Electrodeposition of Ni(OH)2 reinforced polyaniline coating for corrosion protection of 304 stainless steel

Accepted Manuscript Full Length Article Electrodeposition of Ni(OH)2 reinforced polyaniline coating for corrosion protection of 304 stainless steel Li Jiang, Junaid Ali Syed, Yangzhi Gao, Hongbin Lu, Xiangkang Meng PII: DOI: Reference:

S0169-4332(18)30157-0 https://doi.org/10.1016/j.apsusc.2018.01.145 APSUSC 38284

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

30 September 2017 10 January 2018 16 January 2018

Please cite this article as: L. Jiang, J. Ali Syed, Y. Gao, H. Lu, X. Meng, Electrodeposition of Ni(OH)2 reinforced polyaniline coating for corrosion protection of 304 stainless steel, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.01.145

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Electrodeposition of Ni(OH)2 reinforced polyaniline coating for corrosion protection of 304 stainless steel

Li Jiang†, Junaid Ali Syed†, Yangzhi Gao, Hongbin Lu*, Xiangkang Meng* Institute

of

Materials

Engineering,

National

Laboratory

of

Solid

State

Microstructures, College of Engineering and Applied Sciences, Nanjing University, Jiangsu, People’s Republic of China.

________________________ * Corresponding author. E-mail: [email protected] (H. B. Lu) /[email protected] (X. K. Meng) Tel: (+86) 25 8368 5585. Fax: (+86) 25 8359 5535. † Both the authors contributed equally in this work.

1

Abstract In the present paper, polyaniline (PANI) coating was electropolymerized in the presence of phosphoric acid with subsequent deposition of Ni(OH) 2 particles. The Ni(OH)2 reinforced PANI coating significantly enhances the corrosion resistance of 304 stainless steel (304SS) in comparison with the pristine PANI coating. The galvanostatically deposited Ni(OH)2 particles fill the pores of the pristine PANI coating and improves the coatings hydrophobicity which decreases the diffusion of aggressive media. Importantly, the Rp values of Ni(OH)2 reinforced PANI coating is much higher than that of pristine PANI coating and the Ni(OH)2 reinforced PANI coating presents a long-term anti-corrosive ability (360 h) in 3.5 wt.% NaCl solution. The prolonged corrosion protection of Ni(OH)2 reinforced PANI coating is attributed to the improved physical barrier as well as the facile formation of passive oxide film that sustain the anodic protection of the coating.

Keywords: Anti-corrosive; Polyaniline; Ni(OH)2; Pores; Reinforcement

2

1. Introduction One of the major problems associated with the industrial sector and contemporary society is the corrosion of stainless steel (SS) that roots a world with massive economic loss annually. Different polymer coatings have been developed to replace the toxic chromium coatings, however, conducting polymers based coating was reported as the best fit [1-4]. Polyaniline (PANI) distinguishes itself from other conducting polymers due to its environmental stability, economical production, and adjustable high conductivity [5-9]. The anti-corrosive performance of electroactive PANI coating was initially reported by Deberry [10]. Later on, various researchers employed different alloys and metals such as SS, Al, Ti and Cu, etc., to study the different applications of PANI [1, 11-16]. PANI coating not only offers a physical barrier to block the aggressive ions but it also promote the formation of passive oxide layer on the metal surface, which stabilizes the metal for long time [17-19]. Generally, PANI exhibits two different states i.e., oxidized and reduced states [20, 21], and can be easily transformed between the two states under suitable conditions [22]. The corrosion of metal leads to the change in the surface potential that facilitates the reduction of PANI and formation of an oxide layer on the metal surface with anodic protection [23-26]. However, some reports found that the electropolymerized PANI coating is porous in nature and it is difficult to achieve long-term anti-corrosive property due to the porous structure of PANI coating [27, 28]. In recent years, some researchers reported the investigations focused on metal electrodeposition in PANI matrix [29, 30]. Many studies were directed to the 3

metal/polymer systems and several practical metals have been employed, such as, Pt [31], Ag [32, 33], Cu [34, 35], Pd [36, 37], Ni [38] and Au [39]. According to Tsakova’s work [40], the metal ions can travel through the porous structure of the polymer coating and the metal nuclei were formed at the polymer/metal interface. However, the above discussed metals were not very suitable for corrosion protection because their addition in polymer coating would initiate the cathodic reaction and accelerate the corrosion process. In contrast, the stable and passive hydroxide such as Ni(OH)2 is an appropriate candidate for corrosion protection, which can be prepared easily by electrodeposition method and would be suitable to reinforce in conductive polymer coating [41, 42]. So far, the conducting polymer/metal hydroxides for corrosion protection of SS are rarely reported. Previously, our group demonstrated the enhanced anti-corrosive performance of electrodeposited phosphomolybdic acid doped PANI coating for 304SS [9]. However, it is still challenging to achieve the long-term protection ability due to the porous nature of the electrodeposited PANI coating. In the present work, we proposed two step electrodeposition process; initially PANI was electropolymerized on 304SS in phosphoric acid solution, followed by the galvanostatic deposition of Ni(OH) 2 particles to fill the pores and enhance the anti-corrosive performance of the coating for 304SS.

