- Email: [email protected]
Effects of graphene on the corrosion evolution of zinc particles in waterborne epoxy zinc-containing coatings
Progress in Organic Coatings 140 (2020) 105531
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Effects of graphene on the corrosion evolution of zinc particles in waterborne epoxy zinc-containing coatings
T
Shiyu Huanga, Gang Konga, Bo Yangb, Shuanghong Zhangb, Chunshan Chea,* a b
School of Material Science and Engineering, South China University of Technology, Guangzhou, 510640, China Guangzhou Special Pressure Equipment Inspection and Research Institute, Guangzhou, 510663, China
ARTICLE INFO
ABSTRACT
Keywords: Graphene Waterborne Zinc-containing coating Corrosion evolution
In this paper, graphene/ waterborne epoxy zinc-containing coatings with different graphene contents were prepared. The corrosion resistance properties of the graphene/ waterborne epoxy zinc-containing coatings were investigated by electrochemical impedance spectroscopy (EIS), immersion test and neutral salt spray test. The results showed that addition of 0.6 wt% graphene into the coating could remarkably improve its cathodic protection performance and barrier performance comparing with the coating without graphene. In addition, the effects of graphene on the corrosion evolution of zinc particles in the coating were observed by the field-emission scanning electron microscopy (FE-SEM) coupled with energy dispersive spectroscopy (EDS). The results showed that the zinc particles near the interface between steel substrate and the coating corroded firstly after the corrosive media diffused in the coating and reached the interface due to the cathodic protection to the steel, and then zinc particles continued to be corroded from the interface to upper part of the coating since they could provide the cathodic protection to the substrate thanks to the electrical connection of graphene. Meanwhile, Xray diffraction (XRD) patterns confirmed the corrosion products of the zinc particles were mainly consisted of Zn5(OH)8Cl2·H2O.
1. Introduction With the rapid development of marine engineering, such as ships, offshore wind power and offshore drilling platforms, the corrosion protection of steel structures has received increasing attention. Generally, the application of protective coating is one of the most effective approach used to protect steel structures anticorrosion [1–5]. Unlike the conventional coatings, zinc-rich coatings can provide an active cathodic protection for steel substrates even if the existence of mechanical damage in the binders during the its application or service time [6–8]. Among them, the zinc-rich waterborne epoxy coatings, which have the environmentally friendly binder and the low volatile organic compounds emitting, will be the future development trend of the paints [9]. In order to ensure zinc-rich coatings supplying a longer sacrificial service, an outstanding electron transport circuit between zinc particles and the metal matrix for galvanic action must occur, which means the ratio of pigment volume concentration (PVC)/critical pigment volume concentration (CPVC) should be > 1 [10]. However, the presence of a heavy amount of zinc particles in the coating results in many problems with high porosity, weak adhesion strength to the metal matrix and poor surface leveling [11]. Besides, the electrical connection
⁎
between zinc particles and steel substrate rapidly weakens during its service time as the result of the oxidation of zinc particles. Consequently, the sacrificial cathodic protection of the coating gradually dies away. Many attempts were performed to extend the cathodic protection duration of zin-rich coatings via the incorporation of electrically conductive fillers, such as carbon black [12], carbon nanotubes [11,13–15] and polyaniline [10] to reduce zinc particles concentration. Graphene, a monoatomic layer graphite sheet, is made up of sp2 hybridized carbon atoms closely packed into a two-dimensional (2D) honeycomb crystal structure [16]. In recent years, graphene shows the huge potential in anticorrosion coatings attributed to its exceptional properties, for example, ultrahigh specific surface regions, super impermeability and excellent conductivity [17–23]. Therefore, graphene is considered as a good electron conductor to form a galvanic cell between the iron substrate and the zinc particles even when the zinc particle concentration decreases [24–27]. Several attempts focus on investigating the role of graphene in enhancing the corrosion resistance of the zinc-rich coatings. Cheng [17] and Hayatdavoudi et al. [27] found similar conclusions that the addition of graphene can effectively enhance the activation rate of zinc particles and extend the cathodic protection duration of zinc-rich
Corresponding author. E-mail address: [email protected] (C. Che).
https://doi.org/10.1016/j.porgcoat.2019.105531 Received 28 November 2019; Received in revised form 22 December 2019; Accepted 26 December 2019 0300-9440/ © 2020 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
were added at 3000 rev/min and dispersed uniformly. Preparation of component B: the additive was added into the waterborne epoxy resin and stirred at 800 rev/min for sufficient dispersion. Graphene/ waterborne epoxy zinc-containing paint was obtained by mixing homogeneously component A and component B at 800 rev/min. The carbon steel panels with 3.5 × 3.5 × 0.2 cm3 dimensions were mechanically polished using 400, 600, 800 and 1000 mesh sandpaper to eliminate traces of surface oxide. Then the steel substrate is further cleaned with ethanol. Finally, the prepared paints were applied on the surface of the steel panels by air spray. The coated samples were cured at room temperature for 7 days. The coated samples with a Dry film thickness of 40 ± 3 μm were selected for further studies.
coating. Ding et al. [25] found that the zinc-rich coating in which magnetic graphene was self-aligned by uniform magnetic field during the film formation significantly enhanced the diffusion resistance of corrosive medium, but weakened the galvanic protection of the coatings. Ding et al. [24] studied the water permeation mechanism of graphene/zinc-rich coatings, and found that the corrosion potential of the coating has a period of fluctuation during the early stage of immersion, which is attributed to the shielding effect of graphene. At present, there are currently studies on the graphene/zinc-rich coatings that tend to investigate the improvement of the corrosion resistance performance of zinc-rich coating by introducing graphene. However, the effects of graphene on the corrosion evolution of zinc-rich waterborne epoxy coating, especially the zinc particles in the coating, have not been reported. In this paper, the effects of graphene on the anticorrosion performance of waterborne epoxy zinc-containing coatings were investigated by electrochemical impedance spectra (EIS), immersion testing and neutral salt spray test. The corrosion evolution of zinc particles in the composite coatings was observed by field-emission scanning electron microscopy (FE-SEM) coupled with energy dispersive spectroscopy (EDS).
2.3. Characterization The electrochemical measurement immersed in 3.5 % NaCl solution was carried out via an electrochemical workstation (CHI-660E, CH Instrument, Chenhua Co., China) with a conventional three electrode configuration consisted of a platinum as counter electrode, saturated calomel electrode (SCE) as reference electrode and coating sample as working electrode. The region of working electrode is 9 cm2. Electrochemical impedance spectroscopy (EIS) was carried out with a sinusoidal voltage signal of 15 mV and a frequency range of 10 mHz −100 kHz. In addition, the EIS results were fitted by Zview 3.0a software. Neutral salt spray test was conducted according to GB/T 1771–2007 in a salt spray chamber (Dongguan Zhongzhi Testing Instruments Co. Ltd, China). A 5 % NaCl solution was used for salt spray test with a constant chamber temperature of 35 °C. Corrosion evolution for the graphene/waterborne epoxy zinc-containing coating was characterized by field emission Scanning Electron Microscopy (FE-SEM, Nova NanoSEM 430, FEI, USA) with an accelerating voltage of 20.0 kV coupled with Energy Dispersive Spectroscopy (EDS). The cross-section morphologies of the coating samples were obtained by embedding the immersed coating samples in epoxy resin. The chemical component of the coating samples after immersion was characterized by X-ray Powder diffractometer (X'Pert3 Powder, PANalytical, The Netherlands) with Cu Kα radiation (λ = 0.15418 nm).
2. Experimental 2.1. Materials Graphene powders (layer thickness:1.5 nm; layer number: 3–5) were provided by Xiamen Knano Graphene Technology Co., Ltd and used without further purification. Zinc particles (spherical shapes; average particle diameters:3−5 μm) were provided by Guangdong Hosen New Materials Co., Ltd. Waterborne epoxy resin (Epikote 6520WH-53) and Waterborne epoxy curing agent (Epikure 8538-Y-68) were provided by Hexion Specialty Chemicals Inc. Other solvent and additive were provided by Shanghai Aladdin Bio-Chem Technology Co., LTD. 2.2. Preparation of graphene/ waterborne epoxy zinc-containing coating Five types of graphene/ waterborne epoxy zinc-containing coatings were prepared, and the ratio value of PVC/CPVC of the coating was 0.32. The graphene contents were 0, 0.4 %, 0.6 %, 0.8 % and 1 % by the weight of binder and were marked as WEZC, WEZC-0.4 %G, WEZC-0.6 %G, WEZC-0.8 %G and WEZC-1 %G, respectively. The graphene/ waterborne epoxy zinc-containing coating formulation corresponds to a two-component epoxy-polyamide primer. Preparation of component A: the waterborne epoxy curing agent, the suitable solvent and the graphene powder were stirred homogeneously at 2000 rev/min until graphene completely dispersed in the curing agent. Then zinc particles
3. Result and discussion 3.1. EIS measurement The protective properties and electrochemical properties of the coatings were investigated by EIS measurements. Fig. 1 depicts the Nyquist and Bode plots of all coating samples after 66 days immersion. In the case of WEZC (Fig. 1a1), the diameter of the semicircles in low
Fig. 1. EIS and fitting results of all coating samples: (a1, b1) WEZC, (a2, b2) WEZC-0.4 %G, (a3, b3) WEZC-0.6 %G, (a4, b4) WEZC-0.8 %G and (a5, b5) WEZC-1 %G. 2
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 2. The Equivalent circuit models.
