Troubleshooting Hexavalent Chrome Black

Troubleshooting for Chromate Black 6

1. Issue: Iridescent Colors on Parts

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

   • 1.2: Insufficient Immersion Time: Increase the immersion time for the chromate.

2. Issue: Parts are Black but Dull

   • 2.1: Insufficient Part A: Gradually increase Part A by 0.5–1%.

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

3. Issue: Green Parts

   • 3.1: Insufficient Part B: Gradually increase Part B by 0.5–1%.

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

4. Issue: Parts are Shiny Brown

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

   • 4.2: High Solution Concentration: Dilute the solution by 25%, then adjust Part A, Part B, and pH.

5. Issue: Sooty Coating on Parts

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

   • 5.2: Insufficient Part A: Gradually increase Part A by 0.5–1%.

6. Issue: Excessive Addition of Part B

   • 6.1: Chloride Contamination: Check rinse water and mixing water for cleanliness.

   • 6.2: Cyanide Contamination: Check rinse water and mixing water for cleanliness.

   • 6.3: Copper Contamination: Check rinse water and mixing water for cleanliness.

7. Issue: Zinc Coating Flaking Off in Crevices or Low Spots

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

   • 7.2: High Solution Concentration: Dilute the solution by 10% and add water.

   • 7.3: Insufficient Zinc Thickness: Increase the zinc coating thickness.

   • 7.4: Nitrate Contamination: Check rinse water and mixing water for cleanliness.

   • 7.5: Over-Immersion Time: Reduce the immersion time.

8. Issue: White Precipitate Formation

   • 8.1: Chloride Contamination: Check rinse water and mixing water for cleanliness.

   • 8.2: Cyanide Contamination: Check rinse water and mixing water for cleanliness.

Usage Instructions

• CHROMATE BLACK 80 PART A: 60–70 cc/L

• CHROMATE BLACK 80 PART B: 80–100 cc/L

• Immersion Time: 40–120 seconds

• Solution Temperature: Room temperature

• pH: 1.6–2.6

• Agitation: Ensure continuous agitation or aeration during operation

Notes

• Use distilled or DI water for mixing or adding to avoid chromate precipitation and premature solution degradation.

• Immersion time should be 60 seconds as a baseline, adjusted based on the parts and solution age.

• Recommended Technique: If new solution shows less blackening, pre-soak 3–5 parts in the solution for 5–10 minutes to allow zinc dissolution before actual immersion.

Process Steps

1. Post-Zinc Plating: Ensure parts have a thickness of more than 8 microns.

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

3. Dip in Concentrated Phosphoric Acid (0.5% v/v) or Sulfuric Acid (0.25% v/v): For 2–3 seconds (helps extend chromate bath life).

4. Clean Water Rinse: Rinse with clean water.

5. Dip in Distilled Water: For 2–3 seconds.

6. Dip in Pre-Mixed Black Chromate Solution: For 60 seconds.

7. Clean Water Rinse: Rinse with clean water twice.

8. Dip in Distilled Water: For 2–3 seconds.

9. Dip in Chromate Coating (Optional).

10. Dry: Air-dry with warm air (temperature not exceeding 60°C).

11. Oil Coating (Optional): Use a rolling mill for coating.

Solution Adjustment

• Adjust Part A and Part B based on analysis.

• During operation, control pH by adjusting Part A and Part B, or use sulfuric acid (50% v/v) and sodium hydroxide (30% w/v) if chromate levels are adequate.

Adjusting Solution Ratios

• CHROMATE BLACK 80 PART A: Adjust based on analysis results.

• CHROMATE BLACK 80 PART B: Adjust to 1.3 times the amount of Part A.

Recommendations

1. If parts turn red or reddish during dipping, add sodium hydroxide (30%).

2. Pre-rinse parts in distilled water before black chromate immersion to remove residual chromate film.

3. Parts should have a zinc thickness of at least 5 microns or approximately 40 minutes in rolling barrels.

4. Avoid oven drying; use warm air or fan drying.

5. Do not soak parts in hot water after chromate treatment.

6. Chromate-coated parts are more scratch-resistant after 24 hours of storage.

7. If green or brown colors appear, increase dipping time by 2–5 seconds, but avoid over-dipping to prevent zinc film removal.