Re: Anodes?
+1^^ Galvanic corrosion happens when a boat is plugged into shore power. Since you don't plug your boat in and leave it in the water, no protection is needed
I disagree, You are describing stray current corrosion not Galvanic corrosion.
Galvanic corrosion : Corrosion that occurs at the anode of a galvanic cell.
You may remember high school chemistry class where a battery was created by connecting two dissimilar metals with wire and immersing the whole contraption in water thereby activating magnetic fields and starting an electrochemical process causing current to flow.
On a boat with bronze, aluminum, galvanized and stainless steel that are connected with grounding wires and immersed in the lake...... you accomplish the same thing. The more noble metal is the "cathode", the less noble, the "anode". In this process the less noble metal gives up electrons to the more noble thus weakening the metal, otherwise known as "galvanic corrosion".
The "sacrificial" anodes on your shafts, trim tabs etc. are supposed to sacrifice themselves thereby protecting expensive metal parts. This is why it's important to keep your anodes or "zinc's" in good condition and never paint them.
A vessel suffering from galvanic corrosion is usually the source of it's own problem, although two vessel's linked by shore power grounds can create a galvanic cell between two very close boats.
2. Stray Current corrosion : Corrosion that results from an electrical source causing a metal in contact with an electrolyte (water) to become anodic with respect to some other metal in the same electrolyte.
In simple terms a wire touches something it shouldn't, like a faulty bilge pump float or degraded wiring lying in the bilge sending current into the water, causing one metal to give up electrons and corrode. Again any vessel suffering from this type of corrosion is likely the master of it's own disaster but the culprit could also be a neighboring vessel. This type of corrosion can can eat metals at an alarming rate. I know of one 42' motoryacht that lost both shafts, both rudders and both propellers in a space of less than two weeks.
Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring through the common ground on board or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shorepower grounds there is potential for widespread damage. Aside from concerns of corrosion there is also potential for electrocution if shorepower cords are allowed to lie in the water let alone the fools that leave their shorepower cord plugged in at the dock while they go out for an afternoon cruise.
Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a slower rate than DC. This has been a long argued issue by people who know a lot more about this than me. Ground fault protection systems, galvanic isolators, isolation transformers and impressed current systems are some of the various methods attempting to combat corrosion.
Salt water is generally regarded as a more serious breeding ground for marine corrosion as the salt makes it more conductive however, polluted fresh water can be even more conductive with the right contaminants.
