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Application

Sacrificial anode protection: Electrochemical protection achieved by obtaining a protective current from a corrosion cell formed by connecting an auxiliary anode to the protected metal.

This method is often used to prevent rust. Sacrificial anode protection essentially involves attaching a more reactive metal piece, usually zinc or magnesium. This means that the more reactive metal will be oxidized to form metal ions, thereby reducing iron and preventing it from turning into iron hydroxide (rust). Thus, the more reactive metal "sacrifices" itself for the iron. This is a common method typically used to prevent large steel objects, such as underground pipelines and oil tankers, from rusting.

System Requirements

Sacrificial anodes are generally only economically applied to structures requiring small protective currents and in environments with low soil resistivity. Additionally, they are valuable when there is no power supply or when it is uneconomical to use one.

Sacrificial anode materials suitable for soil are primarily magnesium, while zinc and aluminum are used in seawater. To maintain stable current output and reduce anode grounding resistance, sacrificial anodes in soil should be surrounded by chemical backfill, 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 spacing 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 via 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 area;

2. The anode polarization rate should be low, with stable potential and current output;

3. The anode material must 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, and not pollute the environment, posing no public hazard.

Materials

As sacrificial anode materials, they must meet the following requirements:

1. Have a sufficiently negative and stable potential;

2. Exhibit low self-corrosion rates and uniform corrosion, with high and stable current efficiency;

3. Have a high electrochemical equivalent, meaning a large current output per unit weight;

4. Experience minimal anode polarization during operation, dissolve uniformly, and have easily detachable products;

5. The corrosion products should not pollute the environment or pose public hazards;

6. The materials should be widely available, easy to process, and inexpensive.

Common types of sacrificial anodes include magnesium-based, zinc-based, and aluminum-based alloys.

Zinc is a relatively common metal in daily life. In the periodic table, zinc has an atomic weight of 65.4, a density of 7.14, a valence of 2, and a melting point of 420 degrees Celsius.

Zinc is a negatively potential metal with a standard potential of -0.76V. In seawater, the stable potential of high-purity zinc shifts negatively. The corrosion rate of zinc varies with pH: it is higher when pH is less than 6 or greater than 12, but relatively low within the pH range of 6–12.

Impurities can significantly affect the anode corrosion rate and behavior of zinc. The presence of impurities can form local corrosion cells, which may lead to the formation of hydroxides on the zinc surface, creating a robust protective layer that prevents further dissolution. This layer formation mechanism can be utilized in cathodic protection systems.

Cathodic protection for ships includes external protection of all underwater parts, including attachments and open areas, as well as internal protection of various cabin pipelines and bilges.