2 min read · Updated 30 June 2026
Galvanic corrosion is the slow electrical corrosion that eats a boat's underwater metals — and sacrificial anodes are the cheap defence against it. Whenever different metals sit together in seawater, they form a natural battery that corrodes the weaker one. Anodes work by giving themselves up first, protecting the propeller, shaft, keel bolts and fittings.
How galvanic corrosion works
Seawater is an electrolyte, so when two different metals are electrically connected in it, they form a galvanic cell — a natural battery. In that cell:
- The less noble (more reactive) metal corrodes preferentially.
- The more noble metal is protected.
On a boat, the various underwater metals — propeller, shaft, keel bolts, fittings — can set up these cells, and without protection the weaker metal steadily corrodes away.
Sacrificial anodes
The fix is a sacrificial anode — a lump of a reactive metal deliberately fitted to the underwater metals so it corrodes in their place. Because the anode is less noble than what it protects, it becomes the part of the cell that gives itself up, wasting away while the keel, prop, shaft and fittings are spared.
Why they're called "zincs"
Traditionally anodes were made of zinc, so sailors often call them "zincs" whatever the metal:
- Zinc — traditional, suits seawater.
- Aluminium — now widely used in seawater too.
- Magnesium — used in fresh water.
The right anode metal depends on the water the boat sits in.
Checking and replacing
Anodes are inspected regularly and replaced as they waste away — commonly at the annual haul-out, alongside the antifouling. A good rule: replace an anode before it's more than about half gone, so it keeps protecting. A fully wasted anode protects nothing, leaving the boat's valuable underwater metals exposed.
Why it matters
Without anodes, the least noble underwater metal — perhaps an expensive propeller, shaft or keel fitting — corrodes away instead, causing serious and costly damage over time. That's why anodes, though small and cheap, are considered essential protection and a standard part of a boat's maintenance. For the vocabulary, see the sailing terms glossary.
Frequently asked questions
- What is galvanic corrosion?
- Galvanic corrosion happens when two different metals are electrically connected in seawater, which acts as an electrolyte, forming a natural battery or galvanic cell. In that cell the less noble (more reactive) metal corrodes preferentially while the more noble one is protected. On a boat, the various underwater metals — propeller, shaft, keel bolts, fittings — can set up these cells, and without protection the weaker metal steadily corrodes away.
- What is a sacrificial anode?
- A sacrificial anode is a lump of a reactive metal — traditionally zinc, and also aluminium or magnesium — fitted to a boat's underwater metals so that it corrodes in their place. Because the anode metal is less noble than the metals it protects, it becomes the part of the galvanic cell that gives itself up, corroding away while the propeller, shaft, keel and fittings are spared. The anodes are then checked and replaced as they waste away.
- Why are anodes called zincs?
- Because zinc was the traditional metal used for sacrificial anodes, so sailors often call anodes zincs regardless of the metal. Zinc suits seawater, while aluminium anodes are now widely used too, and magnesium anodes are used in fresh water, because the right anode metal depends on the water the boat sits in. The nickname zincs has simply stuck as a general term for sacrificial anodes.
- How often should anodes be replaced?
- Anodes are inspected regularly and replaced when they have wasted away significantly — commonly checked and renewed at the annual haul-out, though how fast they erode depends on the boat, the metals aboard and the water. A good rule is to replace an anode before it is more than about half gone, so it keeps providing protection. An anode that has completely wasted away is no longer protecting anything, leaving the boat's valuable underwater metals exposed.
- What happens if a boat has no anodes?
- Without sacrificial anodes, the boat's underwater metals are left to fight galvanic corrosion unprotected, so the least noble of them — which might be an expensive propeller, shaft or keel fitting — corrodes away instead. Over time this can cause serious and costly damage, weakening or destroying important components. This is why anodes, though small and cheap, are considered essential protection and are a standard part of a boat's maintenance and haul-out routine.