Ask Sail: Galvanic Problems

In the August 2010 issue, Nigel Calder discussed zinc corrosion. As recommended I installed a galvanic isolator in an attempt to stop rapid zinc corrosion and blistering
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William March, Madison Center, Connecticut

Q: In the August 2010 issue, Nigel Calder discussed zinc corrosion. As recommended I installed a galvanic isolator in an attempt to stop rapid zinc corrosion and blistering and “popping” of spots on my lead keel. No change. I disconnected the rigging-to-keel bonding wire that ran to the engine, and the keel popping didn’t occur last season, but the zincs were completely gone. You mention that zinc loss can occur even with an isolator “for various reasons.” What can those be?

Nigel Calder Replies
A: Galvanic isolators have a couple of sets of diodes wired in parallel to conduct electricity in opposite directions. These block electrical currents being driven by voltages up to around 1 volt, which effectively means all galvanically generated current is blocked. However, let’s say there is leakage from the AC system into the grounding system. The diodes can then be “biased” into conducting, at which point the galvanic isolator is effectively useless. To combat this, all galvanic isolators that comply with American Boat and Yacht Council (ABYC) standards include a mechanism (generally a capacitor) that allows stray AC currents to bypass the diodes while still blocking DC currents. (This is OK, because AC current is not supposed to cause corrosion: the ABYC standard requires a galvanic isolator to be able to conduct up to 3 amps AC without becoming conductive to DC currents.) However, given a substantial leak from the AC system, this blocking mechanism can be overwhelmed, at which point the galvanic isolator becomes conductive to DC currents and once again is not providing any galvanic protection.

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