Effect of Mn2+ additive on the zinc phosphating of 2024-Al alloy

Zinc phosphate (ZPO) conversion coatings formed on 2024-T3 aluminum alloy, after dipping in Mn2+-containing ZPO coating baths for different times, have been studied by X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning Auger microscopy. An optimal coating (in terms of thickness and morphology) is formed when the alloy is immersed for 3 min in a ZPO coating solution containing 2000 ppm Mn2+. Comparisons are made with coatings formed from a Ni2+-containing ZPO bath. Both Ni2+ and Mn2+ additives decrease the coating crystal size, with Mn2+ giving a greater reduction, and the coating formed from the Mn2+-containing solution is thicker than that formed with Ni2+. The coatings are heterogeneous in depth with Mn2+ depositing during the early stages of coating, while precipitation of Ni2+ occurs mainly later in the coating process. Patterns of deposition across the surface are also affected significantly by the composition of the ZPO coating solution. Mn2+ in the coating bath gives the most even distribution of coating across the different microstructural regions on the surface.