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Re: [TowerTalk] tower leg ground

To: towertalk reflector <towertalk@contesting.com>
Subject: Re: [TowerTalk] tower leg ground
From: Alan NV8A <nv8a@att.net>
Date: Wed, 16 Jan 2008 07:55:29 -0500
List-post: <mailto:towertalk@contesting.com>
On 01/15/08 12:51 am K4SAV wrote:

>> ............BUT it's advisable to separate the 
>> bronze clamp from the tower leg (presumably galvanized) by a layer of 
>> stainless steel to minimize the galvanic (corrosive) action: .........

> ---------------
> I have asked this question on a couple of other reflectors and got no 
> answer, so I will try this one also.  Can anyone explain how putting a 
> stainless steel shim between a copper wire and a zinc plated tower 
> reduces galvanic corrosion of the tower?
> 
> Obviously the stainless steel shim will have little corrosion, but I 
> don't care about the shim.  I need to protect the tower.
> 
> When I went thru this mental exercise, here is what I found.
> 
> The data in the table below came from the galvanic series for metals 
> which you can find on the internet.
> 
> In the galvanic series, in flowing sea water, in order from least noble 
> to more noble:
> Zinc is -.98 to -1.03
> Aluminum alloys are -.76 to -1.0
> Steel is -0.60 to -0.71
> Copper is -.3 to -.57
> Tin is -.3 to -.33
> Stainless steel -.57 to 0.0 (depending upon the alloy)
> 
> You have at least three goals in avoiding galvanic corrosion.
> 1) Maximize the contact area of the least noble metal exposed to the 
> electrolyte (water),
> 2) Minimize the potential difference between the metals in contact, and
> 3) Keep the water out.
> 
> The least noble metal will corrode.  The higher the potential 
> difference, the faster it will corrode. If you look at the list above, 
> you will see there is no commonly available metal less noble than zinc 
> (your tower). In order to minimize corrosion of the tower plating you 
> should put something as closely matched to the tower as possible.  Seems 
> to me that should be aluminum.   Another zinc plated shim might work but 
> probably would need more frequent replacement because of the copper to 
> zinc junction.  Still, shims are easy to replace, a tower leg is not.
> 
> As far as maximizing the area of the least noble metal (the tower), in 
> the case of a shim, the contact area is the same for both metals because 
> water may collect within the junction but the whole tower is not going 
> to immersed in a liquid.  This item may be important within a clamp 
> (depending on how it is made) but for a shim there is no difference.
> 
> As for keeping the water out, sealing the whole thing in duct seal seems 
> to work for me.
> 
> If you see any error in this please post any corrections.


Obviously the term "stainless steel" covers a wide range of alloys, so 
some may be more suitable than others. I don't recall where I read the 
recommendation originally, but I notice that Polyphaser seems to follow 
the same principle, with a SS shim between the copper wire or strap and 
a galvanized tower leg:

http://www.polyphaser.com/productdetail.aspx?item=TK-1

Sealing with duct seal or "liquid electrical tape" wouldn't hurt either.

73

Alan NV8A
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