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Maintenance

Anodes and protection against galvanic corrosion

How cathodic protection works, how to match the anode material to the water and when to replace it.

The superyacht Lady Lau in Bonifacio

Galvanic corrosion happens when two different metals are electrically connected and immersed in a common electrolyte (water), forming a cell. The less noble metal (the anode) corrodes, protecting the more noble one (the cathode). On a yacht the parts at risk are the propeller shaft, the propeller, the seacocks, the rudder, the ladders and the engine heat exchangers. Cathodic protection means deliberately adding an even less noble metal — a sacrificial anode — which gives itself up, corroding instead of the valuable parts.

Author: Maciej Baniewicz · Boat & Yard editorial team

Methodology: The material was prepared from the sources given and from practical service rules; the parameters have to be confirmed in the manual for the specific manufacturer or model.

The choice of anode material depends on the type of water and is the key decision. In salt water zinc anodes are used, in fresh water magnesium (zinc passivates in fresh water and stops protecting), and in brackish water (river mouths, the coastal Baltic) usually aluminium anodes, which work over a wide range of salinity. Using zinc in fresh water is a common mistake that leaves the protected parts unprotected and corroding. Aluminium is now often recommended as universal and less harmful to the environment.

The effectiveness of the protection depends on electrical continuity (bonding). All protected underwater metal parts should be connected to the anode by a common bonding conductor, so that they form one system. The anode must have a clean, bare metal contact surface — never paint the anode or the contact face, because paint insulates and switches the protection off. Check as well that the anode is firmly tightened onto the protected part (for example the collar on the shaft), because a loose contact breaks the circuit.

Anodes come in many formats matched to the component: ring anodes for the shaft, propeller and prop nut anodes, hull plates, outboard and heat exchanger anodes (engine anodes are often hidden inside the casing — easy to forget). Match the shape and mass to the protected area — an anode that is too small wears out early and stops protecting, one that is too large does no harm but is uneconomical. Do not forget the anodes inside the cooling system of an engine with a heat exchanger.

Checking and replacing is seasonal routine. A practical rule: replace an anode when it is about 50% consumed, without waiting for it to disappear, because the remains have poor contact and little output. If anodes vanish unusually fast, that signals a problem: stray currents from the electrical system, the wrong material, or the influence of neighbouring boats in the marina (current from the quay). An anode that does not wear at all is suspicious too — it may be insulated by paint or a poor contact.

In marinas with shore power there is the added risk of corrosion from leakage currents between boats through the common earth conductor. Galvanic isolators or isolation transformers are used here; they break the path of the corrosive DC current while keeping the shock protection intact. This matters especially for yachts that stay connected to shore power for long periods. The level of protection is checked with a meter and a reference electrode (hull potential against Ag/AgCl or Cu/CuSO4).

To sum up good practice: match the anode material to the water, ensure a clean contact and continuity of the bonding, check the anodes every season (propeller and shaft anodes more often), never paint them and replace them at 50% wear. Keep spare anodes on board and note how fast they are consumed — a sudden change is an early signal of a problem. Manufacturers such as MG Duff, Vetus and Volvo Penta publish catalogues for selecting anodes for specific parts and shafts.

Material prices — products and options

Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.

Sacrificial anodes (zinc / aluminium / magnesium by water type)
Shaft, propeller and hull anodes matched in size
Engine / heat exchanger anodes
Cable and terminals for bonding (equipotential connections)
A wire brush / sandpaper for cleaning the contact
Spanners for fitting the anodes
A galvanic isolator or an isolation transformer (shore power)
A meter with a reference electrode for checking the potential

Recommended materials and sources