pH-Sensitive Polymer Conjugates for Anticorrosion and Corrosion Sensing

Environmentally Friendly Photoluminescent Coatings for Corrosion Sensing

Although an increasing number of studies are being devoted to the field of corrosion, with topics from protection to sensing strategies, there is still a lack of research based on environmentally eco-friendly materials, which is essential in the transition to sustainable technologies. Herein, environmentally friendly composites, based on photoluminescent salts dispersed in vegetable oil-based resins, are prepared and investigated as corrosion sensing coatings. Two salts NaA, where A- is a lanthanide complex anion (with Ln = Nd3+, and Yb3+), are incorporated into the resins as active functional fillers and different coatings are prepared on carbon steel substrates to assess their functional properties. The influence exerted by a corrosive saline solution on the morphology, structural, and photoluminescent properties of the coatings is evaluated, and their suitability for the practical detection of the early corrosion of coated carbon steel is demonstrated. The photoluminescence of the composite coatings depends on the corrosion time, with the effect being more important for the coatings doped with Nd3+. The present work shows that the composites obtained are suitable candidates for corrosion sensing coating applications, offering improved sustainability.

Controlled Release of Hydrophilic Drug from Hollow Nanodots

Here the challenge of limited encapsulation efficiency of ionizable hydrophilic molecules in silica materials is addressed. Two effective strategies are showcased that allow high encapsulation efficiency of salicylic acid, while simultaneously maintaining the morphology and particle size of silica nanocapsules. These promising approaches involve the formation and encapsulation of a prodrug or the complexation of the hydrophilic payload with a hydrophobic moiety to form a complex that is dissociated in acidic conditions. Well-defined core-shell silica nanocapsules with a diameter of 6 nm are obtained and exhibited an encapsulation efficiency of over 90%. High amounts of salicylic acid are released in acidic conditions from silica nanocapsules entrapping the prodrug or the complex, leading to pH-responsive characteristics. This work demonstrates promising strategies for the encapsulation and the controlled release of hydrophilic fertilizers, pesticides or drugs.

Polyaniline Microspheres with Corrosion Inhibition, Corrosion Sensing, and Photothermal Self-Healing Properties toward Intelligent Coating

A smart coating integrating functions of corrosion inhibition, self-healing, and corrosion-sensing was developed based on a polyaniline (PANI) microsphere-loading corrosion sensing probe (8-hydroxyquinone, 8-HQ). The PANI microsphere was prepared in a facile one-pot process via the combination of photopolymerization and an emulsion template. The 8-HQ-loaded PANI microsphere achieved three synergetic effects simultaneously: corrosion inhibition, corrosion sensing, and photothermal self-healing abilities. Benefiting from the corrosion inhibition effect of PANI, the coating with the PANI microsphere exhibited significantly enhanced anticorrosion behavior. After soaking in NaCl solution for 35 days, its impedance was maintained at 1.26 × 109 Ω·cm2, nearly 3 orders of magnitude higher than that of pure resin coating. Meanwhile, the encapsulated 8-HQ exhibited pH-responsive release behavior thanks to the pH-responsive characteristics of PANI, which could chelate with Al3+ ions to form 8-HQ-Al3+ coordinates with a conspicuous fluorescence, achieving a real-time corrosion diagnosing function. Moreover, benefiting from the photothermal property of PANI, the coating with the PANI microsphere displayed rapid crack closure behavior under NIR light irradiation, and the healing efficiency could reach 83.56% under near-infrared irradiation. This work presents an innovative strategy for fabricating an intelligent self-healing, self-reporting, and anticorrosion coating, which provides a new vision to prolong the lifetime of metals.

Corrosion-Responsive Self-Healing Coatings

Organic coatings are one of the most popular and powerful strategies for protecting metals against corrosion. They can be applied in different ways, such as by dipping, spraying, electrophoresis, casting, painting, or flow coating. They offer great flexibility of material designs and cost effectiveness. Moreover, self-healing has evolved as a new research topic for protective organic coatings in the last two decades. Responsive materials play a crucial role in this new research field. However, for targeting the development of high-performance self-healing coatings for corrosion protection, it is not sufficient just to focus on smart responsive materials and suitable active agents for self-healing. A better understanding of how coatings can react on different stimuli induced by corrosion, how these stimuli can spread in the coating, and how the released agents can reach the corroding defect is also of high importance. Such knowledge would allow the design of coatings that are optimized for specific applications. Herein, the requirements and possibilities from the corrosion and synthesis perspectives for designing materials for preparing self-healing coatings for corrosion protection are discussed.

