Comment by Rodney on 13 Mar 2009 at 16:31:11
HDPE is corrosion resistant to supercritical saturated CO2 (for examples 100 bar 25 degrees Celsius and almost saturated with water), since the pH of carbonic acid is in the range of 3 to 4. Carbonic acid is even in these circumstances a weak acid.
Sulphuric acid is a nasty one. For a given concentration sulphuric acid is 500.000 times stronger than Carbonic Acid. For long term operation at very low pH, fluoro polymers (PVDF, ETFE) maybe better.
Note that in high pressure applications the above materials are used in combination with stronger structural materials, such as metals. In that case, the situation at the metal - polymer interface is important for the overall life time. You can think of corrosion reaction with metal, pressure built-up in voids (giving rise to delamination), etc. This is also reason why real composite materials (i.e. glass reinforced epoxy, GRE) are sometimes preferable over polymer - metal laminates: they have the intrinsic potential to be more stable on the long term (at least if the pH does not become too low).