Investigating the corrosion protection mechanisms of graphene

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Investigating the corrosion protection mechanisms of graphene modified zinc-rich epoxy coatings in 3.5 wt% NaCl solution Lu Shen, Yong Li, Wenjie Zhao, Lijing Miao, weiping xie, Huanming Lu, and wang kui ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01821 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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Corrosion Protection of Graphene-Modified ZincRich Epoxy Coatings in Dilute NaCl Solution Lu Shena,b, Yong Lib, Wenjie Zhaoa*, Lijing Miaob, Weiping Xieb, Huanming Lub, Kui Wangb* a

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of

Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo, 315201, China b

Public Technology Service Center, Ningbo Institute of Materials Technology and Engineering,

Chinese Academy of Science, Ningbo, 315201, China KEYWORDS: Graphene, Zinc-Rich Coatings, Corrosion Protection Mechanisms, PANI.

ABSTRACT. In order to hold the excellent corrosion resistance of zinc-rich epoxy coatings with reduced zinc content, five types of graphene modified zinc-rich epoxy coatings (G-ZRCs) were prepared. Effect of zinc content on the corrosion protection behaviors of G-ZRCs was investigated using electrochemical impedance spectroscopy (EIS). Open circuit potential (OCP) was also utilized to deepen the understanding of anticorrosion mechanism of the as-prepared composite coatings. The results confirmed that addition of 0.3 wt% graphene could make the coating with 40 wt% (G-40ZRC) and 55 wt% zinc (G-55ZRC) provide cathodic protection for a short period of time. The EIS and OCP results showed a dominant physical barrier protection


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mechanism for the coatings without zinc (G-ZRC) while there was a mixed mechanism for G40ZRC and G-55ZRC and a dominant cathodic protection mechanism for the coatings with 70 wt% and 85 wt% zinc (G-70ZRC and G-85ZRC). Furthermore, the graphene modified zinc-rich coatings and corresponding corrosion products immersed in a 3.5% NaCl solution for 100 days were also characterized, which could further reveal anticorrosion mechanisms of the graphene modified zinc-rich coatings and passivation effect of polyaniline (PANI).

Introduction Steels are the one of the most important materials, which are widely used in our daily life. However, the corrosion phenomenon of steels in industrial application process must be taken seriously since it can cause huge economic losses. Common methods to protect the steels against corrosion is to use protection coatings or add corrosion inhibitor that can adsorb on the surface forming a protection layer1-2. The application of anticorrosive coatings is a cheap and effective method for protect steels against corrosion3-4. The barrier, adhesion as well as inhibiting functions have significant effects on the whole service lifetime of coatings. Diffusion of corrosive agents including water, oxygen and chloride ion into coating/substrate interface could make the coating blister, weaken the stability of adhesion bond and promote the degradation of the coating5-7. In the process of anticorrosion, protective coatings could prevent the diffusion of water and oxygen and eventually fail due to the existence of inherent defects8. Among various kinds of coatings (organic, inorganic and hybrid coatings)9-11, zinc-rich coatings (ZRCs) have received considerable attentions since 1930s. They are generally applied in severe environments since their outstanding property of protecting metals can be maintained even after slight mechanical damage of the coatings12. For ZRCs, polymer matrix barriers against


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corrosive agents. Among them, epoxy resins are the most commonly used binders for ZRCs because of their outstanding mechanical, chemical stability and high adhesive strength13. Anticorrosion properties of ZRCs mainly adhere to two protection mechanisms. Firstly, they could show cathodic protection through anodic dissolution when zinc particles contact with steel substrate. Secondly, corrosion products including ZnO, Zn(OH)2 as well as Zn5(CO3)2(OH)6 formed since zinc particles could react with O2, H2O and CO2 in the experiments. These corrosion products could seal the pores in the coatings and play a role of barrier against corrosive medium14. It is well known that cathodic protection lasts for a short time and polymer binders don't have good electrical conductivity. Hence, a large number of zinc particles (> 80 wt.%) contacting well with each other and also with substrate are necessary to prolong cathodic protection time. Nevertheless, excessive loading of zinc particles can cause the increase of porosity of coatings, decrease of coating flexibility, and weaker adhesion to the steel substrate1516.

