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