How come?

If the O in CO2 is oxygen, why don't my kegs all oxidize?

The Force (molecular bonding) is strong in that one...

hotwatermusic said:

If the O in CO2 is oxygen, why don't my kegs all oxidize?

Click to expand...

In fact they are all oxidized and that is why they don't rust (are stainless). Their surfaces are coated with a thin layer of chromium oxide which 'passivates' them protecting the iron underneath from attack by the oxygen in the air when wet.

But suppose the metal were not passivated. CO2 would still not attack the iron because carbon is a stronger reducing agent than iron is or, put another way has greater affinity for oxygen than iron does to the point that carbon (in the form of coke) is used to remove the oxygen from iron ore (reduce it) in the smelting process

2Fe2O3 + 4C + O2 --> 4Fe + 4CO2

I think the OP was questioning why the beer in his kegs don't oxidize from the CO2. As mentioned, the oxygen in the CO2 molecule is too strongly bound and cannot liberate itself to oxidize the beer.

That would certainly be a more reasonable question to ask than the one he did. If that's what he meant then the 'strongly bound' explanation is OK. Since CO2 represents the most oxidized state of carbon it is reasonable to ask why it cannot oxidize something else and it can under the proper conditions. When it does so it is reduced to something like CO or ultimately CH4, in which carbon is in the most reduced state possible. Note that if CO is burned to produce CO2

2CO + O2 ---> 2CO2 + heat

heat is evolved (CO is a fuel). The same is true of methane

CH4 + 2O2 ---> CO2 + 2H2O + heat

This makes it clear that to reduce CO2 will require an input of energy (heat or electricity) to break the strong carbon-oxygen bonds (note that they are double bonds). That energy is not available in a cold keg of beer.

^^ minor corrections to equations to make them balance. Commentary was perfect

2CO + O2 --> 2CO2 + heat

Also

CH4 + 2O2 ---> CO2 + 2H2O + heat

[edit: apparently AJ fixed his equations above... probably while I posted this]

Thanks. I did indeed mean to ask about the beer and not the keg itself. Sorry for the poor wording.
However since I had a bonus question answered, may I ask for some further elaboration?
I have some Firestone kegs that do oxidize. They were handed down from a guy who had been brewing since the seventies and my belief is that the chromium oxide layer that AJ mentioned may have been damaged and the stainless was subjected to some pitting from the use of bleach.
Is that a reasonable assumption?
I will also be awarding bonus points should someone be able to answer with them sciency capital letters with the arrows and parentheses and what not.

hotwatermusic said:

Thanks. I did indeed mean to ask about the beer and not the keg itself. Sorry for the poor wording.
However since I had a bonus question answered, may I ask for some further elaboration?
I have some Firestone kegs that do oxidize. They were handed down from a guy who had been brewing since the seventies and my belief is that the chromium oxide layer that AJ mentioned may have been damaged and the stainless was subjected to some pitting from the use of bleach.
Is that a reasonable assumption?
I will also be awarding bonus points should someone be able to answer with them sciency capital letters with the arrows and parentheses and what not.

Click to expand...

Bleach will damage SS for sure. It ate the end of a dip tube and created a hole in the bottom of a my cornie keg. I left a bit of bleach down there for at least a week.

Bar Keepers Friend (oxalic acid) restores stainless steel very well. If you're seeing any rust on those kegs, it's because the oxidized layer has been removed by contaminants, or free iron as been added by cleaning with steel wool, file, etc. The acid in BKF will take care of all of that, revealing the chromium on the surface, which will readily oxidize and protect your shiny friend.

Yes, once the passivating layer has been damaged the underlying iron is indeed subject to corrosion and it seems to be widely accepted that chlorine is capable of causing this damage. Another good way to damage it is to cut it, nick it or abrade it with plain iron or steel as by cleaning with steel wool or drilling with ordinary steel based drills (unless the workpiece is re-passivated).

It's like asking if CO is just part of CO2, why is CO2 no poisonous.

It is but CO2 and CO kill you in different ways.

passedpawn said:

Bleach will damage SS for sure. It ate the end of a dip tube and created a hole in the bottom of a my cornie keg. I left a bit of bleach down there for at least a week.

Bar Keepers Friend (oxalic acid) restores stainless steel very well. If you're seeing any rust on those kegs, it's because the oxidized layer has been removed by contaminants, or free iron as been added by cleaning with steel wool, file, etc. The acid in BKF will take care of all of that, revealing the chromium on the surface, which will readily oxidize and protect your shiny friend.

Click to expand...

OK, thanks, great info. I have BKF and will try using it. The kegs don't seem to show signs of rust with normal usage. However if I do a Starsan treatment and then put them in storage when I do go to use them again they have a "stain" if you will. It comes off with a green scrubby, I don't use steel wool. I generally just run a starsan cycle through and push out with CO2 right before filling to avoid this problem on these particular kegs.

Technically, the CO2 you purchase from homebrew stores, or wherever, isn't 100% pure CO2. I believe its 99.5% or 99.9% pure CO2, so in that sense your kegs are oxidizing. Whether or not its detrimental, or able to be detected is a different animal.