Actually, some researchers have used carbon black17, aluminum pigment18, carbon

nanotube19-20 and graphene oxide21 as conductive fillers. The results showed that conductivity of the coatings was improved to a certain extent. However, the additive content of these fillers was relatively large due to their poor electrical conductivity, which could make the adhesion strength between filler and coating worse. Graphene is a single carbon layer of the graphite structure in its idealized form according to the IUPAC definition22. In the last decade, it has become a highly popular 2-dimentional material owing to its excellent properties such as good electronic conductivity, high mechanical strength, thermal conductivity and great specific surface area23. The unique microstructure and tiny particle size have made graphene material to be an extraordinary high performance additive of some coating systems24-25. Recently, some studies have been performed to investigate the


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mechanism of the graphene modified zinc-rich composite coatings for the protection of steel substrates26-27. They added graphene powder to zinc-rich nanocomposite coatings with large content of zinc (80 wt%) and discussed the effect of amount of graphene on the corrosion protection behaviors of zinc-rich nanocomposite coating. In view of previous researches, our hypothesis is that suitable addition of graphene in ZRCs could enhance the electrical conductivity between zinc particles and metal substrate and thus improving the utilization rate of zinc particles28. Additionally, the lamellar structure of graphene nanosheet with impermeable ability and chemical inertness makes it as an ideal candidate to improve the barrier properties of ZRCs. In this work, G-ZRCs with a fixed amount of 0.3 wt% graphene and five zinc contents (0, 40 wt%, 55 wt%, 70wt% and 85wt%) were systematically fabricated on the Q235 steel. PANI as noncovalent dispersion agent was used to promote the dispersion of graphene in the ZRCs. A passivated oxide layer forming on the surface of steels could protect the substrate against corrosion. The corrosion protection behaviors of G-ZRCs were evaluated by EIS and OCP measurements. Evaluation of the corrosion performance of these systems was also observed by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy to verify the electrochemical results. The role of graphene and zinc particles in the G-ZRCs played at different corrosion stages was revealed clearly and would pave a way for exploring highly efficient anti-corrosion coatings. 2. Experimental 2.1 Materials


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Polyaniline was purchased from Aladdin Industrial Corporation. Tetrahydrofuran (THF), sodium chloride, xylene were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemical reagents and solvent were used as received without purification. Epoxy resin 601 and curing agent (650-polyamide) were kindly supplied by SM Chemical Industry Co., Ltd. China. Graphene was purchased from Morsh Co., Ltd. China. Zinc particles are kindly supplied by Xin Weilin Zinc Co., Lit. China. 2.2 Preparation of PANI-G dispersion The method for preparing PANI-G suspension is as follows. 0.0385 g PANI was dissolved in 15.4 mL THF and then sonicated for 0.5 h. A dark blue dispersion was obtained. After that, 0.077 g of graphene was mixed with PANI/THF dispersion and with sonication for 3 h to obtain homogeneous PANI-G dispersion. In this work, the graphene content in the coatings was 0.3 wt%. In our previous work29, we found that the appropriate addition of graphene in the coating is about 0.5 wt%. It is reported that zinc rich nanocomposite coating with 0.4 wt% graphene exhibited superior corrosion protection performance as compared with zinc rich coatings without and with 0.1 wt% graphene particles26, which verified that the content of graphene in coatings had an effect on the corrosion protection performance of the coatings. We chose 0.3 wt% graphene, which was closed to the values in literatures. 2.3 Preparation of G-ZRCs Firstly, the work electrodes were degreased by sonicating in acetone for 0.5 h and polished by 400, 800, and 1200 mesh sandpapers, respectively. Then the electrode surfaces were rinsed by distilled water, quickly dried with nitrogen and kept dry before coating. The graphene modified coatings containing 0 wt%, 40 wt%, 55wt%, 70 wt% and 85 wt% of zinc were prepared as


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follows. A precalculated epoxy 601 was added to the prepared PANI-G dispersion with agitation for 15 min and sonication for 0.5 h to get a homogeneous mixture. Afterwards, THF in the mixture was eliminated by rotary evaporation. A precalculated zinc particles were added to the above mixture and carefully stirred for 10 min. Subsequently, curing agents (30 wt% of epoxy 601) was added into the mixture, and then the mixture was stirred for 15 min. After that, the paints were degassed in a vacuum oven to eliminate the air bubbles existed in the paint at room temperature. G-ZRCs with different component were coated on the work electrodes by using a wire bar coater, and the coatings were cured in ambient conditions for 4 days. The process was shown in Fig.1. The coatings thickness was controlled by 65±2 μm.

