Does air go bad?

[mpgunner]

This is a great discussion. I've got 3 tanks in my garage ready to go but I hope they won't go over a couple of months before they are used. This is a good reminder to make sure they are always cycled. Thanks

 

[bleeb]

I don't know where you got the idea that corrosion is self sealing. Rust is very porous. It certainly doesn't make an effective barrier to oxygen. Yes, we do seem to be posting past each other, but to answer your question, yes I believe that you can corrode a pound of iron if a tank which has moisture in it sits for several years. I have seen old pieces of metal corrode away entirely. Besides, if the tank is under pressure it will accelerate the corrosion.

Can I throw another question into the mix? Hopefully there's a more tank-knowledgeable metallurgist out there with the answers.

Over the years, I've gotten the impression that rust in tanks (mainly steel) can be roughly classified into two broad categories. The more common one is a few spots or a light coating of surface rust. Is this what people are referring to when they say "flash rush"? I'm guessing this is partly catalyzed by humidity in the fill air. A decent fill is fairly dry, so this rarely causes major problems, either O2-wise or structurally.

The more dangerous form is a wet fill including salt, where enough water is introduced to condense. Liquid salt water will happily chew its way deep into the metal, especially given a few years to do it. I would imagine even a few drops could cause significant structural problems, although to get the roughly 1 lb of steel mentioned earlier in this thread for O2 to start being an issue, you'd be talking about roughly 4 cu.in. Sounds like a lot, but across the bottom of an 8" dia. flat-bottomed tank, that would be only 0.08" deep.

Comments? Further thoughts?

 

[captain]

I don't know where you got the idea that corrosion is self sealing.
.

Actually in some instances it is. Many bridiges and other steel structures are left unpainted because of the type of steel used (not stainless steel)forms a rust film that is the protection from further rusting. There is a bridge over the Mississipi River not far from me that is built with that type of steel and is unpainted. It is entirely covered in rust. I am not saying the steel used for a tank has the same properties but a rust film, if it does not stop further rusting it greatly slows the rate.

 

[divechilly]

Hey Guys,

I didn't mean to start a chemistry battle! I appreciate all the input. Let's go diving!

 

[dannobee]

Whoa, you guys are really going out there in theories.

The bottom line is this; If the tanks were filled at a reputable shop, there is nowhere near enough moisture in them to ever start any kind of reaction that would even reduce the O2 content from 20.8% to 20.7%, well within the accuracy of O2 analyzers that we use.

The dewpoint on a lot of scuba air is WAY below zero, and often below -50 deg when tested. There's no moisture to speak of. No moisture=no rust or corrosion. Now, if the tanks were filled off of a poorly maintained home compressor, a compressor used for paintball fills without a filter, or worse yet, a dive boat compressor, all bets are off.

That said, I have a plethora of tanks in my garage, along with my own compressor, and happily dive a few with "old" air or nitrox in them, just to get rid of it, then I'll take the valve off and peek inside. (I'm talking 2 or 3 year old air) I have yet to see any steel tanks that I've filled that have even a wisp of flash rust in them, much less flaky rusticles growing in them.

But hey, to each his own. And if you feel better dumping the air and refilling it, have at it.

 

[ClayJar]

The dewpoint on a lot of scuba air is WAY below zero, and often below -50 deg when tested. There's no moisture to speak of.

Ah, but the testing of which you speak is done at 1 ata, no? As pressure increases, so does dewpoint (which is the whole reason for the condensate drains on the compressor, of course).

 

[dannobee]

Ah, but the testing of which you speak is done at 1 ata, no? As pressure increases, so does dewpoint (which is the whole reason for the condensate drains on the compressor, of course).

Maybe we're saying the same thing? The condensate drains are on the compressor to keep the next stage from ingesting water, as it falls out of suspension at higher heat and pressures. Obviously the water falling out of suspension isn't compressible and can damage the compressor.

OK, let me put it another way. A dew point of -50 deg equates to 67 ppm of water AFTER the final filter. Or, to put it another way, there could be 10 times as much carbon dioxide in your tank than water and still pass "grade e" specs. With properly maintained filters, the dew point is usually closer to -100 deg. There could be more oil and hydrocarbons in there than water (and still pass "grade e" specs) If there is any moisture in the tank, it didn't come from a properly maintained compressor system.

 

[EastcoastdiverLLC]

Could you please provide a reference to a dive accident involving tank corrosion leading to a hypoxic mix?

I did a search and I couldn't come up with anything. I believe that it is hypothetically possible, but has it ever actually happened?

Not trying to start any huge debate here. I just always error on the side of caution and you may very well be correct... but...

Just wanted you to have this information. Again, I'm not here to debate the point, just trying to give this diver some sound advice.

