Potential pH Diagram at Elevated Temperatures for Metal-CO2/H2S-Water Systems and the Application for the Corrosion of Pure Iron
In the analysis of corrosion behaviors of metals in oil & gas well and chemical plants containing CO2 and/or H2S 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-CO2/H2S-H2O system. The partial pressure of CO2 or H2S 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 H2CO3 and H2S (aq) were used in the program, since chemical species such as H2CO3, HCO3- and CO32- are produced when CO2 or H2S gas dissolved are thermodynamically in the equilibrium state. Then, the corrosion of pure iron in CO2 environments was discussed based on the potential-pH diagram for Fe-CO2-H2O system and the solubility of FeCO3. The solubility decreased as the temperature increased, and the pure iron showed the maximum corrosion rate because the FeCO3 film was partially produced on metal surface around 100°C, and then the corrosion rate decreased above 150°C because of the protective FeCO3 film layer. Finally, the solubility of FeS in Fe-H2S-H2O system is very low compared with that of FeCO3, 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.