Study of the formation of zinc phosphate layers on AA5754 aluminium alloy

Zinc phosphating of aluminium alloys is a conversion treatment that can be used for automotive applications to improve the adhesion properties and the corrosion resistance. In this paper the influence of the aluminium pretreatment on the formation of the zinc phosphate layer is studied The growth of the zinc phosphate layer was examined on degreased, etched and electropolished aluminium AA5754 surfaces, using a Scanning Electron Microscope (SEM). A batch phosphating procedure, representative for the industrial process, was introduced. The surface condition of the aluminium, determined by the selection of the pretreatment procedure, has an influence on the number and sizes of the zinc phosphate crystals present on the aluminium surface. The formation of the layer can be divided into two steps: the initiation and the growth. It is shown that especially the initiation step is influenced by the condition of the substrate prior to phosphating. By means of high sensitivity and high resolution techniques (AFM and ToF-SIMS), it was possible to follow up the initiation process, as these techniques are suitable to detect the nucleation agents as well as the initiating crystals.