An Inhibitor-Loaded LDH- and MOF-Based Bilayer Hybrid System for Active Corrosion Protection of Aluminum Alloys

A novel inhibitor-loaded bilayer hybrid system based on the LDH inner layer and MOF outer layer is designed on an aluminum alloy 2A12 surface to improve corrosion performance. The hybrid film system covers the inherent cavities and intercrystalline defects of the LDH film using the affinity between the LDH and the MOF compounds. The results demonstrate that the LDH-inhI precursor film is entirely covered by new Zn-based MOF microrods. The LDH-inhI precursor film is partially dissolved and recrystallized in favor of MOF crystal growth to strengthen the binding adhesion between LDH and MOF films. The LDH-inhI/MOF-inhII bilayer film shows significantly enhanced corrosion resistance through the synergistic action of LDH and MOF nanocontainers doped with different corrosion inhibitors (vanadates, 2,5-furandicarboxylic acid, and benzotriazoles). Due to the multiple loadings of the MOF film and the sustained-release of the LDH film, this method provides an effective approach to developing new anticorrosion systems and enhancing both the barrier ability and active corrosion protection performance of LDH-based conversion treatments.

A Novel Coating System Based on Layered Double Hydroxide/HQS Hierarchical Structure for Reliable Protection of Mg Alloy: Electrochemical and Computational Perspectives

Growing research activity on layered double hydroxide (LDH)-based materials for novel applications has been increasing; however, promoting LDH layer growth and examining its morphologies without resorting to extreme pressure conditions remains a challenge. In the present study, we enhance LDH growth and morphology examination without extreme pressure conditions. By synthesizing Mg-Al LDH directly on plasma electrolytic oxidation (PEO)-treated Mg alloy surfaces and pores at ambient pressure, the direct synthesis was achieved feasibly without autoclave requirements, employing a suitable chelating agent. Additionally, enhancing corrosion resistance involved incorporating electron donor-acceptor compounds into a protective layer, with 8-Hydroxyquinoline-5-sulfonic acid (HQS) that helps in augmenting Mg alloy corrosion resistance through the combination of LDH ion-exchange ability and the organic layer. DFT simulations were used to explain the mutual interactions in the LDH system and provide a theoretical knowledge of the interfacial process at the molecular level.

Synthesis of Silane Functionalized LDH-Modified Nanopowders to Improve Compatibility and Enhance Corrosion Protection for Epoxy Coatings

Novel modified Zn-Al LDH/epoxy coatings are synthesized and applied to steel substrates, providing active corrosion protection and improved barrier properties. This protective coating is made by combining Epon 828 as a polymer matrix with modified layered-double-hydroxy (LDH) nanoparticles acting as corrosion inhibitor containers. To synthesize the coatings, nitrate was intercalated into Zn-Al-LDH layers through an aqueous co-precipitation method to obtain Zn-Al LDH-NO3, and decavanadate replaced nitrate within the LDH layers through an anion exchange process to obtain Zn-Al LDH-(V10O28)6-. The intercalated LDH was functionalized by silanization with (3-aminopropyl)triethoxysilane (APTES) to increase the compatibility of the LDH inhibitor nanocontainers with epoxy resin and produce a protective coating. To protect the mild steel substrate, functionalized LDH nanopowders were dispersed into the epoxy resin, mixed with a polyamide hardener (Epikure 3571), and applied and cured to the metal surface. Surface morphology, structure, and chemical composition were determined for the modified LDH nanopowders using scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Corrosion protection of the coating system was studied using long-term immersion testing and potentiodynamic polarization studies in a 3.5 wt.% NaCl solution.

Modification of Mg/Al-LDH by vanadate: effects on tribological properties and corrosion resistance

In this study, Mg/Al layered double hydroxide (LDH) composite coatings were prepared on the surface of anodized 1060 aluminum alloy by an in situ growth method, and then the vanadate anions were embedded in the interlayer corridor of LDH by an ion exchange process. The morphology, structure and composition of the composite coatings were investigated using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry and Fourier transform infrared spectroscopy. Ball-and-disk friction wear experiments were carried out to measure the coefficient of friction, the amount of wear, and the morphology of the worn surface. The corrosion resistance of the coating is studied using dynamic potential polarisation (Tafel) and electrochemical impedance spectroscopy (EIS). The results showed that the LDH composite coating with unique layered nanostructure as a solid lubricating film can effectively improve the friction and wear reduction performance of the metal substrate. Chemical modification treatment by embedding vanadate anions in the LDH coating leads to the change of LDH layer spacing and the increase of interlayer channels, resulting in the best friction and wear reduction and corrosion resistance of the LDH coating. Finally, the mechanism of hydrotalcite coating as a solid lubricating film for friction and wear reduction is proposed.

