There are different classification methods for metal corrosion, generally as follows.
1、 Classification according to the principle of corrosion:
1. Chemical corrosion The corrosion caused by the chemical action between the metal surface and the environmental medium is called chemical corrosion. Its characteristic is that there is no corrosion current generated in the process of chemical action. Corrosion of metal in dry gas medium and non-conductive liquid medium (such as alcohol and petroleum) is chemical corrosion.
2. Electrochemical corrosion The corrosion of metal in conductive liquid medium due to electrochemical action is called electrochemical corrosion, and corrosion current is generated during the corrosion process. Atmospheric corrosion, seawater corrosion and soil corrosion are all electrochemical corrosion. For example, for the corrosion of carbon steel in the electrolyte, if the Fe3C potential is positive and the Fe potential is negative, they will form a corrosion micro-cell. The Fe3C is the micro-cathode and releases hydrogen, and the Fe is the micro-anode and is corroded into iron ions. For example, a battery formed by combining two or more metals with different potentials and contacting with the electrolyte solution is called a large battery. The metal with positive potential (such as copper) is a good cathode, and the metal with negative potential (such as iron) is corroded as an anode.
It should be pointed out that sometimes it is difficult to clearly distinguish chemical corrosion from electrochemical corrosion. For example, electrochemical corrosion generally occurs in molten 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 corrosion damage form:
1. Uniform 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 serious than that caused by local corrosion.
2. Local corrosion When the metal is corroded, the corrosion effect is concentrated in a certain area, while the rest of the metal is hardly corroded. The damage caused by this kind of corrosion is serious. Local corrosion can be divided into galvanic corrosion, crevice corrosion, pitting corrosion, intergranular corrosion, selective corrosion, wear corrosion and corrosion fatigue.
Various forms of corrosion damage.
3、 Classification according to specific corrosive environment:
1. Atmospheric corrosion The corrosion of metals in the atmosphere and any wet gas is the most common corrosion.
2. Corrosion in electrolyte solution Corrosion of natural water and most aqueous solutions to metals, such as acid, alkali and salt water solutions to metals.
3. Corrosion of ships and marine facilities in seawater.
4. Soil corrosion Corrosion of metal structures buried underground, such as underground metal pipelines and underground cables.
5. Molten salt corrosion Corrosion occurs when a metal contacts molten salt, such as the corrosion of molten salt during heat treatment.
6. Organic atmosphere corrosion Metals corrode in the organic atmosphere emitted by some non-metallic materials such as plastics and rubber, for example, zinc and cadmium electrodeposits corrode in organic carboxylic acid atmosphere.
7. Biological corrosion Corrosion of certain bacteria, marine organisms and human body to metals.
8. Corrosion of metals in other special environments in high-temperature pure water, space and other environments.
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