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Anti-corrosion Performance of Three Dimensional Hierarchical PANI@BN Nanohybrids Niladri Sarkar, Gyanaranjan Sahoo, Rashmita Das, Gyanaranjan Prusty, Deepak sahu, and Sarat K Swain Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04887 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 5, 2016

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Industrial & Engineering Chemistry Research

Anti-corrosion Performance of Three Dimensional Hierarchical PANI@BN Nanohybrids Niladri Sarkar, Gyanaranjan Sahoo, Rashmita Das#, Gyanaranjan Prusty, Deepak Sahu and Sarat K Swain*

Department of Chemistry, Veer Surendra Sai University of Technology, Burla, Sambalpur-768018, Odisha, India # Presently working as Senior Research Fellow at Department of Instrumentation and Electronics Engineering, Jadavpur University, Kolkata-700098, India * Corresponding author, E-mail: [email protected] Fax: 91-663-2430204, Phone- 91-9937082348

1

ACS Paragon Plus Environment


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ABSTRACT Herein, a facile and effective dilution polymerization route was adopted to prepare microscale hierarchical PANI@BN nanohybrids with a surface textured similar to that of the Aloe-vera leave. Synthesized samples were characterized by FTIR, XRD, FESEM, HRTEM, UV-Visible absorption and TGA/DTG. The hydrophobic nanohybrids with extremely rough surface offered a high barrier for moisture and corrosive environments. Potentiodynamic polarization measurement of PANI@BN/PVA coated steel showed the large shifting in corrosion potential to the anodic region with respect to PANI/PVA. The corrosion inhibition efficiency (IE%) of PANI@ BN/PVA coating on mild steel in 3.5 wt% of NaCl, 1 M HCl and 1 M H2SO4 were calculated from the respective tafel plots. The mechanistic investigation of anti-corrosion performance was carried out through EIS analysis. The higher IE% of the synthesized nanohybrids with PVA coating formulation indicated the superior anti-corrosion performance on mild steel was due to synergetic effect between PANI and BN nanoparticles.

KEYWORDS: Hierarchical structure, electrochemical impedance spectroscopy (EIS), Tafel plots, corrosion, Nanohybrids.

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1. INTRODUCTION Metallic corrosion is one of the serious issues that people are facing ever since the use of metals with huge loss in economy and materials too. Hence, designing of several environment friendly anti-corrosion coatings with superior protection efficiency is still a hot topic of advanced research. Among different conducting polymers, Polyaniline (PANI) is known to be the most potential material in corrosion protection

1,2

and it is because of their easiness of synthesis,

chemical redox reversibility , nontoxic property, good environmental and chemical stability, high electrical conductivity and low cost.3,4 The protection efficiency of PANI based anti-corrosion coatings has already been assured for scratched areas of the coating also where, the bare steel surfaces are in direct contact with the corrosion susceptible environment effectively replace the toxic chromium (VI)

7

5,6

and thereby,

containing coatings. Recently Yang et al.8 has

investigated the efficient anti-corrosion act of polyaniline nanofibers on mild steel in 3.5 wt % saline media. Regarding the protection mechanism of PANI on steel, different mechanistic pathways has been proposed such as corrosion inhibitors, shift of electrochemical interface, anodic protection and barrier protection etc.9-14 According to Kinlen et al., PANI containing coating on steel surface creates an anti-corrosive shell due to the formation of an insoluble iron oxide (Fe2O3) layer at the interface of PANI coating and treated steel surface.15 On the other hand, Torresi et al. has suggested that the anti-corrosion performance of PANI on a metal surface is facilitated due to the formation of a second physical barrier against the penetration of corrosive environment. This fact is supported by the formation of an anion storage at the metal interface in presence of PANI and supposes to release anions when some fracture occurs to the coatings.16 The degree of corrosion inhibition efficiency of PANI also depends on the metal substrates onto which, it is coated and coating formulation.

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However, it is not easy due to the brittleness of PANI. PANI is sparingly soluble in different organic solvents such as N-methylpyrrolidone (NMP), m-cresol and insoluble in water. Moreover, PANI is non-fusible even by heating up to their decomposition temperature. Although electro polymerization offers some advantages

17-19

in getting uniform coating on the metal

surfaces; there is a probability of oxidation or dissolution of corrosion-susceptible metals in the potential domain of the electropolymerization of polyaniline. Hence, a well-known technique of metal coating is to spread the suspensions of microparticles on the large metal surface and to dry them20,21, like painting. To improve the anti-corrosion performance of PANI several strategies has been taken into account. The efficiency of anti-corrosion performance of PANI pigmented vinyl acrylic coating on steel surface in neutral, acid and alkaline media has been successfully reported by Sathiyanarayanan et al.22 On the other hand, the high anticorrosion performance of zinc rich primers containing PANI has been recently established by Meroufel et al.23 The performance efficiency of PANI as anti-corrosion coating can be increased by the addition of other pigments24 and anti-corrosive agents in paints.25 But mostly these are in the macro particulate form. In polymer based coatings, the presence of porosity gives rise to corrosion on the substrates because of the attack of ions through the pores. Hence, utility of various nanoparticles like titanium oxide (TiO2), layered silicates 26-28, graphene 29 within PANI network to increase the barrier performance of PANI based coating has already been established. On the other hand, nano boron nitride (BN) shows excellent properties such as high decomposition temperature, high lubricating effect, chemically inertness, thermal conductivity, oxidation resistance, non wettability and limited surface activities.30, 31 Nano BN has also been proven as efficient fillers to create hurdle in the path of the gas permeability for polymer nanocomposites.32 As per the report of Chen et al.33 BN nanoparticles is Boron nitride (BN) 4


