# Anticorrosive Properties of Poly(o-phenylenediamine)/ZnO Nanocomposites Coated Stainless Steel

Authors: Aisha Ganash
Journal: Journal of Nanomaterials
 (2014)

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## Abstract

Poly(o-phenylenediamine) and poly(o-phenylenediamine)/ZnO (PoPd/ZnO) nanocomposites coating were prepared on type-304 austenitic stainless steel (SS)  using H2SO4 acid as electrolyte by potentiostatic methods. Fourier transforms infrared spectroscopy and scanning electron microscopy techniques were used to characterize the composition and structure of PoPd/ZnO nanocomposites. The corrosion protection of polymer coatings ability was studied by Eocp-time measurement, anodic and cathodic potentiodynamic polarization and impedance techniques in 3.5% NaCl as corrosive solution. It was found that ZnO nanoparticles improve the barrier and electrochemical anticorrosive properties of poly(o-phenylenediamine).


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## Body

## 1. Introduction

Corrosion is an undesirable natural process that arises since the use of metallic materials; therefore, serious efforts to prevent this phenomenon are ongoing through this century. Three approaches are commonly applied to reduce the rate of corrosion including cathodic protection, anodic protection (passivation), and application of barrier coatings [1]. Protective coatings have been widely used for metal corrosion control. The use of conducting polymers for the inhibition of corrosion is an area which is very recently gaining increasing attention [2].A successful electropolymerization on active metal generally requires a passive layer to prevent the dissolution of the metal and at the same time to oxidize monomers into polymers. Sometimes, a primer polymer coating has also been needed to ensure a firm binding deposit onto the base metal. Polyaniline, polypyrrole, and their derivatives have been widely investigated because of the low potential for polymer formation and easy preparation and stability of the formed films. It was shown that ring-substituted conducting polymers [3] such as poly(o-anisidine), poly(o-toluidine), poly(m-toluidine), and poly(o-chloroaniline) can improve the corrosion performance in aggressive environment like straight line polymers such as PANI and PPy. Series of studies have reported that polymer films based on thiophene [4], o-phenylenediamine [5], N-methyl pyrrole [6], 2,5-dimethoxyaniline [7], and other heterocyclic compounds reduce the corrosion rate of steel and iron in sulfuric acid and chloride solutions. Phenylenediamine PoPd is considered as one of the most investigated conducting polymers and also attracted much interest in many studies with various practical applications because of its high conductivity, outstanding air stability, and special physical, chemical properties compared with other conducting polymers [5, 8–12].Conducting polymer/inorganic oxide nanocomposites have recently attracted great attention due to their unique microstructure, physiochemical and electrooptical properties, and a wide range of their potential usage in battery cathodes and in the construction of nanoscopic assemblies in sensors and microelectronics [13, 14]. It is believed that, with the development of material science, ZnO has found further applications in many fields [15, 16]. ZnO is a new multifunctional inorganic material, which has stable physical and chemical properties, high oxidation activity, ease of synthesis, and widely potential applications in many research areas [17]. ZnO plays an important role in the chemical industry, electronics, optics, and many other areas because of its catalysis, optical, magnetic, and mechanical special features [18]. Thus, the nanocomposites consisting of polymer and ZnO have attracted lots of interest in their excellent properties and potential applications [17, 19, 20]. Other researchers reported that the presence of ZnO nanoparticles (NPs) can affect the electropolymerization process where ZnO acts as an incorporating agent [21]. Since composites made from inorganic nanomaterials exhibit indispensable constancy, this design can be effectively employed for specific applications. A number of researches investigated conducting polymer/ZnO nanocomposites, and several research groups have focused their research on the investigation of PPy/ZnO nanocomposites and their applications. Hosseini et al. synthesized PPy/ZnO nanocomposites by electrochemical method, in which they found that pure PPy film could not protect the mild steel perfectly but the introduction of nanosized ZnO into the PPy system has dramatically increased the corrosion resistance of mild steel [19]. Joshi et al. prepared ZnO nanowires modified PPy films by drop casting glass substrates using colloidal solution containing PPy and ZnO nanowires in different weight percentages, and they found that the composites had highly selective and sensitive properties used as chlorine sensors [17]. PPy/ZnO composites film could be also synthesized by using electropolymerization method and the composite film exhibited an improved xanthine biosensing [20]. Mahmoudian et al. studied the effect of the addition of ZnO nanoparticles in electropolymerization of poly(N-methyl pyrrole ) on steel substrates from oxalic acid [22]. Chen et al. found that the PPy/ZnO nanocomposites coating had relatively better charge-carrying capacity, better conductivity, and higher corrosion protection properties than that of pure PPy [23]. PPy/ZnO composites film could be also synthesized by using electropolymerization method and the composite film exhibited an improved xanthine biosensing [20]. Nanostructured core-shell ZnO/PANI composite system was successfully synthesized byin situ polymerization method and it was found that ZnO nanoparticles improve the barrier and electrochemical anticorrosive properties of polyaniline and the addition of polyvinyl chloride increases the barrier effect of polyaniline coating [24].To the best of our knowledge, hitherto no literature has been published for PoPd/ZnO nanocomposites coating as corrosion prevention for stainless steel in chloride medium. For this, recent work involves electrochemical synthesis of nanocomposites coating consisting of PoPd and ZnO by potentiostatic technique on 304-stainless steel material.The structure and properties of PoPd/ZnO nanocomposites coating were studied and the anticorrosive properties of PoPd/ZnO nanocomposites coating on metal sample were investigated in 3.5% NaCl at room temperature.


