Active corrosion protection performance of an epoxy coating applied on the mild steel modified with an eco-friendly sol-gel film impregnated with green corrosion inhibitor loaded nanocontainers

Applied Surface Science 440 (2018) 491–505

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Active corrosion protection performance of an epoxy coating applied on the mild steel modified with an eco-friendly sol-gel film impregnated with green corrosion inhibitor loaded nanocontainers M. Izadi a, T. Shahrabi a,⇑, B. Ramezanzadeh b,⇑⇑ a b

Department of Materials Science Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran Department of Surface Coatings and Corrosion, Institute for Color Science and Technology (ICST), P.O. Box 16765-654, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 5 October 2017 Revised 2 January 2018 Accepted 20 January 2018

Keywords: Green nanocontainers Active inhibition Epoxy coating Hybrid sol-gel film EIS SEM-EDS

a b s t r a c t In this study the corrosion resistance, active protection, and cathodic disbonding performance of an epoxy coating were improved through surface modification of steel by a hybrid sol-gel system filled with green corrosion inhibitors loaded nanocontainer as intermediate layer on mild steel substrate. The green inhibitor loaded nanocontainers (GIN) were used to induce active inhibition performance in the protective coating system. The corrosion protection performance of the coated panels was investigated by electrochemical impedance spectroscopy (EIS), salt spray, and cathodic disbonding tests. It was observed that the corrosion inhibition performance of the coated mild steel panels was significantly improved by utilization of active multilayer coating system. The inhibitor release from nanocontainers at the epoxysilane film/steel interface resulted in the anodic and cathodic reactions restriction, leading to the lower coating delamination from the substrate and corrosion products progress. Also, the active inhibition performance of the coating system was approved by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and energy dispersive X-ray (EDS) analysis on the panels with artificial defects. The inhibitive agents were released to the scratch region and blocked the active sites on the metal surface. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction Epoxy resins are widely used as protective system on the metallic substrates due to different advantages, i.e. excellent chemical resistance, strong adhesion to the metal surface, high corrosion resistance, and good chemical, thermal and mechanical properties [1,2]. Despite the mentioned properties, the insufficient barrier performance against corrosive agents is one of the most disadvantages of the epoxy coatings [3]. Blistering and interfacial adhesion bond destruction in the coating/metal interface occur by corrosive moieties diffusion into the epoxy coating/metal interface [4,5]. Not only the coating deterioration but also the adhesion bonds destruction influence the barrier and inhibiting properties of the epoxy coatings at long exposure times. So, it is necessary to enhance the corrosion protection performance of the coating by various

⇑ Corresponding author. ⇑⇑ Corresponding author. E-mail addresses: [email protected] (M. Izadi), tshahrabi34@modares. ac.ir (T. Shahrabi), [email protected], [email protected] (B. Ramezanzadeh). https://doi.org/10.1016/j.apsusc.2018.01.185 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.

methods [6]. One of the commonly used procedures to induce the inhibitive protection in the epoxy coatings is the incorporation of nanoparticles, pigments, and capsules [7,8]. Chromate based pigments induce effective corrosion protection in the epoxy coatings [9]. However, high toxicity of chromate species guiding the researchers toward the development of new eco-friendly corrosion inhibitive systems [10,11]. Among the different protective agents, the inhibitive micro/nano containers are utilized by many researchers to replace the chromate pigments and induce active corrosion protection in the epoxy coatings. Ghazi et al. [12] used benzimidazole (BIA) and zinc cations intercalated montmorillonite (MMT) clay particles as an active agent in the epoxy-ester coating. They showed that the highest corrosion resistance was obtained by using the mixture of Zn-MMT + BIA-MMT clay particles. The complex formation between BIA+ and Zn2+ cations was suggested as the cause of active performance of the epoxy-ester coating. CaCO3 micro beads were modified with different corrosion inhibitors and utilized by Snihirova et al. [13] to induce an active performance in the water based epoxy coating. The better inhibition ability of the epoxy coating in the presence of cerium ions, and the active inhibition potential of coating system were confirmed by

