Black oxide coating is a surface treatment that converts the surface of a metal part to a black oxide film. Also commonly referred to as blacking or bluing, it is famously the surface finish of choice in the manufacturing of firearm components.
Black oxide coating is a traditional technique that renders the surface of metal parts an attractive and well appreciated deep blue or near black. The coating method is most closely associated with firearms, which raises awareness of it and deepens association with precision, toughness and durability.
The process is used across most ferrous metals and can be used to great cosmetic effect on stainless steel and Aluminum, which it leaves matte surfaced and close to black in most cases.
This is a very common finish on scientific instruments and military equipment. In this context, the finish has become associated with high-tech and reliability, adding a perception of value to products.
Variability in film thickness is somewhat a feature of the process, and this adds a color intensity variation to the film which is widely perceived as low-tech, old-school and appealing, rather than a quality deficit.
What is Black Oxide?
The black oxide coating process requires immersion of metal parts in an alkaline solution, typically containing sodium hydroxide, nitrate, and nitrites.
The solution reacts with the metal surface to produce a black oxide layer, which provides good corrosion resistance, some wear/abrasion resistance and a consumer appreciated black or blue finish.
Black oxide coating is advantageous, compared with other coating methods such as painting or electroplating, as it barely alters the dimensions of parts, retaining manufacturing precision.
Black oxide finish can be used on a wide range of metals, including steel/iron, brass, copper and aluminum.
Black oxide coating benefits from the fact that it is a simple and cost-effective process. Thickness and uniformity of the black oxide layer can be affected by factors such as the concentration of the solution, the temperature and duration of the immersion, and the surface preparation of the metal part.
It is therefore important to control these variables closely, to achieve consistent results.
What is the Process of Applying Black Oxide Finish?
The black oxide coating process consists of these steps:
- Cleaning. Remove any oil, grease, dirt, or other contaminants with an alkaline cleaning solution or by using a degreasing agent.
- Rinse. Use deionized water to wash away the cleaning agent and detached residues.
- Activation. Immersion in dilute sulfuric acid, to remove any remaining surface oxides and prepares the metal surface for the coating.
- Blacking. Immersion in the oxidizing agents, usually sodium hydroxide and sodium nitrate/nitrite. This creates the black oxide layer, thickness defined by a) concentration of oxidizers and b) duration of exposure..
- Rinse. Wash off the oxidizers with deionized water.
- Seal. Coated parts may be surface sealed to enhance the black oxide coating, improving corrosion and durability.
Types of Black Oxide Finish for Precision Machined Parts
There are several variants within the black oxide finish family for precision machined parts, each with their own variations in properties:
- Hot black oxide finish: This is the most widely employed black oxide finish in precision machined parts. The coating results from immersion in a hot alkaline solution containing sodium hydroxide, nitrate and nitrites. This creates a black iron oxide layer on the surface.
- Mid-temperature black oxide finish: This uses a lower temperature immersion process with the same solutions. It results in a more uniform finish than the hot process and is less likely to cause dimensional changes in the parts through expansion and stress relief.
- Room temperature black oxide finish: This uses a milder alkaline coating solution, at room temperature. It is a slower process than the hot or mid-temperature methods, generally reserved for parts of the highest precision that must not be heated.
- Cold black oxide finish: This method uses a cold blackening solution that is brushed or sprayed onto parts. It is typically used for small parts that are difficult to immerse in a hot or cold solution, because of the inability to conceal or minimize the effect of suspension or immersion basket marking.
All black oxide coatings can have a sealing process after blacking, applying a seal coat over the black oxide finish, for better corrosion resistance and improved durability of finish.
Materials for Black Oxide Finish
Black oxide finishes are suited to a wide range of metal surfaces:
- Steel: Blacking is widely used to coat steel parts in the automotive, aerospace, and construction industries. The process creates a moderate level of corrosion resistance and good cosmetic appeal.
- Stainless steel: Blacking works well on stainless steel components. The process leaves parts with an attractive, matte black finish which also provides additional corrosion resistance, where relevant.
- Aluminum: Blacking can be used on Aluminum parts, giving them a similarly attractive black finish that is highly corrosion resistant.
- Copper and its alloys: Surface treatment of Copper and brass gives good results in forming additional corrosion resistance, where relevant.
- Zinc (plated) parts: Blacking adds additional corrosion resistance to the already good performance of Zinc plating on steel – and the same hard wearing black finish.
