Corrosion and Protection of Metallic Materials

1. Introduction and Scope

Corrosion is a natural and spontaneous process of degradation or failure that occurs in almost all metal materials in contact with the surrounding environment. The corrosion of metallic materials remains a major technological, economic, and social challenge because their durability or safety often limits their service lifetime under increasingly demanding exposure conditions [1].

The environmentally assisted degradation of metals and alloys in civil engineering structures has a detrimental effect on their mechanical and functional properties, which may lead to the collapse of infrastructure, such as the Morandi bridge in Genoa, which collapsed on 14 August 2018, causing the death of 43 people; the corrosion of stainless steel tanks, such as the Bhopal gas disaster in India 1984, which killed over 8000 people [2]; or environmental consequences, such as the Prestige Oil Spill in Galicia in 2002, which released around 77,000 tons of fuel oil into the sea [3]. Thus, issues such as corrosion monitoring, inspection, prevention, and life prediction are essential to maintaining the integrity of many critical components and installations [1,4].

In 2016, corrosion-related economic losses were estimated to comprise approximately 3.4% of global Gross Domestic Product [5,6]. One estimate is that 15–35% of the associated cost could easily have been saved by implementing currently available corrosion prevention and control methods (material selection and protection) [7].

On the other hand, ecological aspects are becoming increasingly important, such as the proper utilization of raw materials [1] and environmental protocols seeking to reduce fuel consumption and greenhouse gas emissions. For example, magnesium (Mg) alloys have the lowest density among all metallic materials used for structural applications, with a density of approximately two-thirds that of aluminum (Al) and one-quarter that of steel [8]. However, their poor resistance to corrosion and wear is one of the significant drawbacks of the widespread application of Mg alloys.

These challenges have prompted a search for a deeper understanding of corrosion mechanisms and corrosion protection in a variety of applications; however, further knowledge is required [1,4]. As part of this scenario, this Topic, “Corrosion and Protection of Metallic Materials”, presents several studies and reviews of the latest developments in surface science and engineering research on corrosion and degradation phenomena and the most suitable procedures to protect metallic materials.

2. Contributions

Twenty-one articles have been published that investigate different aspects of the degradation of metallic and polymeric coatings and their mechanisms. One of these is a review that focuses on natural polyphenols. The corrosion and protection of various metals have been studied, e.g., copper (Cu); different steels, including stainless steel; Mg alloys; Al alloys; and depleted uranium (DU). Methods for corrosion mitigation, including inhibitors, tannins, organic and inorganic coatings, metallic coatings, and surface modification and processing methods, have also been addressed. Different corrosion mechanisms have been addressed in this Topic, such as intergranular corrosion failure, stress corrosion cracking (SCC), hydrogen embrittlement, wear mechanisms, and atmospheric corrosion. A set of methods for corrosion analysis have been applied in the aforementioned studies, including structural and surface characterization techniques (Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM)) and electrochemical measurements (Open Circuit Potential (OCP) measurements, Potentiodynamic Polarization (PDP) curves, and Electrochemical Impedance Spectroscopy (EIS)).

In the first article, Zhu and Zhang [Contribution 1] designed and synthesized a novel system based on an amorphous metal–organic framework (AMOF) modified by 2-mercapto-1-methyl imidazole (MMI) and tested it as a corrosion inhibitor for Cu in NaCl solution. The results showed that the AMOF had a high loading capacity for MMI. The release of MMI from the AMOF effectively inhibited Cu corrosion.

Recent developments in utilizing natural polyphenols, particularly tannins, to protect steel from corrosion were reviewed by Sesia et al. [Contribution 2]. Based on the existing literature, they discussed the use of polyphenols as primers in coating systems, adhesion promoters, rust converters on steel substrates, and additives in polymeric coatings and corrosion protection coatings. The authors emphasized the potential of polyphenolic compounds, which are natural, non-toxic, and biodegradable compounds derived from plant sources, as effective and environmentally friendly corrosion inhibitors, focusing on the preparation, improvement, and development of green and sustainable strategies for corrosion protection.

The impact of surface roughness on the static corrosion behavior of J55 carbon steel in CO₂-containing geothermal water at 65 °C was examined by Bai et al. [Contribution 3]. In this study, the authors characterized the morphology of non-exposed and corroded specimens and analyzed the elemental composition of the corrosion product layer formed. The corrosion rates were obtained using the weight-loss method, and the maximum corrosion depth was measured. It was concluded that a higher surface roughness led to increased corrosion rates in the first days of exposure, owing to the presence of protruding areas that tended to act as anodes. After longer periods of exposure, a complete corrosion product layer was formed, which inhibited ion transport. The corrosion products were primarily composed of FeCO₃ with a small amount of CaCO₃, which was not significantly affected by surface roughness.

