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The message below is from
Harry Francis. Harry, I DO think
we should post this. I found it
fascinating and very educational. I’ve
never known this stuff but always wondered. Thanks!!!
Ed
Burkhead
http://edburkhead.com/
ed -at- edburkheadQQQ.com (change -at- and remove the
QQQ)
-----Original Message-----
From: Harry Francis
Sent: Wednesday, June 23, 2004
9:21 AM
To: ed
Subject: Re: [COUPERS-FLYIN]
center section corrosion
Ed,
I think the comments are great....but
perhaps you should add some comment about intergranular corrosion problems.....
The cause of this goes back to the manufacturing process, where
it appears one of the suppliers of aluminum was perhaps less careful in their
manufacturing of the aluminum. However, it may be too involved for the average
reader.
On Making aluminum, the bauxite is
heated, and lime is added to remove impurities such as sulfur, iron, etc....
This slag is removed from the molten aluminum. IF all slag is not removed, then
the resulting extrusions contain the slag particles..... mainly Calcium Aluminum
Sulfates and/or Calcium Ferrite/Ferrous Sulfates. This compound is called
ettringite. on subsequent exposure to moisture it expands at pressures
approaching 30,000 psi....internal corrosion.
Now, why last for 50 years before
reacting ??? I suspect, that a removal of even slight layers
of the metal on an extrusion due to surface corrosion repair ( ie: mice pee)
now opens the slag deposits to moisture, allowing the ettringite to
react.....and expand, destroying the beam...Maybe a new drill hole for
attaching equipment, fasteners, etc....
It seems that several manufacturers
supplied aluminum parts to ERCO, so it is difficult to determine which
company was responsible....or perhaps the long term life of Ercoupes was
not considered ( as Fred has been quoted as expecting the Coupe's life to not
exceed 10 yeas at most.)..... so the material was suitable for this life
expectancy, etc....
Surface corrosion is expected on
aluminum parts...that is what PROTECTS THE ALUMINUM... the formation of a
waterproof coating due to exposure to air. So, if it's dull, that's
good.....clean it, and it gets dull again, trying to protect itself...clean
again and expose the surface to the air, and it gets dull
again.../..etc..etc.. This self protection property of aluminum alloys is
what makes it so suitable for aircraft use.....UNLESS, we get mice pee, salt
water, acid rain, etc that make soluble water compounds that overcome this
natural protection....
I can see a potential problem with over cleaning
main spars ( opening hidden pockets of ettringite), which were off no
problem; now exposed to the atmosphere ( moisture) and destroying a
previously good spar!
Now, what to do?
Cleaned and prepared with corrosion
protection coatings solves the problem, I Think.
BTW, this is the same chemical reaction
that causes concrete to expand and rupture.
Interestingly, the Chinese have been
making ettringite for probably more than a thousand years.... They use it ( it
is also available in the USA/World) as a blastless demolition agent, to quarry
rocks, destroy inside concrete structures, etc where use of explosives is not
suitable... It is a neat material, but not if left in aluminum extrusions.....
Recently large blocks of stone were
examined in the Boston area, where the rocks had depressions ( half holes along
the edges).....where the stone was broken into smaller pieces. I suspect these
stones were left there by the Chinese in 1422/23, and were originally used as
ballast stones in their large trade ships..... similar stones are found on the
ocean bottom in Bimini.... See: www.1421.tv 1421,
The Year The Chinese Discovered America.
Regards,
Harry
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