Biomimetic redox-responsive smart coatings with resistance-release functions for reverse osmosis membranes

Membrane fouling induces catastrophic loss of separation performance and seriously restricts the applications of reverse osmosis (RO) membranes. Inspired by the mussel structure, polydopamine (PDA) and cystamine molecules (CA) with excellent anti-fouling properties were used to prepare accessible, biocompatible, and redox-responsive coatings for RO membranes. The PDA/CA-coated RO membranes exhibit a superior water flux of 65 L m-2 h-1 with a favourable NaCl rejection exceeding 99%. The water permeability through the PDA/CA-coated membrane is much higher than that of most membranes with similar rejection rates. Due to the formed protective hydration layers by PDA/CA coatings, anti-fouling properties against proteins, polysaccharides and surfactants were evaluated separately, and ultralow fouling properties were demonstrated. Moreover, the disulfide linkages in CA molecules can cleave in a reducing environment, yielding the degradation of PDA/CA coatings, thereby removing the foulants deposited on the coatings. The degradation endows the coated membranes with satisfying longtime anti-fouling properties, where the flux recovery reaches up to 90%. The construction of redox-responsive smart coatings not only provided a promising route to alleviate membrane fouling but can also be upscaled for use in numerous practical applications like sensors, medical devices, and drug delivery.

Fast-release kinetics of a pH-responsive polymer detected by dynamic contact angles

Polymers conjugated with active agents have applications in biomedicine, anticorrosion, and smart agriculture. When the active agent is used as a drug, corrosion inhibitor, or pesticide, it can be released upon a specific stimulus. The efficiency and the sustainability of active agents are determined by the released kinetics. In this work, we study the fast-release kinetics of 8-hydroxyquinoline (8HQ) from a pH-responsive, random copolymer of methyl methacrylate and 8-quinolinyl-sulfide-ethyl acrylate [P(MMA-co-HQSEA)] by hydrolysis of the β-thiopropionate groups. We used contact angle measurements of sliding drops as an elegant way to characterize the release kinetics. Based on the results gained from ¹H nuclear magnetic resonance measurement, fluorescent intensity measurement, and velocity-dependent contact angle measurement, we found that both the hydrolysis rate and polymer conformation affect the release kinetics of 8HQ from a P(MMA-co-HQSEA) film. Polymer chains collapse and further suppress the release from the inner layer in acidic conditions, while polymer chains in a stretched condition further facilitate the release from the inner layer. As a result, the cumulative release rate of 8HQ is higher in the basic condition than in the acidic condition.

A pH-Sensitive Lignin-Based Material for Sustained Release of 8-Hydroxyquinoline

The fabrication of pH-sensitive lignin-based materials has received considerable attention in various fields, such as biomass refining, pharmaceuticals, and detecting techniques. However, the pH-sensitive mechanism of these materials is usually depending on the hydroxyl or carboxyl content in the lignin structure, which hinders the further development of these smart materials. Here, a pH-sensitive lignin-based polymer with a novel pH-sensitive mechanism was constructed by establishing ester bonds between lignin and the active molecular 8-hydroxyquinoline (8HQ). The structure of the produced pH-sensitive lignin-based polymer was comprehensively characterized. The substituted degree of 8HQ was tested up to 46.6% sensitivity, and the sustained release performance of 8HQ was confirmed by the dialysis method, the sensitivity of which was found to be 60 times slower compared with the physical mixed sample. Moreover, the obtained pH-sensitive lignin-based polymer showed an excellent pH sensitivity, and the released amount of 8HQ under an alkaline condition (pH = 8) was obviously higher than that under an acidic condition (pH = 3 and 5). This work provides a new paradigm for the high-value utilization of lignin and a theory guidance for the fabrication of novel pH-sensitive lignin-based polymers.

Double Stimuli-Responsive Conducting Polypyrrole Nanocapsules for Corrosion-Resistant Epoxy Coatings

Stimuli-responsive nanocapsules, which can respond to various environmental stimuli and release their encapsulated payload on demand, have attracted wide interest in different fields in recent years. In this work, a novel kind of polypyrrole (PPy) nanocapsules is fabricated and loaded with zinc salt corrosion inhibitors. The synthesized PPy nanocapsules respond to two different external stimuli (pH- and redox-responsive) and can control the release of their encapsulated corrosion inhibitors. The nanocapsules can detect the micro-environmental pH or surface-potential changes associated with the corrosion initiation of the metal substrate. When introduced into a protective epoxy coating, the fabricated PPy nanocapsules inhibit the anodic and cathodic corrosion reactions. The superior corrosion resistance and active corrosion protection effects of the epoxy-PPy-Zn coatings are further demonstrated via electrochemical and long-term immersion tests. The low-frequency impedance, coating resistance, and oxide film resistance increase after about 400 h of exposure in a 3.5 wt % NaCl solution, reflecting the enhanced corrosion protection properties and excellent repairing performance of the coating. Furthermore, the epoxy-PPy-Zn coating can avoid the pitting corrosion of 304 stainless steel. Overall, we have fabricated double stimuli-responsive PPy nanocapsules via a simple and effective strategy and incorporated them into a corrosion-resistant epoxy coating for protecting Fe-based metal substrates.

