All You Need to Know About Black Oxide

Welcome to IDEAL's ultimate guide to Black Oxide Coating Finishing in CNC machining!
Our blog is a comprehensive resource for anyone interested in learning more about this versatile and affordable surface finish option.

What Is Black Oxide?

Generally, black oxide or black iron oxide refers to the oxide of iron known as magnetite (Fe3O4) that is formed when a ferrous metal is exposed to special oxidising chemicals. It is a type of conversion coating, thus the surface layer of a metal is formed into its oxide through conversion. The layer thickness on a finished part is between one and two micrometres.

The thin layer or coating along the part’s surface provides advantages such as greater corrosion protection, an attractive look and a smooth finish. As a result, many applications such as automotive parts use the black oxide coating process.

The blacking process works for some non-ferrous metals, such as zinc and copper, as well.

Types of Black Oxide Coating?

Black oxide processes are easily split into 3 categories. Hot, mid-temperature and cold or room temperature process. The differences between hot and medium-temp processes are basically just temperature. They both form the same blackened magnetite surface finish on ferrous parts, but in the hot black oxide process, the chemical bath is heated to a boil where the mid-temp process is kept below a boil. The reason for the difference is that the hot process is faster (can be done in less than 10 minutes), but the boiling creates undesirable fumes. The mid-temp process can take anywhere from 20 to 60 minutes, but because the chemical bath isn’t boiled, far fewer fumes are produced.

As with most things, there are trade-offs. The hot process typically produces a more durable surface than the mid-temp. There can also be slight differences in appearance between the two, but you wouldn’t be able to distinguish which process was used by just a visual inspection.

The black sheep (ridiculous pun intended) of the family is the cold process. Sometimes called cold blue, cold bluing, or cold black oxide, all the names are a bit of a misnomer. First, it isn’t technically cold, it’s room temperature. Second, it isn’t a chemical conversion that forms an iron oxide layer. It’s actually a copper selenium compound coating the surface. The “cold” process doesn’t usually produce a finish that is as durable or consistent in appearance as the higher temperatures cousins. Cold black oxide solutions are useful for touch-ups on black oxide parts.

When Is Black Oxide Coating Used?

Black oxide coating is typically used in various applications for its functional and aesthetic benefits. Here are some common uses:

Corrosion Resistance: It provides a degree of rust and corrosion protection, making it suitable for metal parts exposed to moisture.

Wear Resistance: The coating can enhance the wear resistance of metal components, extending their lifespan in demanding applications.

Aesthetic Appeal: The black finish improves the appearance of products, making them more visually appealing, which is important for consumer goods and decorative items.

Reduced Glare: The matte finish of black oxide can reduce glare, making it ideal for parts involved in sight-sensitive applications, such as optics.

Adhesion Promotion: It can improve the adhesion of paint, lubricants, and other surface treatments to the metal substrate.

Fasteners and Hardware: Commonly used on screws, bolts, and fittings for both functional and aesthetic reasons.

Is Black Oxide Coating Good? Advantage and Disadvantages？

Advantages

Cost – One of the big advantages of black oxide is its cost. Especially relative to other protective finishes. This is a case of getting what you pay for however, since black oxide doesn’t offer the same level of protection as finishes like zinc plating or powder coating.

Dimensions – Since black oxide doesn’t leave a thick film on the surface, it’s an excellent finish for tightly toleranced parts. Machine tools and fixtures are commonly black oxide finished for this reason and the fact that they are usually in controlled environments and regularly maintained.

Appearance – In many products a particular appearance is important. You can’t always paint or powder coat parts, especially when tolerances are important, so black oxide can give a dark, low-reflectivity look. Black oxide is not always uniform in appearance though, so don’t expect it to look as good as powder coat or that all black oxide finished parts will match.

Lubricity – The surface of black oxide is very porous, meaning it absorbs and holds onto the after-finish sealant (often oil). This has the benefit of providing more lubricity between moving parts than other finishes.

Hydrogen Embrittlement (or lack of) – Without getting too technical, hydrogen embrittlement is an effect that’s caused by certain chemical and electroplating processes where hydrogen molecules get absorbed into the material making it more brittle. Cracks from hydrogen embrittlement can be a serious issue for structural parts when they get certain finishes. An advantage for black oxide is that it doesn’t suffer from hydrogen embrittlement issues.

Disadvantages

Corrosion Resistance – While a black oxide finish is porous and absorbs oil well, it’s only that oil (or other sealant) that offers any protection against corrosion. As far as finishes go, black oxide is one of the worst options for corrosion resistance.

Durability – While a proper black oxide finish shouldn’t flake or rub off, it’s not as durable as a lot of other finish options.

