Zincate Replacement, an Economical a Sustainable Alternative for the Metal-Coating of Aluminum

Abstract

Aluminum parts are of great value for many industries as they are cost-effective, very lightweight combined with a good mechanical strength. However, when parts are exposed to corrosive and abrasive conditions it is preferred to protect them with anodizing, powder coating, or for technical critical applications with nickel-phosphorus (NiP) coatings. Most preferred for functional parts such as scroll compressors, connectors,... is electroless NiP (EN) as it is has a low thickness variation in the final deposit and does not need the application of current like galvanic NiP coatings.Depositing an adherent layer of electroless NiP onto aluminum needs an alloy-specific pre-treatment. The largest challenge in pre-treatment lies in the very fast formation of Al2O3 oxide under neutral pH conditions of 4 - 9. For decades a very effective way to inhibit the re-oxidation of aluminum is the use of alkaline zinc as interim coatings that dissolve in the electroless NiP electrolyte before NiP deposition starts. Commercial solutions are optimized to produce an as thin and homogenous as possible zinc deposit to minimize the drag-in of zinc into the NiP electrolyte. At levels of 80 - 200 mg/l zinc starts to lower the adhesion and plating rate of deposited EN coatings. As results the nickel process needs to be discarded before nickel deposition capacity of electrolyte is fully reached (~ 40 g/l plated Ni). The impact of Zn contamination is most critical as lower the thickness of the EN layer is. At very thin thicknesses below 4 µm the increase of zinc is very large, and electrolytes need to be discarded at 6-12 g/l plated nickel creating higher chemical waste, maintenance effort, and cost. Due to technical changes in many applications, different and more parts will be composed of aluminum. In contrast to established applications NiP that exhibit EN layers > 5 µm new applications arise with the upcoming EV and Battery market that request thinner NiP layers. The most extreme case is reel-to-reel plating of thin (< 50 µm) foils with NiP deposits below 4 µm.In the present work, the application of a new chemical concept of aluminum pre-treatment for low-thickness EN coatings will be presented that reduces the number of needed process steps compared to double zincate by at least 2 steps (excl. rinses) without the need for special equipment such as pulse plating and others. Differences in the initiation of electroless NiP deposition and the correlated adhesion will be evaluated. As a model case, the application of thin NiP layers (< 4 µm) on pure Aluminum foils will be shown and discussed. Figure 1