Synthesis and Anticorrosive Properties of Novel PVK-ZrO2 Nano Composite Coatings on Steel-Substrate

A novel nanocomposite coating containing poly (N -vinyl carbazole) (PVK) and zirconia (ZrO2) nanoparticles was synthesized using chronoamperometry. The effect of ZrO2 nano particles into the PVK matrix and its mechanical, corrosion protection ability on 316L SS substrates were studied. The incorporation of the nanoparticles was endorsed by attenuated total reflectance infrared spectroscopy (ATR-IR) and EDAX analysis. The spheroidal morphology and core-shell structure were revealed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The thermal studies exhibited the stability of coating. The anticorrosion performance of PVK-ZrO2 nanocomposite coatings in 3.5 weight (wt.)% NaCl medium was performed with potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS) studies. The corrosion rate of PVK-ZrO2 nanocomposite coated 316L SS was found to be 0.58 times lower than uncoated 316L SS and a significant decrease in corrosion potential was realized from −0.211 VSCE for uncoated 316L SS to 0.112 VSCE for PVK-ZrO2 nanocomposite coated 316L SS substrates. The results of this study clearly ascertain that the PVK-ZrO2 nanocomposite has an outstanding potential to protect 316L SS against corrosion in a 3.5 wt.% NaCl environment. [DOI: 10.1380/ejssnt.2018.5]


INTRODUCTION
In the recent past, ever since the first suggestion made by MacDiarmid (1985), Conducting polymers (CP) have been employed in the development of smart corrosion inhibiting coatings because of their good environmental stability and chemical resistance to the metal surface [1]. There are numerous reports on the passivating effect of electro active polymers on steels in various environments. There is substantial evidence that electroactive polymers, such as polyaniline (PAni), polypyrrole (PPy), polythiophene etc and its derivatives can offer protection to steel in certain corrosion environments [2][3][4]. However, all these polymer coatings have a serious practical issue, that is, permeation of corrosive species such as oxygen, water and ions to some extent. Incorporation of metal oxide nanoparticles into the conducting polymers minimizes the penetration of these corrosive ions and enhances the corrosion resistance. In the past, several oxide nanoparticles such as; alumina (Al 2 O 3 ) [5], manganese dioxide (MnO 2 ) [6], vanadium pentoxide (V 2 O 5 ) [7], titania (TiO 2 ) [8], ferric oxide (Fe 2 O 3 ) [9], tungsten oxide (WO 3 ) [10], zinc oxide (ZnO) [11], silica (SiO 2 ) [12] and zirconia (ZrO 2 ) [13] were incorporated into CP matrix and its corrosion protecting ability was explored. For instance, poly (o-toluidine) /ZrO 2 nanocomposite coating was effective to control the corrosion rate of mild steel [14]. The Electrochemical polymerization can be carried out to form N -vinyl carbazole (NVC) films on various metal substrates. The corrosion protection offered by selfassembled films of carbazole on copper (Cu) in NaCl solution were recently studied [16]. Exploring the literature confirms that the incorporation of various nanoparticles into PVK matrix enhances properties like antimicrobial [17], charge carrier transport [18], super capacitance [19], photoconductivity [20], optical [21], electrical conductance [22], thermal stability [23] and electrochemical [24]. It is evident from the literature that few investigations were carried out in the past to validate the corrosion protection offered by the PVK nano composite films on steels. Among the inorganic oxide nano particles, ZrO 2 is known for its high strength, high fracture toughness, wear resistance, high hardness and chemical resistance [25]. Several research groups have studied the influence of the ZrO 2 nano particles on the corrosion behavior [26,27]. The influence of ZrO 2 nanoparticles on the surface and electrochemical behavior of polypyrrole nanocomposite coated 316L SS in simulated body fluid was reported [14]. However, quantitative studies on the effect of ZrO 2 nano particles in PVK coatings as poten-ISSN 1348-0391 © 2018 The Surface Science Society of Japan (http://www.sssj.org/ejssnt) tial material for corrosion protection are scanty. Hence, it becomes important to investigate the following objectives (i) to electropolymerise PVK coatings on 316L SS using chronoamperometric technique, (ii) to synthesize PVK-ZrO 2 nano composite coatings by incorporating the ZrO 2 nano particles into the PVK matrix using chronoamperometric technique (iii) to examine the surface, thermal and mechanical characteristics of the nanocomposite coatings and (iv) to examine the protection ability of PVK and PVK-ZrO 2 nano composite coatings in 3.5 wt. % NaCl medium. The results of this work were compared with our earlier work to elucidate the properties of the PVK-ZrO 2 nanocomposite coatings and the effect of varying weight ratio of ZrO 2 nanoparticles [5].

