Black oxide vs phosphate: which finish for screws, nuts and bolts?
Both are conversion coatings we apply in-house. This page summarises the differences that actually change performance and cost.
Black oxide vs Phosphate
Short answer: black oxide for colour and zero size change, phosphate for oil retention and wear
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.
Black oxide versus phosphate comparison
| 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 |
How to choose
Black oxide
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.
Phosphate
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.
Limits before putting it on the drawing
- Neither is standalone outdoor corrosion protection — consider zinc plating for exposed parts.
- Thickness and salt-spray figures depend on bath chemistry, sealing oil and geometry; confirm with trial pieces.
- Unoiled black oxide rusts very quickly — always specify the sealing oil or wax.
- Phosphate adds measurable thickness; allow for it on tight-tolerance threads.
What to send the finisher
- 1Part type, material and lot quantity/weight
- 2Reference standard or spec (if any) and target thickness
- 3Sealing oil type and corrosion requirement
- 4Downstream step: painting, bonding or immediate assembly
- 5Sample pieces for a trial run before volume orders
Frequently asked questions
Do you apply both black oxide and phosphate in-house?
Yes. Both are conversion coatings we run in-house on small fasteners — screws, nuts and bolts. Send the part and the target finish and we can advise which one suits it.
Which resists corrosion better?
Neither protects bare metal; both rely on a sealing oil. Oiled phosphate holds rust-preventive oil longer, so it usually out-lasts oiled black oxide. For outdoor exposure, consider zinc plating instead.
Will the coating change my part dimensions?
Black oxide is under ~1.5 µm, so dimensions are effectively unchanged — good for tight tolerances. Phosphate adds about 2–15 µm, so allow for it on close-tolerance threads.