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Old 24-01-2010, 05:22   #1
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The Life of Zinc

Howdy folks:
Just got around to replacing zinc anodes (+ 1 aluminum anode) on my steel hulled 42-footer. I am tied to a dock in a salt water commercial harbor. Four months on the high seas and the zincs were on for 8 months. They were down to 10% left.
I have also added a couple of hunks of zinc over the side hanging by copper wire while I stay here for the next couple of months.
What is normal life for zincs on a glass boat and or steel boat?
Thanks for any info/ advice.
Pete
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Old 24-01-2010, 05:33   #2
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It really depends upon the boat AND the environment (stray currents) where you are. I had a shaft zinc on for 14 months and it was only 50% wasted... on a boat with an isolation transformer and zero metal below the waterline except for shafting. Also in a marina with a good electrical system.

Your experience at 8 months is a clue. If you have another at-sea interval try to measure that for the "best" life you can expect from your zincs.

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Old 24-01-2010, 06:07   #3
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Where you at, Pete?

Even though Zinc & Aluminum are very close on the galvanic series, I generally wouldn’t mix anode types, as the less noble Zinc (anodic) may corrode to protect the more noble Aluminum (cathodic) metal.

This has likely been happening, if you find the Zinc anodes eroding more quickly than the Aluminum.

Pure zinc has a theoretical capacity of 372 Ampere Hours per pound. That means at 1 amp,it would take 372 Hours to consume a pound of zinc. Now,zinc operates at about 95% efficiency so it would actually be approx.
353 AmpHrs.

On a steel hull, the current requirements for protection (to about -800 to -1050 mv) could be between .3 ma (milliamp) and 6 ma. per sq. ft. for submerged protection, depending on coating quality or lack thereof and very little water flow.

So, for example, a 25lbs zinc anode, putting out 2 amps would last approx. (25lbs x 353 amphrs/ 2 amps/24hrs) or
8825/2= 4412.5/24= 183.8 days.
Keep in mind that this is all theoretical and has many variables

Anodes should be replaced at about 50% erosion.
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Old 24-01-2010, 07:08   #4
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Slightly off topic (sorry). What do you do with the removed anode? Does anyone know a way of recycling them?

P.
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Old 24-01-2010, 13:43   #5
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Gord:
I am in Iquique, northern Chile. It is mainly an industrial port with a good-sized anchovy fishery as well as inshore fishermen. I am here for the paragliding which is great (no rain, ever). Took 4 months to get here from Portland, OR. It's a shitty sail/motor from Ecuador down but I knew that. From here I will head to Easter Island/Pitcairn and west.
I will digest your answer to my query (thank you) with a dictionary, a pile of Nigel Calder books and maybe a digimeter by my side.
Pete
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Old 24-01-2010, 16:08   #6
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Quote:
Originally Posted by fishwife View Post
Slightly off topic (sorry). What do you do with the removed anode? Does anyone know a way of recycling them?

P.
I have recycled hundreds of pounds at a local scrap metal dealer. If you have just a small amount, not sure what you can do with it. Find a recycle bin on the street domewhere maybe?
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Old 24-01-2010, 16:44   #7
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Pete, I am at a marina that is about four years old in the middle Chesapeake Bay. I have a isolation transformer and my diver has not replaced my zinc in two years. Fiberglass hull.
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