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nanoparticles are noncytotoxic to HEK-293, the human embryonic kidney cells and do not slow down the cell production even after 4 days.33 In recent times, hydrophobic and atomically thinlayered nanomaterials such as graphene

34,35

or hexagonal boron nitride (h-BN)

36

have been

established for their effectiveness in anti-corrosion performance for the longevity of marine instruments. In present work, we report a facile and effective protocol for dilution polymerization of aniline in presence of aqueous emulsion of nano boron nitride to fabricate micro-scale 3D hierarchical PANI@BN nanohybrids. The experimental conditions and stoichiometry of aniline, nano BN and oxidant are adjusted in such a manner that during polymerization nano BN wrapped hierarchical PANI@BN nanohybrids are only formed. The designed nanohybrids are structurally characterized and its suitability as anti-corrosion coating on mild steel is carefully investigated with three-electrode system in acidic and saline media.

2. EXPERIMENTAL PROCEDURE 2.1 Materials Aniline (Central Drug House Pvt. Ltd., New Delhi, India) was distilled under vacuum prior to use. Boron nitride (BN) nanopowder (M.W~ 24.82) of average particle size of 70 nm was purchased from Sisco Research Laboratories Mumbai, India. Ammonium persulfate (Central Drug House Pvt. Ltd., New Delhi, India) was purified by the recrystallization from ethanol. Other chemicals used for this purpose were of analytical grade and used as such. Double distilled water was used for the preparation of all solutions.

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2.2 Synthesis of PANI Nanofibers and PANI@BN Nanohybrids For the synthesis of PANI nanofibers, 0.0037 g of aniline was dissolved in 50 mL of HCl solution (1 mol/L) and the concentration of aniline in reaction vessel after adding APS containing HCL HCl (1 mol/L) solution (ANI: APS = 2: 1) was maintained below 0.008 mol/L. Solution mixture was maintained at low temperature (0 oC) for two days to complete the polymerization process.42. Here we followed a smart approach for the synthesis of hierarchical PANI@BN nanohybrids in presence of colloidal BN nanoparticles with the same reaction parameters as we had for the synthesis of PANI nanofibers. In a typical procedure, the BN nanopowder was first suspended in 30 mL of double distilled water under ultrasonication for 30 min (120W/60kHz) to reduce the aggregation of BN nanoparticles. The desired amount (ANI: BN= 37: 1) of aniline (0.037 g) was dissolved in 50 mL of HCl solution (1 mol/L). Then these two solutions were mixed together and stirred for 2 h. After that, 50 mL of HCl solution (1 mol/L) containing APS ((NH4)2S2O8) with a molar ratio of 2: 1 with respect to aniline, was added rapidly to the well dispersed suspension with stirring for 2 min at 0 oC in an ice-water bath and kept for 2 days at this temperature at undisturbed condition. The resulting dark-green product was collected by filtration and washed thoroughly with double distilled water and methanol for several times until the washing solution become colorless. Finally, fine dark green powder was obtained by vacuum drying at 60 o C for 12 h. 2.3. Preparation of PANI@BN/ PVA Containing Coatings on Mild Steel The square specimen (3 cm× 1.5 cm× 0.05 cm) mild steels (MS) samples were polished by sand paper. After rubbing with sand paper, all steel samples were subjected to ultrasonication in a solution of acetone and ethanol to degrease before coating. The elemental composition of the treated steel samples is given in Table S1. PVA (0.5 g) was dissolved in distilled water (10 ml) 6


