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Canada Metal / Martyr Anodes

Canada Metal (Martyr Anodes) is the world's largest anode manufacturer. It is number one in magnesium (fresh water), zinc (sea water) and aluminum (salt and brackish water such as the Black Sea and the Sea of Marmara). Anodes for pleasure boats and engines from Volvo, Bombardier, Mercury and other well-known makers are produced by Canada Metal.

Official catalog Martyr Catalogue 2025 View catalog

Which anode type should I use?

Choose zinc, aluminum or magnesium according to the water. Magnesium is for fresh water, zinc for sea water and aluminum for salt and brackish water (Black Sea, Sea of Marmara).

For Mercruiser boats with AC shore power and no galvanic isolation, use the Merc-A table. For Mercruiser boats without shore power or galvanic isolation, use the Merc-B table.

Anode color guide

Aluminum

Salt and brackish water (Black Sea, Marmara).

Magnesium

For fresh-water boats.

Zinc

Classic protection in sea water.

Useful notes on anodes

Anode weight calculation

For steel and aluminum hulls the required anode weight (lb) is:

Anode weight (lbs) = [(Wetted surface) × (Current density) × (Time in water)] / [(Capacity) × 1000]

  • Current density: painted steel 1.5–5.0 mA/sq-ft; painted aluminum 0.2–3.0 mA/sq-ft
  • Time in water: hours in the replacement interval (1 year = 8,766 hours)
  • Capacity: Mil-Spec zinc 368, Mil-Spec aluminum 1,108 amp-hours/lb
  • Wetted area: full displacement 1.0 × LWL × (Beam + Draft); medium 0.75; light 0.50

Source: ABYC Corrosion Certification Study Guide.

Zinc anode standard Mil-Spec A-18001K

US military standard Mil-Spec Zinc Alloy A-18001K requires zinc alloyed with small amounts of aluminum and cadmium, with iron limited to 0.005 percent. The alloy forms a thin zinc-oxide film that keeps fresh alloy in contact with sea water so the anode stays electrochemically active. Zinc anodes that do not meet this standard can foul and fail to protect the metals they are fitted to.

What is galvanic corrosion?

Galvanic corrosion occurs when dissimilar metals, such as a steel shaft and a bronze propeller, sit in an electrolyte like sea water. Current flows from the more noble metal to the less noble one and the less noble metal is lost.

Zinc is among the least noble metals. Fitted to the hull, propeller, rudder or engines, it takes the corrosion. That is why these anodes are called sacrificial anodes. Inspect them regularly and replace them at 50 percent of original size. An anode that never wears is either poor quality or not making metal-to-metal contact.

Zinc vs aluminum anodes

Mil-Spec zinc and Mil-Spec aluminum anodes protect steel and aluminum boats in salt and brackish water. Zinc has usually been cheaper; exchange prices have narrowed that gap.

  • Zinc MIL-A-18001K: −1050 mV, 0.258 lbs/in³, 368 amp-hours/lb
  • Aluminum Mil-A-24779(SH): −1100 mV, 0.105 lbs/in³, 1,150 amp-hours/lb
  • Service life vs zinc: aluminum about 120%
  • Cost vs zinc: aluminum about 80%

Voltage is versus silver / silver-chloride electrodes in sea water.