LDH-Based "Smart" Films for Corrosion Sensing and Protection

In a "smart" corrosion-protective coating system, both the active anti-corrosion and the early corrosion detection of underlying metals are highly required. It is practical significant to develop materials that possess self-detecting of the early local corrosion and self-healing of coating defects simultaneously. The organic compound 8-hydroxyquinoline (8HQ) is an effective inhibitor and a fluorescent sensor probe for corrosion of aluminum alloy. Therefore, a layer double hydroxide (LDH) nanocontainer film loaded with the 8HQ was developed for the active corrosion protection purpose of aluminum alloy AA2024. In corrosive environments, the 8HQ are released from LDH film to inhibit the corrosion process, leading to the loss of the complexation with Al³⁺ ions in LDH laminates, thus turning off fluorescence. Results show that the LDH film loaded with 8HQ composites can improve the anti-corrosion performance of the film by releasing corrosion inhibitors on demand. Simultaneously, due to the complexation of 8HQ and Al³⁺ ions, the LDH film is fluorescent at the initial stage under ultraviolet light, and then becomes non-fluorescent at the corrosion sites, indicating the corrosion evolution process of the coating. The 8HQ-loaded LDH film with self-healing and self-detecting dual functions provides promising opportunities for the effective corrosion protection of aluminum alloy due to its "smart" and multifunctional properties.

Layered double hydroxides for corrosion-related applications—Main developments from 20 years of research at CICECO

This work describes the main advances carried out in the field of corrosion protection using layered double hydroxides (LDH), both as additive/pigment-based systems in organic coatings and as conversion films/pre-treatments. In the context of the research topic “Celebrating 20 years of CICECO”, the main works reported herein are based on SECOP’s group (CICECO) main advances over the years. More specifically, this review describes structure and properties of LDH, delving into the corrosion field with description of pioneering works, use of LDH as additives to organic coatings, conversion layers, application in reinforced concrete and corrosion detection, and environmental impact of these materials. Moreover, the use of computational tools for the design of LDH materials and understanding of ion-exchange reactions is also presented. The review ends with a critical analysis of the field and future perspectives on the use of LDH for corrosion protection. From the work carried out LDH seem very tenable, versatile, and advantageous for corrosion protection applications, although several obstacles will have to be overcome before their use become commonplace.

Molecular Dynamics Model to Explore the Initial Stages of Anion Exchange involving Layered Double Hydroxide Particles

A classical molecular dynamics (MD) model of fully unconstrained layered double hydroxide (LDH) particles in aqueous NaCl solution was developed to explore the initial stages of the anion exchange process, a key feature of LDHs for their application in different fields. In particular, this study focuses on the active corrosion protection mechanism, where LDHs are able to entrap aggressive species from the solution while releasing fewer corrosive species or even corrosion inhibitors. With this purpose in mind, it was explored the release kinetics of the delivery of nitrate and 2-mercaptobenzothiazole (MBT, a typical corrosion inhibitor) from layered double hydroxide particles triggered by the presence of aggressive chloride anions in solution. It was shown that the delamination of the cationic layers occurs during the anion exchange process, which is especially evident in the case of MBT^-.

Environmentally Friendly Layered Double Hydroxide Conversion Layers: Formation Kinetics on Zn-Al-Mg-Coated Steel

Phosphate- or chromate-based industrially produced conversion layers, while effectively increasing adhesion for organic coatings and corrosion resistance, come at the cost of environmentally problematic and harmful treatment solutions and waste. In this respect, layered double hydroxide (LDH)-based conversion layers offer an environmentally benign alternative without toxicologically concerning compounds in the treatment solution. Here, we study an LDH conversion layer on Zn-Al-Mg-coated steel (ZM-coated steel), which was produced by immersion into a carbonate- and magnesium-containing alkaline solution. The mechanism and kinetics of the conversion layer formation were investigated with in situ open circuit potential measurements, cyclic voltammetry (CV), and scanning electron microscopy (SEM). Acceleration of the LDH layer formation through high convection in the treatment solution was found. This was attributed to a higher oxygen availability at the metal/solution interface because no diffusion-limited state during the layer formation is reached due to high convection. The importance of oxygen within the kinetics indicates a corrosion-like mechanism, with cathodic and anodic sites on the steel sample. The LDH formation happens by co-precipitation of ions present in the treatment solution and dissolved ions from the ZM-coated steel. With CV, SEM, and X-ray diffraction, the growth of the LDH conversion layer was investigated with respect to the immersion time. It was found that after 30 s, the sample surface was almost fully covered with an LDH layer, and with the increasing immersion time, the layer grows in thickness. Increased understanding on the kinetics and mechanism of the LDH conversion layer formation on ZM-coated steel gives rise to a targeted optimization of the treatment solution and process parameters.

Mechanism of action and toxicological evaluation of engineered layered double hydroxide nanomaterials in Biomphalaria alexandrina snails

Layered double hydroxide (LDH) nanomaterials have recently become immense research area as it is used widely in industries. So, it's chance of their release into natural environment and risk assessment to nontarget aquatic invertebrate increasing. So, the present study aimed to synthesize and confirm the crystalline formation of Co-Cd-Fe LDHs and Co-Cd-Fe/PbI₂ (LDH) and then to investigate the toxic impact of the two LDH on the adult freshwater snails (Biomphalaia alexandrina). Results showed that Co-Cd-Fe/PbI₂ LDH has more toxic effect to adult Biomphalaria than Co-Cd-Fe LDHs (LC₅₀ was 56.4 and 147.7 mg/L, 72 h of exposure, respectively). The effect of LC₂₅ (117.1 mg/L) of Co-Cd-Fe LDHs exposure on the embryo showed suppression of embryonic development and induced embryo malformation. Also, it showed alterations in the tegmental architectures of the mantle-foot region of B. alexandrina snails as declared in scanning electron micrograph. Also, exposure to this sublethal concentration caused abnormalities in hemocyte shapes and upregulated IL-2 level in soft tissue. In addition, it decreased levels of nonenzymatic reduced glutathione (GSH), catalase (CAT), superoxide dismutase (SOD), caspase-3 activity, and total protein content in significant manner. Glutathione S-transferase (GST) activity was not affected by LDH exposure. It caused histopathological damages in both glands of snails and also caused a genotoxic effect in their cells. The results from the present study indicated that LDH has risk assessment on aquatic B. alexandrina snails and that it can be used as a biological indicator of water pollution with LDH.