## 2. Experimental Procedure

### 2.1. Materials and Electrochemical Cell

O-Phenylenediamine (98%, Fisher) was used as received. Sulfuric acid (>98%, Fluka) was used for electropolymerization. ZnO nanoparticles with an average diameter < 100 nm were purchased from Aldrich. An electrochemical cell with a three-electrode was used for electrochemical measurements; 304 SS rod (6 mm in diameter), a platinum sheet, and a Metrohm Ag/AgCl electrode (in 3 M KCl) were used as working, counter, and reference electrodes, respectively. The chemical composition (Wt. %) of the 304-stainless steel used in this study was 0.03C, 0.02S, 0.041P, 0.43Si, 8.08Ni, 0.34Mo, 1.45Mn, 0.50Cu, 18.23Cr, 0.82N, and 0.13Co. The 304-stainless steel substrates were polished with a series of emery papers, followed by thorough rinsing in acetone and double distilled water, and dried in air. Prior to any experiment, the substrates were treated as described and freshly used with no further storage.


### 2.2. Electrodeposition of Polymer Composite on 304 SS

Electrochemical experiments were carried out using an AUTOLAB PGSTAT 30 potentiostat/galvanostat connected to personal computer through a USB electrochemical interface. The electrodeposition of PoPd on 304 SS electrodes was carried out potentiostatically (change of current versus time at constant value of potential of 1.2 V versus Ag/AgCl). Electrodeposition experiments were conducted at room temperature up to 30 min (1800 s), using a freshly prepared solution containing 1 M sulfuric acid and 0.15 M o-phenyelendiamine in the presence and absence of 30 mg ZnO nanoparticles for each deposition. At the end of each experiment, the working electrode (304 SS) was coated by reddish brown thin film of PoPd and PoPd/ZnO nanocomposites.For characterization of polymer layer the Fourier transforms infrared (FTIR) spectra of the polymer film were measured using a PerkinElmer FTIR spectrometer. The morphology of the polymer film was investigated by scanning electron microscopy (SEM) using a Jeol JSM-6360LVSEM.


### 2.3. Corrosion Tests

The corrosion resistance property of bare and coated stainless steel was evaluated in 3.5% NaCl at room temperature by open circuit potentialEocp-time curves, Tafel polarization, and electrochemical impedance spectroscopy (EIS).The polarization curves were recorded with a scan rate of 1 mV s−1, where the initial potential was the corrosion potential value reached after one hour of exposure time. The polarization curves were obtained starting from the open circuit potential (OCP) and varying the potential up to 200 mV in anodic branch of the Tafel plot and down to −200 mV cathodic branch of the Tafel plot.Electrochemical impedance spectroscopy data were obtained at corrosion potential over a frequency range from5×104 Hz to 0.1 Hz using AC amplitude of 10 mV around the open circuit potential by using the FRA software. All electrochemical experiments were performed and the data were collected after one hour immersion for 304 SS in corrosive solution at room temperature.The analysis of the impedance spectra was done by fitting the experimental results to equivalent circuits using the nonlinear least-square fitting procedure. The quality of fitting to equivalent circuit was judged by Chi-squared(χ2) values of 10−3or less.


## 2.1. Materials and Electrochemical Cell

O-Phenylenediamine (98%, Fisher) was used as received. Sulfuric acid (>98%, Fluka) was used for electropolymerization. ZnO nanoparticles with an average diameter < 100 nm were purchased from Aldrich. An electrochemical cell with a three-electrode was used for electrochemical measurements; 304 SS rod (6 mm in diameter), a platinum sheet, and a Metrohm Ag/AgCl electrode (in 3 M KCl) were used as working, counter, and reference electrodes, respectively. The chemical composition (Wt. %) of the 304-stainless steel used in this study was 0.03C, 0.02S, 0.041P, 0.43Si, 8.08Ni, 0.34Mo, 1.45Mn, 0.50Cu, 18.23Cr, 0.82N, and 0.13Co. The 304-stainless steel substrates were polished with a series of emery papers, followed by thorough rinsing in acetone and double distilled water, and dried in air. Prior to any experiment, the substrates were treated as described and freshly used with no further storage.


## 2.2. Electrodeposition of Polymer Composite on 304 SS

Electrochemical experiments were carried out using an AUTOLAB PGSTAT 30 potentiostat/galvanostat connected to personal computer through a USB electrochemical interface. The electrodeposition of PoPd on 304 SS electrodes was carried out potentiostatically (change of current versus time at constant value of potential of 1.2 V versus Ag/AgCl). Electrodeposition experiments were conducted at room temperature up to 30 min (1800 s), using a freshly prepared solution containing 1 M sulfuric acid and 0.15 M o-phenyelendiamine in the presence and absence of 30 mg ZnO nanoparticles for each deposition. At the end of each experiment, the working electrode (304 SS) was coated by reddish brown thin film of PoPd and PoPd/ZnO nanocomposites.For characterization of polymer layer the Fourier transforms infrared (FTIR) spectra of the polymer film were measured using a PerkinElmer FTIR spectrometer. The morphology of the polymer film was investigated by scanning electron microscopy (SEM) using a Jeol JSM-6360LVSEM.


