Electrochemical Model of Mild Steel Corrosion in a Mixed H2S/CO2 Aqueous Environment in the Absence of Protective Corrosion Product Layers

The present study has been conducted to investigate the electrochemistry of mild steel corrosion in a mixed hydrogen sulfide/carbon dioxide (H2S/CO2) aqueous environment, and develop an electrochemical model to simulate the experimental results. The experiments were designed to determine the effect of H2S on CO2 corrosion for short-term exposures of a few hours before any interference from iron sulfide corrosion product layers happened. Tests were conducted at different H2S concentrations, ranging from 0 to 10% in the gas phase at 1 bar total pressure at pH 4 and pH 5. Mechanisms related to H2S/CO2 corrosion have been examined by using different techniques such as linear polarization resistance (LPR) using the scan rate 0.125 mV/s), potentiodynamic sweeps (scan rate 1 mV/s), and comparison of experimental results with electrochemical model predictions. Results indicate that the presence of H2S could affect both cathodic reactions and the anodic reaction. An electrochemical model was developed for a mixed H2S/CO2 system, which was calibrated with new experimental results and compared to data found in the open literature. The model predictions fit experimental data well for short exposures (measured in hours) but overestimate the experimental results for longer term exposures (measured by days and weeks) due to the formation of an iron sulfide corrosion product layer, which is not accounted for in the present model.