Effective release of corrosion inhibitor by cellulose nanofibers and zeolite particles in self-healing coatings for corrosion protection
Graphical abstract
Introduction
Polymer coatings have been used for corrosion control in underground structures, gas and oil pipelines, vehicles, offshore structures, etc. [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. The polymer coatings are composed of materials such as alkyd resins, epoxy esters, polyurethanes, and silicone alkyds. The disadvantages of these materials in polymer coating include poor chemical resistance (alkyd, silicone), poor gloss retention, a tendency to chalk (epoxy ester), and high cost (polyurethane) [11]. A plain polymer coating on a metal surface acts as a protective barrier impeding the effect of corrosive species from the surrounding environment. Plain polymer coating systems, however, lose the ability to function if the coating is damaged or breached. Intensive research efforts have resulted in smart coating systems that are self-healing. Coatings with the ability to self-heal protect the underlying substrate even when the coating is damaged.
With these new systems, self-healing coatings are being reported. There are two common strategies of self-healing coatings for corrosion protection. The first strategy is the polymerization of a damage site via the addition of agents of polymerization in a polymer coating. The second strategy involves the addition of corrosion inhibitor into a polymer-coating matrix. Both strategies utilize a container to accommodate either a polymerization agent or a corrosion inhibitor [11]. The containers for these materials are various forms of fibers: nanoparticles, porous structures, microvascular networks, nanocapsules, and core-shell nanoparticles [[12], [13], [14], [15], [16]]. Polymer coatings release active materials such as polymerization agents or corrosion inhibitors at the site of local damage, which continuously protects the underlying metal substrate. The driving forces that cause a release of active material from a container onto the damage site include mechanical stress, heat, electrostatic force, light, and a change in the pH [17]. Thus, materials that are sensitive to corresponding driving forces are utilized [6,12,[18], [19], [20], [21], [22], [23], [24]].
The most common and effective driving force for inhibitor release is a change in pH due to the nature of corrosion processes that change the pH of the surrounding area. A change in pH is a common factor in the corrosion process, and, thus, it can be adjusted for the controlled release of inhibitor from a container. The containers for inhibitor vary and include particles, capsules, and fibers. A polyelectrolyte-based nanocapsule has been employed as a pH-sensitive container that releases inhibitor as the local pH changes and terminates the release as the pH is restored to the initial level due to the healing process [[25], [26], [27]]. Mesoporous silica has served as an inhibitor container that is effective in a seawater environment, because it features a high release of inhibitor under a high pH [28]. Coatings with various levels of pH containing cellulose nanofibers as containers of corrosion inhibitor have been investigated [21]. Factors such as the desorption and adsorption of the corrosion inhibitor on the surface of the cellulose nanofibers are sensitive to a change in pH, and, thus, the inhibitor adsorption and desorption occurs at both low and high pH, respectively, which has resulted in an effective process of self-healing [21]. Fiber has the potential for biological applications due to similarities to microvascular systems, and the loading capacity of inhibitor is high in this system.
Recently, ion-exchanger materials have been investigated for use as self-healing coatings [[29], [30], [31], [32], [33], [34], [35]]. Ion exchanger materials either adsorb or desorb metallic ions that dissolve during the corrosion process. For instance, ion exchanger materials could adsorb the ions from a salt solution and prevent the corrosion process. Zeolite has a crystalline aluminosilicate structure that contains IA and IIA such as calcium, magnesium, potassium, and sodium, which allows it to be either a natural or a synthetic ion-exchanger [36]. Wang et al. have reported a smart epoxy coating containing Ce-MCM22-zeolites that exhibits high resistance to corrosion due to the release of Ce3+ ions from MCM22 zeolite based on the ion exchange mechanism [31]. Dias et al. reported that the addition of Ce-enriched zeolite microparticles to a hybrid sol-gel could enhance the anti-corrosion protection of a coating [33]. Several ion-exchangers are pH sensitive. A pH-responsive zeolitic imidazole framework (ZIF) has been applied to the self-healing of corrosion. In that system, naturally pH-sensitive ZIF-7 nanoparticles reacted rapidly to a change in pH by releasing linkers as healing agents [37]. Yugao et al. added the inhibitor benzotriazole to ZIF-7 nanoparticles, and the inhibitor was released at the site of local damage as the pH changed due to the corrosion process [38]. These reports have emphasized the function of zeolites in the self-healing process, but in this case the morphology of the inhibitor’s container was nanoparticles with a morphology that would not allow control of the release rate. A fiber bio-mimicking host, or container, for an inhibitor such as zeolite has not been investigated yet nor has the release rate of an inhibitor from zeolite by controlling the pH.
In the present study, a polymer coating of cellulose nanofibers, zeolite particles, and corrosion inhibitor was applied to carbon steel. A scratched specimen with the coating was immersed in a corrosive solution to elucidate the self-healing properties by monitoring the polarization resistance. The release behavior of the corrosion inhibitor from the coating was measured via mass changes to the polymer. The self-healing mechanism of the cellulose nanofibers and zeolite particles in the polymer coating was thoroughly examined and is reviewed here.
Section snippets
Materials
Sodium oleate (FUJIFILM Wako Pure Chemical Corporation) was used as a corrosion inhibitor. Cellulose nanofibers with a diameter of 100−500 nm (CELISH KY-100 G, 10 % in water, Daicel FineChem Ltd.) served as a pathway for the corrosion inhibitor (Fig. 1a), as previously reported [21]. We used cellulose nanofibers with a 10 wt.% solid content dispersed in water. Zeolite particles with a mean diameter of 2 μm and a pore size of 1 nm (Molecular sieves 13X (sodium aluminosilicate), Sigma-Aldrich Co.
Polarization resistance of scratched coatings
Fig. 2 shows the typical electrochemical impedance spectroscopy (EIS) of scratched C1-S8 and C1-S8+Z0.5-S0.4 coatings following immersion for 24 h. The impedance in the scratched C1-S8 coating showed a semicircular shape (Fig. 2a), and the polarization resistance of the scratched coating was calculated as the difference of impedances at high and low frequencies. The impedance of the scratched C1-S8+Z0.5-S0.4 coating became larger than that of the C1-S8 coating, which showed the effect of the
Conclusions
Sodium oleate as a corrosion inhibitor was adsorbed onto cellulose nanofibers and zeolite particles, which were then mixed with a polymer resin. Various mixtures of polymer coatings were then applied to carbon steel substrates. The coatings were then scratched with a knife to expose the substrates, which then were submerged into a corrosive solution. The self-healing properties of the coatings were evaluated via the electrochemical impedance of the scratched coatings when exposed to the
Data availability statement
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
CRediT authorship contribution statement
Akihiro Yabuki: Conceptualization, Methodology, Supervision. Masato Kanagaki: Investigation. Chikara Nishikawa: Investigation. Ji Ha Lee: Writing - original draft, Writing - review & editing. Indra Wahyudhin Fathona: Writing - review & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This research was partially supported by JSPS KAKENHI Grant Numbers JP26420705 and JP20H02485; and, by the “Innovation inspired by Nature” Research Support Program, SEKISUI CHEMICAL CO., LTD.
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