Curious Zincs

“I’ve had my boat, which has a Volvo Penta saildrive engine, for eight years. For the first six years, when I hauled the boat for annual maintenance I found the zinc anode on the saildrive was slightly eroded but was still firmly attached. I changed the zinc annually.

However, when I hauled the boat two years ago, I found the anode was loose and the erosion had occurred primarily around the mounting screws. I’ve continued to see erosion around the screws, while the rest of the anode doesn’t seem any more eroded than it was during the first six years. Can you tell me why this might be happening and how to correct it?”

Donald Sorensen, Sequim, Washington

Nigel Calder replies : I talked to the folks at Volvo Penta’s headquarters in Sweden. They assume that original Volvo Penta anodes are being used, which means there has been no change in the composition of the anode material. They suspect that external galvanic influences may be causing the corrosion. As designed, all Volvo Penta saildrive packages are electrically isolated from the engine in order to minimize external influences. Of course, it is possible this isolation has somehow been compromised; this could happen if some dirt between the engine and flywheel housing gets moist. To test for this put the probes of a sensitive ohmmeter on the engine block and on the saildrive. The resistance should be very high or infinite. If there’s low resistance, there is an electrical connection of some sort that needs to be traced and broken.

Even if the drive is correctly isolated, in some circumstances stray current flowing through the water can cause corrosion by entering the saildrive at one point, passing through the unit, then exiting at another point. The exit points are where the corrosion occurs.

Here are some things to think about: Did your problem start after you moved the boat to a different dock space? Did it start when a different boat took up residence near you? That boat could have had an electrical problem. Or perhaps something changed in the marina itself. Were any new steel pilings or other structures installed? Tracking down these seeming unrelated causes calls for an electrician who is skilled in corrosion detection.

My gut feeling is that your problem involves galvanic rather than stray-current corrosion. I say this because stray current will corrode whatever metal discharges it back into the water, while galvanic corrosion will initially be concentrated at the zinc, which is what is happening in your case. That’s why I would first carefully check to see if the saildrive is electrically isolated. You can do this yourself with a good multimeter.

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