Troubleshooting Hexavalent Chrome Black
วิธีการแก้ไขสีดำโครม 6 เบื้องต้น
Troubleshooting for Chromate Black 6
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.
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.
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.
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.
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%.
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.
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.
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
Post-Zinc Plating: Ensure parts have a thickness of more than 8 microns.
Clean Water Rinse: Rinse parts twice with clean water.
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).
Clean Water Rinse: Rinse with clean water.
Dip in Distilled Water: For 2–3 seconds.
Dip in Pre-Mixed Black Chromate Solution: For 60 seconds.
Clean Water Rinse: Rinse with clean water twice.
Dip in Distilled Water: For 2–3 seconds.
Dip in Chromate Coating (Optional).
Dry: Air-dry with warm air (temperature not exceeding 60°C).
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
If parts turn red or reddish during dipping, add sodium hydroxide (30%).
Pre-rinse parts in distilled water before black chromate immersion to remove residual chromate film.
Parts should have a zinc thickness of at least 5 microns or approximately 40 minutes in rolling barrels.
Avoid oven drying; use warm air or fan drying.
Do not soak parts in hot water after chromate treatment.
Chromate-coated parts are more scratch-resistant after 24 hours of storage.
If green or brown colors appear, increase dipping time by 2–5 seconds, but avoid over-dipping to prevent zinc film removal.