A. Chemicals
Analytical grade N -vinyl carbazole (VC) monomer, acetonitrile (ACN), LiClO 4 and all other chemicals used in this study were purchased from Sigma Aldrich. ZrO 2 nano particles were synthesized using the procedure reported elsewhere [27].

Deposition of coating and electrochemical characterization
All the electropolymerization processes and electrochemical corrosion experiments were carried out with electrochemical workstation CHI 760D in a standard cell equipped with 316L SS as working electrode, platinum (counter electrode) and standard calomel electrode (SCE) as reference electrodes. Prior to the deposition of the coating, 316L SS substrates with dimensions 12 × 15 × 2 mm were abraded up to 600 # using silicon carbide (SiC) emery sheets and ultrasonicated in acetone for about 15 min. to remove the impurities. It was further rinsed with water and air dried. The 316L SS samples were passivated in 50 % nitric acid for about 10 minutes and used for electrochemical polymerization. Electro polymerization of vinyl carbazole (VK) on the working electrode (316L SS) was performed with the chronoamperometric technique by applying a constant potential of 1.2 V SCE for 1000 s in ACN with 0.1 M LiClO 4 as supporting electrolyte [5]. All the potentials were measured with respect to SCE electrode. For the incorporation of ZrO 2 nanoparticles into the PVK matrix, 1, 2 and 3 wt.% of ZrO 2 nano particles with respect to monomer (N -vinyl carbazole) concentration was dispersed in the acetonitrile solution containing monomer and LiClO 4 by ultrasonication method and electropolymerization was carried out to prepare the PVK-ZrO 2 nanocomposite coatings, the coating solution was magnetic stirred gently for adequate dispersion of ZrO 2 nanoparticles into the PVK matrix during the electropolymerization. Coated 316L SS substrates were removed from the solution and rinsed with acetonitrile, to remove unreacted monomer molecules and dried in air. The PVK nanocomposite (1, 2 and 3 wt.%) coated substrates were labeled as PVKZ-1, PVKZ-2 and PVKZ-3 respectively.
Potentiodynamic polarization curves were recorded over the potential range of −0.5 to 1.0 V SCE at a constant sweep rate of 1 mV/s. The values of corrosion potential (E corr ), corrosion current density (i corr ), anodic (β a ) and cathodic (β c ) Tafel slopes were extracted from polarization plots and reported in Table 2. The ASTM-G (102-89) was used to calculate the corrosion rate (CR) from the polarization parameters using the following equation [13].
Where, K1 − 3.27 × 10 −3 mm g/µA yr, CR is the corrosion rate (mm/y), i corr is the corrosion current density (µA cm −2 ), EW is the equivalent weight of the substrate and D is the density (g cm −3 ) of the substrate. The protection efficiency (PE) of the PVK-ZrO 2 coatings was determined using the following equation [13] The coating porosity is one of the important parameters, which strongly influences the corrosion protection behavior of the coatings. By determining the coating porosity one can estimate the overall corrosion resistance of the coated substrate [28]. The porosity of PVK-ZrO 2 nanocomposite coating on 316L SS substrates was determined from potentiodynamic polarization resistance measurements using the following relation [13] where, P is the total porosity, R ps is the polarization resistance of the 316L SS, R pc is the measured polarization resistance of polymer coated 316L SS, ∆E corr is the difference between corrosion potentials and β a is the anodic Tafel slope for uncoated 316L SS substrate.
Electrochemical impedance spectroscopy measurements were carried out in 3.5 wt.% NaCl solution at room temperature (25 ± 2 • C) using an electrochemical system, Frequency Response Analyzer (FRA). The test specimens were allowed to attain equilibrium in NaCl solution under open circuit potential (OCP) for 45 minutes prior to the electrochemical tests. EIS spectrum of PVKZ-1, PVKZ-2 and PVKZ-3 were carried out with the amplitude 10 mV in the frequency range between 100 kHz-0.01 Hz. Using Zsimpwin software and non-linear least-square fitting procedure, the charge transfer resistance (R ct ), double layer capacitance (C dl ) and coating resistance (R c ) values were determined by fitting the experimental results to equivalent circuits (EC).

