Does a nitrox tank rust more inside?

[DA Aquamaster]

Basically what Doc Intrepid said. Aluminium is a much more useful metal for scuba tanks because it's lighter, cheaper, and when it does oxidise - the equivalent of rusting - the oxidisation process actually protects the metal underneath. You will notice that brand new scuba tanks are bright and shiny but they oxidise very rapidly and go a dull whitish sort of colour - again, the same process makes aluminium window frames so durable.

As indicated above oxidation of an aluminum surface is a self limiting process where the oxidation itself tends to seal off the underlying aluminum from any further oxidation - but only to a point.

If you have water, and in particular salt water, in an aluminum tank it is going to oxidize and fail very quickly (and the same is true for a steel tank).

Aluminum is also lighter than steel but it is not as strong, so the walls of an aluminum tank need to be at least twice as thick - offsetting the weight savings of the lighter metal. Aluminum also has fatigue and life cycle limits that do not apply to steel tanks, and aluminum is much softer and much more prone to damage that may result in it being condemned. So when you look at the bigger picture, aluminum does not look so hot as a tank material despite the closed nature of the oxidation process.

For example:

1. A steel X7-100 is smaller in overall size than an AL 80 and only weighs a pound or so more, but holds an honest 100 cu ft rather than the 77 cu ft in an AL 80.

2. There are 100 year old welding tanks still in service that still pass hydro test and still qualify for plus ratings - not exactly what one would expect from a steel tank that spent 100 years filled with O2. There are as of yet no 100 year old aluminum tanks, but I seriously doubt that there will be many 100 year old AL tanks in active service given the track record of AL tanks to date.

3. In Indian Springs where cached bottles are left in the cave for 2 months at a time, they switched to steel tanks as they held up much better than aluminum tanks when constantly submerged - again not what you would intuitively expect.

Again, a high percentage of O2 by itself does not increase oxidation in an AL tank or rusting in a steel tank. You also need moisture in the tank for the oxidation to occur regardless of the FO2 or PO2.

Now in a steel tank used for O2 service is makes sense to tumble the tank during the O2 cleaning process to remove all traces of rust for three reasons:

1. Rust attracts moisture and will concentrate it against the steel under the rust.
2. Rust is flammable and is a potential fuel source (rusty steel wool makes great kindling)
3. Rust particles, if entrained in the O2 system at high velocity, could cause a park if they impact another surface and consequently provide an ingition source.

Aluminum tanks, on average, do not require quite as much prep for O2 service.

As for cost, hot rolled steel plate currently sells for about $.40 to $.58 per pound while raw aluminum costs about $.65 per pound, so even when you correct for the inevitably higher costs of properly alloying both metals, the cost of steel is still cheaper per pound than aluminum and the number of pounds per tank is very close so the material cost difference is minimal.

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Contrary to popular opinion breathing a tank down to the point where you cannnot get any more will not result in water entering the tank. Very few if any regs will breathe a tank down to ambient pressure and even if one did, water could not flow upstream unless the lever in the second stage were depressed. So in the real world you would have to breathe the tank down as far as possible under water, then depress the purge to open the system and then go deeper by another 50 - 100 feet to increase the ambient pressure enough to overcome the pressure still remaining in the tank. That is quite frankly just not going to happen in the real world. I'm surprised Myth Busters has not busted that one yet.

What will result in water getting into a tank is a compressor with a faulty moisture separator and/or filter that does not properly dry the air. The other way water commonly gets into a scuba tank is by water being present in the valve or in the fill whip when the tank is filled. If the fill operator does not crack the valves on both the tank and fill whip to blow out any moisture that may be in them, that moistrue will be blwon in the tank during the fill. Both are dive shop related failures so it is somewhat ironic that dive shop associated instructors still preach the "don't breathe the tank donw to zero" line.

 

[scubadude79]

One of the things they stressed in our scuba class was to avoid, whenever possible, depleting a tank completely of air....

Hoomi,
Good point. I remember that from OW and I always try to store my tank at a minimum of 500 PSI if it is there.

Thanks!

 

[scubadude79]

An Aluminum tank is probably already oxidized on purpose by the manufacturer. They probably have taken steps to make the oxidation layer as thick as possible to strengthen the tank.

Didn't know that, Thanks!

 

[BabyDuck]

i think you're mixing two true things.

the cleaning you're seeing advertised for nitrox tanks isn't about rust or oxidation - it is for partial pressure blending. tanks need to be 'o2 clean' (cleaned without any hydrocarbons or hydrocarbon residue - correct me, larry & others, if there's more to it than this) if you're going to make nitrox by filling them with oxygen *then* air to get nitrox, which is 'air with more oxygen'. if the shops bank their nitrox (make large batches that they store in big big tanks that can then be used to fill your tanks), this isn't needed but lots of shops make you have it done anyway.

so the cleaning you're hearing about isn't about the tanks as much as it is about what's going *in* the tanks.

 

[DA Aquamaster]

Hoomi,
Good point. I remember that from OW and I always try to store my tank at a minimum of 500 PSI if it is there.

Thanks!

Tanks have burst discs to ensure that they vent any gas safely rapther than rupture in a fire. Given that an aluminum tank will begin to soften as it loses is heat treatment at temperatures as low as 350 degrees, it makes more sense from a fire safety point to either store the tank nearly empty (300-500 psi) or full, so that it will either not develop enough pressure to do major damage if it ruptures or safely vent the tank before it is in danger of rupturing. Storing an aluminum tank somewhere in the middle is a bad idea, from a fire safety perspective.

