Dr E. Ramanathan PhD
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Standard Operating Procedure (SOP)
Title: Trication Zinc Phosphating Process for ED Coating
Application: Suitable for ferrous substrates prior to Electro-Deposition (ED) Coating
Objective: Ensure consistent phosphate coating with fine crystalline structure and good paint adhesion and corrosion resistance
1. Material Preparation
- Substrate: Mild steel, CRCA, HRPO, castings
- Incoming Inspection: Check for oil, rust, scale
2. Process Flow
| Step No | Process | Parameters | Remarks |
|---|---|---|---|
| 1 | Degreasing | ||
| → Type: Alkaline / Emulsion / Solvent | |||
| → Temp: 55–65°C | |||
| → Time: 5–10 min | |||
| → Conc: 3–7% (alkaline) | Remove oil, dirt, machining fluids | ||
| Spray or dip method | |||
| 2 | Water Rinse – 1 | ||
| → Type: Freshwater | |||
| → Time: 1–2 min | Overflow tank preferred | ||
| 3 | Water Rinse – 2 | ||
| → Type: Freshwater | |||
| → Time: 1–2 min | Ensures complete detergent removal | ||
| 4 | Derusting (if required) | ||
| → Type: Inhibited HCl or phosphoric acid or citric acid | |||
| → Temp: RT–40°C | |||
| → Time: 5–10 min | |||
| → Conc: 5–10% | Only for rusty components | ||
| 5 | Water Rinse – 3 | ||
| → Type: Freshwater | |||
| → Time: 1–2 min | Must be clean to avoid acid drag | ||
| 6 | Activation / Conditioner | ||
| → Zirconium/titanium/talc-based | |||
| → Temp: RT–35°C | |||
| → Time: 1–2 min | |||
| → Conc: 0.5–1.0% | Promotes fine grain phosphate | ||
| 7 | Trication Zinc Phosphating | ||
| → Temp: 45–60°C | |||
| → Time: 5–10 min | |||
| → Conc: as per supplier | |||
| → pH: 2.2–2.8 | |||
| → TA: 20–30 points | |||
| → FA: 1.0–1.5 points | |||
| → FA/TA: 1:10 to 1:15 | Use of Ni+Mn+Zn based formulation; monitor sludge regularly | ||
| 8 | Water Rinse – 4 | ||
| → Type: Freshwater | |||
| → Time: 1–2 min | Overflow type, clean water | ||
| 9 | Passivation (Optional) | ||
| → Type: Chromic / non-chromic (zirconium-based) | |||
| → Temp: RT | |||
| → Time: 0.5–1 min | Enhances corrosion resistance | ||
| 10 | DI Water Rinse (Optional) | ||
| → Conductivity: <30 μS/cm | Optional but recommended | ||
| 11 | Drying | ||
| → Temp: 100–120°C | |||
| → Time: 10–15 min | Ensure dry film before ED coating | ||
| 12 | ED Coating | As per paint manufacturer’s specifications |
3. Bath Maintenance & Controls
- Degreaser: pH, oil drag-out, surfactant content
- Phosphate Bath: FA, TA, Fe/Zn content, sludge level, temperature
- Rinses: Water conductivity (<300 μS/cm), clarity
- Activation: pH, concentration, iron contamination
4. Safety & Environment
- Use PPE: gloves, goggles, aprons
- Handle acids and phosphates with caution
- Provide proper ventilation and exhaust
- Treat effluents as per local pollution norms
5. Documentation
- Record pH, FA/TA, temperature, concentration daily
- Maintain sludge removal log
- ED performance test after coating (salt spray, adhesion)