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with continuous stirring for 30 min at 50 oC and then in an ultrasonicator for another 10 minutes at room temperature. The 10 % (w/v) of PANI@BN nanohybrids powder was crushed and slurry was made with the prepared PVA solution and then sonicated for 30 min. After sonication, a uniform dispersion of PANI@BN nanohybrids in PVA solution was observed with no settling even after 1 h at undisturbed condition. The mild steel samples were painted with the PANI@BN-PVA coating formulation and dried for 30 min at room temperature followed by air baking in an oven at 50 oC for 4 h to form a uniform film of thickness 1.5 µm.. Same synthetic protocol was adopted for coating of PANI-PVA on mild steel. 2.4. Standard characterization techniques Structural investigation of the synthesized PANI and PANI@ BN nanohybrids were carried out by examining the observed X-ray diffraction (XRD) patterns through Rigaku X-ray machine operating at 40 kV and 150 mA. UV-visible spectra of the synthesized samples and BN nanoparticles in dimethyl sulfoxide (DMSO) solvent were measured with a Shimadzu UV-2550 UV-visible spectrophotometer using a quartz cell with 1 cm path length. To investigate the functional group attachment of the synthesized PANI and PANI@BN nanohybrids, FTIR spectra of the prepared samples (in form of KBr pellets) were recorded using a Shimadzu IR Affinity-1 Fourier infrared spectrophotometer in the range of 4000 cm-1 to 400 cm-1. The surface texture of the synthesized samples was examined with field emission scanning electron microscope (FESEM) from Jeol Ltd., Japan (model 5200 with magnification of x 30000). The structural design of the samples was studied using high-resolution transmission electron microscope (HRTEM) operated at 120 kV (Tec-nai 12, Phillips, Hillsboro, OR, USA). The average particle size (in terms of particles diameter) distribution profile of the synthesized PANI@BN nanohybrids was speculated from the dynamic light scattering (DLS) study, using a model BI7



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200SM instrument (Brookhaven Instrument Corporation). The surface topology of the synthesized PANI and PANI@BN nanohybrids was carried out from AFM measurements using pico plus 5500 ILM AFM, with a piezoscanner maximum rang of 100 µm. Sample specimens for AFM measurement were prepared by making a thin film of purified samples on a mica foil. The thermal stability of the synthesized nanohybrids was investigated using thermogravimetric apparatus (DTG-60, Shimadzu Corporation, Japan) under nitrogen purge with a heating cycle of 10

o

C/min.

All

electrochemical

measurements

were

performed

by

three-electrode

electrochemical cell using Autolab Potentiostat/Galvanostate101 (Netherlands). In a three electrode electrochemical system, Pt electrode was used as counter electrode, whereas; Ag/AgCl electrode was used as a reference electrode. Tafel plots of the mild steel, PANI/PVA coated steel and PANI@BN/PVA coated steel were investigated in different corroding media, such as 1M H2SO4, 1 M HCl and 3.5 wt % NaCl solution by using the coated steels as working electrode. At room temperature (26 ± 2 oC), 100 mL of 1M HCl/1 M H2SO4/3.5 wt % NaCl solutions was used as electrolyte solution in the electrochemical experiments. The exposed area of the working electrode to the electrolytic solution was 4.5 cm2. After the immersion for 15 min to the electrolyte solution, Tafel plots were generated by sweeping the potential from −0.67 V to + 0.67 V with respect to the corrosion potential at a scan rate of 10 mV·s−1. To confirm the reproducibility of the potentiodynamic polarization measurements, samples with each microstructure were subjected to the same test for at least three times. The electrochemical impedance spectroscopic (EIS) analysis was carried out in 3.5 wt % NaCl

(aq)

with the same

electrode configuration in the frequency range of 2 MHz to 100 MHz at open circuit potential with perturbation amplitude of 5 mV. Electrochemical parameters were evaluated with the help of the software Nova (version 1.1) by fitting the obtained impedance data according to a suggested equivalent circuit model. The surface morphology of uncoated and coated steels 8


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before and after the exposure to the 3.5 wt% NaCl medium was examined through scanning electron microscope from Hitachi SU3500, Japan.

3. RESULT AND DISCUSSIONS 3.1. Formation Mechanism of 3D Hierarchical PANI@BN Nanohybrids The structural evolution of individual PANI nanostructures consists of three consecutive steps: In the first step, small molecular aggregations i.e, nucleates are formed. In the next step, nucleates are self assembled and organized to form a larger aggregates. In the final step, these larger aggregates are grown to form PANI chains. 37,38 The aniline dimers and semidines are the first oxidation products of aniline oxidation. In next step, aniline trimers with phenazine moiety are formed.39 These small molecular aggregations are oligoaniline and are termed as nucleates. At high acidic conditions (pH< 2.5), nucleates are converted to initiation centers that start the subsequent propagation of PANI chains. The organized self-assembly of hydrophobic nucleates into 1-D stacks by the interplay of π-π interactions, hydrogen bonding, and cross-linking leading to the formation of PANI nanofibers. The growth of polyaniline chains is generally directed perpendicularly from the single stack of nucleates to produce the whole body of nanofibers.40 Hydrophobic front of the nanofibers act as the initiation centre for the formation of new nucleates, thus extending the 1-D columnar structure. The branching of nanofibers is occurred due to absorption of free nucleates.