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electrochemical impedance spectroscopy (EIS) and local electrochemical impedance spectroscopy (LEIS) measurements, respectively. Montemor et al. [14] used layered double hydroxides and cerium molybdate hollow nanospheres as inhibitors (mercaptobenzothiazole) reservoirs to prepare active epoxy coating. The effective corrosion inhibition, active inhibition ability, and synergistic effect of the epoxy coating in the presence of both types of the nanocontainers were proved by EIS, scanning vibrating electrode technique (SVET), and scanning ion-selective electrode technique (SIET). The emeraldine base and salt forms of polyaniline (doped by hydrofluoric acid and camphorsulfonic acid) were added to the epoxy coating by Zhang et al. [15] to enhance the corrosion protection performance of coating. The EIS measurement results confirmed the better corrosion resistance of the camphorsulfonic acid-doped PANI coating. The corrosion protection of mild steel was investigated by a large number of researchers. Refait et al. [16] investigated the corrosion of mild steel coupons at the seawater/sediments interface. The corrosion protection performance of mild steel sample was investigated by Kahyarian et al. [17] in the aqueous acetic acid solution. They showed that the electrochemical reactions rate was decreased in the presence of acetic acid. Dohare et al. [18] used expired Tramadol as a corrosion inhibitor for mild steel in 1 M HCl solution. The better corrosion protection in the presence of higher concentration of Tramadol was confirmed by electrochemical tests. In recent years, green organic compounds are investigated as corrosion inhibitors due to the many disadvantageous of synthetic inhibitors such as complicated synthesis process and toxicity [19,20]. So, the efforts for finding green organic compounds as cheap and non-toxic corrosion inhibitors have been performed in several researches [21,22]. So, it seems that utilization of green corrosion inhibitor loaded nanocontainers (GIN) in the epoxy coatings can be used as effective way to enhance the protection performance of the coatings. Nitrogen containing inhibitors are effective agents in corrosion protection of metal substrates [23]. The plant extracts containing polar functions with N, S, and O in the conjugated structure which are adsorbed on the metal surface with the polar groups, leading to the inhibitive surface film formation [24]. There is different heterocyclic compounds in the Nettle plant which can interact with steel surface and reduce the corrosion rate of metal [25]. Also, the Nettle constituents can chelate with transition metals such as Zn2+ and make complexes which absorb on the metal surface. Therefore, combination of the GIN and zinc acetate can be used as an efficient way to induce active inhibitive protection in the epoxy coating. However, there is several limitation in order to incorporate additives directly into the epoxy coatings such as blistering and lack of adhesion between the containers and epoxy matrix [26]. Also, the adhesion of epoxy coating to the metal surface can be improved by surface pretreatment, i.e. using surface modification or intermediate layers. Among various surface pretreatment methods the use of eco-friendly sol-gel coatings has attracted high consideration of the researchers in recent years. Alkoxy silanes are mediators with two alkoxy (X) and organofunctional (Y) groups in their structures which are written as X3Si (CH2)nY. The alkoxy group (ex: methoxy, ethoxy, etc.) is hydrolysable agent which can attach to inorganic materials (such as metallic substrates) and the organofunctional groups (ex: amino, vinyl, hydroxyl, etc.) react with polymeric materials (such as primer organic coatings) [27]. This coupling action of silanes plays an important role in adhesion improvement of organic coatings on metallic substrates. Also, the silane films with dense and crosslinked network siloxane chains (Si-O-Si bonds) act as a good barrier against water, oxygen, and other aggressive agents which diffuse to the substrate/coating interface [28]. However, there is no inhibitive protection in the silane films. So, it is necessary to enhance the corrosion protection performance of this coating by

Fig. 1. Schematic illustration of GIN-EP coating on MS substrate.

incorporation of active agents into the silane films. The mesoporous silica nanoparticles encapsulated with p-coumaric acid (CA) were utilized by Wang et al. [29] to induce the active inhibition performance in the ZrO2-SiO2 sol-gel coating. They used two sol-gel layer with the smart nanocontainers and Ce(IV) salt to apply bi-layer coating on the AA2024 panels. They showed that the combination of CA and Ce(III) produce a synergistic inhibition effect, providing significant active inhibition functionality. Ding et al. [30] synthesized smart nanocontainers based on the installation of the supramolecular assemblies to load 8-hydroxyquinoline as corrosion inhibitor. They applied the hybrid organic-inorganic sol-gel coating on the AZ31B panels and showed the satisfactory anticorrosion performance of designed hybrid system. Ding et al. [31] used silica nanoparticles to entrap 2-hydroxy-4-methoxyacetophenone as corrosion inhibitors. The active protection of sol-gel coating was proved by electrochemical analysis. Regarding to the proposed works, the bi-layer hybrid coating containing GIN and zinc acetate is considered as intermediate inhibitive layer to induce an active performance to the epoxy system coating. The active multilayer coating layout is illustrated in Fig. 1. In this work, a protective organic system based on GINs as a new reservoir was developed. Also, the zinc acetate was used to induce the synergistic effect with inhibitor molecules existing in the GIN structure. The active hybrid sol-gel film containing GINs was prepared and characterized by field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), and energy dispersive spectrometry (EDS) analysis. Electrochemical impedance spectroscopy (EIS) measurements, cathodic disbonding, and salt spray tests were employed to investigate the corrosion inhibition performance of the active coating. Also, scanning electron microscopy (SEM), EDS, and EIS measurements (on the coated samples with artificial defects) were used to study the active performance and active inhibition mechanism in the organic coating system. 2. Materials and methods 2.1. Materials In this study the Nettle leaves, commonly known as Urtica Dioica, is used as a green source of corrosion inhibitors. For many years the Nettle leaves, which is an herbaceous perennial flowering plant, has been used as a source of medicine, food, and fiber. Flavonal glycosides such as quercitin, as well as carotenoids, chlorophyll, vitamins (C, B and K), histamine, serotonin, acids (e.g. carbonic and formic acid), and minerals (e.g. calcium, magnesium, and potassium) have been considered as the main components existed in the Nettle leaves structure [32]. According to literature, histamine, serotonin and quercitin are some of the most important and potent inhibitors existed in the Nettle leaves extract. The

