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.