Troubleshooting Trivalent Chrome Black

Troubleshooting for Chromate Black 3

1. Issue: Poor Corrosion Resistance After Chromating with Black Chromate 3

   • 1.1: pH Too High (>2.6): Adjust pH to 1.8–2.4 using 50% sulfuric acid.

   • 1.2: pH Too Low (<1.5): Adjust pH to 1.8–2.4 using 50% sodium hydroxide.

   • 1.3: Insufficient Chromate Concentration: Increase the chromate concentration.

   • 1.4: Temperature Too Low: Warm the solution to 28°C.

   • 1.5: Chromate Solution Depleted: Add more chromate solution or mix a new batch.

   • 1.6: Excess Zinc or Contaminants: Dilute or prepare a new solution if contaminants are too high.

   • 1.7: Low Drying Temperature: Adjust the drying time or temperature to 70°C.

2. Issue: Light Chromate Coating (Gray, Blue, or Iridescent Colors)

   • 2.1: pH Too High (>2.6): Adjust pH to 1.8–2.4 using 50% sulfuric acid.

   • 2.2: Inadequate Agitation: Increase agitation or use air to improve mixing.

   • 2.3: Insufficient Immersion Time: Increase the time the parts are immersed.

   • 2.4: Low Chromate Concentration: Adjust the concentration to the correct level.

3. Issue: Spotty or Uneven Chromate Coating

   • 3.1: Excessive Acidity in Rinse Water: Check and replace rinse water if it’s too acidic.

4. Issue: Sooty Appearance on the Chromate Coating

   • 4.1: Excessive Chromate Concentration: Dilute the solution.

   • 4.2: pH Too Low (<1.5): Adjust pH to 1.8–2.4 using 50% sodium hydroxide.

Chromate Solution Preparation for Mixing

• Zinc Black Chromate 3 (471): 80–150 cc/L

• pH: 1.8 - 2.2

• Immersion Time: 40–80 seconds

• Supercoat L (Coating): 50–150 cc/L (for rolling barrels: 50–250 cc/L)

• Immersion Time for Coating: 30 seconds

Process Steps:

1. Post-Zinc Plating: Ensure parts have a thickness of at least 8 microns.

2. Clean Water Rinse: Rinse the parts with clean water.

3. Dip in Concentrated Nitric Acid (0.25–0.5% v/v): For 30 seconds.

4. Clean Water Rinse: Rinse with clean water.

5. Dip in Black Chromate Solution 3 (471): For 40–80 seconds.

6. Clean Water Rinse: Rinse with clean water.

7. Hot Water Dip (70–80°C): For 30 seconds.

8. Dip in Chromate Coating (Supercoat L): For 30 seconds.

9. Dry: Either air-dry or bake until dry.