Technology and principle of blackening treatment

Why should the hardware be blackened? Mainly rust prevention

As we all know, most metals are chemically reactive, but in fact they are easy to oxidize, which is easy to rust in layman’s terms.

Common metals will show different colors after rusting. The following are some common colors of rusted metals:

1. Iron: Iron usually appears reddish brown or orange brown after rusting.

2. Steel: The rusty color of steel is usually reddish brown or orange brown, which is similar to iron.

3. Copper: Copper appears green after rusting. This is the result of copper oxidation to produce patina (copper oxide).

4. Bronze: Bronze usually appears green or green-blue after rusting.

5. Aluminum: When aluminum rusts, it will form a fine layer of white oxide, but it will not show obvious rust like iron.

These are just the rusty colors of some common metals. In fact, different metals and oxidation reactions under different environmental conditions may produce different colors.

Blackening treatment is a process of blackening the metal surface, also known as “black oxide”. This treatment is mainly used to improve the appearance, corrosion resistance and wear resistance of metals.

Blackening treatment is usually achieved by two methods: chemical method and physical method.

1. Chemical method:
-Pickling method: the metal is soaked in acid solution, and the oxide layer and impurities on the surface are removed by the action of acid. Then, it is soaked in alkaline solution, so that an opaque oxide film is formed on the metal surface, forming black.
-Alkaline washing method: metal is soaked in alkaline solution to form a layer of black oxide on the metal surface.

2. Physical law:
-High-temperature oxidation method: the metal is heated to a high temperature, so that the surface of the metal undergoes an oxidation reaction to generate a black oxide. This method is often used for metal materials such as iron and steel.
-Wet electrochemical method: By applying current in a specific electrolyte, the metal surface undergoes oxidation reaction to form black oxide. This method is often used for metal materials such as aluminum and copper.

The principle of these methods is to form a dense oxide layer on the metal surface, which has a black appearance and certain corrosion resistance and wear resistance. By controlling the treatment parameters and process conditions, different colors and effects of blackening treatment can be realized.

Will blackening change the size of the workpiece? How much will it change?

-Will change, and the change generally increases the material, and the thickness is micron. It is below 0.01. It rarely reaches 0.01, in layman’s terms, it is less than 1 silk.

Blackening treatment usually has little change in the size of the workpiece, but the specific change depends on the treatment method and process conditions. Here are some common situations:

1. Chemical treatment: The size of most chemical treatments varies little, usually in the range of several microns to several tens of microns. This is because the chemical treatment mainly produces a thin film on the surface of the metal, which has no obvious change in the volume of the metal.

2. High-temperature oxidation treatment: High-temperature oxidation may cause certain dimensional changes, because metal will expand at high temperature. The specific dimensional change depends on the thermal expansion coefficient of the metal and the treatment temperature. Generally speaking, the size change is small, usually within a few percentage points.

3. Wet electrochemical treatment: Wet electrochemical treatment has little influence on the size, usually in the range of several microns to several tens of microns. This is because when the current is applied in the electrolyte, the oxidation reaction of the metal surface mainly occurs on the surface and does not cause obvious size change.

It should be noted that blackening treatment may form a film on the metal surface, and the thickness of this film will be added to the size of the workpiece. But in general, the thickness of this film is very thin, which has little influence on the dimensional change of the whole workpiece.


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