frequency domain continued to decline, suggesting the corrosion resistance of WEZC decreases continuously. In terms of WEZC (Fig. 1b1), WEZC-0.4 %G (Fig. 1b2), WEZC-0.8 %G (Fig. 1b4) and WEZC-1 %G (Fig. 1b5), it should be noted that two evident time constants were observed in the Bode plots after 14days immersion, which indicating that the aggressive species passed through the matrix and started the corrosion reaction under the steel substrate [28]. However, the Bode plots of WEZC-0.6 %G (Fig. 1b3) did not change significantly after 14days immersion. Since the lowest frequency (|Z|0.01 Hz) impedance modulus was responsible for the impermeable capability of the coating [29], the value of impedance modulus for WEZC (Fig. 1b1) significantly decreased from 3.291 × 107 Ω cm2 (1day) to 5641 Ω cm2 after 66 days immersion. Meanwhile, the WEZC-0.6 %G (Fig. 1b3) exhibited the highest impedance modulus during the whole immersion period, and its value of impedance modulus was 1.287 × 108 Ω cm2 at the initial immersion, then decreased gradually to approximately 2.883 × 106 Ω cm2 after the 66day immersion, which was nearly 3 orders of magnitude larger than that of the WEZC. With respect to WEZC-0.4 %G, WEZC-0.8 %G and WEZC-1 %G (Fig. 1b2, b4 and b5), their values of impedance modulus were lower than that of WEZC-0.6 %G, but higher than WEZC. The observed results may be attributed to the layered structure of graphene which can block the diffusion of water and increase the length of diffusion path [30]. EIS data were fitted with equivalent circuit models (Fig. 2) used to characterize the corrosion behavior of the coating samples. Among the models, the equivalent circuit model (Fig. 2a) was used to simulate the corrosion behavior before the corrosive medium reaches the steel base. Moreover, the model (Fig. 2b) was employed to describe the service stage of the coating after the corrosion reaction takes place at the interface between the steel substrate and the coating. Here, Rs is the solution resistance, while Rpo and Rct represent the pore resistance and charge transfer resistance of the coating. CPEc and CPEdl denote the coating capacitance and double-layer capacitance, respectively. The Warburg element (W) was introduced to describe the diffusion behavior of the coating samples. Fig. 3 shows the variation in the value of Rpo and Rct of the coatings as a function of immersion time. Generally, Rpo and Rct are related to the barrier properties of a coating and the electrochemical corrosion of zinc particles, respectively [31,32]. As shown in Fig. 3a, as the immersion time extended, the Rpo value of the coatings gradually declined, implying the deterioration in corrosion resistance.
Importantly, Coatings with graphene showed a higher value of Rpo than the pure WEZC, and WEZC-0.6 %G exhibited the highest values of Rpo during the immersion period. This can be explained by the percolation of graphene on filling the free volume of the coating and improving the barrier performance of the coating [33]. Interestingly, Fig. 3b shows that the Rct value has a similar variation with Rct, indicting graphene acts as a barrier layer to protect the zinc particles and steel substrate from corrosion [32]. 3.2. Immersion test 3.2.1. Surface macro-morphology The surface photos of the coating samples after 66 days immersion in 3.5 wt% NaCl solution are presented in Fig. 4. As seen from the photos, the blisters occurred on the surface of the coatings decreased first and then increased with the graphene content during the immersion. Red rust caused by corrosion on steel substrate beneath the coating could lead to the formation of the blisters. Significantly, WEZC (Fig. 4a) demonstrates the largest size and number of blisters than other coating samples. By contrast, less blisters appeared on the surface of the graphene-containing coatings. In particular, the least number of blisters were observed on the surface of WEZC-0.6 %G (Fig. 4c), which clearly confirms that the coating with 0.6 wt% graphene exhibited the best protectiveness. This phenomenon is potentially due to the inclusion of moderate graphene that decelerate the corrosive species penetration through the coating by forcing the corrosive medium to travel a longer tortuous path to reach the substrate. However, the excessive graphene addition leads to the agglomeration of graphene in the coating, resulting in the high porosity, which significantly deteriorates the corrosion resistance of the coating [34–36]. The immersion result in Fig. 4 was in a good agreement with the above EIS results in Fig. 1. 3.2.2. Cross-section micro-morphology The corrosion evolution of zinc particles in the WEZC-0.6 %G were observed by FE-SEM coupled with EDS. Fig. 5 shows the cross-section micrographs images and EDS mapping spectra of the WEZC-0.6 %G at different immersion time. As seen from Fig. 5, many zinc particles were completely isolated with each other in this formulation, indicating a poor electrical connection existed between the zinc particles and the steel base. After 35days of immersion (Fig. 5a), no iron corrosion
Fig. 3. Evolution of pore resistance (a) and charge transfer resistance (b) for all coating samples over 66 days of immersion. 3
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 4. The Surface macro-morphology of all coating samples after 66days immersion in 3.5 wt% NaCl solution: (a) WEZC, (b) WEZC-0.4 %G, (c) WEZC-0.6 %G, (d) WEZC-0.8 %G and (e) WEZC-1 %G.
products were found at the interface between steel base and the coating. Notably, a large amount of cotton-like grew as the corrosion products were found on the surface of the zinc particles which located at or near the interface. However, a few cotton-like corrosion products could be found on the surface of the rest of the zinc particles which located far away from the metal substrate surface, especially the isolated zinc particles. Fig. 5a1–a3 shows the EDS mapping spectra corresponding to the Fig. 5a. As mentioned, a higher content of element O and Cl could be observed on the surface of those zinc particles which in contact with metal substrate and located in the range of about 5 μm
from the interface of metal substrate. This suggests that the corrosion of the part of zinc particles near the interface was occurred and the corrosion products were formed on the particles. This result may contribute to the addition of the highly conductive graphene and a better electrical connection between zinc particles to each other and the substrate, which forms a conductive network in the coating, leading to the formation of the galvanic cell at the interface between the zinc particles and the steel base, and consequently the zinc particles play a easily accessible sacrificial anode to protect the steel base from corrosion. So that more and more isolated zinc particles can participate in 4
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 5. Cross-section micromorphology images for WEZC-0.6 %G immersed in the 3.5 %Nacl solution at (a) 35days, (b) 42days, (c) 66days. The corresponding EDS mappings showing the distribution of element Zn (a1-a3), O (b1-b3), Cl (c1-c3) and Fe (c4).
5
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
less red rust occurred in the scribes on the WEZC-0.6 % G (Fig. 7c), revealing the effective role of graphene on the cathodic protection properties enhancement of zinc-containing coating. These observations reveal that inclusion of graphene into the WEZC could increase the cathodic protection performance of the coating. 3.3.2. Cross-section micro-morphology In order to further characterize the sacrificial anode performance of the zinc particles, all coating sample with X- scribes were subjected to the neutral salt spray accelerated corrosion test. Afterward, the crosssection of the scribe regions after salt spray test was investigated by FESEM. Fig. 8 reveals the SEM Cross-section morphology images of all coating samples exposed to the 5 %NaCl salt spray test for 24 h. As seen from Fig. 8, the zinc particles were highly embedded and dispersed in the waterborne epoxy matrix. One can observe that the scratch region of WEZC (Fig. 8a) was completely covered by iron corrosion product, which suggests that zinc particles could not play any sacrificial anode protection to the steel base. The iron corrosion products in the scribe decreased as the graphene content in the coating increases (Fig. 8b, c). When the graphene content in the coating is 0.6 % (Fig. 8c), less iron corrosion products can be found in the scribe. Fig. 9a depicts a highmagnification SEM image of the dashed box labeled as 1 in Fig. 8c. Fig. 9b–d depicts the EDS mapping results of the element Zn, O and Cl, respectively. As seen from Fig. 9, the corrosion products covered the zinc particles and have higher concentration of element O and Cl. The observed result suggests that the zinc particles can provide the sacrificial anode protection to the steel base in the scribe from corrosion, so the steel base in the scribe didn’t corrode but the zinc particles near the scribe corroded. It may be concluded that the presence of the high conductive graphene in the coating plays an effective role on the galvanic action in contact with the zinc particles and the steel base in the scribe and let the zinc particles has the sacrificial anode properties to the steel base. However, when the graphene content is continuing to increase in the coatings (Fig. 8d, e), it is noticed that there were more iron corrosion products not only emerged at the scribe regions, but also in the interface between the coating and the substrate. Since the excessive graphene in the coating is easy to agglomerate due to its high aspect ratio and strong van der Waals forces, the coating may form more pores and the agglomerated graphene may weaken its high conductive property and barrier to the diffusion of corrosive media in the coating [36,37].