Reusability and stability of a novel ternary (Co-Cd-Fe)-LDH/PbI2 photoelectrocatalytst for solar hydrogen production

The design of highly active and cost-effective photoelectrocatalysts for effective hydrogen generation becomes a mandatory issue due to the demands on sustainable solar fuels. Herein a novel ternary Co-Cd-Fe LDH/PbI2 nanocomposite (T-LDH/PbI2NC) was fabricated by combining strategies of doping and in-situ loading of ternary Co-Cd-Fe LDH. The morphological, structural, and optical properties of PbI2, T-LDH, and T-LDH/PbI2 NC were studied by different techniques. LDH narrows the bandgap of the nanocomposite to 2.53 eV which prolongs the lifetime of the photo-induced electrons. Subsequently, the use of T-LDH/PbI2 NC improves the photoelectrocatalytic (PEC) H2 production rate. T-LDH/PbI2 NC shows a catalytic H2 production rate of 107.53 mmol h-1 cm-2 with IPCE% of 83.8% for 307 nm and 67.3% for 508 nm. The ABPE% reaches its supreme of 4.24% for - 0.58 V and 5.41% for - 0.97 V, these values are the highest values yet for LDH-based photocatalysts. The influences of the operating temperature and monochromatic illumination on the PEC performance were studied. Also, the electrochemical surface area, thermodynamic parameters, and Tafe slopes are calculated to label the hydrogen evolution mechanism. Moreover, the stability and reusability of the T-LDH/PbI2 NC photoelectrode were investigated. This work not only illustrated a simplistic and accessible way to produce a new category of highly efficient photocatalysts compared to the previously reported LDH-based PEC catalysts but also demonstrates a new point of view for improving PEC performance towards industrial water splitting under sunlight irradiation.

Performance Evaluation of Layered Double Hydroxides Containing Benzotriazole and Nitrogen Oxides as Autonomic Protection Particles against Corrosion

Layered double hydroxides (LDH) are lamellar structures with positively charged laminates and charge-compensating interlayer anions. The ion-exchange capacity of LDHs makes them as promising hosts for corrosion inhibitor anions with stimulus-responsive release and self-healing anticorrosion. In the current work, LDHs loaded with two different corrosion inhibitors (nitrogen oxides and benzotriazole) were evaluated for their ion-exchange capacity and autonomic protection against corrosion on carbon steel. Studies on nitrogen oxide-loaded LDH (NOx-LDH) showed that nitrogen oxides were successfully intercalated in LDH structure, which were released in chloride media. Open Circuit Potential (OCP) results showed that NOx-LDH extract shifted OCP to nobler values, indicating the protection of metal. For benzotriazole-loaded LDH (BTZ-LDH), the results indicated the presence of benzotriazole in the structure, but its release was not observed. OCP results showed no significant increase of carbon steel protection, corroborating with the conclusion that benzotriazole ions did not migrate to metal surface. Considering these results, the insertion of NOx-LDH in an automotive primer was proceeded, under three different concentrations (0.2. 1.0, and 3.0%). Electrochemical impedance spectroscopy (EIS) showed that the more effective NOx-LDH concentration on corrosion delay was 0.2%, which better balanced protection level conferred by LDH with a possible loss on effectiveness of coating due to increase in porosity.

Layered double hydroxides intercalated with methyl orange as a controlled-release corrosion inhibitor for iron in chloride media

In this study, the corrosion inhibition properties of nanocontainer-type layered double hydroxide (LDH) are evaluated on iron that is immersed in a 3.5% NaCl aqueous solution. LDH ZnAl-NO3 was synthesized via coprecipitation. The material presents satisfactory crystallinity with a Zn/Al ratio of 2:1. Methyl orange (MO) has been added into the synthesis process by exchange with nitrate ions and/or by adsorption of MO onto LDH surfaces (LDH-MO). Iron was immersed in solutions with various concentrations of LDH and LDH-MO ranged 1–6 gl−1, and the corrosion inhibition properties were investigated using linear sweep votammetry, electrochemical impedance spectroscopy (EIS), and SEM. Based on pitting potential studies, LDH has demonstrated inhibition of the pitting corrosion process, and the optimal concentration was identified as 2 gl−1. The presence of MO in LDH provides excellent anticorrosive properties with a mixed inhibition mechanism. The corrosion potential of LDH-MO presents more noble values and exchange current densities that are one order of magnitude less than those of the bare iron after 72 h of immersion in a 3.5% NaCl aqueous solution. EIS results corroborated that the corrosion resistance increased when 2 gl−1 of LDH-MO was in solution. SEM images support the anticorrosive behaviour of the LDH-MO.