Benito-Santiago et al. [Contribution 4] studied the synthesis and characterization of cerium oxide (CeO₂) coatings on a bioabsorbable AZ31-Mg-based alloy for corrosion protection and biocompatibility with pre-osteoblast cells. This study examined previous anodization of the AZ31 alloy with NaOH, the application of a CeO₂ coating through chemical conversion treatment, and the evaluation of its electrochemical behavior in Hank’s solution, as well as its structural properties and biocompatibility. They concluded that the anodization process formed a Mg(OH)₂ film, which increased the corrosion resistance of the AZ31 alloy. The CeO₂ coating further enhanced the alloy’s corrosion resistance and improved its biocompatibility, sealing the passive film of Mg(OH)₂ and promoting osteoblast proliferation on the modified surface compared to bare samples.

Liu et al. [Contribution 5] developed a novel method for preparing high-molecular-weight waterborne epoxy–acrylic emulsions with improved stability and coating film properties. The process involves synthesizing epoxy resins, grafting acrylic monomers onto the epoxy resin, and neutralization to obtain stable emulsions. They investigated the optimum conditions for emulsion preparation, including a grafting temperature of 120 °C; a reaction time of 5 h; specific dosages of epoxy resin, methacrylic acid (MAA), and benzoyl peroxide (BPO); and the optimal neutralization degree and temperature. The emulsions showed excellent mechanical and chemical resistance, with a storage stability of over a year and excellent dilution, mechanical, and freeze–thaw stability. This makes them promising for use in various coating applications.

Li et al. [Contribution 6] prepared a photopolymerized re-healing polyaniline-modified epoxy resin coating material to protect carbon steel from corrosion. The coating material exhibited low water absorption and improved corrosion resistance under visible light irradiation. This study involved the synthesis of graphene oxide (GO) mixed with TiO₂ to extend the light response range, and the structural features were identified using various techniques. The protective properties of the coating were demonstrated through several tests, including EIS and PDP. The presence of TiO₂ reduces the corrosion potential in NaCl solution. By utilizing visible light excitation, the authors showed higher corrosion resistance and structural compactness in the coatings compared to the composite dried in the dark, indicating the potential for effective corrosion prevention using light energy.

Amaya Dolores et al. [Contribution 7] investigated the nature and origin of the “gold dust defect” (GDD) on ferritic stainless steel (FSS) using Electron Backscatter Diffraction (EBSD), Scanning Transmission Electron Microscopy (STEM), Monochromated Electron Energy-Loss Spectroscopy (EELS), and machine learning approaches to analyze two FSS samples with varying Al contents. They found that the GDD was correlated with poor recrystallization of the surface of the affected samples associated with the presence of cracks between the elongated grains and the matrix. The edges of the cracks were rich in chromium oxides and MnCr₂O₄ spinel. Additionally, the incorporation of Al improved the quality of the passive layer on the steel surface, reducing the damage induced by intergranular corrosion and GDD.

Xiong et al. [Contribution 8] studied the effect of molybdenum (Mo) content on the hydrogen evolution reaction (HER) and hydrogen diffusion behavior in 1400 MPa grade high-strength bolt steel. They used computational simulations, PDP, hydrogen permeation experiments, and thermal desorption spectroscopy to study the adsorption behavior of hydrogen on the surfaces of two steels with different Mo contents. Increasing the Mo content was shown to significantly promote hydrogen adsorption and absorption processes, while hydrogen desorption and recombination were slightly promoted, facilitating hydrogen entry behavior and increasing the overall hydrogen content in the steel. This was attributed to the stronger bonding ability of Mo-H compared to Fe-H. The results suggest that increasing the Mo content improved the HER kinetics and hydrogen trapping ability of the steel.

Li et al. [Contribution 9] evaluated the wear and corrosion resistance of CrYN coatings in artificial seawater. These coatings were prepared using multi-arc ion plating under various bias voltages, and characterization techniques such as XRD and SEM were employed to analyze the composition and microstructure of the coatings. The tribological and electrochemical behaviors of the coatings were assessed using a ball-on-disk tribometer, EIS, and PDP. They found that the addition of yttrium (Y) led to grain refinement, improved coating quality, and increased microhardness. The coatings deposited at −100 V were relatively smooth and dense and showed the best corrosion resistance, characterized by the highest corrosion potential, largest impedance values, low self-corrosion current, and low friction coefficients, making them ideal for marine applications.

Jiang et al. [Contribution 10] examined the passivation behavior of Q235 carbon steel at temperatures of 20, 40, and 60 °C in a simulated concrete pore solution. Various techniques, such as electrochemical testing, microscopy, and XPS, were used to analyze the growth and composition of the passive film under different temperature conditions. They observed a two-stage passivation process consisting of a rapid passivation phase before 24 h and a stable phase thereafter, with faster passivation rates, more Fe³⁺ compounds, and more stable semiconductor properties of the passive films at higher temperatures. It was concluded that the passive film composition changed with temperature, producing better and thicker passive films with more Fe₂O₃ at higher temperatures.