Fig. 6. XRD patterns for the corrosive products of WEZC-0.6 %G after 66day immersion.
active cathodic protection process [27]. On the other hand, the corrosion products of zinc particles formed in the coating could also seal the pores in the coating and the barrier performance of the coating could be improved. When the immersion time was extended to 42 days, as shown in Fig. 5b, no iron corrosion products could be found at the interface, indicating that the steel base was still well protected. However, the corrosion process of zinc particles in the coating extends from the coating-steel base interface to the upper part of the coating, and the zinc corrosion products gradually becomes larger. Additionally, EDS mapping results (Fig. 5b1–b3) demonstrated a larger accumulation of O and Cl element appeared on the zinc particles surface and most of zinc particles located in the middle and bottom region of the coating have corroded, indicating that a good electrical connection existed between the zinc particles and the steel base [24], and more zinc particles provided the cathodic protection to the steel base from corrosion. This observation also suggested the inclusion of high conductive graphene was benefited to enhance the electrical connection of the zinc particles separated from each other in the coating and the barrier performance of the coating could be improved. When the immersion time was extended to 66 days, as shown in Fig. 5c, most zinc particles in the coating have corroded, and a layer of corrosion products occurred between the metal substrate and the coating. EDS mapping results (Fig. 5c1–c4) revealed the Cl and O element continue to increase. Interestingly, the O element appeared in the steel base, and the value of impedance modulus of WEZC-0.6 %G remarkably decreased (Fig. 1b3), indicating the zinc particles could not provide the cathodic protection to the steel base anymore and the steel started to corrode. Additionally, the corrosion products on the surface of the zinc particle in WEZC-0.6 %G after 66-days immersion is mainly consisted of simonkolleite (Zn5(OH)8Cl2·H2O) according to XRD analysis (Fig. 6).
3.4. Corrosion protection mechanism Fig. 10 illustrates the corrosion protective mechanism of the scribe and non-scribe regions of the graphene/ waterborne epoxy zinc-containing coating. In the initial stage (Fig. 10a), the corrosive media are more difficult to penetrate in the coating due to the good impermeable barrier property of the graphene and the epoxy resin in the non-scratch region. However, the corrosive media are easy to reach on the surface of steel base in the scratch region, but the steel will not be corroded since the zinc particles in the coating have good electrical connection with each other and the steel base due to the high conductive graphene and can provide the cathodic protection to the steel base in the scratch region. So, the zinc particles near the scratch region will be corroded. In the second stage (Fig. 10b), in term of the non-scribe region, the corrosive media finally reaches at the interface between the coating and the steel base by diffusing through longer path in the coating due to the barrier property of the graphene sheet. However, the iron at the interface will not be corroded since the existence of graphene forms a conductive network of the zinc particles themselves and the steel base, and the zinc particles near the interface can provide cathodic protection to the steel base and be corroded. The zinc particles near the scribe region constantly consumed for prolonging the galvanic protection. In
3.3. Neutral salt spray test 3.3.1. Surface macro-morphology Neutral Salt spray test was carried out to investigate the cathodic protection performance of all coating samples. Fig. 7 depicts the photographs of all coatings sample with X- scribes after 168 h exposure in neutral salt spray chamber. As can be seen, With the graphene content increases, the scale of red rust decreased first and then increased. Among them, the scale of red rust at scribes is the largest for WEZC (Fig. 7a) in comparison with coatings with graphene, exhibiting the poor cathodic protection performance of WEZC (Fig. 7a). Particularly,
6
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 7. The surface macro-morphology of all coating samples exposed to the 5 %Nacl solution for 168 h: (a) WEZC, (b) WEZC-0.4 %G, (c) WEZC-0.6 %G, (d) WEZC0.8 %G and (e) WEZC-1 %G.
the third stage (Fig. 10c), the zinc particles are corroded gradually from the interface to the upper of the coating in no-scratch region and continuing to provide the cathodic protection to the steel base due to the existence of the graphene. On the other hand, the corrosion products of zinc could fill the pores and defects of the coating and the barrier property of the coating can be enhanced. In the scribe region, the zinc particles near the scratch are covered by the corrosion products and lost its cathodic protection to the steel, and the corrosion of the steel base in the scratch starts. In the last stage (Fig. 10d), in no-scratch region, the corrosive media continue to penetrate to the interface, most zinc
particles are corroded and no cathodic protection to the steel base. The coating fails to protect the steel and the steel start to corrosion. 4. Conclusion The present study reveals the influence of graphene on the corrosion behavior of waterborne epoxy zinc-containing coating. In the initial corrosion stage, graphene acts a barrier role against the penetration of corrosive media due to their positive impermeability. As the corrosion time increases, graphene plays an electrical bridge to connect zinc 7
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 8. SEM Cross-section micrographs images of all coating samples exposed to the 5 %Nacl solution for 24 h: (a) WEZC, (b) WEZC-0.4 %G, (c) WEZC-0.6 %G, (d) WEZC-0.8 %G and (e) WEZC-1 %G.
particles, especially the isolated particles, and the steel base, leading to form a galvanic cell between those zinc particles and the steel. As a result, the zinc particles near the iron substrate play an easily accessible sacrificial anode to protect steel substrate against corrosion. Afterward, zinc particles continue to provide cathodic protection from the interface to upper part of the coating along the diffusion path of the corrosive medium thanks to the electrical connection of graphene. The addition of graphene can lower the concentration of zinc particles in the coating which can provide the cathodic protection to the steel.
Author statement The author's contribution to the article are as follows: Shiyu Huang: Methodology, performing the experiments, data and evidence collection, Writing - Original Draft, Gang Kong: Ideas, Writing Review & Editing, Bo Yang: Supervision, Shuanghong Zhang: Visualization, Chunshan Che: data analysis, communications with editorial process.
8
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 9. SEM morphology of the zinc particle and elemental maps obtained from the dashed box labeled as 1 in Fig. 8c.
Fig. 10. Illustration of corrosion protective mechanism for graphene/ waterborne epoxy zinc-containing coatings.
9
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Declaration of Competing Interest [18]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[19]
Acknowledgements
[20]
The authors would like to acknowledge the Guangdong Hosen New Materials Co., Ltd. for their special contribution in the coating preparation and Yi Shen for her contribution in the drawing of the schematic representation of corrosion protective mechanism.
[21]
References
[22]
[1] A.A. Javidparvar, R. Naderi, B. Ramezanzadeh, Epoxy-polyamide nanocomposite coating with graphene oxide as cerium nanocontainer generating effective dual active/barrier corrosion protection, Compos. Part B Eng. 172 (2019) 363–375, https://doi.org/10.1016/j.compositesb.2019.05.055. [2] M. Motamedi, M. Ramezanzadeh, B. Ramezanzadeh, S. Saadatmandi, Enhancement of the active/passive anti-corrosion properties of epoxy coating via inclusion of histamine/zinc modified/reduced graphene oxide nanosheets, Appl. Surf. Sci. 488 (2019) 77–91, https://doi.org/10.1016/j.apsusc.2019.05.180. [3] S. Pourhashem, A. Rashidi, M.R. Vaezi, M.R. Bagherzadeh, Excellent corrosion protection performance of epoxy composite coatings filled with amino-silane functionalized graphene oxide, Surf. Coat. Technol. 317 (2017) 1–9, https://doi. org/10.1016/j.surfcoat.2017.03.050. [4] B. Ramezanzadeh, A. Ahmadi, M. Mahdavian, Enhancement of the corrosion protection performance and cathodic delamination resistance of epoxy coating through treatment of steel substrate by a novel nanometric sol-gel based silane composite film filled with functionalized graphene oxide nanosheets, Corros. Sci. 109 (2016) 182–205, https://doi.org/10.1016/j.corsci.2016.04.004. [5] D. Yu, S. Wen, J. Yang, J. Wang, Y. Chen, J. Luo, Y. Wu, RGO modified ZnAl-LDH as epoxy nanostructure filler: a novel synthetic approach to anticorrosive waterborne coating, Surf. Coatings Technol. 326 (2017) 207–215, https://doi.org/10.1016/j. surfcoat.2017.07.053. [6] S.Y. Arman, B. Ramezanzadeh, S. Farghadani, M. Mehdipour, A. Rajabi, Application of the electrochemical noise to investigate the corrosion resistance of an epoxy zincrich coating loaded with lamellar aluminum and micaceous iron oxide particles, Corros. Sci. 77 (2013) 118–127, https://doi.org/10.1016/j.corsci.2013.07.034. [7] M. Jalili, M. Rostami, B. Ramezanzadeh, An investigation of the electrochemical action of the epoxy zinc-rich coatings containing surface modified aluminum nanoparticle, Appl. Surf. Sci. 328 (2015) 95–108, https://doi.org/10.1016/j.apsusc. 