Investigation on Corrosion Behaviour of Metallic Materials

The rapid industrial and technological progress of modern society has intensified the exploitation and conversion of natural resources into consumer and industrial goods. This continuous expansion of human activity exerts increasing pressure on the environment and often undermines ecological awareness and responsibility [1,2]. To achieve sustainable development, it is therefore essential to integrate environmental criteria and sustainability principles into all areas of human endeavour, including production, transportation, culture, and governance [3,4].

While many environmental challenges can be mitigated through technological innovation, a deeper transformation in societal values and understanding of the relationship between humanity and nature is equally necessary to establish a rational and harmonious coexistence [5,6]. This is especially true in industries where material degradation directly impacts safety, economy, and ecology. Corrosion of metallic materials remains a global concern, resulting in major economic losses, safety hazards, and environmental consequences [1,7]. The development of sustainable corrosion-protection systems has therefore become a key focus of research and industrial application [8].

Latest advances emphasise the use of green corrosion inhibitors (GCIs), i.e., environmentally friendly, biodegradable or naturally derived compounds that prevent corrosion through adsorption, film formation or complexation mechanisms [2,3,9]. Studies have shown that GCIs can effectively replace traditional toxic inhibitors while maintaining or enhancing performance [10,11]. In parallel, research on eco-sustainable coatings has gained attention, where inhibitors are incorporated into organic or hybrid matrices to enable controlled release and self-healing functionality [12,13,14,15]. The multifunctionality of GCIs enables their integration into next-generation protective systems such as nanostructured, self-healing, and biopolymer-based coatings [6,13].

In recent years, metal and metal oxide nanoparticles (MNPs), including Ag, Cu, ZnO, TiO₂, and Al have been extensively incorporated into composite coating systems to improve their anticorrosion, antimicrobial, and mechanical performance. The uniform dispersion of nanoparticles within the coating matrix significantly enhances barrier integrity, decreases corrosion current densities, inhibits microbial biofilm formation, and increases surface hardness and wear resistance [16,17,18]. Despite these advantages, several challenges persist, such as nanoparticle aggregation, insufficient interfacial adhesion, uncontrolled ion release, environmental and cytotoxicity concerns, and limitations in large-scale manufacturing [19,20]. Contemporary research, therefore, focuses on developing multifunctional coating architectures through green synthesis methods, surface functionalization strategies, and comprehensive long-term durability assessments [16,18,21]. Overall, MNP-based composite coatings demonstrate substantial improvements in corrosion protection and mechanical stability, while ongoing investigations aim to optimise their long-term reliability, environmental safety, and economic feasibility [17,20,22]. This Special Issue on “Investigation on Corrosion Behaviour of Metallic Materials” compiles recent studies that address these emerging advances and challenges within this evolving research field.

The article by Žbulj et al. investigated the use of Lady’s mantle flower extract (LMFE) as an eco-friendly corrosion inhibitor for carbon steel in CO₂-saturated brine. Electrochemical tests, potentiodynamic polarisation and electrochemical impedance spectroscopy showed that LMFE achieved over 90% inhibition efficiency at optimal concentrations of 4 g/L (static) and 5 g/L (flow). Surface analysis (SEM and FTIR) confirmed that LMFE adsorbs onto the steel, forming a protective film. The extract was found to be highly biodegradable (BOD₅/COD = 0.96) and only mildly toxic (19.34%), indicating its suitability as a green, biodegradable corrosion inhibitor for use in petroleum and similar industries [23].

In another article, Wang et al. investigated a mixed corrosion inhibitor composed of imidazoline (IM), sodium molybdate, and sodium dodecylbenzenesulfonate (SDBS) for protecting mild steel in a 3.5 wt.% NaCl solution. Using orthogonal experimental design and electrochemical methods (EIS and polarisation tests), the research found that the optimal mixture IM (100 mg/L), sodium molybdate (50 mg/L), and SDBS (100 mg/L), produced the best corrosion inhibition, achieving an impedance of 1.8 kΩ·cm². SEM and EDS analyses confirmed the formation of a dense, uniform protective film on the steel surface. Among the inhibitors, IM had the strongest influence on corrosion resistance, while SDBS enhanced film formation, and molybdate provided supplementary protection [24].

In their work, Yan et al. examined how varying cobalt (Co) content affects the structure and performance of NiTiAlCrCoxN films deposited on stainless steel using magnetron sputtering. All films had a face-centred cubic (FCC) structure with a (200) preferred orientation. Increasing Co content led to decreased hardness but increased elastic modulus. The film with a Co molar ratio of 1.4 showed the best cavitation erosion resistance, having the lowest mass loss (0.72 mg) and highest elastic modulus (253.22 GPa). This improvement was attributed to enhanced solid solution strengthening and better elasticity that reduced microjet impact during erosion [25].

