I always assumed that the limit on aluminum tanks was due to the bad alloy. I assumed that when the common 20-year limit on aluminum tanks began to appear, it was meant to make sure none of the bad alloy were filled, and that people just kept the rule even when it was way more than 20 years since that the alloy was discontinued.
I was wrong.
I did an internet search and found articles from long ago that said aluminum tanks wear out and should be discontinued after [X] number of years, regardless of the alloy. I am going from memory now, and I think the shortest recommended time period was 15 years. That would have been on the oldest article I found. Newer articles suggested longer time periods.
Most of the articles came from Undercurrent magazine. What source did they cite? A dive shop in South Florida--Force E. In other words, somebody at Force E offered that opinion, and they published it world-wide. It might have been whoever was on duty the day the article's author was in there. I don't know.
Interestingly enough we had a thread on ScubaBoard a couple years ago about the Force E shop in Pompano Beach, FL refusing to fill a 20-year old aluminum tank. I went into that shop and asked, and the people there that day said they had no such policy. They would gladly fill a properly tested and inspected 20-year old aluminum tank. My take away from it is that on the day the ScubaBoard poster went for a fill, whoever was on duty that day rejected it based on a memory of a policy from many years past. That thread got a lot of views, and that shop now has its complete fill policy posted for all to see. I haven't been in it since last winter, and I don't remember it, but it does not have a 20-year limit on aluminum tanks.
Summary: Decades ago, someone at Force E in South Florida believed that aluminum scuba tanks wear out and should be discarded after a certain number of years, and someone writing for Undercurrent magazine published that as a general recommendation. That happened more than once, with different numbers of years suggested. It is likely that the recommendation, no longer believed by the shop that originated it, was picked up by some shops around the world, and in some places the policy is still in effect.
