The anode: first-class anti-rust

Have you ever noticed any blocks of metal hanging from the hull of a ship? Those are anodes. They protect the ship from corrosion or rust.

Maritime & Nautical Stories & blogs

Rust and corrosion: enemy of shipping

When you immerse metal in a conductive liquid such as water, a small electric current is created that causes corrosion or rust. This is a problem for ships. With ships, the hull, rudder and keel are especially vulnerable. Corrosion can cause pitting there. Even under the paint layer, the steel may gradually begin to dissolve. At best, this causes loss of efficiency; at worst, it causes serious rust leaks.

Not every type of metal rusts at the same rate. High-quality metals such as stainless steel, bronze or gold corrode less quickly than ordinary steel or aluminium.

Roest en corrosie tasten de romp, het roer en de kiel van een schip aan.

The solution? The anode!

Fortunately, there is a solution to delay the rusting of the ship: the anode. The idea is simple: you divert the electric current to pieces of inferior metal that you attach to the ship. These blocks, the anodes, will rust away first, protecting the ship. This is called 'cathodic protection' in the jargon.

The British chemist Sir Humphrey Davy, also known as the inventor of the mining lamp, first applied this system in England in 1824.To protect wooden ships from the dreaded pile worm, among other things, the ship is given a lining of copper plates underwater. These copper plates are in turn protected by blocks of soft iron. The least noble metal, the soft iron, dissolves, but the copper remains free of corrosion.

Now, steel ships and bronze propellers are protected by zinc or aluminium blocks on the ship's wall: the anodes. Only from the first half of the 20th century did shipbuilders generally apply this technique. Anodes can be found on almost all ships today: even on polyester ships for protection of the bronze propeller and propeller shaft.

Anodes should never be painted, because otherwise you change the tension of the metal. The attachment of the anode should be of ordinary steel, preferably welded to the vessel. Do you hardly recognise the anode because it is almost completely rusted or dissolved? That means it works!

Anodes op de romp van een schip beschermen de metalen bekleding tegen roest en corrosie.

An anode for every type of ship

So the choice of materials is very important in the construction of a ship. Designers and shipyards will always try to use as many materials as possible with equal tension. When this is not possible, the materials should be insulated as far as possible.

The number of anodes needed varies from ship to ship. For each new ship, specialist companies prepare an anode plan. In this, they specify how many anodes are needed, what weight they should be and where on the ship they should be placed. It depends on a great many factors: the size of the ship, the materials used, the area of the ship that is underwater and the sailing area. For example, a steel ship sailing primarily in salt water usually receives zinc anodes. If that same steel ship sails primarily in fresh or brackish water, the anode is usually aluminium.

An anode needs replacement when it has rusted away 70 to 80%. In practice, an anode lasts about 5 years. That roughly coincides with the period of class renewal, the inspection of the ship. At that time, the ship will also receive some new anodes.

The cost price for an anode depends on the weight and/or material. A 4-kg aluminium anode costs about €30. For an average tug at Port of Antwerp-Bruges, the total cost of replacing all the anodes comes to about €2,500.

Anodes leiden de corrosie af en gaan zelf roesten. Daarom moeten ze regelmatig vervangen worden.

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