Black Oxide 101: What You Need To Know

Black oxide, also known as blackening, is a type of chemical-conversion coating applied to ferrous metals and is valued for its corrosion resistance and aesthetic appeal. In addition to these characteristics, black oxide coatings also impart better abrasion resistance, increased hardness, and thermal stability to parts, all in a more cost-effective manner compared to other coating techniques such as electroplating or painting.

This article will discuss black oxide coating, its importance, purpose, applications, and advantages.

What Is Black Oxide?

Black oxide is a coating applied to ferrous materials like steel or iron to improve the substrate’s corrosion and wear resistance. Additionally, the coating adds an aesthetically appealing matte black finish to parts. Black oxide coatings are a type of chemical-conversion coating and are created through a chemical reaction between iron atoms on the substrate surface and oxidizing salts in a chemical bath. While the actual inventor of black oxide coatings is unknown, the coatings have been applied by metalworks and blacksmiths for centuries.

What Is the Importance of Black Oxide?

The significance of black oxide lies in its ability to improve the corrosion resistance of metal surfaces while providing a sleek appearance. Beyond aesthetics, the coating also offers enhanced durability and wear resistance. These characteristics make black oxide coatings valued in various industrial applications.

What Is the Purpose of Black Oxide? 

The purpose of black oxide is multifaceted and encompasses both functional and aesthetic benefits. Not only does black oxide provide added corrosion resistance to ferrous metals, but it also improves their abrasion resistance and increases their hardness. Additionally, black oxide provides a visually appealing matte black, decorative finish that further adds to their value in different applications.

How Is Black Oxide Different From Other Metal Coatings?

Black oxide coating is distinguished from other metal coatings in its application to parts. Unlike painting, powder coating, or galvanizing — which are processes that blanket the substrate with a protective layer that is not chemically bonded to it — black oxide coatings are created through a chemical reaction. Black oxide does not form a distinct layer on the surface; instead, it becomes an integral part of the metal, offering a more robust and uniform finish.

What Are the Principles of Black Oxide?

The principle of black oxide involves a chemical-conversion process applied to ferrous metals, such as steel and iron, to create a thin, protective layer of black iron oxide on the surface. 

How Does Black Oxide Work? 

Black oxide works through a chemical-conversion process applied to the ferrous metal to create a thin, protective layer of black iron oxide on the metal surface. After thoroughly cleaning and preparing the metal’s surface, the metal is immersed in a blackening solution containing oxidizing salts. The metal chemically reacts with the salts to create the black iron oxide layer. The resulting black oxide coating provides corrosion and wear resistance and a matte black finish.

What Are the Physical Properties of Black Oxide Coatings?

The physical properties of black oxide coatings are listed below:

1. Increased Hardness: Black oxide coatings enhance the hardness of the underlying metal. This increased hardness is valued in applications in which resistance to abrasion and wear is crucial, providing a durable surface that withstands mechanical stress.
2. Aesthetic Appeal: Beyond their practical physical enhancements, black oxide coatings also add to the aesthetic appeal of components in the form of a matte black finish.

What Are the Thermal Properties of Black Oxide Coatings?

The thermal properties of black oxide coatings are listed below:

1. Thermal Stability: Black oxide coatings exhibit favorable thermal stability which allows them to withstand elevated temperatures without compromising their integrity. This property is essential in applications in which the coated metal may be exposed to heat, such as in engine components or high-temperature industrial environments.
2. Heat Dissipation: Black oxide coatings are effective at dissipating heat. This is particularly useful for components in which managing heat is crucial for optimal performance, such as in electronic devices or machinery.

Is Black Oxide Magnetic? 

No, black oxide itself is not magnetic and will not affect the magnetic properties of metals. Magnetic metals coated with black oxide will retain their magnetic properties while similarly coated non-magnetic metals will remain non-magnetic.

Is Black Oxide Rusty?

