Mercury - Quicksilver 821631T3 Magnesium Anode Kit Freshwater Use MerCruiser R, MR, Alpha, Bravo, X, XR & XZ Drives 821631T3. Overview. Freshwater corrosion may not be as visible as saltwater damage, but it's just as harmful over time. The Quicksilver 821631T3 Magnesium Anode Kit is designed specifically to protect your MerCruiser sterndrive from corrosion in freshwater environments.. Made from high-purity magnesium, this OEM kit provides the strongest level of galvanic protection available for