The Process of Black Oxide Coating for CNC Steel

In the competitive world of precision manufacturing, the final finish of a CNCmachined part is as critical as its dimensional accuracy. For steel components, black oxide coating stands out as a highly soughtafter surface treatment, offering a perfect blend of aesthetics, functionality, and costeffectiveness. As a onestop solution for CNC machining parts, we leverage this process to deliver superior products that meet the rigorous demands of global industries.


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The black oxide process, also known as blackening, is a conversion coating. This means it transforms the surface of the steel into a protective layer, rather than just applying a coating on top. The process is meticulous and involves several key stages:

1. Alkaline Cleaning and Degreasing: The CNCmachined steel parts are first thoroughly cleaned to remove all contaminants, such as cutting oils, coolants, and shop dirt. Any residue will prevent the uniform formation of the oxide layer.

2. Rinsing: The components are rinsed in clean water to wash away any remaining cleaning chemicals, ensuring a pristine surface for the subsequent chemical reaction.

3. Pickling: The parts are immersed in an acid bath, typically hydrochloric or sulfuric acid. This step removes any light surface rust, scale, or inorganic impurities, further activating the steel surface.

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4. Final Rinsing: Another thorough rinse is critical to neutralize and remove all acidic residues, which could otherwise contaminate the black oxide bath.

5. Black Oxide Application (The Key Step): The cleaned and activated parts are immersed in a hot, alkaline aqueous solution containing oxidizing salts. The temperature is carefully controlled, usually between 275°F and 295°F (135°C 146°C). A chemical reaction occurs where the iron on the surface of the steel converts to magnetite (Fe₃O₄), which is the black oxide layer. This layer is integral to the metal, not a deposited coating, so it will not chip, peel, or flake.



6. PostTreatment Sealing: After blackening, the parts are sealed to maximize corrosion resistance and enhance the appearance. The most common sealant is oil or wax, which is absorbed into the slightly porous oxide layer, creating a robust barrier against moisture.

Why Choose Black Oxide for Your CNC Steel Parts?

Corrosion Resistance: The sealed black oxide finish provides excellent protection against rust in various environments.
Minimal Dimensional Change: The coating is incredibly thin, typically only 0.00002 to 0.00004 inches (0.5 1 micron) thick. This is a crucial advantage for precision CNC parts with tight tolerances, as it does not alter the part's critical dimensions.
Aesthetic Appeal: It delivers a uniform, deep matte black finish that is visually appealing and reduces light glare, ideal for consumer products, optical instruments, and military hardware.
Reduced Galling and Friction: The finish provides a mild lubricity, which is beneficial for moving components like gears and fasteners.
CostEffective: Compared to other finishes like plating or powder coating, black oxide is a relatively lowcost process for the value it provides.

As your trusted onestop manufacturing partner, we integrate this specialized finishing process seamlessly into our production flow. From initial CNC machining to the final black oxide coating, we ensure every component meets the highest standards of quality and performance. Partner with us to enhance the durability, functionality, and marketability of your steel parts.