41-43

In homogeneous phase, one dimensional growth of the

nanofiber is preferred instead of starting new nanofibers. The random aggregation of nucleates, which would result in a granular morphology, is suppressed by lowering the concentration of aniline as well as by the lowering of temperature.44 But due to slight overgrowth of polyaniline on the preformed nanofibers, it will lead mostly irregularly shaped agglomerates. In presence of 9



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aqueous emulsion of boron nitride nanopowder, due to strong π-π interaction with the boron nitride surface and the nucleate oligomer during polymerization of PANI, the organized self assembly of preformed nucleates into 1-D columnar structure is hampered and it follows the hierarchical structural evolution of PANI chains with the core of the hierarchical tube filled up with the self assembled boron nitride nanoparticles. The morphological evolution of PANI nanofibers and 3D hierarchical PANI@BN nanohybrids is schematized in Figure 1.

Figure1. Genesis of hierarchical PANI@BN nanohybrids.

3.2. XRD Analysis X-ray diffraction study (XRD) of PANI, nano BN and PANI@BN nanohybrids are presented in Figure 2(a). The synthesized pure PANI is amorphous in nature and gives a broad peak centered at 2θ value of 25 o may be ascribed due to the periodicity parallel to the polymer chains of PANI.45 This confirms the lower crystallinity of emeraldine salt of PANI as compared to emeraldine base. The nano BN shows the crystalline peaks at 2θ value of 26.7

o

along with

two small peaks at 41.66 o and 55.36 o (inset figure 2-a) respectively. These crystalline peaks of nano BN can be indexed to the (002), (100) and (004) planes respectively. Due to strong π-π 10


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interactions between the π electron clouds of nano BN and the benzene, quinoid rings of the PANI backbone, some sort of crystallinity is developed within hierarchical PANI@BN nanohybrids and can be evidenced from the generation of crystalline peaks at 2θ values of 19.20o and 25.38o apart from characteristic crystalline peaks of nano BN. Hence, a well crystalline phase within PANI is developed due to the ordered arrangement of PANI layers which in turn is triggered by the strong π-π interactions between nano BN and the PANI backbone. The strong ππ interaction is also evidenced from the UV-visible spectrum of PANI@BN nanohybrids (Figure S1 in supporting information). 3.3. FTIR Analysis The interaction established between the BN nanoparticles and PANI is shown by the FTIR spectra (Figure 2-b). The bands at 1555 cm−1 is assigned to the ring quinoid ring (Q) C-C stretching in the semiquinonoid (NH=Q= NH) structure, while the band observed at 1463 cm−1 is due to benzonoid (BZ) ring C-C stretching in N-BZ-NH units (“B-band”) 46,47. The intensity ratio of Q-band at about 1555 cm−1 and B-band at 1463–1470 cm−1 corresponds to that of emeraldine salt form. The bands at 1285–1299 and 1070 cm−1 are assigned to C-N+• stretching in SQ segment and C-N stretching of secondary aromatic amine respectively. All of the above peaks are distinct characteristics of the ES-PANI backbone. On the other hand, FTIR spectrum of nano BN shows two dominant peaks at ∼780 and ∼1512 cm -1, assigned as B-N-B bending and B-N stretching mode respectively. 48,49 Due to strong π-π interaction, the FTIR peaks for hierarchical PANI@BN are shifted with respect to PANI nanofibers.

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Figure 2: (a) Powder XRD pattern and (b) FTIR spectra of PANI, nano BN and PANI@BN nanohybrids 3.4. Morphology and Surface Topology Analysis The low magnification FESEM image (Figure 3-a) discloses that the as obtained dark-green sample is composed of a large quantity of irregularly shaped PANI nanostructures with varying length and diameter. In higher magnification FESEM image (Figure 3-b), it is interestingly found that these nanostructures are branched 1D nanofibers. FESEM image of pure BN nanoparticles is shown in inset of Figure 3(c). Since the FESEM technique is inadequate to see the internal features of nanofibers, the samples are subjected to HRTEM analysis. From HRTEM TEM images as shown in Figure 4(a) and b of PANI, the presence of polyaniline nanofibers and a thin layer of oligoaniline sheet covering the nanofiber surfaces are observed. Nanofibers are observed with almost uniform diameter of 50 nm. From Figure 3(c) and (d) of PANI@BN nanohybrids, the distinct 3D hierarchical morphology with larger in length (several µm) is noticed. As shown in Figure 4(c-e) of PANI@BN nanohybrids, unique Aloe-vera leaf textured shapes are observed throughout with an inner diameter more than 100 nm. The appearance of wide inner diameter 12


