Formation of a chrome-free and coloured conversion coating on AA6063 aluminium alloy

A new passivation process has been developed for producing a chrome-free and coloured conversion coating on aluminium alloy AA6063. The method is based on the reaction of the aluminium alloy with a potassium hexafluorozirconate based solution at room temperature. The conversion coating obtained has a grey colour, which enables visual assessment of the coating development and coating quality during processing. The colouring effect is a significant advantage over the zirconium and titanium based conversion coatings currently used in the industry, which are largely colourless. The new treatment process is simple and allows a uniform coating to be formed within a few minutes. The composition and the surface morphology of the coatings were characterized. The corrosion resistance of the coated samples was evaluated by electrochemical tests. The results have shown that the treatment yields a zirconium based conversion coating with a microcrystalline structure, which is desirable for paint adhesion, and substantially improves the corrosion resistance of the aluminium alloy.