by Dr E. Ramanathan
📌 Zinc Phosphating – Key Parameters
- Free Acid (FA): Unreacted phosphoric acid. Controls aggressiveness.
- Total Acid (TA): Total acid content, includes both free and bound (complexed) acid.
- TA/FA Ratio: Indicates bath age and buffering. Ideal around 8–12.
- Free Iron (Fe²⁺): Accumulates with use. Should be controlled; excess inhibits coating.
- Nitrite Accelerator: Boosts coating formation and regulates iron pick-up.
✅ Recommended Operating Ranges – Cold & Hot Immersion Zinc Phosphating
| Parameter | Cold Immersion | Hot Immersion |
|---|---|---|
| Bath Temp | 25–40°C | 50–65°C |
| Free Acid (FA) | 0.8 – 1.2 points | 1.0 – 2.0 points |
| Total Acid (TA) | 8 – 12 points | 12 – 18 points |
| TA/FA Ratio | 8 – 12 | 8 – 12 |
| Free Iron (Fe²⁺) | < 0.5 g/L | < 0.3 g/L |
| Nitrite Dose | 0.1 – 0.3 g/L (Sodium Nitrite) | 0.2 – 0.5 g/L (Sodium Nitrite) |
| Zinc Content | 1.0 – 1.5 g/L | 1.5 – 2.5 g/L |
| Coating Weight | 1.0 – 2.5 g/m² | 2.0 – 4.5 g/m² |
📈 Correlations Summary
| When… | Then… |
|---|---|
| Free Acid too high (>2.0) | Coating becomes coarse; risk of under- or over-etching |
| TA/FA too low (<6) | Bath becomes unstable; poor coating formation |
| Free Iron too high (>0.5 g/L) | Reduces coating weight and uniformity |
| Nitrite too low (<0.1 g/L) | Coating is thin and patchy |
| Nitrite too high (>0.5 g/L) | May cause over-acceleration; dusty or soft coatings |
If you want more details please WhatsApp to me @ 9444929163 to fix up an appointment.
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