Development of Sustainable Inhibitors for Corrosion Control

Metal degradation due to corrosion is a major challenge in most industries, and its control and prevention has to maintain a balance between efficiency and cost-effectiveness. The rising concern over environmental damage has greatly influenced this domain, as corrosion prevention should comply with the waste regulations of different regions. In this respect, a fundamental question is which modern synthetic materials are more viable from the point of view of their effectiveness. Therefore, this paper is aims to provide an advanced and holistic review of corrosion prevention and control methods. Corrosion prevention techniques have become extensive; however, the literature indicates that polymer coatings, nano-composite coatings, and encapsulation techniques consistently provide the most efficient and feasible outcomes. Therefore, this review article examined the phenomenon of corrosion inhibition mainly from the perspective of these three techniques. Moreover, this research utilized secondary qualitative methods to obtain data and information on comparative techniques. It is found that due to the rapid development of novel materials, corrosion inhibition techniques need to be developed on scales that are more general, so that they could be applied to varying environments. The self-healing coatings are generally based on epoxy-resins incorporated with synthetic compounds such as inhibitor ions, amino-acids, or carboxylic acids. These coatings have become more widespread, especially due to bans on several traditional prevention materials such as compounds of chromium (VI). However, self-healing coatings are comparatively more costly than other techniques because of their method of synthesis and long-term durability. Therefore, although self-healing nanomaterial-based coatings are viable options for limited usage, their utilization in large and complex facilities is limited due to the costs involved. Amino acids and other biological macro-molecules provide another option to attain environmental sustainability and long durability, especially due to their origins being most of naturally occurring compounds such as lignin, cellulose, and proteins.

Difunctional Silicon Dioxide Combined with Graphene Oxide Nanocomposite to Enhance the Anticorrosion Performance of Epoxy Coatings

The nanocomposite BTA-SiO₂-GO was fabricated for the purpose of metal corrosion protection. Herein, the BTA-loaded mesoporous silica nanocontainers were prepared through a facile one-step synthetic method. Subsequently, graphene oxide (GO) was combined with the resultant BTA-SiO₂ compound because GO had a superior barrier property and impermeability. We must note that the double functional groups exist on SiO₂. Benzotriazole (BTA), as an inhibitor, can be loaded into the nanocontainer and GO can also be modified by it, resulting in excellent dispersion in epoxy coatings, which were conducive to enhancing its anticorrosion performance. In this way, the nanocomposite endows the coating system with both self-healing and physical barrier abilities. The EIS results indicated that the impedance value of the BTA-SiO₂-GO composite coatings was up to 1.2 × 10⁹ Ω cm², which indicated excellent corrosion resistant properties.

Novel ZrO2-glycine nanocomposite as eco-friendly high temperature corrosion inhibitor for mild steel in hydrochloric acid solution

We report the green synthesis of novel ZrO₂-Glycine nanocomposite referred to as ZrO₂-Gly NC followed by its characterization using X-ray diffraction (XRD), Fourier transforms infrared (FT-IR) spectroscopy, SEM/EDX, and transmission electron microscopy (TEM) techniques. Further, the inhibition effect of the varying concentration of ZrO₂-Gly NC on the corrosion of mild steel (MS) in 1 M HCl was investigated by weight loss and electrochemical measurements at 40-80 °C. The percentage inhibition efficacy of NC increased with the increase of concentration and temperature and reached about 81.01% at 500 ppm at 70 °C which decreased at 80 °C and exhibited 73.5% inhibition efficiencies. According to the polarization measurements, the investigated ZrO₂-Gly NC works as a mixed-type inhibitor with predominantly inhibiting cathodic reaction. Also, the adsorption isotherm analysis indicated that the adsorption was spontaneous and followed the Langmuir adsorption isotherm. Furthermore, the contact angle measurement revealed the water-repelling property of the investigated inhibitor. The surface morphological study via SEM-EDS micrograph affirmed the appearance of a smooth surface in presence of inhibited media suggesting the formation of protective film by the adsorption of ZrO₂-Gly NC on the surface of the MS even at higher temperature.

