RadioBanter - View Single Post - Restoring a Hy-Gain Yagi...

April 29th 09, 07:38 AM posted to rec.radio.amateur.antenna

"lagagnon" wrote in message news:817062c5-
...

I am about to refurbish an old Hy-Gain TH3-Mk3 Thunderbird tri-band
yagi. This yagi has been used in a coastal environment and thus
the aluminum is slightly pitted and most of the connecting
hardware needs replacing.

I figure steel wool would work fine for the aluminum tubing, I
know how to test the traps, but I am wondering whether to use
stainless steel or galvanized hardware? Which would react least
over time with the aluminum? Also, I remember many years ago there
is an electrical joint compound stuff for using between the tubing
sleeves - can anyone please give me some brand names of this
stuff?

Any other ideas appreciated....

Larry VE7EA

Boy, the partial truths have been flying hot and heavy here. Allow
me to try to explain what is right and what is wrong...

You do _NOT_ want to use steel wool. Aluminum is anodic with
respect to steel. When you use steel wool, you imbed steel fibers
in the aluminum as many have noted. But what happens next is where
most posters are wrong. The aluminum around the steel fiber
corrodes protecting the steel from rusting. Thus your problem
becomes worse. Eventually the aluminum corrodes enough that it no
longer makes good electrical contact to the steel and the steel will
rust — or else fall out of the pit in the aluminum. You are left with
highly pitted aluminum.

I would suggest emery cloth, sandpaper, or a Scotchbright pad to
polish the aluminum. I would avoid stainless steel brushes too as
most grades of stainless are even more cathodic than steel with
respect the aluminum. Likewise avoid brass or copper brushes too.
For large areas, sand blasting works well if you use a fine sand and
not too much pressure.

As to hardware, I suggest hot-dipped galvanized bolts and nuts.
Zinc is slightly more anodic than aluminum, so neither material will
corrode much when in contact with each other. Hardware with a heavy
cadmium plating is also suitable, but due to the toxicity of cadmium
it is a little harder to find. Zinc-plated hardware offers minimal
protection because the zinc coating is so thin. If the hardware is
used in an application where minimal strength is needed, consider
aluminum hardware itself.

There are many joint compounds on the market for joining aluminum to
aluminum. Noalox and Penetrox are two common brands. These
compounds are typically a petroleum grease containing zinc particles.
These particles are usually produced by spraying molten zinc and air
through a nozzle. This produces small, irregular particles whose sharp
edges pierce the aluminum oxide surface film when the aluminum
pieces are tightly clamped together. This produces a good
electrical contact, and the zinc is very compatible with the
aluminum from a corrosion standpoint.

Whatever you do, avoid any contact of copper with the aluminum.
Where the transmission line connects, be sure and heavily coat any
copper solder lugs with solder. Try to keep a hot-dipped galvanized
washer between the lug and the aluminum.

Send me an email and I can send you some good articles on corrosion
that you might find interesting and informative. My experience
comes from many years working in the chemical industry. Even so, I
found the plant metallurgist to be a vital resource in material
selection for the instrumentation I designed.

--
73, Dr. Barry L. Ornitz WA4VZQ