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Old 18-04-2015, 10:00   #1
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Crumbling Anodes?

I have anodes on my bow thruster and on my Isotherm SP fridge through-hulls which I never change -- because they don't waste. I clean them up with a wire brush and that's that.

Now for the very first time, these anodes are worn, but not like the usual anode wasting, which is a clean wearing down with pitting. These anodes are as if chemically corroded -- crumbling into white powder. And a fair amount of marine growth left over from last summer.

Should I be concerned? The Isotherm through hulls don't even necessarily need anodes, according to the manual. They are optional; "just in case you might have any concern". And indeed they are made of copper or bronze and not electrically tied to anything; I'm not sure why they would need anodes any more than any other through hull needs an anode.

Maybe it's because of the fresh water in the Baltic last summer? I got as far as Vyborg, where there's already no salt in the water.

Any experience this? The other anodes (prop, shaft, hull) look normal with the normal type of wasting and at the normal rate.
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Old 18-04-2015, 11:52   #2
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Re: Crumbling Anodes?

Dockhead,
Severe and aggressive corrosion appears as you describe above. If you're in a marina, you may be in close proximity to boats which are not protected properly. If you're on the hook, it could be lack of adequate/consistent protection on your boat. In both cases above, it probably is the combination of oxygen levels, saltwater and localized elements in the water. Here's an interesting article that describes the effects better than I would. I have experienced this twice in a well protected boat.
www.gewater.com/...wa...


GE Infrastructure Water & Process Technologies

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Good luck and good sailing.
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Old 18-04-2015, 12:01   #3
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Re: Crumbling Anodes?

Dockhead,

I suspect you nailed it on the head with you freshwater excursion theory, especially as you normally hang on a mooring and haven't seen this before. In freshwater zinc anodes form zinc oxide and hydroxide pretty quickly, and and both are a powdery white substance. The colder the water the more the balance toward hydroxide.

Pretty much all of the anode folks will tell you zinc for salt water, magnesium for fresh (with the aluminium proponents thrown into the mix for good measure).

For your other anodes they probably looked like this last summer, but activity has caused the hydroxide to slough off and normal wasting to show up. For the inactive anodes no activity means the hydroxide stuck around.
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Old 18-04-2015, 12:10   #4
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Re: Crumbling Anodes?

Quote:
Originally Posted by Dsanduril View Post
Dockhead,

I suspect you nailed it on the head with you freshwater excursion theory, especially as you normally hang on a mooring and haven't seen this before. In freshwater zinc anodes form zinc oxide and hydroxide pretty quickly, and and both are a powdery white substance. The colder the water the more the balance toward hydroxide.

Pretty much all of the anode folks will tell you zinc for salt water, magnesium for fresh (with the aluminium proponents thrown into the mix for good measure).

For your other anodes they probably looked like this last summer, but activity has caused the hydroxide to slough off and normal wasting to show up. For the inactive anodes no activity means the hydroxide stuck around.
That sounds plausible. I hadn't thought about the normal activity causing the sloughing off of the white stuff.
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Old 18-04-2015, 12:17   #5
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Re: Crumbling Anodes?

If you form a perfect coat of hydroxide on your anode there will be no activity beneath, as the coating provides a barrier. But getting that perfect coating is pretty difficult, and seawater usually finds a few cracks and crevices to provide an electrical path to the zinc beneath. Once that happens the concentrated activity at those locations causes spalling of the oxide/hydroxide coating, which opens more surface area to activity and so the cycle continues.
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