No, black oxide itself will not rust. If black oxide is applied to a ferrous material, like a steel alloy, it provides additional protection against corrosion and the formation of rust (iron oxide). Corrosion protection is a common application of black oxide coatings.

Where Is Black Oxide Used?

Black oxide is used in various industries and applications due to its unique combination of corrosion resistance, improved hardness, and attractive appearance. Some notable areas in which black oxide finds application include: aerospace, automotive, electronics, and firearms.

In What Industries Is Black Oxide Most Commonly Used? 

Numerous industries use black oxide coatings for a variety of applications. The different industries in which black oxide is most commonly used are listed below:

1. Automotive: Black oxide is used for fasteners, brackets, wheel components, brake components, trim pieces, and more.
2. Aerospace: Black oxide is used for fasteners and brackets, landing gear components, avionic components, trim pieces, and more.
3. Electronics: Connectors, contacts, housings, enclosures, and circuit board components can be coated with black oxide.
4. Tooling: Hand tools, measurement gauges, jigs, dies, fixtures, tool holders, and more can be coated with black oxide.
5. Fasteners: Screws, bolts, nuts, washers, and threaded rods are often coated with black oxide in industrial environments.
6. Industrial Equipment: Machine components, fasteners, rollers, gears, shafts, and more are often black oxide coated.

How Is Black Oxide Used in the Automotive Industry? 

Black oxide is often used in the automotive industry to coat parts like: engine components (camshafts, cylinder heads), brake components (rotors, calipers), interior and exterior trim pieces, and wheel components (lug nuts, valve stems). Not only do black oxide coatings add corrosion resistance to these components, they also add aesthetic appeal.

How Is Black Oxide Applied in the Field of Electronics?

Black oxide is used in the electronics industry to coat parts like: connectors and contacts, switches, housings, fasteners (screws and brackets), and heat sinks. Black oxide coatings add corrosion resistance and an aesthetic appeal to these parts.

What Are the Different Types of Black Oxide? 

There are two main types of black oxide: hot black oxide and cold black oxide. The two types are described in more detail below:

1. Hot Black Oxide: Hot black oxide coating is the most commonly used type and involves immersing parts in a boiling bath of alkaline salts, nitrites, and nitrates. The bath’s temperature range is typically 275–295 ºF depending on the exact salts used. Applying a black oxide coating above this temperature range will result in a red or rusty appearance while coatings below this temperature range will result in a longer oxidizing time.
2. Cold Black Oxide: Compared to hot black oxide, cold black oxide coatings occur at room temperature. However, unlike hot black oxide which relies on salts and the oxidation of iron on the material’s surface, the cold black oxide process immerses parts in a bath containing phosphoric acid and copper-based and selenium-based solutions. Despite being easier to impose cold black oxide coating onto parts compared to hot black oxide, cold black oxide coatings do not have the same level of quality.

What Are the Materials Used With Black Oxide?

Compared to other types of metal coatings, black oxide is low-tech. To apply black oxide coatings, oxidizing salts like sodium hydroxide and alkaline cleaners are required. Additionally, the two main components manufacturers should have are a salt bath to immerse parts and a temperature control system to regulate the temperature of the bath.

Is Black Oxide Suitable for Tool Steel? 

Yes, black oxide is suitable for tool steel and offers enhanced corrosion resistance and wear resistance to parts. The coating prolongs the life of tool steel, making it a valuable choice in the manufacturing and machining industries. Hand tools, machine dies, gears, and other components fabricated from tool steel benefit from black oxide coatings.

What Is the Process of Black Oxide?