2014.12.034. [8] S. Shreepathi, P. Bajaj, B.P. Mallik, Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: role of Zn content on corrosion protection mechanism, Electrochim. Acta 55 (2010) 5129–5134, https://doi.org/10.1016/j. electacta.2010.04.018. [9] S. Liu, L. Gu, H. Zhao, J. Chen, H. Yu, Corrosion resistance of graphene-reinforced waterborne epoxy coatings, J. Mater. Sci. Technol. 32 (2016) 425–431, https://doi. org/10.1016/j.jmst.2015.12.017. [10] B. Ramezanzadeh, M.H. Mohamadzadeh Moghadam, N. Shohani, M. Mahdavian, Effects of highly crystalline and conductive polyaniline/graphene oxide composites on the corrosion protection performance of a zinc-rich epoxy coating, Chem. Eng. J. 320 (2017) 363–375, https://doi.org/10.1016/j.cej.2017.03.061. [11] S.M. Park, M.Y. Shon, Effects of multi-walled carbon nano tubes on corrosion protection of zinc rich epoxy resin coating, J. Ind. Eng. Chem. 21 (2015) 1258–1264, https://doi.org/10.1016/j.jiec.2014.05.042. [12] A. Meroufel, S. Touzain, EIS characterisation of new zinc-rich powder coatings, Prog. Org. Coat. 59 (2007) 197–205, https://doi.org/10.1016/j.porgcoat.2006.09. 005. [13] A. Gergely, Z. Pászti, J. Mihály, E. Drotár, T. Török, Galvanic function of zinc-rich coatings facilitated by percolating structure of the carbon nanotubes. Part II: Protection properties and mechanism of the hybrid coatings, Prog. Org. Coat. 77 (2014) 412–424, https://doi.org/10.1016/j.porgcoat.2013.11.004. [14] Y. Cubides, S.S. Su, H. Castaneda, Influence of zinc content and chloride concentration on the corrosion protection performance of zinc-rich epoxy coatings containing carbon nanotubes on carbon steel in simulated concrete pore environments, Corrosion 72 (2016) 1397–1423, https://doi.org/10.5006/2104. [15] Y. Cubides, H. Castaneda, Corrosion protection mechanisms of carbon nanotube and zinc-rich epoxy primers on carbon steel in simulated concrete pore solutions in the presence of chloride ions, Corros. Sci. 109 (2016) 145–161, https://doi.org/10. 1016/j.corsci.2016.03.023. [16] F. Yu, L. Camilli, T. Wang, D.M.A. Mackenzie, M. Curioni, R. Akid, P. Bøggild, Complete long-term corrosion protection with chemical vapor deposited graphene, Carbon N. Y. 132 (2018) 78–84, https://doi.org/10.1016/j.carbon.2018.02.035. [17] L. Cheng, C. Liu, D. Han, S. Ma, W. Guo, H. Cai, X. Wang, Effect of graphene on
[23]
[24] [25]
[26]
[27]
[28]
[29] [30]
[31] [32] [33] [34]
[35]
[36]
[37]
10
corrosion resistance of waterborne inorganic zinc-rich coatings, J. Alloys Compd. 774 (2019) 255–264, https://doi.org/10.1016/j.jallcom.2018.09.315. J.H. Ding, H.R. Zhao, Y. Zheng, X. Zhao, H. Bin Yu, A long-term anticorrsive coating through graphene passivation, Carbon N. Y. 138 (2018) 197–206, https://doi.org/ 10.1016/j.carbon.2018.06.018. Y. Hayatgheib, B. Ramezanzadeh, P. Kardar, M. Mahdavian, A comparative study on fabrication of a highly effective corrosion protective system based on graphene oxide-polyaniline nanofibers/epoxy composite, Corros. Sci. 133 (2018) 358–373, https://doi.org/10.1016/j.corsci.2018.01.046. B. Ramezanzadeh, E. Ghasemi, M. Mahdavian, E. Changizi, M.H. Mohamadzadeh Moghadam, Covalently-grafted graphene oxide nanosheets to improve barrier and corrosion protection properties of polyurethane coatings, Carbon N. Y. 93 (2015) 555–573, https://doi.org/10.1016/j.carbon.2015.05.094. B. Ramezanzadeh, Z. Haeri, M. Ramezanzadeh, A facile route of making silica nanoparticles-covered graphene oxide nanohybrids (SiO2-GO); fabrication of SiO2GO/epoxy composite coating with superior barrier and corrosion protection performance, Chem. Eng. J. 303 (2016) 511–528, https://doi.org/10.1016/j.cej.2016. 06.028. B. Ramezanzadeh, S. Niroumandrad, A. Ahmadi, M. Mahdavian, M.H. Mohamadzadeh Moghadam, Enhancement of barrier and corrosion protection performance of an epoxy coating through wet transfer of amino functionalized graphene oxide, Corros. Sci. 103 (2016) 283–304, https://doi.org/10.1016/j.corsci. 2015.11.033. N.N. Taheri, B. Ramezanzadeh, M. Mahdavian, Application of layer-by-layer assembled graphene oxide nanosheets/polyaniline/zinc cations for construction of an effective epoxy coating anti-corrosion system, J. Alloys Compd. 800 (2019) 532–549, https://doi.org/10.1016/j.jallcom.2019.06.103. R. Ding, Y. Zheng, H. Yu, W. Li, X. Wang, T. Gui, Study of water permeation dynamics and anti-corrosion mechanism of graphene/zinc coatings, J. Alloys Compd. 748 (2018) 481–495, https://doi.org/10.1016/j.jallcom.2018.03.160. R. Ding, S. Chen, N. Zhou, Y. Zheng, B. jun Li, T. jiang Gui, X. Wang, W. hua Li, H. bin Yu, H. wen Tian, The diffusion-dynamical and electrochemical effect mechanism of oriented magnetic graphene on zinc-rich coatings and the electrodynamics and quantum mechanics mechanism of electron conduction in graphene zinc-rich coatings, J. Alloys Compd. 784 (2019) 756–768, https://doi.org/10.1016/ j.jallcom.2019.01.070. R. Ding, X. Wang, J. Jiang, T. Gui, W. Li, Study on evolution of coating state and role of graphene in graphene-modified low-zinc waterborne epoxy anticorrosion coating by electrochemical impedance spectroscopy, J. Mater. Eng. Perform. 26 (2017) 3319–3335, https://doi.org/10.1007/s11665-017-2790-8. H. Hayatdavoudi, M. Rahsepar, A mechanistic study of the enhanced cathodic protection performance of graphene-reinforced zinc rich nanocomposite coating for corrosion protection of carbon steel substrate, J. Alloys Compd. 727 (2017) 1148–1156, https://doi.org/10.1016/j.jallcom.2017.08.250. N. Arianpouya, M. Shishesaz, M. Arianpouya, M. Nematollahi, Evaluation of synergistic effect of nanozinc/nanoclay additives on the corrosion performance of zinc-rich polyurethane nanocomposite coatings using electrochemical properties and salt spray testing, Surf. Coat. Technol. 216 (2013) 199–206, https://doi.org/10. 1016/j.surfcoat.2012.11.036. A. Amirudin, D. Thieny, Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals, Prog. Org. Coat. 26 (1995) 1–28, https://doi.org/10.1016/0300-9440(95)00581-1. A.C. Stoot, L. Camilli, S.A. Spiegelhauer, F. Yu, P. Bøggild, Multilayer graphene for long-term corrosion protection of stainless steel bipolar plates for polymer electrolyte membrane fuel cell, J. Power Sources 293 (2015) 846–851, https://doi.org/ 10.1016/j.jpowsour.2015.06.009. L.K. Wu, J.T. Zhang, J.M. Hu, J.Q. Zhang, Improved corrosion performance of electrophoretic coatings by silane addition, Corros. Sci. 56 (2012) 58–66, https:// doi.org/10.1016/j.corsci.2011.11.018. S. Teng, Y. Gao, F. Cao, D. Kong, X. Zheng, X. Ma, L. Zhi, Zinc-reduced graphene oxide for enhanced corrosion protection of zinc-rich epoxy coatings, Prog. Org. Coat. 123 (2018) 185–189, https://doi.org/10.1016/j.porgcoat.2018.07.012. Y.H. Yu, Y.Y. Lin, C.H. Lin, C.C. Chan, Y.C. Huang, High-performance polystyrene/ graphene-based Nanocomposites with Excellent Anti-corrosion Properties, (2014), https://doi.org/10.1039/c3py00825h. M. Ganjaee Sari, M. Shamshiri, B. Ramezanzadeh, Fabricating an epoxy composite coating with enhanced corrosion resistance through impregnation of functionalized graphene oxide-co-montmorillonite Nanoplatelet, Corros. Sci. 129 (2017) 38–53, https://doi.org/10.1016/j.corsci.2017.09.024. L. Gu, S. Liu, H. Zhao, H. Yu, Facile preparation of water-dispersible graphene sheets stabilized by carboxylated oligoanilines and their anticorrosion coatings, ACS Appl. Mater. Interfaces 7 (2015) 17641–17648, https://doi.org/10.1021/ acsami.5b05531. L.C. Tang, Y.J. Wan, D. Yan, Y.B. Pei, L. Zhao, Y.B. Li, L. Bin Wu, J.X. Jiang, G.Q. Lai, The effect of graphene dispersion on the mechanical properties of graphene/epoxy composites, Carbon N. Y. 60 (2013) 16–27, https://doi.org/10.1016/ j.carbon.2013.03.050. L.X. Gong, Y.B. Pei, Q.Y. Han, L. Zhao, L. Bin Wu, J.X. Jiang, L.C. Tang, Polymer grafted reduced graphene oxide sheets for improving stress transfer in polymer composites, Compos. Sci. Technol. 134 (2016) 144–152, https://doi.org/10.1016/j. compscitech.2016.08.014.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Effects of graphene on the corrosion evolution of zinc particles in waterborne epoxy zinc-containing coatings
T
Shiyu Huanga, Gang Konga, Bo Yangb, Shuanghong Zhangb, Chunshan Chea,* a b
School of Material Science and Engineering, South China University of Technology, Guangzhou, 510640, China Guangzhou Special Pressure Equipment Inspection and Research Institute, Guangzhou, 510663, China
ARTICLE INFO
ABSTRACT
Keywords: Graphene Waterborne Zinc-containing coating Corrosion evolution
In this paper, graphene/ waterborne epoxy zinc-containing coatings with different graphene contents were prepared. The corrosion resistance properties of the graphene/ waterborne epoxy zinc-containing coatings were investigated by electrochemical impedance spectroscopy (EIS), immersion test and neutral salt spray test. The results showed that addition of 0.6 wt% graphene into the coating could remarkably improve its cathodic protection performance and barrier performance comparing with the coating without graphene. In addition, the effects of graphene on the corrosion evolution of zinc particles in the coating were observed by the field-emission scanning electron microscopy (FE-SEM) coupled with energy dispersive spectroscopy (EDS). The results showed that the zinc particles near the interface between steel substrate and the coating corroded firstly after the corrosive media diffused in the coating and reached the interface due to the cathodic protection to the steel, and then zinc particles continued to be corroded from the interface to upper part of the coating since they could provide the cathodic protection to the substrate thanks to the electrical connection of graphene. Meanwhile, Xray diffraction (XRD) patterns confirmed the corrosion products of the zinc particles were mainly consisted of Zn5(OH)8Cl2·H2O.
