ZrO2 composite in 3.5% NaCl

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ScienceDirect Materials Today: Proceedings 14 (2019) 279–287

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ICRAMC_2018

Corrosion resistance behavior of PVDF/ZrO2 composite in 3.5% NaCl S. Devikala*, P. Kamaraj and M. Arthanareeswari Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur 603 203, India.

Abstract Marine structures come into contact with the seawater. Corrosion occurs on the walls of these structures and can quickly lead from surface damage to aesthetics impairment. The main cause of the corrosiveness are chloride ions which facilitate initiation and propagation of pits. Stainless steels provide a wide range of applications in seawater environments. They are used for the construction of heat exchangers, tanks and capacitors for electrical systems, refrigeration systems for power plants, tidal power, wind turbines localized at sea. Mild steel find its role in almost every product created from metal due to its low cost and easy availability. The polymeric materials are having multiple adsorption sites for bonding with metal surface and provides higher inhibition efficiency than the corresponding monomers. In the present work, the corrosion inhibition of mild steel in 3.5% NaCl by PVDF/ ZrO2 composites were studied. The composite samples were characterized by using XRD, FTIR and SEM. Potentiodynamic polarization and electrochemical impedance spectroscopic techniques were used to explore the enhanced corrosion inhibition of composites. The corrosion inhibition efficiency of PVDZr composites were found to increase with increase in concentration of ZrO2. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2nd International Conference On Recent Advances In Material Chemistry. Keywords: PVDF; ZrO2; corrosion inhibition; XRD;

1. Introduction PVDF is typically a semi crystalline polymer that is approximately 50% amorphous. PVDF is a polar polymer with excellent chemical, mechanical and electrical properties [1-4]. PVDF has been widely used in many fields, such as ultra filtration and microfiltration membranes, electrode binder in lithium ion batteries, microwave transducers and * Corresponding author. Tel.: +91 9444844369; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2nd International Conference On Recent Advances In Material Chemistry.

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its unique applications as piezoelectric and pyroelectric materials. It’s strong piezoelectric response, chemical and mechanical durability make it a valuable material for sensors and actuators. The piezoelectric effect consists of a linear coupling between an applied electric field and an induced strain. In other words, an input of mechanical energy will produce an electrical polarization. Zirconia (ZrO2) is an oxide which has a high tensile strength, high hardness and corrosion resistance. The spatial arrangement of the atoms in zirconia is characterized by distinct crystallographic structures, characterizing a property known as polymorphism. Its three phases, or crystal structures, are characterized by specific geometry and dimensional parameters: monoclinic, tetragonal and cubic. Zirconia based ceramics are routinely used in structural applications in engineering, such as manufacture of cutting tools, gas sensors, refractories and structural opacifiers [5]. To meet structural demands, zirconia is doped with stabilizers to achieve high strength and fracture toughness [6]. The properties of the polymers and the ceramics could be exploited in the corrosion and gas sensing studies. The new synthesized polymer composites could show significant change in properties such as electrical, anticorrosive and gas sensing properties. Steels are the most important engineering materials and cover a wide range of alloys based on iron and carbon. Mild steel (MS) having >2 % carbon is of particular interest due to its high weldability, durability, and easy annealing capacity. MS find its role in almost every product created from metal due to its low cost and easy availability. The major industries use corrosion inhibitors are oil and gas exploration and production, petroleum refining, chemical manufacturing, water treatment, and the product additive industries to control the metal dissolution and acid consumption [7]. Industrial corrosion inhibitors are generally of organic or inorganic compounds. Organic inhibitors establish their inhibition via adsorption whereas inorganic compounds act as anodic inhibitors and the metallic atoms enclosed in the film improves its corrosion resistance. The extent of inhibition depends on factors such as functional groups, electronic structure, steric factors [8]. The polymer materials are having multiple adsorption sites for bonding with metal surface and provides higher inhibition efficiency than the corresponding monomers [9]. 2. Experimental 2.1 Materials used All chemicals used were of Analar grade and were used as received from the supplier without further purification. The following chemicals were used in this study: Polyvinylidene difluoride (PVDF) (Alfa Aesar) and Zirconium dioxide (ZrO2) (Alfa Aesar). 2.2 Preparation of PVDZr Composites A definite quantity of PVDF was dissolved in dimethyl formamide followed by the addition of a known quantity of ZrO2 and then it was made into a paste in an agate mortar and was subjected to heat at 80 °C for 30 minutes in a hot air oven and made into a powder. PVDZr composites were prepared in the following proportions of PVDF and ZrO2: PVDZr 1 – 9:1, PVDZr 2 – 8:2, PVDZr 3 - 7:3, PVDZr 4 -6:4, PVDZr 5 – 5:5 and PVDZr 6 – 4:6. 2.3 COATING OF MILD STEEL SUBSTRATES WITH POLYMER COMPOSITES Substrate Preparation Mild steel panels were used in corrosion studies. The mild steel panels were polished with emery paper and immersed in 10% dilute sulphuric acid (pickling) at 70-80 ° C for 30 minutes to remove the rust and mill scale. The pickled panels were rinsed thoroughly in deionised water and then rinsed with acetone to remove the acid residues present on it after pickling. The pickled and rinsed mild steel panels were subjected to coating immediately. A pasty solution of polymer composites were coated on surface treated mild steel panels using a Apex Spin Coating unit (SCU 2005) and the panels were sintered in a hot air oven for 30 minutes at certain temperatures. Then these plates were used for corrosion studies.