The steps for the process of applying black oxide coatings are listed below:

1. Remove contaminants like: dirt, grease, rust, dust, and oils through a thorough cleaning of the material. An alkaline detergent or acid cleaning may be used depending on the material and its condition.
2. Once cleaned, immerse the part in a blackening solution containing specific oxidizing salts. The solution composition may include: sodium hydroxide, nitrates, nitrites, and other chemicals. The part is held at a controlled temperature during the immersion process, typically ranging from 285–305 °F, depending on the specific blackening solution used.
3. The chemicals in the solution react with the iron on the surface of the part and create magnetite (Fe3O4). The degree of blackening depends on the time the part is immersed in the solution. Typically, the part is immersed anywhere between 5–40 minutes.
4. Once the allotted immersion time has passed, remove the part from the salts and thoroughly rinse with water.
5. Apply a sealing compound or oil to black-oxide-coated metal. This step is crucial for enhancing the coating's corrosion resistance and durability.
6. Allow the sealed black-oxide-coated metal to dry completely. This ensures the sealing agent adheres effectively and contributes to the overall stability of the coating.
7. Once dried, a visual inspection to ensure the uniformity and quality of the black oxide coating is completed. Additionally, tests like the smut test or relative humidity test, are conducted to validate the coating’s adhesion and achieve the desired outcomes in corrosion resistance.

Is Black Oxide Coating Eco-Friendly? 

No. While black oxide coatings are generally the more eco-friendly option compared to other coating types like painting or electroplating, the black oxide coating process utilizes some chemicals that are harmful to the environment. Chemicals like: nitrites, sodium hydroxide, and others are hazardous but are generally less harmful than chemicals used in other coatings.

Is Black Oxide Suitable for All CNC Machining Materials? 

No. While black oxide is suitable for ferrous metals, it is not suitable for non-ferrous CNC machining materials, like aluminum or titanium. It is crucial to consider factors such as material compatibility when designing parts for a specific application.

Is Black Oxide Suitable for All Types of Metals Used in 3D Printing?

No. Black oxide is suitable for ferrous metals used in 3D printing (as a surface finish) but not non-ferrous metals. Steel, stainless steel, and steel alloys can be coated with black oxide but non-ferrous 3D printing metals like aluminum or titanium cannot be blackened.

What Are the Advantages of Black Oxide? 

Black oxide coatings have many advantages for different industries. The advantages of black oxide coatings are listed below:

1. Significantly enhance the corrosion resistance of ferrous metals, protecting them from rust and other forms of corrosion.
2. Provides an attractive, uniform, matte black finish which makes it popular for decorative applications in various industries.
3. Contribute to improved hardness and wear resistance of the metal, which, in turn, helps to extend its lifespan in challenging environments.
4. Are not thick, which makes them an ideal finish for parts with high tolerances.
5. Compared to other coating methods like electroplating or powder coating, black oxide is often a more cost-effective solution for enhancing the performance and appeal of metal components.

What Are the Disadvantages of Black Oxide?

Despite their many advantages, black oxide coatings also have some drawbacks. The disadvantages of black oxide coatings are listed below:

1. Is specifically designed for ferrous metals and may not be as effective or suitable for non-ferrous materials.
2. Applying black oxide coatings may be labor intensive due to the need to clean parts before the coating process and seal parts after. The sealing process is necessary to enhance corrosion resistance but adds a step to the overall coating process.
3. Compared to other finish options, black oxide coatings are not as durable and have a shorter service life.

What Are the Common Misconceptions of Black Oxide?

Some common misconceptions about black oxide coatings are described below:

1. Some may believe that black oxide coatings do not provide adequate corrosion resistance. However, when black oxide is properly prepared and sealed, it forms an effective protective layer against corrosion.
2. There is a perception that black oxide coatings have weak adhesion and are prone to chipping or peeling. However, when applied following proper surface preparation and process parameters, black oxide forms an integral layer with the metal which ensures strong adhesion. Adhesion issues are often related to insufficient cleaning or incorrect application procedures.

Is It Pricey To Get Black Oxide Coating?

The cost of black oxide coating varies depending on factors such as: the size and complexity of the components, the type of black oxide coating applied, and the quantity of items being coated. 

Summary

This article presented black oxide, explained it, and discussed its importance and various properties. To learn more about black oxide, contact a Xometry representative.

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