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Anodic dissolution of carbon steel in solution mixtures of simulated CO2–H2S in the presence of formic acid (0–500 ppm) was investigated using electrochemical methods such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results show that the corrosion rate increases with an increase in the concentration of formic acid. Potentiodynamic polarization data revealed that the increase in cathodic current density plays a significant role in the increase of carbon steel corrosion rate. A combination of equivalent circuits was employed to model the impedance spectra of the carbon steel electrode acquired at open circuit potential and different overpotentials. The EIS data obtained at various overpotentials were further analyzed by reaction mechanism analysis (RMA) approach to interpret the dissolution mechanism. A multi-step reaction model including two dissolution paths and ferrous intermediate adsorbed species determined the dissolution behaviour of carbon steel. The best fit RMA kinetic parameters were retrieved using an optimization technique and the surface coverage of adsorbed species was estimated. Surface morphology of corroded steel surfaces was also examined with field emission scanning electron microscope demonstrating aggressive uniform corrosion of carbon steel in the presence of formic acid.

The effect of soft cations (Na+, K+, NH4 +) on the corrosion behavior of carbon steel in chloride (Cl−) media was investigated using various electrochemical and nonelectrochemical techniques. Data revealed that the corrosion rate is higher in the case of ammonium chloride solution following the order NH4 +>K+>Na+. The polarization and electrochemical impedance spectroscopy (EIS) measurements indicated a significant influence of cations on both the corrosion rate and the mechanistic reaction pathway of carbon steel dissolution in Cl− media. The EIS measurement showed two time constants in all the cases. Electrical equivalent circuit was employed to model the EIS data. The surface morphology and the corrosion products were also analyzed. It was found that the hydration energy of cations and the initial pH of the solution play a critical role in describing the effect of cations present in Cl− media on carbon steel corrosion.