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Why anodes, and what material?
Anodes: a brief summary.
Sacrificial anodes are necessary to protect the exposed metal in seawater. Whether it’s a steel hull, stainless shaft, bronze propeller or thru-hull, without sacrificial anodes these metals would lose electrons either to each other, or another material in the environment through the electrolytic seawater. The sacrificial anodes do two things: they turn their less noble galvanic selves into the anode of the system so that they donate their electrons through seawater instead of other boat parts, and these anodes can also deposit their electrons onto the metal they are ‘protecting’- a process similar to electroplating. Current through the shore ground or generated from another source (from the new vessel down the dock) will occasionally speed up the process of decay, but is not nearly as common or severe as the suspicion of it is. Stray current leaving your vessel, although rare can be a concern as well, and that evidence can be noted through dezincification or paint halos around fittings.
Sacrificial anodes need to be balanced (not over or under protected) with the amount of exposed metal underwater, and special considerations also need to be given to the salinity and hull material, especially wood, when choosing how much mass and what anode material to use.
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ZINC or ALUMINUM ?
A common question locally.
Zinc is still the more common anode worldwide on large ships as well as recreational vessels, which doesn’t make them better, just more commonly available. There are other factors specific to our region to consider- largely the freshwater element. When exposed to freshwater, zinc forms an oxide layer which isolates it from any further corrosion, reducing its effective protection. During the rainy season, the surface layer of Puget Sound (especially Shilshole and the Duwamish area) can become more fresh/brackish than saltwater too. It took some time for us to get on board with aluminum, but the alloyed material of the improved aluminum anodes makes them more effective for both salt and fresh water- especially in our local environment. Over the years, we have seen good results with aluminum, and aluminum is also less toxic to our local waters, so that’s nice to the fish too. Overall, our recommendation is aluminum anodes, and close to 50% of boats on our schedule have gone to aluminum. If you want to stay with zinc that’s fine too, but never mix anode materials; a vessel should either be all aluminum, or all zinc, all the time.