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

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.