1. Introduction With the rapid development of marine engineering, such as ships, offshore wind power and offshore drilling platforms, the corrosion protection of steel structures has received increasing attention. Generally, the application of protective coating is one of the most effective approach used to protect steel structures anticorrosion [1–5]. Unlike the conventional coatings, zinc-rich coatings can provide an active cathodic protection for steel substrates even if the existence of mechanical damage in the binders during the its application or service time [6–8]. Among them, the zinc-rich waterborne epoxy coatings, which have the environmentally friendly binder and the low volatile organic compounds emitting, will be the future development trend of the paints [9]. In order to ensure zinc-rich coatings supplying a longer sacrificial service, an outstanding electron transport circuit between zinc particles and the metal matrix for galvanic action must occur, which means the ratio of pigment volume concentration (PVC)/critical pigment volume concentration (CPVC) should be > 1 [10]. However, the presence of a heavy amount of zinc particles in the coating results in many problems with high porosity, weak adhesion strength to the metal matrix and poor surface leveling [11]. Besides, the electrical connection
⁎
between zinc particles and steel substrate rapidly weakens during its service time as the result of the oxidation of zinc particles. Consequently, the sacrificial cathodic protection of the coating gradually dies away. Many attempts were performed to extend the cathodic protection duration of zin-rich coatings via the incorporation of electrically conductive fillers, such as carbon black [12], carbon nanotubes [11,13–15] and polyaniline [10] to reduce zinc particles concentration. Graphene, a monoatomic layer graphite sheet, is made up of sp2 hybridized carbon atoms closely packed into a two-dimensional (2D) honeycomb crystal structure [16]. In recent years, graphene shows the huge potential in anticorrosion coatings attributed to its exceptional properties, for example, ultrahigh specific surface regions, super impermeability and excellent conductivity [17–23]. Therefore, graphene is considered as a good electron conductor to form a galvanic cell between the iron substrate and the zinc particles even when the zinc particle concentration decreases [24–27]. Several attempts focus on investigating the role of graphene in enhancing the corrosion resistance of the zinc-rich coatings. Cheng [17] and Hayatdavoudi et al. [27] found similar conclusions that the addition of graphene can effectively enhance the activation rate of zinc particles and extend the cathodic protection duration of zinc-rich
Corresponding author. E-mail address: [email protected] (C. Che).
https://doi.org/10.1016/j.porgcoat.2019.105531 Received 28 November 2019; Received in revised form 22 December 2019; Accepted 26 December 2019 0300-9440/ © 2020 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
were added at 3000 rev/min and dispersed uniformly. Preparation of component B: the additive was added into the waterborne epoxy resin and stirred at 800 rev/min for sufficient dispersion. Graphene/ waterborne epoxy zinc-containing paint was obtained by mixing homogeneously component A and component B at 800 rev/min. The carbon steel panels with 3.5 × 3.5 × 0.2 cm3 dimensions were mechanically polished using 400, 600, 800 and 1000 mesh sandpaper to eliminate traces of surface oxide. Then the steel substrate is further cleaned with ethanol. Finally, the prepared paints were applied on the surface of the steel panels by air spray. The coated samples were cured at room temperature for 7 days. The coated samples with a Dry film thickness of 40 ± 3 μm were selected for further studies.
coating. Ding et al. [25] found that the zinc-rich coating in which magnetic graphene was self-aligned by uniform magnetic field during the film formation significantly enhanced the diffusion resistance of corrosive medium, but weakened the galvanic protection of the coatings. Ding et al. [24] studied the water permeation mechanism of graphene/zinc-rich coatings, and found that the corrosion potential of the coating has a period of fluctuation during the early stage of immersion, which is attributed to the shielding effect of graphene. At present, there are currently studies on the graphene/zinc-rich coatings that tend to investigate the improvement of the corrosion resistance performance of zinc-rich coating by introducing graphene. However, the effects of graphene on the corrosion evolution of zinc-rich waterborne epoxy coating, especially the zinc particles in the coating, have not been reported. In this paper, the effects of graphene on the anticorrosion performance of waterborne epoxy zinc-containing coatings were investigated by electrochemical impedance spectra (EIS), immersion testing and neutral salt spray test. The corrosion evolution of zinc particles in the composite coatings was observed by field-emission scanning electron microscopy (FE-SEM) coupled with energy dispersive spectroscopy (EDS).
2.3. Characterization The electrochemical measurement immersed in 3.5 % NaCl solution was carried out via an electrochemical workstation (CHI-660E, CH Instrument, Chenhua Co., China) with a conventional three electrode configuration consisted of a platinum as counter electrode, saturated calomel electrode (SCE) as reference electrode and coating sample as working electrode. The region of working electrode is 9 cm2. Electrochemical impedance spectroscopy (EIS) was carried out with a sinusoidal voltage signal of 15 mV and a frequency range of 10 mHz −100 kHz. In addition, the EIS results were fitted by Zview 3.0a software. Neutral salt spray test was conducted according to GB/T 1771–2007 in a salt spray chamber (Dongguan Zhongzhi Testing Instruments Co. Ltd, China). A 5 % NaCl solution was used for salt spray test with a constant chamber temperature of 35 °C. Corrosion evolution for the graphene/waterborne epoxy zinc-containing coating was characterized by field emission Scanning Electron Microscopy (FE-SEM, Nova NanoSEM 430, FEI, USA) with an accelerating voltage of 20.0 kV coupled with Energy Dispersive Spectroscopy (EDS). The cross-section morphologies of the coating samples were obtained by embedding the immersed coating samples in epoxy resin. The chemical component of the coating samples after immersion was characterized by X-ray Powder diffractometer (X'Pert3 Powder, PANalytical, The Netherlands) with Cu Kα radiation (λ = 0.15418 nm).
2. Experimental 2.1. Materials Graphene powders (layer thickness:1.5 nm; layer number: 3–5) were provided by Xiamen Knano Graphene Technology Co., Ltd and used without further purification. Zinc particles (spherical shapes; average particle diameters:3−5 μm) were provided by Guangdong Hosen New Materials Co., Ltd. Waterborne epoxy resin (Epikote 6520WH-53) and Waterborne epoxy curing agent (Epikure 8538-Y-68) were provided by Hexion Specialty Chemicals Inc. Other solvent and additive were provided by Shanghai Aladdin Bio-Chem Technology Co., LTD. 2.2. Preparation of graphene/ waterborne epoxy zinc-containing coating Five types of graphene/ waterborne epoxy zinc-containing coatings were prepared, and the ratio value of PVC/CPVC of the coating was 0.32. The graphene contents were 0, 0.4 %, 0.6 %, 0.8 % and 1 % by the weight of binder and were marked as WEZC, WEZC-0.4 %G, WEZC-0.6 %G, WEZC-0.8 %G and WEZC-1 %G, respectively. The graphene/ waterborne epoxy zinc-containing coating formulation corresponds to a two-component epoxy-polyamide primer. Preparation of component A: the waterborne epoxy curing agent, the suitable solvent and the graphene powder were stirred homogeneously at 2000 rev/min until graphene completely dispersed in the curing agent. Then zinc particles
3. Result and discussion 3.1. EIS measurement The protective properties and electrochemical properties of the coatings were investigated by EIS measurements. Fig. 1 depicts the Nyquist and Bode plots of all coating samples after 66 days immersion. In the case of WEZC (Fig. 1a1), the diameter of the semicircles in low
Fig. 1. EIS and fitting results of all coating samples: (a1, b1) WEZC, (a2, b2) WEZC-0.4 %G, (a3, b3) WEZC-0.6 %G, (a4, b4) WEZC-0.8 %G and (a5, b5) WEZC-1 %G. 2
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 2. The Equivalent circuit models.
