Reference

Black Oxide Spec

How black oxide works on steel, the hot, mid-temp, and cold processes, why it adds almost no thickness, and how to call it out per MIL-DTL-13924 or AMS 2485.

Black oxide is a conversion coating, not a plated-on layer. On steel it converts the surface into black magnetite (Fe3O4), so it adds essentially no dimensional thickness, which makes it a good black finish for tight-tolerance parts like fasteners, gears, and tooling. The trade-off is modest corrosion resistance: the finish itself is thin and porous, so protection depends heavily on the supplemental oil or wax seal applied afterward. The main processes are hot, mid-temperature, and cold, governed by MIL-DTL-13924 and AMS 2485.

Black oxide processes compared

All three convert the surface to a black oxide, but at different bath temperatures and with different durability. Hot black oxide gives the truest black and best adhesion; cold black oxide is a room-temperature deposit and is more of a cosmetic blackening.

Process	Typical bath temp	Notes
Hot	~285 F (~141 C)	True magnetite conversion; deepest black, best adhesion and durability. Covered by MIL-DTL-13924 and AMS 2485
Mid-temperature	~200-250 F (~95-120 C)	Lower fume and energy than hot; deposited black oxide with good appearance; results vary by process
Cold	~room temperature	Room-temp blackening (often selenium/copper based); fast and low-cost but thinner, less durable, more cosmetic

Temperatures are typical and vary by proprietary bath. Confirm the process and specification a shop runs for your parts.

Thickness, tolerance, and corrosion

Because it grows from the base metal rather than plating over it, black oxide is prized where plating buildup would be a problem.

Property	Typical	Why it matters
Added thickness	Sub-micron (~5-10 millionths in / ~0.1-0.25 um)	Effectively no dimensional change, so it fits tight-tolerance and threaded parts
Corrosion resistance	Modest; depends on the seal	The oxide alone offers little protection; the post-dip oil or wax does most of the work
Supplemental seal	Oil, or wax / dry-to-touch topcoat	Displaces water and fills porosity; choose based on handling, appearance, and lubricity needs
Appearance	Uniform black, low reflectivity	Reduces glare and improves cosmetics without masking dimensions

Corrosion life is highly dependent on the sealant and service environment. Salt-spray hours are not guaranteed by the black oxide alone.

How to call it out on a drawing

Name the standard and, where it applies, the class and the sealant. Example for steel: "Black oxide per MIL-DTL-13924, oil sealed." For hot black oxide you may cite "Black oxide per AMS 2485." Note that black oxide is for ferrous metals; stainless can be blackened but uses different chemistry, and aluminum uses a black anodize or black chromate instead. If parts are high-strength steel, specify any required bake for hydrogen-embrittlement relief.

Values reflect MIL-DTL-13924, AMS 2485, and common practice. Bath temperatures, added thickness, and corrosion performance vary by process and sealant; confirm what a shop can hold on your steel and parts before release.

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Frequently Asked Questions

How much thickness does black oxide add?

Almost none. Black oxide is a conversion coating that grows from the steel surface, typically on the order of 5 to 10 millionths of an inch (roughly 0.1-0.25 um). That sub-micron buildup is why it is used on threads, gears, and tight-tolerance parts where plating thickness would cause fit problems.

Does black oxide prevent rust?

Only modestly, and only when sealed. The black magnetite layer itself is thin and porous, so most of the corrosion resistance comes from the supplemental oil or wax applied after blackening. Unsealed, or in a wet or salty environment, black oxide offers limited protection.

What is the difference between hot and cold black oxide?

Hot black oxide runs in an alkaline bath around 285 F and forms true magnetite, giving the deepest black and best adhesion and durability; it is covered by MIL-DTL-13924 and AMS 2485. Cold black oxide is applied near room temperature and is a thinner, more cosmetic blackening that is faster and cheaper but less durable.

What specifications govern black oxide?

MIL-DTL-13924 covers black oxide coatings on ferrous metals (carbon, low-alloy, and corrosion-resistant steels) and defines classes by process. AMS 2485 covers hot black oxide coating. Call out the standard, and add the sealant and any embrittlement-relief bake on your drawing.

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