Ramírez et al. [Contribution 11] evaluated a new two-layer anticorrosive system using tannins from pine bark and ZnO nanoparticles superficially modified using chemical methods to enhance the protection of metallic structures from corrosion. This study included film evaluations based on ISO and ASTM standards, along with EIS characterizations at various exposure times, including 0 h, 720 h of accelerated exposure, and 4 months in a corrosive environment classified as C3 (medium aggressive), according to ISO 9223 standards [9]. The results suggest that the combination of tannins and encapsulated ZnO nanoparticles provides a viable alternative to traditional coatings, maintaining high-performance characteristics despite slight decreases in film properties. The authors also considered the impact of meteorological variables, such as relative humidity, temperature, and solar radiation, on the performance of the coating under real atmospheric conditions.

Bonfil et al. [Contribution 12] analyzed the corrosion behavior of carbon steel B450C and low-chromium ferritic stainless steel SS430 when exposed to an extract solution of supersulfated cement (SS1) over 30 days. The extract solution was considered a “green” alternative to Portland cement. Changes in the OCP, pH, and formation of corrosion products were identified using various analytical techniques. The results showed that the initial pH of the solution dropped from 12.38 to 7.84 after exposure for up to 30 days, leading to significant changes in the OCP of carbon steel, which became more negative. This resulted in the formation of various iron oxides as corrosion products, including γ-FeOOH (lepidocrocite), α-FeOOH (goethite), and Fe₂O₃ (hematite). In contrast, the OCP of SS430 shifted toward more positive values, maintaining a passive state, with the presence of Cr(OH)₃ and FeO as corrosion products and crystals of CaCO₃, NaCl, and KCl.

Yang et al. [Contribution 13] identified the failure mechanism of a 316 stainless steel heat-exchanger tube in a geothermal water environment after one year of service. The failure analysis revealed that the primary cause was due to chloride-induced SCC. The lack of nickel (Ni) and Mo in the tube material, which was detrimental to passivity, and the presence of chlorides in the service environment combined with residual stress contributed to the SCC. The failure mechanism was identified as localized anodic dissolution, with cracks initiating from the external tube and propagating in a mixed mode of transgranular and intergranular SCC. This suggests measures for mitigating SCC failure, including verifying the material composition and microstructure of steel parts before service, using more SCC-resistant steels, reducing chloride ion concentrations in industrial condensed water, and relieving residual stress through solution treatment.

Stojanović et al. [Contribution 14] evaluated the durability and corrosion properties of a waterborne coating system on mild steel dried under atmospheric conditions with accelerated infrared (IR) radiation drying techniques. They discussed the environmental benefits of waterborne coatings and explored the effectiveness of IR technology in reducing drying time without compromising the anticorrosive properties and durability of waterborne coatings. They found that coatings with zinc showed better overall protection, and IR drying significantly reduced drying times while maintaining coating quality. It was concluded that the microstructure and porosity of the coatings remained intact, confirming the effectiveness of the drying process. This suggests that this research could serve as a foundation for future studies on IR-dried waterborne coatings.

Shan et al. [Contribution 15] presented a novel method for preparing mechanically robust superhydrophobic surfaces on a 2024 Al alloy. The method is a two-step process involving chemical etching and stearic acid modification in which an Al alloy sheet is immersed in NaOH, the first etchant, followed by Na₂CO₃, the second etchant, resulting in a rough surface with hierarchical regular microscale dents that contain some nanoscale fibers. The superhydrophobic surfaces prepared using this method exhibited improved mechanical durability and corrosion resistance compared to conventional methods. The authors also noted that the surface retained a silvery color, in contrast to the black appearance of the surfaces prepared via one-step etching methods, suggesting a more favorable esthetic and functional outcome.

Novotný et al. [Contribution 16] characterized the manufacturing of geopolymer coatings on Al substrates using brush application. Eight different geopolymer coatings were prepared on an AlMgSi Al alloy substrate and examined for their properties, such as morphology, thickness, adhesion, microhardness, and thermal expansion. It was found that the coatings displayed varying thicknesses from 1.5 to 11 µm, good adhesion to the substrate, and minor defects, with some coatings exhibiting microscopic cracks and bubbles that did not negatively affect the adhesion of the coating. Additionally, the geopolymer coatings exhibited different behaviors in terms of thermal expansion, with some coatings exhibiting positive effects on reducing expansion after repeated thermal loading. Overall, this work provides valuable insights into the potential industrial applicability of geopolymer coatings as a viable alternative to traditional coatings.

