I am a bit late to this, but FYI: Galvanic corrosion is caused by two different types of metal in water that are electrically connected. The “less noble” metal is the one that corrodes, which is why we use zincs (or magnesium or aluminum). There has to be a path between the metals for this to happen. This can get much much worse when on shorepower because all the boats in the marina end up wired together via the green ground wire connection. If no one buys new zincs but you, guess who supplies the entire marina with anode material? YOU DO! Galvanic isolators are 100% needed for any boat on shore power for this reason.
The other form of corrosion is stray current corrosion. This does not need different metals to work, just electricity flowing through the water. The current moves metal from one place to the other, this is how electroplating works. This can be very complicated to track down and can do a vast amount of damage very quickly. There are a lot of different ways this can happen, but imagine a boat with the keel connected to the ground bus, a negative ground engine, and a SSB ground plate with DC connection through to the radio. We now have 3 different underwater metals connected to the negative side of the DC system. If the onboard ground bus and wiring are not perfect, some current will flow through the water too. Joe Coquina
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