Uses and Applications of Black Oxide

Black oxide is used in different industries due to its corrosion resistance and strength, plus its versatile finish for different materials. With its unique set of advantages and disadvantages, there are certain parts particularly suited to black oxide.

Gears – Tight tolerances, contact surfaces and the fact they are commonly bathed in oil make black oxide a common finish for gears.

Firearms – Black oxide is frequently used on firearms where precision fits between parts and high lubricity are desirable. Most firearms are regularly cleaned and oiled so a high level of corrosion protection isn’t necessary.

Fasteners – Screws, nuts, bolts are all available with a black oxide finish. Dimensional stability and lubricity work well in threaded parts. The dark appearance can help fasteners stand out less in applications where that’s a benefit. Black oxide fasteners that are used for exterior applications are likely to rust very quickly.

Tools and Fixtures – Hand tools and fixtures are used in controlled environments and regularly cleaned and oiled. Black oxide holds onto that oil and provides more protection than uncoated steel parts.

Materials Used with Black Oxide Coating

Here are some materials that are compatible with a black oxide coating:

Carbon Steel

Black oxide coating is commonly used on carbon steel parts due to its durability and corrosion resistance. It is commonly applied to machine parts, tools, and fasteners.

Stainless Steel

Stainless steel is a common material that is often used in harsh environments due to its corrosion resistance. Black oxide coating can provide an additional layer of protection to stainless steel parts.

Copper

Black oxide coating is often used on copper parts to create an ebonol c finish. This finish is commonly used on plumbing fixtures, electrical components, and decorative items.

Zinc

Black oxide coating can also be used on zinc parts to create an ebonol z finish. This finish is commonly used on hardware, automotive parts, and decorative items.

Brass

Brass is a material that is often used for decorative purposes. Black oxide coating can be used on brass parts to create a unique and durable finish.

Aluminum

Black oxide coating can be applied to aluminum parts to create a black finish. This finish is often used for decorative purposes and can provide an additional layer of protection against corrosion.

The Black Oxide Process

The black oxide coating process, also known as black oxide finishing or black oxide treatment, involves several steps to create a protective and aesthetically appealing layer on metal surfaces. Here’s an overview of the process:

Preparation

The metal surface must be cleaned to remove any dirt, grease, oxidation, or previous coatings. This is typically done using solvents, alkaline solutions, or abrasive methods like blasting.

Rinsing

After cleaning, the parts are rinsed with water to remove any residues from the cleaning agents.

Oxidation

The cleaned metal is immersed in a hot alkaline solution containing oxidizing agents. Common oxidizers include sodium hydroxide and sodium nitrite.

During this step, the surface of the metal reacts with the solution to form magnetite (Fe3O4), a black iron oxide.

Sealing

After the oxidation process, parts are typically sealed in a light oil or water-based polymer to enhance corrosion resistance and improve the appearance.

This can also help to further protect the black oxide layer and provide lubrication.

Drying

The components are finally dried to remove any moisture, ensuring the protective layer remains intact and adheres properly.

The end result is a durable black oxide finish that not only enhances visual appeal but also provides some level of corrosion and wear resistance. The process can be adapted for different metals, primarily ferrous metals like steel and iron, but also for some non-ferrous metals.

FAQs around Black Oxide Coating

Is black oxide coating expensive?

The cost of black oxide coating can vary depending on the size, complexity, and quantity of the parts being coated. Generally, the cost of black oxide coating is relatively low compared to other types of coatings, making it a cost-effective option for many applications. However, the actual cost will depend on a variety of factors, so it’s best to consult with a professional coating service provider to get an accurate quote for your specific needs.

Is black oxide coating environmentally friendly?

Black oxide coating is generally considered to be a more environmentally friendly option than some other types of coatings, such as electroplating or painting because it is a chemical conversion process that does not involve the use of heavy metals or other harmful materials. The black oxide coating process uses a solution of sodium hydroxide, nitrites, and other chemicals to convert the surface of the metal into a black oxide finish. While these chemicals can be hazardous if not handled properly, they are generally less harmful than some of the chemicals used in other types of coatings.

What is the alternative to black oxide coating?

While there are plenty of applications where black oxide can be an appropriate finish, it’s not right for everything. If you want a black finish but need better protection, consider black zinc plating or powder coating. If you have aluminum parts, anodizing is an excellent alternative and is available in black.

Conclusion

At IDEAL, we excel in providing top-quality custom machining services tailored to meet your project needs. Our advanced technology and skilled team ensure your specifications are met with exceptional accuracy and efficiency.

Contact IDEAL today to see how we can assist with your next project!