On Tue, 12 Jan 2010 12:23:01 -0800 (PST), Tom Horne
wrote:

What would be different is that the
electrical connection would no longer be occurring at a point of
contact between dissimilar metals which would be separated from each
other by the non conductive washer of the dielectric union.

Hi Tom,

I would offer that this non-conductive washer will not solve what you
perceive to be a problem IF it has any water (solution) that can
bridge it.

I still had lightning down conductors connected to the bottom
of both pieces of pipe.
This specific information guarantees a current path for the galvanic
action, IF water wets both sides of the insulated washer and joins
them.

Instead of washers (this is all pretty vague in the geometry), you
should go for insulated stand-offs to increase the separation so as to
allow water to wash off rather than to bead up and join the two
metals. An oversize (as in very wide) washer might do.

Hose down the join and look at the water's wetting of the join.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:

I still had lightning down conductors connected to the bottom
of both pieces of pipe.

This specific information guarantees a current path for the galvanic
action, IF water wets both sides of the insulated washer and joins
them.


That's true, but it also suggests that sacrificial action can be worked. If
water with ionic contaminants bridges the gap (across the dielectric) to
close a local circuit, cause current flow and corrosion, then it's a loop
that presumably does not place current in a signal line, or any resulting
noise (but considering that a galvanic battery is not generally noisy, that's
probably irrelevant). Jimmie mentions interposing brass between copper and
galvanised (zinc coated) plumbing, and surely that would be to reduce
potential between dissimilar metals and so reduce current and corrosion at
each junction, but ultimately one metal will gain metal from the other in the
join, (usually as a mess of salts), but so long as the one that loses is the
negative electrode in the loop, and is a sacrificeable part, then until it
needs replacing, the only other maintenance needed might be a wire-brushing
and regreasing. Short of ensuring same-metal interfaces, I can't think of
another strategy except to seal out moisture, and ideally gas too.