Guide 06 Marine engineering and ship repair
Cathodic protection on ships: anodes vs ICCP
Sacrificial anodes and impressed current cathodic protection (ICCP) both stop a ship's steel hull corroding in seawater, but work differently. Anodes corrode in place of the hull and need no power; ICCP uses a DC supply, inert anodes and reference electrodes to control protection automatically, which suits larger hulls.
- Topic
- Marine engineering and ship repair
- Checked
- 23 September 2026
- Sources
- 4
- Questions
- 5
01
Sacrificial anodes against ICCP
Sacrificial anodes, made of zinc, aluminium or magnesium alloy, are more reactive than steel, so in seawater they corrode in place of the hull. They need no power supply or controls, but they are consumed over time, add a small amount of drag, and cannot be adjusted once fitted.
Impressed current cathodic protection uses a DC power supply feeding inert anodes, commonly platinised titanium, together with reference electrodes that measure hull potential. A controller raises or lowers the anode current automatically to hold the hull at the right protection level, which is why ICCP suits large hulls better than a fixed set of sacrificial anodes would.
02
How often hull anodes are replaced
Hull anodes are sized to last the planned interval between dockings, so they are inspected at every docking and renewed once substantially consumed. A widely cited marine reference treats about five years as a typical replacement life for hull anodes.
Unusually fast wastage points to a specific problem rather than normal service life: stray current from another system, a paint breakdown exposing bare steel nearby, or an electrical bonding fault, and any of these is worth investigating rather than simply fitting a bigger anode.
03
Reference electrodes in an ICCP system
In an ICCP system, reference electrodes, often zinc cells, are fitted port and starboard between the anodes, at points where hull potential is likely to be lowest. They measure the hull-to-seawater potential continuously so the controller can respond to it.
That feedback lets the controller raise or lower anode current to keep the hull cathodic without over-protecting it, which matters because too much current can damage the paint system around the anodes rather than protect the steel underneath it.
04
Checked at the same dry docking as the coating
Cathodic protection is not inspected on its own: a class surveyor examining a ship in dry dock checks anodes and coating together with the sea chests, sea valves and bottom shell, as part of the standard hull survey scope rather than a separate visit.
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Cathodic protection on ships: anodes vs ICCP
What is the difference between sacrificial anodes and ICCP on a ship?
Sacrificial anodes of zinc, aluminium or magnesium alloy corrode in place of the hull steel and need no power, but they are consumed, add drag and cannot be adjusted. Impressed current cathodic protection (ICCP) uses a DC supply, inert anodes such as platinised titanium and reference electrodes to control protection automatically, suiting large hulls.
How often should hull anodes be replaced?
Hull anodes are sized to last the planned interval between dockings, so they are inspected at every docking and renewed when substantially consumed. One widely cited marine reference treats about five years as a typical replacement life. Unusually fast wastage points to stray current, a paint breakdown or a bonding fault.
What is a reference electrode in an ICCP system?
In an ICCP system, reference electrodes, often zinc cells, are fitted port and starboard between the anodes where hull potential is likely to be lowest. They measure the hull-to-seawater potential so the controller can raise or lower anode current automatically, keeping the hull cathodic without over-protecting it and damaging the paint.
What does unusually fast anode wastage usually mean?
Anode wastage much faster than the planned five-year or so replacement life usually points to a specific cause rather than normal service: stray current from another electrical system, a breakdown in the surrounding paint exposing bare steel, or a fault in the hull's electrical bonding. Each is worth checking rather than just fitting bigger anodes.
Does a class surveyor check cathodic protection during dry docking?
Yes. In dry dock the surveyor examines anodes and coating condition together with the outside bottom and shell, rudder and pintle clearances, propeller, sea chests and sea valve fastenings, as part of the standard hull survey rather than as a separate inspection.
Checked against
Sources
- 01Hull corrosion and impressed current cathodic protection (ICCP) on ships, Marine Insight
- 02What is a sacrificial anode, and why ICCP systems are a better solution, Evac (excerpt)
- 03UR Z21 Rev.4, Surveys of Propeller Shafts and Tube Shafts, International Association of Classification Societies, hosted by Türk Loydu
- 04About IRClass, Indian Register of Shipping
This guide explains how the system works in general, as the documents above describe it, and was last checked on 23 September 2026. Rules and thresholds change: the governing document is the authority, not this page.
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