What is Zinc Phosphating (Pretreatment chemicals) ?
Zinc phosphating is a chemical conversion coating where a steel or iron surface reacts with a zinc phosphate solution to form an insoluble crystalline phosphate layer. This layer improves corrosion resistance, paint adhesion, and wear performance.
Chemistry Behind the Process:-
The basic reaction is between the metal surface, phosphoric acid, and zinc ions, typically:
**Fe + H₃PO₄ → FePO₄ + H₂↑
(iron reacts with phosphoric acid to form iron phosphate and hydrogen gas)
Zn²⁺ + PO₄³⁻ → Zn₃(PO₄)₂↓
(zinc ions combine with phosphate to form insoluble zinc phosphate)
Accelerators like Oxy-boost salt, are often added to speed up the reaction and improve coating quality.
Process Flow:–
Step-by-step zinc phosphating treatment:
1- Degreasing / Cleaning:-
* Alkaline or solvent cleaner removes oils and contaminants.
* Done via immersion or spray.
2- Water Rinse:-
* Prevents contamination of next stages.
3- Pickling (if needed):-
* Acidic treatment removes rust or mill scale (RUSTOSHINE).
4- Activation (optional):-
* Fine zinc salts prepare surface for uniform crystal formation.
5- Zinc Phosphate Application:-
* Immersion or spray in a bath containing:
* Gardoline-Z
* Oxy-boost
* Temp: RT to 40–60°C
* Time: 5–15 minutes
6- Water Rinse:-
* Removes loose particles or residues.
7- Passivation (Optional):-
* Passivate-C or other agents seal the surface for extra corrosion resistance.
8- Drying: –
* Heated air or ovens used to remove moisture.
Advantages:-
* Excellent paint adhesion base.
* Enhanced corrosion resistance.
* Uniform, controlled coating thickness.
* Compatibility with oils, waxes, and lubricants.
Disadvantages:–
* Generates sludge (waste management needed).
* Surface preparation is critical.
* Acidic solutions require safety precautions.
Applications:-
Automotive: Chassis, underbodies, fasteners, brake components.
Appliances: Refrigerators, washing machine parts.
Industrial Tools: Cutting tools, machine parts.
Military Equipment: Weapons, vehicles.