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At 08:15 AM 10/01/2001 -0400, Richard Goodwin wrote:
>I don't know about that. Personally I wouldn't trust AC to be
>completely harmless to metals submerged in seawater. Anybody know for
>sure?
>
>Dick
Arild replies:
I'm an ABYC certified Marine Electrical Technician. As part of my work I
get involved with investigating insurance claims.
On several occasions I have encountered situations where stray AC current
caused corrosion. No DC current was measured, nor was any DC circuits
involved. The faulty circuit was clearly an AC circuit and rewiring the AC
circuit corrected the problem.
It is especially noticable in aluminum since the rate of corrosion is
faster in that metal, compared to bronze or stainless steel.
During ABYC technical committee meetings we have discussed the question of
whether or not AC current causes corrosion or if only DC currents do.
The comparison with an electro-plating cell is not completely
equivalent. In an electro-plating tank you have a controlled environment,
a strong acid electrolyte and very close proximity of electrodes. You do
not have a free flow of electrolyte which could carry away ions, nor do
you have any impurities to which the metallic ions can attach. The
currents and voltages involved are much greater than what you normally
experience in a boat.
Even in a test tube, the results are not conclusive. Hence the ongoing
debate among experts.
However, in the real world some of the ions are lost into the surrounding
water and there is a net loss of mass from the more active metal. And
some mass is lost in the process of creating hydrogen gas.
Some purists will argue that what is observed as apparently being AC is
actually rectified, pulsating DC ; but this argument gets a little obscure
and is best left to the engineering and physics text books. We are
discussing overall observed results here.
You will notice a faster rate of corrosion with DC than AC, because of
the constant flow of ions.
Regards
Arild
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