What is phosphating?
Phosphating is a chemical surface treatment that forms a thin, crystalline phosphate layer on steel or metal, providing both corrosion protection and a key for paint adhesion. The workpiece is dipped in or sprayed with a phosphate bath, after which the phosphate crystals deposit onto the metal surface.
How it works and the main types
During the process the acidic phosphate bath reacts with the metal surface, creating an insoluble layer of metal phosphate. Two common variants are distinguished:
- Iron phosphating – a thin, fine-crystalline layer suitable as a base under paint; light to moderate corrosion protection.
- Zinc phosphating – a thicker, coarser layer offering stronger corrosion resistance, often used beneath demanding coating systems.
Function of the phosphate layer
The layer serves two main purposes:
- Corrosion protection: the phosphate layer insulates the metal and slows down rust formation.
- Adhesion base: the porous, rough surface grips a later wet paint or powder coating, so the topcoat adheres better and lasts longer.
Use in construction
Phosphating is widely applied to builder’s hardware, hinges, locks, steel profiles, and prefabricated steel structures. It often forms an intermediate step in the pre-treatment process before powder coating or stove enamelling.
Related terms
- Powder coating
- Galvanising
- Corrosion
- Passivation
- Primer
- Builder’s hardware
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