## 2.3. Corrosion Tests

The corrosion resistance property of bare and coated stainless steel was evaluated in 3.5% NaCl at room temperature by open circuit potentialEocp-time curves, Tafel polarization, and electrochemical impedance spectroscopy (EIS).The polarization curves were recorded with a scan rate of 1 mV s−1, where the initial potential was the corrosion potential value reached after one hour of exposure time. The polarization curves were obtained starting from the open circuit potential (OCP) and varying the potential up to 200 mV in anodic branch of the Tafel plot and down to −200 mV cathodic branch of the Tafel plot.Electrochemical impedance spectroscopy data were obtained at corrosion potential over a frequency range from5×104 Hz to 0.1 Hz using AC amplitude of 10 mV around the open circuit potential by using the FRA software. All electrochemical experiments were performed and the data were collected after one hour immersion for 304 SS in corrosive solution at room temperature.The analysis of the impedance spectra was done by fitting the experimental results to equivalent circuits using the nonlinear least-square fitting procedure. The quality of fitting to equivalent circuit was judged by Chi-squared(χ2) values of 10−3or less.


## 3. Results and Discussion

### 3.1. Electropolymerization of Polymer Composite Films on 304 SS

Figure1 depicts the potentiostatic deposition recorded of o-phenylenediamine on 304 SS from 1 M sulfuric acid and 0.15 M o-phenylenediamine in the presence and absence of 30 mg ZnO nanoparticles in order to compare the structure and anticorrosive property of polymer composite coating with pure PoPd; the figure represents current-time transit during electropolymerization at potential 1.2 V for 1800 s. It can be seen that the time taken for formation of electropolymerized coating is very short.Figure 1
Potentiostatic polymerization of pure PoPd (a) and PoPd/ZnO nanocomposites coating (b) at potential 1.2 V for 1800 s.The comparison between Figures1(a) and 1(b) shows that the decrease of current density of the monomer oxidation in the presence of ZnO is much more than that in the absence of ZnO. The decrease of electrolyte conductivity due to the presence of ZnO [25] and the increasing resistance of the polymer coating during the polymerization process are the reasons for this phenomenon. In addition, ZnO might act as a barrier and reduce direct interaction between monomers and the steel surface as working electrode [22].


### 3.2. Spectroscopic Characterization

The electropolymerized PoPd in the presence and absence of ZnO on steel surface was scraped off and Fourier transform infrared spectroscopy (FT-IR) spectra of the polymer powders were recorded in Figure2. Curve (a) gives the FT-IR spectrum of pure PoPd in which the absorption peaks at 1502 cm−1 and 1582 cm−1 are attributed to the C=C stretching mode of benzonoid and quinoid rings in PoPd chains, respectively [24]. The peaks at 1311 cm−1 and 3437 cm−1 are assigned, respectively, to the C–N and N–H stretching bonds of secondary amine group. In the curve (b) for PoPd/ZnO nanocomposites coating, the peak at 579 cm−1 came from ZnO bond in ZnO nanoparticles [26]. The peak at 3437 cm−1 for nanocomposite (Figure 2(b)) is stronger than that of in pure PoPd (Figure 2(a)). This may be due to the overlapping of peak related to the stretching of O–H bonding between ZnO nanoparticles and PoPd molecules with the absorption peak of N–H bond in PoPd chains [27]. The absorption peaks coming from ZnO were detected in PoPd/ZnO indicating that the ZnO nanoparticles had loaded in PoPd successfully.Figure 2
FTIR spectra (4000–200 cm−1) of pure PoPd (a) and PoPd/ZnO nanocomposites coating (b).


### 3.3. Morphological Characterization

Figure3(a) shows SEM image of PoPd and PoPd/ZnO nanocomposites coating ((b, c, and d) at different magnification), respectively. Other researchers reported that a net of broad lines characterizes the morphology of PoPd layers. These lines are similar to the conductive lines that appeared in transparent films of ladder-conductive polymer benzimidazobenzophenathroline [28].SEM images of pure PoPd (a) and PoPd/ZnO nanocomposites coating (b), (c), and (d).
(a)
(b)
(c)
(d)Figures3(b), 3(c), and 3(d) are consisting of white granular with size of around 100 nm. After ZnO nanoparticles were introduced into the composite system, the ZnO particles during synthesis were distributed randomly into the net lines among PoPd, which made the PoPd coating dense and preferred to prevent the direct contact of corrosive solution.


### 3.4. Comparison in Terms of Protection of 304 SS

#### 3.4.1.Eopc-Time Measurements

E
ocp-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 H2O depends on Eocp 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, Eocp 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 [29]. Figure 4 shows Eocp-time curves of bare 304 SS electrode, pure PoPd, and PoPd/ZnO nanocomposite in chloride solution. As seen from Figure 4, in case of bare 304 SS potential values slightly increased toward the more positive value due to the formation of corrosion products including oxides on the surface, the potential values of pure PoPd coating decreased rapidly toward the more negative direction due to the attack by the aggressive ions that were present in the solution. This implies that significant amounts of electrolyte solution have passed through coating and reached the surface of underlying metal, causing the start of corrosion process [30]. The Eocp value of PoPd/ZnO nanocomposite coated SS is more anodic than that of pure PoPd coating during exposure time and this suggests that this coating polymer maintains its oxidative state and provides corrosion protection to the metal [31]. In addition, this shows that O2 reduction reaction (cathodic reaction) takes place at the polymer surface. In other words, this confirms the displacement of the electrochemical interface (e.g., from metal-electrolyte interface to polymer-electrolyte interface) [32]. The constant value of open circuit potential of PoPd/ZnO nanocomposite under the given experimental conditions indicates a barrier behavior of the PoPd/ZnO nanocomposite, which limits the diffusion of the corrosive species towards the underlying steel substrate. Hosseini et al. found that Polypyrrole/ZnO nanocomposite coating was able to keep its excellent barrier property and there was no evidence for notable corrosion of underlying metal [19].Figure 4
Potential-time behaviour of bare 304 SS (a), pure PoPd (b), and PoPd/ZnO nanocomposites coating (c) in 3.5% NaCl solution.