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during the structural evolution of hierarchical PANI@BN nanohybrids, evidence the presence of self assembled BN nanoparticles within the core structure. The self-assembled BN nanoparticles are found to be wrapped with the thin layers of PANI and it is clearly viewed from HRTEM TEM images (Figure 4-e). The wall thickness of the synthesized micro-scale hierarchical PANI@BN are found to be 50 nm. HRTEM image (inset Figure 4e) of PANI@BN nanohybrids shows the crystalline dark dots on the surface of the nanohybrids with a lattice fringe of 0.35 nm and confirms the presence of BN nanoparticles. Elemental compositions and the presence of nano boron nitride within the hierarchical PANI@BN nanohybrids are confirmed through EDS analysis (Figure 3-e). The crystalline phases of nano BN within hierarchical PANI@BN could easily be identified through selected-area electron diffraction (SAED) pattern (Figure 4-f). Surface roughness is further analyzed by using atomic force microscopy (AFM). The surface roughness of the synthesized samples is calculated by measuring the peaks height in the out of plane (z) direction, using sophisticated AFM software. The three dimensional AFM images (Figure 5 a,b) are captured with a scan rate and scan size of 0.5 lines/s and 0.5µm-5µm respectively. Images are processed by flatten using Pico view 1.12 version software. From the image, it is found that the surfaces of nanohybrids show elevated peaks, evidencing extremely rough surfaces. The average roughness of PANI is found to be 41.08 nm (Figure 5-a). Further, the average roughness of the nanocomposite in z direction is 97.41 nm (Figure 5-b). The increased roughness may be beneficial for the hindrance of the mass transportation through the nanocomposite.

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Figure 3: FESEM images of (a, b) PANI and (c, d) PANI@BN nanohybrids; (e) EDS spectra of PANI@BN nanohybrids (inset of (c): FESEM image of pure BN nanoparticles)

Figure 4: HRTEM TEM images of (a, b) PANI and (c-e) PANI@BN nanohybrids; (f) SAED pattern of PANI@BN nanohybrids (inset of (e): HRTEM image of PANI@BN nanohybrids) 14


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Figure 5: 3D AFM images of (a) PANI and (b) PANI@BN nanohybrids (Inset diagrams I and II are the respective water wettability variation)

3.5. Hydrophobicity of Hierarchical PANI@BN Nanohybrids Hydrophilic character of polyaniline (PANI) chains is attributed by the presence of different hydrophilic functional moiety such as SO3H- , SO42- etc. On the other hand, a honeycomb like structure with alternate arrangement of boron and nitrogen atoms imparts strong hydrophobicity to boron nitride nanoparticles. Hence, incorporation of boron nitride nanoparticles within the core structure of PANI@BN makes the material hydrophobic in nature. Figure 5a-I and b-II (insets) illustrate the contact angle variation of a water drop on PANI film and PANI@BN nanohybrids film. The observed contact angle is significantly increased from ∼42° to ∼80°. It is possibly due to the incorporation of nano BN within PANI chains that modified the PANI@BN 15



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microstructure which leads to different functional group coverage on its surface. The rough surface topology of PANI@BN, shown in Figure 5b, may be accounted as another reason of such observed phenomenon. This result is in accordance with the earlier report.50 The presence of fractural surface with enlarged surface area is generally assigned as a good repellent to any liquid. This factor opened up a new way to the delicate and smart adjustment of PANI wettability in many technological applications 3.6. TG Analysis Figure 6a shows the thermal decomposition profile of pure PANI, whereas; Figure 6b shows the corresponding to DTG curve with majority of weight loss for respective steps. Thermal decomposition of pure PANI occurs in a three step weight loss processes. The first thermal degradation, just below 100 oC is attributed to the expulsion of water molecules from surface of polyaniline nanofibers whereas; the second thermal degradation at around 200-300 oC is supposed to be due to the elimination of acid dopant (HCl) and oligoaniline from the PANI backbone. The final degradation starting at around 300- 400 oC is because of the structural decomposition of the PANI backbone. The excellent thermal stability of pure PANI can be assigned from the gradual weight loss over the wide range of temperature. In TG (Figure 6-b) of hierarchical PANI@BN, it follows the same three steps decomposition pattern, but the residue left at each decomposition temperature is increased due to the incorporation of nano BN within PANI chains. The residue amount left at 700oC for pure PANI is 45 % by weight, whereas for PANI@BN this value is almost increased to 10 % higher, i.e. 55%. It concludes that the synthesized pure PANI exhibits a good thermal stability which is enhanced by the incorporation of nano BN within hierarchical PANI@BN. Similar trend has been reported for Na+montmorillonite intercalated polyaniline nanocomposites.51 16


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Figure 6: (a) TG analysis of PANI, nano BN and PANI@BN nanohybrids (b) Combined TG and DTG curves of PANI@BN nanohybrids.

3.7. Potentiodynamic Polarization Measurements The potentiodynamic polarization measurements for uncoated mild steel and PVA, PANI/PVA and PANI@BN/PVA coated mild steel as working electrodes in different electrolytic solution such as 1M HCl, 1 M H2SO4 and 3.5 wt.% NaCl aqueous solutions are shown in Figure 7(a, b & c) respectively. Values of the electrochemical corrosion parameters, such as corrosion current density (Icorr), corrosion potential (Ecorr), corrosion rate (CR) and polarization resistance (Rp) in different corrosion conditions are presented in Table 1. Stearn-Geary equation (equation-1) is employed to calculate the polarization resistance (Rp) of the coating materials from the respective Tafel plots.52, 53

.

