By happenstance, i stumbled upon this thread just by browsing the net. I've spent the past half hour reading this thread and it's various 'theories' and 'speculation' with the occasional 'fact' mixed in. I registered on these forums just to add my two cents.
I am a "chemical" engineer, with a minor in chemistry, so I may be able to add a bit of credible information.
First off, in regards to the 'carbonic acid' argument:
Carbonic acid has negligible effect on aluminum. Basically NR. This is why you see soft drinks safely canned in aluminum.
Carbonic acid *Does* indeed corrode iron. It corrodes [stainless]steels as well. It depends on the grade of steel as to how much corrosion will take place given an amount of carbonic acid and other factors. I cannot comment on the grade of any of your tanks as I do not own one, but lets just say better safe than sorry.
In regards to the 'water in co2' vs 'co2 in water' - This all involves thermodynamics and some fairly complicated mathematics for those not familiar with it, along with phase diagrams and critical temps and pressures of co2 and h2o. For the sake of simplicity, lets just say that the link illustrated in a previous post have to do with the amount and concentration of each species in question relative to pressure and temperature. It's of importance because the pressure inside of your tank will be changing as you are venting the gas. Given that with varying pressures you will have varying concentrations of carbonic acid, the question to ask is 'could there be a point at which the right conditions exist for the inside of my tank to corrode?'
The answer to such a question is yes. To what degree depends on far too many factors to calculate, including the amount of h2o present in the tank, the pressures, the grade of steel, etc. Short answer is that water inside of a steel tank pressurized with co2 is not an ideal situation. This of course is of little importance if you are using an aluminum tank.
In regards to the 'cold dry ice stress', I'm sorry but I cannot give you any answers on this. Yes, it is a fact that metals will have different properties at different temperatures, but whether or not aluminum/steel would be degraded enough by the given temperatures I do not know. It's something chemE's and as AJ is an EE, we were never taught. MechE's on the other hand deal a lot in this area, and now I am interested enough that I'll need to ask a buddy of mine to see if he knows anything about it. If i ever get a definitive answer, I'll make sure to post back