Fig. 1 A scheme to illustrate the fabrication of the G-ZRCs

2.4 Characterization


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The UV-vis spectra of PANI and PANI-G dispersion were obtained with Lambda 950 spectroscope. Raman spectra were obtained through the confocal Raman spectrometer (Super LabRam II system) using the wavelength of 532 nm. The morphologies of graphene, PANI, PANI-G hybrid, surfaces and fracture surfaces of coatings were characterized by SEM (FEI QUANTA 250, USA). The morphology of PANI-G hybrid was also estimated by scanning probe microscopy (SPM, VeecoMultiMode/NanoScopeIIIa) and Transmission electron microscope (TEM, Tecnai F20, USA). X-ray photoelectronic spectroscopy was performed to estimate the chemical structure of surfaces of coatings with a Kratos AXIS ULTRA Multifunctional XPS using Al (mono) Kα radiation (1486.6 eV) under 1.2×10-9 Torron. The crystal structure of the steel surfaces was evaluated by X-ray diffraction (XRD) with a Bruker AXS X-ray diffractometer. The electrochemical measurements of the coatings on Q235 electrodes were conducted with CHI-660E electrochemical work station by utilizing a three-electrode cell (Q235 electrode of 1 cm2 area, platinum sheet with 2.5 cm2 area and saturated calomel electrode). EIS data at different time intervals were collected in the range of 10 mHz to 100 kHz using an alternating current signal with amplitude of 20 mV.


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Fig. 2 (a-c) SEM results of graphene nanosheets, PANI and PANI-G hybrid, (d)TEM result of PANI-G hybrid and its corresponding SAED, (e)(f) AFM images of PANI-G hybrid and graphene nanosheets. 3. Results and discussion 3.1 Preparation and morphology of PANI-G hybrid Non-destruction dispersion of graphene nanosheets is necessary in order to maintain the perfect sp2 structure of graphene carbon atoms. It is well known that graphene sheets tend to aggregate due to the strong interlayer van der Waals interactions. Hence, it is enormously significant to disperse graphene in solvent or polymer matrix in order to achieve the best performance30,31. Since both graphene and PANI are solid powder, we could not directly measure the thickness of graphene only using PANI as binder. THF is a kind of organic solvent, which


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has been used to dissolve graphene32. In this work, we observed that graphene nanosheets could be stably dispersed in THF with about 5 mg/mL utilizing PANI as dispersing agent. The amount of PANI in all of the coatings was the same and the amount was very small (0.15 wt%). Therefore, the difference of anticorrosive properties among the coatings was independent of PANI. The morphology of graphene, PANI and PANI-G hybrid were evaluated by SEM, TEM and SPM, respectively. It is observed from Fig. 2b that the PANI powder exhibited a typical nanofibrous structure. The powder is uniformly attached to graphene sheets which could promote the dispersion of graphene, as shown in Fig. 2c. TEM image of PANI-G hybrid illustrated that wrinkled graphene sheets presented a typical exfoliated layer structure confirming an effectively disperse graphene sheets. Selected area electron diffraction (SAED) pattern presented a clear hexagonal spots pattern (inset of Fig. 2d), indicating that graphene was well-crytallized. SPM images corroborated the thickness of PANI-G nanosheets was in the range of 1-2 nm while the thickness of graphene sheets in THF was much more greater (19.60 nm and 23.20 nm) (Fig. 2ef). Based on the above results, it can be concluded that well exfoliated graphene sheets were successfully prepared by employing PANI as noncovalent dispersing agent.


