Scuba Cylinder Long-Term Storage: Fact and Fiction

[Akimbo]

I nominate this thread be made "sticky" at the top of this forum.

It was (is?) common practice for manufacturers of large (tube trailer size) cylinders to dry and pressurize them with pure nitrogen to 25-50 PSI, post-hydro and cleaning, before paint. The one plant I saw where these were being made used Nitrogen from a cryogenic source, probably more because of the huge volumes required than dryness of the gas. These cylinders were 20" long and have ports at both ends. They were hydroed, cleaned, emptied, dried, and pressurized when vertical so it was relatively easy to dry the insides compared to a single-port Scuba cylinders -- except maybe for needing a bridge crane and a stair case.

I can't say if this procedure was prompted by these reports or just common sense manufacturing methods. Obviously in this case, cylinders were stored, shipped, and most often used horizontal.

 

[VooDooGasMan]

Cleaning the steel tanks has been a thing that got me in the past, Instead of taking LDS word for things I precheck before take in and see what they tell me. Then dive it and empty and check again.


I was amazed how if a flash rust occurred, it passed for them, or a little bit past flash. now if o2, nitrox was the gas filled, tumble it.

I had one guy tell me he did his dive class through college, and got bad air do to rust in tank and messed his lungs up a bit and went to hospital over it.

A month ago a guy that I dive with got his son certified, he got two old al's 80, I opened up and put tank light in and said that ones toast, then put in the other ans said I might dive that one ut it is on its way also. I have one tank that is like the less of his, I use it to blow off, or blow up rib.


He wanted to dive it and I would not fill but about 500 psi back in, and filled other. I told him to give them back and get newer tanks. now he just told me has got a 110 steel tank japan steel made., I told him to take in to lds and tumble and clean if needed and it will be good for a while.

I have hilti attachments that I brush my tanks with for light rust. Yet would not like to do for anyone else's tank and be responsible.

There are so many ways I have stored tanks and they have all been a very well vis Inspection.

I am dropping off a couple of al 80's to day to a guy that can get them filled at his dive shop, they have a year left on hydro, then a couple more later. I am sending the old ones to others that can use them. with my wife now gone I have no need to have so many tanks for a 3, or 4 days diving. Ten al's and ten steels, should be enough for me, plus storage and pony bottles.


There is alot you can learn just buy having so many tanks, and taking care of them yourself.

I doubt I will ever hydo my tanks again as I will not use them that much, although I do over fill them and they have been fine.

Rinse after diving
Blow out valve after rinse
Leave in shade for storage
Not to hot when filling
Breathe it down to 1000 psi for easier filling
Use christo-lube on valves



Happy Diving

 

[mikes351]

The mechanism that causes tanks to deform over a long period of static fill time is called "creep".
E=ce^(-Q/Rt) where E is the creep rate, c is a constant, Q is the activation energy, R is the ideal constant, and t is the temperature.

For metals like steel this rate (dependent on metal strain, force/ area) ) is very very small unless under high temperatures. It is however a factor. Creep occurs in three stages, the last of which is catastrophic failure however in a metal like steel this requires several hundred years at least.
Steel SCUBA tanks as far as my research shows use ANSI 41xx alloys. Which according to the database I have access to (used 4135)have a max elastic strength of 1210 MPa and a max elongation of 6-12% which are quite high.
An aluminum tank made from a 5xxx alloy (this case 5456) has a max elastic strength of 103-131 MPa and elongate 12-16%.
What the means? Well scuba cylinders are very strong and do not deform permanently easily.
Just a note, 5456 costs $2.00 a Kg compared to $0.85 for the steel however steel is harder to work and corrodes easily.

Interesting information you present, now I will have to learn more about this issue. Thanks always good to learn new information!

 

[Reef_Haven]

I nominate this thread be made “sticky” at the top of this forum.

I agree. Please make this thread a stcky!

With the recent tank explosion death in Tampa, this thread really hits home.

 

[Shftrdog]

Nicely done

 

[Notso_Ken]

Actually, from my chemist background, and supported by your explanations of the answers, the correc answer is almost uniformly "None of the above", or "It depends." If it is a steel tank, the impact is much more pronounced. Aluminium does not corrode the same as steel. Lay an Al on its side or upright, it does not matter. For a steel tank it certainly does in the presence of any moisture.


