Disappearing Anodes

[Poppy]

A pal of mine has just had his Pegasus 800 (yes, a saily thing) lifted at the end of his season to find that all the anodes have disappeared entirely! They were all new in the spring!

Apart from PADI Pikeys, can anybody suggest why they have dissolved in such a short time?

He has been told that it is because he is moored near a boat with mains electric connection - indeed his neigbour fits this description. Is this a reasonable suggestion? Surely if this is the case, there must be a fault of some kind on that boat?

Any suggestions?

[Antares_9]

Sorry Poppy but this one raises more questions before a really informed answer can be given.

Type of water moored in, type of anode used (e.g. Magnesium, Aluminum, Zinc) is he also connected to shore power.

If he is not connected to shore power then it is not the other craft causing the issue even if it has an earth leak, if he is connected then a galvanic isolator should help. Has he moved from fresh to more saline water and not changed from mag to ali or zinc?

Also see the later posts in the following discussion.

http://www.thenorfolkbroads.net/phpBB3/viewtopic.php?f=68&t=9488

[Antares_9]

Just a quick addition, if they were of a suitable material and of sufficient size and have gone completely then he has no way of knowing just how long his underwater gear has gone unprotected in a clearly highly electrolytic environment. He should have it carefully checked for dezincification, especially the skin fittings and prop. In fact I would just replace the skin fittings with new quality SS ones and new SS valves just for my own peace of mind were it my ship.

[senator]

Another thing to consider is proximity to other metal objects, things like steel pilings and even steel boats.

[Antares_9]

Another thing to consider is proximity to other metal objects, things like steel pilings and even steel boats.

Only if you and either the steel boat or the piling are earthed to the same shore power Ian, otherwise there is little or no effect.

[senator]

Thought the whole point of electrolysis was that 2 dissimilar metals placed in close proximity in a saline solution will result in an electrical current flowing from the anode to the cathode or the most reactive to the least metal and in the process destroying the anode.

from what I had gleaned this was a slower process to that of galvanic corrosion which relied on the introduction of an electric current from another source but non the less part of the destructive process to the most noble of metals.

Is this wrong and if so does that mean if the likes of skin fittings and other bits aren't earthed that we can do away with anodes altogether?

[Poppy]

Thanks for the input so far Guys.

The Anodes in question are (were!) zinc, and the boat is moored in Horning, so in fresh water ( should be Magnesium?), and the craft is not connected to a mains supply. He has been saltside for 7 - 10 days once this year.

[Antares_9]

No Ian, it’s simply that a steel boat is not steel at all as far as the electrolysis or corrosion is concerned; it’s paint, epoxy or such which is presented to the electrolyte not the steel. If however all is strung together with shore power earth with a leak it is an entirely different matter. As far as the skin fittings go, if they were similarly protected with an epoxy coat inside and out then yes they would not need anodes. Just look at the damage done to an O/D that has had its coating poorly neglected versus one that has had regular touch ups. A steel boat with it's surface exposed would obviously cause havoc to it's own lesser noble skin fittings and those around it.

[Antares_9]

Thanks for the input so far Guys.

The Anodes in question are (were!) zinc, and the boat is moored in Horning, so in fresh water ( should be Magnesium?), and the craft is not connected to a mains supply. He has been saltside for 7 - 10 days once this year.

I really can’t see how that happened with zinc (good job they eren't mag) in a low saline environment. Assuming he fitted big enough ones then they really must have something other than the natural process affecting them. Are the embedded steel retaining washers still present? With that information it is probably worth a check on stray currents in the marina.

Or is he just a typical yottie and fitted one the size of a half dollar bought at some boat jumble for £2.50?

[Poppy]

Or is he just a typical yottie and fitted one the size of a half dollar bought at some boat jumble for £2.50?

Somehow I don't think so - I'll have to ask French Marine about that !

[senator]

Thanks for that David but how much of the steel would have to be exposed to start the process?

Is it a case of a nasty scratch or would you need to be presenting a larger surface area than that?

A nasty gouge exposing bright metal would do it Ian, untill it got a nice coat of oxide.

[Antares_9]

A nasty gouge exposing bright metal would do it Ian, untill it got a nice coat of oxide.

[senator]

So rust will act as a barrier? good news as I think the piles in Reedham have now received a good dosing of protection

[Antares_9]

Well you know what happens whet electrical contacts corrode Ian. But it is not an insulator in the same way as paint or epoxy.