The effect of pH and role of Ni2+ in zinc phosphating of 2024-Al alloy: Part II: Microscopic studies with SEM and SAM

Abstract

Coatings formed on 2024-T3 aluminum alloy were studied by scanning electron microscopy (SEM) and scanning Auger microscopy (SAM) after dipping in zinc phosphating (ZPO) baths at different acidities, with or without the Ni2+ additive. The objective was to learn more about the ZPO coating mechanism on the different microstructural regions of 2024-T3. When the initial coating solution pH is 4 (optimal acidity), a slower etching rate at the Al–Cu–Fe–Mn intermetallic particle causes significant precipitation of ZnO, which differs from the coating on other regions of the surface where phosphate predominates. The larger crystals (∼μm dimension) on the matrix and the Al–Cu–Mg particle contain more phosphate compared to other areas on the surface. When Ni2+ is added to the coating solution, the Al–Cu–Mg particle is more thickly coated compared to when the Ni2+ is not present. The slower rate of precipitation when Ni2+ is present in the coating solution increases the exposure of the alloy substrate to the acidic environment, so allowing more dissolution of Mg and Al from the Al–Cu–Mg particle. This results in the particle becoming more cathodic in nature, and therefore more coating deposits at this location. Evidence from SAM supports the presence of NiAl2O4, hypothesized in Part I, forming at coating pores later in the process.