Black Oxide Service
Resolute Precision Manufacturing coordinates black oxide coating for steel and stainless-steel precision machined components. Black oxide provides a uniform matte black appearance, mild corrosion resistance with oil supplementation, and essentially zero-dimensional change making it ideal for precision parts where coating thickness would affect fit. We deliver black oxide coated parts with full certification from a single purchase order. Request a QuoteWhat Black Oxide Is and What It Does
Black oxide is a conversion coating a chemical reaction that converts the iron on the surface of steel into magnetite (Fe3O4), a black iron oxide compound. Unlike plating or paint, black oxide does not deposit a layer on the surface; it converts the existing surface material. The result is a matte black coating that is chemically bonded to the steel and adds essentially no measurable dimension to the part.
Black oxide provides three primary benefits: appearance (uniform matte black finish that reduces light reflection and is aesthetically preferred in many applications), mild corrosion resistance (substantially improved when supplemented with oil, wax, or VCI film), and reduced galling on precision sliding surfaces by providing a thin lubricating reservoir in the porous oxide layer.
Hot Black Oxide vs. Cold Black Oxide
Two black oxide processes are used depending on specification and quality requirement:
- Hot black oxide — parts are immersed in caustic soda solution at 285°F to 295°F; produces a true conversion coating; preferred for corrosion resistance and specification compliance; required by MIL-DTL-13924 Class 1
- Cold black oxide — room-temperature chemical conversion; faster and lower cost; produces thinner coating with lower corrosion resistance; suitable for appearance applications where corrosion performance is secondary
Specifications
| Standard | MIL-DTL-13924 Class 1 (hot process) and Class 3 (cold process) |
| Process | Hot black oxide applied at 285°F–295°F; cold black oxide process performed at room temperature |
| Dimensional Change | Essentially zero (0.0001″ or less); no significant impact on precision fits |
| Corrosion Resistance | Minimal without supplemental oil; 200+ hours salt spray protection with oil or VCI film |
| Materials | Carbon steel (1018, 4140, 4340), alloy steel, and stainless steel (300 series) |
| Surface Appearance | Uniform matte black, non-reflective finish |
| Post-Treatment | Oil or wax treatment recommended to improve corrosion resistance in service |
| Temperature Sensitivity | Not suitable for aluminum, zinc, or copper alloys; use an appropriate coating process for those materials |
| ISO | ISO 9001:2015; outside suppliers qualified under RPM’s approved vendor procedures |
Common Black Oxide Applications
Black oxide is the standard surface treatment for a broad range of steel machined components:
- Tooling and fixtures — non-reflective finish on production tooling; minimal dimensional change preserves fit
- Fasteners and hardware — standard black oxide finish on screws, bolts, and socket hardware
- Hydraulic and pneumatic valve components — matte finish combined with oil reservoir in oxide layer
- Gears, cams, and precision mechanisms — anti-galling surface treatment with near-zero dimensional impact
- Medical and dental instruments — matte black finish on reusable steel surgical instruments
- Defense and firearm components — non-reflective black finish to specification
Frequently Asked Questions
Does black oxide provide adequate corrosion protection on its own?
Black oxide without supplemental treatment provides only minimal corrosion protection typically 2 to 5 hours in salt spray testing. When supplemented with oil, wax, or VCI film, corrosion protection improves to 200 hours or better. For outdoor or wet environment applications, zinc plating or electroless nickel provides substantially better corrosion protection.
Can black oxide be applied to stainless steel?
Yes. Stainless steel can be black oxide treated in a specialized hot process. The result is a black finish on stainless with the same near-zero dimensional change. This is common for stainless surgical instruments and food-contact hardware where a non-reflective black finish is required without galvanic corrosion concerns.
Will black oxide affect the hardness of my heat-treated parts?
No. Hot black oxide processing at 285°F–295°F is well below any tempering temperature for common tool steels, alloy steels, and stainless steels. The mechanical properties established by heat treatment are not affected by black oxide processing.
What is the dimensional impact of black oxide on precision threads?
Black oxide produces essentially zero dimensional change less than 0.0001″ which has no measurable impact on thread fit. Precision threaded components, gage surfaces, and close-tolerance fits are routinely black oxide processed without dimensional concerns.