An introduction to aluminum alloys used in SCUBA tanks

1. 1
Clearing the Air
on Aluminum Alloy Tanks
March 2016
Prepared By: Alan Levine

2. Tank Composition
• The most common SCUBA tanks
are made of:
– Steel
– Aluminum Alloy
• Aluminum Alloys generally consist
of either:
– 6351 alloy
– 6061 alloy

3. 6351 Aluminum Alloy
• 6351 alloy typically had 200-500 ppm of
lead
• LUXFER tanks built in 1982 and 1983 have
higher lead concentrations than those built
in later years
• The 6351 alloy is susceptible to “sustained
load cracking” which was previously called
“room temperature grain boundary creep”

4. Sustained Load Cracking
• Sustained load cracking (SLC) was
discovered in SCUBA tanks in 1983
– Cracks from SLC develop slowly
– The cracks can be aggravated by hydro
testing
– Typically these cracks form in the crown of
the tank

5. Sustained Load Cracking
• Sustained Load Cracking is a metallurgical
phenomenon that occasionally develops in
pressure vessels and structural
components
• SLC is not a manufacturing defect; it is a
phenomenon associated with certain alloys
and service conditions

6. Sustained Load Cracking
• Crack growth is reported
to be very slow by
LUXFER, a major
manufacturer of
aluminum high pressure
cylinders
• SLC Cracks are reported
to develop over periods
of eight years or more

7. Sustained Load Cracking
• Cracks in the neck can generally be
observed during routine visual inspections

8. Sustained Load Cracking
• Cracks in the body and shoulder may be
more difficult to detect
• Cracks in the neck can be
usually be identified with
non-destructive testing
using eddy current testing

9. 6351 Aluminum Alloy
• The easiest way to determine the alloy in a
tank is by date of manufacture
– Walter Kidde SCUBA 1972 through
December 31, 1989
– LUXFER 1971 through May 31,
1981

10. 6351 Aluminum Alloy
• The 6351 alloy can no longer be used in
tank manufacturing
• Current 6351 alloy tanks are still valid for
service if they pass the required tests
(i.e. annual visual, annual eddy current
testing, hydro testing every 5 years along
with visual and eddy current testing)

11. 6351 Aluminum Alloy
• Some air stations no longer fill
cylinders made from 6351 alloy
– This is a personal decision, not
the law
• It is illegal to condemn a cylinder
without proof that it is damaged
beyond allowable limits
– Simply being constructed of 6351
alloy is not a reason to condemn
a tank

12. 6351 Aluminum Alloy
• A great source for more information on the
6351 alloy look is the PSI company website
http://www.psicylinders.com/inspectors/library
/18-a-short-review-of-6351-alloy-aluminum-
cylinders
• PSI is considered by many to be the
experts in the field on SCUBA tanks

13. Disclosure
The information in this slide presentation is
intended to be an introduction to aluminum
alloys used in SCUBA tanks and is for
general use only and is not intended to
replace the regulations or provide guidance
associated with the use, inspection or
maintenance of cylinders constructed of
aluminum alloy.

14. 1
The LighthouseCompanies
www.lighthousecompanies.net
Highland Park, IL | Dallas,TX | Janesville,WI
847-257-5056 alan@lighthousecompanies.net