frequency domain continued to decline, suggesting the corrosion resistance of WEZC decreases continuously. In terms of WEZC (Fig. 1b1), WEZC-0.4 %G (Fig. 1b2), WEZC-0.8 %G (Fig. 1b4) and WEZC-1 %G (Fig. 1b5), it should be noted that two evident time constants were observed in the Bode plots after 14days immersion, which indicating that the aggressive species passed through the matrix and started the corrosion reaction under the steel substrate [28]. However, the Bode plots of WEZC-0.6 %G (Fig. 1b3) did not change significantly after 14days immersion. Since the lowest frequency (|Z|0.01 Hz) impedance modulus was responsible for the impermeable capability of the coating [29], the value of impedance modulus for WEZC (Fig. 1b1) significantly decreased from 3.291 × 107 Ω cm2 (1day) to 5641 Ω cm2 after 66 days immersion. Meanwhile, the WEZC-0.6 %G (Fig. 1b3) exhibited the highest impedance modulus during the whole immersion period, and its value of impedance modulus was 1.287 × 108 Ω cm2 at the initial immersion, then decreased gradually to approximately 2.883 × 106 Ω cm2 after the 66day immersion, which was nearly 3 orders of magnitude larger than that of the WEZC. With respect to WEZC-0.4 %G, WEZC-0.8 %G and WEZC-1 %G (Fig. 1b2, b4 and b5), their values of impedance modulus were lower than that of WEZC-0.6 %G, but higher than WEZC. The observed results may be attributed to the layered structure of graphene which can block the diffusion of water and increase the length of diffusion path [30]. EIS data were fitted with equivalent circuit models (Fig. 2) used to characterize the corrosion behavior of the coating samples. Among the models, the equivalent circuit model (Fig. 2a) was used to simulate the corrosion behavior before the corrosive medium reaches the steel base. Moreover, the model (Fig. 2b) was employed to describe the service stage of the coating after the corrosion reaction takes place at the interface between the steel substrate and the coating. Here, Rs is the solution resistance, while Rpo and Rct represent the pore resistance and charge transfer resistance of the coating. CPEc and CPEdl denote the coating capacitance and double-layer capacitance, respectively. The Warburg element (W) was introduced to describe the diffusion behavior of the coating samples. Fig. 3 shows the variation in the value of Rpo and Rct of the coatings as a function of immersion time. Generally, Rpo and Rct are related to the barrier properties of a coating and the electrochemical corrosion of zinc particles, respectively [31,32]. As shown in Fig. 3a, as the immersion time extended, the Rpo value of the coatings gradually declined, implying the deterioration in corrosion resistance.
Importantly, Coatings with graphene showed a higher value of Rpo than the pure WEZC, and WEZC-0.6 %G exhibited the highest values of Rpo during the immersion period. This can be explained by the percolation of graphene on filling the free volume of the coating and improving the barrier performance of the coating [33]. Interestingly, Fig. 3b shows that the Rct value has a similar variation with Rct, indicting graphene acts as a barrier layer to protect the zinc particles and steel substrate from corrosion [32]. 3.2. Immersion test 3.2.1. Surface macro-morphology The surface photos of the coating samples after 66 days immersion in 3.5 wt% NaCl solution are presented in Fig. 4. As seen from the photos, the blisters occurred on the surface of the coatings decreased first and then increased with the graphene content during the immersion. Red rust caused by corrosion on steel substrate beneath the coating could lead to the formation of the blisters. Significantly, WEZC (Fig. 4a) demonstrates the largest size and number of blisters than other coating samples. By contrast, less blisters appeared on the surface of the graphene-containing coatings. In particular, the least number of blisters were observed on the surface of WEZC-0.6 %G (Fig. 4c), which clearly confirms that the coating with 0.6 wt% graphene exhibited the best protectiveness. This phenomenon is potentially due to the inclusion of moderate graphene that decelerate the corrosive species penetration through the coating by forcing the corrosive medium to travel a longer tortuous path to reach the substrate. However, the excessive graphene addition leads to the agglomeration of graphene in the coating, resulting in the high porosity, which significantly deteriorates the corrosion resistance of the coating [34–36]. The immersion result in Fig. 4 was in a good agreement with the above EIS results in Fig. 1. 3.2.2. Cross-section micro-morphology The corrosion evolution of zinc particles in the WEZC-0.6 %G were observed by FE-SEM coupled with EDS. Fig. 5 shows the cross-section micrographs images and EDS mapping spectra of the WEZC-0.6 %G at different immersion time. As seen from Fig. 5, many zinc particles were completely isolated with each other in this formulation, indicating a poor electrical connection existed between the zinc particles and the steel base. After 35days of immersion (Fig. 5a), no iron corrosion
Fig. 3. Evolution of pore resistance (a) and charge transfer resistance (b) for all coating samples over 66 days of immersion. 3
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 4. The Surface macro-morphology of all coating samples after 66days immersion in 3.5 wt% NaCl solution: (a) WEZC, (b) WEZC-0.4 %G, (c) WEZC-0.6 %G, (d) WEZC-0.8 %G and (e) WEZC-1 %G.
products were found at the interface between steel base and the coating. Notably, a large amount of cotton-like grew as the corrosion products were found on the surface of the zinc particles which located at or near the interface. However, a few cotton-like corrosion products could be found on the surface of the rest of the zinc particles which located far away from the metal substrate surface, especially the isolated zinc particles. Fig. 5a1–a3 shows the EDS mapping spectra corresponding to the Fig. 5a. As mentioned, a higher content of element O and Cl could be observed on the surface of those zinc particles which in contact with metal substrate and located in the range of about 5 μm
from the interface of metal substrate. This suggests that the corrosion of the part of zinc particles near the interface was occurred and the corrosion products were formed on the particles. This result may contribute to the addition of the highly conductive graphene and a better electrical connection between zinc particles to each other and the substrate, which forms a conductive network in the coating, leading to the formation of the galvanic cell at the interface between the zinc particles and the steel base, and consequently the zinc particles play a easily accessible sacrificial anode to protect the steel base from corrosion. So that more and more isolated zinc particles can participate in 4
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 5. Cross-section micromorphology images for WEZC-0.6 %G immersed in the 3.5 %Nacl solution at (a) 35days, (b) 42days, (c) 66days. The corresponding EDS mappings showing the distribution of element Zn (a1-a3), O (b1-b3), Cl (c1-c3) and Fe (c4).
5
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
less red rust occurred in the scribes on the WEZC-0.6 % G (Fig. 7c), revealing the effective role of graphene on the cathodic protection properties enhancement of zinc-containing coating. These observations reveal that inclusion of graphene into the WEZC could increase the cathodic protection performance of the coating. 3.3.2. Cross-section micro-morphology In order to further characterize the sacrificial anode performance of the zinc particles, all coating sample with X- scribes were subjected to the neutral salt spray accelerated corrosion test. Afterward, the crosssection of the scribe regions after salt spray test was investigated by FESEM. Fig. 8 reveals the SEM Cross-section morphology images of all coating samples exposed to the 5 %NaCl salt spray test for 24 h. As seen from Fig. 8, the zinc particles were highly embedded and dispersed in the waterborne epoxy matrix. One can observe that the scratch region of WEZC (Fig. 8a) was completely covered by iron corrosion product, which suggests that zinc particles could not play any sacrificial anode protection to the steel base. The iron corrosion products in the scribe decreased as the graphene content in the coating increases (Fig. 8b, c). When the graphene content in the coating is 0.6 % (Fig. 8c), less iron corrosion products can be found in the scribe. Fig. 9a depicts a highmagnification SEM image of the dashed box labeled as 1 in Fig. 8c. Fig. 9b–d depicts the EDS mapping results of the element Zn, O and Cl, respectively. As seen from Fig. 9, the corrosion products covered the zinc particles and have higher concentration of element O and Cl. The observed result suggests that the zinc particles can provide the sacrificial anode protection to the steel base in the scribe from corrosion, so the steel base in the scribe didn’t corrode but the zinc particles near the scribe corroded. It may be concluded that the presence of the high conductive graphene in the coating plays an effective role on the galvanic action in contact with the zinc particles and the steel base in the scribe and let the zinc particles has the sacrificial anode properties to the steel base. However, when the graphene content is continuing to increase in the coatings (Fig. 8d, e), it is noticed that there were more iron corrosion products not only emerged at the scribe regions, but also in the interface between the coating and the substrate. Since the excessive graphene in the coating is easy to agglomerate due to its high aspect ratio and strong van der Waals forces, the coating may form more pores and the agglomerated graphene may weaken its high conductive property and barrier to the diffusion of corrosive media in the coating [36,37].