Effect of Microstructure on Layered Double Hydroxides Film Growth on Mg-2Zn-xMn Alloy

The poor corrosion resistance of magnesium (Mg) alloys significantly restricts their wide applications. The preparation of a layered double hydroxides (LDHs) film can provide effective corrosion protection for Mg alloys. Nevertheless, research on the effect of the Mg alloy microstructure on LDHs film growth is paid less attention, which was studied in detail in this work. Herein, a Mg-2Zn-xMn alloy with different Mn contents was produced, and an LDHs film was then synthesized on their surfaces. The addition of Mn causes a different microstructure in the Mg-2Zn-xMn alloy, which is gradually refined with increasing Mn content, further affecting the surface morphology, surface chemistry, and corrosion protection of the LDHs film. When the Mn content is 1 wt.% (x = 1), the LDHs film presents the best corrosion protection, with the lowest corrosion current density. No obvious corrosion product could be observed by the naked eyes on the surface. By contrast, severe corrosion occurs on the Mg-2Zn-0Mn alloy (x = 0). Finally, the LDHs film growth mechanism was proposed.

Designing a non-hazardous nano-carrier based on graphene oxide@Polyaniline-Praseodymium (III) for fabrication of the Active/Passive anti-corrosion coating

In this work, graphene oxide (GO) based nano-platforms were applied as a non-hazardous solid container with high encapsulating capacity and controllable release activity of eco-friendly inhibitor. For the first time, the adsorption and release properties of the praseodymium cations (Pr³⁺) on GO nanosheets functionalized with polyaniline (PANI) were investigated. The Pr³⁺ cations adsorption/desorption capacity of GOPANI nano-sheets was assessed by Inductively Coupled Plasma (ICP), X-ray photoelectron spectroscopy (XPS), Field Emission Scanning Electron Microscopy (FE-SEM), and High-resolution transmission electron microscopy (HR-TEM) techniques. The obtained results proved that the container modified with Pr³⁺ cations at pH of 7, 600 ppm of adsorpt, and 1 mg/cc of adsorbent dosage provided the highest capacity of inhibitors adsorption/release rates. The adsorption capacity of the GO-PANIs reached more than 500 mg/g. Also, the modified carrier desorbed about 70 % of loaded Pr³⁺ cations in the corrosion simulated condition. The self-healing anti-corrosion ability of the constructed containers in an organic-inorganic hybrid coating (OIHC) was shown by electrochemical analyses results. The resistance of coating with the loaded carriers has increased about 1 order of magnitude in comparison with the neat silane. Moreover, the scratched coatings containing the inhibitor loaded GO-PANIs showed extraordinary total resistance of about 25 Kohm. cm².

Use of synergistic mixture of chelating agents for in situ LDH growth on the surface of PEO-treated AZ91

The principal possibility to grow layered double hydroxide (LDH) at ambient pressure on plasma electrolytic oxidation (PEO) treated magnesium alloy AZ91 in the presence of chelating agents is demonstrated for the first time. It avoids hydrothermal autoclave conditions, which strongly limit wide industrial application of such coating systems, and the presence of carbonate ions in the electrolyte, which lead to the formation of "passive" non-functionalizable LDH. A combination of chelating agents (sodium diethylenetriamine-pentaacetate (DTPA) and salicylate) were introduced to the treatment solution. The role of each additive and the influence of treatment bath composition on the LDH formation processes are discussed. A synergistic effect of DTPA and salicylate during LDH formation is discovered and its possible explanation is proposed.

Layered Double Hydroxide Protective Films Developed on Aluminum and Aluminum Alloys: Synthetic Methods and Anti-Corrosion Mechanisms

This work reviews the characteristics of layered double hydroxides (LDHs) in the context of protective thin films to enhance the corrosion resistance properties of aluminum alloys. A discussion is made in detail about the LDH protection mechanism and the effect of synthesis approaches on LDH structural variations and the corresponding anti-corrosion behavior. LDHs anion-exchange behavior to host inorganic/organic anions makes them a potential material to investigate for anti-corrosion film. This unique advantage and the availability of a wide range of metal oxide-based layers, interlayer anions, and self-healing properties make LDH family an attractive choice for the development of compact LDHs based smart coating systems.

Use of ZnAl-Layered Double Hydroxide (LDH) to Extend the Service Life of Reinforced Concrete

This work investigated the use of ZnAl-layered double hydroxide (LDH) intercalated with nitrate or nitrite ions for controlling the corrosion of steel in reinforced concrete. The work started by analyzing the stability of the powder in the 1–14 pH range and the capacity for capturing chloride ions in aqueous solutions of different pH. The effect of the ZnAl-LDH on the corrosion of steel was studied in aqueous 0.05 M NaCl solution and in mortars immersed in 3.5% NaCl. It was found that the LDH powders dissolved partially at pH > 12. The LDH was able to capture chloride ions from the external solution, but the process was pH-dependent and stopped at high pH due to the partial dissolution of LDH and the preferential exchange of OH^– ions. These results seemed to imply that ZnAl-LDH would not work in the alkaline environment inside the concrete. Nonetheless, preliminary results with mortars containing ZnAl-LDH showed lower penetration of chloride ions and higher corrosion resistance of the steel rebars.