#### 3.4.2. Potentiodynamic Polarization Measurements

Although the Tafel extrapolation method is usually insufficient for understanding the protective properties of electroactive conducting polymer coatings, this method may be used to compare corrosion performances of various polymer coating types [33]. Figure 5 shows Tafel curves of bare 304 SS electrode, pure PoPd, and PoPd/ZnO nanocomposite. The values of corrosion potential (Ecorr), corrosion current density (icorr), anodic and cathodic Tafel constants, and corrosion rate (CR) are obtained from GPES software and reported in Table 1.Table 1
Polarisation parameters for bare 304 SS, pure PoPd, and PoPd/ZnO nanocomposite.
Polarization parameters
Bare 304 SS
Pure PoPd
PoPd/ZnO nanocomposite
E
corr/V
−0.200
−0.113
−0.036
j
corrosion/μA cm−2
0.870
0.036
0.010
ba (mV dec−1)
298
101
112
bc (mV dec−1)
79
70
83
Corrosion rate/μm year
1.91
0.835
0.236Figure 5
Tafel plot of bare 304 SS (a), pure PoPd (b), and PoPd/ZnO nanocomposites coating (c) in 3.5% NaCl solution.However, both redox process of the electroactive conducting polymer and the corrosion of steel must have contributed to theicorr values [34, 35]. Therefore, icorr represents the exchange in the current density values, here. The data in Table 1 shows that current density represented as icorr for pure PoPd and PoPd/ZnO nanocomposite is less than bare 304 SS. When the pure PoPd coating is compared with PoPd/ZnO nanocomposite, we found the value of icorr in case of composites is less (icorr = 0.010 μA cm−2) than the value for PoPd coating (icorr = 0.036 μA cm−2) indicating the highest corrosion protection properties. The decrease in the corrosion current indicates the formation of protective films on metal surfaces. This effect increases in the presence of ZnO nanocomposite. This result shows that PoPd and PoPd/ZnO nanocomposite inhibit the anodic reaction by acting as a barrier layer between electrode surface and corrosion environment.Similar observation was also reported by Mahmoudian et al. [22]. Also by looking to the value of Ecorr, the analysis of data shows the increase in the positivity in this order:PoPd/ZnO nanocomposite > pure PoPd > Bare 304 SS.It is now apparent that the conducting polymer accepts electrons from the metal and gives them to oxygen. This reaction generates the formation of a passive oxide layer at the polymer/metal interface, which lowers the corrosion rate and shifts theEcorr to values that are more positive. The result of the potentiodynamic polarization measurements for the PoPd coating shows a shift in the Ecorr to more positive value and reduces the corrosion rate of SS. The positive shift in Ecorr indicates the protection of the metal surface by the polymer layer. Thus, the protection effect of polymer layer seems to be related to its passivation effect as well as its barrier behavior. The potentiodynamic polarization measurement of PoPd/ZnO nanocomposite coating exhibits a shift in the Ecorr to a more positive value (−0.036 V) and reduces the corrosion rate of SS. In this case, the PoPd/ZnO nanocomposite coating acts as the more effective barrier as it is more stable and corrosion protective in chloride solution [36].The positive shift inEcorr confirms that the 304 SS covered by PoPd/ZnO nanocomposite coating depressed the anodic current of the corrosion reaction and offered greater resistance to corrosion [37].As a result, the introduction of ZnO nanoparticles into to PoPd coatings led to exhibiting high corrosion protection properties. Similar result was obtained by Chen et al., in which they found that the insertion of ZnO nanoparticles improved the corrosion properties of polypyrrole [23]. It can be concluded that both electrochemical protection and barrier effects cause the corrosion protection of the composite coatings. The electrochemical protection is caused by the increase of the corrosion potential and the formation of a passive layer on the metal surface.


#### 3.4.3. Electrochemical Impedance Spectroscopy Study

Figure6 shows the Nyquist impedance curves of PoPd in the presence and absence of ZnO nanoparticles coated 304 SS electrodes. These experimental Nyquist impedance curves were fitted well to the equivalent circuit models inserted in Figure 6. These equivalent circuit models consist of solution resistance (Rs), charge transfer resistance (Rct), pore resistance for the coating (Rpore), constant-phase element for double layer (CPEdl), and constant-phase element for the coating film (CPEc). Here, constant-phase element (CPE) is used instead of the capacitance (C) to define the homogeneities in the system [38, 39]. The fitting results of pure PoPd coating and PoPd/ZnO nanocomposite were tabulated in Table 2.Table 2
Impedance parameters for pure PoPd and PoPd/ZnO nanocomposite.
EIS parameters
Pure PoPd
PoPd/ZnO nanocomposite
Rs/Ω cm2
14.05
10.44
Rf/kΩ cm2
20.96
8.38
Rct/Ω cm2
507
211.4
Qf-Y (μF cm−2)
145.6
210.7
Qf-n1
0.945
0.894
Qdl-Y(μF cm−2)
636
2148
Qdl-n2
1.00
0.759Figure 6
Nyquist plot pure PoPd (a) and PoPd/ZnO nanocomposites coating (b) in 3.5% NaCl solution.The film resistance value obtained for PoPd/ZnO nanocomposite was relatively low with respect to pure PoPd coating, which might be related to the electronic distribution channels of PoPd/ZnO nanocomposite [23].The charge-transfer resistance value for PoPd/ZnO nanocomposite was lower than that of pure PoPd coating, which was associated with the surface morphology of PoPd/ZnO nanocomposite coating. Introduced ZnO nanoparticles in the polymer matrix lead to the decrease of the resistance for charge transfer during electrochemical reaction and thus charge transfer resistance of the entire electrode system due to the increase of the surface area. The film capacitance of PoPd/ZnO nanocomposite coating was found to be higher than that of pure PoPd. The results showed that PoPd/ZnO nanocomposite coating had better conductivity and charge storage capability [23]. Other authors reported the effect of the nanosize additives to improve the anticorrosion electrochemical barrier layer properties of polymers for diffusion of solvent [40–43]. The increase of PoPd surface area in the reaction medium is one of the advantages of the polymerization of PoPd in the presence of nanoparticles. One of the special properties of electroactive polymers is their ability to interact with the ions liberated during the corrosion reaction of steel in the presence of Cl− ion [44–46]. So, their performance increases with the increase of surface area.