Surface characterization
Attenuated total reflectance infrared (ATR-IR) spectroscopy analysis (Perkin Elmer Spectrum Two Spectrometer via reflection mode) was employed in the spectral range of 400-4000 cm −1 to characterize the electro polymerized PVK-ZrO 2 coated substrates. Surface morphologies of PVK-ZrO 2 coated substrates were investigated with scanning electron microscope (SEM) (Hitachi model S3400), was generally operated at 15 keV accelerating voltage and secondary electron imaging mode. Energy dispersive X-ray (EDAX) analysis was used to find out the presence and distribution of elements in the PVK-ZrO 2 nanocomposite coatings, transmission electron microscopy (TEM) was performed using a JEOL-200FX at an accelerating voltage of 200 kV. The samples were dispersed in N -methyl pyrolidone (NMP) and copper grids were then prepared by dipping them into the NMP solution.

Hardness test
Vickers micro hardness measurements were performed on PVK-ZrO 2 coated surfaces at a load of 0.5 kgf with a dwell time of 10 s using the MATSUZAWA model MMT-X series micro hardness tester. The Vickers hardness (HV) value is the quotient obtained by dividing the indentation force by the square mm area of indentation observed in the microscope. For each substrate, at least 10 measurements were made at various sites and an average value is reported.

Adhesion test
The scotch tape test was performed on PVK-ZrO 2 nanocomposite coated 316L SS substrates to evaluate the adhesion strength in accordance with ASTM D 3359. The typical procedure for the scotch tape test was given as follows. A lattice pattern with six cuts in each direction is made, with a separation of 1.0 mm and 25 grids were generated. At room temperature adhesive tape was placed on the grids. The scotch tape was then removed after 1 min with a firm, steady pulling action. The adhesion strength of the coatings was estimated by inspecting the grid area using the illuminated magnifier and counting the number of squares peeled off compared to the total number of squares. Each test result was repeated at least three times. Adhesion tests were performed before and after the polarization studies for comparison.

Thermal Characterization
The thermal stability of the PVK-ZrO 2 nanocomposite was investigated, using a SDTQ-600 WATERS at a heating rate of 5 • C min −1 under nitrogen (N 2 ) atmosphere (flow rate 50 ml/min).

A. Electrochemical polymerization of PVK and PVK-ZrO2 nanocomposite
Choronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped and the resulting current from faradic processes occurring at the electrode was monitored as a function of time [29]. Electropolymerization of PVK-ZrO 2 nano composite was In general increase in current suggests the nucleation processes and decrease in current is correlated to the degradation process. The steady-state current indicates the competition between the polymerization and the degradation processes [30,31]. The nature of the current change varied significantly as the ZrO 2 nano particles were introduced into the coating electrolyte. Thus, compared to the PVK [5], the current density decreased for all the composites. The decrease in current density is attributed to the influence of nano ZrO 2 particles and its varying wt.% and henceforth the polymer growth and morphology.

B. ATR-IR spectroscopy studies
Attenuated total reflection (ATR) infrared spectra of the PVK-ZrO 2 nano composite is shown in Fig. 2. It is compared with the IR data of our previous finding with respect to the synthesis of PVK [5] (see also Table S1 and Fig. S1 in Supplementary materials). PVK-ZrO 2 composite IR spectrum is almost identical to that of the PVK with similar peaks with a slight shift in the stretching bands, which indicates that some interaction exists between PVK and nano-ZrO 2 . The main characteristic peaks of the PVK backbone are assigned as follows: 1222, 1450 and 1478 cm −1 are attributed to C-C stretching of the vinyl carbazole and vinylidene groups respectively [32,33]. The characteristic absorption band corresponding to ring vibration of NVC moiety was found to be 1448 cm −1 [33,34]. The peaks at 1635 and 3044 cm −1 are attributed to aromatic stretching of rings present in carbazole [33]. group [33,34]. In addition, the band around 510 cm −1 is assigned to the Zr-O-Zr stretching mode while the one in the 800 cm −1 region is related to the Zr-O bending vibration [35]. This confirms the incorporation of ZrO 2 into the PVK matrix resulting in formation of PVK-ZrO 2 nano composites.

