In most cases, blackening treatment on steel, also known as black oxidation, can enhance corrosion resistance to a certain extent. However, the protective effect remains somewhat limited. To significantly improve both corrosion resistance and lubrication of the blackened layer, additional measures like oil immersion or subsequent rust prevention treatment are still necessary.
To enhance the rust resistance of steel components, a process is employed to oxidize the steel's surface, creating a dense and smooth layer of iron oxide using potent oxidizing agents. This thin layer of ferro-ferric oxide effectively shields the interior of the steel from oxidation. The iron oxide formed at high temperatures, around 550°C, has a sky-blue hue and is known as "bluish treatment," which is commonly used in weapon manufacturing. In industrial production, the bluish treatment is also a popular choice.
The ability to generate this dense and smooth iron trioxide layer on steel surfaces depends on the selection of a strong oxidizing agent. This potent oxidant typically comprises sodium hydroxide, sodium nitrite, and trisodium phosphate. When in a molten state, they are applied to steel components for bluish treatment, while their aqueous solutions are used for blackening treatment.
Precision-machined parts crafted from iron-based materials, such as stainless steel alloys, can specify black oxide as a surface treatment to provide additional protection.
What is Black Oxide?
Black oxide, also known as oxide black, is a conversion coating formed through a chemical reaction when a part is immersed in an alkaline salt solution, usually at an elevated temperature. This reaction between the stainless-steel alloy and the solution generates a layer of magnetite on the part's surface. Magnetite acts as a protective barrier, preventing moisture from corroding the alloy surface. Since magnetite is black, the process is termed "black oxide."
The solution used in this process may vary slightly depending on the metal being treated. The treatment can be applied using cold, room temperature, or hot processing methods, and it may require pre-treatment of components based on their specific requirements.
One of the advantages of black oxide is that it forms an exceedingly thin layer of magnetite, serving as a moisture barrier. The typical thickness ranges from 5 to 10 millionths of an inch (.000005 inches to .000010 inches), ensuring that the process does not affect the part's dimensional stability. Furthermore, black oxide treatment does not remove or deposit any metal from the part's surface, and unlike painted or electroplated treatments, it does not chip or flake.
Black oxide can also undergo post-treatment processes, such as oil, wax, or varnish, depending on the application and the desired appearance of the final part, ranging from a matte to a glossy finish. Post-treatment can also improve lubrication, making parts run smoother and enhancing their connection with matching components. The oil adds further corrosion protection. If black oxide is not specified for any post-treatment, the process is typically finished with oil.
The Applications of Black Oxide Can Be Specified
Components requiring compliance with MIL-DTL-13924D specifications for military purposes.
Surgical instruments in environments with reduced light to alleviate eye strain.
Gears, fasteners, and hardware in applications necessitating corrosion resistance.
Black hardware for construction and furniture assembly, adding an aesthetic touch to the final product.
Crucial factors to consider when specifying black oxide treatment include:
Duration of protection.
Indoor or outdoor usage.
Desired appearance (matte or glossy).
Environmental conditions (humidity, steam, temperature).
Intended use of the product.
Various testing options are available for black oxide treatment, and they can be specified on the part drawing, but they may add to the time and cost of the process:
This test checks for the presence of black powdery residues on parts when rubbed by hand after the treatment.
Relative Humidity Test: This procedure employs the ASTMD 2247 standard specification to evaluate corrosion resistance concerning water resistance of coatings at 100% relative humidity.