Deng et al. proposed a novel anticorrosion method combining impressed current cathodic protection (ICCP) with graphene-based coatings for marine atmospheric environments. The graphene coating acts as an intermediate conductive layer, forming an electrolytic cell with the metal substrate that improves corrosion resistance, especially at coating defects where an electrolyte film forms. Experiments show the optimal protection voltage is 0.6 V and the best graphene coating thickness is 20 μm. This combined method provides better corrosion protection and lower corrosion rates than coatings alone, making it effective for long-term marine applications [26].

Stojanović et al. investigated the physical and chemical properties of high-temperature silicone-based polymer coatings, both solvent-borne and water-borne, applied to carbon steel with different surface roughness. The research evaluates corrosion protection, adhesion, hardness, thermal stability, and electrochemical behaviour under conditions simulating wood-burning stoves, including salt spray, humidity, and high-temperature cycling tests. Key findings include better adhesion on sandblasted surfaces, superior performance of inorganic zinc ethyl silicate coatings, generally higher barrier properties of solvent-based coatings compared to water-based ones, and excellent thermal stability of all coatings tested. The coatings maintained good appearance and adhesion after high-temperature exposure, making them suitable for high-temperature applications despite challenges in salty environments. Overall, the study concludes that the choice of surface preparation and coating type critically influences performance, and that silicone-based high-temperature coatings, especially inorganic zinc ethyl silicate, are suitable for indoor high-temperature applications such as wood-burning stoves. However, caution is advised in salty environments due to reduced durability [27].

In another article, Landek et al. demonstrated that DC plasma nitriding of AISI 316L stainless steel at 430 °C enhances surface hardness and alters surface roughness, with longer nitriding durations leading to increased hardness. However, extended nitriding also results in the formation of a brittle, hard layer that accelerates wear under micro-abrasion conditions. While nitriding slightly reduces corrosion resistance compared to untreated steel, the overall corrosion stability remains largely unaffected by the nitriding duration. Therefore, optimising nitriding time is essential to achieve a balanced improvement in wear resistance without significantly compromising corrosion performance [28].

In their work, Li et al. introduced a bionic surface structure model inspired by the pit-shaped microtextures found on the abdomen of the dung beetle, designed to enhance the wear resistance and extend the operational lifespan of drill pipes. Using finite element analysis (FEA), the stress distribution and wear behaviour of both the bionic pitted structure and a conventional smooth structure were simulated. The results demonstrated that the bionic design significantly optimised the stress distribution, leading to an 81.3% increase in service life compared with the ordinary structure. To experimentally validate the simulation, the surface of a 7075 aluminium alloy drill pipe was modified by laser cladding with nickel (Ni) powder. Wear tests were subsequently carried out to examine the wear mechanisms and surface damage characteristics of the cladded layer. Microstructural, compositional, and microhardness analyses revealed that the cladding layer predominantly consisted of the Al₃Ni₂ intermetallic phase, which imparted high surface hardness. A distinct transition zone was observed between the cladding layer and the substrate, consisting primarily of softer aluminium, thereby forming a gradient interface that improved the component’s ability to withstand cyclic mechanical loading. Moreover, the bionic pit structure demonstrated an inherent capacity to trap and store wear debris, thereby mitigating abrasive interactions and reducing secondary wear. This self-adaptive feature contributed to an additional 70% improvement in service life, underscoring the synergistic effects of bionic surface engineering and laser cladding technology in enhancing the durability and performance of drill pipes under demanding operational conditions [29].

In another article, Stojanović et al. compared the properties of industrial topcoats dried under atmospheric conditions and using infrared (IR) radiation. Five different solvent-borne topcoats were tested, four based on polyurethane (PUR) and one based on polysiloxane (PSX). The results show that IR-dried topcoats generally have better properties compared to atmospherically dried ones. The drying time was significantly reduced from 5–8 h to just 10 min when using IR radiation. The adhesion of the topcoat to the metal substrate was also improved with IR drying. Mechanical properties like hardness and impact resistance were similar or slightly better for IR-dried topcoats. Electrochemical measurements showed higher initial corrosion resistance for IR-dried coatings, though some topcoats showed better long-term stability with atmospheric drying. Thermal stability was generally improved for IR-dried topcoats. The glass transition temperature (Tg) increased for PUR-based topcoats with IR drying, while it decreased for the PSX-based topcoat, indicating the sensitivity of PSX to IR radiation. FTIR analysis confirmed higher crosslinking for IR-dried topcoats. Overall, the results demonstrate the benefits of using IR radiation for curing industrial topcoats, though the specific polymer composition and presence of certain additives can affect the outcomes [30].

