Hi Kurt,
Congratulations on the new call. It is _way_ more cool than the
old one ;-)
To: <towertalk@contesting.com>
>Date: Sat, 02 Jan 1999 01:19:57 -0800
>From: Kurt Andress <NI6W@contesting.com>
>
>Bill Aycock wrote:
>
>SNIP
>
>> The effect does exist, and I still want to pursue this thread,
>> to find documentation of the effect, if it can be found. It
>> is important that we avoid detrimental effects when we can,
>> and this thread has opened some eyes (mine, at least) to a
>> problem that had little PR before (at least, to my
>> knowledge). If any of you come across a reference, please let
>> me know.
>
>I'd also appreciate more input on this subject!
>
>> I wonder what the effect of "NoAlOx", or similar treatments,
>> has on this effect?
>>
>
>From the data I have been able to gather on the anti-oxidant
>compounds, it appears that NoAlox, Penetrox, and Ox-Gard
>compounds for use with aluminum use zinc particles suspended in
>a variety of petroleum based vehicles.
Specifically regarding penetrox:
There are several different products sold under this name. They
have VERY different characteristics.
Penetrox "A" is zinc particles in a petroleum based vehicle.
Probably best suited for indoor use making copper or brass screw
clamp connections to aluminum wire. Incompatible with some types
of rubber and plastic.
Penetrox "A-13" is zinc particles suspended in a "synthetic"
vehicle. "Synthetic" is not defined on the page I was looking at
<http://www.galco.com/digest/burnchem.htm> but I'm assuming that
this is some form of silicone based oil or grease. If it is
silicone grease, and it has characteristics similar to Dow
Corning's "High Vacuum" grease, it would be much less likely to
evaporate or wash out of a connection than the petroleum based
vehicle. This would make it the stuff of choice for connections
exposed to the weather if the application involved a clamped
connection between dissimilar metals. This stuff is compatible
with rubber and plastics.
Penetrox "E" is copper particles suspended in the synthetic
vehicle. It is primarily for copper to copper connections. In
particular, it is used to make gas tight compression connections
for ground system conductors. This is also probably the stuff
(or something very similar with a petroleum base more likely)
that caused all of the problems when used on aluminum antenna
elements. Please be careful NOT to use this type on aluminum to
aluminum connections. Particularly if they are exposed to
weather on a long term basis.
Snip... (long (and useful) treatment of the operation of
petroleum based joint compounds)
Although the synthetic vehicle zinc based compounds make some
sense for connecting copper wire or lugs to aluminum, I can't
help wondering why we would use any of these zinc containing
compounds when clamping aluminum to aluminum in an antenna
element joint. While it is true that zinc is relatively close to
aluminum in the electrochemical series, it is not as close as
aluminum is to itself.
Why introduce an electrochemical corrosion problem (even a
relatively low potential one) where none exised before? Why risk
making a connection that depends on zinc particles that will be
sacrificed (in a salt atmosphere) or will cause aluminum to
deteriorate (in an acidic atmosphere) once the vehicle is lost.
If the connection actually is made by trapping zinc particles,
then once they are gone, what guarantees the retention of
sufficient clamping force on the aluminum to aluminum joint that
remains?
My personal preference for protecting these joints is to do the
following:
1. Using a Scotchbrite pad that is covered with Silicone grease,
shine up the surfaces that are to be clamped. This breaks
the oxide on the aluminum surface and immediately protects
the exposed metal from exposure to the atmosphere so it
doesn't immediately reoxidize. Use a grease like Dow's High
Vacuum grease that is insoluble in water (even contaminated
water) and resists evaporation for extremely long time
periods and in extreme environmental conditions.
2. Using a (clean) finger coated with the same grease, spread an
even coating (can be quite thin) of the grease over the
cleaned area. This is to guarantee 100% coverage of the
surface and permit you to feel and remove any pieces of grit
or metal filings that might contaminate the joint. I think
we can all agree that a contaminated joint is a bad thing.
3. Assemble the pieces to be joined and apply the clamping
force. For hose clamp type joints, keep trying to rotate the
pieces relative to one another as you tighten the clamp until
this is no longer physically possible. This will guarantee
that there is sufficeint clamping force to expel the grease
out of the connection zones. It will also permit more
surface area to participate in the connection zones as the
pieces deform when worked against one another.
For riveted joints, you take what you get in terms of
clamping force and contact zone area. But so far, this seems
to be entirely adequate if the rivets are sized and installed
properly.
When you are done, you will have a connection that contains no
dissimilar metals and has a relatively permanent occlusive
barrier to moisture and gasses entering the joint. The
connections that I have made using the above method have been the
ones that gave me the least amount of trouble for the longest
period of time.
73, Eric N7CL
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