Electrochemical Mechanism and Model of H2S Corrosion of Carbon Stee

The mechanism of carbon steel corrosion in H2S environment (sour corrosion) has been investigated since 1950s and still poorly understood now. This obstacle hinders the development of an effective protocol for corrosion control and protection in sour environment. The goal of this thesis project is to understand H2S corrosion mechanism through systematic experimental studies and to build a mechanistic corrosion model to simulate the corrosion process at different conditions, including H 2S partial pressure, pH, flow rate, and temperature.;The first part of the project investigated electrochemical behavior of carbon steel corrosion in pure H2S environments. Initially, the uniform H2S corrosion mechanisms were experimentally studied in short term corrosion experiments (lasting 1-2 hr) before any significant interference from iron sulfide corrosion product layers occurred. Corrosion rates were obtained by linear polarization resistance (LPR). Mechanisms related to H 2S/CO2 corrosion were investigated using potentiodynamic sweeps and by comparison with electrochemical modeling. LPR results showed that corrosion rates increased with increasing temperature, partial pressure of H2S, flow rate and decreasing pH. Results of potentiodynamic sweeps show the presence of H2S could affect both cathodic reactions and the anodic reaction. An electrochemical model was developed and can be used to predict the effect of temperature, pH, pH2S and flow on corrosion mechanisms of mild steel in aqueous solutions containing H 2S in the absence of protective iron sulfide layers.;In the second part of the project, the combined action of H2S and CO2 corrosion was investigated. Experiments were conducted at the different H2S concentrations ranging from 0 to 10% in the gas phase at 1 bar total pressure at pH 4.0 and pH 5.0. Results showed that the presence of H2S slowed the charge transfer kinetics related to H2CO3 and H2O reduction reactions at the steel surface. An electrochemical corrosion model was developed for a mixed H2S/CO2 system which was calibrated with new experimental results and compared to data in the open literature.;The third part of the project investigated the effect of the iron sulfide corrosion product layer on H2S corrosion and kinetics of iron sulfide formation. The existence of the thin "inner" iron sulfide layer and its effect on H2S corrosion were clarified based on literature research. The effect of the "outer" iron sulfide layer was investigated using a new experimental set-up which permitted continuous replenishment of fluid to control the surface water chemistry, especially the pH. The effect of pH, flow rate, and temperature on iron sulfide corrosion product layer growth and corrosion rate was examined. High pH, low flow rate and increased temperature lead to a higher precipitation rate of iron sulfide on the steel surface and to the formation of a protective iron sulfide layer.;Finally, a comprehensive mechanistic transient model of uniform CO 2/H2S corrosion of carbon steel has been developed, covering three main processes underpinning corrosion: aqueous chemical reaction in the bulk solution, electrochemical reactions including the mass transport between the bulk solution and the steel surface, and a corrosion product growth model for iron carbonate and iron sulfide layers.