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Fig. 3 (a) UV-vis spectra of PANI and PANI-G hybrid, (b) Raman spectra of PANI, G and PANI-G hybrid, (c) Full spectrum of PANI-G hybrid, (d) N 1s core level spectrum, (e) C 1s core level spectrum. 3.2 Characterization of the PANI-G suspension Fig. 3a showed UV-Vis results of PANI and PANI-G hybrid dispersed in THF. The peaks of PANI located at 313 nm and 604 nm corresponded to π-π* transition of benzene unit and n-π* transition of benzenoid to quinoid, respectively33. With regard to PANI-G hybrid, the peak located at 604 nm shifted to 597 nm, confirming the π-π interaction between PANI molecules and graphene nanosheets. Fig. 3b presented Raman spectra of PANI, graphene and PANI-G hybrid, respectively. The characteristic peak of PANI around 1410 cm-1 was attributed to C-N+ stretching of radical cation. The strong peak located at 1490 cm-1 was the characteristic peak of benzene ring. The bands


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located at 1595 cm-1 and 1617 cm-1 corresponded to quinoid and semiquinoid bands, respectively34. G band and D band of graphene were located at 1580 cm-1 and 1345 cm-1. For PANI-G hybrid, the G band of graphene shifted from 1580 cm-1 to 1583 cm-1, which indicated charge transfer existed between graphene and PANI. Moreover, 2D band of PANI-G shifted to 2718 cm-1 compared with graphene due to the π-π interaction between graphene and PANI. XPS measurement was utilized to analyze the chemical composition of PANI-G hybrid (Fig. 3c-e). Fig. 3c presented that C, N and O elements existed in PANI-G hybrid. Among them, N came from amine group of PANI, and O was from the residue of graphene. Fig. 3d presented the N 1s core level spectrum of PANI-G hybrid, which confirmed the existence of protonated nitrogen (-N+=) from the doped state of PANI. In Fig. 3e, EB of sp2 carbon atoms (C=C) and sp3 carbon atoms (C-C) centered at 284.5 eV and 285.2 eV, respectively. So the peak positions of them are relatively close. In addition, the fwhm (full width at half maximum) of sp3 peak was 1, and the fwhm of sp2 was 1.1, which illustrated not all of sp3 (C-C) peak was wrapped in sp2 (C=C) peak. The weak peaks located at 286.04 eV, 287.2 eV and 288.45 were corresponded to C-O/C-O-C, C=O and O-C=O of graphene, respectively. The peak centered at 291.05 eV was attributed to π-π* band of graphene and PANI35. 3.3 Morphology of G-ZRCs Pigment volume concentration (PVC) of individual coating was adjusted so as to achieve a total zinc content of 0, 40 wt%, 55 wt%, 70 wt% and 85 wt% (marked as G-ZRC, G-40ZRC, G55ZRC, G-70ZRC and G-85ZRC, respectively). In order to observe the fracture surfaces of the coatings, all of the coatings were immersed into liquid nitrogen for several seconds before the breaking process. Graphene is much lighter than zinc, paint is very viscous after THF in the


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solution was removed by rotary evaporation and zinc particles were added to the above solution. So graphene could not concentrate at the surface of the coating. The cross-section photos could prove it (Fig. 4). In the cross-section photos, we could observe graphene dispersed uniformly in the coatings. In terms of G-ZRC (Fig. 4a), the fracture surface was not smooth because of the existed graphene nanosheets in the coating. The fracture surface of coatings got rougher with more content of zinc particles. Moreover, it is observed that graphene nanosheets were homogeneously dispersed in the coatings.

Fig. 4 SEM images of fracture surfaces of (a)G-ZRC, (b) G-40ZRC, (c) G-55ZRC, (d) G70ZRC, (e) G-85ZRC. 3.4 Anticorrosion performance of the PANI-G reinforced zinc-rich epoxy coatings 3.4.1 Open circuit potential (OCP) Electrochemical properties of G-ZRCs coated electrodes can be evaluated just by measuring the OCP values of the coatings12, 17. Fig. 5a presented the variation trend of OCP for G-ZRCs