Fig. 6. XRD patterns for the corrosive products of WEZC-0.6 %G after 66day immersion.
active cathodic protection process [27]. On the other hand, the corrosion products of zinc particles formed in the coating could also seal the pores in the coating and the barrier performance of the coating could be improved. When the immersion time was extended to 42 days, as shown in Fig. 5b, no iron corrosion products could be found at the interface, indicating that the steel base was still well protected. However, the corrosion process of zinc particles in the coating extends from the coating-steel base interface to the upper part of the coating, and the zinc corrosion products gradually becomes larger. Additionally, EDS mapping results (Fig. 5b1–b3) demonstrated a larger accumulation of O and Cl element appeared on the zinc particles surface and most of zinc particles located in the middle and bottom region of the coating have corroded, indicating that a good electrical connection existed between the zinc particles and the steel base [24], and more zinc particles provided the cathodic protection to the steel base from corrosion. This observation also suggested the inclusion of high conductive graphene was benefited to enhance the electrical connection of the zinc particles separated from each other in the coating and the barrier performance of the coating could be improved. When the immersion time was extended to 66 days, as shown in Fig. 5c, most zinc particles in the coating have corroded, and a layer of corrosion products occurred between the metal substrate and the coating. EDS mapping results (Fig. 5c1–c4) revealed the Cl and O element continue to increase. Interestingly, the O element appeared in the steel base, and the value of impedance modulus of WEZC-0.6 %G remarkably decreased (Fig. 1b3), indicating the zinc particles could not provide the cathodic protection to the steel base anymore and the steel started to corrode. Additionally, the corrosion products on the surface of the zinc particle in WEZC-0.6 %G after 66-days immersion is mainly consisted of simonkolleite (Zn5(OH)8Cl2·H2O) according to XRD analysis (Fig. 6).
3.4. Corrosion protection mechanism Fig. 10 illustrates the corrosion protective mechanism of the scribe and non-scribe regions of the graphene/ waterborne epoxy zinc-containing coating. In the initial stage (Fig. 10a), the corrosive media are more difficult to penetrate in the coating due to the good impermeable barrier property of the graphene and the epoxy resin in the non-scratch region. However, the corrosive media are easy to reach on the surface of steel base in the scratch region, but the steel will not be corroded since the zinc particles in the coating have good electrical connection with each other and the steel base due to the high conductive graphene and can provide the cathodic protection to the steel base in the scratch region. So, the zinc particles near the scratch region will be corroded. In the second stage (Fig. 10b), in term of the non-scribe region, the corrosive media finally reaches at the interface between the coating and the steel base by diffusing through longer path in the coating due to the barrier property of the graphene sheet. However, the iron at the interface will not be corroded since the existence of graphene forms a conductive network of the zinc particles themselves and the steel base, and the zinc particles near the interface can provide cathodic protection to the steel base and be corroded. The zinc particles near the scribe region constantly consumed for prolonging the galvanic protection. In
3.3. Neutral salt spray test 3.3.1. Surface macro-morphology Neutral Salt spray test was carried out to investigate the cathodic protection performance of all coating samples. Fig. 7 depicts the photographs of all coatings sample with X- scribes after 168 h exposure in neutral salt spray chamber. As can be seen, With the graphene content increases, the scale of red rust decreased first and then increased. Among them, the scale of red rust at scribes is the largest for WEZC (Fig. 7a) in comparison with coatings with graphene, exhibiting the poor cathodic protection performance of WEZC (Fig. 7a). Particularly,
6
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 7. The surface macro-morphology of all coating samples exposed to the 5 %Nacl solution for 168 h: (a) WEZC, (b) WEZC-0.4 %G, (c) WEZC-0.6 %G, (d) WEZC0.8 %G and (e) WEZC-1 %G.
the third stage (Fig. 10c), the zinc particles are corroded gradually from the interface to the upper of the coating in no-scratch region and continuing to provide the cathodic protection to the steel base due to the existence of the graphene. On the other hand, the corrosion products of zinc could fill the pores and defects of the coating and the barrier property of the coating can be enhanced. In the scribe region, the zinc particles near the scratch are covered by the corrosion products and lost its cathodic protection to the steel, and the corrosion of the steel base in the scratch starts. In the last stage (Fig. 10d), in no-scratch region, the corrosive media continue to penetrate to the interface, most zinc
particles are corroded and no cathodic protection to the steel base. The coating fails to protect the steel and the steel start to corrosion. 4. Conclusion The present study reveals the influence of graphene on the corrosion behavior of waterborne epoxy zinc-containing coating. In the initial corrosion stage, graphene acts a barrier role against the penetration of corrosive media due to their positive impermeability. As the corrosion time increases, graphene plays an electrical bridge to connect zinc 7
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 8. SEM Cross-section micrographs images of all coating samples exposed to the 5 %Nacl solution for 24 h: (a) WEZC, (b) WEZC-0.4 %G, (c) WEZC-0.6 %G, (d) WEZC-0.8 %G and (e) WEZC-1 %G.
particles, especially the isolated particles, and the steel base, leading to form a galvanic cell between those zinc particles and the steel. As a result, the zinc particles near the iron substrate play an easily accessible sacrificial anode to protect steel substrate against corrosion. Afterward, zinc particles continue to provide cathodic protection from the interface to upper part of the coating along the diffusion path of the corrosive medium thanks to the electrical connection of graphene. The addition of graphene can lower the concentration of zinc particles in the coating which can provide the cathodic protection to the steel.
Author statement The author's contribution to the article are as follows: Shiyu Huang: Methodology, performing the experiments, data and evidence collection, Writing - Original Draft, Gang Kong: Ideas, Writing Review & Editing, Bo Yang: Supervision, Shuanghong Zhang: Visualization, Chunshan Che: data analysis, communications with editorial process.
8
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Fig. 9. SEM morphology of the zinc particle and elemental maps obtained from the dashed box labeled as 1 in Fig. 8c.
Fig. 10. Illustration of corrosion protective mechanism for graphene/ waterborne epoxy zinc-containing coatings.
9
Progress in Organic Coatings 140 (2020) 105531
S. Huang, et al.
Declaration of Competing Interest [18]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[19]
Acknowledgements
[20]
The authors would like to acknowledge the Guangdong Hosen New Materials Co., Ltd. for their special contribution in the coating preparation and Yi Shen for her contribution in the drawing of the schematic representation of corrosion protective mechanism.
[21]
References
[22]
