When to replace anodes and where they go: inspection checklist

The main rule: half the weight

Most manufacturers recommend replacing an anode once it has lost about half of its original weight or volume. Beyond that the protection current drops. The anode may also be gone before the next lift, and then the propeller, shaft or hull starts to corrode. If the next inspection is a long way off, replace earlier. On ships, at each dry-docking replace every anode that will not last until the next one.

A useful habit: note the original weight of each anode and weigh the anodes you remove. The weight is often cast on the anode or given in the specification. This tells you the real consumption rate on your vessel and lets you choose an anode weight that matches the maintenance interval.

Where anodes are fitted

Inspection checklist

• Wear: how much of the original weight or volume is left. Half or less — replace.
• Wear pattern: even wear is normal. Heavy pitting on one side — check the wiring and neighbouring metals.
• Dense white or grey crust on an almost intact anode: possible passivation, typical of zinc in brackish water. Clean the anode and consider switching to an aluminium alloy — see zinc vs aluminium anodes.
• Paint on the anode: remove it or replace the anode.
• Fasteners: bolts, nuts and collars intact and tight, no corrosion under the fixing.
• Contact: resistance between anode and structure, measured with a multimeter, close to zero.
• Surrounding metal: pitting on the propeller, pinkish patches on brass parts (dezincification), paint blisters next to the anode — signs of protection problems.
• Alloy: all anodes on the structure are the same metal — zinc or aluminium.

How often to check

• Boats — every time the boat is lifted, at least at the start and end of the season.
• Ships — at every dry-docking and diver inspection; tank anodes during tank inspections.
• Engine heat exchangers — according to the engine maker’s service schedule.
• Quays and sheet piling — during scheduled diver inspections. Protection can also be checked without lifting anodes, by measuring the structure potential.

How to replace an anode properly

• Use the same alloy as on the rest of the structure. When switching from zinc to aluminium, replace all anodes at once.
• With the same alloy, weight and size no smaller than before: a lighter anode will run out sooner. An aluminium anode of the same shape is much lighter than a zinc one but lasts at least as long.
• Remove the old anode and clean the mounting surface back to bare metal.
• Do not paint the new anode, and keep paint off it when the hull is painted.
• After fitting, check the contact with a multimeter.

Can’t find the right anode, or is the vessel an older one? Send us the old anode, a photo with dimensions or a drawing — we will make an anode of the same shape and weight.

Anodes made to sample or drawing

We cast zinc and aluminium anodes of any shape and weight: hull, tank, shaft and propeller, quay anodes. For regular fleet maintenance we can keep the items you need in line with your replacement schedule.