…………….. (1)

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Where, βa and βc are the anodic and cathodic slopes (∆E/∆log I) of the anodic and cathodic curves of the respective Tafel plots and the corrosion current (Icorr) are determined from the intersection point of the linear portions of the anodic and cathodic components of the Tafel plots. The rate of corrosion (CR) for different coating formulations is calculated as equation 2.54

⁄

⁄ . ………… ⁄! . "

(2)

Where, V is the valence, M is the molecular weight, 3270 is a constant and D is the density. The corrosion inhibition efficiencies (IE %) of the PANI containing coatings on mild steel surfaces are calculated from the following equation 3.8

#$ %

& '()'*'+,-

× 100%………….. (3)

Where, Icorr(inhibition) and Icorr are the values of corrosion current density of inhibited and uninhibited specimens, respectively The higher values of βa as compared to βc , for coated steel indicate that the application of external current strongly polarizes the anode. The development of PANI and PANI nanostructure containing coating on the steel surface highly influence the βa values, as a consequence of which the anodic dissolution is decelerated. On the other hand, pure PVA-coated mild steel just has a low Rp value 2.75 kΩcm2. Such lower value of polarization resistance (Rp) may be attributed from the poor barrier property of the coating material on mild steel in absence of any filler. When pure PVA coated mild steel is immersed in saline media (3.5 wt % NaCl

(aq)

solution),

corrosion assisting small molecules, such as O2 and H2O or ions (Cl-) can easily penetrated into the PVA film and comes in the direct contact with the steel surface, so that the process of corrosion takes place immediately. Once PANI nanofibers are introduced in coating formulation, 18


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polarization resistance is maintained to a high value as 32.58 kΩcm2, indicating better corrosion protection than PVA coating. But on introducing hierarchical PANI@BN nanohybrids into the coating formulation, the value of Rp is increased unexpectedly from 32.58 kΩcm2 to 170 kΩcm2 leading to corrosion inhibition efficiency of about 98 %. Table 1: Calculation of different corrosion parameters from the respective Tafel plots in different corrosion environment Sample

Ecorr (mV)

Βc (mVdec-1)

βa (mVdec-1)

Rp (kΩcm2)

Icorr (µA/cm2)

IE %

CR (mm/year)

3.5 wt % NaCl solution Steel

-520

87.4

95.60

1.73

11.48

-

0.418

Steel-PVA

-479

85.1

100

2.75

7.24

36.9

0.263

Steel-PANI/PVA

-265

172

156

32.58

1.09

90.50

0.039

139.6

179.5

170

0.2

98.25

0.007

Steel-PANI@BN/PVA -69

1 M HCl solution Steel

-446

125

85

1.91

11.49

-

0.418

Steel-PVA

-419

92

130

2.62

8.91

22.4

0.324

Steel-PANI/PVA

-158

148

160

8.84

3.38

70.5

0.123

190

230

43.9

1.03

90.95

0.037


1 M H2SO4 solution Steel

-458

92.9

225

0.967

29.51

-

1.075

Steel-PVA

-400

82.9

166

2.24

10.71

63

0.390

Steel-PANI/PVA

-215

213

305

21.69

2.51

91.49

0.091

108

121

126.42

0.19

99.30

0.007


19



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Figure 7: Tafel plots of uncoated steel, PVA-coated steel, PANI-PVA coated steel and PANI@BN-PVA coated steels in (a) 3.5 wt% NaCl (b) 1 M HCl and (c) 1 M H2SO4 solution;(d) Variation of OCP with time to the exposure of 3.5 wt% NaCl solution for coated and uncoated steels. The open circuits potentials (OCP) of bare steel, steel-PVA, steel-PANI/PVA and steelPANI@BN/PVA are determined from these plots recorded after different times of exposure (50 days) to hot saline (3.5 wt% of NaCl) environment are shown in Figure 7(d). OCP measurements are carried out for 50 days with an interval of 24 h. Reproducible results are obtained from the 20


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several measurements of the same samples. OCP measurement versus time provides an indication of the stability of the coating during exposure to a corrosive solution. The initial observed OCP of PVA coated mild steel is resided on the cathodic region as like bare steel samples, but the value of OCP is more positive (− 479 mV vs Ag/AgCl) than that of bare steel (−520 mV vs Ag/AgCl). After 50 days of exposure to saline environment, the OCP value of PVA coated steel shifts to −577 mV vs Ag/AgCl, i.e. to the more cathodic region. This gradual decreasing trend of OCP with exposure time to saline environment is not identified for PANI containing coatings. Moreover, PANI-PVA or PANI@BN-PVA coated steel shows a self recovery effect. This recovery effect is ascribed as the initial decrease in the OCP values and then sudden increase in the OCP values to the more anodic region after some time (after 20 days). Such shifting of OCP values to more noble direction is more pronounced for nano BN incorporated PANI@BN nanohybrids than PANI-PVA coated steel samples. High OCP values of PANI@BN-PVA coated steels as compared to that of bare steel, plain PVA as well as PANI/PVA coated mild steels even after 50 days of the immersion to saline environment clearly indicates the high corrosion resistance of PANI@BN/ PVA coating. 3.8. Electrochemical Impedance Spectroscopic (EIS) Analysis The mechanistic pathway of anti-corrosion performance of PANI@BN/PVA coating on mild steel can be better understood from electrochemical impedance spectroscopy (EIS). It not only provides the information about the electrode processes, but also gives the nature of capacitance formed at the interface between PANI@BN/PVA coating and mild steel surface. Figure 8(a) shows the representation of Nyquist plot of bare steel, PVA, PANI/PVA and PANI@BN/PVA coated steel in 3.5 wt % of NaCl solution. As per the equivalent circuit model, the working electrode (PANI-PVA and PANI@BN/PVA coated steel samples) are composed of 21



