Magnesium Anodes

Magnesium anodes come in two main alloys. The standard Mg-Al-Zn alloy (AZ63 type) contains 5–7 % aluminium, 2–4 % zinc and 0.15–0.7 % manganese, with iron limited to 0.005 %, copper to 0.08 % and nickel to 0.003 %. The high-potential Mg-Mn alloy contains 0.5–1.5 % manganese and less than 0.05 % aluminium. The applicable standards are ASTM B843 and MIL-A-21412.

In the ABS guidance, the standard alloy runs at about −1.5 V against an Ag/AgCl reference electrode and the high-potential alloy at about −1.7 V. Measured against steel's −0.80 V protection criterion, that is a driving voltage of roughly 0.7–0.9 V, against only about 0.2 V for zinc. ABS describes magnesium as suitable for fresh water because of this high potential, and unsuitable for highly conductive sea water.

Magnesium's electrochemical capacity is about 1,200 Ah/kg, but its current efficiency usually stays around 50 %: a large part of the metal dissolves through self-corrosion without producing protective current. At about 1.7–1.8 kg/dm³ it is the lightest of the three metals.

Magnesium anodes are for fresh water only: river and lake craft, yachts kept in fresh water and the fresh water side of heat exchangers are their main uses. In fresh water, zinc and aluminium become coated with an off-white oxide crust and stop working, while magnesium stays active. Our sacrificial anode guide explains which alloy suits which water.

Magnesium is not used in sea water or brackish water: its high potential over-protects and it is consumed very quickly. In brackish water choose aluminium anodes, and in sea water aluminium or zinc anodes. If a boat is moving to salt water, switch it to a suitable alloy.

Magnesium is not accepted in ship tanks: the ABS guidance allows it only in fresh water tanks not adjacent to cargo tanks, and US rules (46 CFR 35.01-25) prohibit it in cargo tanks carrying flammable or combustible liquids. ABS also notes that magnesium is not used on the aluminium hulls of seagoing ships; on aluminium boats and drives, use magnesium only in fresh water and as the manufacturer recommends.

Our magnesium anodes come in hull, shaft and rudder models. They are fixed by bolting or welding their steel inserts; a shaft collar is made of two halves bolted together around the shaft. A shaft collar must not unbalance the shaft or restrict the water flow to the strut bearing.

The surface is never painted, and its electrical contact with the hull must stay low-resistance for its whole life. Replace a magnesium anode when about half of it has gone, or when what remains will not last until the next haul-out, and fit magnesium again.