New effects of TiO2 nanotube/g-C3N4 hybrids on the corrosion protection performance of epoxy coatings
Graphical abstract
Introduction
Polymer coatings are widely used for the corrosion protection of steel surfaces in a marine environment due to their low cost, scalability, and versatility [[1], [2], [3], [4]]. Although organic coatings act as direct barriers against the diffusion of corrosive agents, these coatings have some disadvantages including hydrolytic degradation and creation of holes and defects during the curing step, which decrease their service life [[5], [6], [7]]. Therefore, the development of polymer coatings for protecting steel structures from corrosion has gained considerable attention [[8], [9], [10]]. Meanwhile, nanomaterials can be used as nanofillers in a polymer matrix to overcome the shortcomings of polymer coatings as they improve the barrier performance by filing the pores, micro-cracks, and defects of the organic matrix [[11], [12], [13], [14]].
Among different inorganic nanomaterials used for enhancing the corrosion resistance of polymer coatings, titanium dioxide (TiO2) nanoparticles are suitable candidates because of their intrinsic properties such as inertness, chemical resistance, photostability, shielding effect against ultraviolet rays, and precursor availability [[15], [16], [17], [18]]. Researchers have shown that when TiO2 nanoparticles are dispersed in polymer matrix, the mechanical properties of the coating (i.e. abrasion, hardness, and friction resistance) enhance and the water permeation in the coating decreases [19]. Besides, the corrosion resistance of polymer coating significantly improves by adding TiO2 nanoparticles owing to the enhancement of the barrier performance and increase in the cross-linking density of polymer coatings in the presence of TiO2 nanofiller [[19], [20], [21], [22], [23], [24]]. According to a study by Shafaamri et al. [25], epoxy coatings modified with TiO2 nanoparticles provide high barrier performance owing to the physically interact of TiO2 nanoparticles with the epoxy matrix. Meanwhile, one of the approaches for improving the performance of TiO2 nanopowders in polymer coatings is based on morphological alteration using TiO2 nanotubes with large surface area and a tubular structure because the TiO2 nanotubes can provide higher chemical stability as compared to the conventional TiO2 nanoparticles [26,27]. However, these particles have a strong tendency to agglomerate in the polymer matrix because of their large surface area, and the hydrophilic TiO2 nanomaterials can increase the hydrophilicity of the coating [28,29].
Moreover, different researchers have used graphitic carbon nitride (g-C3N4) as a corrosion-resistant nanofiller in polymer coatings. g-C3N4 is a two dimensional, low cost, non-toxic, and highly stable semiconductor, and g-C3N4 sheets act as a barrier against diffusion of corrosive agents inside polymer coatings [[30], [31], [32], [33]]. The exfoliated g-C3N4 nanosheets with good dispersion in polymer matrix can increase the diffusion path of corrosive ions, and enhance the corrosion resistance of coatings [34]. Accordingly, Karimi et al. [35] used a polystyrene/g-C3N4 anti-corrosion coating on copper. Moreover, Malav et al. [36] developed polyimide/g-C3N4 composite coatings with enhanced corrosion resistance. Further, Chen et al. [37] used g-C3N4 as dispersant to improve the dispersion quality of graphene in epoxy matrix. Therefore, the hybrids of TiO2 and g-C3N4 can be used as efficient nanofillers with the advantages of both materials to develop coatings with higher corrosion resistance. Furthermore, the use of these hybrids is a feasible method to prevent the aggregation of nano-TiO2 and change the closely stacked g-C3N4 sheets into their loose state in epoxy coatings [4]. Thus, herein, the TiO2 nanotube/g-C3N4 hybrids are investigated as nanofillers in a polymer matrix to study their effect on the corrosion protection performance of epoxy coatings.
Since the performance of polymer nanocomposite coatings strongly depends on the filler concentration, chemical reactions between the filler and the polymer matrix, and the dispersion quality of nanofillers in these coatings [12,15,20,38], these parameters are examined to obtain nanocomposite coatings with high corrosion resistance. Hence, the surface of the hybrids can be chemically modified to achieve well-dispersed nanofillers in a polymer matrix [11,39]. An appropriate method for modifying the surface of nanomaterials for their application in polymer coatings is grafting a layer of silane coupling agent on them with similar chemical molecular chains as those of the polymer matrix [12,40]. The incorporation of silane coupling agent could enhance the interfacial adhesion strength and the dispersion quality of the nanofiller in the polymer matrix [41]. The adsorption and interaction of organosilane on TiO2 nanoparticles can lead to formation of strong Si-O-Ti bonds and functional groups on the surface of TiO2 nanoparticles [42,43]. In this regard, Zhang et al. [44] reported that silane-functionalized TiO2 nanoparticles can be homogeneously dispersed in fluorinated acrylic copolymers to prepare toxin-free paints with good adhesion strength and low surface energy for application on ship hulls [44]. Xia et al. [41] utilized the advantages of polydopamine and silane coupling agent on g-C3N4 nanosheets for increasing the corrosion resistance of waterborne epoxy coatings.
