What is the principle of ship cathodic protection?

The core principle of ship cathodic protection is to convert hull metal (such as steel) into a cathode in a corrosive battery through electrochemical means, thereby inhibiting its oxidation reaction (i.e., corrosion).

This technique is mainly achieved by two methods: the sacrificial anodic method and the applied current method.

1. Sacrificial anode method (passive protection)
Principle: Install metal blocks with a more negative potential (more lively) on the hull (such as zinc, aluminum or magnesium alloys) to form a galvanic battery with the hull steel. At this point, these metal blocks preferentially corrode as a "sacrificial anode", releasing electrons to flow to the hull, making the hull protected as a cathode.
Reaction process :
Anode (zinc/aluminum): Zn → Zn²⁺ 2e⁻ (self-dissolved)
Cathode (hull): O₂ 2H₂O 4e⁻ → 4OH⁻ (inhibits the oxidation of iron)
Features :
No external power supply required, simple installation and maintenance;
Suitable for small vessels or areas with complex structures (e.g. ballast tanks);
The anode will gradually wear out and need to be replaced regularly.

2. Applied current method (active protection, ICCP)
Principle: Using an external DC power supply (potentiostat) and an auxiliary anode, electrons are forcibly injected into the hull so that it is always in the cathode state. The current flows from the auxiliary anode to the hull surface through seawater, inhibiting the corrosion reaction.
Core Components :
DC power supply (automatically adjusting output);
Auxiliary anodes (e.g., MMO mixed metal oxide anodes);
Reference electrodes (e.g., silver/silver chloride electrodes for monitoring hull potential and feedback control).
Advantages :
The current is adjustable to adapt to different navigation environments (such as high-salinity seas);
It is suitable for large ships and has a design life of more than 20 years.
Significantly reduce maintenance costs and docking frequency.

3. Key parameters and standards
Protection Potential: Hull potential is typically maintained in the range of -0.85V to -1.20V (relative to CSE) to ensure effective protection.
Current density: Depending on the coating condition, it is generally about 15 mA/m², and it may be as high as 110 mA/m² (such as ballast tank) when the coating is damaged.
Zonal protection strategy: High corrosion areas such as the bottom, stern, and propeller need to focus on the deployment of anodes or enhance current output.

4. Practical application and development trend
Modern large ships often use hybrid protection schemes: the applied current system(ICCP) is the main one, sacrificing the anode as a local supplement (such as near the rudder and propeller). 

At the same time, combined with the intelligent monitoring system, the current is adjusted in real time to improve protection efficiency and safety. 

The International Maritime Organization (IMO) and major classification societies have listed it as a mandatory safety measure, which can be called the "lifeline" of ship anti-corrosion.