Dr E. Ramanathan PhD
Prerequisites for Trication Phosphating ensure the formation of a fine, uniform, adherent phosphate coating. They include surface, chemical, and process conditions:
1. Surface Cleanliness
- Thorough degreasing: Remove oils, greases, and dirt.
- Derusting or pickling: Eliminate rust and mill scale.
- Surface must be free from oxides, carbon, weld flux, and old paint.
2. Proper Rinsing
- Use clean water rinses after degreasing and pickling.
- Final rinse water should have a conductivity below 50 µS/cm to avoid salt contamination.
3. Activation (Optional but Recommended)
- Use of a titanium phosphate or colloidal activator before phosphating.
- Promotes uniform grain size and dense crystalline structure.
4. Surface Temperature
- Preferred substrate temperature: 25–40°C.
- Avoid condensation or wet surfaces during processing.
5. Phosphating Bath Parameters
- Free acid: ~1.0–1.5 points
- Total acid: ~10–20 points
- Acid ratio (T.A./F.A.): ~10–15
- Iron concentration: <1.5 g/L
- Zinc, Nickel, Manganese content: as per formulation (typically ~0.5–1 g/L each)
- Temperature: 45–60°C
- Time: 3–10 minutes (depending on bath type)
- pH: 2.5–3.2
6. Water Quality
- DI or RO water for final rinse and phosphating bath make-up (to minimize scale and sludge).
7. Equipment Readiness
- All process tanks and pipelines should be clean and resistant to acid corrosion.
- Use of polypropylene, SS316, or rubber-lined tanks is ideal.
8. Agitation and Filtration
- Gentle agitation ensures uniform coating.
- Sludge removal system must be in place to avoid bath contamination.
9. Post-Treatment
- Water rinse followed by passivation or sealing rinse (e.g., chromic acid or non-chrome sealer).
Ensuring all these conditions leads to optimal trication phosphate coating for corrosion resistance and paint adhesion.