4

2. Materials and methods 2.1 Materials Nickel nitrate was purchased from Sinopharm Chemical Reagents Co. Ltd. Aniline, phosphoric acid and the rest of the chemicals were obtained from Shanghai Aladdin Bio-Chem. Technology Co. Ltd and Shanghai Lingfeng Chemical Reagents Co. Ltd. The Senda Decoration Materials Co. Ltd has provided the type 304SS plates and their composition are presented in Table 1. A series of SiC papers were used to polish 304SS plates followed by washing with acetone and ethanol, respectively. Table 1 2.2 Electrodeposition of coatings The Metrohm PGSTAT 302N electrochemical workstation was used to perform the electrochemical analyses at room temperature. The electrochemical cell was used for analyses and electrodeposition consist of three electrodes, 304SS plate as working electrode, Ag/AgCl (saturated KCl) reference electrode and platinum counter electrode. The exposed area of 304SS electrode was 1 cm2 and the remaining was sealed. The electrodeposition of PANI coating on 304SS in the presence of phosphoric acid was performed via cyclic voltammetry (CV) technique. The mixture of 0.1 M aniline in 0.5 M phosphoric acid was used for electropolymerization of PANI. The electrodes were placed in the aniline solution mixture and PANI was electrodeposited by CV with 24 cycle number at a rate of 20 mV/s and the potential window is −0.4 to 1.3 V. In the case of Ni(OH)2 reinforced PANI coating, the electrodeposited pristine PANI coating on 304SS was followed by the galvanostatically deposition of Ni(OH)2 5

particles at a current density of 1 mA cm-2 for 20 min in 0.4 M nickel nitrate solution. The Ni(OH)2 reinforced PANI coating was tagged as PANI+Ni(OH) 2. 2.3 Instrumentation The X-ray photoelectron spectroscopy (XPS) was used to characterize the electrodeposited Ni(OH)2 and the passive oxide layer in coating/304SS interface, which was performed by Thermo Fisher Scientific XPS system. The morphology of the coating was analyzed by a JOEL JSM-6510 scanning electron microscope (SEM). The static contact angle (CA) of the coating was measured by a Kruss Kontaktwinkle DSA100 setup. The electrochemical behavior and the corrosion resistance of the uncoated and coated 304SS were evaluated in 3.5 wt.% NaCl solution. To stabilize the exchange of ions between samples and electrolyte, the sample was immersed in 3.5 wt.% NaCl solution for 15 min before electrochemical measurements. The potential was swept at the rate of 1 mV/s for potentiodynamic polarization measurements. The amplitude of 10 mV was used for EIS measurements with the scanned frequency of 10 5 to 10-2 Hz and the Nyquist diagrams were simulated by Zview software.

Fig. 1

3. Results and discussion 3.1 Characterization of electrodeposited coatings The presence of phosphoric acid in electropolymerization of aniline would cause phosphating of the 304SS surface which enhances the linkage between the metal

6

surface

and

the

polyaniline

layer

as

described

previously

[9].

The

electropolymerization of aniline exhibits different stages. Initially, the protonated aniline monomer was formed in the presence of H3PO4 and adsorbed on the working electrode. Later on, the oxidation of the free radicals at the surface of the electrode generates the dimers. Finally, the oligomers were formed due to the oxidation of these dimers followed by the polymerization of aniline [43]. However, the electrodeposited PANI coating is porous, the presence of pores would allow the transport of ions, and these ions could easily approach the coating/SS interface, further weakens the corrosion resistence of the coating. Therefore, we galvanostatically deposited Ni(OH)2 particles in PANI coating to fill the pores within the coating and improve the coatings corrosion resistance. Fig. 1 shows the scheme of Ni(OH)2 electrodeposition process. When the PANI coating was immersed in the nickel nitrate solution, nickel and nitrate ions would enter into the pores of the coating due to the conducting ability of PANI. The ions were approaching the surface of 304SS under the applied electrodeposition current, followed by the reactions mentioned in Eqs. (1) and (2) [44]. 2

i2

2

e

4

i

0

(1) (2)

The pH value at the electrode interface would increase due to the reduction of nitrate ions [45, 46], while the increasing number of hydroxide ions and high pH value during the electrodeposition process further facilitate the deposition of Ni(OH) 2 species [44]. As a result, the reaction product Ni(OH) 2 reinforced into the pores of 7