Fabrication of dual anti-corrosive polyaniline microcapsules via Pickering emulsion for active corrosion protection of steel

A novel kind of inhibitor-loaded polyaniline (PANI) microcapsule was prepared by Pickering emulsion photopolymerization using polyaniline particles as the Pickering emulsifier. In our strategy, water-dispersible polyaniline nanoparticles were firstly synthesized using a micelle template method and used to stabilize oil-in-water emulsions, in which the oil phase contained photo-crosslinkable and pH sensitive monomers and a photo-initiator. Under UV light, the pH-responsive monomers underwent photo-polymerization and crosslinking and converted to microcapsule shells. During this process, polyaniline nanoparticles were trapped in the microcapsule shells, leading to the formation of PANI microcapsules. The structure and morphology of the synthesized PANI microcapsules were analyzed using FTIR spectroscopy, SEM, and EDX mapping. The inhibitor (mercaptobenzothiazole, MBT) was subsequently incorporated into the PANI microcapsule as a functional core and demonstrated pH-sensitive releasing behavior. With the anti-corrosive PANI as the microcapsule wall and the inhibitor MBT as the core, the as-prepared MBT loaded PANI (MBT@PANI) microcapsule could afford dual corrosion protection, allowing smart protection of metals when exposed to corrosive conditions. The MBT@PANI microcapsules were embedded in UV-cured coating for protecting steel. The corrosion protection performance of the coating with MBT@PANI microcapsules was evaluated using the electrochemical impedance spectroscopy technique and salt spray test, which demonstrated the synergistic inhibition effect of the PANI wall and the loaded MBT in improving anti-corrosion performance of the coating.

Design of Nanostructured Protective Coatings with a Sensing Function

Nanostructured multilayered coatings for metals are prepared to simultaneously provide a function of corrosion mitigation and of corrosion sensing for copper substrates. Silica nanocapsules, embedded in one layer of the coating, are used as a host for a corrosion inhibitor and as a sensor, which detect changes of pH value and release inhibitors via an optical signal. Furthermore, another layer in the coating exists in a nanonetwork loaded with another corrosion inhibitor, which is impregnated with a hydrophobic polymer. We demonstrate that a specific arrangement of layers leads to an optimum anticorrosion and sensing performance while the sensing signal can be prolonged for a long time. It is the first time that the fluorophore detecting corrosion is conjugated to the nanosensor and that nanofibers and nanocapsules are used simultaneously to load and release corrosion inhibitors for anticorrosion applications.

Intelligent Polymers, Fibers and Applications

Intelligent materials, also known as smart materials, are capable of reacting to various external stimuli or environmental changes by rearranging their structure at a molecular level and adapting functionality accordingly. The initial concept of the intelligence of a material originated from the natural biological system, following the sensing–reacting–learning mechanism. The dynamic and adaptive nature, along with the immediate responsiveness, of the polymer- and fiber-based smart materials have increased their global demand in both academia and industry. In this manuscript, the most recent progress in smart materials with various features is reviewed with a focus on their applications in diverse fields. Moreover, their performance and working mechanisms, based on different physical, chemical and biological stimuli, such as temperature, electric and magnetic field, deformation, pH and enzymes, are summarized. Finally, the study is concluded by highlighting the existing challenges and future opportunities in the field of intelligent materials.

Stimulus Responsive Zeolitic Imidazolate Framework to Achieve Corrosion Sensing and Active Protecting in Polymeric Coatings

Metal substrates beneath polymeric coatings are susceptible to localized corrosion, which could result in lifetime reduction and catastrophic failure without timely repair treatment. In situ detection of corrosion and repair coating defects are in high demand yet challenging to fulfill so far. Herein, we report a smart polymeric coating by integrating nanosensors into the coating matrix, which is capable of efficient corrosion sensing and active anticorrosion protecting. The nanosensors were constructed by zeolitic imidazolate framework encapsulated with the polyethylene glycol-tannic acid complex. The morphology, chemical constitution, and stimulus responsiveness of nanosensors were systematically analyzed. The generation of local corrosion beneath coating can be promptly sensed and reported by a conspicuous purple color derived from tannic-iron ion coordinates. Meanwhile, local electrochemical impedance spectroscopy results proved that the metal degradation process at the defected interface can be largely inhibited, exhibiting active anticorrosion property. Furthermore, the constructed smart coating possessed superior impermeability and long-term protective performance under simulated seawater and harsh salts spray conditions. This feasible and effective strategy based on simple nanosensors to engineer smart coatings paves a new way to develop high environmental adaptability protective materials with protecting, corrosion sensing, and self-healing functions.