Ken

Scuba Cylinder Long-Term Storage: Fact and Fiction

So what is the bottom line? Fatal outcomes are rare but I would not want to be that rare case. Corrosion is common. Less common is corrosion that is significant enough to warrant tumbling or other service. Even less common is corrosion or wear that is significant enough to cause a cylinder to fail hydro. If I paid $500 for a steel cylinder, I would want it to last forever.

So based on the best evidence, here are my answers to the quiz. But do not follow my advice. Either follow the recommendations of experts or read the literature and make your own informed decision.

QUIZ.

1. When storing a scuba cylinder for long periods of time, should the tank be stored full or almost empty?

b. Almost empty

Storing a cylinder almost empty helps to reduce corrosion because of the reduced oxygen partial pressure. While this is not so important with an aluminum cylinder, it can make a big difference with a steel cylinder. Also, storing a cylinder almost empty just makes sense from the perspective of reducing permanent expansion although the effects are minimal.

2. When storing a scuba cylinder for long periods of time, in what position should it stored?

a. Upright

Steel cylinder should be stored vertically in an upright position to minimize the effects of corrosion. If a steel cylinder is stored on its side and there is water present in the cylinder, the thin side walls and the large contact area can significantly corrode the cylinder side wall in as little time as 3 months. In the Battelle study, corrosion reduced side wall thickness to less than 1/3 of the orginal wall thickness in some cases. The Battelle study concluded that corrosion that results from lying a cylinder on its side probably represents the greatest hazard to personnel. It seems that aluminum cylinders can be stored in any position (except inverted) because aluminum cylinders are much less prone to severe corrosion.

3. When storing a scuba cylinder for long periods of time, does 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 per se. Obviously the fraction of oxygen affects the partial pressure but the total pressure has a relatively greater effect. The presence of water is a prerequisite for corrosion, though. A dry breathing gas does not corrode.

4. Conversely, when storing a scuba cylinder for long periods of time, does the cylinder affect the breathing gas?

a. Yes

The URI corrosion study demonstrated definitively that oxygen is consumed and carbon monoxide is generated during the corrosion process in steel cylinders. No such effect has ever been demonstrated with aluminum cylinders. There was one documented death from a corrosion-induced hypoxic mix in a cylinder that apparently developed over a period of only 3 months. Before using a steel cylinder that has been stored long-term, either dump the gas and refill or analyze the gas for oxygen and carbon monoxide content.

5. When storing a scuba cylinder for long periods of time, does a little bit of moisture affect the tank?

a. Yes

Moisture is the enemy. More water equals more corrosion. It is a self-limiting process because the water is consumed during the corrosion process. Steel tanks are more susceptible to corrosion than aluminum tanks, but aluminum tanks are more susceptible to galvanic corrosion of the neck threads in a salt water environment.

6. When we talk about long-term storage of scuba cylinders, what time period constitutes long term?

a. Three months

A lot can happen in three months.

 

[BlueTrin]

Thanks @Doc Harry

 

[Puppeteer]

Copper ions are known to promoted galvanic corrosion. However, URI failed to explain why the steel cylinders with the same fresh (tap) water did not suffer substantial galvanic corrosion like the aluminum cylinders. The combination of brass valves and copper ions should have produced galvanic corrosion in the steel tanks just as it did in the aluminum tanks.

The reason the copper ions would not have affected the steel tanks in the same way is because the copper would have cemented almost immediately on the steel surface removing it from solution.

This is an electrochemical reaction where copper ions become copper metal and iron metal become iron ions.

This reaction won't happen with aluminium spontaneously.

 

[GiraffeMarineSalvage]

Thank you for sharing that info, I always appreciate the chance to consider improvements to how I treat my tanks and personal practices..
Already was or moving forward will be incorporating elements of those recommendations with substantial exception of personally weighing pros/cons of having a tank ready to dive (even if not expecting to dive a tank anytime soon, ie between 3mo to expiration of 1yr vis) FAR outweighs any theoretical shortening of tank life as a result of storing full for me personally...
Also, purely speculating stoichiometry, if one is already confirming gas mix after long term storage is what it was supposed to be in terms of % O2 in air, EAN, or trimix I couldn't help but wonder if and when CO & CO2 could raise to a dangerous level in a steel tank without a noticable decrease in O2 %. Once I had a moment to do some quick math it became very clear the standard precision for measuring O2 (or O2/He/N2/residual gases of O.1% ie 1000ppm may catch something dramatic BUT it would be completely unreliable catching CO increasing to a deadly level...).