The owner of the dive shop I work with also does hydro testing and did 13,000 tanks last year.
I am not sure how many of these tanks required an eddy current test or how many other tanks had an eddy current test as part of an annual VIP rather than hydro test, but in the average year he will have 3 or 4 tanks either fail the eddy current test or have a visible crack in the neck or shoulder area that is detected before the test. Most of these are medical oxygen tanks with tightly radiused shoulders and 1/2" valve openings. It's pretty rare to have a scuba tank fail and he has had only one in the last year or two.
The risk of a crack in a tank with 6351 alloy is real, but with current hydro testing, proper eddy current inspection at an annual or 18 month interval and proper fill procedures, the odds of a catostophic failure are very, very remote.
I would not personally buy a used AL 6351 alloy scuba tank if I did not know who onwed it and how they treated it (that goes for any AL tank) but I would have no qualms about continuing to use one that I own and that was properly filled, hydro'd and inspected.
8buck:
I know Walter Kiddie had some bad alloys I have three of there tanks and two by luxfier. When lux bought walter kiddie back in late 80's i think they put out a list of the t-6 al that was bad. Also in my lds yesterday we were talking about this. and they said when a company buys out another company their stamp wich is good for like 15 years i think, when it goes out they have the option of just having it stamped 3al over the name.
Walter Kidde tanks have heavier walls than other AL 80's and demonstrate significantly less expansion during a hydro test. If I had to own a 6351 alloy tank, I'd want it to be a Walter Kidde.
If the tank is stamped ICC, or DOT already, the question of whether the manufacturer is still in business is a moot point. There is no need to stamp anything out.
Some of the LDS confusion here may be occurring because of what can happen with tanks stamped with an exemption number. An exemption normally has to be renewed every 2-3 years and if the company goes out of business, then the exemption will most likely not be picked up by another company and will expire. The same thing could potentially occur if one company buys another and chooses not to renew the acquired company's exemptions. And of course a company could decide for what ever reason not to renew its own exemptions (although this makes the customers who bought the tanks from the company really crabby). Once an exemption expires and is not renewed, the tank cannot be hydro tested and put back into service and will essentially not be legally useable after the current hydrotest date expires.
What makes this more confusing in the case of scuba tanks is that 6351 alloy tanks made under the SP6498 exemption were granted an permanent DOT approval and no longer have to have the exemption renewed. These tanks should have the exemption numbers stamped out and replaced with DOT 3AL at the next hydro to reflect this. This blanket approval has been in effect for a LONG time but we still see tanks that come in without the DOT 3AL stamp meaning they have not been hydro'd (and probably not used) in a long time.
