Life of aluminum -- age or # of cycles based?

[SparticleBrane]

For all you metallurgist / engineering types out there -- is the life of an aluminum cylinder based off of the age of the cylinder, or the number of use cycles? Does the metal lose the stretching ability just due to the natural aging process, or does this occur due to constantly filling and draining the cylinder? Hopefully my question makes sense.

I am hoping it is the latter -- a friend of mine found a "new" used Al80 on craigslist locally for $60 and sent me the link. Turns out it is 9 years old and was used once as a paintball transfill cylinder, and then it just sat there. The owner said it was drained the day after he got it since he used it to fill his paintball gear, so it hasn't been sitting there full for 9 years; apparently it is completely empty (or close to it). It has never been in water so it really looks brand new.

Thus, this got my mind thinking -- Luxfer says their cylinders are "cycle-tested in excess of 100,000 cycles at service pressure", but many people say the 'maximum life' of an aluminum cylinder is 15-years. Somehow I'm thinking this is a throwback to dive shops in Florida not filling aluminum cylinders 15-years old and older due to worries about SLC/neck-cracks, rather than logically thinking about the service life of the cylinder.



[-]Being a paintball guy, he didn't know if the cylinder was Luxfer or Catalina (I asked but never got a response ). If it's Catalina, I don't want it. Any guesses? The font doesn't look quite like the font on my Luxfer Al80s...[/-]

edit: Based on the serial number prefix of "AS" (and the different font of neck markings) it would appear this is a Catalina, so I just told him I would pass. Oh well...

 

[Scared Silly]

SB, your intuition is correct. With metals used as a structural element their life is based of the number cycles at a particular stress level. Now this is given they are properly maintained and not subject other outside influences. This is in general and not an absolute.

Now this is not the case with all materials. For instance, nylon based products are affected by ultraviolet light and oxygen. Over time UV light weaken the bonds while oxygen not surprisingly oxidizes the nylon such that it weakens and becomes more brittle. These are influence that are hard to protect it from.

FWIW - My undergrad is in MechEngr. and I looked at the aging process of materials as one of my projects.

 

[SparticleBrane]

Thanks for the info! Too bad I won't be getting this tank...oh well...

 

[muddiver]

The rejection of a tank for service is based on the hydro test perfomance. The test gives an indication of how much rebound is left in the material of the cylinder. Rebound decreases with use and at a certain point there is a last fill that will expand the cylinder perminately thus wearing it out.

 

[SparticleBrane]

The rejection of a tank for service is based on the hydro test perfomance. The test gives an indication of how much rebound is left in the material of the cylinder. Rebound decreases with use and at a certain point there is a last fill that will expand the cylinder perminately thus wearing it out.

Right -- I'm familiar with all that. I suppose my question came about from wondering if just sitting around empty, the "life" of aluminum cylinders was being 'used up' just due to some sort of natural aging process in the metal, or if their aging is caused by usage.

eg -- if I put a brand new Al80 empty in a closet for 50 years, and then brought it out to use, would it have the same service life, all things being equal, as an Al80 that just came off the assembly line?


Sometimes the rejection of a tank for fills at a shop, as wrong as it is, is based off of age and perceived danger rather than knowledge.

 

[WD8CDH]

Catalina tanks are just fine, in fact Catalina never used the "bad alloy" used by Luxfer and others back in the 80's and earlier.

By "age" aluminum tanks would last longer than steel because of slower corrosion, but in general, a 6061-T6 aluminum tank or a steel tank that hasn't rusted or corroded have practically unlimited life. I wouldn't be worried in the least about a 50 year old tank not being safe (after a hydro, of course).

Some composite tanks have a 15 year or 30 year life due to resin breakdown.

 

[SparticleBrane]

Catalina tanks are just fine, in fact Catalina never used the "bad alloy" used by Luxfer and others back in the 80's and earlier.

Right -- in this particular instance I was planning on using it for a stage bottle and Catalina 80s aren't known to 'ride' very well, hence why I passed on the deal.

 

[themagni]

Aluminum doesn't have a stress knee, so in theory an Al tank will degrade faster than a steel one. However, since steel corrodes much faster than Al, they don't last as long for diving. You could probably buy stainless steel tanks, but remember that stainless stains less. Could you buy titanium tanks? Wow, that would be blasphemously expensive.

In other words, a steel tank would be limited by its age; Aluminum by its use. As with all diving gear (or, more accurately, anything at all), taking good care of it and following the maintenance schedules will make it last a lot longer. With regular testing and inspections, it is highly unlikely (finite, non-zero chance) that the tank (any material) itself will fail. The failure points are significantly more likely to be the o-rings, valves, and other moving parts.

 

[Gilldiver]

However, since steel corrodes much faster than Al, they don't last as long for diving.

In other words, a steel tank would be limited by its age; Aluminum by its use. .

Should I tell that to the 1960 galvinised steel 72 that just past hydro with a + or the ungalvinised steel 1920's vintage O2 and He tanks in my cascade?

In the real world, all tank life is "On condition" you can ruin a steel or aluminum tank in less then a year or have them last 100's of years depending on the conditions they are used under.

A steel or aluminum tank filled to the rated pressure is well below the yeld and should last tens of 1,000's of cycles. But the same tank with a bit of water in it or with cave fills may not make it past 1 or 2 hydros.

 

[halocline]

Pete, how did you get the + rating? Do you have the REE number for LP72s? If you do have a number, or your hydro facility does, I would be very much interested in that.

BTW, I'd buy that catalina tank if it were local.