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Subject: Re: TWL: Aluminum Boat Problems in the News
From: Arild Jensen (elnav@XXX.XXX)
Date: Thu Feb 21 2002 - 02:10:01 EST
At 10:10 PM 02/20/2002 -0800, Kim Boyce and Eric Thoman wrote:
>You may have heard in the last couple of days about some hull cracking and
>corrosion issues discovered on aluminum boats built in the Seattle area.
>>> snip<<<
>According to my friend and Naval Architect, the issue is huge and may
>effect dozens if not hundreds of aluminum boats recently delivered . . .
>>> snip<<<
>I was told that the supplier of the material has stated that they never
>claimed it was intended for marine use. You wonder how it could have become
>so common so quickly.
REPLY
I mentioned picking up a book on Corrosion while at IBEX.
It has several chapters on metallurgy, alloys and physical properties.
With reference to 5083 Alloy it says in part:
MArine - Grade Aluminum Alloys
5000 series Wrought Alloys ( A95xxx) and Casting alloys ( A05xx.x)
The 5000 series alloys have good welding characterisitcs and excellent
corrosion resisttance in the marine environment.
The most popular marine alloys of this group are 5052, 5083, 5086, and a
newer member of the family 5456.
Europeans seem to prefer 5083 and 5454, respectibvely, for these
applications ( hull and deck)
There is really not a great deal of diffference between 5083 and 5086 but
US builders find 5086 a littel easier to bend.
Further on there is a table giving comparative strenghts of these same
alloys with 5083 having the highest strenght values.
Considering that the rest of the book appears to be very well researched
and documented with refernces to ASTM and similar professional
certification or testing associations and standard setting bodies, I'm led
to believe that this book reflected mainstream thingking at the time it was
written. The book is copyright 2001 ISBN 0-07-155019-4
REAding this would certainly give the reader the impression that 5083 is a
known and designated "marine" application alloy with desirable
characteristics such as strenght and corrosion resistance.
It certainly looks like some unforseen ageing characteristic of the alloy
has finally come to light.
Being a "wrought" alloy the material is finished with a physical stress
treatment like rolling or extrusion. Not cast, then cooled in a mold which
is primarily a heat treatment process.
Could it be that a sizable mill run had some undetected impurity or that
some erroneous settings of the processing mills is responsible for the
failures detected in some of these vessel.
It wouldn't be the first time that a supposedly identical alloy made in
one country had minute differences compared to the same alloy produced in
another country and thus by a slightly different process and mill formula.
Regards
Arild
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