PANI as well as deposited on the coating surface (supported by SEM images in Fig. 3). The electrodeposition of Ni(OH)2 may have two tendencies, at first it fills the pores of the coating and secondly it decelerates the electron transport within the porous network of PANI matrix due to its less electroactive nature. The result of Ni(OH)2 deposition depends on both the conducting properties and morphology of PANI coating under the given experimental conditions [47, 48]. Fig. 2 The XPS measurements were carried out to confirm the successful deposition of Ni(OH)2 species. The XPS survey and N 1s, P 2p and Ni 2p XPS spectrums of PANI+Ni(OH)2 coating are shown in Fig. 2. Fig. 2a demonstrates the composition of PANI+Ni(OH)2 coating, which contains five kinds of elements, i.e., Ni, O, N, C and P. The XPS spectrum of N 1s with three decomposed peaks is shown in Fig. 2b, the fitted peaks at 399.1 and 400.0 eV belong to quinoid di-imine (-N=) and benzenoid diamine nitrogen (-NH-), respectively. The corresponding nitrogen peak at 401.1 eV indicates the positively charged nitrogen (N+) in the doped PANI structure [49, 50]. In addition, the doping degree of the coating can be calculated by the ratio of N+ to the total nitrogen content and the acquired N+/N ratio is 0.24. The fitted peak at 132.6 eV in the P 2p XPS spectrum (Fig. 2c) can be ascribe to the doped H3PO4 molecule in PANI matrix, which indicates that the H3PO4 has been doped into PANI effectively. Fig. 2d shows the Ni 2p XPS spectrum of PANI+Ni(OH) 2 coating that decomposed into four peaks. The fitted peaks at 855.5 and 873.1 eV with 17.6 eV spin-energy separation belongs to Ni 2p 3/2 and Ni 2p 1/2 respectively, the other two peaks at 861.3

8

and 879.4 eV are assigned to the respective satellite peaks of Ni 2p

3/2 and

Ni 2p

1/2.

The XPS peaks representing the characteristics of Ni(OH) 2 phase [46, 51] and these results indicate that the Ni(OH)2 has been electrodeposited in the H3PO4 doped PANI coating successfully. Fig. 3 Fig. 3a and 3b shows the morphology of pristine PANI and PANI+Ni(OH)2 coating. During electropolymerization process, PANI tends to deposit within the region of high electrochemical activity, which leads to the non-uniform coating. As shown in Fig. 3a, the PANI coating is not compact with numerous pores on the surface (magnified inset). This coating structure would allow the aggressive media to permeate inward and initiate the corrosion process. In contrast, PANI+Ni(OH) 2 coating exhibits less pores with visible Ni(OH)2 particles as shown in Fig. 3b, these particles have different size and successfully deposited within the pores of PANI (magnified inset). The existence of Ni(OH)2 particles in PANI coating modified the porous structure of PANI coating and hinders the diffusion pathway of aggressive ions. The surface wettability of the coating is associated to its chemical composition and surface morphology, the hydrophobicity of coating could improve its anti-corrosive performance due to restricting adsorption of water onto the coating surface [52, 53]. Therefore, the wettability of the coating was characterized by evaluating the contact angles (CAs) of water droplets and the results are shown in Fig. 3c and 3d. It is observed that the pristine PANI coating exhibits a relatively 9

hydrophilic behavior with CA = 42.6 ± 0.1° (Fig. 3c) due to its porous structure. In contrast, the CA of PANI+Ni(OH)2 coating is 102.3 ± 0.8° as shown in Fig. 3d, i.e. the incorporation of Ni(OH)2 particles in PANI matrix allows the coating achieves hydrophobicity properties. The hydrophobicity of PANI+Ni(OH)2 coating will effectively preventing the adsorption of water onto the coating surface and ion exchange with the electrolyte, further enhances its corrosion resistance. Fig. 4

3.2 Anti-corrosive performance of the coatings The anti-corrosive ability of the coating is often represented by potentiodynamic polarization curve and it is used to evaluate the corrosion potential (Ecorr) of the tested samples [54, 55]. The high Ecorr value demonstrate that the tested sample is difficult to corrode in the aggressive environment [56]. Fig. 4a shows the polarization curve of the coated and uncoated 304SS samples, the Ecorr value of uncoated 304SS was found to be -421 mV and for pristine PANI coating the value was -247 mV. The increase of Ecorr value for the pristine PANI coated 304SS is due to the anti-corrosive nature of PANI as compared to uncoated 304SS. However, the PANI+Ni(OH)2 coating has the most positive Ecorr value (-22 mV), the results reflect that the PANI+Ni(OH)2 coating can prevent the inward diffusion of aggressive ions and enhance the corrosion protection for the 304SS substrate due to the effective physical barrier [8]. Besides the transformation in different states of PANI, the main reason for the enhanced corrosion resistance of PANI+Ni(OH)2 coating is that the Ni(OH)2 particles modified the porous structure of PANI and improved the hydrophobicity of the coating. Therefore, the 10