Fighting corrosion with stimuli-responsive polymer conjugates

Corrosion is a financial and enviromental plague which leads to the deterioration of our infrastructures. Using corrosion inhibitors at low concentrations in coatings is one effective method for preventing corrosion. Inspired by the development of polymer-drug conjugates, corrosion inhibitors are incorporated in various polymer structures to create novel materials for hindering corrosion. We discuss the strategies to covalently integrate corrosion inhibitors in polymer structures to form polymer-inhibitor conjugates. Inhibitors are conjugated to polymers via non-labile or stimuli-labile linkages to allow the release of the inhibitors upon onset of corrosion. The application and anticorrosion performance of representative polymers are also discussed.

Acid-cleavable polymers for simultaneous fast and slow release of functional molecules

Hydrophobic copolymers are designed to respond to acid stimuli for both simultaneous rapid and sustained release of multiple cargos.

Controlling release kinetics of pH-responsive polymer nanoparticles

Polymers with pH-responsive properties display anticorrosion performance.

Self-reporting of payload release in polymer coatings based on the inner filter effect

New polymeric nanoparticle sensors are developed for monitoring the release of non-fluorescent payloads in coatings by the naked eye.

Magnetically Navigated Core-Shell Polymer Capsules as Nanoreactors Loadable at the Oil/Water Interface

Polymer core-shell nanocapsules with magnetic nanoparticles embedded in their oil cores were fabricated and applied as nano(photo)reactors. Superparamagnetic iron oxide nanoparticles (SPIONs) coated with oleic acid were first synthesized and characterized structurally, and their magnetic properties were determined. The capsules with chitosan-based shells were then formed in a one-step process by sonication-assisted mixing of (1) an aqueous solution of the hydrophobically derived chitosan and (2) oleic acid containing the dispersed SPIONs. In this way, magnetic capsules with a diameter of approximately 500-600 nm containing encapsulated SPIONs with an average diameter of approximately 20-30 nm were formed as revealed by dynamic light scattering and scanning transmission electron microscopy measurements. The composition and magnetic properties of the formed capsules were also followed using dynamic light scattering, electron microscopies, and magnetic force microscopy. The water-dispersible capsules, thanks to their magnetic properties, were then navigated in a static magnetic field gradient and transferred between the water and oil phases, as evidenced by fluorescence microscopy. In this way, the capsules could be loaded in a controlled way with a hydrophobic reactant, perylene, which was later photooxidized upon transferring the capsules to the aqueous phase. The capsules were shown to serve as robust reloadable nanoreactors/nanocontainers that via magnetic navigation can be transferred between immiscible phases without disruption. These features make them promising reusable systems not only for loading and carrying lipophilic actives, conducting useful reactions in the confined environment of the capsules, but also for magnetically separating and guiding the encapsulated active molecules to the site of action.

Efficient Graphene/Cyclodextrin-Based Nanocontainer: Synthesis and Host-Guest Inclusion for Self-Healing Anticorrosion Application

Cyclodextrin, with a hydrophobic inner cavity and a hydrophilic exterior, is often used to encapsulate a widest range of guest molecules based on host-guest inclusion interactions. Graphene, an emerging nanobuilding material, exhibits great potential for numerous applications because of its superior characteristics. Herein, we synthesized a novel graphene/β-cyclodextrin-based supramolecular nanocontainer with excellent inhibitor encapsulating capacity and high impermeable properties. The benzotriazole (BTA)-loaded nanocontainers were then used to endow coating system with excellent passive and active anticorrosion performance. Local electrochemical impedance spectroscopy (LEIS) was performed to characterize the self-healing behavior of composite coatings. Results indicated that the protective capability of the scratched coatings can be recovered through BTA release from containers. Furthermore, the long-term corrosion resistance of container-based coating was largely improved as observed from EIS. The effective healing process involves two conditions: (1) the release of BTA from containers and formation of adsorption layers on exposed metal surfaces and (2) the impermeable graphene nanosheets greatly impeded the electrolyte penetration and corrosion extension around the scratch. This novel graphene/β-cyclodextrin-based nanocontainer endows polymer coating with efficient self-healing functionality and durable anticorrosion property.

Hemiaminal ether linkages provide a selective release of payloads from polymer conjugates

Hemiaminal linkages allow for a selective and pH-responsive release of triazoles from polymer conjugates.