Galvanic Soup

This launch was previously in fresh water, but with a new owner and new location in salt water the protection of underwater metals by sacrificial anodes needed to change

Repairing the rubrail on a small rowboat the other day triggered a multi-day thought process about the metals used on boats.  To start, it was a simple question: did the aluminum pop rivets used to attach the rail fail first by corrosion, or did they fail by blunt force trauma? My guess is that once the pop rivets began to corrode, they couldn’t stand the strain of repeated pounding the rail took. Should I replace the rivets with a larger size—or perhaps another material altogether?

The next day, corrosion and compatibility were still on my mind as I inspected the bottom of a small launch. The boat had come out of the brackish Hudson River and would be moving to the salt water of Boston Harbor this summer.  When I mentioned to the owner that the underwater zincs would have to be changed, the response was that we needn’t bother since they “looked fine.”

The owner did not understand that the sacrificial anodes were in fact aluminum and not actually zinc. That might be appropriate for freshwater, but it won’t protect the boat’s underwater metals in the even more corrosive galvanic soup that is saltwater.

I tried to tell him that not only were there different types of sacrificial anodes (zinc, aluminum, and magnesium) but that choosing the right type depended on both the environment (freshwater, brackish or salt), and the type of boat and which underwater metals it had. A wooden boat with bronze fittings would require different protection than a fiberglass boat with an aluminum outdrive.

He wouldn’t listen. I needed research to support my explanation, a third party source that could corroborate my assertions and guide any boater on how to protect their particular boat type wherever it floated.

I found just what I was looking for at

I learned about bonding systems that wire underwater metal (props, struts, seacocks, trim tabs, and the like) on boats to a common ground for the purpose of isolating them from galvanic action. I learned that bonding system grounds must be separate from DC electrical system grounds used for onboard electronics. I also learned that boats in a marina can collectively add to corrosion if not properly protected and also that simply connecting to shore power can cause problems. I also discovered that you can overprotect a boat and cause physical damage to the hull (alkali delignification) and make your anti-fouling bottom paint less effective with the wrong type of anodes. And that a certain amount of voltage is inherent in both freshwater and saltwater and what these acceptable ranges are, in each environ.

The little boat that started the thought. To connect to charts that will help you determine the right anode based on your boat and location lick on the links on the left.

So don’t rely solely on my advice. If you’re buying a boat that is changing locations, or have zincs that are consumed faster than you think realistic, or you suspect improper wiring and electrical problems are corroding the metals on your boat, I highly recommend you visit You’ll get the advice, tools (corrosion reference electrodes), charts, and products you need to determine how best to protect your boat from costly corrosion problems. And who knows, you may even be able to convince a stubborn owner that corrosion can be controlled and how best to do it.

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