Phosphating turns the surface of steel into a thin layer of crystalline phosphate in a hot chemical bath. The layer holds paint and oil very well. Iron and zinc phosphate are used under paint and powder coating; zinc phosphate with oil protects fasteners from rust; manganese phosphate holds oil on gears, cams and engine parts to reduce wear during running-in.
Quick facts
- Types
- Iron, zinc, manganese phosphate
- Base metals
- Mainly steel and cast iron
- Coating weight
- From under 1 g/m² (iron) to over 15 g/m² (manganese)
- Colour
- Grey to dark grey or black
- Used with
- Paint, powder coat, oil or wax
- Usually priced
- Per kg or per batch
Which phosphate
| Type | Best for |
|---|---|
| Iron phosphate | Cheap pretreatment before painting |
| Zinc phosphate | Pretreatment for better paint life; with oil on fasteners |
| Manganese phosphate | Wear and anti-galling on gears, cams, pistons |
What to tell the plater
- Phosphate type and coating weight, or the standard
- Whether oil, paint or powder follows
- Material and part size
- Threads or faces that must stay bare
- Quantity
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Questions
- Does phosphating stop rust on its own?
- Only a little. It works best with oil, paint or powder coating over it.
- Phosphating or blackodising?
- Both are thin dark finishes. Phosphate holds oil and paint better and suits wear parts; blackodising gives a more even black look.
- Does phosphating change part size?
- Slightly: a few microns, more for heavy manganese phosphate. Close fits may need allowance.
The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 23 October 2026.