A. Hovestad

Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure

The chemical conversion treatment for aluminium alloys based on the immersion in cerium chloride/hydrogen peroxide solutions is one of the possible alternatives to the chromate conversion process for the corrosion protection of aluminium alloys. The nucleation and growth of the cerium-based conversion coating on AA2024 was studied using complementary surface analysis techniques, as atomic force microscopy in the Kelvin probe mode (SKPFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of both the intermetallic particles and the copper in solid solution was considered. According to this aim, different surface preparation procedures prior to the conversion process were investigated. Acid pickling and Ce-based deoxidising, as well, were found to cause the formation of copper or copper oxide deposits, i.e. copper smut, on the AA2024 surface. While the intermetallics do not act as preferential nucleation sites, the deposition of copper on the surface is a necessary condition in order to form a thick cerium oxide film. On the other hand, the copper smut strongly decreases the adherence of the conversion layer to the aluminium substrate.

Investigation of the chromate conversion coating on Alclad 2024 aluminium alloy: effect of the pH of the chromate bath

The parameters of the chromate bath, like temperature, pH, and fluoride content, strongly affect the morphology and chemical composition of the chromate conversion coating and as a consequence have a large influence on its corrosion performance. In this paper, electrochemical impedance spectroscopy (EIS) was used in combination with other techniques to investigate the role played by the pH of the chromate bath on the properties of the chromate film formed on Alclad 2024 aluminium alloy. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE) have shown the formation of a thicker and less dense chromate layer when the pH of the chromate bath is changed from 2.4 to 1.2. The analysis of the EIS spectra have highlighted that this change in pH leads to the formation of more protective and more resistant chromate corrosion products (CCP) inside the defects of the chromate film. When a thin, dense and protective layer of CCP is formed in the defects, the corrosion behaviour of the chromate conversion coating improves for two main reason: (a) further attack of the defects is avoided or delayed; (b) the change in volume caused by the formation of the CCP is limited resulting in a low level of stress in the film, which as a consequence is not detached from the aluminium substrate.