Working a couple of days a week at a marine supply shop offers a window into what’s happening within the local boating community. It’s part of what I like about the job. So this week as I watched cases of bottom paint and hundreds of boat zincs walk out the door, I knew that spring commissioning season had arrived.

But what, exactly, are those zincs doing for us? Many people slap them on without knowing precisely what they do, when just a little science helps explain it all.

A shaft zinc installed on a propeller shaft protects both the propeller and shaft from corrosion. Note the zinc installed on the rudder to protect it. (Click for larger image.) Photo courtesy Wikimedia Commons

To understand galvanic corrosion, we first have to talk about nobility of metals and the galvanic scale. And no, I’m not talking about a Game of Thrones episode. Gold (as you might expect) sits near the very top of the nobility scale, while metals such as zinc and magnesium are very near the bottom. The basic premise of galvanic corrosion is this: when you immerse two metals with different nobilities in an electrolyte (such as saltwater), you set up a very small galvanic circuit. Put zinc and gold in a saltwater pool and the zinc will start corroding, giving up bits of its electrons to this circuit.

Here's a common example of a combination that can set up a galvanic circuit on a boat: a stainless steel shaft with a bronze propeller. When you immerse and connect these two dissimilar metals together and then place them in saltwater, a tiny electrical current sets up. The less noble metal (in this case, your expensive bronze propeller) is going to be the one to donate its electrons to that circuit, which means after time it will weaken, pit, and perhaps even eventually break under load.

This is where those handy zincs you picked up at the marine supply store come in. The only thing you need to do to break up the galvanic circuit between your prop shaft and your propeller is to introduce a metal that is less noble to the equation. And yes, you guessed it, zinc is way less noble than stainless steel or bronze. Install a metal collar fashioned from zinc on your shaft and you’ve broken the circuit. Now the zinc will corrode, leaving your prop and shaft to happily do their jobs.

These pencil zincs are used to prevent galvanic corrosion in an engine. Photo by Gary Reich

Another place on boats where galvanic corrosion can set up is in engines. Think of all the dissimilar metals in the saltwater cooling circuit of an outboard or inboard diesel engine—aluminum, iron, steel, and more. That’s where zinc tabs for outboards and pencil zincs for inboard engines come in. Introduce that zinc to the cooling circuit and it will corrode well before everything else does. That's why these zincs are often called "sacrificial."

There is certainly much more to discuss when it comes to galvanic corrosion on boats. Here are a few more articles you might find helpful: