It’s a fair bet that every PMY reader knows about galvanic corrosion—or at least the basics of it: Drop two dissimilar metals into water and you get a transfer of ions that creates an electrical current (electrolysis), resulting in a process (galvanic corrosion) that eats away the “less noble” of the two metals. (For a list of common metals, see chart on page 47.)
But even if you don’t understand the exact chemistry, you at least know that every time you haul your boat you need to check (if not replace) your sacrificial zincs, those cheap, least-noble chunks of silvery metal that are designed to melt away before the important stuff does.
As long as some portions of your zincs remain every time your boat comes out of the water, you can probably assume that everything’s fine and continue to do things as you always have. But what happens when the yard guy finishes power-washing your pride and joy, and lo and behold, the zincs are there and you still have pitting and oxidation on some of your underwater metallic components, such as props, shafts, trim tabs, and rudders? Worse, maybe the galvanic acne is so bad, you have to replace the component.
If that’s the case you’ve got a problem that you can’t solve by just bolting on more zincs. If certain components are being damaged by electrolysis, it’s probably because they’re not being protected by your boat’s bonding system, of which zincs are a part. In a proper system, every underwater metallic object is connected, usually by a bronze strap inside the boat, to one or more sacrificial anodes located outside the boat and in contact with the water. These are often located on the transom and clamped to the prop shaft. (Actually, zinc is an inaccurate term as modern anodes are made of an alloy that is only partly zinc.) If everything in your bonding system is done right, the only component that can get pitted (or disappear) in the presence of a galvanic reaction should be this anode. Of course, once this anode is gone, all bets are off.
There are several reasons why a metallic component may not be connected to your bonding system: (A) your boat’s manufacturer didn’t do a proper job at the time of construction; (B) you added or replaced something and forgot to attach it to the system; or (C) somehow the bonding strip broke, interrupting the continuity with the sacrificial anode. Regardless of the cause, if you’ve found a component that’s clearly been attacked by galvanic corrosion, your next step is obvious: attach it to the bonding system. But if the corrosion is just beginning or the component has been painted, you may not be able to see the damage. And some corrosion may occur inside a component, where you can’t see it. (More than one through-hull fitting has failed at sea without warning due to galvanic corrosion.) That’s why it’s a good idea to annually verify that all your boat’s underwater metallic components are properly bonded.
You need just two things to do the job: a digital multimeter, which you can buy at any electronics supply store, such as Radio Shack and something called a corrosion-reference electrode, which is available from a number of specialty outlets, such as www.getaprop.com, for around $150. Basically, this is a silver-alloy anode with a ten-foot-long lead. (Extensions are available.) Drop the anode end into the water and plug the other end into the positive outlet of your multimeter. Plug the probe that came with the multimeter into the negative outlet, and you’re ready to begin checking your system.
This article originally appeared in the March 2009 issue of Power & Motoryacht magazine.