Anything above ambient pressure will be fine - 100 psi is more than enough for a storage fill and enough to avoid the LDS wanting to do a VIP, so in that regard 500 psi is overkill. So I'd store mine long term with 100 psi rather than 500 psi or close to 3000 psi and certainly not anywhere between 500 and 2750 psi or so.

An Aluminum tank is probably already oxidized on purpose by the manufacturer. They probably have taken steps to make the oxidation layer as thick as possible to strengthen the tank.

I was going to let this one slide, but since people seem to be taking it on faith it needs to be addressed.

Aluminum tanks are heat treated. AL tanks are now made from 6061-T6 aluminum alloy. "6061" refers to the alloy itself and "T6" refers to the heat treatment it recieves. The heat treatment is what makes the aluminum in the tank stronger.

Oxidation occurs naturally over time, but it does not add strenght and is something that would be better not occurring at all. But as it is a self limiting process in the abscence of liquid moisture, it does not pose a problem. The oxidation we are talking about here is a dulling of the shiny finish to a flatter grey finish that essentially seals the underlying aluminum. If you get oxidation producing a whitish powder, you are going well beyond that closed process and have a problem. Seeing that type of whitish powder on the walls of an AL tank during a VIP would indicate a problem and would be an indicator that tumbling the tank is in order.

 

[DA Aquamaster]

i think you're mixing two true things.

the cleaning you're seeing advertised for nitrox tanks isn't about rust or oxidation - it is for partial pressure blending. tanks need to be 'o2 clean' (cleaned without any hydrocarbons or hydrocarbon residue - correct me, larry & others, if there's more to it than this) if you're going to make nitrox by filling them with oxygen *then* air to get nitrox, which is 'air with more oxygen'. if the shops bank their nitrox (make large batches that they store in big big tanks that can then be used to fill your tanks), this isn't needed but lots of shops make you have it done anyway.

so the cleaning you're hearing about isn't about the tanks as much as it is about what's going *in* the tanks.

Marci is correct in that the goal of O2 cleaning a tank is to ensure that there are no hydro carbons or any rust or oxidation residues that could provide either fuel for an O2 fire or provide entrained particles that couple provide an ignition source by creating sparks somewhere in the fill system, valve or regulator.

Certain materials for lines, and fittings are specificed for fill systems used with high percentage O2 systems as are pipe diameters and bend radiuses to reduce the potential for sparks occurring if entrained particles do end up in the system.

Tumbling the tank to remove any oxidation or rust that could provide fuel for a fire or a source of entrained particles is just a continuation of the safety process of eliminating fuel and ignition sources (as you cannot remove the oxygen) and a fire cannot exist without all three.

She is also correct that special cleaning is not needed for tanks used with mixes of 40% or less, but that some shops make you get it anyway, for reasons that have a lot more to do with cach flow than safety.

The qualifier here is that if a tank or valve will ever see an O2 mix over 40% it needs to be O2 cleaned. This means that if a shop uses partial pressure blending where 100% O2 (or any O2 mix over 50%) is put into the tank first, then topped with air to get the desired mix, then the tank must be O2 cleaned. Shops that fill from a bank or use stick or membrane systems to produce nitrox do not need to have the tank O2 cleaned - but again often require it anyway.

 

[Hoomi]

The 500 PSI figure I remember from our training was primarily in regards to making sure we finish the dive with air still in our tanks, just on the concept of always being sure we're not pushing the limits and winding up with no air before we're safely back at the surface. Basically, it was stressed that if we ended the dive with very little air remaining, we probably hadn't given ourselves sufficient margin for anything unexpected.

I do agree with a previous reply on this topic that, even taking a tank to 0 PSI on the guage, it is not likely the tank could suck in water or humid air. Probably the only realistic way it could would be to run it down to 0 while diving in a lake at higher elevation, then driving down to sea level, and opening the valve there. Perhaps part of the reason the LDS stressed making sure we leave a little air in the tank was so that the tank valve could be cracked open a bit to blow off any possible moisture with some of the remaining dry air inside, before hooking to the fill station.

Extra precautions may not accomplish much, but they certainly don't hurt, either.

 

[bleeb]

She is also correct that special cleaning is not needed for tanks used with mixes of 40% or less, but that some shops make you get it anyway, for reasons that have a lot more to do with cach flow than safety.

"cach flow"?

 

[Doc Intrepid]

DA meant "cash" flow.

Good cach, tho'!

 

[DA Aquamaster]

Gotta love typos...

I should add one more thing to the aluminum versus steel for O2 use issue.

Steel tanks are often low pressure. For exmaple a steel 45 operates at 2640 psi (2400 psi service pressure plus 10%) while an aluminum 40 has a 3000 psi service pressure. In some places you just can't get 100% O2 at 3000 psi. Many places can't and many places just won't boost O2 that high.

So if the most you can get is 2400 psi, you'll get 41 cu ft in your steel 45 and only 32 cu ft in an aluminum 40.

The same advantage carries over to partial pressure blending of high percentage deco mixes, where you will need less psi of O2 in a low pressure steel tank than in a 3000 psi aluminum tank. it makes transfills easier with out a booster and allows either a higher percentage mix or a greater volume fill with whatever pressure O2 is available.

So steel tanks offer several advantages compared to aluminum tanks, even with 100% O2 use.