The effect of pH and role of Ni in zinc phosphating of 2024-Al alloy: Part I: Macroscopic studies with XPS and SEM

Coatings formed on 2024-T3 aluminum alloy were studied by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) after dipping in zinc phosphating (ZPO) baths at different acidities, for different lengths of time, and with or without Ni²⁺ additive. The overall objective was to learn more about the role of Ni²⁺ on the ZPO coating mechanism, particularly since this additive is believed to improve corrosion protection for the Al alloy. Secondary phosphates dominate the coatings when the Ni-containing solution is adjusted to starting pH values of either 3 or 5, while tertiary phosphate is predominant at pH 4. AlF₃ precipitates during the early stages of the coating process. Ni²⁺ has two main roles in the mechanism. First, the rate of increase in local solution pH is retarded by the slower kinetics of reactions involving Ni²⁺ compared to Zn²⁺, leading to thinner ZPO coatings when Ni²⁺ is present in the coating solution. Second, most Ni²⁺ deposition occurs during the later stages of the coating process, by nickel phosphate deposition and/or by formation of a Ni-rich oxide.