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CNC Parts Surface Treatment Black Oxide
Black oxide coatings belong to the category of conversion coatings, which involves a chemical process to form an oxide coating. Its name “black oxide” is derived from the black or matte appearance it gives to metal surfaces. The black oxide process involves dipping the machined metal part into an alkaline solution, which then converts the iron on the surface to magnetite, resulting in a black coating. This black oxide coating method can be used on a variety of metals, including stainless steel, copper, zinc and solder.
Black Oxide Process
Black Oxide is a conversion coating that is compatible with steel, stainless steel and copper and has a black appearance that slightly improves the corrosion resistance of parts.
- The parts are first machined and deburred.
- Use an alkaline solution to clean the parts.
- Rinse with water.
- Black Oxide. Immerse parts in a hot (135-145ºC) alkaline bath for 5 to 40 minutes depending on part size.
- Rinse with water.
- Post-treatment. Sealing the oxide surface with oil, lacquer or wax. This gives the part a desirable aesthetic appearance and improves its corrosion resistance.
Black Oxide Types
Type | temp | course of events | manifestations | appearances |
Thermal Black Oxide | High temperature, typically around 141°C (286°F) | It converts the surface of ferrous materials into magnetite (Fe3O4). | The coating is a natural black, opaque, magnetic material with good abrasion and corrosion resistance. | The coating is more even and darker in color. |
Medium temperature black oxide | Temperature range: 90-120°C | It also converts to magnetite, but at a lower processing temperature. | Does not produce corrosive gases, abrasion resistance and corrosion resistance may be slightly inferior to thermal oxidized black. | The coating color may be slightly lighter than thermal black oxide. |
Cold Black Oxide | indoor temperature | Copper selenide was used as the black oxide formulation. | The coating is not as resistant to abrasion and corrosion as hot black oxide, but it is suitable for materials that cannot withstand high-temperature processing. | Coating color may be light and may be uneven. |
Conventional Black Oxide | Room temperature or slightly higher. | Obtained by immersing the metal in a solution containing sodium hydroxide and sodium nitrite. | It is the most basic and common black oxide coating with moderate performance and average corrosion and abrasion resistance. | Coating is darker but may be uneven. |
CNC Surface Finish Black Oxide Requirements
Surface treatment material requirements
Materials have a significant impact on the acceptance of black oxide coatings. Black oxides are typically applied to metals such as steel, stainless steel and iron. Some metals (e.g., certain aluminum alloys) may require additional processing because they bond less efficiently to black oxide coatings.
Part Shape
The geometry of the part will affect the application of the coating. Complex shapes or hard-to-reach parts may require special considerations, such as the use of masking or selective coating techniques. Parts with deep grooves or complex features may require careful planning to ensure a uniform coating.
Part Application
The thickness of the black oxide layer depends on the part application. While black oxide coatings are typically thin, ranging from 0.0002 to 0.0005 inches, thicker coatings can be applied to parts that require additional corrosion or wear protection. A coating that is too thick may affect the size and function of the part, so the coating process must be precisely controlled.
Example of a black oxide part
Boiling black or blackening, or bluing, as a traditional surface treatment process, with its high-quality appearance, convenient film-forming conditions, unique in the surface treatment process. It is generally used for the surface treatment of steel workpieces such as springs, mechanical parts, stamping parts, screws, and automobile parts.