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What is Black Oxide Coating? All You Need to Know

Black oxide coating is a treatment method used to make the surface of components made of steel appear black by forming a black film on the surface through a chemical reaction. Black oxide coating offers various advantages, including providing corrosion resistance, being cost-effective, maintaining dimensions, and having strong adhesion properties. In this article, we will understand what is black oxide coating and provide a detailed explanation of the principles, and main features.

What is Black Oxide Coating?

Black oxide treatment is a process that forms a black Fe3O4 oxide film (commonly known as “black rust”) on the surface of iron and steel.

In the black oxide process, steel components are immersed in a solution containing approximately 35-45% sodium hydroxide, along with an oxidizing agent and a reaction accelerator. The temperature is then set to about 130-150°C. This leads to the oxidation of the surface and produces a substance known as sodium ferrate. Subsequently, this sodium ferrate is reduced, leading to the formation of the Fe3O4 oxide film on the surface of the components. Typically, the parts are further treated with rust-preventing oil (by immersion or application). The final result is an appearance that resembles something as if it were dyed black.

Advantages of Black Oxide Treatment

Black oxide treatment has the following features and finds applications in various fields:

Improved Rust Resistance

The film on the surface of components is robust and stable and suppresses the formation of red rust (corrosion that significantly damages iron surfaces) to some extent. While it may not provide the same level of rust resistance as zinc or nickel plating, black oxide treatment is recommended when you want to impart a certain level of rust resistance while maintaining an aesthetically pleasing appearance. Additionally, post-processing with rust-preventing oil retains oil on the film’s surface, improving the lubricity of tools and mechanical components.

Reduced Susceptibility to Peeling

Black oxide treatment is a process that chemically transforms only the surface of components into another substance. The new surface, then, does not peel off easily. It can be considered a relatively long-lasting surface treatment.

Minimal Impact on Dimensional Accuracy

In black oxide treatment, the formation rate of the film is not very rapid, resulting in a film thickness of approximately 1-2μm. Furthermore, the film forms from the surface of the component towards the interior. The treatment temperature is relatively low, not exceeding 150°C, eliminating concerns about part deformation due to heat. Therefore, the dimensions of the components remain largely unchanged before and after black oxide treatment. This advantage makes black oxide treatment commonly employed for components that require high precision.

Cost-effective

Black oxide treatment boasts cost-effectiveness due to the low raw material cost of the treatment solution and the absence of electrodes or fixtures. Processing numerous components at once in a basket further reduces the processing cost per unit. Regardless of variations in size and shape, processing different components at once while keeping costs down makes black oxide treatment a preferred choice when handling various components economically.

Considerations When Applying Black Oxide Treatment

Black oxide treatment offers numerous advantages, but there are some considerations to keep in mind:

Not all materials are suitable for black oxide treatment

Black oxide treatment is most suitable for iron-based materials. However, castings, quenched parts, wire-cut components, and similar materials may result in a black color with a reddish tint rather than a complete black. Additionally, alloy steels that contain high levels of elements like chromium or nickel may achieve a gray appearance rather than black.

Rust progresses if the oil film is compromised

The Fe3O4 black oxide film itself has limited rust resistance. As mentioned earlier, the typical practice is to treat the film’s surface with rust-preventive oil. However, if this oil film is compromised, external moisture and air can penetrate the fine pores within the Fe3O4 black oxide film, leading to the advancement of red rust. It’s important to recognize that black oxide treatment has its limits when it comes to rust prevention.

Conclusion

Black oxide treatment is a surface treatment method that forms a ferric oxide (Fe3O4) film on the surface of iron and steel components. It provides a beautiful appearance, resembling that of something “dyed black,” while preserving the dimensional accuracy of the parts. It offers advantages such as cost-effectiveness compared to other treatment methods and the ability to provide a certain level of rust resistance.

Black oxide treatment is ideal for applications where maintaining dimensional accuracy, achieving an aesthetically pleasing black appearance, and desiring a certain level of rust resistance are paramount. Specifically, it is employed in various applications, including steel products like screws and bolts, a wide range of tools, precision machinery components, molds, automotive parts, interior items, and art pieces. However, it’s worth noting that while it can provide rust resistance, it may not endure outdoor use in extremely humid environments or near the sea. Hence, it may not be suitable for components used in such conditions.