Study of the phosphating treatment of aluminium surfaces: role of the activating process

In this paper the relation between the activating step and the formation of the zinc phosphate layer is studied on aluminium substrates pretreated in different ways. For the first time direct proof of the presence of the titane phosphate nucleation agents on the metal surface was found. It is shown that during the activation titanium phosphate particles are adsorbed flat on the aluminium surface, having typical dimensions of approximately 7–8 nm in thickness and 0.1–0.3 μm in length. The aluminium surface condition, which was varied by the pretreatment (decreasing, etching and electropolishing) has an important influence on the amount of particles adsorbed on the aluminium surface. The titanium phosphate nuclei act as nucleation agents as the first generation of zinc phosphate crystals nucleate immediately on these centres. These small nucleated crystals grow laterally in a 2D direction leading to flat walled crystals contacting the aluminium substrate. The orientation and form of these zinc phosphate crystals is dominated by the form of the nucleation agent, resulting in an epitaxial growth mechanism. The differences in the surface condition after activation are reflected in the amount of zinc phosphate crystals which are initiated.