PANI+Ni(OH)2 coating significantly blocks the permeation of corrosive species and act as a potential barrier against corrosion to protect 304SS surface [52, 57]. The curves of open-circuit potential (Eocp) versus time for uncoated and coated 304SS in 3.5 wt.% NaCl solution are shown in Fig. 4b. Initially, the Eocp of uncoated 304SS was found to be -201 mV as shown in Fig. 4b. After 24 h of immersion in 3.5 wt.% NaCl solution, the rise in Eocp value was noticed due to the formation of a passive oxide layer on 304SS surface [58]. Later on, the destruction of passive oxide layer was observed, which results in an apparent drop in Eocp values after 216 h immersion. The pristine PANI coating presents a higher Eocp values with initial value of -120 mV due to PANI redox and anti-corrosive nature in comparison with the bare 304SS. The initial Eocp value of PANI+Ni(OH)2 coating is 274 mV, much higher than the value of uncoated and pristine PANI due to the reinforcement of Ni(OH) 2 in PANI matrix. A drop in Eocp values of pristine PANI coating was observed after 200 h of immersion, however, the Eocp value of PANI+Ni(OH)2 coating remains higher than the values of uncoated and pristine PANI coated 304SS during the prolonged immersion time. This is because the presence of Ni(OH)2 in PANI coating (Fig. 3b) hinders the electrolyte diffusion and blocks the intrusion of aggressive media. These results imply that the PANI+Ni(OH)2 could protect the 304SS surface from corrosion for a long time and stabilizes high Eocp values during the whole immersion process. Fig. 5 Fig. 5 shows the Nyquist and Bode plots of uncoated and coated samples in 3.5 wt.% NaCl solution as well as equivalent electric circuits used to fit the Nyquist plots.

11

It can be observed that the Ni(OH)2 reinforced coating shows the profound increase in impedance in comparison with the pristine PANI coating and uncoated 304SS (Fig. 5a). The Nyquist diagram of bare 304SS shows a capacitive loop with one time constant in the corresponding Bode plot and its equivalent electric circuit is shown in Fig. 5c. Where Rs represents the solution resistance, CPEdl is the constant phase element ascribed to the double layer capacitance, the charge-transfer resistance Rct includes the SS/oxide layer and oxide layer/solution interface charge transfer resistance, the resistance of the oxide layer and the pore resistance of the oxide layer [59]. The pristine PANI coating shows one small capacitive loop at high frequency region and a straight line in the low frequency region (magnified inset in Fig. 5a), which is due to the charge diffusion within the conductive PANI layer [17]. Thus, the equivalent electric circuit used for the pristine PANI coating contains a Warburg diffusion impedance element (Zw) as shown in Fig. 5d, the elements Rpore and CPEc represent the pore resistance and capacitance of the pristine PANI coating, respectively. In the case of PANI+Ni(OH)2 coating, two time constant of EIS response were observed. The equivalent electric circuit shown in Fig. 5e is used to model the EIS plot. Fig. 5 indicates that the proposed equivalent electric circuits gives best fitting to the experimental data. The impedance of CPE is equated as [13]: n

(3)

where Y0 is the CPE, j is the imaginary unit, and

is the angular frequency of the

C

0

applied AC voltage, in the case of exponential term n = 0-1 and n = 1 for an ideal 12

capacitance. The impedance of the element Zw is defined as [60]: sn

Rcoth

sn

(4)

where s λ2/D, with λ represents the diffusion layer thickness, D the diffusion coefficient. R is the diffusion resistance, while n is the coefficient for W (0 < n < 1). Fig. 6 Table 2 Fig. 6 shows the Nyquist and Bode plots of pristine PANI and PANI+Ni(OH) 2 coatings immersed in 3.5 wt.% NaCl solution for prolonged immersion time. Fig. 6a indicates that the pristine PANI coated 304SS exhibits one capacitive loop with a diffusion line (inset of Fig. 6a). However, second time constant were clearly observed in the corresponding Bode plots after 168 h of immersion due to the accumulated corrosion products within the pores and the formation of passive oxide layer. After 168 h of immersion, the EIS data were modeled with the equivalent electric circuit as shown in Fig. 5e. In the case of PANI+Ni(OH)2 coating, the shape of its Nyquist and Bode plots remains unchanged during 360 h of immersion and exhibit large corrosion resistance. The fitting results of the EIS data for the pristine PANI and PANI+Ni(OH)2 coating during the prolonged immersion obtained by using the equivalent electric circuit models (Fig. 5c-e) are compiled in Table 2 and Table 3. The values of n associated with element CPE are in the range of 0.8-1. The impedance of pristine PANI coating increased in the first 168 h (Fig. 6a), which is attributed to the anodic protection of conductive PANI coating and formation of the oxide layer on the coating/SS interface. In addition, the aggressive ions permeated 13

