Mercury - Aluminum Trim Tab Anode Corrosion Protection & Steering Aid Cast Aluminum 31640T4. Overview. Corrosion is one of an engine's biggest enemies, and the Mercury Marine 31640T4 Aluminum Trim Tab Anode is built to stop it in its tracks. Designed for Mercury and Mariner 30-200HP outboards and MerCruiser sterndrives (except Bravo), this OEM anode protects your lower unit from galvanic corrosion while also providing a small hydrodynamic trim force to help balance steering. Made from high-quali