? Where is your part right now?

Not made yetWe machine the steel part and black-oxide it in one order — the coating barely moves a dimension, so tight fits survive it.Machine + coat from one order →Already madeSend us your steel or stainless parts. Black oxide is a conversion coating — it darkens the surface rather than depositing metal on it.Check if your part qualifies →Still comparing processesBlack oxide against paint, plating and bluing — the decision is usually about whether you can accept a dimensional change or not.Compare against other coatings →

SURFACE FINISHING

Black Oxide Service

Steel Stainless steel Matte black 0 – 30 µm

A chemical conversion coating that grows a microscopic layer of magnetite (Fe3O4) on ferrous parts. It darkens the surface, cuts reflection and adds corrosion resistance — and because the layer is only 0 to 30 µm, it is one of the few coatings you can put on a part with critical dimensions.

Get a quote Check if my part qualifies

0 – 30 µmThe published thickness range of the magnetite layer

Steel & stainlessThe ferrous materials this process covers

Matte blackMinimises reflection — the reason it is used on optical and tool parts

Conversion coatingIt does not build up the way plating does

Two identical steel parts side by side, one as machined and one with a black oxide finish — Same part, before and after — black oxide darkens the surface in place rather than coating it with a separate layer.

Sequence

Black oxide is one of the last steps — and it barely moves the drawing

The layer grows from the steel itself, so it follows every machined feature instead of covering them. That is what makes it usable on threads and fits where a plated layer would have to be masked.

STEP 1MachineMill, turn or fabricate to drawing

STEP 2Deburr & cleanScale, oil and oxide stop the conversion reaction where they sit

STEP 3Any welding or heat treatmentDo it before coating — heat burns the layer off at the joint

STEP 4Black oxideYou are here — 0 to 30 µm of magnetite

STEP 5Oil or sealThe usual post-step, to complete the corrosion protection

Because the layer is so thin, black oxide is often chosen precisely because it does not need masking — threads stay usable. The trade-off is protection: a conversion layer is thinner than a plating, so it is usually finished with an oil or seal. See dimensional change.

Fit check

Can we black-oxide your part? Three things decide it

Material, condition and what happens after. The third one is where most orders go wrong — welding after coating destroys the layer at the joint.

Material

Yes — steel and stainless steel
No — aluminium, copper, brass, zinc, plastics
Not listed: ask first

Black oxide works on ferrous surfaces because the layer is iron oxide. Non-ferrous parts need a different process — see materials.

Condition & size

Rust, scale and heat-treat oxide interfere with the reaction
Previously plated or painted parts have to be stripped first
Maximum part envelope: To confirm

Tell us if the part has been heat treated, welded or plated before, and the largest dimension.

After the coating

Do not weld after coating — the heat destroys the layer locally
Oil or seal is the usual finishing step to complete protection
Handling a dry coated part can leave marks until it is sealed

Tell us the service environment so the right post-treatment can be specified.

Specifications

Black oxide: 0 – 30 µm of magnetite on steel and stainless steel

Every value below is taken from our published specification for this process.

Materials

Steel · stainless steelFerrous surfaces — the layer is iron oxide formed from the metal itself.

Colour

BlackPublished colour for this process.

Texture

MatteNon-reflective, which is the point on optical and tool parts.

Layer thickness

0 – 30 µmPublished range for the magnetite (Fe3O4) layer.

Function

Reduces reflection, resists corrosionBoth are stated functions of the finish, not incidental effects.

Post-treatment

To confirmOil or seal specification is being confirmed with the shop.

Why the thin layer matters: 0 to 30 µm is roughly a tenth of a typical plated layer. On a threaded or press-fit part that difference decides whether masking is needed at all — see dimensional change.

Dimensional change

What 0 – 30 µm does to a thread or a press fit

Black oxide forms from the surface rather than sitting on top of it, so the growth is a fraction of what plating adds. On most parts that is small enough to ignore — on a fine thread it still has to be considered.

Where it acts	Layer effect	Effect on the fit	What to do
Threads	0 – 30 µm on the flanks	Much less fill than plating — most threads stay usable without masking.	Flag fine-pitch threads if the assembly is critical.
Bores and press fits	0 – 30 µm on the wall	A light interference fit generally survives the layer.	Check very tight tolerances against the 30 µm upper bound.
Ground and lapped faces	Follows the surface	The layer conforms to the surface rather than smoothing it.	Black oxide will not improve a surface finish.
Rust or scale before coating	Reaction is blocked	Oxide left on the surface shows as a patch that did not take.	Remove scale and rust before the parts arrive.

