Corrosion of Aluminum wiring

Hello,
I am confused. How come insulated aluminum cable corrodes so badly yet overhead aluminum lines seem to last years.
The supply to my house failed over the week end. The repair crew arrived and figured out that the break was 23.5m back from my meter box. Wandering back though the garden the linesman noticed a "new" bush which was about the right distance from the meter box. We had planted it last spring. He pronounced that the break would be there.
He dug up the bush and started clearing the hole. He quickly found the cable which was between 6 to 10 inches below the surface. Clearing more earth from the hole, using a spade, the linesman brushed against the cable. It broke clean in two.
He explained that this was common enough. I had apparently nicked the cable when planting the bush in the spring and this had allowed mosture to seep into the cable. The recent rain was the final straw and the cable failed.
The cable consisted of an inner aluminum core surrounded by insulation, a copper mesh braid and another outer layer of insulation. The inner core had apparently totally disintegrated in the space of about 8 months. The copper braid had as good as failed as well.
Can anybody explain how this happens given that aluminum overhead lines are constantly exposed to mosture.
--
Fergus McMenemie Email: snipped-for-privacy@twig.demon.co.uk.
Technical Basis Phone:(UK) 07721 376021
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Fergus McMenemie wrote:

Aluminium is a very reactive metal. In open air, it forms a closely bound oxide layer, which protects it from further oxidation despite it's reactivity.
In soil, this layer is under constant attack from acidic soil water, and is constantly eroded. It doesn't last long once the insulation has gone.
--
Grunff


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As well as the likely absence of the protective oxide layer which Grunff mentioned, the presence of the copper close to the aliminium in your soil (plenty of dissolved carbonate, nitrate, and other anions to help) is likely to set up a nice electrolytic reaction to help the aliminium (and the copper) turn into nice soluble salts. (Allegedly. It's twenty- mumble years since I did any chemistry, and I always got much better with the organic than the inorganic; so correction and ridicule from the better-informed would be welcome and deserved, respectively).
Stefek
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On 4 Nov 2003 23:13:05 GMT, snipped-for-privacy@hp.com wrote:

Trying to solder aluminium is fun too :-)
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It's called the electrolytic effect and also occurs when boats are in the water. The aluminium acts like a sacrificial anode when an arc of electric current is passed between it and the copper braiding. The arc then sets up a hot spot and the two metals eventually burn through. Very common.
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Fergus McMenemie wrote:

Its not moisture. Its oxygen. That on air forms a nice tough aluminium oxide layer, and thats it....unless of course some weak acid gets in...in which case that dissolves the aluminium oxidejust about as fast as it forms...
My guiess is you lost waterproofing somehow when the bush was planted, and that plus acid soiil and water did the rest.

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On Wed, 05 Nov 2003 11:41:04 +0000, The Natural Philosopher wrote:

<snip>
Aluminium is pretty much OK when used by itself for overhead lines. The problems come with connections between dissimilar metals or with the build-up of oxides in joints.
Cu/Al joints were made either using split-bolt mechanical connectors - line taps - covered with denso tape to stop ingress of moisture, or much better these days using compression fittings. Bimetal Cu/Al compression fittings are made with a friction weld between the two metals, and these are not prone to corrosion, for some reason that I never did fully understand.
All compression fittings have an abrasive grit-grease inside, and the conductors are (should be!) scratch brushed before making the connection, to break down the surface oxidation.
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very well put, and as ACSR conductor has now been replaced by all alliminum alloy the need for compression anchor clamps has been reduced from 16 individual joints on a 400kv quad bundle to 4. There are 61 strands in a piece of conductor and all the inner strands are greased to help prevent corrosion, and as said this grease must be cleaned off and brushed back to bear metal before jointing.
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Hi Wanderer,
Do you know someone is spoofing your name and e-mail address, and is sending out these posts.
<<http://groups.google.com/groups?q= snipped-for-privacy@wcpjdwvg.com&ie=UTF-8&oe=UTF-8& hl=en&btnG=Google+Search>>
Just to let you know.
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On Wed, 05 Nov 2003 22:29:13 GMT, BigWallop wrote:

Had someone doing it a few weeks ago, but I complained to their ISP and I think their plug was pulled. Just tried the link you posted but that got me nowhere, and a quick search with my name only brought up the groups I post to.
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I think the other link was still under a session with my machine and was still linked only to me. Sorry.
--
www.basecuritysystems.no-ip.com

Outgoing mail is certified Virus Free.
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On Thu, 06 Nov 2003 08:30:21 GMT, BigWallop wrote:

Could you email me, the addy is live. I had a look through the posts your link threw up but couldn't see anything obvious that looked as though it appeared to come from me, unless I'm missing something obvious.
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Can you try your own e-mail address on the google to see what it brings up. The e-mail address I searched for was another one used by this creep sending posts about Christina Agulerra (?) and I've just found my real e-mail address had been used as well. I've sent the obligatory report to the abuse on three ISP's and I'm sure there'll be more people doing the same.
I only noticed it when I was going to answer one of your posts and when I looked through the group again, this one popped it's ugly head up. Of course I deleted it without thinking and thought you were a fan of hers too. :-))
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I remember about 12 or 15 years ago watching an LEB lineman splicing a new feed to a house into the main cable in the street. Apart from being quietly gobsmaked at the whole procedure of working live (fitter standing on a rubber mat with knobbles on it of the sort you used to buy at Halfords as a car floor mat, which the fitter's mate dried off with a blast from the propane blowtorch before putting it down in the bottom of the hole) I was surprised that he was soldering the joint between the Ali spur and the copper main cable. I asked him about it but forget what the explanation was - maybe something in the flux?
The soldering itself was a performance: pouring molten solder from a ladle over the joint, catching the run-off in another ladle!
-- John Stumbles -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+ The most dangerous component in a car is the nut that holds the steering wheel
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On Wed, 5 Nov 2003 22:54:06 -0000, "John Stumbles"

You mean you haven't worked on live wires before? Chicken ;)
I have bought me a decent set of electrical pliers which have insulated handles with little protusions to stop your fingers wandering too close to the metal bits. Very reassuring when cutting into cable, even if you have switched the mains off first.
PoP
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wrote:

D'you mean "I hadn't before" or "I haven't since"?! Anyway: Yes and Yes (though not always intentionally :-)

I have various electrical/electronic tools with bits missing from them ....
John S
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