Zinc Phosphate & Oil (Phos & Oil)
Zinc phosphate conversion coating with protective oil — the lowest-cost steel fastener finish, thin and black, 2–72 hours neutral salt spray by specification, for handling, storage and dry ambient service.
Last updated: 2026-11
Specifications
Phosphate & oil is the economy finish for steel fasteners: a liquid-bath chemical conversion process that grows a thin black zinc-phosphate crystal layer, followed by a protective oil application. The coating itself is not a corrosion barrier — the oil is — so the protected life fits handling, packing, storage, and everyday dry ambient service, which is what most unexposed fastener work needs. It slightly improves lubricity over plain steel and carries very low hydrogen-embrittlement risk, since no electrolysis is involved. When a buyer's drawing says "black" or "phosphate" on carbon-steel bolts and screws, this is the finish being described.
| Process | Zinc phosphate liquid-bath chemical conversion + protective oil topcoat |
|---|---|
| Reference standards | MIL-DTL-16232 (with oil / with corrosion preventative) and GMW3179 (with oil / with corrosion preventative) |
| Corrosion resistance | 2–72 h neutral salt spray by specification — GMW3179 oil: 24 h; GMW3179 with corrosion preventative: 72 h; MIL-DTL-16232 oil: 2 h; MIL-DTL-16232 with corrosion preventative: 72 h |
| Appearance | Thin black coating |
| Lubricity | Slightly increased vs plain steel; no general COF requirement — GMW3179 offers a friction-controlled topcoat option at COF 0.13 ±0.03 |
| Hydrogen embrittlement | Very low risk — non-electrolytic process; hardened parts processed per specification to detect and eliminate hydrogen |
| Relative cost | Lowest of the common steel fastener finishes (reported plating-shop comparison) |
Materials
- Carbon steel fasteners
- Alloy steel fasteners
- Case-hardened fasteners
Ordering & documentation
Documents
Shipments are backed by Mill Test Certificate (MTC), Dimensional Report, Salt Spray Test Report on request, with full batch traceability from raw material to finished product. Quality management follows the owner-confirmed compliance list: ISO 9001:2015, RoHS, REACH. Exact document sets are confirmed on the order.
Quotation
- The fastener family (Zinc Phosphate & Oil (Phos & Oil)) plus the standard designation from your drawing — DIN / ISO / JIS / ASTM / ANSI, or a drawing for custom parts.
- Size (diameter and length), thread type (metric or inch), property class or grade, and coating or finish.
- Quantity per size and your target delivery date.
- Destination country, and any required documents (for example a mill test certificate) so the quote includes them.
Sample & supply
Minimum order quantity is per batch, carried with the fastener order. Finishing run adds 5–10 days to the fastener lead time. Quantities or variants outside the standard range are confirmed at quotation.
Storage & shipping
Parts are packed for export and shipped factory-direct to buyers in Europe, Asia, the Americas and the Middle East. Store fasteners dry and protected until installation, and keep coated parts in their packaging until use; specific packing and preservation requirements can be stated on the order and confirmed at quotation.
Frequently asked questions
Is phosphate & oil enough for outdoor or wet service?
No — plan on it for handling, storage and dry ambient exposure only. The corrosion resistance of the common specifications tops out at 72 hours of neutral salt spray, and the oil layer that does the protecting is consumable. For outdoor, wet or saline service, step up to zinc electroplate (standard thickness classes) or beyond, depending on the hours the spec demands. Phos & oil remains the correct, lowest-cost call for internal machinery, furniture, and any assembly where the fastener lives its life dry.
What is the MOQ and lead time for this finish?
Finishing runs per batch carried with the fastener order and adds 5–10 days to the fastener lead time. Coated parts follow the finish line that processes them, so confirm the exact schedule against quantity and finish class at quotation.
What should an RFQ for this finish state?
State the base fastener (grade, diameter, length), the coating standard and class (or the process plus thickness/color requirement where no standard callout exists), any salt-spray or friction requirement, and quantity per size — plus destination country and required documents such as a mill test certificate.
Related knowledge
- Reading a Fastener Coating Callout: ISO 10683 Designations, Salt-Spray Hours and What to Confirm With the Plater
- Zinc-Plated vs Hot-Dip Galvanized Fasteners: Which Coating for Which Environment
- Zinc-Flake Coating for Fasteners: When the Evidence Supports It, and What to Verify at the Coater
- Zinc-Flake vs Zinc-Nickel for High-Strength Fasteners: What the Evidence Settles on Hydrogen, Salt Spray, Friction and Specs
Request a Quote
Get factory-direct pricing from the manufacturer.
What to include in your inquiry
- ✓ Standard and property class (e.g. DIN 912, ISO 4762, class 8.8) — or a drawing for custom parts.
- ✓ Sizes, lengths and thread type (metric or inch).
- ✓ Quantity per size and your target delivery date.
- ✓ Destination country, plus any coating or finish requirement.
- ✓ Any documents you need with the shipment (e.g. mill test certificate) so the quote includes them.
- ✓ Your inquiry goes straight to the factory team by email and gets a reply within 24 hours.
- ✓ Factory-direct pricing
- ✓ Standard and custom specifications
- ✓ Factory-direct export to buyers in Europe, Asia, the Americas and the Middle East
- ✓ Full fastener family support