// FINISHING — BLACK OXIDE
Black Oxide Coating
Chemical blackening for mild and stainless steel — a matte-black surface conversion that adds essentially no dimension, with light corrosion resistance and a clean look.
Black oxide is a chemical conversion process that reacts with iron in the steel surface to produce magnetite (Fe₃O₄), a black compound integrated into the metal rather than deposited on top of it. The result is a uniform matte-black appearance with minimal thickness — typically less than 0.0001" per side — making it the correct choice when you need a low-reflectivity or aesthetic surface treatment without any dimensional impact on machined features, threads, or close-tolerance interfaces.
What black oxide does and does not do
Black oxide provides light corrosion resistance in dry or mildly humid indoor environments. It is not a stand-alone treatment for outdoor or wet exposure — parts intended for those environments are typically oiled or waxed after blackening to improve moisture resistance. What it does exceptionally well is produce a consistent, attractive matte-black surface at low cost with no measurable effect on part geometry.
- Matte-black finish — decorative and low-reflectivity
- No significant dimensional buildup — safe on threads and close-tolerance features
- Mild corrosion resistance, enhanced with post-treatment oil or wax
- No hydrogen embrittlement risk (unlike some electroplating processes)
- Fast turnaround relative to electroplated finishes
Ferrous materials
Black oxide is a process for ferrous metals — it relies on the iron content in the steel to form the magnetite layer. It is applied to mild steel and stainless steel. Low-carbon and medium-carbon steels blacken readily. Stainless steel uses a different chemistry (hot alkaline process) but produces the same matte-black result.
Part size capability
Our finishing partner NorTex handles parts up to 24" × 20" for black oxide. Larger parts should be discussed at the quoting stage.
| Process | Hot alkaline black oxide (chemical conversion) |
|---|---|
| Appearance | Matte black, uniform |
| Dimensional buildup | Less than 0.0001" per side (negligible) |
| Max part size | 24" × 20" |
| Substrates | Mild steel (low/medium carbon) · Stainless steel |
| Corrosion resistance | Light (indoor) — enhanced with post-treatment oil or wax |
Black oxide is commonly combined with our CNC milling and CNC turning work for tooling, jigs, fixtures, and mechanical components where a clean black appearance matters. For steel parts needing more robust corrosion protection, see zinc plating. For stainless steel parts, passivation addresses free-iron contamination rather than appearance.
// FAQ
Frequently asked questions
No. Black oxide adds less than 0.0001" per side — effectively zero from a tolerance standpoint. It is safe for threads, close-tolerance bores, and precision mating surfaces without any pre-machining allowance.
Black oxide alone provides only light corrosion resistance and is not rated for outdoor or wet exposure. For most applications, a light coat of oil or wax is applied after blackening to seal the surface and provide meaningful indoor protection. For outdoor or aggressive environments, zinc plating or other coatings are more appropriate.
Yes. Stainless steel blackens using a hot alkaline process specifically suited to its chromium-oxide surface. The result is visually similar to mild steel black oxide — a uniform matte-black finish.
Black oxide adds essentially zero thickness, which matters for threaded parts, precision features, and close-tolerance assemblies. It also provides a more durable, integral surface than paint in many wear situations, and does not chip or peel. Powder coat is better for impact and outdoor corrosion resistance; black oxide is better when dimensions must be maintained exactly.
Send us the hard one.
Upload your drawing or STEP file and we'll come back with pricing and lead time — from a single high-mix part to full production runs, held to exacting tolerances.