#### 3.4.4. The Mechanism of Protection and the Role of ZnO

The electroactive organic coatings could provide anodic protection to stainless steel, by reducing themselves and stabilizing the more protective oxide formation. This behavior must have enhanced the passivation and accelerated the repair of passivity when it is broken [47]. It was shown [48] that the undoping process of conducting polymer works as a cathodic reaction, whereas the metal oxidation acts as an anodic reaction. This process may result in the positive shift of corrosion potential by forming a passive film at the metal-polymer interface.The formed passive film on the surface protects the steel from corrosion and maintains the doping state of the polymer. The conducting polymer can also be reoxidized by atmospheric or dissolved oxygen as the reaction. Therefore, reoxidation of the conducting polymer is necessary for the polymer to continue in its role as electroactive coating(1)poly0+n4O2+nH+⟶polyn++n2H2O
The equation obviously shows the importance of oxygen reduction reaction of the polymer surface. Introducing ZnO in the nanocomposite increases the surface area of nanocomposite and this leads to increasing the surface area of PoPd; this can increase the ability of the PoPd/ZnO nanocomposite coating to interact with the ions liberated during corrosion reaction and/or it causes an increase in the rate probability for the occurrence of cathodic reduction of oxygen on the PoPd and enhances effectively the polymer oxidation and the formation of the oxide film. In addition, the incorporated ZnO nanoparticels also enhance the passivation of the coated SS in acidic solutions.


## 3.1. Electropolymerization of Polymer Composite Films on 304 SS

Figure1 depicts the potentiostatic deposition recorded of o-phenylenediamine on 304 SS from 1 M sulfuric acid and 0.15 M o-phenylenediamine in the presence and absence of 30 mg ZnO nanoparticles in order to compare the structure and anticorrosive property of polymer composite coating with pure PoPd; the figure represents current-time transit during electropolymerization at potential 1.2 V for 1800 s. It can be seen that the time taken for formation of electropolymerized coating is very short.Figure 1
Potentiostatic polymerization of pure PoPd (a) and PoPd/ZnO nanocomposites coating (b) at potential 1.2 V for 1800 s.The comparison between Figures1(a) and 1(b) shows that the decrease of current density of the monomer oxidation in the presence of ZnO is much more than that in the absence of ZnO. The decrease of electrolyte conductivity due to the presence of ZnO [25] and the increasing resistance of the polymer coating during the polymerization process are the reasons for this phenomenon. In addition, ZnO might act as a barrier and reduce direct interaction between monomers and the steel surface as working electrode [22].


## 3.2. Spectroscopic Characterization

The electropolymerized PoPd in the presence and absence of ZnO on steel surface was scraped off and Fourier transform infrared spectroscopy (FT-IR) spectra of the polymer powders were recorded in Figure2. Curve (a) gives the FT-IR spectrum of pure PoPd in which the absorption peaks at 1502 cm−1 and 1582 cm−1 are attributed to the C=C stretching mode of benzonoid and quinoid rings in PoPd chains, respectively [24]. The peaks at 1311 cm−1 and 3437 cm−1 are assigned, respectively, to the C–N and N–H stretching bonds of secondary amine group. In the curve (b) for PoPd/ZnO nanocomposites coating, the peak at 579 cm−1 came from ZnO bond in ZnO nanoparticles [26]. The peak at 3437 cm−1 for nanocomposite (Figure 2(b)) is stronger than that of in pure PoPd (Figure 2(a)). This may be due to the overlapping of peak related to the stretching of O–H bonding between ZnO nanoparticles and PoPd molecules with the absorption peak of N–H bond in PoPd chains [27]. The absorption peaks coming from ZnO were detected in PoPd/ZnO indicating that the ZnO nanoparticles had loaded in PoPd successfully.Figure 2
FTIR spectra (4000–200 cm−1) of pure PoPd (a) and PoPd/ZnO nanocomposites coating (b).


## 3.3. Morphological Characterization

Figure3(a) shows SEM image of PoPd and PoPd/ZnO nanocomposites coating ((b, c, and d) at different magnification), respectively. Other researchers reported that a net of broad lines characterizes the morphology of PoPd layers. These lines are similar to the conductive lines that appeared in transparent films of ladder-conductive polymer benzimidazobenzophenathroline [28].SEM images of pure PoPd (a) and PoPd/ZnO nanocomposites coating (b), (c), and (d).
(a)
(b)
(c)
(d)Figures3(b), 3(c), and 3(d) are consisting of white granular with size of around 100 nm. After ZnO nanoparticles were introduced into the composite system, the ZnO particles during synthesis were distributed randomly into the net lines among PoPd, which made the PoPd coating dense and preferred to prevent the direct contact of corrosive solution.


