towards chromium free anodizing processes, specifically the tartaric-sulfuric acid (TSA) one, as a viable alternative. Besides, sealing treatments of the Al anodic layer in chromate solutions are another source of toxic residues and their replacement for environmentally friendly alternatives are also of great interest. Surface treatments with cerium ions are being largely studied for the corrosion protection of Al alloys due to their good anticorrosion properties associated to the lack of toxic residues generated. In this investigation, a hydrothermal treatment in aqueous solution with cerium ions has been proposed as a post-treatment for TSA anodizing, and its effects on the AA2524-T3 alloy corrosion resistance investigated. This alloy presents a chemical composition similar to the AA2024 but with lower and more controlled amounts of alloying elements. The sealing treatment was carried out by immersion in aqueous solutions with cerium ions at the boiling temperature. The effect of Ce on the characteristics of the surface film formed, such as morphology and corrosion resistance, was investigated by SEM and electrochemical impedance spectroscopy. The results were compared to those of sealing in boiling water. The results showed that the sealing treatment in solution with Ce(III) ions presented a less stable behavior than the sealing with only boiling water. However, the results of the EIS experiments showed recovery of the protective properties of the system, indicating that some self-healing properties must be imparted to the system.