Hybrid Sol-gel Coatings for Corrosion Mitigation: A Critical Review

The corrosion process is a major source of metallic material degradation, particularly in aggressive environments, such as marine ones. Corrosion progression affects the service life of a given metallic structure, which may end in structural failure, leakage, product loss and environmental pollution linked to large financial costs. According to NACE, the annual cost of corrosion worldwide was estimated, in 2016, to be around 3%-4% of the world's gross domestic product. Therefore, the use of methodologies for corrosion mitigation are extremely important. The approaches used can be passive or active. A passive approach is preventive and may be achieved by emplacing a barrier layer, such as a coating that hinders the contact of the metallic substrate with the aggressive environment. An active approach is generally employed when the corrosion is set in. That seeks to reduce the corrosion rate when the protective barrier is already damaged and the aggressive species (i.e., corrosive agents) are in contact with the metallic substrate. In this case, this is more a remediation methodology than a preventive action, such as the use of coatings. The sol-gel synthesis process, over the past few decades, gained remarkable importance in diverse areas of application. Sol-gel allows the combination of inorganic and organic materials in a single-phase and has led to the development of organic-inorganic hybrid (OIH) coatings for several applications, including for corrosion mitigation. This manuscript succinctly reviews the fundamentals of sol-gel concepts and the parameters that influence the processing techniques. The state-of-the-art of the OIH sol-gel coatings reported in the last few years for corrosion protection, are also assessed. Lastly, a brief perspective on the limitations, standing challenges and future perspectives of the field are critically discussed.

In situ growth of a CaAl-NO3−-layered double hydroxide film directly on an aluminum alloy for corrosion resistance

In this study, a calcium–aluminum-layered double hydroxide (CaAl-LDH) thin film was grown on an AA6082 aluminum alloy, for the very first time, by using a facile in situ growth method in an effort to investigate the CaAl-LDH structural geometry and corresponding corrosion resistance properties.

Highlights on the Catalytic Properties of Polyoxometalate-Intercalated Layered Double Hydroxides: A Review

Layered double hydroxides (LDH) are an extended class of two-dimensional anionic materials that are known for their unique lamellar structure, versatile composition, and tunable properties. The layered architecture allows the intercalation between the positively charged sheets of a vast variety of anionic species, including oxometalates and polyoxometalates (POM). The hybrid composites that were developed using POM and LDH show great advantages when compared to both parent materials causing the appearance of new functionalities, which may lead to remarkable contributions in many areas of application, especially in catalysis. The current review paper emphases all of the crucial works already existing in literature that are related to the large group of POM-LDH solids and their use as catalysts for fine organic synthesis. The new trends in the development of the POM-LDH catalysts are highlighted based on the overview of 121 scientific articles that were published between 1984 and 2019. The main topics are focused primarily on the synthesis, characterization, and the catalytic applications of different LDH systems hosting polyoxometalates with low, medium, and high nuclearity. The intense exploration of the POM-LDH field has led to the obtaining of countless effective catalysts used in various types of reactions, from condensation, esterification, halodecarboxylation, to oxidation and epoxidation.

Nanocontainer-Based Active Systems: From Self-Healing Coatings to Thermal Energy Storage

We highlight the development of nanocontainer-based active materials started in 2006 at the Max Planck Institute of Colloids and Interfaces under the supervision of Prof. Helmuth Möhwald. The active materials encapsulated in the nanocontainers with controlled shell permeability have been first applied for self-healing coatings with controlled release of the corrosion inhibitor. The nanocontainers have been added to the paint formulation matrix at 5-10 wt % concentration, which resulted in attaining a coating-autonomous self-healing ability. This research idea has attracted the attention of many scientists around the world (>1500 publications during the last 10 years) and has already been transferred to the commercialization level. The current trend in nanocontainer-based active systems is devoted to the multifunctionality of the capsules which can combine self-healing, antibacterial, thermal, and other functionalities into one host matrix. This article summarizes the previous research done in the area of nanocontainer-based active materials together with future perspectives of capsule-based materials with antifouling or thermoregulating activity.

One-step synthesis and growth mechanism of nitrate intercalated ZnAl LDH conversion coatings on zinc

An approach for the synthesis of ZnAl-NO3 LDH conversion coatings on zinc in an aqueous acidic Al(NO3)3/NaNO3 solution is demonstrated for the first time. The growth mechanism has been investigated using time resolved structural, microstructural and analytical methods. A LDH growth model involving both electrochemical and chemical processes is suggested.

