When impure metals come into contact with electrolyte solutions, a galvanic reaction occurs, and the more active metals lose electrons and become oxidized. This type of corrosion is called electrochemical corrosion.
From a macroscopic point of view, when two materials with different potentials form the anode and cathode of corrosion, under the action of the surrounding electrolyte, the anode with high potential becomes the victim, while the cathode with relatively low potential is protected. From a microscopic point of view, between two different organizations, between the basic phase and the inclusions and precipitation phases in the steel, such an anode-cathode "micro-battery" is also formed. One side is dissolved and the other side is protected. Even the unevenness caused by various defects such as scratches on the surface of the material can cause corrosion. This is the simplest principle of material corrosion.
Electrochemical corrosion is the metal and electrolyte forming two electrodes to form a corrosion galvanic cell. For example, iron and oxygen, because the electrode potential of iron is always lower than that of oxygen, iron is the negative electrode and is corroded. The characteristic is that many small bulges of varying diameters will form on the surface where oxygen corrosion occurs, and the secondary layer is black powdery ulcer corrosion pits.
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