An Impressed Current Cathodic Protection (ICCP) system protects the underwater hull from corrosion by making the ship’s hull the cathode of an electrochemical cell. A rectifier supplies controlled DC current to inert anodes, while the hull receives the return current and is protected from corrosion. ICCP systems on ships use a DC source and inert anodes such as MMO/titanium, with automatic regulation based on hull potential measured by reference electrodes.

Working:

1. AC supply is fed to a transformer-rectifier unit.
2. The rectifier converts AC to low-voltage DC.
3. The positive terminal is connected to inert anodes (usually titanium/MMO) fitted externally on the hull.
4. The negative terminal is connected to the ship’s hull.
5. Current flows from anodes → seawater → hull.
6. The hull becomes cathodic, so corrosion of hull steel is prevented.
7. Reference electrodes (silver/silver chloride / zinc type) measure hull potential.
8. The automatic controller adjusts output current so hull potential remains within the protective range, avoiding under-protection or over-protection.

Main components

• Transformer/rectifier
• Automatic control panel
• Inert anodes
• Reference electrodes / potential sensors
• Hull bonding cables and monitoring arrangement

Rudder

1. The rudder may be electrically insulated by bearings/pintles, so bonding is required.
2. Protection is ensured by:
   • flexible bonding straps / cables across rudder stock, carrier bearing or pintles
   • sometimes supplementary sacrificial anodes on rudder
3. This ensures the rudder remains electrically continuous with the hull and receives cathodic protection.

Propeller

1. The propeller shaft is often electrically insulated from the hull by the oil film in stern tube and bearings.
2. Therefore, ICCP current may not protect the propeller effectively.
3. Protection is ensured by fitting a shaft earthing / shaft bonding device:
   • slip ring on shaft
   • silver/graphite brushes to hull earth
4. This provides electrical continuity between shaft/propeller and hull, and also prevents bearing pitting due to shaft potential. A turning propeller is often insulated from the hull by the lubricating oil film, so a shaft earthing device with brushes and slip ring is used to avoid bearing damage and improve protection.

• Do not overprotect the hull: Excess current can damage paint coating and may cause hydrogen effects on high-strength steel.
• Maintain electrical continuity: Ensure proper bonding of rudder, shaft, stabilizers, thrusters, sea chests, etc.
• Inspect anodes and reference cells regularly: Keep them clean, undamaged, and properly insulated from hull structure where required.
• Check and calibrate control system: Reference electrodes and controller must be tested periodically for correct hull potential.
• Avoid stray current interference: Careful cable insulation and earthing arrangement to prevent corrosion of nearby fittings.
• During dry dock: Switch off ICCP before docking/undocking and inspect anodes, shields, and hull coating condition.