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.

A chemical stability map is advanced by incorporating ion complexation, solubility, and chemical trajectories to predict ZnO, Zn(OH)₂, ZnCO₃, ZnCl₂, Zn₅(CO₃)₂(OH)₆, and Zn₅(OH)₈Cl₂·H₂O precipitation as a function of the total Zn content and pH of an NaCl solution. These calculations demonstrate equilibrium stability of solid Zn products often not considered while tracking the consumed and produced aqueous Zn ion species concentrations through chemical trajectories. The effect of Cl-based ligand formation is incorporated into these stability predictions, enabling enhanced appreciation for the local corrosion conditions experienced at the Zn surface in chloride-containing environments. Additionally, the complexation of Cl⁻ with Zn²⁺ is demonstrated to compete with the formation of solid phases, making precipitation more difficult. The present work also extends the chemical stability diagram derivations by incorporating a Gibbs–Thompson curvature relation to predict the effect of nanoscale precipitate phase formation on species solubility. These thermodynamic predictions correlate well with experimental results for Zn corrosion in full and alternate NaCl immersion, and have far-reaching utility in a variety of fields requiring nanoscale, semiconductor, and/or structural materials.

Pigmented coatings developed for the corrosion protection of engineering alloys have been successful in the field of corrosion control, and thus are ubiquitous in application. Supporting this success are an array of methods to diagnostically assess these systems and measure their state of degradation. These methods assist the development of next generation coatings, in-service evaluation, and post-mortem analysis of performance. This review comprehensively explores these modern assessment methods, used to screen candidate corrosion prevention pigments, assess pigment protection efficiency, quantify overall pigment effectiveness, and/or predict coating service lifetime. Coating service life assessment is conducted based on the class of coating considered, as categorized by (1) chemical inhibition, (2) galvanic protection, (3) high impedance ion barrier, or a combination of these protection mechanisms. Exciting new in-situ and operando methods are growing in use to enable high fidelity measurement of chemical inhibitor release and deposition, such as micro and in-situ Raman spectroscopy and inductively coupled plasma atomic emission spectrometry (ICP-AES), to name a few. In the galvanic protection coating class, service life and the state of coating performance are now being quantified through methods such as galvanostatic pulse and accelerated cycle testing. Galvanic throwing power is assessed by scanning vibrating electrode technique (SVET), multi-electrode array (MEA), and scanning kelvin probe (SKP). The performance of high impedance ion barrier coatings has traditionally been understood through use of electrochemical impedance spectroscopy (EIS) and equivalent circuit modeling, which have been applied to all coating classes. Finite element analysis (FEA) models the combined effects of coating resistance, water layer thickness, and pigment potential on galvanic coupling and/or inhibitor species release as well. The development of effective corrosion prevention pigments and the in-service evaluation of their efficiency/effectiveness demands a broad collection of techniques. These methods now include new advances in thermodynamic modeling via chemical stability diagrams to assess pigment function in specific conditions, in-situ pH measurement, as well as energy dispersive (EDS) and x-ray photoelectron spectroscopic (XPS) analyses to determine elemental distribution and corrosion product formation. These techniques and more are explored in this review to document the state-of-the-art in the field.

AA5xxx-series alloys are commonly utilized as lightweight replacements for steel components in marine applications, but experience precipitation of an anodic β phase (Al3Mg2) on grain boundaries at temperatures as low as 40°C (313 K). When stressed in aggressive environments, the preferential dissolution of this intermetallic leads to intergranular stress corrosion cracking (IGSCC). Susceptibility to IGSCC is assessed in the S-L orientation for differing compositions (AA5083 and AA5456) and material tempers (-H131, -H116, and -SHTQ) sensitized at 100°C (373 K) to Nitric Acid Mass Loss Test (NAMLT) values of 24 mg/cm2 and 40 mg/cm2. The more Mg-rich AA5456 shows less susceptibility to IGSCC than AA5083 for a constant -H116 temper. In AA5083, the -H131 temper exhibits the greatest susceptibility to IGSCC, followed by the -H116, and the -SHTQ conditions. These composition and temper effects are pronounced at NAMLT 24 mg/cm2, but are less traceable at NAMLT 40 mg/cm2. Reduced IGSCC susceptibility scales with decreasing yield strength. Susceptibility to IGSCC is interpreted in the context of a Hydrogen Enhanced Decohesion model, to relate microstructural changes to the governing hydrogen-based mechanisms. The lack of similitude in IGSCC behavior at constant NAMLT values suggests that this metric is a poor proxy for IGSCC susceptibility across different compositions/tempers.

The effect of loading orientation (S-L versus T-L) on intergranular stress corrosion cracking (IGSCC) behavior was assessed in AA5083 and AA5456 in two tempers (-H116, -H131) at nitric acid mass loss test (NAMLT) levels of 24 mg/cm2 and 40 mg/cm2. Regardless of temper or composition, loading in the T-L orientation demonstrated reduced susceptibility to IGSCC as a result of delamination toughening caused by fissure propagation in the L-T plane. Electron backscatter detection analysis confirmed these fissures to be intergranular, and the fissure size/density increased in plane strain constraint conditions toward the center of the specimen. Occurrence of these out-of-plane fissures was validated by testing on NAMLT of 10 mg/cm2 and 67 mg/cm2 AA5456-H116, although delamination propagation occurred only under full immersion in 0.6 M NaCl, and not testing (fatigue or slow rising displacement) conducted in moist air. These delamination phenomena are understood further through application of a hydrogen-enhanced decohesion-based micromechanical model assessing H effects at the crack tip.