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Nettle leaves were collected from plants that grow in the Northern Foothills of Alborz Mountain, Iran. The collected leaves were washed with deionized water and dried in room temperature for one week. The dried leaves were powdered and heated in distilled water for 4 h at 70 °C. The supernatant was separated by a filter paper and heated at 60 °C until was turned to a pasty material. The obtained green paste was dried in room temperature and stored at 4 °C in the refrigerator. The collected leaves were washed with deionized water and dried in room temperature for one week. The dried leaves were powdered and heated in distilled water for 4 h at 70 °C. The supernatant was separated by a filter paper and heated at 60 °C until was turned to a pasty material. The obtained green paste was dried at room temperature and stored at 4 °C in the refrigerator. The GIN was synthesized through l-b-l process, using polyelectrolyte to entrap the green corrosion inhibitors [33]. In a brief description, the positively charged polyaniline (PANI) was deposited on the negatively charged Fe3O4 nanoparticles. In the second step, the Nettle molecules were loaded on the Fe3O4-PANI core-shell as inhibitor agents. Finally, the polyacrylic acid (PAA) layer was deposited on the inhibitor loaded core-shell to entrap the Nettle molecules and reduce the destructive interactions between inhibitor and coating constituents. Tetraethylorthosilicate (TEOS, C8H10O4Si), teriethoxymethylsilane (TEMS, C7H18O3Si), acetic acid (C2H4O2) and zinc acetate (Zn(O2CCH3)2 were purchased from Merck Co. Ethanol (C2H5OH, 99%) was supplied by Ameretat Co. (Iran). Epoxy resin (Araldite GZ7 7071X75: solid content: 74–76%, epoxy value: 0.15–0.17, density: 1.08 g/cm3) and amido polyamide curing agent (CRAYAMID 115) were supplied by Saman Co. and Arkema Co., respectively. Mild steel (CK10) panels (wt.%: 0.1% C, 0.45% Mn, max 0.4% Si, and about 99% Fe) with 100
35
1 mm dimensions were purchased from local company. 2.2. GIN-epoxy coating preparation The steel panels were mechanically abraded by emery papers and then were pretreated by NaOH solution (25 g/L) at 55 °C for 7 min. Chemical pretreatment by NaOH solution was performed to create additional hydroxyl groups on the metal surface and enhance the adhesion of silane coating to the steel panels. The silane layer was prepared by using the mixture of TEOS (25 wt. %), TEMS (25 wt.%), deionized water (30 wt.%), and ethanol (20 wt.%) through a two-step deep coating process. Two deep coating vessels containing zinc acetate (ZA) and GIN (GIN concentration: 1 g/l) were used to apply bi-layer silane coatings. The silane coated

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steel panels were dried at 60 °C for 4 h. In the next step, the mixture of epoxy resin and polyamide hardener (at a ratio of 1.3/1 w/w) was utilized to apply the epoxy organic coating on the steel panels. In this regard, the epoxy/hardener was mixed with toluene as diluent to reduce the viscosity and the obtained mixture was applied on the steel panels by a film applicator (coating thickness: 100 lm). Finally, the coated panels were cured at room temperature and post-treated at 100 °C. Three different coatings were applied on the samples to investigate the inhibitive performance of the steel panels including epoxy coating without silane, epoxy coating with neat silane bi-layer (Silane-epoxy), and epoxy coating with silane bi-layer containing ZA and GIN (GIN-epoxy). 2.3. Characterization 2.3.1. Coating characterization The chemical composition and structure of the GINs containing hybrid sol-gel film were characterized by FT-IR spectroscopy (SHIMADZU, wavenumber region: 4000–400 cm1), FE-SEM (TESCAN MIRA 3), EDS, and AFM analysis. 2.3.2. EIS measurements A three electrode cell (reference electrode: Saturated Calomel electrode (SCE), counter electrode: platinum in 1 cm2 area, working electrode: steel samples in 1 cm2 area) was used to perform the EIS measurements. The EIS data were obtained at open circuit potential (OCP) in the frequency range of 100 kHz to 10 mHz and at amplitude sinusoidal voltage of ± 10 mV. The three samples (Epoxy, Silane-epoxy, and GIN-epoxy) were immersed in 40 mL NaCl solution (3.5 wt.%) and EIS measurements were performed over time. Also, the coated samples were scratched by a surgery knife (dimension: 2 cm
20 lm) and exposed to the saline solution (3.5 wt.% NaCl) to investigate the active performance of the GIN-coating. Data measurements and analysis were done by Nova (version 1.6) and Z-View2 software, respectively. All EIS tests were repeated three times to obtain the standard deviation values. 2.3.3. Salt spray test Salt spray test was performed in salt spray cabinet in accordance with ASTMB117 standard. So, the coated panels with artificial defect (scratch with a dimension of 3 cm
2 mm, created using a surgery knife) were used to accelerate the corrosion process. All samples were located in test camber at an angle of 45° and exposed to the 5.0 wt.% NaCl solution at 40 °C. The condensate collection rate and relative humidity were of at least 1.0 to 2.0 ml/h

Fig. 2. The top-view FE-SEM images of GIN-epoxy sample.

