Corrosion of metal and rust and rust removal of metal parts

1. Causes of metal corrosion

Corrosion of metals has two major categories: chemical corrosion and electrochemical corrosion.

(1) Chemical corrosion

The phenomenon that a metal material is chemically reacted with an external medium and is damaged is called chemical corrosion. No current is generated during chemical corrosion, just a simple chemical action. Such as the oxidation of the metal surface produced in a dry atmosphere at room temperature. For different metals, the structure and properties of oxides are different. Some can form a fine and stable oxide film on the metal surface to isolate the inner metal from the external medium and protect it. Chromium, aluminum, zinc, etc.; and some oxide layers are loose and easy to fall off, so that the inner metal continues to be corroded by corrosive media, such metal corrosion rate is very fast, such as iron, magnesium, copper and so on. The oxidation rate of the metal is much faster at high temperatures than at low temperatures, and preventing the metal from coming into contact with the oxidizing medium can slow or prevent chemical corrosion of the metal. Common corrosive media are oxygen, hydrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, hydrogen chloride and industrial waste gas.

(2) Electrochemical corrosion

Corrosion caused by current during the corrosion process is called electrochemical corrosion.

Metals are more or less always containing certain impurities. Different metals have different potentials, and different compositions in the same metal also have different potentials. When the metal is in contact with a certain electrically conductive solution, a potential difference occurs, causing a flow of electrons in the solution, causing the metal having a low potential to be first corroded.

Corrosion depends on the chemical properties of the metal itself and on the surrounding medium and metal structure. For example, the wet environment is more likely to rust than the dry environment. The metal with less impurities than the impurities is easily corroded. Under high temperature conditions, it is more likely to rust than the low temperature conditions. The dirty environment is more dirty than the air, and the clean environment is easy to rust. Metal parts Low surface finish, high rust resistance, etc.

2. Corrosion characteristics of common metals

(l) Steel and cast iron

When it starts to rust, the surface is dark gray, gradually forming a yellow-brown powdery rust, and then developing into a piece of brown or brown scar.

(2) Copper and its alloys

The rust is characterized by a brownish red or green thin layer. The main component of green rust generated on the copper surface is basic copper carbonate; copper hydroxide is dark blue, cuprous oxide is red, and copper oxide is black.

(3) Aluminum and its alloys

Corrosion is characterized by white or dark gray spots, sometimes white powder appears in the rust, and then develops, there is a rust pit filled with rust products.

(4) galvanized, cadmium plated, tin plated

The rust of the coating is characterized by a gray, black spot or a white powdery layer.

(5) Blue parts

Blue is itself an artificial oxide protective film. Its rust is brown rust or rust, sometimes not much different from bluish (purple, blue-black, blue-brown).

(6) Phosphating parts

The corrosion characteristics on the phosphating film are the same as those in the steel. Others such as zinc and aluminum alloys have the same characteristics of direct corrosion after phosphating with zinc and aluminum alloy itself.

(7) Sandblasting parts

It is a grayish white uniform fine-grained blasting surface that is rust-colored when corroded.

(8) painted parts

When rust is formed under the paint layer, the paint layer will bulge and foam, which will cause swelling and flaking, and rust will appear.

3. Anti-rust method for metal parts

Commonly used metal parts rust prevention methods include oxidation method, phosphating method, electroplating method, spray coating plastic coating method, and brush coating anti-rust coating method. The first few are generally used in the manufacturing process of parts, and only the anti-rust coating method of brushing is introduced.

(1) Various steel parts

Can be coated with anti-rust paint. First apply the red anti-rust paint (primer) on the surface of the part, and then apply a layer of gray anti-rust paint (top coat), pay attention to the surface of the part can not be painted.

(2) The bearing can be coated with a petrolatum paint containing 1% refined rosin.

(3) Spring springs should not be oiled. If rust marks are found, they can be removed with a soft brush and coated with graphite.

(4) All kinds of bolts can be cleaned with kerosene and coated with a thin layer of neutral mineral waste engine oil.

(5) Various precision parts, tools, etc.

Can be coated with a variety of anti-rust grease.

In the anti-rust work of metal parts, the method of applying anti-rust grease is simple and practical. There are many kinds of finished anti-rust oils on the market. Commonly used are anti-rust oil, anti-rust grease, anti-rust lubrication dual-use grease, vapor phase corrosion inhibitor, peelable plastic, etc., according to different varieties of accessories and different protection requirements. To choose to use.

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Label: Metal parts Corrosive media Phosphating film Steel parts Copper and its alloys

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