Galvanic Coating on Zinc or Zinc Die-Cast

If zinc (e.g., zinc die-cast or galvanized surfaces) is already present as a substrate and an additional galvanic layer (e.g., copper, nickel, chrome, etc.) is to be applied, slightly different requirements apply compared to steel. Zinc is a relatively reactive metal and can be quickly attacked by strong acids or alkalis. In addition, zinc tends to form oxide layers, which can make good adhesion more difficult. Typically—especially for zinc die-cast—a so-called “strike” (adhesion-promoting layer) is used to ensure reliable adhesion for the subsequent topcoat. An alkaline copper electrolyte is used as the strike layer.

The following is a recommended process sequence for galvanically coating zinc or zinc die-cast:

1. Cleaning / Degreasing

   Purpose: Removal of oils, greases, and other contaminants.
   Notes: Use suitable (mild) cleaners, since zinc can be attacked in strongly alkaline solutions. Typically done in an immersion bath, possibly supported by ultrasound or spray cleaning.

2. Rinsing

   Purpose: Removal of residual cleaners to avoid carry-over into subsequent baths.
   Tip: Multi-stage rinsing (cold or warm water) is often useful to thoroughly cleanse the surface.

3. Light Pickling / Activation (Conditioner)

   Purpose: Removal of thin oxide layers and activation of the zinc surface.
   Conditioner: A special mild acidic bath that is lightly diluted (according to the manufacturer’s instructions). This gently treats the zinc surface without excessive etching.

4. (Optional) Brief Rinse

   Purpose: Removal of any pickling or conditioner residues so as not to contaminate the subsequent baths.

5. Adhesion-Promoting Layer (Alkaline Copper Electrolyte)

   Purpose: Deposition of a very thin but strongly adhering metal layer that serves as the foundation for the subsequent coatings.
   Alkaline Copper Electrolyte:
   - Ensures good adhesion to zinc, provided the surface is properly pretreated.
   - Is (compared to cyanide-containing baths) more environmentally friendly and safer.
   - Important: Carefully monitor pH, complexing agents, and temperature.

6. Main Coating

   Options:
   - Additional copper layer (acidic or alkaline) for conductivity and smoothing.
   - Nickel (matte, semi-matte, or bright) for corrosion protection and hardness.
   - Chrome (decorative or hard) for gloss and wear resistance.
   - Other metals or alloys (e.g., brass, tin, bronze) according to requirements.

7. Rinsing / Post-Treatment

   Purpose: Removal of electrolyte residues and additives.
   Optional: Passivation or sealing for additional corrosion protection or specific appearance.

8. Drying / Final Inspection

   Drying: Centrifuge, hot-air oven, or similar methods to avoid water stains.
   Finish: Visual inspection, coating thickness measurement, and adhesion tests.

Brief Summary of the Process Steps

1. Degreasing / Cleaning
2. Rinsing
3. Light Pickling / Activation (Conditioner, diluted)
4. (Rinsing)
5. Adhesion-Promoting Layer (Alkaline Copper Electrolyte)
6. Main Coating (e.g., Copper, Nickel, Chrome, etc.)
7. Rinsing / Post-Treatment
8. Drying / Final Inspection