In the analysis of corrosion behaviors of metals in oil & gas well and chemical plants containing CO
2 and/or H
2S gas, detailed knowledge of a stable corrosion product and its solubility is essential. Thus, a computer program has been developed that can automatically construct the potential-
pH diagram and calculate the solubility of corrosion product at elevated temperatures for Metal-CO
2/H
2S-H
2O system. The partial pressure of CO
2 or H
2S gas in this program was used as the input parameter because that can be measured in laboratory tests and actual fields. The thermodynamic data of H
2CO
3 and H
2S (aq) were used in the program, since chemical species such as H
2CO
3, HCO
3- and CO
32- are produced when CO
2 or H
2S gas dissolved are thermodynamically in the equilibrium state. Then, the corrosion of pure iron in CO
2 environments was discussed based on the potential-
pH diagram for Fe-CO
2-H
2O system and the solubility of FeCO
3. The solubility decreased as the temperature increased, and the pure iron showed the maximum corrosion rate because the FeCO
3 film was partially produced on metal surface around 100°C, and then the corrosion rate decreased above 150°C because of the protective FeCO
3 film layer. Finally, the solubility of FeS in Fe-H
2S-H
2O system is very low compared with that of FeCO
3, and the corrosion rate of the pure iron was also low at 60 and 100°C, but that increased above 150°C. In this case, the phase transition of FeS from α to β was an important factor.
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