Electrochemical corrosion is mainly because the metal on the surface of the stainless steel is less mobile than iron, which causes the iron to be oxidized more severely as the negative electrode of the primary battery.
Electrochemical corrosion is that metal and electrolyte form 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. It is characterized by the formation of many small bulges with different diameters on the surface where oxygen corrosion occurs, and the secondary layer is black powdery ulcer corrosion pits.
The metal material is in contact with the electrolyte solution and corrodes through the electrode reaction. The electrochemical corrosion reaction is a redox reaction. In the reaction, the metal loses electrons and is oxidized. The reaction process is called the negative electrode reaction process. The reaction product is the metal ion entering the medium or the metal oxide (or metal insoluble salt) covering the metal surface; The substance obtains electrons from the metal surface and is reduced, and the reaction process is called the positive reaction process. In the process of the positive electrode reaction, the substances that gain electrons and are reduced are customarily called depolarizers.
During uniform corrosion, there is no significant difference in the probabilities of negative and positive reactions occurring anywhere on the metal surface, and the positions of the surfaces where the two reactions are performed are constantly changing randomly. If there are some areas on the metal surface that mainly carry out negative reactions, and the rest of the surface areas mainly carry out positive reactions, the former is called the negative area, the latter is the positive area, and the negative area and the positive area form a corrosion battery. It is the negative electrode reaction that directly destroys the metal material. Therefore, an external power supply or a wire is often used to connect the protected metal to another metal with a lower electrode potential, so that corrosion occurs on the metal with a lower potential.
