Skip to content 
What Is Black Oxide Coating?
Black oxide coating, also known as blackening or blackening oxide, is a conversion coating for ferrous materials such as steel, copper, and brass. The process involves converting the surface layer of the material into a thin layer of black iron oxide, which provides corrosion resistance and an attractive black finish.
What is the Black Oxide Coating process?
the black oxide coating process requires specialized equipment and expertise to achieve consistent and high-quality results.
Cleaning:
The metal parts to be coated are first cleaned thoroughly to remove any dirt, grease, or other contaminants that may interfere with the coating process. The parts may be cleaned using various methods, such as degreasing, sandblasting, or chemical cleaning.
Pre-treatment:
After cleaning, the parts are usually treated with a pre-treatment solution that helps to prepare the surface for the black oxide coating. This may involve dipping the parts in an acid bath or applying a phosphate coating to the surface.
Black oxide coating:
The parts are then immersed in a hot black oxide solution, which typically consists of a mixture of sodium hydroxide, sodium nitrate, and water. The duration of immersion in the solution varies based on the type of metal and the desired finish. During this time, the black oxide coating forms on the surface of the metal parts through a chemical reaction.
Rinsing and sealing:
After the parts have been coated with black oxide, they are removed from the solution and rinsed thoroughly with water to remove any excess chemicals. To help protect the finish, the parts may also be treated with a sealant or oil.
Quality control:
Finally, the parts are inspected to ensure that they meet the required specifications for the finish. This may involve checking for defects, such as discoloration or uneven coating, and verifying that the parts meet any other requirements, such as hardness or corrosion resistance.
Types Of Black Oxide Finish？
Hot black oxide:
The hot black oxide process uses a solution of alkaline salts to create a black oxide layer on the surface of the Precision metal parts at high temperatures （285°C (545°F) – 315°C (600°F)）. The specific composition of the salt bath used can vary depending on the desired properties of the finished product, but some of the most common salt baths used in the hot black oxide process include:
- Sodium hydroxide (NaOH) and sodium nitrate (NaNO3) bath: This is the most commonly used salt bath for hot black oxide, as it provides good corrosion resistance and wear resistance properties. The concentration of the NaOH and NaNO3 in the solution can be adjusted to achieve the desired finish.
- Potassium hydroxide (KOH) and sodium nitrate (NaNO3) bath: This salt bath produces a black oxide layer that is more uniform and less prone to color variation than the NaOH and NaNO3 bath. This bath is also less likely to produce rust or corrosion on the metal surface.
- Lithium hydroxide (LiOH) and sodium nitrate (NaNO3) bath: This salt bath is used for parts that require a high degree of wear resistance and lubricity, such as bearings and gears. The LiOH and NaNO3 combination creates a harder and more lubricious black oxide layer than other salt baths.
Mid-temperature black oxide:
This type of black oxide finish is achieved at a lower temperature than hot black oxide and is typically used for parts that are sensitive to high heat, such as aluminum or titanium. Mid-temperature black oxide can produce a range of colors, from light gray to deep black, depending on the duration of the process.
Cold black oxide:
Cold black oxide is achieved at room temperature using a solution of copper sulfate, sulfuric acid, and water. This process produces a black finish with good corrosion resistance, but it is less durable than hot black oxide.
Applications for Precision Machined Projects Using Black Oxide Finish
Black oxide finish is a popular choice for precision machined projects that require a durable and corrosion-resistant coating that can withstand wear and tear. Here are some common applications of black oxide finish for precision machined projects:
- Aerospace components: Black oxide finish is often used for aerospace components, such as screws, bolts, and fittings, to provide corrosion resistance and to meet industry standards for surface finish and wear resistance.
- Firearms parts: Black oxide finish is commonly used for firearm parts, such as barrels, receivers, and triggers, to provide a durable and corrosion-resistant coating that can withstand the harsh environment of firearms use.
- Automotive components: Black oxide finish is often used for automotive components, such as gears, shafts, and bearings, to provide a protective coating that can withstand high temperatures, friction, and exposure to corrosive substances.
- Medical instruments: Black oxide finish is commonly used in medical instruments where the absorption of light is critical. The non-reflective surface of the black oxide finish reduces glare and eliminates reflection, which can be a significant problem in surgical procedures where light reflection can cause visual difficulties for the surgeon.
- Precision machinery parts: Black oxide finish is commonly used for precision machinery parts, such as gears, shafts, and bearings, to provide a protective coating that can withstand the harsh operating conditions of machinery use.
Considerations for Using Black Oxide Finish on Precision Machined Parts
Considerations for using black oxide finish include:
- Material compatibility: Black oxide finish is most commonly used on steel, but it can also be used on other materials such as copper, brass, and aluminum. it is important to ensure that the material is suitable for the black oxide process before proceeding.
- Use scenario of parts: The intended use of precision machined parts, such as exposure to the ocean or high temperatures, is a critical factor in determining whether to use the black oxide process
- The desired final appearance: In addition to the functional properties of a precision machined part, the final appearance is also essential, particularly when aesthetics is a critical factor. The black oxide finish offers two distinct final looks, matte or glossy, depending on the type of after-finish applied. Considering the potential limitations in the final appearance of black oxide parts is crucial.
