Skip to content

Finishes / Black oxide coating

Black oxide coating services for steel parts

Black oxide converts the surface of steel into black iron oxide (magnetite) in a hot alkaline salt bath near 140 °C. The layer is about 1 µm, so it barely changes dimensions. On its own it gives little corrosion protection. The oil, wax or lacquer applied afterwards does most of that work.

Buyers specify it on tooling, fixtures, gears, shafts, hand tools, fasteners and machine parts where a uniform black look, low glare and tight fits matter more than outdoor corrosion life.

Send an RFQ for this finishWhat to put on the drawing
  1. Carbon or alloy steel substratebase
  2. Black oxide (magnetite) conversion layerabout 1 µm
  3. Oil, wax or lacquer sealantthin film

The oxide forms from the steel itself. The sealant on top carries most of the corrosion protection.

Section view, not to scale. Thickness values are typical, not promised.

Typical values

Typical ranges, not promised. The spec on your drawing and the process control of the shop set the real numbers.

Thickness
About 0.5 to 1.5 µm
Process
Hot alkaline bath near 140 °C. Mid and room temperature versions are different chemistries
Sealant
Water displacing oil, dry wax or lacquer
Appearance
Matte to semi-gloss black, follows the surface finish underneath
Corrosion
Mild, indoor. Depends on the sealant and on storage
Electrical
Conductive
Handling
Baskets or racks. Suits large lots of small parts

Specs buyers call out

Put the spec and its type, class or grade on the drawing. Shops quote against it, and approvals narrow the list.

MIL-DTL-13924
Black oxide coating for ferrous metalsClass 1 alkaline for steel, Class 4 for stainless
AMS 2485
Black oxide coating, aerospaceProcess and sealant for steel parts
ISO 11408
Black oxide coatings on iron and steelInternational equivalent with test methods
Customer spec
Sealant and testMany drawings only state the sealant type and a humidity or salt spray test

Base materials

Name the alloy and hardness on the RFQ. Preparation changes with the base metal, and some shops only run one family.

Suits

  • Carbon and alloy steel
  • Tool steel
  • Cast iron

Needs care

  • Stainless steel (separate process, often less even)
  • Hardened steel (cleaning method)
  • Parts with brazed or soldered joints

Not typical

  • Aluminum
  • Zinc die castings
  • Parts that must survive outdoor exposure without upkeep

Hardened steel over 39 HRC

The hot alkaline bath itself puts little hydrogen into steel, but acid pickling before it can. For steel over 39 HRC, ask for alkaline cleaning or blasting instead of acid, or a relief bake after any acid step. ASTM B850 is a common reference. State the hardness on the RFQ.

When to specify it, and when to look elsewhere

A good fit for

  • Black, low glare finish with almost no change in size
  • Tooling, fixtures, gears and shafts used indoors
  • Parts that are oiled in service anyway
  • Large lots of small parts at low cost per part

Look elsewhere for

  • Outdoor or wet service: zinc-nickel with a black passivate lasts far longer
  • Parts that must stay dry to the touch, unless a dry wax sealant is accepted
  • Aluminum, which needs a black anodize instead
  • Parts stored for long periods without packaging

What to put on the drawing and RFQ

Six lines that decide whether quotes compare. Missing items come back as questions and add days.

  • Spec and classMIL-DTL-13924 Class 1 or 4, AMS 2485, or your own spec.
  • SealantOil, dry wax or lacquer. It sets both corrosion performance and how the part feels.
  • Process typeSay if only hot black oxide is accepted. Room temperature blackening is a different product.
  • TestHumidity or salt spray test method and duration if your spec has one.
  • HardnessHardness of the part and how it should be cleaned before blackening.
  • PackagingVCI bags or paper for long storage or sea freight.

What moves the price

Shops price by load, lot and labor more than by the chemistry itself.

  • One of the cheapest finishes per part on large lots
  • Lot charge sets the price on small lots
  • Stainless needs a separate process line
  • Lacquer sealant costs more than oil

Plating and coating cost: what drives the price per part

Questions buyers ask

About black oxide coating specifically. For how matching works, see the home page.

Does black oxide prevent rust?

Only mildly. The oxide is thin and porous. The oil, wax or lacquer applied after does most of the protection, and only for indoor or oiled service.

How much does black oxide change dimensions?

About 1 µm or less. It is used on gauges, fits and threads where plating would not fit.

What is the difference between hot and cold black oxide?

Hot black oxide converts the steel surface into iron oxide. Room temperature products deposit a copper selenium compound that is less durable. If your drawing means hot black oxide, write it.

Can stainless steel be black oxidized?

Yes, with a separate process covered by MIL-DTL-13924 Class 4. Color is often less even than on carbon steel.

Black oxide or zinc plating?

Black oxide for fit and look on indoor parts. Zinc or zinc-nickel with a black passivate for corrosion protection. The black oxide vs zinc guide compares them.

Get quotes for black oxide coating

Send one RFQ with the drawing, base material, spec and lot size. We match it with up to 3 shops that run this finish on a line that fits your part, and reply within 24 business hours.

Send my RFQ

Related guides

Other finishes