C. Coating morphology
Surface morphology and elemental composition of PVK-ZrO 2 coated 316L SS substrates were studied using SEM and EDAX analysis and are shown in Fig. 3 (note that the SEM Micrograph of PVK is shown in Fig. S2 in Supplementary materials). The SEM micrographs revealed a significant variation in coating morphology with the incorporation of ZrO 2 nano particles. The morphology of the PVK-ZrO 2 coated 316L SS shows a uniform globular type spheroidal morphology all over the surface. The chemical composition of the PVK-ZrO 2 coated 316L SS as examined by EDAX analysis shows the presence of zirconium thereby indicating the incorporation of ZrO 2 particles into the polymer matrix. Further insight into the microstructure of the PVK-ZrO 2 coatings can be seen from TEM images, which are shown in Fig. 4. In the TEM image of PVK-ZrO 2 coating, the lighter portion showed the polymer whereas, the dark portion indicated the ZrO 2 nanoparticles. TEM images further confirmed the core shell like morphology.

D. Adhesion strength
The durability and longevity of coatings are largely dependent upon their adhesion to the substrate since this determines the ease of removal and plays a significant role in determining the protecting nature of the coatings [36][37][38]. Standard tape adhesion test (ASTM D 3359) was carried to examine the adhesion strength between the interface of PVK-ZrO 2 coatings and the 316L SS substrates and

E. Microhardness test
Vickers Microhardness (Hv) of PVK-ZrO 2 coatings on 316L SS substrates is compared with the microhardness values of 316L SS and PVK are shown in Fig. 6. The hardness values for PVK and PVK-ZrO 2 coatings on 316L SS are in the range of (2452-7355 MPa) a prerequisite for corrosion protection applicationsr [13]. The Hv value for the PVK-ZrO 2 coatings on 316L SS coating is higher than that of the PVK and attributed to the incorporation of ZrO 2 nano particles. The physical interaction between PVK and ZrO 2 nano particles in the coating attributes for the improvement in microhardness [39].

Open circuit potential measurement
Potential-time curves are generally used to understand the efficiency of the conducting polymer during corrosion tests as the kinetics of reduction reactions of O2 and H 2 O depends on the E ocp value of the bare metal. When a metal is coated with a highly doped conducting polymer and as long as the conducting polymer remains in its conductive state, the E ocp value of the conducting polymer/metal system is always above that of the bare metal electrode. When the conducting polymer is reduced, the potential value drops to a lower value [40]. Figure 7 shows Eocp measured for PVKZ-1, PVKZ-2 and PVKZ-3 coated 316L SS substrates in 3.5% NaCl for about 45 min.
The positive shift of the E ocp basically shows the passive state of the underlying metal because of the good corrosion protection ability of the surface film. The E ocp value of PVK-ZrO 2 nanocomposite coated SS shows a similar behavior to that of PoPd/ZnO nanocomposite coated SS which suggests that this coating polymer maintains its oxidative state and provides corrosion protection to the metal [41]. In addition; this shows that O 2 reduction reaction (cathodic reaction) takes place at the polymer surface. In other words, this confirms the displacement of the electrochemical interface (e.g., from metalelectrolyte interface to polymer electrolyte interface) [42]. Hosseini et al. found that polypyrrole/ZnO nanocomposite coating offered excellent barrier property with constant E ocp value [43]. The constant value of the open circuit potential of PVK-ZrO 2 nanocomposite under the given experimental conditions shows the enhanced anticorrosion performance of ZrO 2 nano particles in PVK matrix acting as a barrier for the ingression of the Cl − ions, thereby limiting the diffusion of the corrosive species towards the underlying steel substrate. But when the loading is increased to 3 wt.%, a negative shift is observed which infers the maximum loading limit is only 2 wt.%. The decrease of potential towards the negative direction is due to the rapid diffusion of chloride ions through the coating. Potentiodynamic polarization curves obtained for all the samples in 3.5% NaCl solution are shown in Fig. 8. The plots were drawn to derive the values of different electrochemical parameters like corrosion potential (E corr ), corrosion current (i corr ), Tafel constants (β a and β c ), polarization resistance (R p ), protection efficiency (PE) and corrosion rate (CR) by extrapolating the anodic and cathodic curve using Tafel extrapolation method (Table I).

