Richard Clark wrote in
:

On Sun, 10 Jan 2010 19:37:33 -0800 (PST), Tom Horne
wrote:

If I use a dielectric union to connect to the
galvanized iron mounting but I still bond both the support pipe and
the antenna base to ground can I avoid the galvanic action that
already caused the two pipe threads to gall.

Hi Tom,

Galvanic action occurs at the interface of two dissimilar metals with
an electrolyte between them. Each metal/electrolyte combination
constitutes a "half cell" with a potential characteristic to that
metal. When combined, they form a total potential which supports
current flow and metal migration. You can build a chain of such
reactions through intermediate steps (a chain of connections); but the
current flow still is a product of the electrolyte between them (the
half cell chemistry) and if you can break the chain, you break the
current and the metal migration.

The chemistry is not complex, and it occurs in nature without too much
difficulty - hence the problem with weather and poor sealing. A
sal****er environment is classically the most brutal. Toss in acid
rain too. The solution is fairly simple, but the success of
implementation, as the devil, is in details. One simple problem is
you may seal the joint or connection from the outside elements, but
you may also be sealing the contaminant in with the joint or
connection. Now you may observe that two solutions are required.
There are a variety of products that can be used to flood the joints
to reduce/prevent both these problems. Others will chime in with
commercial names and their experience.

As there is potential developed, and a current flow, you could in
theory detect a bad coupling. Oddly enough, the two metal combination
could by absolutely dry, free of contaminants, and still give a
voltage indication. This would occur on the basis of a temperature
driven migration of current (the Seebeck effect) and you have what is
called a thermocouple. This, however, is a very small potential in
comparison but could still surprise the investigator. Here, the
effect is not chemical but is a heat characteristic of each metal
(which means the two metals combine to create the perceived voltage
through their individual contributions just as in the galvanic cell).
Given that most metal joints in a circuit occupy the same temperature
environment there is no potential difference developed.

Most technicians consider these potentials as inconsequential (barring
the corrosion), but when it comes time to make accurate measurements
of small potentials, they can become galling to the patience.

One consequence of these interfaces becoming contaminated is that you
can also develop a non-linear conductor or a primitive rectifier. If
this is a path for RF currents (a common mode from your transmitter)
you can generate or receive spurious frequencies. Oh happy day.

73's
Richard Clark, KB7QHC


Would sacrificial action be appliable? Magnesium blocks are sometimes used to
prevent chemical action taking place on less reactive metals electrically
connected to them. I'm not sure if it can be applied though, that trick is
usually done with grounded (usually buried) blocks to protect pipelines, etc.