through the pores produced corrosion products, which accumulated within the pores of the pristine PANI coating and fill the pores to a certain extent. However, after 168 h of immersion, the deterioration of the PANI coating was observed in NaCl solution with a decrease in impedance (Fig. 6a) and this phenomenon are in correspondence with the Rpore and Rct values mentioned in Table 2. In contrast, the impedance of PANI+Ni(OH)2 coating was much higher than that of pristine PANI coating during the whole immersion time (Fig. 6b). The increasing trend in Rpore values (Table. 3) of the PANI+Ni(OH)2 coating was observed during immersion due to the reinforcement of Ni(OH)2 particles. The Rct values shows increasing trend initially, followed by stable Rct values and maintains a high value at 360 h of immersion (6.026 MΩ). In addition, the CPEdl value reflects the water spread at coating/SS interface and the delamination of the coating from the substrate [49]. It is found that the CPEdl values of PANI+Ni(OH)2 coating is one order of magnitude smaller than that of pristine PANI coating, suggesting that the PANI+Ni(OH)2 coating has better protection ability in comparison with the pristine PANI coating. The insoluble Ni(OH) 2 particles has obvious passivation effect and can effectively restrict water or oxygen approaching the surface of SS substrate as reported previously [61], therefore, the Ni(OH)2 reinforced PANI coating is more stable and provides much better corrosion protection than the pristine PANI coating during the long-term immersion. Table 3 Fig. 7 Polarization resistance (Rp) was also used to demonstrate the anti-corrosive 14

performance of the different coatings. For pristine PANI coated 304SS, Rp is the sum of Rpore, Rct and Zw-R, in the case of PANI+Ni(OH)2 coated 304SS, Rp equals to Rpore plus Rct [13]. Fig. 7 shows the Rp values of pristine PANI and PANI+Ni(OH)2 coating with increasing immersion time. It is found that the pristine PANI coating shows an increasing trend during the initial 168 h of immersion followed by a decrease in Rp values. Generally, the conductive polymer coating not only acts as a physical barrier to block the aggressive ions but also forms the passive oxide layer on coating/SS interface, i.e., anodic protection. However, the coating is porous in nature and the doped PANI gradually reduced during immersion, which causes an increased diffusion of aggressive media with immersion time, therefore, a decrease in Rp values was observed. This decrease in Rp values indicate the deterioration of the coating and it continuously decreases with immersion time. In contrast, the PANI+Ni(OH)2 coating shows an increase in Rp values within 216 h of immersion and stabilized with Rp value of 6.05

MΩ cm2 after 360 h immersion (Fig. 7). The Rp values of

PANI+Ni(OH)2 coating are several orders of magnitude higher than the values of pristine PANI coating during the whole immersion, which show its durability and long-term corrosion protection ability. Ni(OH) 2 particles in PANI coating not only fills the pores, but it presence in the pores of the PANI matrix may provide effective passivation effect as reported by Qiu et. al [61], hence prolonged the anodic protection of the conductive coating and effectively passivate the underlying substrate. Fig. 8 3.3 Interface analysis 15

In order to further explore the anti-corrosive performance of pristine PANI and PANI+Ni(OH)2 coatings, Fig. 8a-d show the SEM images of the coatings and their respective interfaces after 360 h immersion in 3.5 wt.% NaCl solution. It is observed that the surface morphology of pristine PANI coating after 360 h of immersion (Fig. 8a) is similar to its morphology before immersion (Fig. 2a), which exhibits obvious pores. In contrast, the PANI+Ni(OH)2 coating displays a compact surface without noticeable pores and shows clear Ni(OH)2 particles after immersion (Fig. 8b). However, the Ni(OH)2 particles become more exposed and the overall morphology of the coating shows obvious difference from its morphology before immersion (Fig. 2b) due to the dedoping process of the doped H3PO4 and the ion exchange reaction during the immersion process [4]. In order to explore the underlying interface of the coated samples, the pristine PANI and PANI+Ni(OH) 2 coatings were carefully removed after immersion and their respective coating/SS interfaces were analyzed. As shown in Fig. 8c, the interface of pristine PANI/304SS reveals initiation of pit and crevice corrosion (rectangular area) after immersion due to the conductive nature of PANI and the appearance of pores in the coating surface [62], which weakens the coatings barrier and allows the electrolyte to invade the underlying substrate. However, the interface underlying PANI+Ni(OH)2 coating displays little corrosion as shown in Fig. 8d, implies that PANI+Ni(OH)2 coating provides effective protection for the substrate. The polished 304SS surface morphology before and after immersion were represented in Fig. 8e and 8f to facilitate the comparison. Numerous pits and crevices with corrosion products at 304SS surface were detected after immersion (Fig. 8f) in 16

