Manganese phosphate is a porous crystalline layer that holds oil where it is needed — on the raceway during running-in, when the surfaces have not yet mated and scuffing risk is highest.
| Thickness | 5–15 µm (crystalline, porous) |
|---|---|
| Salt spray | 24–96 h when oiled |
| Max service temperature | ≈200 °C (the oil it retains is the limit) |
| Effect on dimension | Adds thickness — 10–30 µm across a diameter; usually applied before final grind |
| Friction | Lowers during running-in; retains the lubricant film |
| Process temperature | 85–95 °C bath |
| Typical use | Raceways, gears, camshafts, piston rings, parts that must bed in |
The phosphate layer is crystalline and porous at a microscopic scale. Oil soaks into the pores and is held there, so when two new surfaces run together under load — before the asperities have worn smooth — there is still lubricant at the contact. That is why phosphate is specified for the first hours of life, not for the whole of it: once the surfaces have mated, the layer's job is done.
The second effect is that phosphate prevents localised cold welding. Bare steel asperities in a new assembly can weld and tear under high contact pressure, which is the mechanism behind scuffing and seizure. A phosphate layer is softer than the steel and shears instead, protecting the substrate.
| Property | Mn phosphate | Black oxide | PTFE dry film | Zinc plating |
|---|---|---|---|---|
| Thickness | 5–15 µm | 1–3 µm | 5–15 µm | 5–8 µm |
| Mechanism | Porous, oil-retaining | Conversion, cosmetic | Low-friction film | Sacrificial barrier |
| Salt spray (oiled) | 24–96 h | 24–72 h | Binder-dependent | 96–240 h |
| Effect on clearance | Moderate — plan for it | Negligible | Moderate | Significant |
| Best for | Running-in | Appearance, tolerance | Dry running, low friction | Corrosion |
Raceways that must bed in under high contact pressure.
Read →Where the grease film at start-up is marginal.
Read →When no dimensional change is allowed.
Read →When the requirement is low friction rather than oil retention.
Read →Yes, by roughly the coating thickness on each surface — 10–30 µm across a diameter for a 5–15 µm layer. Either coat before final grinding or allow for it in the clearance.
For bearing and gear surfaces, yes: the manganese layer is denser, retains oil better and withstands higher contact pressure. Zinc phosphate is more common on painted steel for paint adhesion.
No. It gives mild protection when oiled, measured in tens of hours of salt spray. Use zinc or zinc-nickel where corrosion is the actual requirement.
Bath temperature, concentration or immersion time out of specification, or a surface that was not properly cleaned. A powdery layer does not retain oil and rubs off in service.
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