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Old 09-14-2014, 09:59 AM   #10
caltexflanc
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City: North Carolina for now
Join Date: Aug 2011
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Just picked up this nice explanation about zinc in fresh water off this morning's T&T digest, courtesy of "Sean":

"Can someone please explain why zinc anodes are not recommended for fresh
water."

Yes. In fresh water, the electrolyte is much weaker and the activity
level of the zinc does not support enough sloughing of the material. In
very short order, the zinc will become coated with an impermeable layer
of zinc hydroxide (the way zinc naturally oxidizes in the presence of
water, unrelated to galvanic action). Once this layer is in place, the
galvanic action will stop entirely, and the zinc essentially becomes
passive -- it provides no anodic protection whatsoever. Whatever else
on the boat is then the next least noble metal, such as an aluminum
part, then becomes the anode for the whole boat.

... The idea is to protect the prop, not to make the anode go away.

If the anode does not "go away" then it's not protecting anything.
That's how it works.
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