V.S. Heat Treatment
Heat treatment & metal finishing experts
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V.S. Heat Treatment
Heat treatment & metal finishing experts
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Both are conversion coatings we apply in-house. This page summarises the differences that actually change performance and cost.
Black oxide vs Phosphate
Black oxide forms a very thin layer (under ~1.5 µm), so part dimensions are effectively unchanged and the finish is a uniform black. Manganese-zinc phosphate builds a thicker crystalline layer (about 2–15 µm) that holds rust-preventive oil, improves run-in wear and acts as a paint base. Neither protects bare — both need a sealing oil.
| Decision factor | Black oxide | Mn-Zn phosphate |
|---|---|---|
| Coating type | Fe₃O₄ conversion layer | Crystalline phosphate conversion layer |
| Typical thickness | Under ~1.5 µm | About 2–15 µm |
| Dimensional change | Effectively none — good for tight tolerances | Slight growth with layer thickness |
| Colour / texture | Gloss to matte black | Dark grey to black, fine matte crystal |
| Corrosion (oiled) | Low — relies on the oil | Better — crystals retain oil longer |
| Oil / grease retention | Fair | Excellent — the main strength of phosphate |
| Paint / adhesive base | Not a strength | Good — crystal surface keys the paint |
| Typical use | Decorative parts, tools, tight-tolerance screws | Friction parts, springs, bolts before painting |
Choose black oxide when the priorities are a uniform black appearance, low cost and unchanged dimensions — furniture screws, hand tools and indoor decorative parts that will carry a sealing oil.
Choose phosphate when parts see friction or run-in, need a layer that holds rust-preventive oil longer, or will be painted or rubber-bonded afterwards — chassis bolts, springs and machine parts.