Black Oxide Coating Thickness on Steel: 0.6–0.9 µm, and Why Dimensions Barely Move

Q: Is black oxide thickness the same on stainless steel and cast iron?

No. Stainless steel blackens through a different hot chemistry and builds a thinner, sub-micron layer with a darker grey-black appearance; cast iron behaves less uniformly because graphite inclusions affect the conversion. Carbon and low-alloy steels give the most consistent 0.6–0.9 µm result.

Q: How should black oxide thickness be specified on a drawing?

Specify the process (hot alkaline black oxide), the substrate, the after-treatment (oil, wax or lacquer), and whether dimensions apply before or after finishing. There is usually no need to tolerance the layer itself — at 0.6–0.9 µm it sits below machining tolerance interest; state instead that critical dimensions apply post-finish.

Specification guide for machined steel parts · Boyang Hardware, Zhongshan, China

Direct answer Hot alkaline black oxide on carbon and alloy steel forms a magnetite layer of roughly 0.6–0.9 µm. Because the coating converts into the metal rather than stacking on top of it, a finished part typically changes dimension by less than 2 µm — which is exactly why black oxide is specified on threads, gauge surfaces and tight-tolerance steel components where plating build cannot be tolerated.

0.6–0.9 µmhot black oxide on steel
<2 µmnet dimension change
<1 µmstainless layer
20×thinner than 15 µm nickel
100%post-finish inspection
1–2 dquotation

Layer thickness by substrate

Table 1 — Typical black oxide layer build by substrate in hot alkaline processing. Values are process-window figures; color and uniformity also vary with surface condition before treatment.
Substrate	Typical layer	Appearance	Notes
Carbon steel (1018, 1045)	0.6–0.9 µm	Uniform deep black	The reference case; most consistent
Low-alloy steel (4140)	0.6–0.9 µm	Deep black	Alloy content has little effect on thickness
Tool steel (hardened)	0.5–0.9 µm	Black to blue-black	Pre-hardening or post-hardening both possible; watch distortion in the hot bath
Stainless steel (304/316)	<1 µm	Grey-black	Different chemistry; less uniform than carbon steel
Cast iron	Variable	Matte black	Graphite inclusions cause color variation; trial recommended
Brass / copper	0.5–1.5 µm	Black-brown	Separate chemistry from steel black oxide

Why the thickness number is so small

Black oxide is a conversion: the bath oxidizes the iron surface in place to magnetite (Fe₃O₄). A portion of the resulting oxide grows outward and a portion replaces metal beneath the original surface — so the "coating thickness" you measure is not what the caliper sees. Net growth on the part is typically under 2 µm, in contrast to electroplating, which adds the full layer on top of the surface (see nickel and chrome thickness data). Process background: the black oxide process step by step.

Thickness vs other finishes on steel

Table 2 — Dimensional impact of common finishes on machined steel, per surface.
Finish	Typical layer	Approx. dimension change per surface	Threads affected?
Hot black oxide (unsealed)	0.6–0.9 µm	<2 µm	No
Nickel plating	5–25 µm	5–25 µm	Yes above ~5 µm unless masked
Decorative chrome (over nickel)	0.5–2 µm	Negligible on top of nickel	Follows nickel
Hard chrome	20–100+ µm	20–100+ µm, then ground	Usually ground after
Zinc plating (reference)	~5–15 µm	5–15 µm	Yes unless masked

What to state on the drawing

Process: "hot alkaline black oxide" — not just "black finish".
Sealant: oil, wax or lacquer, since corrosion performance lives in the seal, not the oxide.
Dimensional reference: whether critical dimensions apply before or after finishing.
Masking: any faces that must remain bare or conductive.

Honest limits: a 0.6–0.9 µm layer is below the reliable resolution of most coating-thickness gauges, so black oxide thickness is controlled by process parameters and coupon testing rather than measured part-by-part — anyone promising a per-part thickness certificate on black oxide should be questioned. And the thin layer offers little abrasion or heavy-corrosion protection: for outdoor or marine exposure, plating or a painted system is the honest recommendation.

FAQs

How thick is black oxide coating on steel?

Roughly 0.6–0.9 µm for hot alkaline black oxide on carbon and alloy steel, with net part dimension change typically under 2 µm.

Is black oxide thickness the same on stainless steel and cast iron?

No — stainless builds a thinner sub-micron grey-black layer via different chemistry, and cast iron colors non-uniformly because of graphite inclusions.

How should black oxide thickness be specified on a drawing?

State the process, substrate, sealant and dimensional reference; the layer itself is normally not toleranced at this scale.