Layer range as published for this process. Because the coating is a conversion layer, the growth per surface is at most the figure shown — not twice it.

Working to a tight tolerance? Send the drawing and we will confirm whether the layer fits inside it.

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What it is

Black oxide against the other ways to get a dark part

The finish is a magnetite layer grown from the steel. That single fact explains what it is good at — and why it is the wrong choice when you need a thick protective barrier.

Two black oxide coated steel mounting brackets with a matte black surface

Black oxide (magnetite)

0 – 30 µm

A chemical coating that covers the surface with a smooth, microscopic layer of magnetite (Fe3O4). It is formed from the steel itself, so it follows every machined feature.

Best for: precision parts, threads and fits, minimising reflection

Not for: parts needing a thick sacrificial layer — the coating is only 0–30 µm.

Close view of a black oxide coated steel block showing the matte black surface

The alternatives

Different mechanisms

If the requirement is colour rather than protection, painting covers it. If it is sacrificial corrosion protection, galvanizing does that. If it is a hard wear surface, plating is the family to look at.

Painting for colour · galvanizing for sacrificial protection · plating for wear

Trade-off: all of those add a layer measured in tens of microns or more — black oxide does not.

Not sure whether black oxide or plating suits the drawing? Send it and describe the service environment.

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Pick or avoid

When black oxide is the right call — and when it is not

Pick black oxide when

The part is steel or stainless steel.
You need a dark, non-reflective surface — the published function of the finish.
The part has threads, bores or fits that cannot tolerate a thick coating.
Corrosion resistance is wanted but a plated layer would be too thick.
The part is a tool, a fastener or a machine component that should look like metal, not painted.

Avoid black oxide when

The material is not ferrous — aluminium, copper, brass and zinc do not take this coating. See materials.
You need sacrificial protection that keeps working at a scratch — that is galvanizing, not a conversion layer.
You need a specific colour other than black — see painting.
You need a hard, wear-resistant surface — see electroplating.
The part will be welded after coating.

Need protection that survives a scratch rather than a colour change? Ask for galvanizing and black oxide quoted side by side.

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Design constraints

What decides whether the black oxide comes out even

Three variables: the condition the part arrives in, what happens after coating, and how the part is handled once it is dry.

Starting condition

Black oxide is a reaction with the steel surface. Rust, heat-treat scale, oil or a previous plating blocks it locally — and the blocked area stays bare. Strip and clean first.

Heat treatment

Heat treatment before coating is fine, but it leaves an oxide layer that has to be removed. Tell us if the part has been hardened or stress relieved.

Welding

Weld before coating, never after. The heat burns the layer off at the joint and the affected zone has to be re-coated locally.

Post-treatment

A dry black oxide surface is not the finished state — an oil or seal is normally applied to complete the corrosion protection. Tell us the service environment.

Handling

The layer is thin and can be marked by handling before the post-treatment. Parts should be packed rather than loose-bagged.

Appearance expectation

The finish is matte and slightly uneven by nature on cast or forged surfaces. If the visible face must be uniform, tell us and it can be oriented and checked.

Drawing notes

What to write on the drawing so the layer comes out even

For black oxide the note is mostly about the condition of the part and what happens after coating — masking is rarely needed because the layer is so thin.

1.  BLACK OXIDE (MAGNETITE) FINISH ALL SURFACES.    LAYER 0-30 um.2.  SURFACES TO BE FREE OF RUST, SCALE AND HEAT-TREAT    OXIDE BEFORE COATING.3.  APPLY OIL / SEAL AFTER COATING: ____________    (SPECIFY PER SERVICE ENVIRONMENT)4.  DO NOT WELD, GRIND OR RE-MACHINE AFTER COATING.    LOCAL REPAIR TO BE RE-COATED.5.  APPEARANCE: MATTE BLACK, NON-REFLECTIVE.    SLIGHT TONE VARIATION ACCEPTABLE ON CAST / FORGED SURFACES.

Examples only. The line that most often goes missing is the post-treatment — black oxide is usually applied with an oil or seal, and the choice depends on the environment.

Materials

Which materials take black oxide — and what to use if yours does not

The coating is iron oxide, so it needs iron. Non-ferrous substrates each have their own darkening or protective route, named below.