Potentiodynamic polarization studies
From the results it is seen that the uncoated 316L SS exhibited corrosion potential (E corr ) of about −0.211 V [5]. The corrosion potential (E corr ) of PVK-ZrO 2 coated substrates shifts towards the positive direction indicates the protection offered by PVK-ZrO 2 coatings. It was also observed that the PVK-ZrO 2 coated 316L SS exhibited lower corrosion current density (i corr = (0.149−0.097) µA cm −2 ) compared to bare 316L SS (i corr = 2.45 µA cm −2 ) indicates that the PVK-ZrO 2 coatings reduce the metal dissolution. The R p values for PVK coated SS shows a higher value than the uncoated 316L SS. Incorporation of ZrO 2 nano particles further increases R p thereby indicating the high protection offered by PVK-ZrO 2 . The corrosion rate of PVK-ZrO 2 coatings decreased with the increased protection efficiency of 98%. These results confirm that the PVK-ZrO 2 coating on 316 L SS act as a highly protective layer, which is attributed to the presence of ZrO 2 nanoparticles in polymer matrix based on the corrosion protection mechanism put forth by several researchers [44][45][46]. But when 3 wt.% ZrO 2 nano particles are added R p drastically decreases, which confirms the loading limit of the nano particles in the PVK matrix. This shows that the decrease in the barrier property of the coating as the diffusion of electrolyte takes place through the coating. Increase in corrosion current and decrease in protection efficiency corroborates with a decrease in R p value for PVKZ-3 coatings. Similar results were obtained by Chen et al., in which they found that the insertion of ZnO nanoparticles improved the corrosion properties of polypyrrole [47]. The coating porosity is one of the important parameters, which strongly governs the anticorrosive behavior of the coatings [48,49]. Therefore, measurement of the coating porosity was determined in order to estimate the overall corrosion resistance of the coated substrate and results are plotted and given in the Fig. 9. The coating porosity decreased with the addition of nano ZrO 2 particles and when the load was increased to 3 wt.% the porosity increased showing the overload of nanoparticles. From the above results it can be con- cluded that both electrochemical protection and barrier effects cause the corrosion protection of the PVK-ZrO 2 nanocomposite coatings.

Electrochemical impedance spectroscopy
Generally, corrosion resistance of the system can be inferred from the total impedance values measured in the low frequency region [50]. In the Bode resistance plot PVKZ-2 coated substrate exhibited higher impedance values in the low frequency region when compared to PVKZ-1 and PVKZ-3. It is inferred that PVKZ-3 shows less capacitive behavior in the middle frequency region and the phase angle value decreased in the low frequency region. This linear portion observed in the low frequency region indicates that the dissolution of the metal ions or natural metal oxides into the solution. However, the steel coated with PVKZ-2, shows an increase in maximal phase angle value with broad time constant suggested that the enhancement of resistive behavior of the protective layer due to the coating on the surface of 316L SS [51,52]. Further, the specimen coated with PVKZ-2 showed an impedance modulus few order of magnitude higher than the PVKZ-1 (Fig. 10). Thus obtained spectra were fitted using the circuit shown in Fig. 11 consisting of electrolyte resistance (R s ), pore resistance (R po ) relating to the diffusion of the electrolyte through the coatings via pore which can affect the barrier properties of the coatings, coating resistance (R c ), charge transfer resistance (R ct ), pore capacitance (CPE po ), coating capacitance (CPE c ), and double layer capacitance (CPE dl ). Constant phase element (CPE) is used instead of "C" to distinguish the deviation from the ideal capacitive behavior (note that the EC curves fitting plots of uncoated PVKZ-1, PVKZ-2 and PVKZ-3 coated 316L SS substrates are given in Fig. S3 in Supplementary materials). R ct values for PVKZ-1 and PVKZ-2 are higher when compared to 316L SS which quantifies the charge transfer resistance offered by the coatings. Very low capacitance (CPE c ) value combined with high resistance value (R c ) for PVKZ-2 indicates enhanced corrosion resistance offered by the coating. However, the Rc decreases and CPEc increases for PVKZ-3 showing the slight decrease in corrosion resistance with the overloading of ZrO 2 nanoparticles. The coatings being the protective passive layer offer effective barrier properties against the penetration of corrosive ions on metal. The decrease in CPE dl values for PVKZ-1 and PVKZ-2 is attributed to the effect of the nano particles offered in the polymer matrix by controlling the diffusion of corrosive ions at the interface. But the increase in CPE dl values for PVKZ-3 shows the overloading and the commencement of delamination. Figure 12 represents the typical TGA and DTA curves with traces of weight loss as a function of temperature under Nitrogen atmosphere. The first and the second weight loss corresponding to 89.06 • C and 105-127 • C is related to the removal of impurities. The thermal tran-