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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