Early ferric hydroxide species are important precursors in the formation of corrosion products, yet their atomistic evolution from small molecular aggregates to crystal-like motifs remains poorly understood. In this work, we adapt a robust quantum mechanical conformer-search method developed by Wang and Vasilyev to Fe(OH)3 oligomers and systematically investigate the most stable structures from the dimer to the decamer. For each oligomer size, diverse candidate geometries were generated and optimized, followed by energy- and geometry-based screening to identify the lowest-energy structures and representative low-energy isomers. The calculations reveal pronounced size-dependent stabilization and structural reorganization during oligomer growth, together with clear changes in Fe coordination environments, Fe–O connectivity, and electronic properties. The evolution of hydroxyl-bridged frameworks and Fe–O polyhedral connectivity provides insight into how local bonding patterns develop with increasing oligomer size. In addition, the relative stabilities of the low-energy structures indicate that specific oligomer sizes may act as favorable intermediates during the early-stage nucleation of corrosion products. These results suggest that the aggregation of Fe(OH)3 does not proceed as a simple monotonic growth process, but instead involves preferred structural motifs at specific oligomer sizes. Comparisons with representative crystalline corrosion products further clarify how early Fe(OH)3 oligomers evolve toward bulk corrosion phases. This study provides a molecular-level picture of the initial stages of corrosion-product formation and offers a computational route to bridge molecular corrosion precursors with experimentally observed crystalline products.

Steel corrosion presents significant economic, environmental, and safety challenges, particularly in chloride‑rich environments. At the atomic level, corrosion arises from competition between protective oxygen adsorption and chloride‑driven depassivation. Although these processes are well characterised on ideal Fe surfaces, the role of surface defects in modulating this balance remains poorly understood. In this study, spin‑polarised density functional theory (DFT) calculations were used to systematically examine defect-controlled corrosion reactivity on the Fe(100) surface. Three surface models are considered: pristine Fe(100) (Fe(100)-P), adatom‑modified Fe(100) (Fe(100)-A), and vacancy‑defective Fe(100) (Fe(100)-V). Adsorption energies and electronic structures are computed to quantify defect-driven changes in O‑ and Cl‑binding at the most stable sites. Oxygen adsorption is found to be strongest on the pristine surface, consistent with effective passivation (Fe(100)-P > Fe(100)-A > Fe(100)-V). Vacancy defects markedly weaken O binding, suppressing protective oxide film formation, while adatoms have only a minor influence. In contrast, Cl adsorption is significantly enhanced at vacancy sites (Fe(100)-V > Fe(100)-P > Fe(100)-A), identifying vacancies as preferential Cl‑accumulation sites that accelerate passive‑film breakdown. These findings establish vacancy defects as dual corrosion promoters that simultaneously hinder passivation and enhance Cl‑induced depassivation. Detailed atomistic and mechanistic analysis of these defect effects will be presented, offering practical guidance for defect-engineered alloy design and advanced surface treatment strategies to achieve superior corrosion resistance.

Ferritic/martensitic 9Cr heat-resistant steels are extensively used in high-temperature steam power systems due to their balanced creep strength and oxidation resistance. However, during start-up, shut-down, hydrotesting, and wet layup conditions, these alloys experience aqueous exposure where electrochemical corrosion and localized attack may compromise structural integrity. Although molybdenum (Mo) is a key alloying element in 9Cr steel families, its independent role in governing passive film stability and breakdown remains unclear due to strong compositional coupling in commercial grades. This study aims to isolate and quantify the intrinsic effect of Mo on passivation and localized corrosion behavior using a systematic model alloy approach. A Fe–9Cr–xMo (x = 0–18 wt.%) model alloy series was developed to maintain a constant chromium backbone while varying Mo as the primary design variable. Electrochemical behavior was evaluated in chloride-containing aqueous environments using potentiodynamic polarization, cyclic polarization, repassivation potential analysis, and critical pitting temperature measurements. Microstructural characterization was performed to correlate electrochemical responses with compositional heterogeneity and secondary phase formation. Mo content modifies passive behavior in a non-linear manner. Intermediate Mo levels enhance passive film stability, increase breakdown and repassivation potentials, and elevate critical pitting temperature, indicating improved localized corrosion resistance. In contrast, higher Mo contents promote microstructural heterogeneity and metastable pitting activity, reducing repassivation efficiency and narrowing the passive stability window. These results suggest a composition-dependent transition from Mo-enhanced passivation to heterogeneity-assisted breakdown. This work establishes a quantitative composition–passivation–breakdown relationship for Fe–9Cr–xMo alloys and identifies a mechanistic threshold in Mo content controlling localized corrosion resistance. The findings provide fundamental insight for optimizing 9Cr-based steels subjected to combined steam and aqueous exposure in energy infrastructure applications.

