Magnesium Hanging Anode — Freshwater Berth

Magnesium Hanging Anode — Freshwater Berth

A suspended magnesium anode on a cable, dropped over the side and bonded to the vessel. For boats lying in a freshwater berth, and for laid-up vessels where the hull anodes are out of the water or already spent.

Why magnesium in fresh water

Fresh water is far more resistive than sea water, and zinc cannot drive enough current through it — it polarises, skins over and stops protecting while still looking perfectly serviceable on the hull. Magnesium is the fresh-water alloy. If your vessel works the Great Lakes, the St Lawrence, or any inland waterway, zinc is the wrong metal no matter what the chandlery stocks.

Sizes and weights

Size	Weight	Notes
Suspended anode with 4 m cable — 1.0 kg	1.0 kg (2.20 lb)	for berthed or laid-up vessels

Dimensions in millimetres with inches alongside. Weights are nominal and confirmed at quotation.

Alloy — ASTM B843

Magnesium anodes are cast to ASTM B843, which covers two alloys, and the choice between them matters more than most people expect:

Alloy	Open circuit potential	Use
AZ63 — standard potential (Al 5.3–6.7%, Zn 2.5–3.5%, Mn 0.15–0.7%)	≈ −1.55 V	The default. Steel and GRP hulls in fresh water, and the right choice near aluminium.
M1C — high potential (Mn 0.5–1.3%, Fe ≤0.03%, Ni ≤0.001%, Cu ≤0.02%)	≈ −1.70 to −1.75 V	High-resistivity water and soil, where the extra driving voltage is needed to push current.

A caution on aluminium

More driving voltage is not better. Aluminium suffers alkali attack and hydrogen blistering of its paint when driven more negative than about −1200 mV, and high-potential magnesium will do exactly that. On aluminium hulls, outdrives and sterndrives use the standard-potential alloy and watch the protection potential. Blistering paint and unusually fast anode wastage are the symptoms of over-protection, not of needing more anode.

How magnesium compares

	Zinc	Aluminium	Magnesium
Water	Salt	Salt and brackish	Fresh only
Capacity	≈ 780 A·h/kg	≈ 2000–2500 A·h/kg	≈ 1100 A·h/kg
Relative life, same size	100	130–150	≈ 30
Specification	ASTM B418	no composition standard — qualified by test	ASTM B843

Magnesium drives harder than either of the others but spends itself doing it — roughly a third of the life of zinc for the same size. That is the trade, and it is why magnesium belongs in fresh water and nowhere else. Put it in salt and it will waste in a season.

Sizing and replacement

Sizing comes from ABYC E-2 for boats and DNV-RP-B401 for ships. Whatever the calculation says, replace at about 50% wasted — output falls away as surface area is lost.

Wrong alloy for your water? See saltwater anodes — zinc to ASTM B418 Type I and aluminium.