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Is Your Aluminum Cylinder Safe? By John Meyer You may have heard stories
about aluminum, cylinders and something called Sustained Load Cracking. The rumors imply that SLC is
a major problem with aluminum scuba cylinders and that it makes them dangerous
to use. All high-pressure cylinders
used in scuba, both steel and aluminum, can become dangerous if they are
physically Abused (i.e., dropped, dented,
or subjected to high heat, as in a house fire), routinely overfilled to
a working pressure Above the manufacture’s
rating for that cylinder or if they are improperly inspected at the routine
intervals Required for both visual and
hydrostatic tests - or not inspected at all. The current stories deal with
a particular aluminum cylinder made of an alloy known as 6351. This metal was used in making
scuba Cylinders, and in the aerospace industry up until 1988. This particular
alloy can develop slow growing cracks
known as “SLC.” It
is not a manufacturing defect; it is a characteristic of the metal. Physical abuse or routine overfill
can encourage the development of SLC. Since the cracks require about six years to grow to the point where they can be dangerous, they can easily be detected By the use of proper visual inspections, and by “eddy current” testing before they are a problem. Eddy current tests can “see” Cracks in metals before the human eye can see them. This alloy is not prone to SLC. You may have heard that these new cylinders were being replace, too. Actually, as “eddy current” Testing was being introduced, inexperienced technicians where rejecting cylinders based on the test because of improperly evaluated Testing results. Manufactures replaced a limited number of tanks so that they could run tests and figure out why they were being Returned. These tests showed no SLC in the tanks, but allowed the industry to provide better training to eddy current test Technicians to make the test more accurate:
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