Material	Black oxide	Notes	Use instead
Steel	Yes	The main substrate — fasteners, tools, machine parts and hardware.	—
Stainless steel	Yes	Darkens as well, though the corrosion benefit is smaller because the base already resists corrosion.	—
Cast iron	Yes	Ferrous, so it takes the coating — but the casting skin has to be removed first for an even result.	—
Aluminium	No	Aluminium has no iron to convert; its black finish is a dyed anodized layer, not a black oxide.	Anodizing (black)
Copper and brass	No	Copper alloys need their own darkening or protective treatments.	Electroplating · Polishing
Zinc-plated parts	No	A zinc layer replaces the steel surface the reaction needs.	Zinc plating

Part in more than one material? Send the assembly and we will route each component to the process that fits it.

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Applications

Where black oxide is used

Our published description places it in hardware, automotive and defence work — the common thread is a metal part that must stay metal-looking and non-reflective.

Black oxide coated steel brackets with a matte black non-reflective surface

Hardware and fittings

Flanges and nuts that have to look like metal, resist corrosion and stay at their dimensions.

Close view of a black oxide coated steel block with a machined face still bright

Tools and machine parts

Where a non-reflective dark surface is wanted and a thick coating would throw the tooling dimensions out.

Steel part before and after black oxide treatment, showing the colour change only

Automotive and defence

Parts that need corrosion resistance and reduced reflection without a change in fit — the published application areas for this finish.

QA and documentation

What you get back with the parts

Purchasing usually needs paper as well as parts. Here is what we can issue today and what is still being confirmed.

Available

Colour and coverage

Visual check for a uniform matte black across the specified faces.

Available

Pre-treatment condition

Confirmation that scale, rust and previous coatings were removed.

To confirm

Layer thickness

Measured magnetite layer within the 0–30 µm range.

To confirm

Salt-spray hours

Corrosion hours with the specified post-treatment. Being confirmed with the shop.

Black oxide guides and comparisons

Longer answers on the questions this page only has room to state.

Black oxide finishes: a precise approach

The magnetite layer explained, with the limits of a conversion coating.

Zinc plating: everything you need to know

The sacrificial alternative — a zinc layer protects steel by being consumed itself.

Chrome, zinc and nickel plating compared

When the requirement is a hard wear surface rather than a colour change.

Galvanizing service

When the part needs protection that keeps working at a scratch.

Passivation: a surface treatment process

The corrosion step usually chosen for stainless steel instead.

All surface finishing options

Compare all 20 finishes by roughness, thickness, corrosion and cost.

FAQ

Black oxide FAQ: the questions engineers ask us

What is a black oxide finish?

A chemical coating technique that covers a metal surface with a smooth, microscopic layer of magnetite (Fe3O4), 0 to 30 µm thick. It minimises the reflection of the part and resists corrosion.

Which materials can be black oxided?

Steel and stainless steel — the process converts the iron in the surface, so the substrate has to be ferrous. Cast iron also takes it once the casting skin is removed. Aluminium, copper, brass and zinc do not, and each has its own route instead.

Will black oxide change my part dimensions?

Very little — the published layer range is 0 to 30 µm, and it forms from the surface rather than sitting on top of it. That is roughly a tenth of a typical plated layer, which is why threads and fits usually survive without masking.

Does black oxide need masking?

Normally not for tolerance reasons, because the layer is so thin. Masking is only needed if a surface has to stay uncoated for a functional reason, such as an electrical contact.

Is black oxide the same as painting it black?

No. Paint adds a layer on top of the part and can chip; black oxide converts the surface itself, so the colour does not peel off — but the protection is thinner and is normally completed with an oil or seal.

How is it applied?

Chemically, by immersion — the part's surface reacts and forms the magnetite layer. That is why the condition of the part matters so much: rust, scale or oil blocks the reaction where it sits and leaves a patch.

Can you black oxide a part that has been heat treated?

Yes, but the heat-treat oxide has to be removed first or the coating will not take evenly. Tell us if the part has been hardened, stress relieved or welded before it arrives.

Do I need anything after black oxide?

Usually an oil or seal to complete the corrosion protection — a dry conversion layer on its own is thinner protection than most service environments want. The specification is being confirmed; tell us the environment and it will be matched.

Get your black oxide quote

Upload a STEP, IGES or PDF drawing. Tell us the material, the surface condition it is in now, and the environment it has to survive.

Get a quote Machine it first

What to send us

Drawing — 2D and 3D files (STEP, IGES or PDF).
Material — steel, stainless steel or cast iron.
Previous treatment — heat treatment, welding, plating or rust that has to be removed.
Service environment — so the oil or seal post-treatment can be specified.
Critical tolerances — anything where the 0–30 µm layer has to be accounted for.
Quantity — number of units, including any spares.

All uploads are secure and confidential. Engineers review and reply with a quotation and lead time.