The article, by Samardžija et al., investigated the use of aluminium (Al), nickel (Ni), and silver (Ag) nanoparticles incorporated into epoxy coatings to improve their anticorrosion, migration, and antibacterial properties. The nanoparticles were characterised using SEM, EDS, and XRF analyses. Electrochemical impedance spectroscopy (EIS) was used to evaluate the anticorrosion performance of the nanocomposite coatings in a 3.5 wt.% NaCl solution. The migration of the nanoparticles from the epoxy coating into simulated wastewater was also studied. The antibacterial activity of the nanoparticles and nanocomposite coatings was tested against Gram-positive (B. subtilis) and Gram-negative (P. aeruginosa) bacteria. The results showed that the nanocomposite with 1% Ag nanoparticles exhibited the best corrosion resistance, followed by the 1% Ni and 1% Al nanocomposites. The Al and Ag nanoparticles demonstrated effective antibacterial activity, while the Ni nanoparticles showed limited antibacterial effects. The Al nanoparticles showed the highest migration from the epoxy coating into the wastewater, indicating their potential for antimicrobial activity. Overall, the 1% Al NP epoxy nanocomposite showed good anticorrosion and antibacterial properties, making it a promising candidate for applications in pipelines [31].

In their article, Samardžija et al. investigated the characterisation of epoxy resin and epoxy paint nanocomposites containing 1 wt.% aluminium nanoparticles (Al NPs) using Scanning Electrochemical Microscopy (SECM). Cross-sectional SECM spectra, along with SEM images and EDS elemental maps, were employed to analyse the distribution of aluminium and chlorine in the 1% Al NP epoxy resin coating and aluminium and oxygen in the 1% Al NP epoxy paint nanocomposite. The SECM analysis revealed notable variations in impedance among the samples: the epoxy resin exhibited a relatively homogeneous electrochemical response, whereas the epoxy paint displayed localised impedance peaks, attributed to the presence of additional components within the paint matrix. The analyses confirmed that the Al NPs were uniformly dispersed in both the epoxy resin and epoxy paint, without agglomeration or air bubbles, indicating excellent compatibility within the polymer matrices. Cross-sectional SECM spectra and EDS maps revealed the distribution of aluminium and chlorine in the epoxy resin coating and aluminium and oxygen in the epoxy paint coating. Electrochemical testing confirmed that the incorporation of Al NPs significantly enhanced the impedance and corrosion resistance of the coatings. This improvement is attributed to a cathodic passivation mechanism, in which oxides formed on the Al NP surfaces over time, effectively sealing pores and providing increased protection against aggressive environments. Among the tested formulations, nanocomposites containing 1.0 wt.% Al NPs exhibited the best combination of mechanical and anticorrosive performance. These findings indicate that the incorporation of Al nanoparticles enhances the corrosion resistance of the coatings, as reflected by the reduced coating capacitance (CPEcoat) values and improved electrochemical stability [32].

In their work, Yan et al. investigated the localised corrosion behaviour of Q125 casing steel in residual acid solutions during oil reservoir acidising, focusing on the effectiveness of Mannich base (Mb) inhibitors. It was found that higher inhibition efficiency occurs in fresh acid compared to residual acid, where localised corrosion and pitting are prevalent due to decreased inhibitor coverage. Analysis revealed that corrosion products like FeCO₃ hindered inhibitor adsorption, promoting corrosion, while electrochemical tests demonstrated that the Mb inhibitor significantly improved corrosion resistance in both fresh and residual acid solutions. The findings underscore the necessity of effective corrosion protection for Q125 casing steel in acidic environments to enhance oil recovery processes [33].

The reviewed literature by Yang [34] provided a comprehensive examination of the development, mechanisms, and performance of plasma electrolytic oxidation (PEO) coatings for enhancing the corrosion resistance of magnesium (Mg) alloys. PEO, also known as micro-arc oxidation, is identified as an efficient and environmentally benign surface treatment capable of producing dense, ceramic-like oxide layers that markedly improve protection in corrosive environments. These coatings typically exhibit a two-layered structure consisting of a porous outer region and a dense inner barrier layer, the latter serving as the primary defence against corrosive attack. Corrosion of PEO-coated Mg alloys generally initiates at inherent structural defects such as micropores, cracks, and discharge channels. These defects facilitate the ingress of chloride ions, which dissolve MgO and lead to the formation of Mg(OH)₂ and subsequently MgCl₂, driving localised corrosion and eventual failure of the coating. Although the chemical composition of PEO coatings varies with electrolyte type, the fundamental corrosion mechanisms are similar. Differences between silicate- and phosphate-based coatings arise mainly from variations in phase composition and discharge behaviour, which influence corrosion pathways and protective performance. Overall, the review highlights two major avenues for improving PEO coating performance: structural refinement to reduce defect density and phase composition engineering to form more stable, corrosion-resistant phases. Advances in these areas continue to push PEO technology toward more robust, scalable, and industrially viable protective coatings for Mg alloys.