(From The 2003 Report on Diving Accidents and Fatalities, Divers Alert Network) this was the most recent I found...

"01-63 Instructor without designated buddy
found on wreck; tanks shown to contain
improper gas mix
Cause of Death: Asphyxia
This 42-year-old male was an experienced dive
instructor who worked in a dive shop and
filled his own tanks. He made a wreck dive
with eight other divers but without a designated
buddy to 112 fsw (33.6 msw).
Approximately 35 minutes into the dive,
another diver found the decedent on the wreck
and unconscious. The stricken diver was
brought to the surface and, after some delay,
they managed to get him into the boat.
Resuscitation efforts were unsuccessful. An
evaluation of the dive equipment revealed that
one of the two tanks the decedent had been
using contained less than 1 percent oxygen.
That particular tank contained several ounces
of rust-colored water, and the tank itself had
extensive rust and pitting throughout its inner
surface. The logical conclusion is that this tank
had not been used recently. The oxygen that
was originally present in the tank was likely
consumed in the oxidation (rust) process. The
tank did not have current visual inspection
and hydrostatic testing. Contributing
factor/significant diagnosis included obesity"

Gary

 

[EastcoastdiverLLC]

Can I throw another question into the mix? Hopefully there's a more tank-knowledgeable metallurgist out there with the answers.

Over the years, I've gotten the impression that rust in tanks (mainly steel) can be roughly classified into two broad categories. The more common one is a few spots or a light coating of surface rust. Is this what people are referring to when they say "flash rush"? I'm guessing this is partly catalyzed by humidity in the fill air. A decent fill is fairly dry, so this rarely causes major problems, either O2-wise or structurally.

The more dangerous form is a wet fill including salt, where enough water is introduced to condense. Liquid salt water will happily chew its way deep into the metal, especially given a few years to do it. I would imagine even a few drops could cause significant structural problems, although to get the roughly 1 lb of steel mentioned earlier in this thread for O2 to start being an issue, you'd be talking about roughly 4 cu.in. Sounds like a lot, but across the bottom of an 8" dia. flat-bottomed tank, that would be only 0.08" deep.

Comments? Further thoughts?

Many/most steel cylinders that have been hydro'd or prepared for oxygen service end up with a light coating of flash rust (this is normal). This powder like coating occurrs during the drying process. Air moving over a highly polished steel surface will rust. Pitting can condem a tank, flash rust will not. I can tell you that rust in any form is not self limiting and as mentioned in an earlier post will continue like a cancer in the presence of moisture and high partial pressures of oxygen. Flash rust can be greatly reduced with various drying methods and chemicals but I wont get into that. Flash rust is harmless just hard to explain...

I was informed during my visual inspection course, that Aluminum Oxide (the aluminum equivelent to rust) is "self limiting" . That oxidation in Aluminum will virtually stop after a white coating has been established. We will simply tumble a tank that has heavy oxidation in order to properly evaluate it.

The bottom line, little or no moisture in your fill = little to no corrosion. Aluminum Oxide is self limiting and rust is not. Pitting beyond acceptable standards in either Aluminum or Steel will condem the tank...

Hope this helps,
Gary

 

[jefffalcone]

Not trying to start any huge debate here. I just always error on the side of caution and you may very well be correct... but...

Just wanted you to have this information. Again, I'm not here to debate the point, just trying to give this diver some sound advice.

(From The 2003 Report on Diving Accidents and Fatalities, Divers Alert Network) this was the most recent I found...

"01-63 Instructor without designated buddy
found on wreck; tanks shown to contain
improper gas mix
Cause of Death: Asphyxia
This 42-year-old male was an experienced dive
instructor who worked in a dive shop and
filled his own tanks. He made a wreck dive
with eight other divers but without a designated
buddy to 112 fsw (33.6 msw).
Approximately 35 minutes into the dive,
another diver found the decedent on the wreck
and unconscious. The stricken diver was
brought to the surface and, after some delay,
they managed to get him into the boat.
Resuscitation efforts were unsuccessful. An
evaluation of the dive equipment revealed that
one of the two tanks the decedent had been
using contained less than 1 percent oxygen.
That particular tank contained several ounces
of rust-colored water, and the tank itself had
extensive rust and pitting throughout its inner
surface. The logical conclusion is that this tank
had not been used recently. The oxygen that
was originally present in the tank was likely
consumed in the oxidation (rust) process. The
tank did not have current visual inspection
and hydrostatic testing. Contributing
factor/significant diagnosis included obesity"

Gary

Is this a typo? Less than 1% O2 and contributing it to rust? Something doesn't add up.

I appreciate you posting this because I asked for it, but it doesn't quite make sense. Everyone who was involved in the chemistry discussion, including a PHD physicist, agreed that it would take at least 2lbs of rust to bring air down to 1%.