1) The cylinder did not fail because Luxfer made it before 1990.
2) Cylinder failures like this are not common ANYWHERE.
The cylinder failed because the self-regulating SCUBA industry isnt doing a very good job of self-regulating and better wake up before that option is taken away. Sustained load cracking, or SLC does happen with 6351 alloy, this is a given. Luxfer was one of several manufacturers that made AL cylinders out of 6351. Theres a long history of different AL alloys. 6351 is just one of them. WE THINK they have a shorter lifetime than the current 6061 alloy, but 6061 hasnt been used nearly as long as the 6351 alloy.
In other words, it appears that the 6351 has a finite lifetime and we just dont know about 6061 yet (but lab results seem to point to the conclusion that its a better alloy).
So, are 6351 cylinders dangerous? No. Not by ANY common measure of safety. After on the order of hundreds of millions of fillings we have somewhere between 20 and 30 cylinders failing from SLC. Hence this is not a common failure ANYWHERE.
If SLC failures happened without any warning even this tiny, statistically insignificant amount could perhaps be considered dangerous. However, SLC typically develops over three to five years, which means you dont just get one or two shots at detecting it during a visual inspection, you get a minimum of three chances to discover SLC and take the cylinder out of service.
This is where the dive shops are failing, and where many are making matters worse. Many dive shops do not have qualified inspectors on staff, and/or consider an inspection a cursory glance in a cylinder to see if theres any corrosion. Inspecting the threads takes time and effort, and many shops dont expend that time and effort and THATS why we have cylinders letting go. You dont need an eddy tester to see if theyre cracks in threads, it just speeds up the inspection process. A good visual inspection will discover cracks. Every time an AL cylinder lets go due to SLC, you know that there are at least three bad inspectors behind the explosion. THATS the problem, not 6351. Everything wears out, with something catastrophic like a cylinder letting go, you better have a good way of finding out when its wearing out. An inspection, if correctly done, does just that.
Where shops are making matters worse is that some shops are starting to refuse to fill 6351 cylinders citing their concern over the alloy. Who knows if this is a genuine concern over the minimum-wage shop monkey that fills cylinders, or the far more likely reason that they can sell a newer cylinder (ka-ching! $) to the typical uninformed diver that believes everything that shop tells em. The result is the same, the shop is admitting that the visual inspection protocol that their self-regulated industry has set up isnt being implemented correctly. Hello big-government regulation if the industry doesnt clean house and soon. Some shops are doing the right thing and offering free eddy current testing of cylinders (
http://fdu.com/tanksafetyalert.htm for example). While this still admits that the current process is broken, theyre at least doing their part to fix it AND theyre not shafting the customer for the industrys failings.
The neck picture in
http://www.airsource-one.com/bea8.jpg shows what is currently thought to be the fingerprint of an SLC fracture. See the shadows on both sides, the right side about half way between the threads and the edge of the picture, the left side almost to the edge of the picture? Current theory is that this is oxidation on the faces of the crack, so these were HUGE cracks before the cylinder went, both extending at least an inch away from the threads.
If you want some good, sane reading on SLC, rather than FUD, turn to Bill High, the president of Professional SCUBA inspectors, inc.:
http://www.psicylinders.com/message.html.
As for the article
http://www.divenewzealand.com/53tank1.html, the fact that hes wet filling is a show of ignorance right then and there, so the articles author immediately becomes suspect.
But to assume that this author really is seeing SLC failures left and right whereas the US as a whole has only seen a little more than 10 brings me to an interesting theory. The US requires a hydro only once every five years where NZ requires them every other year. Could the more frequent testing be the problem down there? It wouldnt be the first time that a government-dictated cure was worse than the disease itself.
Roak