Yougui Zheng

Electrochemical Study and Modeling of H2S Corrosion of Mild Steel

The internal corrosion of mild steel in the presence of hydrogen sulfide (H2S) represents a significant challenge in oil production and natural gas treatment facilities, but the underlying mechanisms involved in H2S corrosion are still not fully understood. This lack of knowledge makes the prediction, prevention, and/or control of aqueous H2S corrosion of mild steel much more difficult. In the present study, H2S corrosion mechanisms were experimentally investigated in short-term corrosion tests (lasting 1 h to 2 h), conducted in a 1 wt% sodium chloride (NaCl) solution at different pH (pH 2 to pH 5), at different temperatures (30°C to 80°C), under various H2S/N2 gaseous concentration ratios (0 to 10%[v]) and flow rates, using a X65 mild steel rotating cylinder electrode. Corrosion rates were measured by linear polarization resistance (LPR). Corrosion mechanisms were investigated by using potentiodynamic sweeps and by comparison with electrochemical modeling. LPR results showed that corrosion rates increase...

Thermodynamic Study of Hydrogen Sulfide Corrosion of Mild Steel

Expanding the basic knowledge required for improved understanding of hydrogen sulfide (H2S) corrosion of mild steel is needed. When it comes to even the most basic chemical descriptors of aqueous H2S systems such as H2S solubility, water chemistry, polymorphism of iron sulfide, the formation or dissolution of iron sulfide scale, and its protectiveness, many open questions persist and more investigation is required. Inconsistencies in prevailing expressions for H2S solubility constant (KH2S), the first dissociation constant (Ka,1), and the second dissociation constant (Ka,2) are reviewed here. By comparing with experimental data, the best prediction model for H2S solubility and dissociation in an H2S–H2O system was identified. Occurrence of iron sulfide polymorphs was experimentally investigated and it was found that, in short-term exposures, mackinawite formed at 25°C, while greigite and pyrite were detected at 60°C. The solubility limits for these iron sulfides were determined. Simplified Pourbaix diagra...

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 ...

A Thermodynamic Model for the Prediction of Mild Steel Corrosion Products in an Aqueous Hydrogen Sulfide Environment

In the present study, a comprehensive thermodynamic model, depicted by Pourbaix diagrams, was developed with the relatively narrow focus on corrosion of mild steel in oil and gas field conditions. This thermodynamic model focuses on predicting the formation of metastable or stable corrosion products in sour environments at elevated temperature up to 250°C, which includes mackinawite (FeS), greigite (Fe3S4), the pyrrhotite group (Fe1–xS, x = 0 to 0.17), and pyrite (FeS2). The model is based on theoretical thermodynamic calculations and data found in the open literature. The appearance of Pourbaix diagram is significantly affected by temperature. Long-term corrosion experiments at two different temperature (25°C and 80°C) were conducted to investigate the corrosion product stability predictions made by the Pourbaix diagrams. The equilibrium state predicted in the Pourbaix diagrams was compared with the quasi-equilibrium state attained in the long-term experiments. To this end, the surface pH, bulk pH, ferro...

Advancement in Predictive Modeling of Mild Steel Corrosion in CO2- and H2S-Containing Environments

Over the past decade, the knowledge related to predicting internal pipeline corrosion for sweet and particularly sour environments has dramatically improved. Advancement in understanding of the cor...

The Role of Iron Sulfide Polymorphism in Localized H2S Corrosion of Mild Steel

ABSTRACT Localized corrosion in sour fields is a challenge persisting in the oil and gas industry, as it has frequently been seen as a cause for catastrophic failures of upstream pipelines. Hence, ...

Investigation of Cathodic Reaction Mechanisms of H2S Corrosion Using a Passive SS304 Rotating Cylinder Electrode

The internal corrosion of pipeline steel in the presence of hydrogen sulfide (H2S) represents a significant problem in the oil and gas industry. Its prediction and control pose a challenge for the corrosion engineers. In previously published research by the same authors, an electrochemical model of H2S corrosion was developed in both pure H2S and H2S/CO2 aqueous systems. An additional electrochemical cathodic reaction, direct H2S reduction, was uncovered based upon the carbon steel corrosion experimental results. However, in the carbon steel corrosion experiments, the Tafel’s region of cathodic reactions experienced interference by the anodic iron dissolution reaction, making the electrochemical kinetics of cathodic reactions unclear. In the present study, experimentation was conducted to better resolve the direct reduction of H2S while minimizing the effect of the anodic reaction by using a passive stainless steel working electrode. The electrochemical kinetics parameters for H2S reduction (i.e., Tafel s...