In addition, note that the TiO2 nanoparticles can absorb UV light (λ ˂ 400 nm, 5% of sunlight) because the band gaps of anatase and rutile TiO2 are 3.2 and 3.0 eV, respectively [45]; however, g-C3N4, with the band gap of approximately 2.7 eV, is a visible-light sensitive photocatalyst [46,47]. Thus, the exfoliation of g-C3N4 and its coupling with TiO2 nanostructures are versatile methods for improving the photocatalytic activity of TiO2 under visible-light irradiation by decreasing the recombination rate of electron-hole pairs. Current studies have focused on the photocatalytic performance of TiO2, g-C3N4, and their hybrids [[48], [49], [50]]. Since the equipments submerged in natural water, even at a depth of more than one meter, absorb light in the blue-green region (~500 nm) of the electromagnetic spectrum [51], it is important to consider the effect of the TiO2/g-C3N4 hybrids on the corrosion protection performance of epoxy coatings both in the dark and under visible-light irradiation. Furthermore, it has been reported that epoxy resins are sensitive to UV-irradiation and chemical alteration, and the delamination of epoxy coatings happen during exposure to sunlight/UV light [[52], [53], [54]]. In this regard, UV and visible-light irradiations can significantly affect the corrosion process of metallic structures, and the steel surfaces show photovoltaic effect under sunlight which is a major corrosion parameter in marine environment [55,56]. Therefore, when photocatalytic nanomaterials are used in polymer coatings, the developed nanocomposite coatings could undergo photo-degradation during their service life.
As mentioned above, TiO2 nanotubes and g-C3N4 nanosheets have potential applications for improving the corrosion resistance of polymer coatings; however, the effect of their hybrids for utilizing the advantages of both TiO2 nanotubes and g-C3N4 in developing corrosion resistant polymer coatings have not been considered. Therefore, the main goal of this research is to investigate the corrosion protection performance of epoxy coatings containing TiO2 nanotube/g-C3N4 hybrids. Moreover, the influence of light irradiation on corrosion is an important factor, which is often neglected in related studies. Since TiO2 nanotubes and g-C3N4 nanosheets are photocatalytic nanomaterials, the effect of their hybrids on the corrosion resistance of nanocomposite coatings under light irradiation should be considered. Herein, TiO2 nanotube/g-C3N4 hybrids were synthesized and their photocatalytic performance was evaluated under visible-light irradiation. Then, to more clearly investigate the effect of visible-light-driven nanofillers on the corrosion resistance of epoxy coatings, the hybrid with the highest photocatalytic performance was used as an anti-corrosion nanofiller in the epoxy coatings. Furthermore, the hybrids were functionalized with 3-(aminopropyl)triethoxysilane (APTES) to study the effects of silane functionalization on the corrosion resistance of polymer coatings. The corrosion resistance of the epoxy nanocomposite coatings loaded with non-modified and silane-functionalized hybrids was examined in a 3.5 wt% NaCl solution both in the dark and under visible-light irradiation. To the best of our knowledge, this is the first time that the TiO2 nanotube/g-C3N4 hybrid is used as anti-corrosion nanofiller in a polymer matrix and the effects of dark and visible-light irradiation on the corrosion protection performance of polymer nanocomposite coatings are investigated.
Section snippets
Materials
All chemicals were used directly without purification in this study. Melamine (C3H6N6, 99.99%), nitric acid (HNO3, 70%), sodium chloride (NaCl), sodium hydroxide (NaOH), hydrochloric acid (HCl, 37%), methanol anhydrous, ethanol (≥99.7%), isopropanol (≥99.7%), and Rhodamine B (RhB) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Titanium tetra-isopropoxide (TTIP, Ti(OCH(CH3)2)4, 97%) was supplied from Shanghai Macklin Biochemical Co., Ltd. (China). APTES (H2N(CH2)3Si(OC2H5)3,
XRD results
Fig. 2 shows the XRD results of exfoliated g-C3N4, TiO2 nanoparticles, TiO2 nanotubes, and the hybrids. The XRD pattern of g-C3N4 shows two diffraction peaks at 2θ = 12.9° and 27.7°, which can be attributed to the (100) interplanar diffraction of tri-s-triazine units and (002) interlayer diffraction of conjugated aromatic systems having graphitic layered structure with 0.322 nm interlayer distance, respectively (JCPDS #87-1526) [30,50,60,61]. Additionally, the diffraction peak at 2θ = 17.7° is
Conclusion
Herein, we investigated the effect of visible-light-driven photocatalytic nanomaterials on the corrosion protection performance of epoxy coatings in the dark and under visible-light irradiation. The TiO2 nanotube/g-C3N4 hybrids containing g-C3N4 in various amounts and silane-functionalized hybrids were synthesized and characterized by XRD, TEM, SEM/EDS mapping, XPS, and N2 adsorption/desorption isotherms. The optical properties and visible-light photocatalytic performance of the synthesized
Funding sources
The present work was financially supported by the CAS President's International Fellowship Initiative (PIFI, No. 2019PE0059), National Natural Science Foundation of China (NSFC, No. 41806090), and National Natural Science Foundation of China (NSFC, No. 41827805).
CRediT authorship contribution statement
Sepideh Pourhashem: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing, Funding acquisition. Jizhou Duan: Conceptualization, Supervision, Writing - review & editing, Funding acquisition. Fang Guan: Resources. Nan Wang: Investigation. Ying Gao: Investigation. Baorong Hou: Supervision.
Declaration of competing interest
We declare that we have no financial and personal relationships with other people or organizations that could have appeared to influence the reported research in this paper.
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