Magnesium has a low current capacity. Like a bowl of nuts, it’s quickly eaten. Zinc and aluminum are akin to a plate piled high with home cooking. The meal lasts longer. So, though magnesium offers more protective electrons faster, it protects for only about one-third the time of zinc, and just one-fifth the time of aluminum. In a hypothetical situation where zinc would last 150 days, magnesium would last but 50 days, while an aluminum anode would have corrosion lined up at the chow line for 225 days.
Additionally, the high rate of electrons served up by magnesium can create too much current — i.e., “overprotect” — when used to protect a metal that itself is already very active, like aluminum, especially one used in waters that are more conductive of electricity. This means salt water and polluted fresh water. In fact, if you take a freshwater boat protected by magnesium anodes to the coast and put it in a slip for a week, you’ll have a catastrophe on your hands.
So, how do you choose which anode type to use on your boat? Cost helps answer the question. Magnesium is more expensive than zinc, and zinc is more expensive than aluminum. Additionally, comparing zinc and aluminum, besides their carrying capacities, which bear on your length of season, zinc forms an insulating oxide film over itself, especially in fresh water, that stops it from working. In fact, if you have an anode that seems never to have worn away, it’s probably oxidized and not doing its job! By mixing iridium and other metals with aluminum, alloys are created that don’t form aluminum oxide.
See the chart (above) showing general guidelines for choosing an anode type based on the metal needing protection and its location.
There are other considerations. For example, if your boat is connected to shore power, and thus connected to all the other boats at the dock, you may choose “weaker” zinc over aluminum to minimize the electrical activity. But if your boat has a galvanic isolator, or you’re not plugged in, the more active anode might be a better choice.
Confused? Here’s the takeaway.
The principles presented should have raised your anode awareness. They give you the basis to raise the right questions. Seek out a mechanic with ABYC “Marine Electrical” or “Marine Corrosion” certification and have the conversation. Don’t just buy an anode kit because it has your engine’s brand on it.