How galvanic corrosion works
Galvanic corrosion needs three things. Two different metals. An electrolyte to carry current between them. And an electrical connection joining them. Your boat hands over all three without anybody trying — aluminum gearcase, stainless prop shaft, bronze through-hull, bolted together into one thing and sitting in the water. Get current flowing and the less noble metal starts giving itself up so the more noble one doesn't have to. Not orange rust, either. It just dissolves, slowly, from the surface in.
An anode is a lump of metal picked on purpose because it's less noble than everything else on the boat. Bond it into the same electrical path and it becomes the metal the current eats, so the gearcase, the trim rams and the prop get left alone. That's the whole design. A pitted, half-gone anode hasn't failed. It's been working. Failure looks like the opposite: an anode still smooth and shiny at the end of a season usually isn't connected, got painted over, or is the wrong alloy.
The alloy has to match the water
Three alloys are in common use, and you can't swap them around. Zinc's the salt-water alloy. Magnesium is the fresh-water one. Aluminum is the brackish compromise, and it works properly in fresh water too, which is why a lot of manufacturers fit it as standard now. Nobody made that up to sell anodes. Every alloy sits at its own voltage next to the metals it's guarding, and how it behaves depends on how well the water conducts. Fit the wrong one and it's not a compromise. The boat has no protection at all.
Zinc's the trap. Put a zinc anode in fresh water and it passivates — a hard, pale crust of oxide spreads across its face, insulates it, and the current it puts out falls to next to nothing. The anode looks fine. It stays in one piece and does nothing. So a lake boat on factory zincs is basically unprotected, while the owner looks at an anode that's barely touched and figures everything's healthy. Lake Winnipesaukee is fresh water end to end, roughly seventy-one square miles of it, no salt anywhere in its twenty-one-mile length. This applies to every boat on it.
Magnesium's the most electrically active of the three, and it gives the strongest protection in low-conductivity fresh water. It's also used up fastest, so check it more often, and keep it off any boat that spends part of the year in salt. Trailering somewhere brackish? Aluminum's the practical default. Never mix alloys on one boat; they end up working against each other instead of against the corrosion. And don't go by color. Read the casting marks.
Where the anodes live
On an outboard, the obvious one is the trim tab above the prop. It's a working anode as well as a steering trim device. There's usually a plate anode on the gearcase, often anodes on the transom bracket and the trim cylinders, and on many engines an internal anode inside the cooling passages or the exhaust housing that nobody ever looks at. Skip one of those hidden ones and even a well-looked-after engine ends up corroded.
Sterndrives carry more, because more of them lives underwater. There's one on the drive itself. Then the gimbal ring or transom assembly, the trim cylinders, and quite often a ring or a disc tucked behind the prop. Keep the boat on a trailer and it spends a lot less time wet, so those anodes can go seasons. Good news, right? Except that's exactly why they get forgotten, painted over in a touch-up, or left to corrode at the mounting face until they've lost electrical contact altogether.
Stray current is a different and worse problem
Galvanic corrosion is the slow background stuff. Stray-current corrosion is another thing entirely: an electrical fault pushing current through the water and out through the boat's underwater metal. Usually it's shore power, a wiring fault in a dock pedestal, a pump wired badly into the bonding, or a neighboring boat on the same dock circuit. It's far more aggressive than galvanic action. It can pit a gearcase badly in a few weeks, not a few seasons.
You'll know it when you see it. Anodes that vanish in one season. A white, powdery bloom on the aluminum castings. Paint peeling off a drive in sheets, with clean metal loss underneath. Damage that's worse on one side of the boat, or that only showed up after the boat moved to a certain slip. New anodes won't fix it, because the anodes aren't the problem. It needs electrical diagnosis of the boat and the dock circuit and then, depending on what turns up, a galvanic isolator or a fix to the shore-power ground path.
What to check, and when
Check every anode three times a year: at spring launch, around midsummer, and when she comes out (on this lake, that's whenever your marina decides to haul). Swap one out at about half gone. Waiting for it to vanish costs you, since a shrinking anode puts out less as it goes. Never paint one. And when a new one goes on, clean the mounting surface down to bare, bright metal on both sides.
Check the fastener too. The anode has to be electrically continuous with the casting it's protecting, so one corroded stud or a grimy mating face and the whole thing's switched off without anyone noticing. Anodes vanishing unusually fast, or not wearing at all? That's a diagnostic result in itself, and worth chasing before the next launch. If a drive's already showing pitting or paint lift, get it looked at while it's still a coating problem, not a casting one.