[1] A.A. Javidparvar, R. Naderi, B. Ramezanzadeh, Epoxy-polyamide nanocomposite coating with graphene oxide as cerium nanocontainer generating effective dual active/barrier corrosion protection, Compos. Part B Eng. 172 (2019) 363–375, https://doi.org/10.1016/j.compositesb.2019.05.055. [2] M. Motamedi, M. Ramezanzadeh, B. Ramezanzadeh, S. Saadatmandi, Enhancement of the active/passive anti-corrosion properties of epoxy coating via inclusion of histamine/zinc modified/reduced graphene oxide nanosheets, Appl. Surf. Sci. 488 (2019) 77–91, https://doi.org/10.1016/j.apsusc.2019.05.180. [3] S. Pourhashem, A. Rashidi, M.R. Vaezi, M.R. Bagherzadeh, Excellent corrosion protection performance of epoxy composite coatings filled with amino-silane functionalized graphene oxide, Surf. Coat. Technol. 317 (2017) 1–9, https://doi. org/10.1016/j.surfcoat.2017.03.050. [4] B. Ramezanzadeh, A. Ahmadi, M. Mahdavian, Enhancement of the corrosion protection performance and cathodic delamination resistance of epoxy coating through treatment of steel substrate by a novel nanometric sol-gel based silane composite film filled with functionalized graphene oxide nanosheets, Corros. Sci. 109 (2016) 182–205, https://doi.org/10.1016/j.corsci.2016.04.004. [5] D. Yu, S. Wen, J. Yang, J. Wang, Y. Chen, J. Luo, Y. Wu, RGO modified ZnAl-LDH as epoxy nanostructure filler: a novel synthetic approach to anticorrosive waterborne coating, Surf. Coatings Technol. 326 (2017) 207–215, https://doi.org/10.1016/j. surfcoat.2017.07.053. [6] S.Y. Arman, B. Ramezanzadeh, S. Farghadani, M. Mehdipour, A. Rajabi, Application of the electrochemical noise to investigate the corrosion resistance of an epoxy zincrich coating loaded with lamellar aluminum and micaceous iron oxide particles, Corros. Sci. 77 (2013) 118–127, https://doi.org/10.1016/j.corsci.2013.07.034. [7] M. Jalili, M. Rostami, B. Ramezanzadeh, An investigation of the electrochemical action of the epoxy zinc-rich coatings containing surface modified aluminum nanoparticle, Appl. Surf. Sci. 328 (2015) 95–108, https://doi.org/10.1016/j.apsusc. 2014.12.034. [8] S. Shreepathi, P. Bajaj, B.P. Mallik, Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: role of Zn content on corrosion protection mechanism, Electrochim. Acta 55 (2010) 5129–5134, https://doi.org/10.1016/j. electacta.2010.04.018. [9] S. Liu, L. Gu, H. Zhao, J. Chen, H. Yu, Corrosion resistance of graphene-reinforced waterborne epoxy coatings, J. Mater. Sci. Technol. 32 (2016) 425–431, https://doi. org/10.1016/j.jmst.2015.12.017. [10] B. Ramezanzadeh, M.H. Mohamadzadeh Moghadam, N. Shohani, M. Mahdavian, Effects of highly crystalline and conductive polyaniline/graphene oxide composites on the corrosion protection performance of a zinc-rich epoxy coating, Chem. Eng. J. 320 (2017) 363–375, https://doi.org/10.1016/j.cej.2017.03.061. [11] S.M. Park, M.Y. Shon, Effects of multi-walled carbon nano tubes on corrosion protection of zinc rich epoxy resin coating, J. Ind. Eng. Chem. 21 (2015) 1258–1264, https://doi.org/10.1016/j.jiec.2014.05.042. [12] A. Meroufel, S. Touzain, EIS characterisation of new zinc-rich powder coatings, Prog. Org. Coat. 59 (2007) 197–205, https://doi.org/10.1016/j.porgcoat.2006.09. 005. [13] A. Gergely, Z. Pászti, J. Mihály, E. Drotár, T. Török, Galvanic function of zinc-rich coatings facilitated by percolating structure of the carbon nanotubes. Part II: Protection properties and mechanism of the hybrid coatings, Prog. Org. Coat. 77 (2014) 412–424, https://doi.org/10.1016/j.porgcoat.2013.11.004. [14] Y. Cubides, S.S. Su, H. Castaneda, Influence of zinc content and chloride concentration on the corrosion protection performance of zinc-rich epoxy coatings containing carbon nanotubes on carbon steel in simulated concrete pore environments, Corrosion 72 (2016) 1397–1423, https://doi.org/10.5006/2104. [15] Y. Cubides, H. Castaneda, Corrosion protection mechanisms of carbon nanotube and zinc-rich epoxy primers on carbon steel in simulated concrete pore solutions in the presence of chloride ions, Corros. Sci. 109 (2016) 145–161, https://doi.org/10. 1016/j.corsci.2016.03.023. [16] F. Yu, L. Camilli, T. Wang, D.M.A. Mackenzie, M. Curioni, R. Akid, P. Bøggild, Complete long-term corrosion protection with chemical vapor deposited graphene, Carbon N. Y. 132 (2018) 78–84, https://doi.org/10.1016/j.carbon.2018.02.035. [17] L. Cheng, C. Liu, D. Han, S. Ma, W. Guo, H. Cai, X. Wang, Effect of graphene on
[23]
[24] [25]
[26]
[27]
[28]
[29] [30]
[31] [32] [33] [34]
[35]
[36]
[37]
10
corrosion resistance of waterborne inorganic zinc-rich coatings, J. Alloys Compd. 774 (2019) 255–264, https://doi.org/10.1016/j.jallcom.2018.09.315. J.H. Ding, H.R. Zhao, Y. Zheng, X. Zhao, H. Bin Yu, A long-term anticorrsive coating through graphene passivation, Carbon N. Y. 138 (2018) 197–206, https://doi.org/ 10.1016/j.carbon.2018.06.018. Y. Hayatgheib, B. Ramezanzadeh, P. Kardar, M. Mahdavian, A comparative study on fabrication of a highly effective corrosion protective system based on graphene oxide-polyaniline nanofibers/epoxy composite, Corros. Sci. 133 (2018) 358–373, https://doi.org/10.1016/j.corsci.2018.01.046. B. Ramezanzadeh, E. Ghasemi, M. Mahdavian, E. Changizi, M.H. Mohamadzadeh Moghadam, Covalently-grafted graphene oxide nanosheets to improve barrier and corrosion protection properties of polyurethane coatings, Carbon N. Y. 93 (2015) 555–573, https://doi.org/10.1016/j.carbon.2015.05.094. B. Ramezanzadeh, Z. Haeri, M. Ramezanzadeh, A facile route of making silica nanoparticles-covered graphene oxide nanohybrids (SiO2-GO); fabrication of SiO2GO/epoxy composite coating with superior barrier and corrosion protection performance, Chem. Eng. J. 303 (2016) 511–528, https://doi.org/10.1016/j.cej.2016. 06.028. B. Ramezanzadeh, S. Niroumandrad, A. Ahmadi, M. Mahdavian, M.H. Mohamadzadeh Moghadam, Enhancement of barrier and corrosion protection performance of an epoxy coating through wet transfer of amino functionalized graphene oxide, Corros. Sci. 103 (2016) 283–304, https://doi.org/10.1016/j.corsci. 2015.11.033. N.N. Taheri, B. Ramezanzadeh, M. Mahdavian, Application of layer-by-layer assembled graphene oxide nanosheets/polyaniline/zinc cations for construction of an effective epoxy coating anti-corrosion system, J. Alloys Compd. 800 (2019) 532–549, https://doi.org/10.1016/j.jallcom.2019.06.103. R. Ding, Y. Zheng, H. Yu, W. Li, X. Wang, T. Gui, Study of water permeation dynamics and anti-corrosion mechanism of graphene/zinc coatings, J. Alloys Compd. 748 (2018) 481–495, https://doi.org/10.1016/j.jallcom.2018.03.160. R. Ding, S. Chen, N. Zhou, Y. Zheng, B. jun Li, T. jiang Gui, X. Wang, W. hua Li, H. bin Yu, H. wen Tian, The diffusion-dynamical and electrochemical effect mechanism of oriented magnetic graphene on zinc-rich coatings and the electrodynamics and quantum mechanics mechanism of electron conduction in graphene zinc-rich coatings, J. Alloys Compd. 784 (2019) 756–768, https://doi.org/10.1016/ j.jallcom.2019.01.070. R. Ding, X. Wang, J. Jiang, T. Gui, W. Li, Study on evolution of coating state and role of graphene in graphene-modified low-zinc waterborne epoxy anticorrosion coating by electrochemical impedance spectroscopy, J. Mater. Eng. Perform. 26 (2017) 3319–3335, https://doi.org/10.1007/s11665-017-2790-8. H. Hayatdavoudi, M. Rahsepar, A mechanistic study of the enhanced cathodic protection performance of graphene-reinforced zinc rich nanocomposite coating for corrosion protection of carbon steel substrate, J. Alloys Compd. 727 (2017) 1148–1156, https://doi.org/10.1016/j.jallcom.2017.08.250. N. Arianpouya, M. Shishesaz, M. Arianpouya, M. Nematollahi, Evaluation of synergistic effect of nanozinc/nanoclay additives on the corrosion performance of zinc-rich polyurethane nanocomposite coatings using electrochemical properties and salt spray testing, Surf. Coat. Technol. 216 (2013) 199–206, https://doi.org/10. 1016/j.surfcoat.2012.11.036. A. Amirudin, D. Thieny, Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals, Prog. Org. Coat. 26 (1995) 1–28, https://doi.org/10.1016/0300-9440(95)00581-1. A.C. Stoot, L. Camilli, S.A. Spiegelhauer, F. Yu, P. Bøggild, Multilayer graphene for long-term corrosion protection of stainless steel bipolar plates for polymer electrolyte membrane fuel cell, J. Power Sources 293 (2015) 846–851, https://doi.org/ 10.1016/j.jpowsour.2015.06.009. L.K. Wu, J.T. Zhang, J.M. Hu, J.Q. Zhang, Improved corrosion performance of electrophoretic coatings by silane addition, Corros. Sci. 56 (2012) 58–66, https:// doi.org/10.1016/j.corsci.2011.11.018. S. Teng, Y. Gao, F. Cao, D. Kong, X. Zheng, X. Ma, L. Zhi, Zinc-reduced graphene oxide for enhanced corrosion protection of zinc-rich epoxy coatings, Prog. Org. Coat. 123 (2018) 185–189, https://doi.org/10.1016/j.porgcoat.2018.07.012. Y.H. Yu, Y.Y. Lin, C.H. Lin, C.C. Chan, Y.C. Huang, High-performance polystyrene/ graphene-based Nanocomposites with Excellent Anti-corrosion Properties, (2014), https://doi.org/10.1039/c3py00825h. M. Ganjaee Sari, M. Shamshiri, B. Ramezanzadeh, Fabricating an epoxy composite coating with enhanced corrosion resistance through impregnation of functionalized graphene oxide-co-montmorillonite Nanoplatelet, Corros. Sci. 129 (2017) 38–53, https://doi.org/10.1016/j.corsci.2017.09.024. L. Gu, S. Liu, H. Zhao, H. Yu, Facile preparation of water-dispersible graphene sheets stabilized by carboxylated oligoanilines and their anticorrosion coatings, ACS Appl. Mater. Interfaces 7 (2015) 17641–17648, https://doi.org/10.1021/ acsami.5b05531. L.C. Tang, Y.J. Wan, D. Yan, Y.B. Pei, L. Zhao, Y.B. Li, L. Bin Wu, J.X. Jiang, G.Q. Lai, The effect of graphene dispersion on the mechanical properties of graphene/epoxy composites, Carbon N. Y. 60 (2013) 16–27, https://doi.org/10.1016/ j.carbon.2013.03.050. L.X. Gong, Y.B. Pei, Q.Y. Han, L. Zhao, L. Bin Wu, J.X. Jiang, L.C. Tang, Polymer grafted reduced graphene oxide sheets for improving stress transfer in polymer composites, Compos. Sci. Technol. 134 (2016) 144–152, https://doi.org/10.1016/j. compscitech.2016.08.014.