Understanding the growth kinetics of passive films is crucial for controlling corrosion resistance. However, the mechanistic origin of the commonly observed current transients remains unclear, and uniquely determining the interfacial kinetic parameters remains challenging. In this presentation, the transient growth behavior of passive films on Fe, Ti, Ni, and Cu was investigated using potentiostatic current density-time (I-t) analysis combined with Mott–Schottky measurements. Within the framework of the point defect model (PDM), a systematic methodology was established to interpret I-t curves. A two-stage double-logarithmic relationship in log I–log t plots is attributed to film growth governed by oxygen vacancies coupled with metal cation interstitials or vacancies, whereas a single-stage behavior indicates control by a single defect type. The slope of the log I–log t plot is determined by the ratio of interfacial transfer coefficients, with a more negative slope corresponding to the formation of a denser passive film with fewer defects. Importantly, integrating I-t analysis with electrochemical impedance spectroscopy (EIS) enables quantitative determination of the kinetic parameters of interfacial defect reactions in the PDM. While I-t transients constrain transport and reaction kinetics during early-stage growth, EIS provides complementary information on interfacial reaction resistances and capacitances under steady-state conditions. Their combined application offers a more reliable and internally consistent strategy for extracting transfer coefficients and rate constants, thereby strengthening parameter identification within the PDM framework.

This work focuses on using representative corrosion damage morphology, determined from fractography, to predict crack propagation behavior in a test article under fatigue load. A dogbone specimen (AA7075-T651) with a simulated fastener hole was pre-corroded using atmospheric corrosion conditions (RH = 80%, salt load = 1000 mg/m², T = 25^o C). A CFRP insert was placed in a through hole on the side surface of the dogbone to represent a fastener system and to promote local corrosion through galvanic coupling. A thin non-conductive separator was placed between the CFRP insert and the AA7075-T651 through hole to prevent physical contact between those two materials. The specimen was salt dosed and placed in an environmental chamber with controlled relative humidity and temperature. A zero-resistance ammeter test of the CFRP/AA7075-T651 couple was performed for 10 days to induce galvanic corrosion damage on the surface surrounding the through hole mouth. Following pre-corrosion, the dogbone specimen was tested under constant amplitude fatigue load (S_max = 25.11 ksi, R = 0.89) in laboratory air; the total fatigue life measured experimentally was 230,135 cycles. Following fatigue testing the failed specimen was inspected for corrosion-related crack initiation sites and final crack shape using SEM imaging. A computational fracture mechanics model, based on the boundary integral method, was developed to simulate the fatigue test. Fractographic imaging was used to size and locate initial flaws in the through hole. To approximate the anticipated short-to-long crack transition, the Hartman–Schijve equation was fitted to a crack growth kinetic data set and used to drive the simulation model. A range of model inputs were selected to understand the sensitivity of fatigue life predictions to the fracture mechanics model inputs used to represent the complex corrosion damage morphology. The computational model produced results that were in reasonable agreement with the experimental test.

This presentation will provide new insights and ideas to address the relationship among environmental exposure parameters, the presence of applied and residual stress, and microstructure/metallurgy performance. Electrochemical screening and sensing techniques have been explored to assess local corrosion reactions, selective dissolution and passivation behavior, and to observe the nucleation of environmentally assisted cracking (EAC). Typical examples and innovative approaches to advance our mechanistic understanding of localized corrosion and EAC are discussed, with the aim of better understanding mechanistic behavior and extending component lifetime. Techniques for measuring corrosion kinetic behavior, passive film composition, and crack-nucleation propensity are introduced, and the application of electrochemical techniques is elaborated. A range of new ideas is presented, including the application of electrochemically controlled micro-/nano-indentation tests, in situ confocal microscopy of hydrogen embrittlement (HE), and 2D/3D/4D corrosion and crack growth measurements using advanced imaging methods. These tests are then combined with chemical fingerprinting of corrosion products using (spectral) imaging approaches. The development of corrosion chemistry is observed over time, yielding information about local passivation behavior under redox conditions. We present work on aluminum alloys, stainless steels, and Ni-base alloys, addressing material behavior for marine exposure, petrochemical applications, and nuclear/radwaste exposure regimes. Results from different characterization techniques will be compared and correlated to their electrochemical responses.

Aluminium alloys have become the material of choice for automotive heat exchangers due to their favourable mechanical properties and corrosion resistance. While the impact of homogenization temperature and duration on AA3xxx alloys has been studied, the specific effect of preheating holding time on clad AA3003mod alloys remains underexplored. This study investigates how isothermal soaking time influences the microstructure and corrosion resistance of AA3003mod alloy clad with AA4045, a configuration used in automotive brazing sheets. To replicate industrial production, hot-rolled and homogenised slabs underwent thermo-mechanical processing. Sandwich samples of AA4045/AA3003mod were preheated at 505 °C for 20, 30, 35, 40, 38 and 45 hours. Corrosion resistance was assessed using potentiodynamic through open-circuit potential (OCP) measurements and potentiostatic polarisation techniques in a 3.5% NaCl solution, with pitting quantitatively analysed via an Olympus Microscope equipped with a 3D imaging software and Scanning Electron Microscopy (SEM). The corrosion rates for all brazed samples exceeded the generally accepted standard of <0.1 mm/y. Pitting severity and current density peaked between 30 and 45 hours of homogenization, indicating reduced nobility. In contrast, samples homogenised for 20 hours exhibited minimal pitting, the shallowest pits, the most noble open-circuit potential, and the lowest current density, demonstrating that extended homogenization significantly deteriorates corrosion resistance of the roll bonded AA4045/AA3003mod sheets.