comparison with its morphology before immersion as shown in Fig. 8e, which indicates that the uncoated 304SS suffers severe corrosion. The results reveal that both the pristine PANI and PANI+Ni(OH)2 coatings can protect the 304SS and the PANI+Ni(OH)2 coating offers superior anti-corrosive performance in comparison with pristine PANI coating. These results are in accordance with the EIS results. Fig. 9 Table 4 In order to investigate the corrosion protection mechanism of PANI+Ni(OH) 2 coating, the composition of the passive oxide layer underlying the pristine PANI and PANI+Ni(OH)2 coatings were characterized by XPS, respectively. The results show that chromium, iron, nickel and oxygen were the main components of the passive oxide layer. Fig. 9 illustrates the XPS spectrum of Cr 2p3/2, Fe 2p3/2, and O 1s in the passive oxide layer of pristine PANI and PANI+Ni(OH) 2 coated 304SS. The fitted XPS peaks with the relative atomic proportion of the passive oxide layers for pristine PANI and PANI+Ni(OH)2 coated 304SS are listed in Table 4, the fitted data allows to compare the significant differences between the passive oxide layer underneath the pristine PANI and PANI+Ni(OH)2 coatings. The Cr 2p3/2 peak around 576.7 eV was fitted with four peaks corresponding to Cr, Cr2O3, Cr(OH)3 and CrO3 (Cr6+) spices as shown in Fig. 9a and 9b [63, 64]. The passive oxide layer of pristine PANI coating shows much higher amount of Cr(OH) 3 than Cr2O3 (Table 4). However, the content of Cr(OH)3 and Cr2O3 in the passive oxide layer of PANI+Ni(OH)2 coated 304SS are rather similar, which is due to the 17

occurrence of the solid transformation of Cr(OH)3 to Cr2O3 with time in the presence of Ni(OH)2 [65]. Moreover, the higher content of Cr6+ in pristine PANI coated 304SS is due to the reaction between water and Cr(OH) 3 on PANI/304SS interface [65], which would result in the removal of chromium element from the coating/substrate interface and leads to rapid degradation of the passive oxide layer beneath the pristine PANI coating [66]. Fig. 9c and 9d show the Fe 2p3/2 XPS spectrums for the pristine PANI and PANI+Ni(OH)2 coating, respectively, the peak of Fe in both spectrums were fitted with three peaks belong to Fe metal, FeO and FeOOH (goehite) species [67, 68]. Table 5 shows that the passive oxide layer beneath the PANI+Ni(OH) 2 coating has lower Fe content than the pristine PANI coated SS, implying a depletion of iron. Moreover, the content of FeOOH in the both passive oxide film are higher than the content of FeO, indicates that FeOOH is the primary iron oxidized species in the passive oxide layer [69]. The O1s XPS spectrums obtained from the passive oxide layer of pristine PANI and PANI+Ni(OH)2 coating are shown in Fig. 9e and 9f, respectively. The spectrum was fitted with three peaks: O2−, hydroxides (-OH) and oxygen in water. The passive oxide layer under the pristine PANI coating exhibits higher content of water and lower content of O2- in comparison with PANI+Ni(OH)2 coating. In addition, Table 5 indicates that the main constituent of O in the passive oxide layer of both pristine PANI and PANI+Ni(OH)2 coating is OH-, which attributes to the formation of Cr(OH)3 and FeOOH [69]. 18

Table 5 also displayed the ratio of Cr2O3/Cr(OH)3 and FeO/FeOOH in the passive oxide layer beneath pristine PANI and PANI+Ni(OH)2 coatings. Table 5 shows that the passive oxide layer of PANI+Ni(OH) 2 coated 304SS exhibits higher chromium content than pristine PANI coated 304SS with an increasing Cr2O3/Cr(OH)3 ratio. This increasing content of chromium oxide indicates the formation of stable and thick passive oxide film underneath the PANI+Ni(OH) 2 coating [67]. The high content of iron element with a large FeO/FeOOH ratio in the passive oxide layer underneath PANI+Ni(OH) 2 coating was also observed, indicating a significant amount of FeO in the passive oxide layer under PANI+Ni(OH)2 coating. Due to the anodic passivation nature of PANI, iron dissolved from the substrate surface in the early stage, then the flux from the oxides would further result in the residual current passivation [70]. Thereby, the decreased content of iron element in the passive oxide layer can be observed after the long-term immersion. However, the substantial amount of iron element in the passive oxide layer underneath the PANI+Ni(OH)2 coating was detected, indicates a lower dissolution rate of these species compared to that of pristine PANI coating. In addition, the high content of water in the pristine PANI passive oxides is due to the direct contact of water and 304SS implies the devastation of the coating during immersion [67]. The above results indicate that the electrodeposited Ni(OH)2 in PANI matrix can facilitate the formation of passive oxide film and sustain its anodic protection during immersion, which verified the EIS results. As a result, the PANI+Ni(OH)2/SS interface exhibits a satisfactory passive oxide layer and provides an effective protection for the 19

underlying substrate.

