Real-Solution Stability Diagrams: A Thermodynamic Tool for Modeling Corrosion in Wide Temperature and Concentration Ranges

A method was developed for construction of stability diagrams for metals in the presence of realistically modeled aqueous solutions. The method was based on a comprehensive thermodynamic model that combines the Helgeson-Kirkham-Flowers (HKF) equation of state for standard-state properties with a solution nonideality model based on the activity coefficient expressions developed by Bromley and Pitzer. Composition-dependent nonideality effects were incorporated into the calculation of predominance areas for dissolved and solid species. Using the combined thermodynamic model, stability diagrams can be computed for systems involving concentrated solutions (i.e., with molalities up to 30 mol/kg) at temperatures up to 573 K and pressures up to 100 MPa. Since the diagrams are based on a realistic thermodynamic model for the aqueous phase, they are referred to as real-solution stability diagrams. In addition to customary potential (E) and pH variables, concentrations of various active species (e.g., complexing agents) can be used as independent variables, making it possible to analyze effects of various compounds that promote or inhibit corrosion. Usefulness of the methodology was demonstrated by generating real-solution stability diagrams for five representative systems (i.e., sulfur-water [S-H₂O], copper-ammonia-water [Cu-NH₃-H₂O], titanium-chlorine-calcium-water [Ti-Cl-Ca-H₂O], iron-sulfur-water (Fe-S-H₂O], and zinc-water [Zn-H₂O]).