Mercury - Magnesium Gearcase Anode Corrosion Protection Magnesium 8M0179229. Overview. There's a certain kind of quiet confidence that comes when your outboard is protected the right way-especially if you spend your time on freshwater lakes where corrosion creeps up slowly and quietly. You don't notice it on that sunny morning when you idle away from the dock, but you feel it years later if the engine wasn't protected. That's why a good anode isn't just a small block of metal; it's peace of mind