Directly answering your question, there has been no documented catastrophic failure of a properly inspected 6351-T6 alloy scuba tank due to sustained load cracking since the eddy current inspection protocols were adopted.
There have been a few cases of properly inspected 6351 T-6 alloy tanks leaking through the shoulder (*more on that below), but again, none have catastrophically failed due to sustained load cracking since the eddy current protocols began being used.
To put the number of pre-eddy current failures and the overall risk in perspective, there were 25 million 6351 T6 alloy tanks made, and a grand total of 19 catastrophically failed, 15 in the US and 4 abroad. That's a failure rate of .000076%. In comparison over the same time period there have been 24 catastrophic failures of steel tanks (due to improper inspection, pitting, etc) - and almost no mention of steel tanks being "dangerous".
Sustained load crack propagation is very slow and the five year interval between re-qualification of 3AL tanks is sufficient to catch that.
* Now...there is one small problem with how 6351 T6 tanks are requalified. For efficiency purposes nearly all RINs perform the visual inspection first - based on the logic that it makes no sense to hydro test a tank that won't pass the visual inspection. The problem is that the eddy current test is done during the visual portion of the requalification. I have personally observed tanks pass the visual eddy portion of a requalification and then have a previously undetectable crack propagate during the hydro test portion of the requalification. In one case, the tank failed through such a crack during the hydro test and came out spitting water through the crack.
My take away on this as it pertains to the local dive shop setting, is to ensure the local dive shop also does an annual eddy current inspection using the Flare Technologies Visual Eddy instrument, the AIT Visual Plus instrument, OR performs an enhanced visual inspection using a Thread Inspection Pipe or the Flare Optic Viewer. Both of the latter visual inspection techniques will detect a crack long before it propagates to the point of failure, and one or the other should also be used to confirm Visual Eddy or Visual Plus results to guard against false positives that occur when poorly trained or over zealous staff find a "crack" during an eddy current inspection. In many cases it's not a crack at all, but rather a normal fold from the manufacturing process.
More importantly, the shop's role with 6351 T6 tanks is to backstop the RIN by performing an additional eddy current or enhanced visual inspection to detect a crack that may have become visible during the hydro test phase of requalification (after the RIN did the required Visual Eddy inspection) as RINs in general are hard headed and don't see the logic of doing the VE inspection last, and the CGA and DOT aren't requiring the VE be done last.
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With all that said as a qualifier, there is no reason for a shop not to do a thorough visual inspection of a properly re-qualified 6351-T6 tank and keep it in service.
However, some shops won't. Sometimes that is based on ignorance and out of control liability fears. In other cases it may be because they lack the equipment to do and eddy current or enhanced visual inspection in the first place. Sometimes, they just want to sell you a new tank, particularly if they have one in stock and getting 18 months or so past it's manufacture date.
What I really don't understand is that some RINs are now refusing to requalify 6351 T6 tanks. That might just be a bad case of lazy, or it may also be due to unfounded fear of liability if they inspect a tank that then fails (a near zero probability event based on the numbers from the pre-eddy current years, and a zero probability event based on the post eddy current numbers).
