Utilization of chemical stability diagrams for improved understanding of electrochemical systems: evolution of solution chemistry towards equilibrium

Predicting the stability of chemical compounds as a function of solution chemistry is crucial towards understanding the electrochemical characteristics of materials in real-world applications. There are several commonly considered factors that affect the stability of a chemical compound, such as metal ion concentration, mixtures of ion concentrations, pH, buffering agents, complexation agents, and temperature. Chemical stability diagrams graphically describe the relative stabilities of chemical compounds, ions, and complexes of a single element as a function of bulk solution chemistry (pH and metal ion concentration) and also describe how solution chemistry changes upon the thermodynamically driven dissolution of a species into solution as the system progresses towards equilibrium. Herein, we set forth a framework for constructing chemical stability diagrams, as well as their application to Mg-based and Mg-Zn-based protective coatings and lightweight Mg-Li alloys. These systems are analyzed to demonstrate the effects of solution chemistry, alloy composition, and environmental conditions on the stability of chemical compounds pertinent to chemical protection. New expressions and procedures are developed for predicting the final thermodynamic equilibrium between dissolved metal ions, protons, hydroxyl ions and their oxides/hydroxides for metal-based aqueous systems, including those involving more than one element. The effect of initial solution chemistry, buffering agents, complexation agents, and binary alloy composition on the final equilibrium state of a dissolving system are described by mathematical expressions developed here. This work establishes a foundation for developing and using chemical stability diagrams for experimental design, data interpretation, and material development in corroding systems. Chemical stability diagrams: predicting corrosion product stability Chemical stability diagrams can predict the final equilibrium conditions of corroding systems based on factors such as pH. A team led by John Scully at the University of Virginia in the U.S.A constructed diagrams to represent the chemical stability of metallic corrosion compounds as a function of pH and metal ion concentration. In magnesium-based alloy systems, the diagrams showed the metal with the harshest equilibrium requirement dictated the final pH. pH buffering ammonia and carbonate species also altered solution chemistry by requiring more metal ions to cause the same change in pH, while metal carbonate compounds seemed more stable at high pH, indicating preferential formation under atmospheric exposure. Utilising chemical stability diagrams in conjunction with other traditional phase diagrams may therefore improve understanding of corroding electrochemical systems.