Black coating is a commonly used surface treatment – but how does it work? What are the advantages of black coating?
Black coating is a common method of chemical surface treatment. This process aims to achieve rust prevention by making an oxide film on the metal surface to isolate the air from the material. Black coating treatment can be adopted when appearance requirements are not too high. The surface of steel parts would be blackened and its temperature should be between 135 ~ 155 ℃.
The advantage of black coating of steel at room temperature is that the process is energy-efficient. The equipment used is simple and easy to operate and free from environmental pollution. It can be used from small to large pieces, complex-shaped and also different types of steel.
We have many years of production practice: Weimi can take care of the whole process.
The procedure for the black coating process:
General chemical degreasing –> washing in a hot water –> washing in clean water –> washing strong corrosion –> washing in clean water (twice) –> chemical degreasing –> washing in hot water –> washing in clean water –> weakening corrosion –> washing in clean water (twice) –> black coating at room temperature –> washing in clean water (twice) –> immersing in dehydrated oil –> inspection.
If iron and steel are treated at room temperature before being blackened chemical degreasing is generally used. The black coating liquid at room temperature is acidic. The surface of the workpiece must be fully moisturized before black coating to effectively ensure the firmness of the black film. Otherwise, it will not be possible to produce a good quality product. It’s proved through many years of practice that whether degreasing tank that is used alone or a low-concentration three-base added degreasing agent: it must be heated at about 50°C to accelerate the degreasing speed. The degreasing and rust removal operations are significantly improved when done within a short period of time (3-5 minutes).
Black coating at room temperature:
Weimi’s years of production have proven that the film’s fastness does not need to wait for 24 hours. You can get a good black film layer that is comparable to high-temperature blackening.
Solution stability: the working solution produces less white precipitate and it is not affecting the quality of the film. It is not necessary to adjust the initial liquid for more than two years without precipitation and the effect remains unchanged.
ZT black coating agent added into the black acrylic coating agent to simplify the production.
The film has good adhesion and corrosion resistance but the color of black film turns to light blue.
When the solution is maintained and adjusted to black at room temperature the surface of the steel is activated to produce Fe2 +. Excess Fe2 + Se2- reacts to form a white precipitate. In the presence of an oxidant, the oxidation of Fe2 to Fe3 also results in precipitation.
Possible defects and solutions for blackening: During the black coating process of steel parts the following defects may occur during operation. These can include:
(1) Loose oxide film: The oxide film should be very dense and firmly bonded to the basic. Oxide film shouldn’t be loose or easy to wipe off. This might happened mainly due to the lack of iron ions in the newly prepared oxidation solution. The phenomenon can be solved by adding old oxidation solution or some clean metal and boiling for 30 min to increase the iron ion concentration of the oxidation solution, and this phenomenon will be eliminated.
(2) Red-brown rust spots on the surface of the workpiece: The oxide film should be dark black, bright and smooth. If red-brown spots or pieces appear on the surface of the workpiece this is mainly due to the excessive concentration of sodium hydroxide in the oxidation solution and the excessive formation of iron oxide. It can be solved by adjusting the composition of the solution and adding some sodium nitrite and water to reduce the concentration of N and H.
(3) The surface of the workpiece is light gray: The surface of the workpiece loses its blackish lustre and becomes light gray which is caused by the low concentration of sodium hydroxide in the oxidation solution. The phenomenon can be solved by adding the appropriate amount of sodium hydroxide.
(4) No oxide film is formed: After 40 ~ 5Min treatment the surface of the workpiece is still not colored and an oxide film is formed. The main cause is the solution temperature being too low and the sodium nitrite concentration being insufficient. The phenomenon can be solved by appropriately increasing the solution temperature and increasing the amount of sodium nitrite.
(5) Scratches on the surface of the oxide film: Occasionally, the joints appear on the black base and the joints are sometimes turn out to be dense. This phenomenon is called “scratching” and it’s caused by insufficient oxidation time. The solution is to extend the oxidation time (soaking time) and add a small amount of sodium hydroxide.
(6) The surface of the workpiece is green: Oxide film on the surface of the workpiece is not black but green. That’s the result of the over-oxidation. It is caused by the temperature of the oxidation solution being too high and the sodium nitrite concentration being also too high. The solution is to add an appropriate amount of cold water.
Note: You must wear old protective gear when adding water, add water slowly to prevent the bath from splashing that might cause burns. Next step: add a small amount of sodium hydroxide to reduce the temperature of the solution and sodium nitrite concentration.
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