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resistors and capacitor. In circuit diagram, Rs is the resistance of the electrolyte, Rp is the polarization resistance, representing the difficulties in the way of corroding the iron surfaces and Q shows an electrical double layer capacitor, arising from the charge accumulation at the interface between steel surfaces and coating polymers. The double layer capacitance and the charge transfer resistance of the electrode are reflected from the semicircle nature of the Nyquist plot. Importantly, the incorporation of boron nitride nanoparticles within the core structure of PANI@BN/PVA coating formulation does not influence the shape of the impedance profile of PANI/PVA coated steel surface. However, the impedance profile of bare steel and PVA coated mild steel is slightly different in nature with two capacitive loops. The single semicircle in the Nyquist profile for PANI/PVA and PANI@BN/PVA coated steel surface indicates the occurrence of a single charge-transfer reaction in high frequency region. The increase in arc length of the Nyquist diagram of PANI@BN/PVA coated steel as compared to PANI/PVA and PVA coated steel indicates the higher corrosion efficiency of PANI@BN/PVA coating formulation. Figure 8(b) shows the Bode plots for bare mild steel, PANI/PVA, PANI@BN/PVA and coated steels in 3.5 wt % NaCl solution. Generally, an ideal capacitance is characterized with a phase angle (α) of − 90° and slope (S) of -1. However, a linear region is observed in the Bode plot of PANI@BN/PVA coated steel sample with slope (S) of 0.87 and phase angle (α) of -73o. The result suggests a protective layer is formed at the interface of working electrode and electrolyte solution. The experimental data is well fitted according to the proposed equivalent circuit. The results are summarized in Table S2. The corrosion inhibition efficiency (IE %) can be calculated on the basis of Rct values by the equation 4.

IE%

345 -3°45 345

× 100………. (4)

22


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Where, Rcto and R

ct

are charge-transfer resistance in the absence and presence of

polymeric coating on steel surfaces, respectively. The “n” values are in the range of 0.84 to 0.99, which is close to unity. The high efficiency of PANI nanofibers in coating formulation with PVA is shown as IE of 95 %, but with the incorporation of boron nitride nanoparticles within the core structure of PANI has remarkably influenced the anti-corrosion performance of PANI@BN/PVA with IE of 99 %.

Figure 8: (a) Nyquist plots of uncoated steel, PVA-coated steel, PANI-PVA coated steel and PANI@BN-PVA coated steels in 3.5 wt% NaCl (b) Bode plots of uncoated steel, PVA, PANIPVA and PANI@BN-PVA coated steel samples in 3.5 wt% NaCl 3.9. Surface Characterization The interface morphology between treated steels and their respective coatings is shown in the Figure S2. Figure S2-(a) shows the SEM image of sand blasted steel sample which exhibits some obvious scratches due to the rubbing of the surface with sandpaper. The coating should be free from any micro voids, which could allow the corroding environment to direct contact of the steel 23



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surfaces. From SEM images, a distinct difference of the interface morphology between treated steel with PANI-PVA and PANI@ BN/PVA coating and also with the uncoated steel sample has been observed. With higher content of PVA in PANI@BN/PVA blend, the interface is appeared as porous morphology (Figure S2-b), whereas; for 10 (w/v %) of PANI@ BN in PVA-coating formulation (Figure S2-c, d), a compact layer of uniform and smooth microstructure is developed on the metal surface, which has improved the corrosion resistance of the material. After 50 days exposure to 3.5 wt % saline environment, the changed surface morphology of the bare mild steel shows a highly corroded surface with some micro-sized particles on the surface. These microsized particulates can be assigned as rust Fe3O4, formed due to the electrochemical reaction of mild steel and aggressive environment (Figure 9c). But when the steel samples are coated with PANI/PVA (Figure 9b) and PANI@BN/PVA (Figure 9d), exposure to saline environment does not make any substantial damage of the treated steel surface because of the corrosion, only tough lamellae are appeared on the surface layer. From figure 9 (a-d), it is clear that the extent of surface depletion is higher for uncoated steel, whereas; almost unaffected for PANI@BN/PVA coated steel surface. This realization is visually reflected in digital picture of the specimen steel with or without coating (Figure 9, I-VII). It could also be seen that the pure PVA-coated steel suffered from infuriating corrosion with the generation of thin layers of red rust (Fe3O4) but it is hardly affected in PANI@BN-PVA coated mild steel.

