The effect of Ti-colloid surface conditioning on the phosphating of 7075-T6 aluminium alloy

Abstract

7075-T6 aluminium alloys panels were surface conditioned in a titanium colloid suspension, under a variety of different conditions, and subsequently these samples were immersed in a ZnO + H₃PO₄ coating mixture, and the phosphate coating layers characterized. Morphologies observed by scanning electron microscopy (SEM) reveal that the coating layer consists of two phases, namely an amorphous phase, which is directly bonded to the substrate, and a crystalline phase, in which larger crystal grains grow on top of the amorphous base. Chemical compositions of the coating layers were analysed by X-ray photoelectron spectroscopy (XPS) and by static and imaging secondary ion mass spectroscopy (SIMS). It is found that the details of the surface conditioning affect the final coating, both in terms of morphology and chemical composition. For example, larger amounts of Zn and P are detected in coatings for which the initial conditioning is done at 40°C, compared with room temperature; a similar statement can be made for surfaces which are water rinsed after the Ti pretreatment, compared with those which are not water rinsed prior to the coating treatment.