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Clearing the Airon Aluminum Alloy Tanks

March 2016Prepared By: Alan Levine

Tank Composition

• The most common SCUBA tanks are made of:– Steel– Aluminum Alloy

• Aluminum Alloys generally consist of either:– 6351 alloy– 6061 alloy

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”

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

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

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

Sustained Load Cracking

• Cracks in the neck can generally be observed during routine visual inspections

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

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

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)

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

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

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.

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