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Advanced Definition
Definition 8: The electrode with a more negative potential is called a sacrificial anode because, as the current continues to flow, the anode material is gradually consumed. As a sacrificial anode material, the metal or alloy must meet the following conditions: (1) The potential must be sufficiently negative to supply sufficient electrons to cause cathodic polarization of the protected metal equipment.
System Requirements
Sacrificial anodes are typically only economically applied to structures with low protective current requirements and in environments with low soil resistivity. Additionally, they are valuable when there is no power supply condition or when it is uneconomical.
Sacrificial anode materials suitable for use in soil are primarily magnesium, while in seawater, zinc and aluminum are used. To maintain stable current output and reduce anode grounding resistance, chemical backfill should be used around sacrificial anodes in soil, mainly composed of 75% calcium sulfate, 20% bentonite, and 5% sodium sulfate. Sacrificial anodes should not be buried in coke. When used in groups, the distance between anodes should be at least 3m. The soil cover thickness above the anode should be at least 0.6m. To measure the off-potential, sacrificial anodes should be connected to the pipeline through a test box. When sacrificial anodes are used near AC traction systems, the continuous AC induced voltage on the anode body should not exceed 20V.
Anode Requirements
1. The potential must be sufficiently negative but not too negative to avoid hydrogen evolution reactions in the cathode region;
2. The anode polarization rate should be low, and the potential and current output should be stable;
3. The anode material should have a high electrical capacity;
4. It must have high current efficiency;
5. It should dissolve uniformly and detach easily;
6. The material should be inexpensive and readily available;
7. The corrosion products produced should be non-toxic, harmless, non-polluting, and pose no environmental hazards.
Understanding
Electrochemical oxidation of metals or alloys. The metal or alloy component is used as the anode, and an electrolytic method is employed to form an oxide film on its surface.

The metal oxide film alters the surface state and properties, such as surface coloring, improving corrosion resistance, enhancing wear resistance and hardness, and protecting the metal surface. For example, aluminum anodizing involves placing aluminum or its alloys in a corresponding electrolyte (such as sulfuric acid, chromic acid, or oxalic acid) as the anode and performing electrolysis under specific conditions and with applied current. The aluminum or its alloy oxidizes at the anode, forming a thin layer of aluminum oxide on the surface, with a thickness of 5–20 micrometers. Hard anodized films can reach 60–200 micrometers. After anodizing, aluminum or its alloys exhibit increased hardness and wear resistance, up to 250–500 kg/mm2, good heat resistance (the melting point of hard anodized films is as high as 2320K), excellent insulation (withstand voltage up to 2000V), and enhanced corrosion resistance (withstand thousands of hours in ω=0.03 NaCl salt spray without corrosion). The oxide film layer contains numerous micropores that can adsorb various lubricants, making it suitable for manufacturing engine cylinders or other wear-resistant parts. The strong adsorption capacity of the film micropores allows for coloring into various beautiful and vibrant hues.

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Pei Yingying 1862587  9268