Hydrostatic Cracking

Information on Sustained Load Cracking in Aluminium Cylinders

What is sustained-load cracking?
  • SLC is a metallurgical phenomenon that occasionally develops in cylinders made of 6351 aluminium alloy, as well as other types of pressure vessels and structural components under stress for sustained periods of time
  • SLC has occurred in cylinders manufactured by various companies, including Luxfer
  • Cylinders that have been mechanically damaged, over-filled or abused are more susceptible to SLC.
  • SLC is not a manufacturing defect; it is a phenomenon inherent in the metal itself.
Are SLC related scuba tank ruptures widespread?
  • No. Of the more than 40 million all-aluminium cylinders manufactured by various companies over the last 45 years, only 20 SLC related or suspected SLC ruptures have been reported to the regulatory authorities around the world. All these ruptures were in cylinders made from 6351 alloy
  • Of these ruptures 12 have been scuba cylinders
How can I tell if my Luxfer cylinder is made from 6351 alloy?
  • The easiest way is to check the original hydrostatic test date stamped on the cylinder crown.
  • Luxfer manufactured 6351-alloy cylinders during the following periods:
  • United States: 1972 through mid-1988
  • England: 1958 through 1995
  • After these dates, Luxfer began making cylinders from a proprietary 6061 alloy, which is not susceptible to SLC.

In Australia, CIG Gas Cylinders manufactured scuba tanks from 6351 alloy from 1975 through 1990, and then switched to 6061 alloy. Luxfer acquired CIG Gas Cylinders in 1997, by which time Luxfer was manufacturing all scuba tanks with its own proprietary version of 6061 alloy. All Luxfer tanks manufactured in Australia under the names “Luxfer” and “Luxfer Australia ” have been made exclusively with Luxfer’s 6061 alloy

Why is it necessary to have my 6351-alloy cylinder inspected with an eddy-current device?
  • When properly used, eddy-current devices contribute significantly to early detection of hard-to-see sustained-load cracks.
  • However, an eddy-current test is not a replacement for a diligent visual inspection by a properly trained inspector.
  • Eddy-current devices are tools to enhance proper visual inspections.
  • Eddy-current devices currently approved by Luxfer for use with Luxfer 6351-alloy cylinders are Visual Plus, Visual Plus II, Visual Plus III, and Visual Eddy. (However, the only eddy-current device approved by Luxfer for use with cylinders made from Luxfer’s proprietary 6061 alloy is Visual Plus III; see question 12, below, for details.)
Why does Luxfer place so much emphasis on inspection?
  • Proper inspection is crucial to scuba tank safety-and it cannot be overemphasized that the quality of inspection is far more important even than the frequency of inspection! An untrained or improperly trained inspector can look at a 6351-alloy cylinder numerous times without detecting SLC. Make sure that your inspector is well-trained and competent.
  • Unfortunately, many untrained or improperly trained technicians continue to inspect cylinders, and no uniform standards for inspector training and certification exist among regulatory authorities around the world.
  • Luxfer is working with industry groups and government agencies to help establish such standards.
If Luxfer 6061-alloy cylinders are not susceptible to SLC, why has Luxfer exchanged 6061 cylinders that have been reported to have cracks?
  • After the introduction of eddy-current technology, Luxfer received reports of cracking in 6061-alloy cylinders. We accepted returns on about 1,200 of these cylinders to conduct extensive tests. Not a single cylinder was found to be cracked.
  • On Luxfer 6061-alloy cylinders, eddy-current devices sometimes show harmless “indications” that lead to “false-positive” readings for SLC.