Layered Double Hydroxide Clusters as Precursors of Novel Multifunctional Layers: A Bottom-Up Approach

The specific microstructure of aluminum alloys is herein explored to grow spatially-resolved layered double hydroxide (SR-LDH) clusters on their surface. Upon chemical modification of LDHs via intercalation, adsorption and grafting with different functional molecules, novel surface-engineered surfaces were obtained. Crystal structure and phase composition were analyzed by X-ray diffraction (XRD) and surface morphology was observed by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectrometry (GDOES) were used to correlate structural changes upon ion-exchange and interfacial modifications with chemical composition and surface profiles of the SR-LDH films, respectively. The protection conferred by these films against localized corrosion was investigated at microscale using the scanning vibrating electrode technique (SVET). LDH-NO3 phase was obtained by direct growth onto AA2024 surface, as evidenced by (003) and (006) XRD diffraction reflections. After anion exchange of nitrate with 2-mercaptobenzothiazole (MBT) there was a decrease in the SR-LDH thickness inferred from GDOES profiles. The subsequent surface functionalization with HTMS was confirmed by the presence of Si signal in XPS and GDOES analyses, leading to an increase in the water contact angle (c.a 144° ± 3°). SVET measurements of the SR-LDH films revealed exceptional corrosion resistance, whereas the bioluminescent bacteria assay proved the anti-microbial character of the obtained films. Overall the results obtained show an effective corrosion protection of the SR-LDHs when compared to the bare substrate and the potential of these films for biofouling applications as new Cr-free pre-treatments.

Triple-Stimuli-Responsive Smart Nanocontainers Enhanced Self-Healing Anticorrosion Coatings for Protection of Aluminum Alloy

Novel acid/alkali/corrosion potential triple-stimuli-responsive smart nanocontainers (TSR-SNs) were successfully assembled to regulate the release of an encapsulated corrosion inhibitor, benzotriazole (BTA), by installing specially structured bistable pseudorotaxanes as supramolecular nanovalves onto orifices of mesoporous silica nanoparticles. In normal conditions, BTA molecules were sealed in the mesopores. Upon any stimulus of acid, alkali, or corrosion potential, BTA molecules were quickly released because of the open states of the supramolecular nanovalves. TSR-SNs as smart nanocontainers were added into the SiO₂-ZrO₂ sol-gel coating to fabricate a stimuli-feedback, corrosion-compensating self-healing anticorrosion coating (SF-SHAC). Compared with the conventional pH-responsive smart nanocontainers synthesized for the SHAC, TSR-SNs not only respond to the pH changes occurring on corrosive microregions but also, and more importantly, feel the corrosion potential of aluminum alloys and give quick feedback. This design avoids wasting smart nanocontainers because of the local-dependent, gradient pH stimulus intensities and obviously enhances the response sensitivity of the SF-SHAC. Electrochemical impedance spectroscopy and salt spray tests prove the excellent physical barrier of the SF-SHAC. Through scanning vibrating electrode technique measurements, the SF-SHAC doped with TSR-SNs demonstrates inhibiting rates for corrosive microcathodic/anodic current densities that are faster than other control SHACs. The new incorporated corrosion potential-responsive function ensures the efficient working efficiency of TSR-SNs and makes full use of the preloaded corrosion inhibitors as repair factors.

Nanocontainer-based self-healing coatings: current progress and future perspectives

Nanocontainers add more functionalities to the standard coating formulations.

A novel composite system composed of zirconia and LDHs film grown on plasma electrolysis coating: Toward a stable smart coating

A novel composite system composed of zirconia and double hydroxide layers (LDHs) was successfully fabricated on the plasma electrolysis (PE) coating. For this aim, the molybdate-loaded LDHs film grown on the PE film of aluminum alloy was modified additionally by zirconia nanoparticles via a facile dip-coating method. The MoO₄^2- anions which were obtained by anion exchange process from the precursor CeMgAl-LDH film, led to decrease the distance between the flakes of LDHs film where a flower-like structure was successfully developed. Moreover, the inclusion of zirconia helped to decrease the size of pores present in the LDHs films. Accordingly, a superior smart protective film was obtained due to the possible synergetic effects between the MoO₄^2- and Ce³⁺ ions released from LDHs film as well as the high chemical stability of zirconia. The LDHs film modified by zirconia can be regarded as a stable smart coating, meaning that it has the ability to control the release of corrosion inhibitors and providing an excellent long-term electrochemical performance as well.

Chelating agent-assisted in situ LDH growth on the surface of magnesium alloy

In situ formation of layered double hydroxides (LDH) on metallic surfaces has recently been considered a promising approach for protective conversion surface treatments for Al and Mg alloys. In the case of Mg-based substrates, the formation of LDH on the metal surface is normally performed in autoclave at high temperature (between 130 and 170 °C) and elevated pressure conditions. This hampers the industrial application of MgAl LDH to magnesium substrates. In this paper, the growth of MgAl LDH conversion coating directly on magnesium alloy AZ91 at ambient conditions (25 °C) or elevated temperatures is reported in carbonate free electrolyte for the first time. The direct LDH synthesis on Mg alloys is enabled by the presence of organic chelating agents (NTA and EDTA), which control the amount of free and/or hydroxyl bound Mg2+ and Al3+ in the solution. The application of the chelating agents help overcoming the typical technological limitations of direct LDH synthesis on Mg alloys. The selection of chelators and the optimization of the LDH treatment process are supported by the analysis of the thermodynamic chemical equilibria.

