Why Your Old Aluminum Tank May Be Rejected for Testing

Reader Ed Leiter (Bar Harbor, ME) wrote in to tell us that when he sent four tanks to a company that handles hydrotesting, it refused to test two of them. The reason given was ‘weak alloy.’ “I was very surprised because the two MKII U.S. Divers Aqualung tanks, 54- and 64-cu-ft. aluminum tanks respectively, had been sent out for testing in the past. The note on them when they were returned untested was ‘Alloy 6351-T6/No test.’ Is the ‘weak alloy’ issue something new to the diving community?”

No, Ed. In fact Undercurrent has written about it before, and because many divers have old 6351 aluminum alloy tanks, it’s a good time for an update. We asked Mark Gresham, CEO of cylinder inspection firm PSI, Inc., about the problem and here’s what he told us.

“While the U.S. government states that no recall of the nearly 25 million 6351 alloy cylinders out there is warranted, the Department of Transportation (DOT) and the National Institute for Occupational Safety and Health called for annual inspection by trained inspectors in 1999.

“Twenty-one recreational and firefighter cylinders have ruptured explosively in the U.S. from sustained load cracking (SLC), with an additional four internationally. SLC usually occurs in cylinders that remain filled for long periods of time. U.S. manufacturers that used 6351 alloy included Luxfer, 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 U.S. but are valid in Canada. Luxfer stopped using 6351 alloy by 1988.

“SLC is a slow progressing process. Government findings on a ruptured cylinder showed one crack had progressed over 8.7 years and the other over 9.3 years. For this reason, the DOT specifies all cylinders made from 6351 alloy be carefully inspected by trained inspectors with special attention given to the threaded neck area.

“Some air stations no longer fill 6351 cylinders and some hydrostatic retesters won’t test these cylinders. While a few dive businesses and hydrostatic retesters have informed divers that 6351 alloy cylinders have been recalled, that is not true. And it is illegal to condemn a cylinder without proof that it is damaged beyond allowable limits. All high-pressure cylinders contain phenomenal energy and 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.”

So, Ed, if your hydrotester didn’t find any damage but just won’t fill the tanks on principle, you might find another hydrotester who will test them for you.