Ce-based conversion coatings on Rheo-HPDC low Si content aluminium alloy

Cerium-based conversion coatings have been considered as an effective alternative to hazardous and carcinogenic chromate-based coatings used in the treatment of metal surfaces such as aluminium alloys. However, there is still considerable debate over the mechanism by which these coatings are formed on different alloys and microstructure features. In the current work, Ce-based conversion coatings were deposited on Rheo-HPDC Al-Si alloys by immersion in water-based solutions of cerium nitrate. Effect of deposition parameters including immersion time and Ce(NO3)3.6H2O concentration on the corrosion resistance and the morphology of coatings was investigated. In addition in some experiments, NaCl was added to the cerium based solution in order to accelerate the deposition of the conversion coating and also to understand the coating formation mechanism. Electrochemical behaviour of the treated aluminium alloys was evaluated in the solution of 0.05 mol/L NaCl using polarization test and electrochemical impedance spectroscopy (EIS) measurements. The morphology and distribution of the cerium-based conversion coatings on the aluminium alloys were studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray photoelectron spectroscopy (XPS). Conversion treated substrates in all solutions were shown to possess improved corrosion resistance in comparison to the untreated samples. Immersion time and Ce(NO3)3.6H2O concentration slightly affect the deposition and the passivation capability of the cerium hydroxide/oxide layers. SEM images revealed that the deposition of cerium is more favoured in some cathodic areas including iron-based intermetallics and/or eutectic silicon phase. This phenomenon helps to block the active interfaces between these cathodic sites and the aluminium matrix, which is prone to localized corrosion in chloride ion containing environments.