A short Review of 6351 Alloy Aluminum Cylinders

Since 1985 numerous articles and government notices have been released describing concerns about cylinders made from 6351 alloy aluminum.  While the U.S. Government has determined no recall of the nearly 25 million 6351 alloy cylinders is warranted, annual inspection by trained inspectors is called for in the 1999 Department of Transportation (DOT) and National Institute for Occupational Safety and Health (NIOSH) bulletins.

A total of 15 SCUBA and fire fighter SCBA cylinders have ruptured explosively in the US from sustained load cracking with an additional 4 at international locations. Although small in numbers, these ruptures caused property damage and personal injury.

Sustained load cracking (SLC), a metallurgical anomaly, occasionally develops in 6351 aluminum alloy high-pressure cylinders.  As the name implies, SLC usually occurs in cylinders that remained filled for long periods of time.  US manufacturers that used 6351 alloy included Luxfer Gas Cylinders, Walter Kidde, Cliff Impact, Norris Industries (SP6688) and Kaiser (SP6576).  The permits for both Norris and Kaiser have expired and those cylinders are no longer approved for use in the US.  Luxfer stopped using 6351 alloy by mid 1988 and now provides a complete listing on its web site of dates when various cylinder models were changed to a proprietary 6061 alloy. Catalina Cylinders began making aluminum cylinders in 1986 using a 6061 alloy that is not susceptible to SLC.

SLC is a very slow progressing process.  Government findings on a ruptured scuba cylinder showed two cracks had progressed, one over 8.7 and the other 9.3 years.  For this reason the DOT specifies all cylinders made from 6351 alloy be carefully inspected by trained inspectors (DOT recommends PSI, Inc inspector training) with special attention given to the threaded neck area.

Quality inspections are the key to cylinder safety whether for SLC or any of the other damage types that precipitate cylinder ruptures. Several tools are now available to aid the inspector when examining cylinder threads.  Initially, the DOT called for a small magnifying mirror and light to assess threads. Now, two excellent tools provide a greatly improved view of the areas where SLC is likely to propagate. One is called a Thread Inspection Pipe (TIP) and a less expensive model called the Flare Optic Viewer (FOV).

In 1996 an eddy current instrument was designed specifically for 6351 alloy aluminum cylinder SLC examination. The Visual Plus instrument (sold by AIT) was closely followed to market by the Visual Eddy instrument (designed and sold by Flare Technology).  Both instruments, when used correctly, will readily identify even very small cracks.  Unfortunately, these instruments are often improperly used by operators causing many good cylinders to be incorrectly condemned. Formal, professional training is essential to ensure technicians can properly operate the system and interpret the results.

A few dive businesses have required eddy current instrument use on steel cylinders. These instruments were not designed for use on steel cylinders and will not work.  Further, there is no history of cracking in steel cylinders. Because of improper instrument use, Luxfer Gas Cylinders has issued a directive stating the proprietary 6061 alloy used on its cylinders made after May 1988 is not subject to SLC and the eddy instruments currently in service will often produce false positives, that is, report a crack where no crack exists.  Formally trained eddy current technicians learn how to avoid false positives.  That training is available at DEMA trade show seminars and at numerous US and Canadian locations by PSI affiliate instructors.

The US DOT has, in its proposed HM220F* rules that all 6351 alloy cylinders be examined at least annually with an eddy current instrument.  If enacted as proposed then those millions of cylinders, including scuba, found to be free of SLC will remain in service.  That same rule will require that those filling cylinders be given some form of protection. Commonly used plastic garbage cans, sheet metal tubs, etc. will not qualify as cylinder filling containers.

Some air stations no longer fill cylinders made from 6351 alloy.  While that is a personal decision, it may be a poor business practice.  A few dive business employees have told cylinder owners that 6351 alloy cylinders have been recalled. NO, scuba cylinders have been recalled.  Others state that the cylinder must be condemned outright.  Such statements border on fraud and it is illegal to condemn a cylinder without proof that it is damaged beyond allowable limits.  The Luxfer 2003 voluntary voucher program allowed owners of older cylinders to replace their cylinders at a reduced cost. That program has ended.

Several of the recent aluminum cylinder ruptures have attracted considerable industry attention while the more than 24 steel cylinder ruptures over the years are forgotten.  The prudent dive industry professional should be very cognizant that ALL high-pressure cylinders contain phenomenal energy and ALL should be given great care.  Every ruptured cylinder had obvious damage whether it was a crack in the threads, damage from excessive heat, extensive corrosion or other abuses.

The solution to safe cylinders is regular quality inspections by trained inspectors and diligent fill station operators.

For more detailed information about cylinder safety visit the PSI/PCI web site at www.psicylinders.com.

Cylinder inspection tools are available directly from PSI/PCI from our catalog (click the tank marked "Order" in the Training and Supplies section of our website or contact us directly at:

Phone: 425.398.4300     email  This e-mail address is being protected from spambots. You need JavaScript enabled to view it.




* This has been incorporated to CFR 49 Title 180, Appendix C

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