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3. Results and Discussion 3.1. XRD The XRD pattern of pure PVDF is shown in figure 3.2 which indicates PVDF has monoclinic structure. The peak positions (2θ = 18.48(0 2 0), 20.18(1 1 0), 26.60(0 2 2), 34.91(1 3 1), 38.81(0 4 1) and 41.41o (2 2 1) and relative intensitites obtained for the polymer match with the JCPDS Card no. 38-1638 file, identifying it as PVDF with monoclinic structure with β phase (Figure 1). The average crystallite size is found to be 0.1549 µm.

Fig. 1 XRD pattern of PVDF

The peak positions (2θ = 30.26 ( 0 1 1), 34.81 (0 1 1), 34.81 (0 0 2), 43.13 (0 1 2), 50.70 (0 2 0), 60.20 (1 2 1) and 74.53o (2 2 0) and relative intensitites obtained for ZrO2 match with the JCPDS Card no. 50-1089 file, identifying it as ZrO2 with tetragonal phase (Figure 2). The average crystallite size is found to be 0.1756 µm.

Fig. 2 XRD pattern of ZrO2

As the ZrO2 content increases, the characteristic composite peaks at 30.26, 50.37, 50.70, 60.2 corresponding to the tetragonal phase ZrO2, are obviously pronounced, and the peaks corresponding to PVDF diminish. The broad peak at region of of 2θ = 15-20°, showing the main crystalline property of PVDF disappears when ZrO2 content increases. This shows that a small amount of PVDF may exist in the composite samples with higher ZrO2 content(Figure 3 (i) – (iii)). The average crystallite size is found to be 0.1367 µm.

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Fig.3 (i)

XRD patterns of PVDZr 1 and 2

Fig.3 (ii)

XRD patterns of PVDZr 3 and 4

Fig.3 (iii)

XRD patterns of PVDZr 5 and 6

3.2. FTIR The observed vibrational band at 1402 cm-1 belongs to the deformation vibration of the CH2 group. The peaks noticed at 873 and 854 cm-1 belongs to the rocking mode of vinylidene group of the polymer. The band at 1066 cm-1 is due to β crystalline phase of PVDF(Figure 4). The bands at 532 and 498 cm-1 may be assigned to the wagging and bending vibration of CF2. The bending vibration of CF2 is observed at 613 cm-1 also [10-11].

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Figure 4 FTIR spectrum of PVDF

Figure 5 FTIR spectrum of ZrO2 Pure zirconia shows IR peaks at 1278, 750, 580, 447 and 416 cm-1. These bands correspond to the various symmetric and anti symmetric structural vibration arising out of zirconia in its pristine state (Figure 5). The peaks at 750 and 514 cm-1are typical Zr-O-Zr asymmetric and Zr-O stretching vibrations respectively [12]. The occurrencce of these bands indicates the formation of crystalline tetragonal phase of zirconia. A broad band at 693 cm-1 that can be attributed to Zr-O stretching. Absorption band at 466 cm-1 is related to the vibration of Zr-O bond in ZrO2 [13]. In the composite PVDZr , the peaks due to ZrO2 at 2204, 2192, 2169, 1973, 1683, 1651, 1434, 750, 686, 665, 650 and 520 cm-1 are shifted to 2225, 2187, 2160, 1964, 1685, 1652, 1415, 737, 677, 667, 650, 642 and 518 cm-1 respectively. This suggests the presence of ZrO2 in the composite. PVDF characteristic peaks are also observed in the composite at 1189, 533 and 491 cm-1 respectively. In addition, some new peaks are also formed. The shifts in the pure PVDF and pure ZrO2 indicate that some interactions have occurred. This may be due to the interaction between the CF2 groups of PVDF and oxygen atoms of ZrO2. On adding ZrO2, significant changes in the spectral features in terms of the appearance of new peaks and the disappearance of existing peaks are observed (Figure 6 (i) - (iii)).

Figure 6 (i) FTIR spectra of PVDZr 1 and 2

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Figure 6 (ii) FTIR spectra of PVDZr 3 and 4

Figure 6 (iii) FTIR spectra of PVDZr 5 and 6 3.3. SEM To establish whether inhibition is due to the formation of a film on the metal surface via adsorption scanning electron photographs were taken. The SEM images of PVDF and PVDZr 6, is shown in the figure 7. This observation clearly proves that the inhibition is due to the formation of an adsorbed film through the process of adsorption of the polymer composite on the metal surface [14].

