Phosphating
Phosphating is a chemical conversion coating of zinc, manganese or iron phosphate that gives steel a crystalline, grey-black surface. It is used as a base for paint and powder coating, and when oiled, for rust prevention and anti-galling on moving parts.
What it is
The steel part reacts in a phosphoric acid solution to form an insoluble phosphate crystal layer bonded to the surface. The porous layer holds paint, oil or wax very well.
Zinc phosphate is widely used under paint and for corrosion protection with oil; manganese phosphate gives a heavier, harder coating for wear, break-in and anti-galling on gears and engine parts; iron phosphate is a light pre-treatment before painting.
Typical applications
- Pre-treatment before painting or powder coating
- Fasteners and springs (phosphate and oil)
- Gears, camshafts and sliding parts (manganese phosphate)
- Cold forming and wire drawing lubrication carrier
- Firearm and defence components
Suitable materials
- Carbon and low-alloy steels
- Cast iron
- Galvanised steel (zinc phosphate)
- Not suitable for stainless steel or aluminium in standard form
Points to consider
- On its own, phosphate gives limited rust protection; it is almost always oiled, waxed or painted.
- Specify the type (zinc or manganese) and coating weight, as their purposes differ.
- High-strength steels may need baking to relieve hydrogen embrittlement.
- The coating adds a little thickness and roughness; allow for it on precision fits.
Frequently asked questions
Is phosphating good for rust protection?
With oil or wax it gives useful rust protection for indoor storage and use. For outdoor exposure it is normally used as a base under paint or powder coating.
What is the difference between zinc and manganese phosphate?
Zinc phosphate is finer and mainly for paint adhesion and general corrosion protection. Manganese phosphate is heavier and harder, used for wear resistance and anti-galling on moving parts.
Why is phosphating done before powder coating?
The crystalline layer greatly improves paint adhesion and slows rust creeping under the coating if it is chipped.