[TowerTalk] tower leg ground

[K4SAV]

>............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: .........
>  
>
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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.

Jerry, K4SAV