Are aluminum tanks from the 1990's ok or are they also destined to be considered unsafe at some point?
There's a lot of confusion around this.
First, any tank made after certain dates in the early 1990's (like 1990, 1991) are made with the same alloy used today: 6061. Tanks before that may (and probably *are*) be made from a different alloy: 6351. That older alloy definitely suffers from Sustained Load Cracking (SLC). Many additional details here:
PSI-PCI - A short Review of 6351 Alloy Aluminum Cylinders
I'm not going to get into arguments for or against 6351, because that's not what you're asking. But's those tanks that have gotten dive shops used to forcing replacement of aluminum tanks. The question, is what about "old" 6061 tanks?
There are three schools of thought with this.
1) 6061 tanks are fine, and they'll always be fine. Go nuts.
2) 6061 tanks *may* be fine, and they may not. Who knows, maybe they'll find SLC or something else?
3) "Old" tanks should be condemned no matter what. Why risk it?
Sadly, it seems that more and more *dive* *shops* are falling into categories #2 and even #3. Why? The most obvious and cynical reason is: they sell SCUBA tanks! They've gotten hooked on the candy of replacing "old" tanks, and why should they stop now? This just gives them a fear-mongering reason to sell more, and the fear gives them a fig-leaf to hid behind: we're doing it for your safety and that of our staff! Think of the children!
The other reason: it lets them under-train their staff. It takes actual effort to get people to understand tank types, ages, brands, dates that are safe, etc. It's real easy to say, "Any tank over 20 years old is dead to us." In fact, there was a big thread on SB recently about a dive shop refusing to fill *steel* tanks because they were older than 20 years.
Is this a cynical reply? You betcha. There's very little evidence that even 6351 tanks in proper hydro and with a proper visual eddy test are unsafe: DOT says that SLC to failure takes more than 5 years, and that proper VE at hydro will catch them. Let alone *any* evidence for failure of 6061.
But in the end, unless you're gonna own your own compressor, you're at the mercy of dive shops both today and in the future. What percentage of shops? Enough that it comes up *routinely* around here. A quarry I've been to a few times was warning people that they would put in the 20-and-out policy for 2019. I haven't been there this year to know if they wen through with it or not. But this is not isolated.
So this is a long-winded answer to say that scientifically, there's no reason to not use a tank from 1995. But practically, you will make your life noticeably harder, both now and likely in the future too.
Edited to add: The responses above me say much the same thing, but without any real reason. Not a shot at their comments -- I agree with them. But the reasons are never really stated, it's just 'that's a bad idea'. (Mainly because IMHO, there is no scientific reason for it. But you can't beat the house...
) Which is why I own as few aluminum tanks as possible - just for deco bottles.
