[TowerTalk] RADIAL WIRE, Aruba corrosion

[Georgens, Tom]

Salt corrosion is a subject close to my heart.  In fact, in about 48 hours,
I will get to see the latest bit of wrath nature has wrought on my station
in Barbados.

Any steel that is not galvanized or stainless will rust in a few months.
Copper split bolts will turn green overnight.  That said, I have never seen
stainless corrode like some others have reported.

One lesson I learned is that all element joints have to be protected.  While
dissimilar metals are part of the problem, my biggest problem has been salt
building up on anything that protrudes into the airflow. It was very common
for salt to build up around clamps and antenna joints.  This weakens the
aluminum and makes it prone to vibration damage.  When I first took over the
station, 4 element tips has broken off and there were a bunch more broken
tips in the storeroom.  On top of that, it makes it nearly impossible to
disassemble the elements.

My solution has been to tape across all element joints using a good tape.  I
then overcoat it with liquid electrical tape.  I was very skeptical of this
stuff but it has worked very well standing up to the water, salt, and UV.  I
have yet (and I emphasize yet) to lose an element tip, and it has the added
benefit of making it easier to take the elements apart.

In the last few years, I have learned many lessons the hard way but things
are getting better.

On the other hand, keeping the computers and incoming phone service working
has been a perpetual problem.

73, Tom W2SC 8P5A





-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com]On Behalf Of Terry Gerdes
Sent: Sunday, February 13, 2005 8:51 AM
To: TOWERTALK at contesting.com
Subject: Re: [TowerTalk] RADIAL WIRE, Aruba corrosion


Having spent 11 years working on radar systems in the Marshall Islands, I'm
a bit of a expert of issues of maintaining antennas in tropical salt
conditions.   In the case of the Marshall's, you are setting 8 degrees north
of the equator.   The temperature is a constant 80 degrees F (plus/minus 5
degrees) year round and there are constant trade winds blowing 10 months of
the year coating all exterior surfaces with warm salt spray.   Keeping
anything outdoors in this environment is a real challenge.  For example, a
new bicycle will last a couple of years and either the frame will rust in to
or the spokes will rust out.

On the antenna side, the real issue is the corrosion due to the dissimilar
metals.  Anywhere there is a dissimilar metal, there is a galvanic reaction
and the stronger material will eat away the weaker material.   For the
amateur antennas, you traditionally have stainless steel bolts holding
together aluminum elements.  The aluminum will simply erode away where it is
in contact with the stainless.   Here is a picture of a less than six-month
old Mosley antenna,  http://www.ab5k.net/images/kwajant2.jpg.   You can see
on the aluminum phasing lines where the stainless bolts are attacking it.

You can see the Mosley boom at  http://www.ab5k.net/images/kwajant1.jpg.
The stainless steel u-bolts are very rusty.  They were treated with
never-seize before they went up.  I believe the use of the never-seize
probably increased the rusting on the bolts due to adding more dissimilar
metals.  That is not really bad as a good coat of rust provides a protective
coating which helps slow down the corrosion a bit.

The corrosion issue is serious and is very hard to stop.  Any antenna taken
into a environment like this needs to be hardened.  For example when
assembling aluminum elements use a aluminum pop rivet rather that a
stainless steel bolt or hose clamp.  You will probably still have some
dissimilar metals due to different alloys in the aluminum but it will slow
down the corrosion.

On areas where you have to have dissimilar metals, due to structural
strength requirements, the use of a sacrificial washer works well.  Rather
than have the stainless bolt erode away the aluminum element, you place a
sacrificial aluminum washer between the stainless bolt and the aluminum
element.  That way most of the corrosion will take place on the sacrificial
aluminum washer.   The sacrificial aluminum washer can be replaced as
needed, assuming you can get the rusted bolt out.

At work at the TRADEX radar, dissimilar metals also caused both major and
minor repairs.
http://www.ab5k.net/images/tradex.jpg.
http://www.ab5k.net/images/TradexFeed.jpg
http://www.ab5k.net/images/TradexFeed2.jpg.


 73 Terry AB5K,      ex KX6OI and V73AQ
www.ab5k.net



> Mike describes the effects of the severe Aruba ocean atmosphere on an
> aluminum vertical over 2 years.  I wonder if an initial assembly using
> thorough greasing of all joints with Noalox or equiv, and periodic touch
ups
> could preserve the connection integrity for a significant period, or
whether
> even then,  the attack by the salty air is not significantly slowed.
> 73, DX, de Pat Barthelow  AA6EG   aa6eg at hotmail.com
>
>
>
> >From: "MIKE GREENWAY" <K4PI at peoplepc.com>
> >To: "Jim Lux" <jimlux at earthlink.net>, <TOWERTALK at CONTESTING.COM>
> >Subject: Re: [TowerTalk] RADIAL WIRE
> Snip;
> On the subject of
> >aluminum oxidizing , we had a Gap Challenger that was put up new in Aruba
> >using the supplied hardware.  In 2 years the aluminum started turning to
> >white oxide powder where the screws go through the tubing.  The holes for
> >the screws go so big and the area around them got so brittle it had to be
> >taken down  73 Mike K4PI
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.

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