Carbon dioxide reacts with water according to the following equation: CO2 + H2O --> HCO3- + H+ As the concentration of CO2 increases, so does the concentration of the H+ ion. This ion can then react with Fe in metals as follows: Fe + 2H+ --> 2H (atom) + Fe2+ As corrosion proceeds, the ferrous ions produced can react with the bicarbonate ions to form ferrous carbonate, which precipitates as a scale.