Complex and mysterious, corrosion is as dear to boaters’ hearts as their credit card balances are. In past marine corrosion articles, I’ve delivered straight science.
This time, I’m taking a colloquial approach. Here goes.
Corrosion is like the uninvited guest who always shows up hungry. At some point, you forget about serving this goat a meal and dish out snacks and junk food to keep him away from the roast beef. With corrosion, you feed the unwelcome guest excess electrons instead of Twinkies. These electrons come ready to eat, “packaged” in the form of what are called sacrificial anodes. Corrosion eats the anode (the Twinkie) instead of your sterndrive, prop shaft or raw water through-hull (the good stuff) because anodic metals dish out electrons as freely as Grandma dishes out Thanksgiving dinner. Know that with each electron consumed, a minute bit of the anode’s metal is consumed instead of a bit of your boat or drive’s metal, hence the sacrificial moniker.
Now, most boaters refer to sacrificial anodes as “zincs,” because zinc has been used for a long time. But there are other “zincs” made of aluminum, magnesium and various alloys with various trade names. In many cases nonzinc anodes outperform actual zinc anodes, depending upon the type of metal you need to protect and the nature of the water in which the metal being protected is immersed. It all has to do with the voltage of the anode when referenced against a standard. The standard is an electrode made of silver/silver chloride. That just means they are relative: The higher the anode’s voltage, the more freely it gives up electrons and the more able it is to feed corrosion. We are talking about negative voltage here, so wrap your mind around the concept of “more negative voltage.”
At a glance, you might conclude that, since magnesium has the highest negative voltage, that’s the anode to use to protect any metal, on any boat. But it’s not.
Anodes create electrical current by setting up a difference in electrical potential. More is better in the sense that it’s the equivalent of a tastier dish that your guest, corrosion, will devour readily. But suppose you run out of snacks?