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Fig. 3. FE-SEM micrographs of (a) cross-sectional area of sol-gel bi-layer, (b) cross-sectional area of GIN containing sol-gel layer (point X), and (c) EDS elemental (Si, Zn) maps of sol-gel bi-layer coating.

per 80 m2 (horizontal collection area) and 95%, respectively. The protective performance of the coating was investigated with the size and distribution of damaged area on the coated samples surfaces. 2.3.4. Cathodic disbonding test Cathodic disbonding test was performed in a three-electrode configuration including Ag/AgCl reference electrode, Pt counter electrode, and the coated steel panels as working electrode. A 9 cm2 area of the coated samples containing an artificial defect (a hole with 4 mm in diameter on the center of samples) was exposed to 3.5 wt.% NaCl solution at pH = 6.5 and the rest area of the steel panels were masked with beeswax-colophony mixture. The cathodic disbonding tests were conducted under polarization potential 1.34 V vs Ag/AgCl for 24 h. After each test, loosened coating was lifted with sharp knife to determine the disbonded area. All cathodic disbonding tests were repeated 3 times and the averages results were reported to ensure the reproducibility. 2.3.5. Surface studies FE-SEM and EDS analysis were performed to study the active and active inhibition ability of the GIN-epoxy. To this end, the Silane-epoxy and GIN-epoxy samples with artificial defect (scribe

dimension: 2 cm
20 lm) were exposed to 3.5 wt.% NaCl solution at pH = 6.5 for 72 h. Then, the immersed samples were rinsed with deionized water and ethanol and were dried in an oven at 50 °C for 12 h. EDS elemental maps were employed to investigate the presence of Zn, C, Fe, and O elements.

3. Results and discussion 3.1. Characterization of active coating The top-view FE-SEM images of GIN-epoxy sample are presented in Fig. 2. Uniform and smooth surface of cured epoxy top-coating is observed in the FE-SEM images. Also, there is no considerable defect on the GIN-epoxy sample. The FE-SEM images of cross-sectional area of the hybrid sol-gel layer containing the nanocontiners is illustrated in Fig. 3. The active layer thickness is about 50 lm and the coating structure is affected by abrading procedure and some cracks can be seen in the coating. According to Fig. 3(a), there are two different structures in the cross-sectional area of the coating. A surface with considerable roughness is observed in area (I) which corresponded to the nanocontainer induced layer. However, the layer in area (II) has a smooth morphology in the absence of the nanocontainers. From Fig. 1, the first

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Fig. 4. EDS analysis data related to point X in the cross-sectional area of active hybrid sol-gel film (the corresponding weight percentage of each element is illustrated beside its peak). Fig. 6. FT-IR spectra of (a) hybrid sol-gel film, (b) active nanocontainers, (c) active hybrid sol-gel film.

hybrid sol-gel layer consists of zinc acetate and the active nanocontainers were applied in the second hybrid sol-gel layer. The uniform dispersion of the nanocontainers is obvious in the

FE-SEM image of cross-sectional area (point X). The oppositely charged polyelectrolyte molecules which applied on the nanocontainers surfaces to entrap the corrosion inhibitors is the main

Fig. 5. The topographic and phase-contrast AFM images of (a, b) hybrid sol-gel film, and (c, d) GIN hybrid sol-gel film.

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factor, leading to uniform dispersion of the containers. The EDS elemental maps were used to show two distinct layers in the solgel bi-layer structure (Fig. 3(c)). The Zn elemental map confirms the presence of the layer containing zinc acetate in area (II).

Also, the Si elemental map shows the silane matrix of sol-gel bi-layer film. The EDS analysis was employed to investigate the composition of hybrid sol-gel layer filled with nanocontainers (point X).

Fig. 7. Bode plots of Epoxy, Silane-epoxy, and GIN-epoxy samples after (a, b) 20 (c, d) 40, and (e, f) 60 days of immersion in the saline solution (3.5 wt.% NaCl); solid lines and marker points represents the fitted and experimental data, respectively.

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According to Fig. 4, O, C, Fe, N, and Si elements are detected in the cross-sectional area of the silane bi-layer coating. The film contains C, O, and Si in its structure. Also, the Fe and a fraction of O elements are corresponded to the Fe3O4 nanoparticles in the active layer composition. Moreover, N and a fraction of C content of hybrid sol-gel layer are due to the Nettle molecules entrapped in the nanocontainers structure. The cross-sectional structure and composition confirm the incorporation of the active nanocontainers in the hybrid sol-gel film structure. The AFM images were employed to study the surface morphology of the MS panels coated with neat and GIN containing hybrid sol-gel film. The 2 lm topographic and phase contrast images of

two samples are shown in Fig. 5. The neat hybrid sol-gel film (Fig. 5(a)) shows uniform surface without any significant color contrast, indicating the homogeneity of the hybrid sol-gel film thickness. The Root mean square (RMS) surface roughness is recorded about 0.25 nm. The phase contrast images are used beside topographic images to confirm the second-phase constituents in the films [34]. There are no obvious second-phase materials in the phase contrast image of neat silane coated panel (Fig. 5(b)). There is a sharper color contrast in the topographic image of the nanocomposite hybrid sol-gel film which indicates more height differences due to the active nanocontainers incorporation into the coating. RMS roughness of coating is measured about 0.65

Fig. 8. Nyquist plots of Epoxy, Silane-epoxy, and GIN-epoxy samples after (a) 20 (b) 40, and (c) 60 days of immersion in the saline solution (3.5 wt.% NaCl); solid lines and marker points represents the fitted and experimental data, respectively.

