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Anode Requirements

1. The potential must be sufficiently negative but not overly so to avoid hydrogen evolution reactions in the cathode region;

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

3. The anode material must have a high electrical capacity;

4. It must exhibit high current efficiency;

5. Dissolution should be uniform and easy to detach;

6. The material should be inexpensive and readily available.

7. The resulting corrosion products should be non-toxic, harmless, environmentally friendly, and pose no public hazard.

Materials

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

1. Possess a sufficiently negative and stable potential;

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

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

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

5. Corrosion products must not pollute the environment or pose public hazards;

6. 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 metal with a negative potential, 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 levels: it is higher when pH is less than 6 or greater than 12, but relatively low within the pH range of 6 to 12.

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

Cathodic protection for ships includes external protection of underwater areas, covering all attachments and exposed surfaces, as well as internal protection for various cabin pipelines and bilges.

I. Cathodic Protection for Areas Below the Waterline of Ships

Implementing cathodic protection on ships without coatings is practically impossible or highly uneconomical due to the required protective current and current distribution. Additionally, an electrical insulation layer should exist between the ship's steel plates and anti-fouling coatings to prevent the electrochemical reduction of toxic metal compounds. Cathodic electrolysis products cannot prevent marine fouling; on the contrary, if cathodic protection is applied, marine organisms may attach to inert copper materials during natural corrosion.

Depending on the scope of protection, full protection or partial protection of the ship's underwater areas should be considered. In partial protection, only the stern is protected, which is particularly vulnerable due to high water flow rates, aeration, and the formation of corrosion cells on components like propellers and rudders. Partial protection can also extend to the bow, but the bow is similarly affected by high water flow rates. Since mechanical damage to coatings often occurs at the bow and midship, full protection of ships using sacrificial anodes or impressed current methods has become increasingly important. Installing sacrificial anodes on the bilge keel (the curved part between the bottom and side of the ship, which provides stability) poses no issues, and their protective range can extend to components like propellers and rudders. Alternatively, these components can be individually protected based on the ship's design and protection method.

In all cases, local or entire aluminum alloy or stainless steel hulls must undergo cathodic protection. This also applies to high-alloy steels containing more than 20% chromium and 3% molybdenum, as they are prone to crevice corrosion under coatings. Cathodic protection design must be tailored to specific conditions.

Pei Yingying   186258   79268