Hello,
Here's some hide to chew. It's a repost of a repost. Might have been covered
here, but I didn't see it. It gives me more to think about when considering
buying tanks at yard sales....
John



A local CA boat captain wrote:
There are two principal alloys of aluminum used for aluminum cylinders
-- 6351 and 6061.  Aluminum cylinders manufactured with 6351 cylinders
had a history of developing Sustained Load Cracks.  Of the millions of
such cylinders manufactured, there was not a very large percentage,
well less than 1%) that developed such cracks, but it was enough to
cause aluminum cylinder manufacturers to stop using this alloy.
Luxfer, Walter Kiddie, and Norris Industries, among others used this
alloy (Catalina has never used it).

Because of the problem, Luxfer transitioned to using only 6061 alloy
in their cylinders.  This transition was completed in August of 1988.
You can check Luxfer's website
(http://www.luxfercylinders.com/news/releases/20030617.shtml) for the
unvarnished word on Luxfer cylinders and their trade-in program.  In
short, Luxfer requires all Luxfer cylinders manufactured with 6351
allow aluminum to have an eddy current test as part of the annual
visual inspection.  The specifically recommend that their 6061 alloy
cylinders NOT be eddy current tested.

As a mater of interest, in August of 2002 the US DOT removed the
authorization for FURTHER MANUFACTURE of aluminum cylinders with 6351
alloy.  This does not mean that existing cylinders have been recalled
or are unsafe.  It just means that no future cylinders can be made of
this alloy.