Table 1 Obtained data from the EIS fitted plots of the Epoxy, Silane-epoxy, and GIN-epoxy samples after (a) 20 (b) 40, and (c) 60 days of immersion in the saline solution (3.5 wt.% NaCl); the mean of three replicates is shown for each panel, ‘‘±” shows the standard deviation. Sample

Immersion time 20 d log (|Z|10

Epoxy Silane-epoxy GIN-epoxy

5.6 ± 0.2 5.9 ± 0.2 7.1 ± 3

40 d

60 d

2 mHz/Xcm )

4.7 ± 0.3 4.9 ± 0.2 6.7 ± 0.4

20 d

40 d

60 d

fb (Hz) 5.5 ± 0.1 6 ± 0.2 6.4 ± 0.1

3.2 ± 0.3 3.3 ± 0.1 3.1 ± 0.2

20 d

40 d

60 d

Phase angle (at 100 kHz) (deg) 3.3 ± 0.4 4 ± 0.2 3.0 ± 0.3

3.2 ± 0.2 3.4 ± 0.3 2.7 ± 0.1

12 ± 3 90 ± 1 89 ± 1

18 ± 2 86 ± 2 90 ± 1

13 ± 4 19 ± 5 90 ± 2

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Fig. 9. Active sites of the Nettle constituents and chelates formation (due to the interaction of charged chemicals in the nettle structure with Fe2+ cations) leading to the inhibitive film formation on the steel surface.

nm. The surface roughness is more than neat hybrid sol-gel film which is related to the nanocontainers addition to the film. Also, the nanocontainers are observed in the phase contrast image of the hybrid sol-gel film (Fig. 5(d)). The nanocontainers size difference observed in the AFM images is related to initially added containers. This difference is due to the hybrid sol-gel layer which

covered the nanocontainers in the film. In the other hand, the nanocontainers convexity is measured by AFM on the film surface. So, the obtained nanocontainers sizes are more than the exact dimensions of the containers [35]. The chemical compositions of the hybrid sol-gel film, active nanocontainers, and active hybrid sol-gel film were characterized to investigate the preparation of the active film. From Fig. 6(a), the peaks observed at 1440 cm1 and 1632 cm1 are related to vibrating OH groups which could be attributed to the remained water molecules in the film structure [36]. The absorption peak centered at 1279 cm1 can be assigned to the vibration of Si-C bond [35]. Two characteristic bands at 1128 cm1 and 1043 cm1 are related to Si-O bond originated from the backbone network of hybrid sol-gel film [35,37]. Also, the weak shoulder appeared around 827 cm1 can be assigned to Si-CH3 bonding [38]. Characteristic peaks at 1294 and 1315 cm1 (Fig. 6(b)), corresponded to vibrations of C-N (aromatic amine) and the resulted peaks at 1504 cm1 and 1574 cm1 are corresponded to quinoid and benzenoid rings, respectively [39,40]. The FT-IR characteristic peaks of GINs are illustrated in Fig. 6(b). The Nettle absorption peaks are observed at 1077 cm1, 1415 cm1, 1622 cm1 which are corresponded to phenolic compounds and amide groups [41,42]. The resulted peak at 1728 cm1 is due to carbonyl stretching, corresponded to polyacrylic acid (PAA) layer in the GIN structure [43].

Fig. 10. (a, b) The Bode and (c, d) Nyquist plots of Epoxy, Silane-epoxy, and GIN-epoxy samples with artificial defects after 1, 2, 4, and 24 h of immersion in the saline solution (3.5 wt.% NaCl); solid lines and marker points represents the fitted and experimental data, respectively.

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Silane-epoxy sample is decreased over time (after 40 days) and the third time constant is observed in the Bode and Nyquist diagrams. The appeared time constant at medium-frequency range could be due to the corrosion products formation at corrosion sites in the epoxy-steel interface [13]. Moreover, the high frequency phase angle decreases to lower value (about 85°), showing the loss of coating adhesion after 40 days of immersion. The increase of breakpoint frequency (fb) is observed after 40 days of immersion (from 3.3 Hz to 4 Hz). The breakpoint frequency increment reveals that the delaminated area at the steel/coating interface is increased [48,49] After 60 days of immersion, the phase angle decreased to lower values (about 20°) and a slight increase in the |Z|10 mHz values is observed (Table 1). From Fig. 7, the |Z|10 mHz values for GIN-epoxy sample is 1 order of magnitude higher than the values for Silane-epoxy sample. Also, the semi-circle diameter of Nyquist plot (as a criterion of corrosion resistance) is much higher than the corresponding values for Epoxy and Silane-epoxy samples at different immersion times. Furthermore, the second and third time constants are observed at 20 and 40 days of immersion, respectively. However, the phase angle is close to 90° at all immersion times, indicating the good barrier and adhesion performance of GIN-epoxy coating. From Table 1, the breakpoint frequency (fb) values for GIN-epoxy sample are lower in comparison with Silane-epoxy sample, indicating the reduction of microscopic delaminated areas [49]. There are cavities, free volumes and porosities in the epoxy matrix which are pathways for electrolyte diffusion to the metal surface. The silane film blocks these pathways and limits the access of electrolyte to the steel surface. Also, by electrolyte diffusion into the epoxy-steel interface, the inhibitive constituents release from the incorporated agents (GINs and ZA) and reduce the corrosion rate. Histamine and serotonin are some typical active inhibitive compounds existed in the Nettle leaves extract [50]. Histamine can be protonated at specific molecule sites in the pH values under 10.5 [51]. The aliphatic amino groups and imidazole rings are two moieties which are responsible for protonation of histamine molecules [51]. Also, the amine groups of serotonin can be protonated in the pH values under 10.5 due to the electrostatic interactions between the p electron cloud of indole ring and the positively charged nitrogen atom [52]. The Nettle constituents released from the nanocontainers can be protonated and interact with Fe2+ and Zn2+ cations to form chelates absorbing as an inhibitive layer on the metal surface as an inhibitive layer [53]. The interaction of Nettle constituents with Fe2+ cations and the active moieties of the each molecule are illustrated in Fig. 9. After investigation of the intact coated samples, panels with artificial defect were studied to better investigate on the corrosion