Lobaz et al. [Contribution 17] developed low-melting phosphate glass coatings to protect structural parts composed of DU from corrosion. In this study, a glass frit was prepared using zinc lead phosphate low-melting glass sprayed on top of a DU surface and fired at 440 °C, and various analysis techniques were used to characterize the glass films and their crystalline structures. A partially crystallized continuous film with complex morphology was formed, with the addition of fillers impacting the barrier properties and diffusion paths for water or oxygen through the film. The coating was resistant to high doses of γ-radiation. The findings of this study support the suitability of glass coatings for shielding against ionizing radiation, making them a promising solution for protecting DU structural components.

Arnoult et al. [Contribution 18] investigated the impact of the Laser Shock Peening (LSP) surface treatment technique on the corrosion behavior of AISI 304L stainless steel in a VVER primary water environment at 280 °C and 8 MPa. LSP treatment was found to induce compressive residual stress on the surface, reaching depths of approximately 1 mm, leading to an increase in the corrosion rate and indicating a detrimental effect of LSP on corrosion resistance. In situ EIS was used to assess the corrosion behavior of the specimens, showing differences in the protective properties of the oxide layer between the LSP-treated and untreated specimens. This research suggested that any degree of cold work, regardless of its nature, results in a higher corrosion rate in high-temperature aqueous electrolytes.

Bártová et al. [Contribution 19] analyzed the effects of 40% cold working and subsequent isothermal annealing times at 650 °C for varying durations on the precipitation of secondary phases in AISI 316L austenitic stainless steel and their impact on intergranular corrosion. The authors analyzed the microstructural changes; the influence of the precipitation of phases such as M₂₃C₆ carbide, sigma, chi, and Laves, primarily inside the grain boundaries and shear bands; and the intergranular corrosion resistance of steel under different annealing conditions and degrees of cold working. It was concluded that longer annealing times led to an increase in secondary phase precipitation, impacting the susceptibility of the steel to intergranular corrosion. The authors also compared experimental data with thermodynamic calculations using Thermo-Calc software v.TCW5 (Thermo-Calc Software, version TCW5, Solna, Sweden) to predict the equilibrium state of steel under different annealing conditions.

Zhao et al. [Contribution 20] investigated the corrosion behavior of 30CrMnSiNi2A steel in artificial seawater and salt spray environments, focusing on how the solution temperature and pH affect the electrochemical behavior. Various tests were conducted to compare corrosion performance, including electrochemical analysis, slow strain rate tensile tests (SSRTs), and immersion tests. The experimental results indicated that the corrosion products were mainly composed of α-FeOOH, γ-FeOOH, and Fe₃O₄, with a notably higher content of Fe₃O₄ in the rust layer from the salt spray environment compared to the immersion environment. The corrosion rate was found to be significantly higher in the salt spray test due to differences in oxygen concentrations, attributed to the decrease in pH, which accelerated cathodic reactions, while temperature changes simultaneously promoted both anodic dissolution and cathodic reduction.

Finally, Zhang et al. [Contribution 21] explored the performance of ultrahigh-molecular-weight polyethylene (UHMWPE) barrier nets in a marine environment, focusing on their aging behavior. The effects of UV irradiation, hygrothermal aging, and salt spray aging on the chemical composition, morphology, thermal stability, and strength of the material were examined. The results indicated that UV energy significantly activates UHMWPE molecules, leading to chain breaking, which reduces the breaking strength more effectively than salt spray. An environmental spectrum was established to simulate real service conditions and evaluate the accelerated ratio of the environmental spectrum compared to the actual service environment, showing that the aging process affects the mechanical properties of the UHMWPE fibers, likely due to the cross-linking and degradation of macromolecular chains. The environmental spectrum developed in this research can be used to simulate real service conditions and evaluate the service life of UHMWPE barrier nets.

3. Conclusions and Outlook

At present, the corrosion of metallic materials remains a major technological and economic challenge because their stability or durability often limit their use under real-life service conditions in more extreme environments. The field of corrosion research is dynamic and ever-evolving, with ongoing efforts to develop new materials, techniques, and strategies to mitigate or prevent corrosion. The collection of research articles presented in this Topic highlights a wide range of scientific advancements. Contributions include understanding the complex interactions between materials and environmental factors, developing environmentally friendly corrosion inhibitors, elucidating the mechanisms of the anticorrosive actions of polymers and paint coatings, characterizing modified surfaces, and refining process techniques. Additionally, these studies emphasize efforts to mitigate negative environmental impacts. As Guest Editors of this Topic, we hope that the articles presented will attract the interest of many scientists, prove valuable for scientists and engineers engaged in the field of corrosion resistance, and serve as a guide to advance future research. In conclusion, ongoing research and advancements in corrosion science are crucial for developing effective strategies to mitigate corrosion-related problems and ensure the durability and safety of materials.