Stray Current in the Bilge, DC, Not AC, Is the Culprit When Metal Fittings Corrode

Bob Loeser, a marine consultant from Palm Island, Florida, was working on a case when someone observed that corroded fittings were a good indication of AC current leaking into the bilge. Bob's experience was that AC current didn't cause corrosion, but his opinion wasn't supported by any books or articles on the subject. On the contrary, a well known book on electrical systems confidently reported that AC current caused "severe corrosion." What the book didn't say was where the statement had come from.

Bob, a former senior marine engineer at Underwriters Laboratories, isn't one to accept an unsupported statement, even when it's published in a well-regarded book. He devised an experiment using two bronze through-hulls, each immersed in a separate 30-gallon tank of salt water. In one tank, he created a circuit by attaching a 10-gauge wire carrying 120 volts of AC current to the through-hull. In the same tank, a second 10-guage wire was attached to a copper plate, which was immersed two feet from the through-hull.

For realism, he attached the through-hull to a pine plank. When the electricity was switched on, the current from the throughhull to the copper plate was 0.62 amperes (AC), which was sufficient to cause a light bulb to glow.

An identical arrangement was used in the second tank to create a DC circuit that caused a navigation light to glow (0.5 amperes).

The test using AC current was continued for 1,200 hours with no visible indication of damage or weight loss to the bronze through-hull. The test using DC current, however, was halted after onto 72 hours because the bronze through-hull was destroyed.

The conclusion: While leaking AC current is a significant electrocution danger, it poses little or no threat to metal fittings. Stray DC current, not AC current, is what corrodes metal fittings.

The reason: Whereas the electrons in DC (direct current) current flow in only one direction, which causes the metal feeding the current into the water to lose electrons and corrode, the electrons in AC (alternating current) current alternate direction back and forth 60 times a second, which prevents any significant loss of electrons from the metal.

From Seaworthy, BoatUS Marine Insurance Report, July 2001, "Back to Basics".

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