What is the difference between bluing and passivation in surface treatment?

Bluing treatment is a method of using chemicals to improve surface hardness and durability, while passivation is a physical method to passivate the sharp part to achieve comfort in use.

Bluing treatment: Bluing is a material protection technology that heats steel in air or directly immerses it in a concentrated oxidizing solution to produce an extremely thin oxide film on its surface, also known as blackening.

The bluing of steel parts can be carried out in molten salts of sodium nitrite and sodium nitrate, or in hot air at high temperatures and superheated vapors of more than 500 °C, and more commonly in concentrated caustic soda with sodium nitrite. The composition of the solution, the reaction temperature and the time of the bluing depend on the composition of the steel matrix. The composition of the blue film is magnetic iron oxide, the thickness is microns, and the color is related to the material composition and process conditions, including gray black, dark black, bright blue, etc.

The bluing film alone is less resistant to corrosion, but it is improved in both corrosion resistance and abrasion resistance after oiling, waxing or varnishing. When blue, the size and finish of the workpiece have little effect on the quality. Therefore, it is commonly used in precision instruments, optical instruments, tools, hardness blocks, etc.

Passivation: The last and key step in chemical cleaning is the purpose of the material to prevent corrosion. For example, after pickling, water flushing and rinsing, the metal surface of the boiler is very clean and very activated, and it is easy to suffer from corrosion, so it must be passivated immediately to form a protective film on the cleaned metal surface and slow down corrosion.

After the metal is treated with oxidizing medium, its corrosion rate is significantly lower than that before the original treatment, which is called metal passivation. The passivation mechanism can be explained mainly by the thin film theory, that is, the passivation is due to the interaction between the metal and the oxidizing medium, and a very thin, dense, well-covering passivation film can be firmly attached to the metal surface during the action. This film exists in separate phases, usually compounds of oxygen and metals.

It plays the role of completely separating the metal from the corrosive medium, preventing the metal from being in direct contact with the corrosive medium, so that the metal basically stops dissolving and forms a blunt state to prevent corrosion.

There is a layer of color film after degreasing phosphating passivation

Landlord: Hello!

Based on my work experience, the following is derived:

We often say that stainless steel passivation is divided into: 1Natural color passivation 2Washable passivation: does not change the size and color of the workpiece, and is applied to stainless steel (200 series, 300 series) stainless iron (400 series).

Pickling passivation: It is a comprehensive product of pickling and passivation, which removes the extremely thin chromium-poor layer metal on the surface of the workpiece at the beginning of passivation. When the oxide scale is removed, the whole reaction is transformed into a passivation process, forming an effective passivation protection.

For stainless steel (200 series, 300 series) The size of the workpiece is changed and a uniform silvery-white color is formed.

Methods to check whether the passivation is qualified: blue dot detection, neutral salt spray test, etc.

The first is to add carbon, the surface is not easy to corrode, the second is to put strong acid, the surface is passivated, it should be what you said, I don't know how to test whether it is qualified.