Every steel hull submerged in seawater corrodes. Cathodic protection exists to reverse this process: instead of the steel losing metal, it begins to receive electrons from another source. There are two ways to do this: sacrificial anodes (also called galvanic anodes) and impressed current cathodic protection (ICCP). Both methods work. The choice depends on the vessel’s size, operating profile, and lifetime cost.
How Cathodic Protection Works, in a Nutshell
Corrosion is an electrochemical reaction: the steel in the hull loses electrons and turns into oxide. Cathodic protection supplies these electrons from the outside, turning the hull into the cathode of a cell. The component that loses metal becomes the anode.
The effectiveness of the protection is measured by the potential of the hull relative to a reference electrode. The DNV-RP-B401 recommended practice, an international standard for cathodic protection design, specifies −0.80 V relative to the Ag/AgCl electrode in seawater as the protection potential for carbon steel. Potentials less negative than this indicate underprotection, i.e., corrosion. Potentials that are too negative (overprotection) can cause paint to peel and, in high-strength steels, lead to hydrogen embrittlement.
Sacrificial (galvanic) anodes
These are blocks made of a metal alloy that is more “active” than steel—usually aluminum, zinc, or magnesium—that are welded or bolted to the hull. They corrode on their own, without any external energy, protecting the surrounding steel.
Advantages
– Simple, no electronics, no power supply
– Low initial cost
– Ideal for small vessels and specific areas (rudder, propeller, sea water intakes, tanks)
Limitations
– The protection is not adaptive: it does not respond to changes in salinity, temperature, speed, or damage to the paint
– Weight and hydrodynamic drag on large hulls
– They need to be replaced periodically, usually during dry docking
– In fresh or brackish water, performance varies significantly depending on the alloy
ICCP — Impressed Current Cathodic Protection
The ICCP uses inert anodes (typically titanium coated with mixed metal oxides, Ti-MMO) powered by a rectifier. Reference electrodes measure the hull’s potential at all times, and the control panel automatically adjusts the current to maintain protection within the optimal range.
Advantages
– Automatic and adjustable protection: compensates for variations in salinity, temperature, speed, and paint wear.
– Few anodes for large hulls; less weight and less drag
– Long anode life: Ti-MMO KC LTD anodes last at least 15 years
– Enables continuous monitoring, and soon remote monitoring, with MCP Pulse.
Limitations
– Higher initial cost
– Depends on electricity and specialized technical maintenance
– If improperly adjusted, it can provide excessive protection and damage the paint. That is why an annual inspection is important
Quick Comparison
| Criterion | Sacrificial anodes | ICCP |
|---|---|---|
| Protection source | Metal Alloy Consumption | Rectifier + inert anodes |
| Automatic adjustment | No | Yes |
| Initial cost | Low | Higher |
| Lifetime Cost (Large Hulls) | Higher (replacements, weight, drag) | Lower |
| Maintenance | Periodic Replacement | Technical Inspection and Calibration |
| Recommended for | Small vessels, localized areas | Medium and large ships, FPSOs, platforms |
| Monitoring | Visual Inspection | Continuous, including remote |
Can you use both of them together?
Yes, and it’s common. Many ships use ICCP on the hull and sacrificial anodes in specific areas, such as sea boxes, thruster tunnels, and tanks. The important thing is for the design to take into account the interaction between the systems.
How to Decide
- Vessel size and type: Above a certain size, the ICCP is usually worth it
- Operating profile: Users who frequently change regions, speeds, or water types will benefit from the automatic adjustment
- Time between dockings: Long intervals favor ICCP
- Current system status: It is often cheaper to reactivate or upgrade an existing ICCP than to replace it
Do you need to evaluate your system?
MCP Marine supplies ICCP systems with Ti-MMO K.C. LTD anodes and a complete line of galvanic anodes made of aluminum, zinc, and magnesium. It also performs inspections, maintenance, and repairs—including anode replacement by submersion —on systems from any manufacturer.
Contact a technician: WhatsApp +55 21 99512-0824 · info@mcpmarine.com.br
By André Macena, Project Director at MCP Marine, with over 16 years of experience in cathodic protection and marine systems.
