Carbonic acid action involves combination of carbon dioxide and water. Though present in pure water, carbon dioxide dissolved in water provides ions that produces free hydrogen. Carbon dioxide in the atmosphere combines with rain water to form carbonic acid (H2CO3):
H2O + CO2 -> H2CO3
Though weak, when carbonic acid is combined with a mineral like calcite (CaCO3) common to limestone, calcium and bicarbonate ions are released and carried off by groundwater.
CaCO3 + H2CO3 -> Ca+2 + 2 HCO3 -
http://octane.nmt.edu/waterquality/corrosion/CO2.htm
The maximum concentration of dissolved CO2 in water is 800 ppm. When CO2 is present, the most common forms of corrosion include uniform corrosion, pitting corrosion, wormhole attack, galvanic ringworm corrosion, heat affected corrosion, mesa attack, raindrop corrosion, erosion corrosion, and corrosion fatigue.