Black oxide coating is generally used on ferrous (iron and steel) components, but it also provides good results on the non-ferrous metals listed, used in some specialist applications such as instrumentation and military equipment.
Common Applications for Black Oxide Finish for Precision Machined Parts
Black oxide finish for precision machined parts is used in a very wide range of applications:
- Firearms: Black oxide finish is commonly used on gun parts, which benefit from a durable, corrosion-resistant black finish that enhances their appearance and diminishes fingerprinting.
- Aerospace: Black oxide finish is used on precision machined parts in the aerospace industry to enhance corrosion and wear resistance in harsh environments.
- Automotive: Black oxide finish is used on automotive parts such as engine components, and transmission and suspension parts to provide corrosion and wear resistance.
- Medical devices: Black oxide finish is used on precision machined medical devices for corrosion resistance and biocompatibility.
- Electronics: Black oxide finish is used in electronics applications to provide a durable and conductive black finish that is corrosion and wear resistant.
- Industrial machinery: Black oxide finish is used in industrial machinery for durability and corrosion resistance under harsh operating conditions.
Pros and Cons of Black Oxide Finish
As with all surface treatment process options, black oxide finish has advantages and disadvantages
Benefits:
- The finish provides excellent corrosion resistance, protecting the surface of the part from rust/corrosion and mild chemical attack.
- These finishes improve wear resistance of surfaces, helping to resist damage from friction, abrasion, mechanical function.
- Black oxide finishes improve the visual appeal of engineered parts.
- These coatings are simple and cost-effective, compared with other surface treatment methods like plating or painting.
- No dimensional change results from the application, making this a good choice for precision machined parts with close tolerance fits.
Problems:
- Limited color options, from translucent mid blue to opaque black depending on process and substrate.
- Achieving good uniformity of finish/thickness can be challenging, especially with complex or irregular parts.
- Black oxide finishes involve the use of chemicals that can be hazardous to human health and the environment, so operator safety measures and waste disposal procedures are paramount.
Kemal: Your Preferred Partner for Precision Machining Projects
Our team has combined experience in the manufacture of CNC machined parts that exceeds 100 years. Our in house services in 3 and 4 axis machining, CNC lathe work etc operate on the highest quality standards and are ready to meet your needs.
For the wider range of services in multi-axis CNC machining and in product finishing, we find it more effective to build connections with supportive partner companies, whose services we seamlessly manage on behalf of our clients.
We will be delighted to talk with you about your needs and evaluate how our services can help you.
Testing Options for Black Oxide Finish
Testing is essential in ensuring that the applied finish is of the right quality and meets the standards and client specifications:
- Salt spray testing evaluates the corrosion resistance of the surface finish. The part is placed in a spray chamber and exposed to room temperature salt mist for a prescribed period.
- The hardness of the coating can be measured using a Rockwell or Vickers hardness test. This quantifies aspects of coating durability.
- Adhesion testing evaluates the bonding strength between the black oxide finish and the base metal. This test involves making a cut in the finish and then using adhesive tape to perform peel testing.
- Surface roughness testing using a profilometer or other surface roughness measurement equipment characterizes the surface variances – although these will also result from substrate roughness before coating.
- Colorimetry is used to evaluate the uniformity, color and consistency of the finish.
These testing methods identify any defects or quality issues and ensure that the finished parts perform as expected.
Important Factors to Consider When Specifying Black Oxide Finish for Your Machined Parts
In selecting for black oxide finishing of parts, you must consider the performance benefits and limitations, and then the differentials between variants of the process, before committing to a path:
- Duration of Protection Required: In aggressive environments, black oxide coating can be vulnerable. Addition of a seal coat can greatly increase chemical survival of the coating.
- Final Application of the Part: What are the corrosion, abrasion and cosmetic requirements the application imposes? Can black oxide treatment meet the specification?
- Application Environment: In particular you must develop a clear understanding of the corrosion or abrasion conditions the part(s) will encounter, to avoid specifying a coating that is not sufficiently robust.
- Desired Aesthetic Appearance: Aesthetic values are not the same for all parts. A tank gunsight has very different aesthetic needs than a laptop chassis, so make sure the intrinsic ‘industrial rugged’ appearance of black oxide coating suits your market.
Black Oxide Finish FAQs
Conclusion
Overall, black oxide coating of metal parts is a low cost, high functioning coating that can serve in a broad range of applications and environments.
Select the process with care and specify with precision – and where your information about the application is complete and fits the deliverables of this process, great results are within reach, for relatively low cost.
