Yacht anodes: choosing zinc, aluminium or magnesium

Sacrificial anodes protect adjacent underwater components by corroding more readily than those components. Their condition, material and contact with the water all matter. An anode that is painted, isolated or unsuitable for the water may not provide the intended protection.

For yachts based in Mallorca, the normal operating environment is salt water, but cruising plans can change the correct choice. This guide covers zinc, aluminium and magnesium, the replacement point, coating mistakes and checks around shafts, propellers, trim tabs, thrusters and drives.

How sacrificial anodes protect underwater components

An anode is deliberately made from a more vulnerable material. It corrodes to protect adjacent components from galvanic corrosion, so some loss of anode material is expected.

Condition has to be interpreted rather than judged by appearance alone. An anode that has lost about half its material is due for replacement under Mercury's guidance. At the other extreme, a completely unworn anode is not automatically healthy. Mercury notes that no visible wear can mean damaging corrosion is occurring on more critical parts.

The inspection therefore needs to answer several separate points:

• Is the anode being consumed? Some corrosion of the sacrificial material shows that it is not simply remaining untouched.
• How much remains? Compare the remaining body with its original form and use the yacht or drive manufacturer's criterion.
• Is it exposed to water? Paint and other coatings prevent the direct contact that the anode needs.
• Does the alloy suit the water? Zinc, aluminium and magnesium are not interchangeable across every operating environment.

Where corrosion concerns include shafting, propellers or drive components, coordinate the anode assessment with a broader shaft and propulsion inspection.

Choosing the alloy for the yacht's actual water

MGDUFF's material guide specifies zinc or aluminium for salt water, aluminium for brackish water and magnesium for fresh water. The yacht manufacturer's anode specification remains part of the decision, so water type alone should not be used to override the specified alloy.

For a Mallorca-based yacht remaining in sea water, MGDUFF identifies zinc or aluminium as the relevant choices. Confirm which of those materials the yacht or equipment manufacturer specifies.

Brackish water needs separate treatment. Zinc can develop the same passivation problem that zinc and aluminium face during prolonged fresh-water exposure. The resulting white oxide crust can seal the anode and stop it working, including when the boat returns to salt water.

Magnesium belongs in fresh water under MGDUFF's guide. It is not a salt-water upgrade. In salt water, its accelerated activity shortens its life and can produce a difficult calcareous deposit.

Deciding when replacement is due

Mercury recommends replacing anodes when about half the material has been lost to corrosion. This is a useful manufacturer threshold, not a substitute for a yacht, drive or equipment manual that gives a different criterion.

Do not rely only on elapsed time. The supplied guidance calls for frequent inspection, particularly in salt water, while the replacement decision depends on material loss and the relevant manufacturer's instructions. A planned dive or haul-out provides an opportunity to compare all exposed anodes rather than assessing only the easiest one to see.

Record each location separately:

• Shaft and propeller anodes: note material loss, coating condition and marine growth.
• Trim-tab anodes: check that they are present, exposed and not painted.
• Thruster anodes: inspect the exposed anodes and record their condition.
• Drive anodes: compare remaining material with the manufacturer's replacement criterion.

A missing anode or one that is more than roughly half consumed needs engineering attention. So does an anode that remains inexplicably untouched while corrosion is visible elsewhere. The second condition can be more serious than an anode showing normal sacrificial loss because an undamaged appearance does not prove that protection is taking place.

Preventing paint and antifouling mistakes

Anodes must remain in direct contact with the water. Never paint, antifoul or otherwise coat their exposed surfaces. During work on nearby underwater gear, mask the anodes or remove them before applying the coating.

This point should be included in the work scope whenever underwater paint is renewed. The gelcoat, paint and antifouling service is closely related to anode condition because coating nearby gear without masking or removing the anodes can leave them covered.

After coating work, inspect every listed location rather than checking a single obvious anode. Shafts, propellers, trim tabs, thrusters and drives may have separate exposed anodes, and each one needs clear water contact.

A thin scum film is different from deliberate paint or coating. Mercury says light scum can be removed with a stiff brush. Do not cover the cleaned surface afterwards.

Paint blistering, white powder and deep pitting on underwater aluminium are signs of galvanic corrosion. These findings call for immediate inspection of the anodes and the wider corrosion system, rather than simply replacing the visibly damaged coating.

What crew can check safely

Crew checks should concentrate on visible condition, material selection and accurate reporting. Carry them out during an appropriate dive or haul-out when the exposed underwater components can be seen.

• Inspect all exposed locations. Check shaft, propeller, trim-tab, thruster and drive anodes. Record material loss, coating condition and marine growth for each location.
• Photograph the condition. Clear images can show whether an anode is missing, coated, heavily consumed or apparently untouched. They also give the engineer a structured view of separate locations.
• Remove light scum. Use a stiff brush on accessible anodes where only a thin scum film is present. Do not paint or coat the cleaned anode.
• Check the recorded alloy. Compare the installed anode material with the yacht's normal water type and the manufacturer's specification.
• Report a water-type change. Tell the engineer if the yacht will move between salt and fresh water for more than two weeks. MGDUFF recommends changing anodes for that transition.
• Look at nearby aluminium. Report white powder, paint blistering or deep pitting rather than treating these as cosmetic defects.

Visible findings can also support investigation through marine electrical and electronics service, particularly where anodes are electrically isolated or remain untouched while corrosion appears elsewhere.

When to call an engineer

Arrange an inspection when the remaining material, water exposure or surrounding corrosion makes the anode's condition uncertain. Do not assume that fitting another anode of the same material is correct if the yacht's operating water has changed.

Call an engineer when:

• Consumption has reached the replacement point. The anode is more than roughly half consumed, or the yacht or drive manual calls for replacement under a different criterion.
• An anode is missing or covered. Painted and coated anodes cannot maintain the required direct contact with water.
• The anode appears isolated. An electrically isolated anode requires investigation rather than a cosmetic clean.
• There is no explainable wear. An untouched anode combined with visible corrosion elsewhere can indicate that critical parts are being damaged.
• Corrosion signs appear. White powder, paint blistering or pitting on underwater aluminium, shafting or drive components warrants immediate anode and corrosion-system inspection.
• The water type will change. Seek advice when moving between Mediterranean salt water and fresh water for an extended period and the correct alloy is uncertain. MGDUFF's change guidance applies when the move lasts more than two weeks.

Blueward Marine can assess anode, marine electrical and propulsion concerns in Mallorca. Submit photographs and fault details through online service booking, or call the 24/7 line on +34 971 251 374.