Galvanic corrosion appears in two forms:
Active - where electric currents from either the mains or batteries/charging system passes through the water and onto a metal part of the boat. If this happens the metal object is likely to be eaten away.
This could happen if a shoreline has a poor connection in the neutral. This would cause a volt-drop so the neutral on the boat and the neutral ashore is at a slightly different voltage. The neutral is earthed so there is a potential current path between boat and "bank".
Always seek specialist advice on mains installation.
We have already looked at problems relating to 12/24 volt positive connections hanging in bilge water and the issue of using a steel hull as a return path.
PASSIVE - when dissimilar metals are immersed (or made damp) they form a battery cell and current flows. The current is "made" by breaking down one of the metals - the one forming the negative plate of the cell.
Those old enough to have started motoring in the 60s will remember shiny trims falling off cars, leaving great rust holes where the clips had been. Galvanic corrosion between aluminium pop rivets and the steel of the body caused this. If you look nowadays, you will find such trims are held in place by little plastic "pockets" which insulate the dissimilar metals.
Providing you do nothing odd, the average GRP boat owner can ignore this if in fresh water because your underwater gear will be made from resistant materials like stainless steel or bronze. (River Weaver etc excepted).
Owners of metal boats MUST take note wherever they are used.
GRP & Wooden boat owners on the sea should take note. Remember a wooden boat's hull fixings are at risk!
The further apart two metals are in the galvanic series, the greater the current that will be produced, and thus the greater the corrosion.
Note how far apart graphite and zinc are - the plates of an ordinary dry cell.
Brass is a mixture of copper and zinc. Note how far apart they are. This is why when you try to change a tap washer in an old house the jumper usually crumbles and looks pink - the zinc has been corroded out of the brass.
The yellow metal on a boat should be gun metal or bronze. Bronze is a mixture of copper and tin that are close together, and thus resist corrosion.
The salt water enhances the electrical action so the effects would be greater.
On Steel boats
Even on fresh water, there is still enough "gap" between steel and bronze to cause problems so action has to be taken.
To combat galvanic corrosion chunks of a very negative metal are fixed to the hull. Zinc for the sea and magnesium for fresh water. These are electrically connected to all the other submerged metal parts of the boat.
The result is the chunk of metal - the anode - is eaten away and the other metal parts are left intact.
On steel/metal fitting is easy - you just weld or bolt the required number of the correct size on. The hull itself ensures electrical connection.
Wood and GRP hulls normally require bolting through the hull and internal bonding cables being run from the anodes to the metal parts.
Some anodes can be obtained in "clamp on" form which allows them to be fitted directly to prop shafts, rudders and P/A brackets. These will not require bonding, but they will only protect items in electrical contact with what they have been fitted to.
Anode manufacturers are happy to advise on optimum size and location.
Never paint or anti-foul anodes - if you do they will not be in electrical contact with the water and will thus not work.
Large Hull Anodes should be inspected every two years.
Small clamp on anodes might need changing mid-season, only experience can tell.
NEVER "Copper" antifoul outdrives or outboard legs. If you do the copper might cause damaging corrosion of the alloy case.