Synthesis and Characterization of Hollow Mesoporous Silica Nanoparticles for Smart Corrosion Protection

Different approaches have been considered for the development of smart anticorrosive coatings by the incorporation of nanocontainers loaded with corrosion inhibitors into the protective layer. Nanocontainers are designed to allow a controlled release of the inhibitor in response to an external stimulus, thus, achieving more efficient and more economical use of the active component. In this case, a pH change is a very interesting stimulus to trigger the release because corrosion processes cause local pH changes. To this end, a special focus has been placed on the use of mesoporous silica nanoparticles (MSN) as nanocontainers due to their interesting characteristics, such as larger surface area, versatile functionalisation, stability, etc. However, the use of hollow mesoporous silica nanoparticles (HMSN), with a large central hole combined with an external mesoporous silica shell, offers an additional advantage due to the higher loading capacity. In the present work, HMSN have been efficiently synthesised, loaded with sodium phosphomolybdate, as a non-toxic alternative to the use of chromates, and encapsulated by a layer of an oppositely charged polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA). The morphology and textural properties of the produced nanocapsules have been studied by different techniques (SEM/EDS, TEM/EDS, Brunauer⁻Emmett⁻Teller (BET) analysis method, ζ-potential). Finally, the releasing capacity and corrosion protection at different pH values have been studied, confirming the smart behaviour of the encapsulated loaded HMSN.

Arsenic removal from water/wastewater using layered double hydroxide derived adsorbents, a critical review

Arsenic pollution has become a worldwide environmental concern. Dangerous arsenic concentrations in natural waters threaten the health of millions of people, and this has received significant attention. Among the various technologies that have been developed for arsenic removal from water, the use of adsorption technology is considered to be a prevailing method, because the adsorption approach usually has high removal efficiency and the advantage of convenience of handling. In recent years, layered double hydroxides (LDHs) have become prime candidates for arsenic removal, due to their hydrophilic nature and cationic layered structures. Research on arsenic removal using LDHs is mainly focused on (1) the influence of the synthesis method and composition of the LDH, (2) the influence of the particle size of the LDH, (3) the influence of the Mg/Al ratio in LDHs, (4) LDH-based hybrids and (5) the competition with other anions. This paper provides a review of the currently available literature focusing on arsenic removal using LDHs for the five parts mentioned above. In addition, based on this overview, a closing section will suggest research efforts for future work. It is expected that this review will provide a summary of the main research in this area, and will also shed light on the direction of future development.

Enhanced Corrosion Resistance of Superhydrophobic Layered Double Hydroxide Films with Long-Term Stability on Al Substrate

A superhydrophobic ZnAl-layered double hydroxide (LDH)-La film was prepared by a hydrothermal method and further modification by laurate anions in this work. Comprehensive characterizations of this film were performed in terms of morphology, composition, structure, roughness, and wettability by scanning electronic microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, three-dimensional laser scanning confocal microscopy. The long-term corrosion protection effect of this superhydrophobic film was investigated deeply by monitoring the changes of the electrochemical impedance spectra for a long time of up to a month in 3.5 wt % NaCl solution. In the meantime, the changes of the contact angle were also recorded with the evolution of the immersion time. The result indicated that the stable superhydrophobic ZnAl-LDH-La film was able to provide efficient protection for the underlying Al substrate for a long time. In addition, the capability of the superhydrophobic surface against harsh conditions, including chemical damages and physical damages, was emphatically investigated. It was found that the superhydrophobic surface was chemically stable toward acid (pH ≥ 3), alkali, and heating, and it also exhibited high ultraviolet (UV) radiation resistance. This superhydrophobic coating maintained superhydrophobicity for 7 days of radiation in an UV chamber equipped with a 40 W UV lamp (λ = 254 nm), indicating superior ability of adapting to outdoor environment. This comprehensive investigation of the superhydrophobic ZnAl-LDH-La film is considerably helpful for researchers and engineers to get deep insight into its potential for practical applications in the field of corrosion and protection.

Sealing of anodized magnesium alloy AZ31 with MgAl layered double hydroxides layers

In this work, anodized magnesium alloy AZ31 with and without boiling water sealing was pre-prepared, and then MgAl-layered double hydroxide (LDH) films were fabricated on it through hydrothermal chemical conversion of the pre-prepared anodic layer. The morphology, structure, and composition of the films were characterized by XRD, SEM, EDS, FT-IR, XPS and GDOES. It was found that the porosity of the films was reduced after in situ fabrication of the LDHs. The effects of boiling water sealing treatment on the anodized substrate were also discussed. Moreover, the polarization curve, EIS, and immersion tests showed that LDHs fabricated on the anodized substrate with boiling water sealing treatment exhibited a significant long period of protection for the substrate.

In this work anodized magnesium alloy AZ31 with and without boiling water sealing was pre-prepared, and then MgAl-layered double hydroxide (LDH) films were fabricated on it through hydrothermal chemical conversion of the pre-prepared anodic layer.

Antimicrobial activity of 2-mercaptobenzothiazole released from environmentally friendly nanostructured layered double hydroxides

AIMS

The objective of this work was to assess the antibacterial effect of 2-mercaptobenzothiazole (MBT), used as model-biocide, immobilized in a layered double hydroxide (LDH) structure, under different conditions of pH and salinity, envisaging possible applications of the system in active antifouling and anticorrosion coatings.

