Classification of metal corrosion: According to the corrosion mechanism of metal, metal corrosion can be divided into chemical corrosion and electrochemical corrosion. Whether it is chemical corrosion or electrochemical corrosion, the essence of metal corrosion is the process that metal atoms are oxidized into metal cations.
chemical corrosion
Chemical corrosion refers to the damage caused by the direct chemical reaction between metal and non-electrolyte. The corrosion process is a pure chemical reaction of pure redox, that is, the corrosion medium directly interacts with atoms on the metal surface to form corrosion products. No current is generated during the reaction, and the process conforms to the law of chemical kinetics.
electrochemical corrosion
Electrochemical corrosion is the damage caused by the chemical interaction between metal and electrolyte solution. During the reaction, the anode loses electrons, the cathode gains electrons, and the electrons flow. The process obeys the basic laws of electrochemistry.
According to the different acidity and alkalinity of electrolyte solution, electrochemical corrosion can be divided into hydrogen absorption corrosion and hydrogen evolution corrosion
Main anticorrosion measures of metal materials
(1) Improve the properties of metals
Choosing different materials to form corrosion-resistant alloys according to different uses, or adding alloying elements to metals to improve their corrosion resistance, can prevent or slow down the corrosion of metals. For example, adding nickel to steel to make stainless steel can enhance the corrosion resistance.
(2) It is an effective method to prevent metal corrosion by forming a protective layer on the metal surface and covering various protective layers to isolate the corrosiveness of the protected metal gas. There are two main protective layers for industrial holes: nonmetallic protective layer and metal protective layer. Generally, the protective layer is formed by the following methods:
1) Phosphating treatment of metal: After degreasing and rust removal, iron and steel products are soaked in phosphate solution with specific components to form a water-insoluble phosphating film on the metal surface. This process is called phosphating. Phosphating film is dark gray to dark gray, and its thickness is generally 5 to 20 microns, so it has good corrosion resistance in the atmosphere. The film has a microporous structure and has a strong adsorption capacity for paint. If it is used as the bottom layer of paint, the corrosion resistance can be further improved.
2) Oxidation treatment of metal: After adding steel products into the mixed solution of Na0H and NaN02 and heating treatment, a blue oxide film (main component Fe3O4) with a thickness of about 0.5 1.5 micron can be formed on its surface to achieve the purpose of steel corrosion protection. This process is called bluing treatment, or bluing for short. This oxide film has great elasticity and lubricity, and does not affect the accuracy of parts. Therefore, parts of precision instruments and optical instruments, such as spring steel, thin steel sheet and thin steel wire, tend to be blue.
3) Non-metallic coating: It is better to spray metal surface with plastic than spray paint. The plastic coating is dense and smooth. Bright color, both anti-corrosion and decorative dual functions. Enamel is a kind of glass enamel with high silica content and excellent corrosion resistance. Therefore, as a corrosion-resistant nonmetallic coating, it is widely used in petrochemical, pharmaceutical, instrumentation and other industrial sectors and daily necessities.
4) Metal protective layer: This is a protective coating formed by plating one metal on the surface of another metal product that needs protection. The former metal is called metallization. Besides electroplating and electroless plating, there are other methods to form metal coatings, such as hot dip plating, thermal spraying, infiltration plating and vacuum plating.
(3) electrochemical protection
Electrochemical protection is one of the important methods of metal corrosion protection, and its principle is to change the potential of corroded metal by using applied current, thus slowing down or inhibiting metal corrosion. Electrochemical protection can be divided into anode protection and cathode protection.
anodic protection
Is to introduce enough anode current to the metal surface, so that the metal is anodized, that is, the potential becomes positive, and the metal is in a passive state.
The dissolution rate is greatly slowed down.
Cathodic protection is to introduce enough cathodic current on the corroded metal surface to polarize the metal cathode, that is, the potential becomes negative to prevent the metal from dissolving.
(4) Corrosion inhibitor method
Corrosion inhibitor method is a common anti-corrosion measure. Adding a small amount of corrosion inhibitor in corrosive environment can physically interact with metal surface.
Chemical action, thus significantly reducing the corrosion of metal materials. Because the corrosion inhibitor does not need special equipment and does not change the properties of metal components, it has the advantages of economy and adaptability, and is widely used in pickling cooling water system, oil field water injection and so on.