Environmentally Assisted Degradation of Metals and Alloys

1. Introduction and Scope

Metallic materials play a central role in ensuring the safety and reliability of critical sectors such as infrastructure, energy, and transportation [1,2,3,4,5,6,7]. However, when metals and their alloys are subjected to mechanical loads while exposed to corrosive media and complex environmental stresses, their mechanical integrity and service lives can be significantly compromised, potentially leading to catastrophic failures [8,9,10,11,12,13]. Environmentally assisted degradation primarily manifests as corrosion, oxidation, hydrogen embrittlement, stress corrosion cracking (SCC), and corrosion fatigue, often occurring synergistically, which poses substantial challenges for prediction and mitigation [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29].

This Special Issue of Metals, titled “Environmentally Assisted Degradation of Metals and Alloys,” brings together the latest research from scientists worldwide, covering a comprehensive spectrum from fundamental mechanisms to engineering applications. Key topics include the corrosion behavior of steels under practical and cyclic conditions, high-temperature oxidation and crack initiation in boiler steels and Ni-based superalloys, hydrogen embrittlement mechanisms and mitigation, stress corrosion cracking and corrosion fatigue in stainless and marine steels, and advances in surface modification and coatings such as phytic-acid/silane composites, organosilane treatments, and ultrasonic-assisted Fe-based amorphous cladding to enhance the corrosion resistance and durability of steels and magnesium alloys in complex service environments.

These contributions not only highlight the multidisciplinary nature of environmentally assisted degradation research, integrating materials science, electrochemistry, and mechanics but also provide new insights for designing durable and reliable metallic systems. The Special Issue further emphasizes the relevance of environmentally assisted degradation studies to critical infrastructure, hydrogen energy, and carbon neutrality initiatives, offering valuable guidance for the design, optimization, and reliability assurance of next-generation high-toughness and sustainable metallic materials and engineering systems.

2. Contributions

A total of fifteen high-quality contributions were published in this Special Issue, encompassing original research articles and reviews that span multiple scales, ranging from atomistic simulations to field-level corrosion studies. Each study provides new insights, methodologies, or theoretical interpretations that collectively advance the understanding of environmentally assisted degradation in metals and alloys.

Several studies focused on corrosion behavior of pipeline and structural steels under practical conditions. Li et al. investigated CO₂ corrosion of X70 pipeline steel under intermittent gas–liquid flow, showing accelerated localized attack under alternating wet and dry conditions (Contribution 1). Benkhedda et al. combined electrochemical testing with ultrasonic inspection to assess external corrosion and defect detection in buried API 5L X60 pipelines (Contribution 2). Chen et al. investigated Cu-Sb-added low-carbon steels, showing that Cu and Sb enhance chloride ion corrosion resistance by increasing corrosion potential, promoting protective compound formation, and facilitating the conversion of Fe²⁺ into α-FeOOH (Contribution 3). Kopylov et al. examined ultrafine-grained AISI 321 steel processed through ECAP, revealing that σ-phase nanoparticle formation enhances strength and stress-relaxation resistance but compromises corrosion performance (Contribution 4).

High-temperature oxidation and its impact on mechanical integrity were examined in several contributions. Burja et al. studied boiler steels at 650 °C, elucidating oxide-scale formation, phase evolution, and conditions that minimize spallation while enhancing long-term oxidation resistance (Contribution 5). Gao et al. investigated FeCrAl alloys, revealing the formation of a multilayer oxide film with Al-rich layers that enhances high-temperature corrosion resistance (Contribution 6).

Hydrogen embrittlement was addressed at microstructural and atomic scales. Zhang et al. employed first-principles calculations to study hydrogen adsorption and diffusion on Fe(110) surfaces doped with alloying elements, proposing strategies to suppress hydrogen uptake and enhance embrittlement resistance (Contribution 7). Zhao et al. examined hydrogen embrittlement in QP980 steel, demonstrating how hydrogen trapping at phase interfaces governs crack initiation and propagation (Contribution 8). Tao et al. reviewed micro-sample testing methods, including small-sized tensile, small punch, and nanoindentation tests, highlighting their effectiveness at characterizing hydrogen embrittlement in limited-size components and providing insights into hydrogen’s influence on mechanical properties and microstructural evolution (Contribution 9).

