Black oxide is a cost-effective surface finish unlike any other, it is a chemical conversion process used to blacken the surface of a part without adding a thick coating. Black oxide has certain properties and characteristics that make it ideal for certain applications.
What is Black Oxid？
Black oxide is a chemical conversion coating applied to ferrous metals, such as steel and iron. The process involves a chemical reaction that forms a layer of black iron oxide (Fe3O4) on the metal surface.
While commonly used for ferrous metals, black oxide can also be applied to certain non-ferrous metals such as stainless steel, copper, and titanium, though the process may vary.
Types of Black Oxide
There are 3 types of black oxide, they include:
- Hot black oxide
- Mid-temp black oxide
- Cold black oxide
Hot Black Oxide
Hot black oxide involves immersing the metal components in a hot alkaline solution typically containing sodium hydroxide, nitrates, and nitrites. The immersion happens at elevated temperatures, typically around 275 to 285 ℉. During this immersion, a chemical reaction occurs between the metal surface and the solution, resulting in the formation of a thick layer of black iron oxide (Fe3O4) on the surface of the metal.
Mid-temp Black Oxide
Mid-temp black oxide is characterized by its operation at moderate temperatures, typically around 194 to 249°F, it does not produce corrosive fumes, providing a safer working environment.
Cold Black Oxide
Cold black oxide is applied at lower temperatures, typically around room temperature of around 68 to 86°F. Cold black oxide does not involve the surface conversion to oxides. In this process, copper selenide is deposited on the surface of metal parts. However, the wear resistance of this black coating is relatively poor.
Processes of Black Oxide
The operation of black oxide can be summarized into the following 6 steps:
Clean
The metal parts undergo thorough cleaning to remove any contaminants, oils, or rust from the surface. This step is crucial for the effectiveness of the subsequent black oxide treatment.
Rinsing
After cleaning, the metal parts are rinsed to remove any residual cleaning agents or contaminants. Rinsing prepares the surface for subsequent treatment steps.
Acid pickling
Acid pickling involves immersing the metal parts in an acidic solution. This step helps in removing any remaining oxides, scale, or other surface impurities. It also etches the surface, creating a more receptive environment for the black oxide treatment.
Rinsing
Following acid pickling, the metal parts undergo another rinsing process to remove any residual acid and prepare the surface for the next stage.
Black oxide chemical bath
The metal parts are immersed in a specially formulated black oxide chemical bath. This bath typically contains specific chemicals, such as sodium hydroxide, nitrates, and nitrites, which react with the metal surface to form a black iron oxide layer. The immersion time and bath composition depend on the desired thickness and properties of the black oxide layer.
Sealing
The final step involves sealing the black oxide coating. Sealing is optional but is often performed to enhance corrosion resistance and improve the durability of the finish. This may involve using a rust inhibitor or applying a protective oil or sealant.
Advantages of Black Oxide
Here are some of the key advantages of using black oxide:
Decorative Black Appearance
Achieves the desired surface appearance, and the color remains stable during temperature fluctuations.
Minimal Dimensional Change
The thickness of the black oxide coating does not exceed 0.75µm, with half of it adding to the part’s dimensions and the other half penetrating the part’s depth.
Strong Adhesion between Coating and Substrate
The black oxide coating adheres firmly to the surface and does not peel off.
Low-right Reflection
Black oxide-coated parts on moving components, surgical instruments, and hand tools reduce glare, minimizing eye fatigue.
Improved Corrosion Resistance
The black oxide coating provides some corrosion protection to parts, but being thin, it does not offer corrosion resistance in highly corrosive environments.
Disadvantages of Black Oxide
Here are 2 key disadvantages of using black oxide:
Limited Corrosion Resistance
Black oxide provides a certain level of corrosion resistance, but it is not as effective as some other surface treatments, such as anodizing or electroplating. In harsh environments or exposure to certain chemicals, the protective layer may break down over time.
Limited Color Options
As the name suggests, black oxide produces a black finish. If you require a different color for aesthetic or functional reasons, black oxide may not be the ideal choice
Design Consideration for Black Oxide
When considering the use of black oxide for metal finishing, several design considerations should be taken into account.
Part Geometry and Size
Consider the geometry and size of the metal parts. Black oxide is well-suited for small to medium-sized parts with relatively simple shapes. Complex geometries or very large parts may pose challenges in achieving uniform coverage.
Smooth Surface
Remove any contaminants, rust, or scale from the metal surface before applying the black oxide treatment. A clean and smooth surface will enhance the quality of the finish.
Material Compatibility
Black oxide is primarily suitable for ferrous metals, such as iron and steel. Ensure that the material to be treated is compatible with the black oxide process. Some non-ferrous metals like aluminum are not suitable for black oxide.
