Black Oxide Coating
Black oxide conversion coating for fasteners — a hot alkaline bath that turns the surface flat black with negligible dimensional change, paired with an oil topcoat for temporary rust protection on indoor and precision hardware.
Last updated: 2026-11
Specifications
Black oxide is a chemical conversion coating: steel parts are immersed in a hot alkaline bath (typically sodium hydroxide with sodium nitrite and other oxidising agents at 140–150 °C) that converts the surface itself rather than adding a layer, which is why thread fit and dimensions are essentially unchanged. On its own the coating does almost nothing against rust — the oil film applied over it is the only part of the system that prevents corrosion, delaying oxidation on the order of days rather than the weeks and years a sacrificial zinc coating delivers. That is why black oxide is the finish for indoor machine parts, tooling, and precision components where appearance and dimension matter more than corrosion life, and where the part lives its life dry and lightly maintained.
| Process | Hot alkaline chemical conversion coating — typically sodium hydroxide with sodium nitrite and other oxidising agents at 140–150 °C |
|---|---|
| Reference standards | MIL-DTL-13924 (black oxide, military specification); ISO 11408 covers chemical conversion black oxide coatings on iron and steel |
| Corrosion protection | Minimal alone — the supplementary oil film is the part of the system that prevents corrosion; typically applied with oil, wax, or lacquer for temporary rust prevention |
| Service temperature | Black oxide (and oil): useful design temperature limit 300 °F |
| Appearance | Flat black |
| Dimensional impact | Conversion process — negligible build-up; used on precision parts where dimension matters |
| Hydrogen embrittlement | Low — generates a small amount of atomic hydrogen, significantly less than acid pickling or electroplating; risk is low for most fastener grades but not zero at the highest strength levels (property class 12.9: precautionary bake 190–230 °C for 4–8 h recommended for safety-critical use) |
| Typical use | Indoor machine parts, tooling, and precision hardware; aesthetics |
Materials
- Carbon steel fasteners
- Alloy steel fasteners
- Case-hardened fasteners (baking considered above 45 HRC)
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 (Black Oxide Coating) 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 black oxide a corrosion finish?
Not in the way zinc or galvanizing is. The black oxide layer itself provides minimal corrosion protection — the oil, wax, or lacquer applied over it does the actual protecting, and only temporarily (the oiled film delays rust on the order of days). Specify black oxide when the requirements are appearance, low dimensional change, and low cost on parts that live indoors or in maintained assemblies; when a drawing needs real corrosion life in wet or outdoor service, a sacrificial zinc coating or galvanizing is the correct call.
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