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immersed in the 3.5 wt% NaCl solution for 100 days. OCP values of G-ZRC lay in the range of 0.5 V ~0 V. Following the 60 days of immersion, the value remained around -0.3 V until the immersion test was over. In order to clarify the effect of graphene on the ZRCs, OCP measurement for ZRC (70 wt% Zn) without the addition of graphene (70ZRC) was also conducted. We found that the OCP values of 70ZRC were positive than the corrosion potential of iron. The coating was prepared in the laboratory by ourselves instead of from manufacturing enterprise. We did not add conductive materials (e.g. carbon black) in the coating. Since zinc content in the coating was not enough to form continuous contact between zinc particles, OCP values of 70ZRC are positive than the corrosion potential of iron. However, it is worth noting that OCP values of G-40ZRC and G-55ZRC lay in cathodic protection region within the first 5 days, which may suggest that graphene improved the electrical connection between the small amounts of zinc particles and enhanced the utilization ratio of zinc particles which allowed to afford cathodic protection to the steel substrate. After immersion for 5 days, the OCP values tended to increase, which was attributed to the formation of corrosive products of zinc. In detail, the oxidation products with poor conductivity could lead to the reduction of electrical connectivity among the zinc particles and between zinc particles and steel substrate. In Fig. 5a, OCP results of G-70ZRC stayed in cathodic protection region for 13 days. Then, the OCP value moved to a more positive region. The OCP values of G-85ZRC stayed in cathodic protection region for a longer time. Coatings with higher zinc content presented superior sacrificial protection since higher zinc content together with graphene nanosheets can do good to promote the electrical connectivity between the zinc particles and metal substrate.


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Fig. 5(a) OCP evolution for G-ZRCs, (b) Evolution of Zf=0.01Hz for different G-ZRCs systems during the 100 days' immersion test. 3.4.2 EIS spectra analysis 3.4.2.1 G-ZRC EIS was utilized to study the anticorrosive performance of the coatings. Generally speaking, a coating's barrier performance could be evaluated according to the impedance modulus at the lowest frequency during the whole immersion test. The initial Zf=0.01Hz value of G-ZRC is 1.52× 1010 Ω·cm2 (Fig. 5b). However, the Zf=0.01Hz value decreased to 3.17 × 106 Ω · cm2 after 60 days of immersion, suggesting a prominent deterioration of the coating after absorbing large amounts of water. Subsequently, the value stayed around it during the whole immersion time. In the process of immersion, graphene nanosheets existed in the coating could make the path of corrosive agents more tortuous due to the physical barrier function of the nanosheets, which could extend the protection time32.


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3.4.2.2 G-40ZRC and G-55ZRC Impedance plots of the ZRCs with different contents of zinc during the immersion test of 100 days were shown in Fig. 6. During the entire immersion test, both G-40ZRC and G-55ZRC exhibited high Zf=0.01Hz (108 ~ 1010 Ω·cm2) which indicated that the two coatings had excellent corrosion protection. The good corrosion protection property should owe to the cathodic protection and low porosity of the coatings which could promote the zinc particles in the coating thoroughly wetted by the epoxy binder. In addition to improving the electrical connectivity among the zinc particles, graphene nanosheets can also make the path to the substrate of the corrosive medium become longer. 3.4.2.3 G-70ZRC It is reported that ZRCs could provide sacrificial or galvanic protection when zinc content in the coating is > 70 wt% (on dry film)15. The impedance spectra for G-70ZRC were shown in Fig. 6a4-c4. In this work, coating system was studied with a mixed amount of 0.3 wt% well-dispersed graphene and 70% zinc content (G-70ZRC). According to the OCP results in Fig.5a, 70ZRC failed to offer cathodic protection. However, G-70ZRC could provide cathodic protection due to the graphene nanosheets added in the coating increased the utilization ratio of zinc particles. According to Fig.6b4, the impedance increased significantly of 60 days and 80 days, and then decreased significantly after 100 days. Based on OCP results, G-70ZRC could provide cathodic protection within 13 days immersion. Then, corrosion production of zinc produced. The impedance reached the maximum after 80 days' immersion and the value of Cc was very high since the number of zinc corrosion production reached the maximum after 80 days' immersion. It is observed that the impedance decreased significantly after 100 days, which illustrates the


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corrosion medium has penetrated the substrate and the coating lost effectiveness. Additionally, it can be seen in Fig. 6c4, there were two time constant after 80 days immersion, confirming the existence of corrosion protection of the coating. However, there was just one time constant after 100 days immersion, which illustrated the porosity of the coating and the foaming zone of the interface between the coating and substrate were very large. The coating has lost protection effect36. For G-70ZRC, porosity increased as the zinc content reached 70 wt%, the graphene nanosheets could not seal all of the pores in the coating. Then, the coating began to absorb water. The Bode plot showed Zf=0.01Hz with a prominent decrease between 104 ~ 106 Ω·cm2 compared with other samples. This phenomenon might be related to a capillary transport mechanism caused by the high porosity of the coating, which makes high galvanic influence by the zinc particles37. It is worth mentioning that the presence of graphene could promote the electrical connectivity among the independent zinc particles and thus provide available zinc active particles. The existence of large numbers of zinc particles brought a high anode to cathode surface area ratio and more dissolution of zinc. It is in consistent with OCP results that the sample could provide cathodic protection within 13 days immersion.