The challenges involved in the black oxide coating process and the corresponding solutions
Black oxide coating can be susceptible to defects that can affect the quality and appearance of the finished product. Here are some common defects and solutions for black oxide coating:
- Uneven coating: Uneven coating can occur when the parts are not properly cleaned or prepared before the black oxide process. The solution is to ensure that the parts are thoroughly cleaned and prepared before the black oxide process and to monitor the coating thickness during the process.
- Discoloration: Discoloration can occur if the parts are not properly rinsed after the black oxide process or if the rinse water is contaminated. The solution is to ensure that the parts are thoroughly rinsed after the black oxide process and to use clean rinse water.
- Blotchy appearance: A blotchy appearance can occur if the parts are not properly agitated during the black oxide process. The solution is to ensure that the parts are properly agitated and submerged during the black oxide process.
- Peeling or flaking: Peeling or flaking can occur if the parts are not properly cleaned or prepared before the black oxide process, or if the coating is too thick. The solution is to ensure that the parts are thoroughly cleaned and prepared before the black oxide process and to monitor the coating thickness during the process.
Quality testing For the Black Oxide Finish
When using a black oxide finish, there are several performance tests that can be used to evaluate the quality and durability of the finish. Here are some common performance tests for black oxide finish:
- Salt spray test: The salt spray test is a common method for evaluating the corrosion resistance of black oxide finishes. The test involves exposing the coated parts to a salt spray mist for a specified period of time and then evaluating the amount of corrosion that has occurred.
- Abrasion resistance test: The abrasion resistance test is used to evaluate the durability of the black oxide finish under conditions of wear and friction. The test involves subjecting the coated parts to a specified level of abrasion and then evaluating the amount of wear that has occurred.
- Adhesion test: The adhesion test is used to evaluate the strength of the bond between the black oxide coating and the underlying metal substrate. The test involves applying a force to the coating and evaluating the amount of delamination or separation that occurs.
- Hardness test: The hardness test is used to evaluate the hardness of the black oxide finish, which can affect its resistance to wear and abrasion. The test involves measuring the hardness of the coating using a specified method.
- Coating thickness test: The coating thickness test is used to ensure that the black oxide coating has been applied to the desired thickness. The test involves measuring the thickness of the coating using a specified method.
Pros and cons of the black oxide finish
Pros:
- Corrosion resistance: Black oxide finish is a highly effective solution for preventing corrosion, making it an excellent choice for parts that will be exposed to harsh environments or corrosive substances.
- Aesthetics: Black oxide finish provides a dark, attractive appearance that is often preferred for certain applications, such as firearms or automotive components.
- Cost-effective: Black oxide finish is a relatively low-cost finishing process compared to other treatments like electroplating or anodizing.
- Wear resistance: Black oxide finish provides a hard, wear-resistant surface that can help extend the life of parts and components.
- Environmentally friendly: Black oxide finish is a non-toxic process that does not use hazardous chemicals or produce harmful byproducts.
Cons:
- Limited color options: Black oxide finish only provides a dark, black color, so it may not be suitable for applications that require a different color.
- Limited scratch resistance: While black oxide finish provides good wear resistance, it is not as scratch-resistant as some other surface treatments.
- Limited thickness control: The thickness of the black oxide finish can be difficult to control, which can affect the performance of certain parts or components.
- Limited durability: Black oxide finish may not be as durable as some other surface treatments, particularly in applications that involve high levels of wear or exposure to harsh environments.
- Limited adhesion: Black oxide finish may not adhere well to certain types of metals, particularly those that have a high level of oxidation or surface contamination.
Selecting the black oxide finish for your precision-machined parts
Surface finishes are essential for enhancing the quality of precision machined parts. Among the various options available, black oxide finish is a popular choice due to its affordability and versatility. However, achieving a successful finish requires expertise in precision machining. At AT-Machining, we have the necessary skills and experience to handle various surface finishing processes, including anodizing, plating, polishing, painting, and sanding. Additionally, we offer end-to-end precision machining services, including rapid prototyping services like CNC machining.
Contact us today to request a quote and we will respond within the next 12 business hours.
FAQ for Black Oxide Finish
Which type of black oxide coating is the best?
hot black oxide is considered the best for achieving a high-quality finish. It produces a thick, durable coating that offers excellent resistance to wear, corrosion, and other forms of damage.
Is black oxide finish durable?
Yes, a black oxide finish is known for its durability. It provides a hard, abrasion-resistant coating that can protect the underlying metal from wear, corrosion, and other forms of damage. However, the durability of the finish also depends on factors such as the quality of the initial surface preparation, the thickness of the coating, and the environmental conditions to which the finished part will be exposed. Proper maintenance and care can also help to ensure the longevity of the black oxide finish.
Which materials are commonly treated with black oxide?
The black oxide treatment can be applied to a wide range of materials, including steel, stainless steel, copper, brass, bronze, and zinc.
Can black oxide coating affect the dimensions of parts?
No, black oxide coating does not increase the dimensions of parts as it is a surface treatment and adds a very thin layer on top of the material. The increase in thickness is negligible and does not affect the dimensional accuracy of the parts.
