Black Oxide
Also: black oxide coating, blackening, bluing, hot black oxide, MIL-DTL-13924, AMS 2485, black oxide finish, magnetite coating
Black oxide converts the surface of steel to a thin black magnetite film in a hot alkaline bath. Near-zero thickness, a matte black look and mild rust resistance once oiled or waxed.
31 shops tagged Black oxide among 3,387 finishing and heat treating companies in the Noramark directory
Black oxide is a conversion coating, not a deposit. Steel is immersed in a boiling alkaline nitrate solution at about 140-150 °C (285-300 °F) for 5-30 minutes, and the surface iron reacts to form magnetite (Fe3O4), a black oxide about 1-2.5 µm (0.00004-0.0001") thick. The film grows into the metal, so there is no measurable dimensional change and no build-up on edges or threads. It is the finish on taps, drills, gauges, fasteners, gun parts and any steel part that must look black and stay to size.
MIL-DTL-13924 is the military spec and AMS 2485 the aerospace one; Class 1 in MIL-DTL-13924 is the standard hot alkaline process for carbon and alloy steel, and other classes cover stainless and special cases. On its own, black oxide gives very little corrosion protection, a few hours of salt spray. The supplementary oil, wax or lacquer topcoat does the protecting: an oiled part survives a day or two of salt spray and indoor storage for years; a dry-to-touch wax gives handling parts a clean finish; lacquer suits display parts. The drawing should say which.
Mid-temperature (about 100-120 °C) and room-temperature "black oxide" processes exist. The mid-temperature bath produces true magnetite with less fuming; the room-temperature products deposit a copper selenide film that looks similar but rubs off more easily and is not a MIL-DTL-13924 coating. Stainless steel is blackened in a separate, hotter bath and comes out a dark grey-black. Free-machining and high-alloy steels sometimes finish reddish or uneven, so ask for a sample when colour matters.
At a glance
| Typical tolerances | Film thickness 1-2.5 µm (0.00004-0.0001"), with no measurable dimensional change: threads, ground fits and gauge surfaces keep their size and finish. The oil or wax topcoat adds nothing measurable once wiped. Hardness of the base metal is unaffected; the bath temperature is well below any tempering temperature. |
|---|---|
| Size limits | Tank-limited, typically 1.5-2.5 m (5-8 ft); small parts in baskets by the thousand. Closed tubes and blind holes must drain or they carry hot caustic out of the tank. Very large parts are blackened by specialist shops or by cold-blackening products in place. |
| Surface finish | Matte to satin black that follows the base surface: a polished part comes out gloss black, a machined part matte black, a blasted part dead-flat black. Colour is a uniform blue-black on plain carbon steel, grey-black on cast iron and hardened parts with scale, and sometimes brown or red-tinged on leaded or high-chromium grades. The film is soft and abrades in handling; the topcoat carries the corrosion resistance. Salt spray: hours dry, 24-100 hours oiled or waxed. |
| Lead time | 1-3 business days as a batch process; often same day for small lots. Stainless blackening and special topcoats may add a day or two. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon, alloy and tool steels, cast and ductile iron (castings with heavy scale come out grey), powder metal after sealing, and case-hardened and through-hardened parts without loss of hardness. Stainless steel in a dedicated bath (MIL-DTL-13924 Class 3 or 4 territory, grey-black). Copper and brass blacken in a different chemistry. Aluminum, zinc and plated surfaces cannot be black oxided. Leaded steels (12L14) and high-chromium alloys may finish brown or blotchy.
What drives the cost
- Lot size: the minimum lot charge dominates small orders; large baskets cost almost nothing per piece
- The topcoat: oil is standard; wax, dry-film and lacquer cost more and take longer
- Cleaning and descaling before the bath, especially on heat-treated or rusty parts
- Stainless or copper alloy variants that route to a separate bath
- Racking of parts that cannot be basket-tumbled (cosmetic or fragile parts)
- Certifications to MIL-DTL-13924 or AMS 2485, and salt-spray coupons
When to use it
- Steel parts that must stay to size: gauges, taps, drills, dowels, ground pins, precision fasteners
- A black appearance on machined steel at the lowest possible cost
- Glare reduction on tools, optics hardware and instrument parts
- Mild indoor corrosion resistance on parts that will be oiled in service anyway (firearms, machine parts, tooling)
- A base for oil retention on sliding parts, and a break-in aid on gears and fasteners
When not to
- Outdoor or wet service: an oiled black oxide part rusts within days of exposure; use zinc, e-coat or powder
- Aluminum, zinc, plated or non-ferrous parts, except brass and copper in their own bath
- Parts that must not carry oil (clean rooms, food contact, vacuum equipment) unless a dry-film topcoat is acceptable
- Wear resistance: the film is soft and wears through immediately
- Colour-matched sets across different steel grades; leaded and high-chromium alloys finish differently
Design tips
- Specify "Black oxide per MIL-DTL-13924 Class 1, supplementary treatment: oil" (or wax, or lacquer); the topcoat decides the corrosion life.
- Finish all machining and grinding first; the film adds nothing, so post-machining leaves bright spots.
- Heat treat before black oxide; hardened parts blacken fine and the bath is below tempering temperature.
- Descale or blast heat-treated parts before blackening if a uniform black matters; scale comes out grey.
- Add a drain hole or note the orientation for blind holes and closed sections so the caustic drains and rinses.
- For a dead-flat black, bead blast first; for gloss black, polish first. The finish follows the surface.
- If the part is 12L14, 416 or a high-chromium alloy and colour matters, ask for a first-piece sample.
Black Oxide by material
Frequently asked questions
- Does black oxide prevent rust?
- Only mildly, and only through its oil or wax topcoat. Dry black oxide fails in hours of salt spray; oiled, it lasts a day or two and protects indoors for years. For outdoor or wet service, choose zinc plating, e-coating, powder coating or galvanizing.
- How thick is black oxide?
- About 1-2.5 µm (0.00004-0.0001"), and it grows into the surface rather than on top of it, so there is no measurable change in dimensions. That is why gauges, taps and precision fasteners use it.
- Can stainless steel be black oxided?
- Yes, in a separate, hotter bath. The result is a dark grey to black conversion film that is thinner and less uniform than on carbon steel and reduces the stainless surface corrosion resistance slightly; many shops prefer to leave stainless bright or use a black PVD coating instead.
- Does black oxide affect hardness or tolerances?
- No. The bath runs at about 140-150 °C, far below any tempering temperature, and the film is a micron or two thick. Hardness, ground fits and thread sizes are unchanged.
- What is the difference between black oxide and bluing?
- Hot bluing is black oxide by another name: the same alkaline nitrate bath, used in the firearms trade. Cold bluing is a selenium-based touch-up product, thinner and less durable. Both differ from black phosphate, which is a heavier, more absorbent grey-black coating.
