Thin oxidized coat protecting SS!! Live and learn. Aluminum oxidizes on the
surface and thereby protects the rest.
There is a type of corrosion for SS called oxygen deprivation corrosion but
is usually of more interest in marine engineering/boats.
Once upon a time long long ago in a land far far away (San Diego California)
I took a sabbatical for over a year and worked as a field service engineer
in marine electronics. I have seen plenty of aluminum to SS contact with
corrosion. Prior to the USA tuna fleet being regulated out of existence
(accelerated by over fishing... One tuna fish recently sold for $70,000 to a
Japanese restaurateur) there were (1980's) several $10,000,000 tuna seiners,
the ones with helicopters on a pilot house roof helipad, that had aluminum
superstructures like British warships but not expecting attack by Exocet
missiles as in the Falklands went with aluminum for its
weight-strength-corrosion modes. Pieces of aluminum channel were welded
onto the aluminum deck to serve as mounts for marine band HF antenna tuners
and vertical HF whips. RF ground connections were terminated to the aluminum
antenna tuner mounts using SS fasteners (through bolted) to secure crimp
connectors (crimped to the coax braid) to the aluminum stand.
Lets see... we have aluminum in contact with crimp connectors (tinned copper
alloy) in contact with tinned copper braid in contact with SS bolts too.
Typically the aluminum in the vicinity of the SS fasteners was cratered
badly as the least noble of the metals sacrificed itself in favor of the
more noble. (Aluminum was the least noble.) Boxes made of glass fiber filled
plastic housing the auto-tuners were bolted to the aluminum stands as well
with no copper involved. Those fasteners corroded too, just not as much as
the case where copper was included. If you could get the attention of an
installer and make an impression on his mind you might get him to apply
silicone RTV (non acetic acid emitting type) to exclude moisture, a key
ingredient in the electrolytic corrosion. and get things to last longer.
Often these connections would not last one fishing trip when not properly
protected from moisture.
Can we learn from this? Maybe if we exclude moisture from the aluminum-SS
contact we will not have an electrolyte and no electrolytic corrosion.
Patrick NJ5G
-----Original Message-----
From: Hans Hammarquist
Sent: Thursday, January 09, 2014 12:30 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Fwd: N2JFS photos of bolts
I was recommended but Universal to use galvanized bolts as the SS are not
available in high enough grade. (I don't remember now what grade they
recommended.) In my mind I will replace the bolts in a few (maybe 10 - 15)
years. That will also give me an idea what condition they are.
I don't know how important the grade issue is and in my case I think I
already have both "belt and suspenders" as I have a self-supporting tower
equipped with guy wires. (The tower should be able to sustain 135 mph even
if the base gives in.)
SS bolts are not a panacea. Even if the bolt doesn't corrode, which it
actually might anyhow, it might damage the surrounding aluminum. Has
something to do with Redox potential. The problem is that the Redox
potential is not stable for aluminum and SS steel, in other word, you can
not predict what will corrode when you connect and aluminum piece to a SS
piece. When dirt cover the two metals it will form the destructive
electrolyte, which will cause the corrosion. Depending on what the dirt
contain you will get different results (place of corrosion). If you can keep
the dirt away (not necessary easy) you can prevent or slow the corrosion
rate (needless to say, really).
As a final note: SS needs access to air to stay stainless. A thin oxide
layer on the steel surface serves as the corrosion prevention. If you cover
the steel and the protective layer get damaged the corrosion can set in very
fast (know that from experience, not from a tower, though).
Hans - N2JFS
-----Original Message-----
From: john <john@kk9a.com>
To: towertalk <towertalk@contesting.com>
Sent: Thu, Jan 9, 2014 9:28 am
Subject: Re: [TowerTalk] N2JFS photos of bolts
I had the same bolt issue with my first Universal tower. I have never seen
corrosion of the base, except in Aruba and it was only surface rust. I had
filled the square tube with concrete too keep if from filling with water.
As I recall the supplied bolts were just poor quality zinc plated.
Aluminum and galvanized steel touch in many areas of my tower systems with
no issues so I do not think it is a dissimilar metal problem. In fact, in
corrosive environments I have seem many instances were stainless steel
destroyed the aluminum it was in contact with so SS hardware has its
problems.
John KK9A - P40A
To: towertalk@contesting.com
Subject:[TowerTalk] N2JFS photos of bolts
From: kr2q@optimum.net
Date: Thu, 09 Jan 2014 12:31:15 +0000 (GMT)
Wow..those photos bring back lots of memories from the late 60's/early
70's..
I had an aluminum (Heights) crankup tower (84 feet?).
I noticed some "rust" one year and pulled out the bolt that came with the
tower.
They looked well on the way to what you showed in your photos.
Yeah...you can't use steel bolts on an aluminum tower..this happens every
time.
So I replaced all of the bolts with SS. Worked great and let me sleep at
night.
However, I was always worried about the Al to Steel junction at the base.
Also, the steel
triangles at the top/bottom of each (telescopic) section "rusted"
(dissimilar
metals - eaten
away) pretty badly. I had SS triangles fabricated, but by the time they
arrived, I went off
to college and wen I returned, I found a greenhouse where my tower used to
be.
:-)
So I bought some taper sections and no longer had a crank up. My dad was
kind
enough
to let me "move the tower" to the other side of the house...again using SS
bolts..but still had
the same issue with the base.
When I got my own place, I bought steel crankup towers. Heavier but no
worries
about the
aluminum eating away the steel.
de Doug KR2Q
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