According to Fig. 6(a-c), the characteristic peaks of the GINs are appeared in the FT-IR spectrum of active hybrid sol-gel film which confirmed the successful incorporation of nanocontainers into the inhibitive film. 3.2. Corrosion performance 3.2.1. EIS measurements The inhibitive performance of the coated samples was evaluated by EIS technique after 20, 40, and 60 days of immersion. The Bode and Nyquist plots obtained after different immersion times are represented in Fig. 7 and Fig. 8, respectively. The impedance measurement data were fitted by two equivalent circuits with two and three time constants as illustrated in Fig. 8. In the equivalent circuits, Rs, Rc, CPEc, Rf, CPEf, Rct, and CPEdl are the solution resistance, coating resistance, coating constant phase element of coating, film resistance, film constant phase element, charge transfer resistance, and double layer constant phase element, respectively. The constant phase element (CPE) is used to explain the non-ideal capacitance properties of the metal surface due to the surface heterogeneity. The surface heterogeneity is resulted from surface roughness, the adsorption of inhibitors, and formation of surface layers on the metals [44]. It can be seen that the impedance modules at low frequency limit (|Z|10 mHz: impedance magnitude at 0.01 Hz) of the epoxy sample is lower than 106 Xcm2 and the corresponding phase angle is about 12° after 20 days of immersion (Table 1). The negligible capacitive performance of the epoxy sample shows the electrolyte diffusion into the epoxy-steel interface and occurrence of the corrosion process [45]. Also, the considerable reduction in the high frequencies (10 kHz) values of phase angle confirms considerable decrease in the coating adhesion and barrier performance [12]. The|Z|10 mHz values are decreased after 40 days of immersion. However, the values of |Z|10 mHz increased after 60 days of immersion which could be related to considerable corrosion products formation. The electrolyte diffusion beneath the coating results in the creation of hydroxyl ions on the cathodic sites which can interact with Na+ cations, forming NaOH [46,47]. NaOH is a strong alkaline agent, leading to the pH rise at the coating/metal interface and as a result the adhesion bonds deterioration occurs. The |Z|10 mHz values for Silane-epoxy sample is one order of magnitude higher than the corresponding values for epoxy sample. Also, the (10 kHz) phase angle is close to 90°, indicating the better corrosion protection performance of the Silane-epoxy sample in comparison with Epoxy sample. However, the second time constant is observed in the Nyquist and Bode plots of Silane-epoxy sample which is due to the electrolyte diffusion into the coating-substrate interface. The |Z|10 mHz values for

Table 2 Extracted data from the EIS fitted plots of the scratched Silane-epoxy and GIN-epoxy samples after 24 h; the mean of three replicates is shown for each panel. Time (h)

Sample

Rct a Xcm2

CPEdl b Y0 (lX1 cm2)

nc

Cdl (lFcm2)

Rf d (Xcm2)

CPEf e Y0 (lX1 cm2)

nf f

1

Silane-epoxy GIN-epoxy Silane-epoxy GIN-epoxy Silane-epoxy GIN-epoxy Silane-epoxy GIN-epoxy

11,325 41,692 9837 43,010 9472 44,043 7568 39,882

70.06 5.835 117.6 3.965 141.01 4.221 0.167 3.487

0.852 0.857 0.825 0.847 0.829 0.823 0.718 0.788

78.79 5.67 143.03 3.85 170.72 4.58 229 4.125

1611 845 124 941 119 982 189 810

59.3 5.83 135.14 8.97 147.58 10.07 0.184 11.45

0.669 0.821 0.785 0.787 0.761 0.777 0.684 0.753

2 4 24 a b c d e f

The The The The The The

standard standard standard standard standard standard

deviation deviation deviation deviation deviation deviation

range range range range range range

for for for for for for

Rct values is between 2.2% and 9.5%. Y0(dl) values is between 3.% and 11%. n values is between 0.2% and 0.9%. Rf values is between 1.5% and 10%. Y0(f) values is between 2.5% and 8.7%. nf values is between 0.3% and 1.1%.

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resistance performance of the GIN-epoxy coating. So, the Silaneepoxy and GIN-epoxy samples with artificial defect were exposed to saline solution (NaCl 3.5 wt.%) for 24 h. The Nyquist and Bode plots of the immersed samples are illustrated in Fig. 10. The CPEdl is the non-ideal capacitance of double layer and consist of Y0 and n which are the admittance and exponent of CPE [54]. Therefore, the ideal double layer capacitance (Cdl) values were calculated from Eq. (1) [55]. fmax is the frequency at which the imaginary component of the impedance reaches to a maximum value.

