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 (KH(2)s), the first dissociation constant (K-a,K-1), and the second dissociation constant (K-a,K-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 degrees C, while greigite and pyrite were detected at 60 degrees C. The solubility limits for these iron sulfides were determined. Simplified Pourbaix diagrams for the H2S-H2O-Fe system have been constructed with different meta-stable and stable iron sulfides (mackinawite, pyrrhotite, greigite, and pyrite) to predict the corrosion products in H2S corrosion of mild steel.