Scuba Cylinder Long-Term Storage: Fact and Fiction

[Doc Harry]

Note: fatigue life and static load are not related. Steel does not suffer from time dependent static stress issues such as creep or "sustain load cracking". As we all know aluminum can suffer from sustain load cracking.

Long term high pressure storage of a steel cylinder that is not subject to corrosion is not a problem.To avoid corrosion very dry gas is the first step (reduced O2 content does help, but is secondary).

I will expand more on this after I finish reading all the articles.

Understand. I changed the word "fatigue" to "weakness."

 

[LeadTurn_SD]

Doc Harry: Just wanted to add my thanks for taking the time to do this!

I had heard conflicting information over the years regarding tank storage, and your write-up is really helpful.

I wonder if this post could/should be made "sticky"?

Thanks again.

 

[Doc Harry]

I don't think much of the sustained load theory. Personally, I believe the publicized failures were due to cycling alloys from defective batches, and which was aggravated by hydro testing. That is, if a small crack starts as the result of a hydrostatic overpressure it can only get worse from there. I bought aluminum tanks in 1972, used them for a year and decided to relegate them to air tool use. They were kept at full press and hydroed for the first time in 1994. Both tanks passed hydro and visual. I continued to use them until 2004 when I discarded them due to bad vibes. They certainly didn't owe me anything and probably would have passed again.

It is my understanding that the sustained load cracking (SLC) observed in the 6351 aluminum alloy was actually the result of load-induced migration of lead molecules within the matrix. The migration and aggregation of lead molecules caused focal weakness and cracking.

Someone correct me if I am wrong.

BTW, all but 3 scuba cylinder explosions that I could document occurred because of SLC. One 6351 3AL aluminum 80 cylinder with a current VIP/hydro exploded during filling at 3,200 PSI to due a manufacturing defect (slag from the manufaturing process was accidentally included in the alloy matrix). Then there are the two corrosion-induced explosions noted above. All the rest are SLC-related explosions.

 

[Gilldiver]

It is my understanding that the sustained load cracking (SLC) observed in the 6351 aluminum alloy was actually the result of load-induced migration of lead molecules within the matrix. The migration and aggregation of lead molecules caused focal weakness and cracking.
.

Where did the Lead come from? There is no lead content in any wrought aluminum alloy that I know of.

The major alloy differences between 6061 and 6351 is that 6351 has slightly higher silicon, iron, manganese, zinc, and titanium.

I have never seen a metallurgical analysis of a crack, but the consensus of the people I work with (in an aircraft manufacturers metals failure lab) is that it most likely is an intergranular corrosion attack leading to the initiation of the crack. This may be at a fold line from the swaging down of the neck. We would need a cracked cylinder to look at so we could open up the crack and have a look with the SEM.

 

[Luis H]

Where did the Lead come from? There is no lead content in any wrought aluminum alloy that I know of.

The major alloy differences between 6061 and 6351 is that 6351 has slightly higher silicon, iron, manganese, zinc, and titanium.

I have never seen a metallurgical analysis of a crack, but the consensus of the people I work with (in an aircraft manufacturers metals failure lab) is that it most likely is an intergranular corrosion attack leading to the initiation of the crack. This may be at a fold line from the swaging down of the neck. We would need a cracked cylinder to look at so we could open up the crack and have a look with the SEM.

How many cracked cylinders would you like?
If you have the Lab, I can supply the cylinders.


Thanks

 

[Luis H]

An understanding of elastic and permanent expansion has helped me to understand that my scuba cylinders should be stored empty to minimize permanent expansion. Storing a tank at full service pressure increases permanent expansion. However, practical considerations require that cylinders be stored with some pressure (e.g., 300 psig) to prevent moisture from entering.

Thanks for the correction, but the statement in bold is not correct.
In the case of steel long term loading below its yield strength does not affect the steel by itself. It may accelerate corrosion but the load in itself does not affect the steel.

The working pressure of most cylinders is normally design to about 60% of the lowest expected yield strength. This is part of the safety factor to account for the fact that no material is totally homogeneous (and for other possible defects).

 

[pescador775]

Yes, Dr Harry, 6351 contains some lead and yes, the theory was that cracks occurred in "migration" areas. However, attempting to discriminate after the fact between migration and defective mix at the outset might be difficult. It is possible that too much lead was used in the first place. There have been explosions, one in Miami, that occurred in absence of any corrosion or slag. In fact, it happened a couple days after hydrostatic test and visual inspection. Every time an anomaly like this happens there is more fancy footwork from the experts. Explanation of the Miami deal was along the lines of "instantaneous, catastrophic failure, not SLC related". The details of the accident sound a lot like the "slag" tank. Are we talking about the same thing here? If so, it is new to me but the wheels keep grinding, even when I'm not watching. However, if there can be multiple explanations of similar accidents there can also be more than one interpretation of "load". Moreover, we might want to focus our attention on SLC as it relates to the other aluminum alloy, 6061; here, Luxfer says "no problem". I agree. It never was.

 

[mikes351]

Thanks Doc, good information

 

[Scared Silly]

Nice summary. In Number 3 the explanation seems to contradict the answer

3. When storing a scuba cylinder for long periods of time, does the composition of the breathing gas (i.e., air, Nitrox, etc.) affect the tank?

a. Yes

Higher partial pressures of oxygen cause higher rates of corrosion. It appears that it is the partial pressure of oxygen that is the major factor, not the percentage of oxygen in the gas."

The problem is the word "composition" which refers to the components and usually their amounts. But then in the explanation you say the partial pressure is the key factor NOT the percentage of O2. But the percentage of O2 contributes to the partial pressure.

As such, I would drop the "not the percentage of oxygen in the gas." in your explanation.



A couple of comments.

Besides the physical reasons for storing a cylinder at a low pressure there is also another, safety from accidental fire and subsequent explosion from heating. At low pressure the gas will expand but by the time it would be lethal the cylinder would failed due to heat. Conversely, at high pressure the gas would expand but blow the burst disk before the cylinder fails. Somewhere in between is the problem area as the pressure could rise enough but not rupture the burst disk but the cylinder weakens from the heat and then ruptures.

The other comment regarding materials and oxidation. Both steel and aluminum oxide. However, aluminum oxide is a barrier and prevents further oxidation whereas steel does not and the process continues.

 

[Doc Harry]

Thanks for the correction, but the statement in bold is not correct.
In the case of steel long term loading below its yield strength does not affect the steel by itself. It may accelerate corrosion but the load in itself does not affect the steel.

The working pressure of most cylinders is normally design to about 60% of the lowest expected yield strength. This is part of the safety factor to account for the fact that no material is totally homogeneous (and for other possible defects).

Every time a cylinder is pressurized to operating load it expands ever so slightly, even steel cylinders. Some of the expansion is permanent. The longer that a cylinder is pressurized, the greater the permanent expansion as a percentage of total expansion. This is what hydro testing checks. Are you saying this is not true? Or are you saying something else?