Avonthediver
Contributor
I've been doing a little research into tanks by Luxfer made during the 1980's, as I have two tank's made in '83. still in great shape and just passed viz again.
But I'm being encouraged to end their long life's by my LDS.
So I have been doing a little reading and just trying to see that after their run for the next year if I should turn them into mail box's.
So I have posted a small read and have a question to the mass' what would you do?
Inspecting Aluminum Scuba Tanks
by Hans Petter Roverud
I do hydrotests and visual inspections for a dive store in Norway. I've seen quite a few cylinders. With few exceptions, those cylinders that are condemned fail the visual rather than the hydro. Aluminum is tricky and treacherous. It doesn't corrode from condensation. However, if it sees salt water, the ensuing damage is obvious in no time. We can stamp out and destroy those tanks without any further checking. According to regulations, we're not allowed to expose aluminum to acid rinsing in an attempt to salvage it. So unless the entire inside surface comes out spotless after a hot water rinse, that cylinder is condemned. No sleep is lost over corroded aluminum!
The really treacherous ones are old aluminum tanks that look OK. They have never seen salt water on the inside and present a flawless shiny surface. They may have been exposed to excessive heat, but this won't show. While the cylinder may still look great, metal fatigue can change the strength of aluminum alloys for a number of reasons. Theoretically, it will show as loss of elasticity during the hydrotest, but I wouldn't bet my life on it. I'm pretty much looking for a reason to condemn any old aluminum tank I test!
These days, the Norwegian Navy throws away cylinders after 10 years of service. A few years ago, a BC inflation cylinder exploded while being stored in a locker. The cylinder was in good standing regarding hydrotesting, yet it failed anyway. It was, however, made of aluminum and older than 10 years. Fortunately, nobody was injured.
Steel is different. Fatigue is less of an issue. When it's bad you can tell at a glance. Usually, the rust kills steel before it fails a hydrotest. Still, a badly rusted steel cylinder may prove to be OK after an acid rinse. Superficial rusting may look messy, but what you really want to know is whether it has progressed to pitting. The point is, what you see is what you get. There's rarely any doubt whether a steel cylinder will pass or not. The only good-looking steel cylinder I've condemned (since it failed the hydrotest) was produced in Germany in 1944. That was not exactly a good year for German industry since WWII had depleted supplies of decent quality steel! Even so, it took almost 50 years to develop fatigue.
Aluminum is more brittle. Old European cylinders have tapered valve threads that seal with a tape. The torque needed for proper sealing transfers as a constant sideways strain to the neck of the cylinder. I've seen aluminum cylinders with tapered threads suddenly develop hairline cracks. Luckily, the air vented rapidly without the valve coming off or the entire tank rupturing. It could be that the torque was excessive, but the point is that aluminum may crack during prolonged strain. Steel would yield, but the brass of the valve stem would be deformed long before the steel would have to yield beyond limits. Another point to remember is that the valve stem has to be isolated from the aluminum by means of tape or grease to avoid galvanic corrosion. This goes for straight as well as tapered valve stems. Brass in direct contact with aluminum means you will sacrifice the aluminum bit by bit due to galvanic corrosion.
Since metal fatigue is so hard to detect in aluminum cylinders, how about adopting the 10 year rule? The cost of condemning an old aluminum tank every 10 years is financially bearable. After all, the tank valve is half the price of a new cylinder. Besides, it's a lot more expensive to replace hands. This being said, I prefer aluminum tanks as side-mounts due to their buoyancy characteristics. I'm in no way suggesting that aluminum is out. I'm just saying that old aluminum tanks are dispensable.
But I'm being encouraged to end their long life's by my LDS.
So I have been doing a little reading and just trying to see that after their run for the next year if I should turn them into mail box's.
So I have posted a small read and have a question to the mass' what would you do?
Inspecting Aluminum Scuba Tanks
by Hans Petter Roverud
I do hydrotests and visual inspections for a dive store in Norway. I've seen quite a few cylinders. With few exceptions, those cylinders that are condemned fail the visual rather than the hydro. Aluminum is tricky and treacherous. It doesn't corrode from condensation. However, if it sees salt water, the ensuing damage is obvious in no time. We can stamp out and destroy those tanks without any further checking. According to regulations, we're not allowed to expose aluminum to acid rinsing in an attempt to salvage it. So unless the entire inside surface comes out spotless after a hot water rinse, that cylinder is condemned. No sleep is lost over corroded aluminum!
The really treacherous ones are old aluminum tanks that look OK. They have never seen salt water on the inside and present a flawless shiny surface. They may have been exposed to excessive heat, but this won't show. While the cylinder may still look great, metal fatigue can change the strength of aluminum alloys for a number of reasons. Theoretically, it will show as loss of elasticity during the hydrotest, but I wouldn't bet my life on it. I'm pretty much looking for a reason to condemn any old aluminum tank I test!
These days, the Norwegian Navy throws away cylinders after 10 years of service. A few years ago, a BC inflation cylinder exploded while being stored in a locker. The cylinder was in good standing regarding hydrotesting, yet it failed anyway. It was, however, made of aluminum and older than 10 years. Fortunately, nobody was injured.
Steel is different. Fatigue is less of an issue. When it's bad you can tell at a glance. Usually, the rust kills steel before it fails a hydrotest. Still, a badly rusted steel cylinder may prove to be OK after an acid rinse. Superficial rusting may look messy, but what you really want to know is whether it has progressed to pitting. The point is, what you see is what you get. There's rarely any doubt whether a steel cylinder will pass or not. The only good-looking steel cylinder I've condemned (since it failed the hydrotest) was produced in Germany in 1944. That was not exactly a good year for German industry since WWII had depleted supplies of decent quality steel! Even so, it took almost 50 years to develop fatigue.
Aluminum is more brittle. Old European cylinders have tapered valve threads that seal with a tape. The torque needed for proper sealing transfers as a constant sideways strain to the neck of the cylinder. I've seen aluminum cylinders with tapered threads suddenly develop hairline cracks. Luckily, the air vented rapidly without the valve coming off or the entire tank rupturing. It could be that the torque was excessive, but the point is that aluminum may crack during prolonged strain. Steel would yield, but the brass of the valve stem would be deformed long before the steel would have to yield beyond limits. Another point to remember is that the valve stem has to be isolated from the aluminum by means of tape or grease to avoid galvanic corrosion. This goes for straight as well as tapered valve stems. Brass in direct contact with aluminum means you will sacrifice the aluminum bit by bit due to galvanic corrosion.
Since metal fatigue is so hard to detect in aluminum cylinders, how about adopting the 10 year rule? The cost of condemning an old aluminum tank every 10 years is financially bearable. After all, the tank valve is half the price of a new cylinder. Besides, it's a lot more expensive to replace hands. This being said, I prefer aluminum tanks as side-mounts due to their buoyancy characteristics. I'm in no way suggesting that aluminum is out. I'm just saying that old aluminum tanks are dispensable.