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Fig. 6 Time-dependant Nyquist and Bode plots of the coatings during the whole immersion test of 100 days. (a1-a3) G-ZRC, (b1-b3) G-40ZRC, (c1-c3) G-55ZRC, (d1-d3) G-70ZRC, (e1-e3)G85ZRC.


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3.4.2.4 G-85ZRC Fig. 6a5-c5 showed the impedance spectra for G-85ZRC at different exposure time. For G85ZRC, high content of zinc particles led to too much porosity of the coating thereby penetration of water and fast activation of zinc particles occurring. The cathodic protection period lasted longer time compared to G-70ZRC because of higher amount of zinc particles which was in accordance with the OCP values. The forming of more conductive paths could be promoted by the higher zinc amount through the connection among zinc particles or the connection between zinc particles and steel substrate by graphene nanosheets. Additionally, the longer cathodic protection period should also be attributed to conductive graphene that can activate zinc particles. In the later stage of immersion, Zf=0.01Hz increased significantly, which was due to the barrier properties provided by large amount of zinc corrosion products produced and the epoxy binder.


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Fig. 7 (a1)(a2)Equivalent electric circuits of the collected EIS results of G-ZRC. (b1)(b2)Scheme illustration of G-40ZRC and G-55ZRC, (c1)(c2)Scheme illustration of G-70ZRC and G-85ZRC. (d1)(d2) Equivalent electric circuits of the collected EIS results of G-40ZRC, G-55ZRC, G70ZRC and G-85ZRC. 3.4.4 Electrical equivalent circuit analysis


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The EIS results of G-ZRCs were fitted by Zview software utilizing the equivalent electric circuits and the scheme illustration were shown in Fig. 7. The equivalent circuit in Fig. 7a1-a2 was employed to fit the results which contained solution resistance (Rs), coating pore resistance (Rc), coating capacitance (Cc), charge transfer resistance (Rct) and double layer capacitance (Qdl). After 100 days' immersion, Rc of G-ZRC decreased from 9.36×109 to 2.49×106 Ω·cm2. Rct of G-ZRC decreased from 2.41 × 1010 to 8.44 × 105 Ω · cm2. It can be observed that barrier performance of G-ZRC significantly decreased after immersing in 3.5 wt% NaCl solution for 10 days, which was brought by the adsorption of corrosive medium and the existence of transport pathways to the substrate. Under this condition, Warburg impedance element (Zw) was introduced to the equivalent circuit (Fig. 7a2). Fig.7b1-b2 showed the scheme illustration of G40ZRC and G-55ZRC. For G-40ZRC and G-55ZRC, the anticorrosion process included of stage I and stage II. Stage I was in the cathodic protection stage, which was a short period. In this stage, part of zinc particles was activated. Subsequently, corrosion protection entered stage II. In this stage, both coatings were with low porosity due to low content of zinc particles. In addition, graphene sheets were enough to seal the holes, which could improve the density of coatings. Besides, corrosion products of zinc formed in stage I played a barrier role to the corrosive agents. Fig. 8a showed the high coating resistance (Rc) values for G-40ZRC and G-55ZRC. The magnitudes during the entire exposure time were 108 ~ 1010 Ω · cm2 and 108 ~ 109 Ω · cm2, respectively. Since the magnitude of Rc is bound up with the amount of the porosity of the coating and the diffusion situation of the corrosive agents in the coating38, these results indicate G-40ZRC and G-55ZRC systems remained almost intact during the whole immersion test. In addition, magnitudes of Rct were in the range of 108 ~ 109 Ω·cm2 for G-40ZRC and G-55ZRC.


