WATER CRAFT

This section provides a brief overview of galvanic anodes and how they apply to water craft. For further advice contact an accredited Marine Surveyor.

Aluminium Vessels

For aluminium used in hull construction, the primary aluminium is mixed with additional elements to create an alloy that has improved mechanical properties and greater corrosion resistance (e.g. Marine Grade Aluminium). This is in contrast to aluminium anode alloys, where additional elements are mixed with the primary ingot to make the anode actively corrode.

Aluminium grades used in hulls can be subject to corrosion from acid and alkali degradation of the natural protective oxide layer on the surface of the aluminium. Damage to this layer can render the hull susceptible to corrosion. Aluminium or zinc anodes and protective coatings help protect aluminium hulls against corrosion.

Alkali problems on aluminium hulled vessels can be the result of too many anodes being fitted. This is best controlled by adhering to the manufacturer’s recommendations on the installation of anodes, by assessing the hull using a reference electrode such as a Rust Seeker or by seeking advice from a corrosion engineer.

It is considered good practice to use a doubler plate for attaching anodes to the hull (see photo on right). This ensures that mechanical damage due to unexpected grounding does not tear studs out of the hull plating, jeopardising the integrity of the hull.

The action of cathodic protection can produce an alkaline solution in the immediate vicinity of the anode, so it is recommended that a sound hull coating be applied to doubler plates and surrounding areas.

Steel Vessels

Steel is a very robust material. If the hull is well maintained with a good marine coating and suitable galvanic anodes, the vessel should last for many years. A good understanding of basic corrosion principles and how they apply to a steel vessel and a suitable maintenance schedule are critical in ensuring that steel vessels are adequately protected against corrosion.

Fibreglass Vessels

Corrosion on fibreglass vessels is usually restricted to the shaft/propeller, rudders and skin fittings. Most corrosion can be controlled by installing shaft anodes or bonding to a hull anode.

To reduce drag on racing yachts, owners often suspend anodes over the side on cables when moored and withdraw them during races. Professional advice should be sought if owners wish to follow this practice, as the vessel has no cathodic protection during races.

Note: Osmosis on fibreglass hulls is the result of osmotic or ionic migration of moisture through poorly applied fibreglass which results in delamination of the fibreglass from the matting. Such defects do not usually occur on hulls manufactured under strict factory conditions. Such damage generally has no relationship to Cathodic protection.