A brief history of black oxide
The exact origin or inventor of black oxide coating is not well documented, but the process of blackening iron and steel has been used for centuries by blacksmiths and metalworkers.
What were the early uses of black oxide coating?
Preventing corrosion. Black oxide coating helps improve metal components’ resistance to corrosion and rust, extending their lifespans and reducing the need for expensive or time consuming repairs or replacements.
Reduced wear. Black oxide enhances the hardness and durability of metal surfaces, reducing wear and tear and improving the performance of metal components in hard working or industrial environments.
Aesthetics. With a smooth, matte finish, black oxide improves the appearance of metal components, making them easier to incorporate into a variety of applications, as their visibility is not an issue.
Reduced light reflection. For a variety of functional and cosmetic applications, a black oxide coating makes surfaces less reflective.
What is black oxide predominantly used for now?
Black oxide is still used to achieve the above results and is well suited for applications that require a durable, low maintenance, and corrosion resistant surface finish, such as:
Fasteners, hardware, and tools. Black oxide is a popular finish for fasteners such as screws, nuts, and bolts, as well as hand tools and other hardware, as it provides a matte, aesthetically pleasing finish that is corrosion resistant, durable, and easy to grip.
Automotive and aerospace components. In the automotive and aerospace industries, black oxide is used as a surface finish for a variety of parts, including engine components, suspension systems, and landing gear.
Industrial equipment. Black oxide is used to finish a wide range of industrial equipment, such as pumps, valves, and machinery, to provide corrosion and wear resistant surfaces.
What Is The Process Of Applying Black Oxide Finish?
The back oxide finish process generally involves five steps which include:
Cleansing
Rinsing
Descaling/Pickling
Black Oxide Finish
After Finish
This section will explain each of these steps in detail.
Cleaning
This process involves preparing the part for the black oxide finish coating. The machinists degrease and clean the part using an alkaline solution.
Rinsing
This process involves washing off the alkaline solution used in cleaning with distilled water.
Descaling/Pickling
This stage is only necessary if the parts have rust or scales. It involves removing all the rust and oxide stains on the finished parts by soaking them in acid.
Neutralizing
This process is only necessary when the finished parts have blind holes, or it is an assembly of parts. This involves rewashing the part with an alkaline solution. This will help prevent bleed-outs which could impair the surface of the part.
De-plating
This conditional stage occurs only when the finished part is plated with another material (e.g., chromium, zinc, cadmium, etc.). The machinist must de-plate the part as the black oxide finish cannot stay on plated parts.
Black Oxide Finish
This is where the machinist finally applies the finish. The machinist dips the finished metal parts into tanks containing various alkaline solutions such as sodium hydroxide, nitrates, and/or nitrates. This stage has three different temperatures, determining the coating material on the part’s surface.
At the hot (141°C) and mild temperatures (90 – 120°C), the process leaves a magnetite coat (Fe2O3) on the part. However, at the cold temperature operating level, the process deposits a copper selenium coat (Cu2Se) on the part surface.
After-Finish
Without the after-finish, the finished part is still highly prone to corrosion. However, this rule does not apply to black oxide stainless steel or brass surfaces. This is because these metals have inherent anti-corrosion properties. However, for other metal parts, the after-finish is quite vital. The common after-finish candidates are oil, wax, and lacquer.
Oil is the commonly used after-finish among machinists. It gives a glossy surface and also increases the lubrication and corrosion properties. Wax gives a duller surface, but its lubrication and corrosion-augmenting properties are even higher. Lacquer is also a good choice, especially if you need to finish the parts quickly, as it dries very quickly.
What’s the difference between black oxide and other surface finishes?
Black oxide differs from other popular surface finishes in several ways.
Aesthetics. While other metal finishes, such as anodizing, electroplating, and powder coating, offer a variety of color options, black oxide provides a unique, matte black finish.
Durability. Black oxide provides a durable, corrosion-resistant surface that is often more wear-resistant than other finishes.
Process. Creating a black oxide finish is relatively simple and cost-effective compared to other finishes, such as electroplating or anodizing.
Design considerations for black oxide parts
Consider these important factors when using black oxide coating.
Coat thickness. The thickness of black oxide coating depends on several factors, including the type of metal being coated, the method used to produce the black oxide coating, and the desired level of protection, with thicker coatings providing increased protection. Typically, however, black oxide coatings are between 0.0002 to 0.0006 inches thick.
Use on stainless steel. It is possible to use black oxide coating on stainless steel. While stainless steel is naturally rust and corrosion-resistant, a black oxide coating can provide additional protection as well as enhance its aesthetic qualities.
Best practices for black oxide coating
Follow these tips and note some design limitations for black oxide coatings.
Complex geometries. Parts with complex geometries and tight corners may not be suitable for black oxide coating, as it can be difficult to achieve a uniform coating on these surfaces.
Surface finish. Black oxide coating works best on surfaces with a smooth, uniform finish, as rougher surfaces can result in a less uniform coating.
Material compatibility. Not all metals are compatible with a black oxide coating. Some metals, such as aluminum, do not react well with the black oxide process, which can result in a poor-quality finish.
Corrosion resistance. While black oxide provides some protection against corrosion, it is not as effective as other finishes such as anodizing or electroplating.
Masking. Black oxide does not have a significant dimensional impact, therefore masking parts is not necessary.