According to the China Aluminum Industry Network, using secondary zinc immersion before electroless nickel plating on aluminum alloys is a common practice. However, this method has several significant drawbacks. First, the zinc layer can easily dissolve in acidic electroless nickel plating solutions, leading to poor adhesion and coating failure. Second, the presence of the zinc layer between the substrate and the nickel layer can lead to galvanic corrosion (Al-Zn-Ni) in corrosive environments, which may result in blistering or peeling of the coating and reduced corrosion resistance. Third, after electroless nickel plating, the material cannot undergo heat treatment at 400°C, as this will cause the zinc layer to leach out, weakening the coating's integrity. Additionally, the large difference in thermal expansion coefficients between aluminum and nickel can create high stress at elevated temperatures, potentially causing cracks between the coating and the base metal.
Instead of zinc immersion, activation treatment offers a more effective and reliable pre-treatment method for electroless nickel plating on aluminum alloys. This process provides a stable and uniform base layer, allowing the nickel coating to adhere well to the material. The results achieved with activation are comparable to those obtained through traditional zinc immersion, making it a promising and innovative approach in aluminum alloy surface treatment.
The activation solution typically contains NiSO₄·6H₂O at 24–30 g/L, HEDP at 40–50 g/L, and a stabilizer (N25) at 25–30 ml/L. The pH should be adjusted to above 12 using NaOH, and the process is carried out at room temperature for 1–4 minutes. Controlling the activation time is crucial, and the optimal duration should be determined experimentally for each specific formulation. The ideal time is when the surface turns uniformly gray, ensuring excellent coating quality during subsequent electroless nickel plating. This method not only improves the performance of the final coating but also enhances the overall reliability and durability of the plated parts.
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