Black oxide, also known as blackening or gun bluing, is a chemical conversion coating produced by immersing steel parts into a concentrated alkaline salt solution heated to approximately 285°F (140°C). Unlike plating or painting, this process does not “add” a layer on top of the metal; instead, it triggers a chemical reaction that transforms the surface of the steel into magnetite (Fe₃O₄).

Best Industries:

Performance Types:

Best Materials:

Industrial black oxide coating (MIL-DTL-13924) for precision components. Our process ensures uniform coverage, oil-retention for rust prevention, and a decorative finish.

High-quality black oxide metal finishing services for steel, stainless steel, and copper. Achieve a sleek matte black finish with enhanced corrosion resistance and zero dimensional change.

Key Advantages: Dimensional Stability

The primary reason engineers specify black oxide is its zero dimensional change. Because it is a conversion coating rather than an additive one, the thickness is negligible (typically 5 to 10 millionths of an inch). This makes it ideal for:

High-precision machined components.
Internal threads and small fasteners.
Complex assemblies where tolerances are too tight for paint or powder coating.

The Role of Supplemental Coatings

In its natural state, black oxide provides very poor corrosion resistance. It is porous by nature, and without intervention, the underlying steel would begin to rust almost immediately when exposed to humidity. To solve this, the finish is treated with “after-finish” sealants:

Oil: The most common sealant; the oil is absorbed into the porous oxide layer to create a moisture barrier.
Wax: Often used when a dry-to-the-touch finish is required.
Lacquer: Used for a more permanent, glossy aesthetic.

Specifications

Specification	Comments
MIL-C-13924C	No dimensional change.
Class 1	Alkaline, oxidizing. for wrought iron, cast and malleable irons, plain carbon, and low alloy steels
Class 3	Fused salt oxidizing. (Not performed at DFW Anodize)
Class 4	Alkaline oxidizing. For 300 series Corrosion resistant steel alloys only.

Industry Applications

Black oxide is widely utilized in the aerospace and automotive industries, particularly for transmission components and hydraulic systems. In these environments, the parts are constantly bathed in lubricating oils, which naturally provides the necessary corrosion protection that the coating lacks on its own. It is the preferred choice for moving parts that require a non-reflective, anti-galling surface but cannot tolerate the thickness of more robust finishes like chrome or zinc plating.

Finish Comparison

	Black Oxide	Zinc Plating (Clear/Black)	Anodizing (Type II/III)
Base Material	Ferrous (Steel, Iron)	Ferrous (Steel, Iron)	Non-Ferrous (Aluminum)
Process Type	Chemical Conversion	Electroplating (Additive)	Electrochemical (Growth)
Dimensional Change	Negligible (< 1 µm)	Significant (5–25 µm)	Moderate (10–50 µm)
Corrosion Resistance	Poor (Relies on oil)	Excellent (Sacrificial)	Excellent (Ceramic-like)
Surface Hardness	No Change	Moderate	Very High (especially Type III)
Common Use	Gears, Bearings, Tools	Bolts, Brackets, Chassis	Aerospace frames, Cases

Black Oxide
Base Material	Ferrous (Steel, Iron)
Process Type	Chemical Conversion
Dimensional Change	Negligible (< 1 µm)
Corrosion Resistance	Poor (Relies on oil)
Surface Hardness	No Change
Common Use	Gears, Bearings, Tools

Zinc Plating (Clear/Black)
Base Material	Ferrous (Steel, Iron)
Process Type	Electroplating (Additive)
Dimensional Change	Significant (5–25 µm)
Corrosion Resistance	Excellent (Sacrificial)
Surface Hardness	Moderate
Common Use	Bolts, Brackets, Chassis

Anodizing (Type II/III)
Base Material	Non-Ferrous (Aluminum)
Process Type	Electrochemical (Growth)
Dimensional Change	Moderate (10–50 µm)
Corrosion Resistance	Excellent (Ceramic-like)
Surface Hardness	Very High (especially Type III)
Common Use	Aerospace frames, Cases

Why Choose One Over the Other?

Black Oxide: The “Precision” Choice

As you noted, this is the go-to for transmission and hydraulic systems. In these cases, the clearance between a gear and a shaft might be so tight that adding even 10 microns of zinc plating would cause the assembly to seize.

Best for: Moving parts, precision gauges, and internal engine components.

Zinc Plating: The “Outdoor” Choice

Zinc acts as a “sacrificial” barrier. If the coating is scratched, the zinc will corrode instead of the steel underneath. However, because it adds a measurable layer of metal, you often have to “over-tap” threads (make them slightly larger) to account for the thickness of the plating.

Best for: Automotive chassis parts, outdoor fasteners, and brackets.

Anodizing: The “Protection” Choice

If your part is aluminum, you would typically use anodizing rather than black oxide. Anodizing actually “grows” an oxide layer out of the base metal. It is incredibly hard and can be dyed various colors (including a deep, lustrous black).

Best for: Lightweight aerospace structures, high-wear surfaces, and decorative consumer electronics.