When he hauled his boat in December, 2022, Gary DeSanctis, president of Active Interest Media Marine Group, publisher of Power and Motoryacht, got an unpleasant surprise: The submerged section of the mounting bracket holding his 200-hp outboard was badly pitted by corrosion, and so was the nose of the lower unit. (Like many outboards, the four-stroke doesn’t tip up far enough to lift the lower unit fully clear of the water.) DeSanctis is careful to keep everything up to snuff, yet he was blindsided by rampant electrons flowing through his underwater metal. This could happen to you, too.
Immersing dissimilar metals that are in electrical contact with each other—through the boat’s bonding system, for instance—in salt water, generates a flow of electrons between them. Salt water is an electrolyte, and this creates essentially a low-voltage battery. The lessernoble, or anodic, metal, e.g., aluminum, magnesium, zinc, gives up electrons to the more noble, cathodic, metal, e.g., silicon bronze, Monel, passive stainless steel, and deteriorates in the process. This is galvanic corrosion, and it’s why
