Microstructural effects on the initiation of zinc phosphate coatings on 2024-T3 aluminum alloy

The initiation of coatings deposited on to 2024-T3 aluminum alloy from supersaturated zinc phosphating solutions has been studied using scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The alloy microstructure, especially associated with the second-phase particles, strongly affects the formation stages of the coating process, where etching of the substrate has a significant role. At the start, zinc phosphate (ZPO) crystals form on the Al–Cu–Mg second-phase particles, rather than on the matrix or on the Al–Cu–Fe–Mn particles, with the initial nucleation appearing at interfaces between Al–Cu–Mg particles and the matrix. In contrast, the formation of the ZPO coating is delayed on the cathodic Al–Cu–Fe–Mn particles, compared to those of the Al–Cu–Mg composition. When the coating process is completed, the whole sample surface is covered with ZPO although its thickness varies at the different micro-regions