Cdl ¼ Y0 :ð2:p:f max Þ

n1

ð1Þ

The resulted data obtained by the fitting process are listed in Table 2. The |Z|10 mHz values for the GIN-epoxy sample is one order higher than the values for Silane-epoxy coatings. Also, |Z|10 mHz values of Silane-epoxy decrease over time despite of corresponding values for the GIN-epoxy sample which have a negligible increment over time (24 h immersion). The higher corrosion resistance of the GIN-epoxy coating is related to the release of active agents from the coating into the scratch region. Two time constants are observed in the Bode-phase diagrams of Silane-epoxy and GINepoxy samples. These time constants are overlapped in Nyquist plots. The low-frequency (LF) time constant is due to the charge transfer resistance. The medium to high frequency range time constant may indicate the presence of the film precipitated on the steel surface inside defect [56]. The second time constant is more obvious after 4 h immersion in the Silane-epoxy coating, indicating the increase of the surface layer thickness. However, the Bodephase plot deviation of GIN-epoxy sample was not changed over time which could be corresponded to the surface film formation consisting of inhibitor complexes. The active inhibition performance of GIN-epoxy multi-layer was investigated by the variation of Rct values at different immersion times. According to Table 2, the Rct values for GIN-epoxy sample were increased after 4 h of immersion in saline solution. However, The Rct values for Silane-epoxy sample are decreased over time in the absence of active agents in the coating. The observed increasing trend in the Rct values for GIN-epoxy sample denotes partial recovery of the surface film and active inhibition effect. Also, the Cdl value for GIN-epoxy is reduced at longer immersion times. The Cdl reduction is attributed to the local dielectric constant reduction and is in a good accordance with Rct values [57,58]. Water molecules can be replaced by Nettle molecules on the metal surface, leading to the local dielectric constant reduction [59,60]. 3.2.2. Surface study FE-SEM-EDS analysis was used to confirm the active performance of the GIN-EP coating. So, the Silane-epoxy and GIN-epoxy samples with artificial defect were immersed in the saline solution. After 72 h of immersion, the scratch region of the samples was investigated by FE-SEM images and EDS elemental maps. The scratch region of Silane-epoxy and GIN-epoxy samples are illustrated in Fig. 11. The corrosion products are obvious in the scratch region of the Silane-epoxy coating while the corrosion product is negligible in the scratch region of GIN-epoxy sample. The EDS elemental maps of scratch regions in the Silane-epoxy and GIN-epoxy samples are shown in Fig. 12. The high oxygen concentration in the scratch region of Silane-epoxy coating shows the drastic corrosion occurrence and confirms the considerable corrosion products creation (Fig. 11(a)). Furthermore, the low oxygen and high iron concentration in the scratch region of GIN-epoxy sample confirm the reduction of corrosion rate in the presence of active agents in the coating structure. In the other hand, the Zn and C detection in the scratch region of GIN-epoxy coating

Fig. 11. FE-SEM micrographs showing surface of scratched regions in the: (a) Silane-epoxy and (b) GIN-epoxy samples exposed to saline solution (3.5 wt.% NaCl) for 72 h.

confirms the release of Zn2+ cations and Nettle molecules from the active coating and is in a good accordance with EIS measurements data resulted from the samples with artificial defects. The Zn2+ cations and Nettle molecules can be dissolved in the electrolyte and transport to the metal surface. The Fe2+ cations and OH anions are produced on the anodic and cathodic active sites, respectively (as it can be seen by Eqs. (1) and (2)).

2H2 O þ 2e ! H2 þ OH Fe ! Fe2þ þ 2e

ðcathodic reactionÞ

ðanodic reactionÞ

ð2Þ ð3Þ

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501

Fig. 12. EDS elemental (Fe, O, Zn, C) maps of (a) Silane-epoxy and (b) GIN-epoxy samples scratched sites exposed to saline solution (3.5 wt.% NaCl) for 72 h.

In the absence of inhibitive agents the corrosion process occurs and the corrosion products fill the scratch region (Fig. 11(a)). In the active GIN-epoxy coating, the released Zn2+ cations interact with the hydroxyl anions and precipitate on the cathodic sites (Eq.

(4)) [61]. Also, the active constituents of Nettle (such as serotonin and histamine) released from GINs could protonate in the saline electrolyte and interact with Zn2+ and Fe2+ cations, forming chelates (as it can be seen by Eqs. (5) and (6)). The interaction of the

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Nettle constituents with Zn2+ and Fe2+ cations is occurred through sharing their lone pair of electrons with 3d empty orbitals of metal cations [53]. The formed chelates precipitate on the anodic sites and diminish the corrosion process. This interpretation is in agreement with EIS (Fig. 10), FE-SEM (Fig. 11(b)), and EDS elemental map (Fig. 12(b)) results, confirming the considerable corrosion protection performance of the GIN-epoxy coating in the saline solution.

Zn2þ þ 2OH ! ZnðOHÞ2

ðat cathodic regionsÞ

C10 H12 N2 O þ Fe2þ =Zn2þ ! Zn2þ =Fe2þ . . . N2 OH12 C10 ðat anodic regionsÞ

ð4Þ

ð5Þ

C5 H9 N3 þ Fe2þ =Zn2þ ! Zn2þ =Fe2þ . . . N3 H9 C5 ðat anodic regionsÞ

ð6Þ

3.3. GIN-epoxy coating properties 3.3.1. Salt spray test The inhibitive performance of the GIN-epoxy coating was investigated by salt spray test. The Epoxy, Silane-epoxy, and GIN-epoxy coated panels with artificial defect were exposed to the salt fog and the time-dependent visual performance of them was studied. From Fig. 13 the corrosion process occurs in the scratch region of Epoxy sample in the presence of oxygen and corrosive ions after 1 week [3]. Also, some blisters are obvious on the epoxy coated panel surface. The saline electrolyte containing corrosive agents diffuse along coating-steel interface, leading to the loss of adhesion. The coating disbonding is increased with time and progressed on the metal surface. The observed blisters are created by cations diffusion into the coating/steel interface through conductive pathways. Subsequently, the cations migrate along the interface to the cathodic regions and neutralize the hydroxyl ions [5]. So, the coating delamination is associated with surface blisters are the main cause which responsible for the epoxy sample degradation. There are no obvious visual changes under salt spray test in Silane/epoxy