METHODS AND RESULTS

Biological effects of MBT immobilized in LDH were assessed by monitoring bacterial bioluminescence of cell suspensions of either Allivibrio fischeri or a recombinant strain of Escherichia coli, as a proxy for bacterial activity. Salinity (1, 2 and 3% NaCl) and pH (4, 5, 6 and 7) of the suspension media were experimentally manipulated and biocide release tests were performed in parallel. The release profiles obtained by UV-visible spectrophotometry indicated a fast release of biocide from MBT@LDH, slightly enhanced in 3% NaCl and under alkaline conditions. However, biological effects were more pronounced at 1% NaCl and at neutral pH.

CONCLUSIONS

The release and toxic effect of MBT immobilized in LDH is dependent on the concentration of solutes in the suspension medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results confirm LDH as a biologically compatible material with potential to be used for biocide delivery.

Inhibition of Steel Corrosion and Alkaline Zinc Oxide Dissolution by Dicarboxylate Bola-Amphiphiles: Self-Assembly Supersedes Host-Guest Conception

For many decorative applications like industrial and architectural paints, prevention of metal substrates from corrosion is a primary function of organic coatings. Triggered release of inhibitor species is generally accepted as a remedy for starting corrosion in case of coatings damage. A polyurethane based coating, doped with bola-amphiphiles of varying molecular weight but with a common head group motif that stems from ring-opened alkenyl succinic anhydride, enables passivation of the defect and mitigates cathodic delamination, if applied on cold rolled steel. An antagonistic effect results from the intercalation of the bola-amphiphiles into layered double hydroxide Zn2Al(OH)6 and subsequent incorporation of the hybrid phase into the organic matrix. In particular higher molecular weight bola-amphiphiles get immobilized through alkaline degradation of the layered framework in the basic milieu at the cathode. By means of sediments from colloidal states it is demonstrated that in-situ formed zinc oxide encapsulates the hybrid phase, evidenced by impeded dissolution of the ZnO based shell into caustic soda. While inhibition of steel corrosion results from a Donnan barrier layer, impeded zinc oxide dissolution is rooted in zinc catalyzed bola-amphiphile hydrolysis and layered deposition of the crystalline spacer diol hydrogenated bisphenol-A.

Two-shell structured PMAA@CeO2 nanocontainers loaded with 2-mercaptobenzothiazole for corrosion protection of damaged epoxy coated AA 2024-T3

In this work, novel two-shell structured inhibitor-loaded poly(methacrylic acid)@cerium oxide (PMAA@CeO₂) nanocontainers were synthesised and characterized. The purpose of the nanocontainers is to increase the corrosion protection provided by an epoxy coating applied to an aerospace alloy (AA 2024-T3). The (PMAA@CeO₂) nanocontainers with diameters of 550 nm were synthesised by a four-step process with the method of distillation precipitation polymerization for the synthesis of the inner PMAA layer, and the sol-gel method for the development of the outer CeO₂ layer. The loaded nanocontainers were characterized by scanning and transmission electron microscopies. The corrosion protection properties of the epoxy coated AA 2024-T3 with 2-mercaptobenzothiazole (2-MBT) loaded PMAA@CeO₂ nanocontainers were evaluated with and without artificial scribes by electrochemical impedance spectroscopy (EIS). The results indicated that the epoxy coating containing the 2-MBT-loaded nanocontainers provided enhanced protection of the AA 2024-T3 substrate.

In Situ Formation of Decavanadate-Intercalated Layered Double Hydroxide Films on AA2024 and their Anti-Corrosive Properties when Combined with Hybrid Sol Gel Films

A layered double hydroxide (LDH) film was formed in situ on aluminum alloy 2024 through a urea hydrolysis method, and a decavanadate-intercalated LDH (LDH-V) film fabricated through the dip coating method. The microstructural and morphological characteristics were investigated by scanning electron microscopy (SEM). The corrosion-resistant performance was analyzed by electrochemical impedance spectroscopy (EIS), scanning electrochemical microscopy (SECM), and a salt-spray test (SST).The SEM results showed that a complete and defect-free surface was formed on the LDH-VS film. The anticorrosion results revealed that the LDH-VS film had better corrosion-resistant properties than the LDH-S film, especially long-term corrosion resistance. The mechanism of corrosion protection was proposed to consist of the self-healing effect of the decavanadate intercalation and the shielding effect of the sol-gel film.

Hierarchically organized Li–Al-LDH nano-flakes: a low-temperature approach to seal porous anodic oxide on aluminum alloys

Li-LDH sealing is accounted for being highly competitive to standard hot-water sealing as referred to reduced treatment temperature and higher corrosion protection efficiency.

Sealing of tartaric sulfuric (TSA) anodized AA2024 with nanostructured LDH layers

Zn–Al LDH-NO₃ was grown on TSA anodized surface of AA2024 aluminum alloy LDH-VOx was obtained from LDH-NO₃via anionic exchange reaction The LDH-VOx layer confers remarkable active corrosion protection.

Mesoporous silica based reservoir for the active protection of mild steel in an aggressive chloride ion environment

Spherical mesoporous silica (m-SiO₂) with well-ordered pores was synthesized by a modified Stöber method using CTAB micelles.

Polyelectrolyte-modified layered double hydroxide nanocontainers as vehicles for combined inhibitors

The dual release of Ce³⁺ and 2-mercaptobenzothiazole from layered double hydroxides modified by the layer-by-layer leads to an improvement of corrosion protection when compared to unmodified layered double hydroxides.