Several contributions examined stress corrosion cracking or corrosion fatigue, highlighting the interaction between mechanical stress and corrosion-induced failure. Gao et al. investigated sensitized FSWed AA5083 Al-Mg alloy, showing that β′-Al₃Mg₂ precipitates along grain boundaries severely degrade corrosion resistance, leading to high susceptibility to intergranular and stress corrosion cracking (Contribution 10). Wang et al. analyzed oxidation-assisted crack initiation in Ni-based single-crystal superalloys under cyclic fatigue, providing mechanistic insights into the synergistic effects of oxidation and mechanical stress (Contribution 11). Ren et al. examined axle steel EA4T under uniaxial cyclic deformation, revealing that dislocation density increases with plastic strain and cycling, evolving from simple lines and pileups to tangles and walls, providing microscopic insights into its ratcheting and cyclic deformation behavior (Contribution 12).

Innovative surface engineering and corrosion protection strategies were also presented. Duan et al. proposed a phytic-acid/silane composite coating for cold-rolled steel, demonstrating enhanced corrosion protection and adhesion as an environmentally friendly alternative to chromate coatings (Contribution 13). Petrunin et al. investigated organosilane-based treatments on carbon steel, achieving superior corrosion resistance and adhesion through tailored silane compositions (Contribution 14). Han et al. studied the ultrasonic-assisted laser cladding of Fe-based amorphous coatings, showing reduced crack formation and improved coating stability in marine environments (Contribution 15).

Collectively, these contributions provide comprehensive insights into environmentally assisted degradation processes, spanning mechanistic, experimental, and computational approaches. They enhance scientific understanding and offer practical guidance for designing and protecting metallic systems under aggressive service conditions.

3. Conclusions and Outlook

This Special Issue comprehensively advances the understanding of the environmentally assisted degradation of metals and alloys under multi-scale and diverse environmental conditions [8,9]. The studies included indicate that the synergistic effects of materials, mechanical stress, corrosive media, temperature, and hydrogen remain the core challenges in predicting and mitigating material degradation. Through the integration of experimental, theoretical, and computational investigations, the authors reveal the microscopic mechanisms of corrosion processes, hydrogen–metal interactions, oxidation phenomena, and stress corrosion cracking and propose innovative strategies in alloy design, surface engineering, and corrosion protection. Related advances can also be found in recent studies [30,31,32,33,34,35,36,37,38].

Future research progress will rely on integrating advanced in situ characterization techniques with data-driven modeling and machine learning approaches to achieve more accurate predictions of the service lives of metallic materials [39,40,41,42,43,44,45,46]. The hydrogen–metal interaction problem, including hydrogen production, storage, and transport, requires intensified research to ensure the safety of future hydrogen infrastructure [47,48,49,50,51,52,53]. Research on the environmentally assisted cracking of metals, particularly pipeline steels, magnesium alloys, aluminum alloys, and titanium alloys, is expected to expand, covering new methods for simulating crack initiation [54], mechanisms of strain shock or strain burst induced cracking [55,56], and the role of sulfate blunted crack tips, thereby enabling effective strategies to suppress crack propagation [57]. In addition, multifactor coupling approaches such as cross-rolling combined with Ca microalloying or I-phase precipitation can be employed to enhance corrosion resistance and resistance to environmentally assisted cracking [3,58]. Furthermore, sustainable protection technologies such as bio-based coatings, intelligent corrosion inhibitors, and corrosion-resistant alloys will play a crucial role in aligning materials design with global sustainability goals [38,59,60,61,62,63,64,65,66].

On behalf of the editorial team, we sincerely thank all the authors for their valuable contributions to this Special Issue and the reviewers for their thorough and constructive evaluations. Special thanks are extended to the Metals Editorial Office for its professional support throughout the publication process. We hope that this Special Issue serves as an essential reference for researchers and engineers working on mitigating environmentally assisted degradation of metals and alloys and inspires future innovative research.