sample after 3 weeks. From Fig. 13(b), the coating delamination initiates and increases with time after 4 weeks. However, blisters aren’t appeared in the presence of silane sol-gel thin film. These two observations show that the disbonding and blistering rates are reduced by silane film utilization as an intermediate layer. The silane coatings are good mediators which can enhance the compatibility of the organic coatings and metal surface [62]. However, there is no active performance in the hybrid sol-gel layer and coating disbonding is inevitable in a long-term salt spray exposure period. The GIN-epoxy sample is intact after 8 weeks of exposure and there is no obvious coating disbonding or blisters on the coated steel surfaces. The better barrier performance and the active inhibition performance could be considered as the reasons of better visual performance of the GIN-epoxy samples against salt spray. The hybrid sol-gel layer in the GIN-epoxy contains nanocontainers based on Fe3O4 nanoparticles. The magnetite nanoparticles could act as barrier against diffusion of corrosive electrolyte into the coating-steel interface and in this way retard the blistering process. Also, the disbonding rate reduction is due to the Nettle molecules and Zn2+ cations releasing from the nanocontainers and inner silane layer, respectively. The heterocyclic compounds of Nettle constituents, which contained lone pair of electrons, could interact with transition metals (such as zinc and iron) and form metal-inhibitor complexes, leading to the anodic sites blocking [55,63]. Also, the hydroxyl ions are consumed by Zn2+ cations resulted in Zn(OH)2 deposition on the cathodic regions. 3.3.2. Cathodic disbonding The cathodic disbonding results are illustrated in Fig. 14. According to Fig. 14(a) the most disbonded area is observed for the epoxy coating which is about 4.9 cm2. The decrease in the delaminated area (Fig. 14(b)) is obvious for the Silane/epoxy coating (disbonded area: 2.5 cm2). The better cathodic disbonding resistance in the presence of hybrid sol-gel layer is attributed to the intermediate performance of the silane molecules which improved the epoxy/metal interfacial adhesion [64,65]. Finally, the results indicate that the lowest cathodic disbonding area is obtained for the GIN/epoxy panels (Fig. 14(c)). The corresponding

Fig. 13. Visual performance of the (a) Epoxy, (b) Silane-epoxy, and (c) GIN-epoxy samples with artificial defects exposed to salt spray test over time.

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Fig. 14. The cathodic disbonding performance of (a) Epoxy, (b) Silane-epoxy, and (c) GIN-epoxy samples after 24 h.

Fig. 15. The different mechanisms preventing cathodic disbonding in the GIN-epoxy sample.

area for the GIN/epoxy coating (0.8 cm2) is 6 times lower than epoxy coating which confirmed the significant improvements in the cathodic performance of the epoxy coating in the presence of the active nanocontainers. The corrosive agents diffuse along the coating-metal interface and corrosion reactions are occurred at active sites. The hydroxyl ions are generated under cathodic process, leading to the cations transportation from the bulk electrolyte to the active sites of polymer/steel interface [5,66]. By diffusion of the metal ions (Na+) and the creation of hydroxyl ions, a strong alkaline environment can be created at the cathodic sites [67]. The ferrous oxide layer in the coating/metal interface is reduced under alkaline condition, leading to adhesion loss between the epoxy coating and steel surface [68]. There are several mechanisms for reducing the coating cathodic delamination rate in the GIN-epoxy coated steel panels which are demonstrated in Fig. 15. The Nettle extract compounds consist of many electron-reach elements which can with interact with Fe2+ cations, resulting in the complex formation on the active sites of steel surface. So, the corrosion reactions are retarded in the presence of inhibitive surface layer, leading to the lower concentration of hydroxyl ions. Also, the zinc cations leached out from the active coating into the interface could interact with hydroxyl ions and prevent the coating delamination. The interaction of the Nettle constituents with Zn2+ cations (through sharing their lone pair of electrons with 3d empty orbitals, leading to the chelate formation

which is deposited on the metal surface) is the third mechanism which could be considered as inhibitive mechanism of the GINepoxy coating against cathodic delamination. Finally, the acetate anions could interact with the cations on the anodic sites in the coating/steel interface and retard the delamination process. 4. Conclusion 1. The preparation of the GIN-epoxy coating was proved by characterization of the hybrid sol-gel structure through FT-IR, FESEM, AFM, and EDS analysis. 2. The EIS and salt spray tests results indicated that the GIN-epoxy is a good inhibitive coating for the steel substrate against saline solution. 3. The active performance of the GIN-epoxy coatings was confirmed through EIS measurements on the coated samples with artificial defect over time. Also, FE-SEM images and EDS elemental maps confirmed the active inhibition performance of the GIN-epoxy coating. 4. The considerable cathodic disbonding performance of the GINepoxy coating was proved by cathodic disbonding test over time. The active agents (Nettle molecules and Zn2+ cations), releasing from the organic coating, is responsible for the significant cathodic disbonding protection.

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