I've been thinking about this for a time now. This thread pushed
me over the edge of being curious enough to actually do some
research. So I took a peek at a galvanic series table of
materials to see where the players actually stand relative to one
another.
There are three bronzes listed on the table I have. They are all
clustered near copper in the series. One of them, silicon
bronze, is actually farther than copper is from zinc. Anyhow,
there is no problem making a clamped connection directly between
bronze and copper.
There are several grades of stainless steel in the table. They
are all clustered in a region which is approximately midway
between copper and zinc (the galvanic coating material).
I'm sure that the UL is referring to the electrical safety of the
bronze to galvanized connection and not necessarily certifying
that there will not be corrosion of the galvanized participant.
The connection will remain safe for UL purposes as long as there
is significantly more material making up the galvanized conductor
than the bronze clamp.
It is no skin off the UL's nose if the galvanized coating on the
pipe is degraded in small areas over the years. They (or the
NEC) specify that the connection must be regularly inspected. It
can be slightly relocated and retightened occasionally to restore
the connection's effectiveness. The galvanized pipe is generally
not a thin wall load bearing structural member.
But the tower owner is concerned that the zinc galvanic coating
NOT be attacked because this can ultimately cause the tower steel
to be exposed to corrosion processes that will weaken it.
Using stainless steel shim as an intermediate material in a
clamped connection between either bronze or copper and zinc will
reduce the galvanic potential difference across the physically
connected materials to approximately half of what it would be if
the copper or bronze were directly connected to the zinc. So
the corrosion process will be approximately half as rapid as it
would be without the stainless.
Whether this is an important consideration depends entirely on
the environment the tower is exposed to. In an arid climate far
from industrial or metropolitan sources of atmospheric
contamination, it is probably not an important consideration.
In a climate that frequently wets the connection, or exoses it to
acid rain or is near salt water, it might be a very important
consideration. In such an environment, unless the connection can
be completely protected from its immediate environment by a
durable moisture barier, a stainless intermediate material should
probably be used. Corrosion can't happen if the electrolyte
can't touch both sides of the connection. And it will happen
only half as fast with the stainless intermediate material.
Of course this or any other kind of clamped connection between
dissimilar materials should never be used below grade where it is
continuously in contact with an electrolyte solution.
73, Eric N7CL
To: <towertalk@contesting.com>
>Date: Fri, 20 Nov 1998 08:12:10 -0500
>From: "w8ji.tom" <w8ji.tom@MCIONE.com>
>
>I'll second this...
>
>> bronze ground clamps are made specifically for attaching to
>> copper, or galvanized steel without corroding, and are
>> U.L. listed for that purpose.
>
>Bronze is a very good material to use for an interface.
>
>73 Tom
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