Oxygen depletion

[john j]

Rust is iron oxide, a compound composed of iron and oxygen.   Rust is a solid at the temperatures we are dealing with.  So when rust is produced, oxygen is removed from the gaseous state and locked into a solid form. 

CO2 does not form.  First of all, there is no source of carbon.  Sure there are minute quantities of carbon in the steel alloy and on the surface from atmospheric contaminants, but not enough to matter.  Secondly, the activation energy of the iron oxidiation reaction (formation of rust) is lower than the carbon oxidation reaction.  In other words, it is far easier to make iron oxide than carbon dioxide at the temperatures we are dealing with.  If it was easy to oxidize carbon at standard temperatures, you couldn't leave bags of charcoal or piles of coal lying around because they would oxidize and turn to ash.  If you leave a piece of bare steel lying around, especially in the presence of water, it rusts quickly.

The tank will also not rust continuously and remove all of the oxygen from the tank.  As the tank rusts, the inside becomes coated with iron oxide.  This oxide layer then blocks oxygen from reaching the metal underneath.  This process is called passivation.  Anodization of aluminum works in the same manner.  Anodization is a process that creates a layer of aluminum oxide over the surface of the aluminum.  Once this layer forms, it blocks the aluminim from further oxidization because the oxygen cannot get to the aluminum underneath.  Oxygen reacts very quickly with aluminum as well, so this would cause problems in aluminum tanks too.  It doesn't however, because the metal becomes passivated with the oxide coating.  Any steel of aluminum tank contains a oxide layer inside.  It is very thin and you can't see it, but it is there.

If you have a lot of liquid water in your tank, the rusting reaction is accelerated, becasue oxygen can dissolve in the water and dissociate into oxygen atoms, which are called free radicals.  They are much more reactive than oxygen gas which is actually a molecule (two atoms of oxygen combined together).  You are still not looking at a rate so fast that the oxygen level in the tank is going to be reduced significantly, because there is so much gas in the tank relative the amount of iron available to the rusting reaction.  If you filled a tank to only a few PSI you might see a small drop in O2 level becasue there isn't much oxygen in the tank in terms of number of atoms.  However, with hundreds of PSI or more in a tank, there are so many oxygen atoms that even if billions and billons of them get convered into iron oxide, the percent loss is incredibly small.


Go ahead and verify on your own because it's good to learn about these things. 

I like Guiness.

[Roger Southwick]

John -- excellent explanation!  Thank you.

[wtdrm]

[john j]

I just spent a few minutes doing a rough calculation.  I made a lot of assumptions, but if you had a steel cylinder about the size of Al 80, filled with 80ft3 of air, and it rusted uniformly inside, consuming 1 millimeter of the tank wall, you would reduce the available oxygen atoms in the gas by 0.00001%.  It's a rough calculation and I invite others to perform their own and compare.  Even if I'm off by as much as a few powers of 10, that still puts it at a tiny loss.  1000 times my estimation is still 0.01% loss.  When the tank is pressurized, there is such a huge number of oxygen atoms in there, that losing some to rust is insignificant.  The numers here are huge, like ten to the power of 27 oxygen atoms in there.

[DiveCaptDean]

Uh.....air...fill...tank.....Uh....Uh....
Me like dive scuba..........

Does anyone else feel ....'tupid?

might i ask what type of engineer...?

DANG

[DiverQueen]

Yo Dive Capt Dean,
Has moving BACK to Minnesota hurt you that bad?

[MAXFACTOR]

John

Thanks for taking the time to enlighten all of us.  I missed your most recent post, or I would have responded sooner.  Obviously,  I concede that you know your physics,  but that will come to no surprise to all of us that either know you or know of you. 

I haven't found any source that would comfirm my belief that CO2 or another gas is produced with any combination of gases, either being produced by the oxidation, or the elimation of 02 and by the source  (you)   I probably won't be able to. 

However, I know I read it somewhere, or had an instructor along the way tell me that, because it's stuck in my head.  Until such time that I can find that or get some other explanation, I FULLY ACCEPT YOUR ANSWER AS  .....WELL.....GOSPEL !   
Obviously you know why I wanted you on my advisory board.  Still Room. 
thanks again to all

MaxFactor

[wukuu]

So what is the cause of the oxygen depletion???  Is it microorganisms getting into the tank, multplying and consuming the oxygen?  What is the standard thought behind what causes this?

[Inspirationdiver]

Just a thought, but most of the gas I dive is hypoxic trimix or heliox, usually about 9% oxygen.  I guess there's a chance that the diver didn't really know the true contents of the tank and it was supposed to be 6% O2 ???

[wukuu]

So are the stories we hear about oxygen depletion nothing more then an urban legend?????