Why Sacrificial Anodes Matter

Put two different metals in seawater and connect them electrically, and you have built a battery. Current flows, and one of the metals dissolves to feed it.

A sacrificial anode is a lump of metal chosen specifically to be the one that dissolves. It is meant to disappear. That is the entire design.

What happens when they are gone

The corrosion does not stop when the anode is consumed. It simply moves to the next most reactive thing in the circuit — your propeller, your shaft, your drive leg, the inside of your heat exchanger.

This is the part owners underestimate. An anode that has completely disappeared is not “due for replacement soon”. It has been doing nothing for however long it has been gone, and something else has been dissolving instead.

Replace at about half consumed, not when they are gone.

The anodes almost nobody checks

Hull and drive anodes are visible and usually get changed at antifouling. The engine’s internal anodes are not visible, and on a great many boats they have never been touched:

• Heat exchanger pencil anode — threaded into the housing
• Aftercooler anode on many turbocharged engines
• Oil cooler and gearbox cooler anodes on some installations

A boat with immaculate hull anodes and an untouched heat exchanger anode is extremely common. The heat exchanger is the thing quietly dissolving.

Choosing the right alloy

A boat that moves between salt and fresh water — up a river system, for instance — needs this considered rather than assumed. So does a boat that has just moved berths.

Contact is as important as the anode

An anode only works if it is electrically connected to what it protects. Paint, corrosion or a poorly cleaned mating face insulates it, and then it sits there looking healthy while the metal it was supposed to protect corrodes.

Never paint an anode. Clean the mating face to bare metal when fitting.

When wastage rate changes, listen

A sudden change in how fast your anodes disappear is diagnostic. Possible causes:

• Stray current from a wiring fault — this is fast and destructive, and it can consume a propeller in a season
• Galvanic interaction through shore power, sometimes from a neighbouring boat
• The wrong alloy for the water you are now in
• Warmer water — tropical berths legitimately consume anodes faster

If yours are disappearing much faster than they used to, investigate rather than simply fitting more.

Frequently asked questions

How often should anodes be replaced? At about half consumed. How long that takes varies enormously with water temperature, salinity, shore power and how the boat is used.

Does my engine have anodes inside it? Very likely — a pencil anode in the heat exchanger at minimum. It is the most commonly neglected item on a marine engine.

Zinc or aluminium? Aluminium is widely used in salt and brackish water; zinc remains common in salt. Magnesium is fresh water only.

Can I paint over anodes when antifouling? No. A painted anode provides no protection at all.

Why are mine disappearing so much faster this year? Something changed — a wiring fault, a new neighbour on shore power, a different berth, or a move to warmer water. Worth an electrical check.