## 3.4. Comparison in Terms of Protection of 304 SS

### 3.4.1.Eopc-Time Measurements

E
ocp-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 H2O depends on Eocp 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, Eocp 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 [29]. Figure 4 shows Eocp-time curves of bare 304 SS electrode, pure PoPd, and PoPd/ZnO nanocomposite in chloride solution. As seen from Figure 4, in case of bare 304 SS potential values slightly increased toward the more positive value due to the formation of corrosion products including oxides on the surface, the potential values of pure PoPd coating decreased rapidly toward the more negative direction due to the attack by the aggressive ions that were present in the solution. This implies that significant amounts of electrolyte solution have passed through coating and reached the surface of underlying metal, causing the start of corrosion process [30]. The Eocp value of PoPd/ZnO nanocomposite coated SS is more anodic than that of pure PoPd coating during exposure time and this suggests that this coating polymer maintains its oxidative state and provides corrosion protection to the metal [31]. In addition, this shows that O2 reduction reaction (cathodic reaction) takes place at the polymer surface. In other words, this confirms the displacement of the electrochemical interface (e.g., from metal-electrolyte interface to polymer-electrolyte interface) [32]. The constant value of open circuit potential of PoPd/ZnO nanocomposite under the given experimental conditions indicates a barrier behavior of the PoPd/ZnO nanocomposite, which limits the diffusion of the corrosive species towards the underlying steel substrate. Hosseini et al. found that Polypyrrole/ZnO nanocomposite coating was able to keep its excellent barrier property and there was no evidence for notable corrosion of underlying metal [19].Figure 4
Potential-time behaviour of bare 304 SS (a), pure PoPd (b), and PoPd/ZnO nanocomposites coating (c) in 3.5% NaCl solution.


### 3.4.2. Potentiodynamic Polarization Measurements

Although the Tafel extrapolation method is usually insufficient for understanding the protective properties of electroactive conducting polymer coatings, this method may be used to compare corrosion performances of various polymer coating types [33]. Figure 5 shows Tafel curves of bare 304 SS electrode, pure PoPd, and PoPd/ZnO nanocomposite. The values of corrosion potential (Ecorr), corrosion current density (icorr), anodic and cathodic Tafel constants, and corrosion rate (CR) are obtained from GPES software and reported in Table 1.Table 1
Polarisation parameters for bare 304 SS, pure PoPd, and PoPd/ZnO nanocomposite.
Polarization parameters
Bare 304 SS
Pure PoPd
PoPd/ZnO nanocomposite
E
corr/V
−0.200
−0.113
−0.036
j
corrosion/μA cm−2
0.870
0.036
0.010
ba (mV dec−1)
298
101
112
bc (mV dec−1)
79
70
83
Corrosion rate/μm year
1.91
0.835
0.236Figure 5
Tafel plot of bare 304 SS (a), pure PoPd (b), and PoPd/ZnO nanocomposites coating (c) in 3.5% NaCl solution.However, both redox process of the electroactive conducting polymer and the corrosion of steel must have contributed to theicorr values [34, 35]. Therefore, icorr represents the exchange in the current density values, here. The data in Table 1 shows that current density represented as icorr for pure PoPd and PoPd/ZnO nanocomposite is less than bare 304 SS. When the pure PoPd coating is compared with PoPd/ZnO nanocomposite, we found the value of icorr in case of composites is less (icorr = 0.010 μA cm−2) than the value for PoPd coating (icorr = 0.036 μA cm−2) indicating the highest corrosion protection properties. The decrease in the corrosion current indicates the formation of protective films on metal surfaces. This effect increases in the presence of ZnO nanocomposite. This result shows that PoPd and PoPd/ZnO nanocomposite inhibit the anodic reaction by acting as a barrier layer between electrode surface and corrosion environment.Similar observation was also reported by Mahmoudian et al. [22]. Also by looking to the value of Ecorr, the analysis of data shows the increase in the positivity in this order:PoPd/ZnO nanocomposite > pure PoPd > Bare 304 SS.It is now apparent that the conducting polymer accepts electrons from the metal and gives them to oxygen. This reaction generates the formation of a passive oxide layer at the polymer/metal interface, which lowers the corrosion rate and shifts theEcorr to values that are more positive. The result of the potentiodynamic polarization measurements for the PoPd coating shows a shift in the Ecorr to more positive value and reduces the corrosion rate of SS. The positive shift in Ecorr indicates the protection of the metal surface by the polymer layer. Thus, the protection effect of polymer layer seems to be related to its passivation effect as well as its barrier behavior. The potentiodynamic polarization measurement of PoPd/ZnO nanocomposite coating exhibits a shift in the Ecorr to a more positive value (−0.036 V) and reduces the corrosion rate of SS. In this case, the PoPd/ZnO nanocomposite coating acts as the more effective barrier as it is more stable and corrosion protective in chloride solution [36].The positive shift inEcorr confirms that the 304 SS covered by PoPd/ZnO nanocomposite coating depressed the anodic current of the corrosion reaction and offered greater resistance to corrosion [37].As a result, the introduction of ZnO nanoparticles into to PoPd coatings led to exhibiting high corrosion protection properties. Similar result was obtained by Chen et al., in which they found that the insertion of ZnO nanoparticles improved the corrosion properties of polypyrrole [23]. It can be concluded that both electrochemical protection and barrier effects cause the corrosion protection of the composite coatings. The electrochemical protection is caused by the increase of the corrosion potential and the formation of a passive layer on the metal surface.