(a)

(b)

Fig. 7 SEM images of (a) PVDF (b) PVDZr 6

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3.4. DSC The Tg of pure PVDF is found to be 38 °C whereas for PVDZr 1 and PVDZr 6 the values are found to be 35 and 31 °C respectively. The Tg of polymer composites is decreased as compared to pure PVDF as shown in the figure. Probably, the mobility of PVDF chain is not constrained due to PVDF-ZrO2 interaction. This interaction may be related to the dispersion of ZrO2 in PVDF matrix. The incorporation of ZrO2 considerably decreases the Tg which enhances the polymer chain motion and increases volume fraction of the amorphous phase, which obviously increases the ionic transport process. It is also observed that composite with higher ZrO2 content, possess low Tg value. This may be attributed to highest conductivity of PVDZr 6 (Figure 8). It can be concluded that the ceramic not only facilitates for ionic conductivity but also interacts with the polymer phase.

3.5 Electrochemical Studies

Fig.8 DSC plots of (a) PVDF (b) PVDZr 1 (c) PVDZr 6

The Icorr of bare mild steel is 35.1 µA/cm2 . The incorporation of ZrO2 in PVDF matrix reduced the corrosion currents of PVDZr 6 to 0.9 µA/cm2. This indicates that the addition of ceramic oxides in PVDF matrix has improved the corrosion resistance. It is seen that the least corrosion current value i.e. a better corrosion resistance is displayed by PVDZr 6 (Figure 9). The corrosion potential Ecorr of bare mild steel is -706 mV. The incorporation of ZrO2 and in PVDF matrix resulted in a positive shift in potential. This indicates that the addition of ceramic oxides in PVDF matrix has improved the corrosion resistance. An increase in ceramic oxides content in the composites PVDZr 6 , resulted in a significant shift to more positive value indicating its better corrosion resistance behaviour compared to all the other 5 composite samples (Table 1). Thus, it’s understood that the addition of ceramic oxides upto 10-60 wt% favours the corrosion resistance [15]. The corrosion rate of bare mild is found to be 10.72 mpy whereas for the composites of PVDZr 1 is found to be 2.95 mpy. For PVDZr 6, the corrosion rate values are found to be 0.64 mpy. This indicates that with increase in the content of ceramic oxides, the corrosion rate decreases.

Fig. 9 Tafel plots of bare mild steel and PVDZr composites

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Table 1 Corrosion parameters obtained from Polarization studies for bare mild steel and coated PVDZr composites System studied

Ecorr (mV)

Icorr (µA/cm2)

Corrosion rate (mpy)

Bare mild steel

-706

35.1

10.72

PVDZr 1

-232

8.5

2.59

PVDZr 2

-230

6.1

1.86

PVDZr 3

-226

4.9

1.49

PVDZr 4

-223

3.1

0.94

PVDZr 5

-216

2.2

0.67

PVDZr 6

-212

0.9

0.29

The Nyquist plots of composites is shown in the figure 10. Figures represent the Nyquist plot. The interception of Z’ in the nyquist plot at higher frequencies is ascribed as electrolytic bulk resistance Rs and at lower frequencies the interception is ascribed as Rct. The Nyquist plot shows the emergence of a second semi circle. The equivalent circuit (Fig. 11) used for fitting the plots obtained for coatings and the fitted values are displayed in tables (Table 2). The Rct values increased significantly when compared to bare mild steel.The Rct value being the highest for PVDZr 6 indicating that the active area available for corrosive attack is less or alternatively the corrosion reistance is better compared to other samples [16]. It is understood from the table that Cdl value is very low for PVDZr 6. This indicates that addition of around 60 wt% of ceramic oxide improved the surface morphology of the coating and decreased the surface defects. The improvement of corrosion resistance could be attributed to the fine surface structure of composite coating compared to bare mild steel due to the incorporation of ceramic oxide particles into polymer [17-19]. The higher values of the Rct obtained for the composite coatings of the present study imply better corrosion protective ability [20].

Fig.10 Nyquist plots for bare mild steel and PVDZr composites Table 2 Electrochemical parameters obtained from Impedance studies for bare mild steel and coated PVDTi composites System studied Bare mild steel PVDTi 1 PVDTi 2 PVDTi 3 PVDTi 4 PVDTi 5 PVDTi 6

Rct (Ohm cm2) 23.4 41.25 57.16 62.44 65.77 68.61 99.11

Cdl (µF) 2.583x10-2 2.530x10-3 2.010x10-3 1.980x10-3 1.121x10-3 1.080x10-3 0.786x10-3

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Fig.11 Equivalent circuit model for PVDF composites Conclusion PVDZr composite acted as a good inhibitor f`or the corrosion of mild steel in 3.5% NaCl. Potentiodynamic curves revealed the mixed mode of inhibition of PVDZr. The inhibition efficiency increases with an increase in concentration of ceramic oxide content. The SEM images of mild steel reveal the formation of adsorbed film of PVDZr.

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