3.4 Mechanism Conductive polymers protect the metal mainly through the anodic protection and barrier effect. The pristine PANI coating obtained through electrochemical deposition is porous in nature and allows the quick diffusion of aggressive species, which results in the destruction of the coating during immersion process. The reduction of conductive PANI forms a passive oxide layer through the reactions as shown in Eqs. (5)-(9) [58]. At the anodic zone, Fe2+ ions released with the formation of oxide layer (Eq. (5)), and Eq. (6) shows the oxygen reduction reaction in the cathodic zone. In addition to transformation in different states (Eqs. (7)-(9)), the PANI redox reaction facilitates the release of the counter ion A−, which can form insoluble particles with Fe2+. In the case of PANI+Ni(OH)2 coating, there are three advantages of the electrodeposited Ni(OH)2 to modify the coatings structure, firstly it fills the pores of the PANI coating, secondly it enhances the coatings hydrophobicity and thirdly it facilitates the passivation of coating/SS interface. The existence of Ni(OH)2 in PANI matrix can sustain its anodic protection to form a stable and a thick passive oxide layer (supported by the interface analysis). e2

e 2

2

4e-

2 -

(5) -

4

2 2

(6) -

2

2e2

2e-

-

(7) (8)

-

(9) 20

Based on the above discussion, it is envisaged that the Ni(OH) 2 in PANI coating modified its porous structure and strengthens the barrier against aggressive species. Furthermore, it facilitated the formation of passive oxide film on coating/SS interface. Therefore, the addition of Ni(OH) 2 in PANI coating sustain the anodic protection and improves the long-term anti-corrosive performance of the coating.

4. Conclusion Ni(OH)2 particles were successfully electrodeposited to modify the porous structure of the electropolymerized pristine PANI coating and enhances the long-term protection ability of the coating in aggressive media. The surface of PANI+Ni(OH) 2 coating is more compact with less pores and the deposition of Ni(OH)2 particles significantly improved the coatings hydrophobicity, resulting in more positive Ecorr values compared to the pristine PANI coating. The Rp values of PANI+Ni(OH)2 coating is much higher than that of pristine PANI coating even after 360 h of immersion due to Ni(OH)2 particles that facilitated the formation of passive oxide film and sustains the anodic protection with barrier effect during the prolonged immersion. The deposition of Ni(OH)2 particles in PANI matrix blocks the access of aggressive media and shows the long-term anti-corrosive performance.

Acknowledgements This work was jointly supported by the National Natural Science Foundation of China (No. 51501088, No. 51771090), the Fundamental Research Funds for the Central Universities (No. 021314380071), and the Science & Technology Support

21

Plan of Jiangsu Province (BE2014039, BY2015080).

22

Figure captions Fig. 1. The brief schematic diagram for Ni(OH)2 electrodeposition process. Fig. 2. XPS survey spectrum (a), N 1s, P 2p and Ni 2p XPS spectrums (b, c, d) of PANI+Ni(OH)2 coating. Fig. 3. The morphology of pristine PANI and PANI+Ni(OH)2 coatings with their respective magnified images (a, b), the contact angle of pristine PANI and PANI+Ni(OH)2 coatings (c, d). Fig. 4. Potentiodynamic polarization curves (a) and time dependence of open circuit potential (Eocp) curves of uncoated and coated 304SS during 360 h of immersion (b) in 3.5 wt.% NaCl solution. Fig. 5. The Nyquist plots of the uncoated and coated 304SS with their magnified inset (a), the comparison of the Bode plots associated with uncoated and coated samples (b), the equivalent electric circuits to fit the impedance diagram of uncoated 304SS (c), pristine PANI coating (d, e) and PANI+Ni(OH)2 coating (e). Fig. 6. Nyquist and Bode plots of pristine PANI coating (a) and PANI+Ni(OH)2 coating (b) during various immersion time. Fig. 7. Polarization resistance (Rp) values of pristine PANI and PANI+Ni(OH)2 coating with different immersion time. Fig. 8. Surface morphology of pristine PANI and PANI+Ni(OH) 2 coatings (a, b) and their underlying interfaces (c, d) after 360 h of immersion, surface morphology of uncoated 304SS before and after 360 h of immersion (e, f). Fig. 9. Cr2p3/2, Fe2p3/2 and O1s XPS spectrum of the passive oxide layer for pristine PANI coated (a, c, e) and PANI+Ni(OH) 2 coated (b, d, f) 304SS after 360 h of immersion in 3.5 wt.% NaCl solution.