### 3.4.3. Electrochemical Impedance Spectroscopy Study

Figure6 shows the Nyquist impedance curves of PoPd in the presence and absence of ZnO nanoparticles coated 304 SS electrodes. These experimental Nyquist impedance curves were fitted well to the equivalent circuit models inserted in Figure 6. These equivalent circuit models consist of solution resistance (Rs), charge transfer resistance (Rct), pore resistance for the coating (Rpore), constant-phase element for double layer (CPEdl), and constant-phase element for the coating film (CPEc). Here, constant-phase element (CPE) is used instead of the capacitance (C) to define the homogeneities in the system [38, 39]. The fitting results of pure PoPd coating and PoPd/ZnO nanocomposite were tabulated in Table 2.Table 2
Impedance parameters for pure PoPd and PoPd/ZnO nanocomposite.
EIS parameters
Pure PoPd
PoPd/ZnO nanocomposite
Rs/Ω cm2
14.05
10.44
Rf/kΩ cm2
20.96
8.38
Rct/Ω cm2
507
211.4
Qf-Y (μF cm−2)
145.6
210.7
Qf-n1
0.945
0.894
Qdl-Y(μF cm−2)
636
2148
Qdl-n2
1.00
0.759Figure 6
Nyquist plot pure PoPd (a) and PoPd/ZnO nanocomposites coating (b) in 3.5% NaCl solution.The film resistance value obtained for PoPd/ZnO nanocomposite was relatively low with respect to pure PoPd coating, which might be related to the electronic distribution channels of PoPd/ZnO nanocomposite [23].The charge-transfer resistance value for PoPd/ZnO nanocomposite was lower than that of pure PoPd coating, which was associated with the surface morphology of PoPd/ZnO nanocomposite coating. Introduced ZnO nanoparticles in the polymer matrix lead to the decrease of the resistance for charge transfer during electrochemical reaction and thus charge transfer resistance of the entire electrode system due to the increase of the surface area. The film capacitance of PoPd/ZnO nanocomposite coating was found to be higher than that of pure PoPd. The results showed that PoPd/ZnO nanocomposite coating had better conductivity and charge storage capability [23]. Other authors reported the effect of the nanosize additives to improve the anticorrosion electrochemical barrier layer properties of polymers for diffusion of solvent [40–43]. The increase of PoPd surface area in the reaction medium is one of the advantages of the polymerization of PoPd in the presence of nanoparticles. One of the special properties of electroactive polymers is their ability to interact with the ions liberated during the corrosion reaction of steel in the presence of Cl− ion [44–46]. So, their performance increases with the increase of surface area.


### 3.4.4. The Mechanism of Protection and the Role of ZnO

The electroactive organic coatings could provide anodic protection to stainless steel, by reducing themselves and stabilizing the more protective oxide formation. This behavior must have enhanced the passivation and accelerated the repair of passivity when it is broken [47]. It was shown [48] that the undoping process of conducting polymer works as a cathodic reaction, whereas the metal oxidation acts as an anodic reaction. This process may result in the positive shift of corrosion potential by forming a passive film at the metal-polymer interface.The formed passive film on the surface protects the steel from corrosion and maintains the doping state of the polymer. The conducting polymer can also be reoxidized by atmospheric or dissolved oxygen as the reaction. Therefore, reoxidation of the conducting polymer is necessary for the polymer to continue in its role as electroactive coating(1)poly0+n4O2+nH+⟶polyn++n2H2O
The equation obviously shows the importance of oxygen reduction reaction of the polymer surface. Introducing ZnO in the nanocomposite increases the surface area of nanocomposite and this leads to increasing the surface area of PoPd; this can increase the ability of the PoPd/ZnO nanocomposite coating to interact with the ions liberated during corrosion reaction and/or it causes an increase in the rate probability for the occurrence of cathodic reduction of oxygen on the PoPd and enhances effectively the polymer oxidation and the formation of the oxide film. In addition, the incorporated ZnO nanoparticels also enhance the passivation of the coated SS in acidic solutions.


## 3.4.1.Eopc-Time Measurements

E
ocp-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 H2O depends on Eocp 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, Eocp 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 [29]. Figure 4 shows Eocp-time curves of bare 304 SS electrode, pure PoPd, and PoPd/ZnO nanocomposite in chloride solution. As seen from Figure 4, in case of bare 304 SS potential values slightly increased toward the more positive value due to the formation of corrosion products including oxides on the surface, the potential values of pure PoPd coating decreased rapidly toward the more negative direction due to the attack by the aggressive ions that were present in the solution. This implies that significant amounts of electrolyte solution have passed through coating and reached the surface of underlying metal, causing the start of corrosion process [30]. The Eocp value of PoPd/ZnO nanocomposite coated SS is more anodic than that of pure PoPd coating during exposure time and this suggests that this coating polymer maintains its oxidative state and provides corrosion protection to the metal [31]. In addition, this shows that O2 reduction reaction (cathodic reaction) takes place at the polymer surface. In other words, this confirms the displacement of the electrochemical interface (e.g., from metal-electrolyte interface to polymer-electrolyte interface) [32]. The constant value of open circuit potential of PoPd/ZnO nanocomposite under the given experimental conditions indicates a barrier behavior of the PoPd/ZnO nanocomposite, which limits the diffusion of the corrosive species towards the underlying steel substrate. Hosseini et al. found that Polypyrrole/ZnO nanocomposite coating was able to keep its excellent barrier property and there was no evidence for notable corrosion of underlying metal [19].Figure 4
Potential-time behaviour of bare 304 SS (a), pure PoPd (b), and PoPd/ZnO nanocomposites coating (c) in 3.5% NaCl solution.