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Fig. 8 Tendency of (a) Rc and (b) Rct values of G-ZRCs, (c) XRD patterns of surface of steel coated by G-ZRCs. Fig. 7c1-c2 showed the two anticorrosion periods for G-70ZRC and G-85ZRC. The anticorrosion process mainly consisted of two stages including cathodic protection period (stage I) and barrier protection period (stage II) provided by corrosion products of zinc which was illustrated in Fig. 7c1-c2. The graphene in the coatings was not enough to seal the holes due to the high porosity of the coatings. In the early stage, G-85ZRC exhibited lower Rct values as compared with G70ZRC, which was brought by a larger number of active zinc particles. However, in the later stage of anticorrosion process, Rc values of G-85ZRC was higher than those of G-70ZRC since large amount of corrosion products of zinc were generated for G-85ZRC, which was more than that of G-70ZRC. Compared with G-40ZRC and G-55ZRC, G-70ZRC and G-85ZRC displayed much lower Rc values which were due to the higher porosity of the coatings. The anticorrosion ability of G-70ZRC and G-85ZRC was not as good as G-40ZRC and G-55ZRC. For G-85ZRC, longer cathodic protection period was provided due to higher amount of zinc particles and uniformly dispersed conductive graphene. In the cathodic protection period (within 30 days' immersion), the impedance was in the range of 102 ~ 104 Ω·cm2 based on EIS results. Since large amount of corrosion products of zinc produced, impedance increased to 105 ~ 107 Ω·cm2 from 30 days to 100 days based on EIS results, which showed the coating provided barrier


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protection in this period. Fig. 7d1-d2 showed the electrical equivalent circuit of G-40ZRC, G55ZRC, G-70ZRC and G-85ZRC. Rs, Rc, Cc, Rct, Qdl and Zw represent solution resistance, coating resistance, coating capacitance, charge transfer resistance of the corroding interface of zinc particles, double layer capacitance and Warburg impedance, respectively26. Zw was included to the equivalent circuit to evaluate the diffusion process through the corroding interface of zinc particles39. For G-40ZRC and G-55ZRC, Warburg impedance element (Zw) was introduced from 30th day and 10th day, respectively. For G-70ZRC and G-85ZRC, Zw was introduced from 10th day and 30th day, respectively.

3.5 Characterization of rust layers on the Q235 steel and coatings after 100 days immersion The coatings on the Q235 steel surfaces were peeled off after 100 days immersion. Subsequently, the crystalline structure, morphology and element composition of the steel surfaces were characterized by XRD, SEM and EDS, respectively. As shown in Fig. 8c, the GZRC, G-70ZRC and G-85ZRC coated substrates were corroded and β-FeOOH was observed. In addition, the substrates formed peaks of Fe2O3 and Fe3O4, which could be used to inhibit the corrosion reaction. It might be the conducting PANI in the coating that could promote the formation of passive oxide. The peak of Fe2(OH)3Cl was observed and the phenomenon might be caused by the Cl- bonding with Fe2O3 and forming Fe2(OH)3Cl40. With regard to the substrates coated by G-40ZRC and G-55ZRC, just the peaks of Fe could be observed, which confirmed that the steel surfaces were not corroded and the results were in consistent with the results of Rc and Rct presented in Fig. 8a-b. The steel surfaces coated by G-ZRC, G-70ZRC and G-85ZRC with small corrosive region were not smooth. By comparison, the steel surfaces coated by G-40ZRC and G-55ZRC without corrosion products presented regular scratches causing by rubbing the


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surface with sandpaper (Fig. 9a2-a3). The corresponding EDS results confirmed chlorides were found on the steel surface, which were in agreement with XRD results in Fig. 8c. Fig. 9b1-b5 showed the surface of coatings after 100 days immersion. As shown in Fig. 9b1, G-ZRC surface was smooth. However, many small holes could be observed which were caused by the traces of water molecules entering the coating (Fig. S1). For G-40ZRC and G-55ZRC, parts of corroded zinc particles were observed on the surfaces. In contrast, large amount of corroded zinc particles were observed on the surfaces of G-70ZRC and G-85ZRC. Fig. 9c1-c5 were the fracture surfaces of the coatings after 100 days immersion. It is observed that graphene sheets existed in G-ZRC in Fig. 9c1. Some corrosion products of zinc were found inside G-40ZRC and G-55ZRC, which confirmed that a short period of cathodic protection occurred in the immersion process. However, a large number of corroded zinc particles products could be seen clearly for G-70ZRC and G-85ZRC, which could further verify that G-70ZRC and G-85ZRC played a role of cathodic protection. Hence, sacrificial properties of the zinc-rich coatings could be proved through the SEM images of surfaces morphology and fracture surfaces of G-ZRCs with different exposure times. The chemical composition of the coating surfaces were identified by XPS analysis for the surfaces before and after immersion. In this work, we took G-ZRC and G-85ZRC as typical examples (the results of others were shown in Fig. S2).


