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Anodes are the cheapest insurance a ship has. They corrode on purpose so that the Hull and the propeller do not. Ship anode replacement in Nagoya is a simple job that keeps the corrosion protection working between dockings.
A sacrificial anode is made of a metal that corrodes more readily than steel. When it is connected to the Hull in sea water, it gives up metal in place of the steel. As long as the anode is in good condition, the Hull is protected. When it is used up, protection stops and corrosion begins at the damaged coating and exposed metal.
Ships carry anodes on the Hull plating, near the stern, on the rudder, around the propeller, in the sea chests and on other underwater fittings. The number and layout are set by the ship's design and class requirements. Some ships also have an impressed current system, which uses electrodes and a power supply instead of sacrificial metal.
The Diver checks every accessible anode, noting its condition, the percentage consumed and any damage to the attachment. The result is recorded with photographs. If an anode is gone or nearly so, it is marked for replacement. An inspection often finds missing anodes after berth contact or heavy fouling.
Anodes can be bolted or welded to the Hull. Bolted anodes have studs and nuts, and replacing them is quick. Welded anodes are attached by a steel strap or core that is welded to the plate. The Diver cuts or breaks the old connection, cleans the surface and welds the new anode, using wet welding. The weld is checked for good electrical contact.
Where the ship has an impressed current system, the Diver inspects the electrodes and reference cells and checks that the coating shields around them are intact. Damage to electrodes is reported. Repair may require specialist parts and the system maker's advice.
A normal anode inspection and replacement takes 2 to 5 hours, depending on how many anodes need work. The ship should send us the anode plan if it has one. We bring the anodes or use the ones supplied by the ship, and the engine room confirms that the propeller is locked.
After the job, you receive photographs of each anode before and after, the number replaced and notes on any other findings. This supports the technical file and any class requirement.
Anodes are made from zinc, aluminium alloy or, in some cases, magnesium. Zinc is common and works well in sea water. Aluminium anodes are lighter and last longer for the same weight, which is why they are often used on large ships and in tanks. The class society and the ship's plan specify which type to use, and we follow that specification. Using the wrong type can reduce protection.
There is no single rule, but many technical managers replace an anode when it has lost about half to two-thirds of its original mass. Anodes near the propeller and rudder wear faster, as the flow is greater. A ship that has been in warm or brackish water may use up its anodes more quickly than expected.
Q1. What does a sacrificial anode do?
It corrodes in place of the steel, protecting the Hull and propeller as long as the anode is in good condition.
Q2. How are anodes attached to the Hull?
Bolted anodes use studs and nuts, while welded anodes are attached by a steel strap or core welded to the plate. Both can be replaced by a Diver.
Q3. How long does an anode inspection and replacement take?
Normally 2 to 5 hours, depending on how many anodes need replacing.
Q4. What happens if anodes are not replaced?
Corrosion begins at damaged coating and exposed metal, which can lead to wasted plating and propeller damage.