23

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30

Table 1 Composition of 304SS plate/ wt.%. Cr

Mn

Si

Ni

Mo

C

P

S

Fe

18.50

0.88

0.59

8.12

0.30

0.05

0.015

0.028

Balance

Table 2 The fitted electrochemical parameters of pristine PANI coated 304SS at different immersion time in 3.5 wt.% NaCl solution. Time (h)

Rs Ω cm2)

CPEc Ω cm-2 s-n )

Rpore Ω cm2)

CPEdl Ω cm-2 s-n )

Rct Ω cm2)

Zw-R Ω cm2)

Rp Ω cm2)

0 1 12

6.522 6.321 7.569

4.571×10-5 4.689×10-5 4.101×10-5

1.343×102 6.052×102 3.677×103

----

----

1.203×102 7.680×103 1.704×104

2.546×102 8.285×103 2.072×104

24 48 72 120

6.911 7.038 8.102 6.932

4.076×10-5 3.902×10-5 3.763×10-5 3.792×10-5

8.598×103 8.509×103 9.257×103 9.007×103

-----

-----

9.329×104 1.117×105 1.314×105 1.922×105

1.019×105 1.202×105 1.406×105 2.012×105

168 216 288 360

9.849 10.48 9.081 13.35

3.826×10-5 3.813×10-5 3.761×10-5 3.778×10-5

1.115×104 1.064×104 1.091×104 1.037×104

4.081×10-5 4.054×10-5 3.446×10-5 3.654×10-5

2.824×105 2.481×105 2.357×105 2.301×105

-----

2.934×105 2.588×105 2.466×105 2.405×105

-1

-1

31

Table 3 The fitted electrochemical parameters of PANI+Ni(OH)2 coated 304SS at different immersion time in 3.5 wt.% NaCl solution. Time (h)

Rs Ω cm2)

CPEc Ω cm-2 s-n )

Rpore Ω cm2)

CPEdl Ω cm-2 s-n )

Rct Ω cm2)

Rp Ω cm2)

0 1 12

9.081 9.234 9.366

3.007×10-5 2.977×10-5 3.149×10-5

2.030×103 5.598×103 8.251×103

8.545×10-6 7.267×10-6 5.724×10-6

4.819×106 4.991×106 5.122×106

4.821×106 4.997×106 5.130×106

24 48 72 120 168 216 288 360

9.619 8.445 8.419 8.632 10.64 13.89 14.71 13.67

3.325×10-5 3.183×10-5 2.874×10-5 3.077×10-5 3.152×10-5 3.536×10-5 4.566×10-5 4.224×10-5

1.045×104 1.276×104 2.118×104 2.302×104 3.022×104 3.025×104 3.131×104 3.211×104

5.701×10-6 5.610×10-6 5.656×10-6 6.125×10-6 5.246×10-6 5.266×10-6 5.366×10-6 5.933×10-6

5.282×106 5.526×106 5.881×106 5.936×106 5.989×106 6.039×106 6.032×106 6.026×106

5.292×106 5.539×106 5.902×106 5.959×106 6.019×106 6.093×106 6.063×106 6.058×106

-1

-1

Table 4 XPS peak position, peak area and content (at.%) corresponding to the passive oxide layer beneath the pristine PANI and PANI+Ni(OH)2 coated 304SS. Spectra Cr 2p3/2

Components

Pristine PANI coated SS Position(eV) Peak area %

Cr Cr2O3

574.63 576.10

477 3795

3.21 25.54

575.25 576.44

1012 8851

4.36 38.15

Cr(OH)3

577.09

7500

50.48

577.16

9119

39.30

CrO3(Cr6+)

578.39

3086

20.77

578.18

4221

18.19

50.59

Cr2O3/Cr(OH)3 Fe 2p3/2

97.05

Fe

707.05

4146

32.56

706.88

4410

14.13

FeO

708.70

1483

11.65

709.70

6466

20.73

FeOOH

711.18

7103

55.79

711.36

20321

65.14

20.88

FeO/FeOOH O 1s

PANI+Ni(OH)2 coated SS Position(eV) Peak area %

31.82

O2-

530.60

36684

30.59

530.12

45319

33.98

OH-

531.48

72407

60.41

531.58

76743

57.55

H2O

532.91

10787

9

533.21

11290

8.47

32

Graphical abstract

33

Research highlights 

Corrosion resistance enhancement of 304SS by electrodepositing Ni(OH)2 particles in PANI coating



Ni(OH)2 tailors the porosity of PANI coating and enhances the coatings physical barrier



Ni(OH)2 sustains the anodic protection of the coating during the long-term immersion



Mechanism involved in durable anti-corrosive performance by Ni(OH)2 reinforced PANI coating was envisaged

34