Scientists discovered a couple of centuries ago that when you fasten two metals together in seawater, electrons will flow from the more active metal, the anode, to the less active, the cathode, due primarily to the difference in the metals’ electrical potential.
The anodes are the sacrificial zinc, magnesium, or aluminum pieces that attach to your drive or lower gearcase, and the cathode is the drive and its components. As the anode supplies light current, it gradually dissolves into ions in the seawater and produces electrons that flow to the cathode, which becomes negatively polarized and protected against corrosion. Simple, right? Kevin Anderson, who may well have the longest job title at Mercury Marine, explained the process more simply.
“Any time you couple two dissimilar metals in water, one will corrode at the expense of the other, and that type of corrosion is called galvanic corrosion,” said Anderson, manager and technical adviser, metallurgy, chemistry, plastics, and corrosion for Mercury Marine R&D. “In the most basic sense, a sacrificial anode is a piece of metal that sacrifices itself by corroding to save the rest of the boat and motor from corroding. So it actually is sacrificial metal that is electrically coupled to the rest of your boat, and primarily your motor.”
As you might imagine, the lifespan of a sacrificial anode varies with where you use your boat. In general, for boats used primarily in salt water, sacrificial anodes should last around nine months, Anderson said. For freshwater environments, anodes last longer, upward of three years. The general rule is that if 50 percent of the mass of the anode is gone, replace it. And be sure to replace all your anodes at the same time.
Where you use your boat also determines what material works best. Boaters who run exclusively in fresh water benefit from magnesium anodes. If you use your outboard or outdrive in salt water, aluminum is best. Likewise, if you regularly go from salt to fresh, aluminum is the way to go.
When replacing anodes, Anderson said you can just remove the old and bolt in the new and they should work fine. Factory kits come with new hardware. If your anodes bolt to a bare metal surface, use a 3M Scotch Brite pad—not a wire brush—to clean the surface where the anode rests to boost the anode’s continuity. If you use a wire brush, you will embed iron particles in the surface, which decrease the effectiveness of the anode slightly, Anderson said.
It’s also a good idea to coat the threads with Mercury Marine’s Anti-Corrosion Grease (Part No. 802867A1). Be sure your bolts are tight, but remember that you are threading stainless steel into delicate aluminum. Hand tight is fine.
Sacrificial anodes aren’t sexy and they won’t make your boat any faster. However, they are essential hardware and they are easy to install, and relatively inexpensive. And if you use the right material for your environment, you can benefit from added protection.