Hydrogen sulphide (H2S) service occurs when a stream containing a partial pressure of H2S exceeds 0.3 Kpa. The presence of cyanides and a high pH are also considered to contribute to a sour environment. Materials for sour (H2S) service should comply with NACE MR0175 (ISO15156) for sour service.
One of the corrodent in the gas is CO2 that dissolves in water (free water) and forms carbonic acid (H2CO3). The partial pressure of CO2, pH and temperature are critical factors in the corrosivity. CO2 plays such a prominent role in oilfield production corrosion; some of the factors considered governing its behavior are concentration, pressure, temperature and composition of water. Pressure increases the solubility to lower pH; temperature decreases the solubility to raise pH.
To predict corrosiveness of gas, the partial pressure of CO2 is used as the basis:
Partial pressure of CO2 (psi) = (total pressure) X (mole fraction of CO2 in the gas)