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Zinc has protected ships for more than a century, and it still does the job well. Zinc anode replacement in Nagoya is straightforward, but it pays to know what you are fitting and when it needs to be changed.
Zinc corrodes steadily in sea water and gives up its metal in place of the steel it is attached to. It is stable, predictable and cheap. Its working potential in sea water is about minus 1.05 volts against a silver-silver chloride reference electrode, which is enough to protect steel. Many class societies and ship designers specify zinc for hulls, rudders and propeller areas, and it is the usual choice for ships that trade between sea and brackish water.
Aluminium alloy anodes deliver much more electrical capacity per kilogram, commonly about 2,500 amp-hours per kilogram against roughly 780 for zinc. This makes them lighter for the same life, and many large ships use them. Aluminium may behave less well in brackish water and in some tanks, and it is not always allowed. The ship's anode plan and class rules decide, and we fit what the plan calls for.
The port area receives fresh water from rivers, so salinity can vary. Zinc generally performs well in these conditions. In very warm water or in mud, some zinc anodes can form a hard film that reduces their output. Using good quality anodes made to a recognised standard reduces the risk. We use anodes with test certificates when the owner supplies them, or supply certified ones ourselves.
Many technical managers replace a zinc anode when it has lost about half to two-thirds of its original mass. Anodes near the propeller and rudder usually wear faster. If a ship has been idle for a long time or has suffered coating damage, wear can be quicker. We measure and photograph each anode and report the percentage left.
Bolted zinc anodes are unscrewed and replaced with new ones. Welded anodes have their steel core welded to the Hull. The Diver cleans the area, welds the new anode and checks the connection. A good electrical connection matters as much as the anode itself. For propeller and shaft anodes, the Diver checks the fixing screws and locking.
We need the anode plan or a list of positions. If it is not available, we inspect and create a simple record. The job usually takes 2 to 5 hours. The report lists each anode, its condition, what was replaced and any damage to the coating around it.
Q1. What is the electrical capacity of zinc compared with aluminium?
Zinc delivers roughly 780 amp-hours per kilogram, while aluminium alloy anodes deliver about 2,500 amp-hours per kilogram.
Q2. At what wear level are zinc anodes usually replaced?
Many technical managers replace them when about half to two-thirds of the original mass is lost. Anodes near the propeller and rudder wear faster.
Q3. What is the working potential of zinc in sea water?
About minus 1.05 volts against a silver-silver chloride reference electrode.
Q4. How long does anode replacement take?
Normally 2 to 5 hours, depending on how many anodes need to be changed.