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Fig. 9 (a1-a5) SEM images of surfaces morphology of steels coated by G-ZRC, G-55ZRC, G70ZRC and G-85ZRC, respectively. (b1-b5) SEM images of surfaces morphology of G-ZRC, G40ZRC, G-55ZRC, G-70ZRC and G-85ZRC.(c1-c5) SEM images of fracture surfaces of G-ZRC, G-40ZRC, G-55ZRC, G-70ZRC and G-85ZRC, respectively. Fig. 10a1-a2 showed the C 1s core level photoemission spectra of G-ZRC before and after immersion. The peaks located at 284.8 eV, 286.03 and 288.03eV are attributed to C-C/C-H bonds, C-O/C-N bonds and C=O bands, respectively41. These functional groups are the


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characteristic functional groups of a conventional epoxy resin. Graphene signals could not be detected obviously since the graphene content in G-ZRC was just 0.3 wt%. Compared with Fig. 10a1, significant change was observed in Fig. 10a2. A new peak located at 288.8 eV in C1s core lever attributed to O-C=O. The formation of the new peak might be due to the hydroxyl bonding between water molecules and hydroxyl groups of epoxy resin42. The results suggested the direct contact with electrolyte could alter the chemical composite of the coating surface. The XPS highresolution spectra for Zn2p3/2 of G-85ZRC are shown in Fig. 10b1-b2. Before immersion, only a part of ZnO could be observed since the surface of zinc is easily oxidized in the air. After 100 days immersion in 3.5 wt% NaCl solution, the peaks at 1022.4 eV, 1022 eV and 1021.7 eV are attributed to Zn5(CO3)2(OH)6, Zn and ZnO, respectively19. The process of zinc dissolution in a neutral solution can be expressed as follows:

Carbon dioxide from the environment adsorbed into the coating and reacted with Zn(OH)2, forming the corrosive products of Zn5(CO3)2(OH)6 and ZnO43.


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Fig. 10 (a1) C1s high-resolution spectrum before immersion for G-ZRC. (a2) C1s high-resolution spectrum after immersion in 3.5 wt% NaCl during 100 days for G-40ZRC. (b1) Zn2p3/2 highresolution spectrum before immersion for G-85ZRC. (b2) Zn2p3/2 high-resolution spectrum after immersion in 3.5 wt% NaCl during 100 days for G-85ZRC. Conclusions In this work, five types of graphene modified zinc-rich epoxy coatings (G-ZRCs) were prepared. The corrosion protection mechanisms of G-ZRCs immersed in 3.5 wt% NaCl solution were clarified. According to electrochemical results, G-ZRC predominantly presented barrier corrosion protection. A mixed corrosion protection mechanism was provided for G-40ZRC and G-55ZRC due to more activated zinc particles caused by the positive effect of graphene and low porosity of the coatings. Finally, a major cathodic protection mechanism was provided for G-


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70ZRC and G-85ZRC because of higher amount of zinc particles and existence of graphene that enhanced electrical connectivity between zinc particles and steel substrate. 70ZRC without graphene could not provide cathodic protection. Hence, it was confirmed that grapheme existed in the coatings had a significant effect on the corrosion protection mechanism of ZRCs. EDS and XRD results showed that protection layer formed at the coating surface brought by passivation effect of polyaniline (PANI). Corresponding Author Corresponding authors: E-mail: [email protected] (W.J. Zhao), [email protected] (K. Wang). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Associated Content Supporting information included SEM image of surface morphology of G-ZRC after 100 days immersion and XPS results of G-40ZRC, G-55ZRC and G-70ZRC before immersion and after 100 days' immersion.

Acknowledgements We express our great thanks to the National Natural Science Foundation of China (51775540), Zhejiang Province Key Technology Project (2015C01006), Youth Innovation Promotion


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Investigating the corrosion protection mechanisms of graphene

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