There are different classification methods for metal corrosion, which are roughly as follows.
1. Classification according to the principle of corrosion
(1)Chemical corrosion The corrosion caused by the chemical interaction between the metal surface and the environmental medium is called chemical corrosion. Its characteristic is that no corrosion current is generated during the chemical action. The corrosion of metals in dry gas media and non-conductive liquid media (such as alcohol and petroleum) is chemical corrosion.
(2)Electrochemical corrosion The corrosion of metal in the conductive liquid medium due to electrochemical action is called electrochemical corrosion, and corrosion current is generated during the corrosion process. Atmospheric corrosion, seawater corrosion, soil corrosion, etc. all belong to electrochemical corrosion. For example, the corrosion of carbon steel in the electrolyte, the Fe3C potential is more positive and the Fe potential is more negative, they will form a corrosion microbattery, Fe3C is a microcathode to precipitate hydrogen gas, and Fe is a microanode to be corroded into iron ions. For example, a battery formed by combining two or more metals with different potentials in contact with an electrolyte solution is called a large battery. A metal with a more positive potential (such as copper) is a good cathode, and a metal with a more negative potential (such as iron) is an anode and is corroded.
It should be pointed out that it is sometimes difficult to clearly distinguish chemical corrosion from electrochemical corrosion. For example, electrochemical corrosion generally occurs in iron steam, but it is converted into chemical corrosion at high temperature, and it is difficult to clearly distinguish its temperature limit.
2. Classification according to the form of corrosion damage
(1) Even corrosion When the metal is corroded, the corrosion effect is evenly distributed on the entire metal surface. The damage caused by such corrosion is relatively less harmful than local corrosion.
(2) Local corrosion When the metal is corroded, the corrosion effect is concentrated in a certain area, but the rest of the metal has little corrosion. The harm caused by this type of corrosion is evenly corroded. Local corrosion can be divided into different types: galvanic corrosion, crevice corrosion, pitting corrosion, intergranular corrosion, selective corrosion, wear corrosion and corrosion fatigue.
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