The principle of aluminum alloy sacrificial anode is to use the potential difference of different metals to provide electrons for the protected metal, so that the protected metal is in a state of excess electrons as a whole, and the potential of each point on the metal surface is reduced to the same negative potential, so that there is no potential difference between the points on the metal surface, no more electron flow, and metal atoms no longer lose electrons and become ions dissolved into the solution. Ultimately, the purpose of slowing down corrosion is achieved. Conditions that aluminum alloy sacrificial anode materials should have:
1. The polarization rate of the anode should be small, and the potential and current output should be stable; in this way, the potential of the sacrificial anode does not move much in the positive direction (the result of anode polarization) during operation; the raw materials are abundant, the production and processing are easy, and the price is low; it is uniformly activated and dissolved during operation, and no insoluble corrosion products are deposited on the surface, so that the anode can work stably for a long time;
2. The sacrificial anode has a sufficiently negative stable potential. That is, there should be a sufficiently large open circuit potential difference between it and the protected metal to prevent hydrogen evolution reaction in the cathode area; it will cause the coating to separate from the pipeline, that is, cathode peeling, which will not only make the anti-corrosion layer ineffective, but also consume a lot of electricity, and cause hydrogen embrittlement fracture of the metal material.
3. The capacitance of the anode material should be large; the corrosion products produced by the sacrificial anode during operation should be non-toxic and harmless, non-polluting the environment, and non-hazardous; the self-corrosion rate (self-dissolution amount) of the aluminum alloy sacrificial anode during operation should be small, and the current efficiency should be high.
Advantages: no need for frequent maintenance; no external power supply; low cost, cost saving; convenient and simple, easy to install; effective provision of uniform current distribution; easy to add anodes in most cases; small current output leads to small or no stray current interference.
Disadvantages: has a lower driving voltage/current; may be ineffective in high resistivity soil environment; it is difficult to replace the waste anode, and the cost is also relatively expensive; more anodes are required for structures with inferior coatings; aluminum alloy sacrificial anodes have a higher cost per ampere current than impressed current cathodic protection due to their lower current efficiency (self-corrosion consumption).