Sustained-load Cracking FAQ for Cylinders in Australia and New Zealand

We have compiled a list of our most frequently asked sustained load cracking questions (and our responses) below. You may read the entire set of questions, or jump directly to a particular question by clicking on a red link:

1. What is sustained-load cracking?
2. How quickly do the cracks grow?
3. How many aluminum cylinders have exhibited SLC?
4. Which types of cylinders are most susceptible to SLC-related ruptures?
5. Are SLC-related scuba tank ruptures widespread?
6. If there have been so few SLC-related cylinder ruptures, why is there so much alarming information about them on the Internet?
7. I’ve seen evidence on the Internet that a Luxfer scuba tank exploded in Perth , Australia , early in 2005. Was SLC involved?
8. I’ve heard that Luxfer have closed their plant in Australia and they are no longer manufacturing scuba tanks. Is that true?
9. Why is it necessary to have my 6351-alloy cylinder inspected with an eddy-current device?
10. Why does Luxfer place so much emphasis on inspection?
11. How can I tell if my Luxfer cylinder is made from 6351 alloy?
12. If Luxfer 6061-alloy cylinders are not susceptible to SLC, why has Luxfer exchanged 6061 cylinders that have been reported to have cracks?
13. How often should my Luxfer 6351-alloy scuba tank be inspected?
14. Should I stop using my 6351-alloy scuba tank?

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SUSTAINED-LOAD CRACKING (SLC)

The following are Luxfer's responses to frequently asked questions about sustained-load cracking (SLC) in aluminum scuba cylinders made of 6351 alloy.

1. What is sustained-load cracking?

• SLC is a metallurgical phenomenon that occasionally develops in cylinders made of 6351 aluminum alloy, as well as in 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.
  

2. How quickly do the cracks grow?

• Very slowly, as extensive research by Luxfer and outside laboratories has shown.
  
• No scientific evidence supports rumors and claims of "fast crack growth."
  
• Cracks typically take eight or more years to grow large enough to cause a cylinder leak.
  
• Because SLC growth is so slow, properly trained inspectors have adequate opportunity to detect cracks during the normal requalification process.
  

3. How many aluminum cylinders have exhibited SLC?

• Out of a total population of approximately 1,073,000 Luxfer scuba cylinders made of 6351 alloy, only 1.25% have exhibited SLC.¹
  
• Out of Luxfer's total 6.1-million population of 6351-alloy cylinders, the SLC rate is slightly less than 0.37%.
  
• While we do not have complete statistics on cylinders manufactured by other companies, industry experts estimate that out of a worldwide population of more than 30 million 6351-alloy cylinders, far less than 1% have exhibited SLC.
  

4. Which types of cylinders are most susceptible to SLC-related ruptures?

• Any type of cylinder manufactured from 6351 alloy could conceivably develop a sustained-load crack over time if subjected to certain conditions.
  
• However, a cylinder is more likely to rupture in applications where pressure is higher and where overfilling and abuse occur more often.
  
• Historically, the applications most susceptible to SLC-related rupture are scuba and SCBA (life support).
  

5. Are SLC-related scuba tank ruptures widespread?

• No. Of the more than 40-million all-aluminum cylinders manufactured by various companies over the last 45 years, only 20 SLC-related or suspected-SLC ruptures have been reported to regulatory authorities around the world. All these ruptures were in cylinders made from 6351 alloy.
  
• Of these ruptures, 12 have been scuba cylinders.
  
• Let's look at the actual record.
  

Aluminum scuba tank ruptures

*Luxfer Gas Cylinders acquired CIG Gas Cylinders, an Australian manufacturer, in 1997. CIG Gas Cylinders had stopped using 6351 aluminum alloy in 1990, and Luxfer did not use 6351 alloy for cylinders that Luxfer subsequently manufactured in Australia under the name Luxfer Australia. Only Luxfer’s proprietary 6061 aluminum alloy—which is not susceptible to sustained-load cracking—was used for Luxfer Australia cylinders.

6. If there have been so few SLC-related cylinder ruptures, why is there so much alarming information about them on the Internet?

• A great deal of misinformation and exaggeration about SLC is attributable to rumors and inaccurate reports spread by word-of-mouth and the trade press, but especially by the Internet, where the proliferation of inaccuracies is widespread and essentially unregulated.
  

7. I’ve seen evidence on the Internet that a Luxfer scuba tank exploded in Perth, Australia, early in 2005. Was SLC involved?

• An older scuba tank did, in fact, rupture in Perth in January 2005, and the likely mode of rupture was SLC. The investigation is ongoing. However, the 6351-alloy tank involved in the accident was not manufactured by Luxfer, but by CIG Gas Cylinders, a company acquired by Luxfer in 1997. CIG Gas Cylinders used 6351 alloy to manufacture scuba tanks from 1975 through 1990, and then switched to 6061 alloy. All scuba tanks manufactured in Australia by Luxfer have been made from Luxfer’s proprietary 6061 aluminium alloy, which is not susceptible to SLC.
  

8. I’ve heard that Luxfer have closed their plant in Australia and that they are no longer manufacturing scuba tanks. Is that true?

• Luxfer has not stopped manufacturing scuba tanks. The Luxfer plant in Australia was closed in 2004 during a consolidation of Luxfer’s global manufacturing capacity. However, Luxfer has maintained a sales office and distribution center near Sydney. The Luxfer tanks now sold in Australia are manufactured at the Luxfer plant in California, which produces more aluminium tanks than any other facility in the world. Luxfer also manufactures scuba tanks in England. Luxfer remains fully committed to supplying the highest-quality tanks to the scuba industry worldwide.
  

9. 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.)
  

10. 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.
  

11. 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.
  

12. 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.
  

13. How often should my Luxfer 6351-alloy scuba tank be inspected?

• In Australia and New Zealand , scuba tanks must be inspected and requalified (including a hydrostatic retest) annually. Moreover, 6351-alloy aluminium scuba tanks must undergo an eddy-current test every two years.
  

14. Should I stop using my 6351-alloy scuba tank?

• As long as your 6351-alloy scuba tank is properly inspected and requalified in accordance with regulations in Australia and New Zealand, there is no reason why you cannot continue to use it.

FOOTNOTES:

¹Statistics are current as of April 15, 2005; statistics are subject to change.