Sustained-load cracking

    Back to topWhat 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.

    Back to topHow 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.

    Back to topHow 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.

    Back to topWhich 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).

    Back to topAre 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, 11 have been scuba cylinders. Six occurred in the United States, and the remainder occurred elsewhere in the world.
    • Let's look at the actual record.

    Aluminum scuba tank ruptures

    Oct-88 Southport, Australia SLC No Luxfer
    Jun-94 Miami, Florida SLC Yes Luxfer
    Jan-98 Corlette, Australia SLC No CIG Gas Cylinders*
    Feb-98 Riviera Beach, Florida SLC Yes Walter Kidde
    Aug-98 Tairua, New Zealand SLC Yes Luxfer
    Dec-98 Tampa, Florida Not determined No Luxfer
    Apr-99 British Colombia SLC No Luxfer
    Mar-00 Key Largo, Florida SLC Yes Walter Kidde
    Jun-00 Miyako Island, Japan SLC No CIG Gas Cylinders
    Jul-04 Oahu, Hawaii Not determined Yes Walter Kidde
    Sep-04 Videlia, California Not determined No Walter Kidde
    Jan-05 Perth, Australia SLC No CIG Gas Cylinders
    Jun-07 Rhode Island SLC No Luxfer

    *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.

    Back to topIf 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.

    Back to topWhy 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 10, below, for details.)

    Back to topHow do I know if the person inspecting my cylinders is properly trained?

    • Luxfer recommends taking your cylinder to an authorized Luxfer service center or to an inspector trained by Professional Scuba Inspectors, Inc. (PSI) or the Association of Scuba Service Engineers & Technicians (ASSET).2
    • It cannot be overemphasized that the quality of inspection is far more important than the frequency of inspection! An untrained or improperly trained inspector can look at a 6351-alloy cylinder numerous times without detecting SLC.
    • 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.

    Back to topHow 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 the above 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.

    Back to topIf 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.

    Back to topHow often should my Luxfer 6351-alloy scuba tank be inspected?

    • DOT requires requalification (hydrostatic retesting and visual inspection) of all aluminum scuba tanks every five years, regardless of alloy.
    • Both the DOT and the U.S. scuba industry recommend an annual visual inspection for all 6351-alloy scuba tanks. Luxfer supports this recommendation.
    • For its 6351-alloy tanks, Luxfer has established a manufacturer's requirement for a visual inspection, including an eddy-current test, at least every 2.5 years.

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