On Jan 12, 2:35*am, Richard Clark wrote:
On Mon, 11 Jan 2010 21:17:13 -0800 (PST), Tom Horne

wrote:
If I use a dielectric union for the joint and bond around it for
lightning safety will that solve the corrosion problem by preventing
direct contact between the two dissimilar metals or would the bonding
cause the same destructive current flow as the direct contact?

Hi Tom,

I don't know what you mean by bond that is both insulative to cathodic
action, and conductive to lightning. *To me, bonded metals don't
introduce a complaint of anticipated galling. *If they are bonded
(soldered, brazed, or welded), you don't expect to disconnect them and
suffer galling problems. *If they are soldered, brazed, or welded,
then the seam can still support cathodic action - it is simply a cell
loaded with the short of the bond if I read it that way.

I don't often find myself in your situation, so I have little to go on
beyond the common discussion. *I have had to deal with these issues
with fine measurement where it is always lingering and it often took
heroic effort (careful, this is hyperbole unless you have to make a
living at it) to succeed.

Jimmie may have something more to offer.

73's
Richard Clark, KB7QHC

Jimmie & Richard
Sorry for not being more clear. I was asking if the use of a
dielectric union to join the two pipes would prevent the corrosion of
the threads of the union now used instead of a galvanized coupling
even if I still had lightning down conductors connected to the bottom
of both pieces of pipe. In other words the two pieces of pipe would
still be joined mechanically by a dielectric union and electrically by
the lightning down conductor. 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.
--
Tom Horne, W3TDH

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.