Passivation is a key factor in the durability of materials. It presents an even more crucial challenge when we are faced with the challenges of environmental and climate transitions. These challenges require control and mastery of resources, sustainability and material manufacturing processes. In this context, the work of Prof. Digby D. Macdonald is both pioneering and important.

Today, it has inspired a great number of studies, and, when they are discussed, Digby D. Macdonald's research inspires many developments, attested to by the emergence of numerous models based on the renowned Point Defect Model.

Of course, this is not to reduce Prof. Digby D. Macdonald's work to passivation alone, but this presentation attempts to provide a brief overview of how his work, and that of his closest collaborator, Prof. Mirna Macdonald, has inspired me in my career as a corrosion specialist in particular and a materials scientist in general.

Beyond the mechanisms of passive film formation, the presentation will mainly focus on illustrating how the theme of passivation can enrich our knowledge of the role of additives on the one hand, and the conceptualisation of anti-corrosion solutions through the optimisation or definition of new materials, on the other hand.

The first topic will focus on the study of the effect of nitrogen on passivation and how the work carried out with Digby D. and Mirna Macdonald has led to an understanding of the so-called nitrogen blocking effect in the transport of passive films. The second example will focus on the strategy developed to develop new materials for the deployment of fuel cells for electromobility based on passive film functionalising. Finally, I will illustrate the various links between passivation and the multifunctionality of materials, based on recent work on tribocorrosion under irradiation.

Atmospheric galvanic corrosion of aluminum alloys continues to challenge accurate prediction and mitigation, particularly in mixed-material systems used in aerospace and marine environments. This includes systems exposed to dynamic atmospheric conditions where wet–dry cycling, chloride deposition, and microstructural heterogeneity govern degradation processes. Aluminum alloys, valued for their strength-to-weight ratio and passivity, are particularly susceptible to localized and galvanic attack when combined with dissimilar materials in aerospace, marine, and energy infrastructure.

Conventional salt-spray or cyclic corrosion tests fail to replicate the complex wet–dry cycling and localized electrochemical interactions that govern long-term atmospheric degradation. This study explores how to integrate a hybrid framework combining accelerated laboratory electrochemical techniques with time-of-wetness (TOW) to better predict the atmospheric galvanic behavior of 6061-T6 aluminum (UNS A96061) coupled with stainless steel (UNS S30400), copper (UNS C11000), and carbon-fiber reinforced polymer composites (CFR PMC).

Zero-resistance ammeter (ZRA) experiments were performed in a controlled-humidity chamber and a cyclic-corrosion test chamber under varying chloride concentrations to simulate different atmospheric exposures. The galvanic current data were correlated with modified Faraday-based equations incorporating measured TOW to estimate corrosion rates in g m⁻² day⁻¹. The aluminum–copper couple exhibited the highest current densities and corrosion rates (up to 12 g m⁻² day⁻¹) under high-chloride conditions, while aluminum–CFR PMC and aluminum–stainless-steel couples showed comparable but lower rates. The integrated approach successfully bridged the gap between accelerated tests and real atmospheric behavior by quantifying the effects of wet/dry cycles on galvanic kinetics.

The findings highlight the potential of TOW-based electrochemical modeling as a predictive tool for assessing material compatibility and optimizing design in dissimilar-metal assemblies. Future work includes more cyclic corrosion chamber testing and outdoor validation at various exposure sites to refine the correlation between laboratory and field data.

During dry storage of spent nuclear fuel, the stainless steel (SS) canisters used for containment are cooled by ambient air that often carries fine salt aerosols. These airborne salts can accumulate on the canister surface and, in the presence of humidity, deliquesce to form concentrated chloride brines that promote localized corrosion and ultimately stress corrosion cracking. The present work investigates the initiation of pitting corrosion and the subsequent pit-to-crack transition in SS304H under simulated canister-relevant environments. Three surface preparation methods, including hand grinding, machine grinding, and milling, were used to create specimens with varying degrees of surface deformation and residual stress. A MgCl2-rich brine was employed to reproduce the chemistry expected after deliquescence at approximately 40 % relative humidity. To generate a range of pit morphologies, specimens were exposed to controlled atmospheric corrosion under constant relative humidity and temperature. The results showed that machine milling introduced a substantial near-surface deformation and tensile residual stress, leading to the formation of the most severe pitting corrosion and long cracks. Additionally, dealloying was found in machine-ground specimens exposed to the simulated brine after only two days. The results highlight the importance of residual stress on localized corrosion and the pit-to-crack transition in atmospheric conditions.