Aluminium Anodes

The conventional alloy is Al-Zn-In: pure aluminium does not stay active in sea water, so zinc and indium are added. The US military standard MIL-DTL-24779 calls for 4.0–6.5 % zinc, 0.014–0.020 % indium and 0.08–0.20 % silicon, with iron below 0.08 % and copper below 0.005 %.

As conservative design values, DNV-RP-B401 uses 2,000 Ah/kg and −1.05 V against an Ag/AgCl reference electrode for aluminium in sea water; the practical values in the ABS guidance are about 2,500 Ah/kg and −1.09 V. Against zinc's 780 Ah/kg, that works out at roughly 2.5 to 3 times less mass for the same charge.

The former European standard for galvanic anodes, EN 12496, was replaced in 2025 by EN ISO 9351, which covers their use in sea water and saline sediments. Our aluminium alloy is LR type approved, and every heat ships with its analysis report.

Aluminium works in sea water, brackish water and seabed mud, and keeps protecting in estuaries where zinc can passivate. It is not normally effective in fresh water, where magnesium anodes are used. Capacity falls as temperature rises: in the ABS guidance it drops from 2,500 Ah/kg at 25 °C to 500 Ah/kg at 80 °C.

On a ship, aluminium anodes are fitted to the hull, the rudder and ballast tanks; in tanks they go mainly in the lower part, as several small pieces rather than one large one. Offshore platforms usually rely on galvanic protection because the design is simple and robust and needs no power, and aluminium is normally the preferred alloy there. Cathodic protection of jetties, quays and harbour piles is covered by ISO 13174.

A caveat for tankers: on ships carrying cargo with a flash point below 60 °C, aluminium anodes in cargo tanks and adjacent ballast tanks are limited to a falling energy of 275 J, because falling aluminium can strike a thermite spark. The limit is set by weight times height above the tank bottom; a 10 kg piece, for example, must sit lower than 2.8 m. The rule does not apply to ballast tanks not adjacent to cargo tanks, and the falling-energy limit does not apply to zinc anodes either.

Aluminium anodes are also cast on a steel insert, which is welded or bolted to the structure. Where one is fixed by any method other than welding, DNV-RP-B401 requires a continuity cable of at least 16 mm², and the resistance to the structure must never exceed 0.1 Ω. On a newbuilding, they are fitted before launching.

The working face is never painted and is protected during coating work; only the back face of flush-mounted types, facing the structure, may be coated. The replacement rule is the same as for zinc: renew it when about half has been consumed or when what remains will not last until the next dry-docking, and replace it with aluminium again.

Take care when removing old stubs: early aluminium alloys contained mercury, which concentrates in the remains, so precautions are needed, especially when heat is used. For general advice on choosing and sizing, see our sacrificial anode guide.