24


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Figure 9: Digital photographs of uncoated, PVA-coated, PANI-PVA coated and PANI@BNPVA coated steel before (I-IV) and after (V-VIII) the exposure to hot saline media; SEM images of steel interface of uncoated steel before (a) and after (c) the 50 days exposure to 3.5 wt % saline media; SEM images of (b) PANI-PVA coated and (d) PANI@BN –PVA coated steel after the 50 days exposure to saline media. 3.10. Mechanism of Corrosion The hierarchical PANI@BN nanohybrids in coating formulation with poly vinyl alcohol (PVA) can improve the adhesion and mechanical properties of the coating by increasing the microscopic interactions between PANI@BN nanohybrids and treated steel sample. The nanohybrids coating on metal surface provides a high barrier for the diffusion of Cl- , H2O and O2 due to the extremely rough surfaces of PANI@BN nanohybrids. The hydrophobic nature of the nanohybrids also prevents the penetration of moisture to the metal surface. The most interesting findings of the present experiments are the self-healing effects. According to Wessling and co-workers13, corrosion protection for PANI occurs via formation of a passivating Fe2O3 layer at the interface of metal and PANI coating. This tendency of forming a passivating layer on to the coated surface is also pronounced for PANI@BN nanohybrids coating and this is reflected from the shifting of potential to anodic sides more as compared to other coated

25



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Page 26 of 35

materials. Apart from this effect, the resulting thin coating of PANI@BN nanohybrids also act as an anion storage due to synergetic effect of PANI and nano BN and provide a electrical barrier for electron transport and thereby slowing down the electrochemical reactions that are responsible for metallic corrosion. All possible aspects of anticorrosion performance of PANI@BN coated mild steel is schematized in Figure 10. The above factors lead much better performance than the earlier report of PANI containing coating.55

Figure 10: Proposed anti-corrosion mechanism of PANI@BN nanohybrids coated steel.

4. CONCLUSION Hierarchical PANI@BN nanohybrids are synthesized in presence of the aqueous emulsion of nano BN by the oxidative polymerization of aniline by ammonium persulphate (APS) in dilute condition. The synthesized PANI@BN nanohybrids are characterized by using XRD, FTIR, FESEM and HRTEM. These nanohybrids show a unique hierarchical morphology rather than nanofibrillar morphology. It may be due to the presence of strong π-π interactions of nano BN with the quinoid and benzene rings in PANI. This is evidenced from UV-visible spectrum of 26


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PANI and PANI@BN nanohybrids. The synthesized PANI@BN nanohybrids are electroactive like pure PANI. The corrosion protection properties of the PANI and PANI@BN nanohybrids coated mild steel exposed to 1 M HCl, 1 M H2SO4 and 3.5 wt % NaCl solutions are investigated by potentiodynamic polarization measurements. Anti-corrosion performance of PANI@BN/PVA coated steel samples in 3.5 wt % of NaCl solution is analyzed through EIS data. Different corrosion parameters such as polarization resistance (Rp), corrosion potential (Ecorr), capacitance (Q), corrosion rate (CR) and corrosion current density (Icorr)

are calculated from the

potentiodynamic polarization measurements and also from the EIS measurements for PVA, PANI-PVA and PANI@BN/PVA coated steel samples in presence of different corrosion media. From this parametric estimation, it is confirmed that PANI@BN nanohybrids with PVA coating formulation exhibits higher anti-corrosion performance than that of PVA and PANI/PVA containing coatings. With incorporation of boron nitride nanoparticles within the core structure of PANI@BN nanohybrids, hydrophobicity as well as the crystallinity of the PANI nanostructure is influenced highly as it is observed from the XRD patterns, FESEM and HRTEM images of PANI@BN nanohybrids. These changed structural aspects of PANI@BN nanohybrids may be encountered for the higher anti-corrosion performance of PANI@BN/PVA coating on mild steel. The interface morphology of treated steels and their respective coatings before and after the exposure of 3.5 wt% of hot saline medium are visualized from SEM images. These images are in good agreement with potentiodynamic polarization measurements and also reveal the higher corrosion inhibition efficiency of hierarchical PANI @BN nanohybrids than pure PANI.

27



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ASSOCIATED CONTENT Supporting Information UV-visible spectra of PANI, nano BN and PANI@BN nanohybrids, Table for the elemental composition of treated steels, electrochemical parameters obtained by fitting of EIS data, SEM images of uncoated and coated steel with different coating composition of PANI@BN nanohybrids and PVA are included in the supporting information. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Fax: 91-663-2430204, Phone- 91-9937082348 .ACKNOWLEDGMENT Authors express their thanks to Department of Science and Technology (Biotechnology) Government of Odisha, India (No. 2332/ST/ST-II-(SC)/34/ for financial support.

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