| Subject: Re: Sea Cock corrosion |
Author:
Brian Henry
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Date Posted: 6/02/05 15:20:59
In reply to:
Nick
's message, "Re: Sea Cock corrosion" on 6/02/05 13:30:27
There are, of course, two forms of galvanic corrosion. The usual one is fairly weak where dissimilar metals used on the boat form the two plates, cathode and anode, with the water as electrolyte, that induces a current that depletes the anode and why we delibrately fix a sacrificial form to protect the less noble element - usually constituents of the propeller and seacocks.
Then there is the more active version that I think must be at work here because of the rate of depletion; stray electical curents in the water from electrical leakage, that can be broken also into two types, local from the ship itself or, more difficult to trace and cure, from another ship or mains power on an adjacent dock fitting.
I am always suspicious of neighbouring boats in my marina that have permanently connected mains power (forbidden by the marina but, well, it is in Italy) because there is a great potential for badly grounded equipment leaking into the water and causing current flow from my fittings and anodes.
I note that I earlier referred to Blakes seacocks as being constructed of manganese bronze - as are most highly specified propellers. I have just checked on the Blakes website and found that they are, in fact, of DZR1, a dezincification resistant brass, which is why you have the "pinkish hue" typical of zinc depleted brass showing the copper residue.
Regards, Brian.
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