## 3.4.2. Potentiodynamic Polarization Measurements

Although the Tafel extrapolation method is usually insufficient for understanding the protective properties of electroactive conducting polymer coatings, this method may be used to compare corrosion performances of various polymer coating types [33]. Figure 5 shows Tafel curves of bare 304 SS electrode, pure PoPd, and PoPd/ZnO nanocomposite. The values of corrosion potential (Ecorr), corrosion current density (icorr), anodic and cathodic Tafel constants, and corrosion rate (CR) are obtained from GPES software and reported in Table 1.Table 1
Polarisation parameters for bare 304 SS, pure PoPd, and PoPd/ZnO nanocomposite.
Polarization parameters
Bare 304 SS
Pure PoPd
PoPd/ZnO nanocomposite
E
corr/V
−0.200
−0.113
−0.036
j
corrosion/μA cm−2
0.870
0.036
0.010
ba (mV dec−1)
298
101
112
bc (mV dec−1)
79
70
83
Corrosion rate/μm year
1.91
0.835
0.236Figure 5
Tafel plot of bare 304 SS (a), pure PoPd (b), and PoPd/ZnO nanocomposites coating (c) in 3.5% NaCl solution.However, both redox process of the electroactive conducting polymer and the corrosion of steel must have contributed to theicorr values [34, 35]. Therefore, icorr represents the exchange in the current density values, here. The data in Table 1 shows that current density represented as icorr for pure PoPd and PoPd/ZnO nanocomposite is less than bare 304 SS. When the pure PoPd coating is compared with PoPd/ZnO nanocomposite, we found the value of icorr in case of composites is less (icorr = 0.010 μA cm−2) than the value for PoPd coating (icorr = 0.036 μA cm−2) indicating the highest corrosion protection properties. The decrease in the corrosion current indicates the formation of protective films on metal surfaces. This effect increases in the presence of ZnO nanocomposite. This result shows that PoPd and PoPd/ZnO nanocomposite inhibit the anodic reaction by acting as a barrier layer between electrode surface and corrosion environment.Similar observation was also reported by Mahmoudian et al. [22]. Also by looking to the value of Ecorr, the analysis of data shows the increase in the positivity in this order:PoPd/ZnO nanocomposite > pure PoPd > Bare 304 SS.It is now apparent that the conducting polymer accepts electrons from the metal and gives them to oxygen. This reaction generates the formation of a passive oxide layer at the polymer/metal interface, which lowers the corrosion rate and shifts theEcorr to values that are more positive. The result of the potentiodynamic polarization measurements for the PoPd coating shows a shift in the Ecorr to more positive value and reduces the corrosion rate of SS. The positive shift in Ecorr indicates the protection of the metal surface by the polymer layer. Thus, the protection effect of polymer layer seems to be related to its passivation effect as well as its barrier behavior. The potentiodynamic polarization measurement of PoPd/ZnO nanocomposite coating exhibits a shift in the Ecorr to a more positive value (−0.036 V) and reduces the corrosion rate of SS. In this case, the PoPd/ZnO nanocomposite coating acts as the more effective barrier as it is more stable and corrosion protective in chloride solution [36].The positive shift inEcorr confirms that the 304 SS covered by PoPd/ZnO nanocomposite coating depressed the anodic current of the corrosion reaction and offered greater resistance to corrosion [37].As a result, the introduction of ZnO nanoparticles into to PoPd coatings led to exhibiting high corrosion protection properties. Similar result was obtained by Chen et al., in which they found that the insertion of ZnO nanoparticles improved the corrosion properties of polypyrrole [23]. It can be concluded that both electrochemical protection and barrier effects cause the corrosion protection of the composite coatings. The electrochemical protection is caused by the increase of the corrosion potential and the formation of a passive layer on the metal surface.


## 3.4.3. Electrochemical Impedance Spectroscopy Study

Figure6 shows the Nyquist impedance curves of PoPd in the presence and absence of ZnO nanoparticles coated 304 SS electrodes. These experimental Nyquist impedance curves were fitted well to the equivalent circuit models inserted in Figure 6. These equivalent circuit models consist of solution resistance (Rs), charge transfer resistance (Rct), pore resistance for the coating (Rpore), constant-phase element for double layer (CPEdl), and constant-phase element for the coating film (CPEc). Here, constant-phase element (CPE) is used instead of the capacitance (C) to define the homogeneities in the system [38, 39]. The fitting results of pure PoPd coating and PoPd/ZnO nanocomposite were tabulated in Table 2.Table 2
Impedance parameters for pure PoPd and PoPd/ZnO nanocomposite.
EIS parameters
Pure PoPd
PoPd/ZnO nanocomposite
Rs/Ω cm2
14.05
10.44
Rf/kΩ cm2
20.96
8.38
Rct/Ω cm2
507
211.4
Qf-Y (μF cm−2)
145.6
210.7
Qf-n1
0.945
0.894
Qdl-Y(μF cm−2)
636
2148
Qdl-n2
1.00
0.759Figure 6
Nyquist plot pure PoPd (a) and PoPd/ZnO nanocomposites coating (b) in 3.5% NaCl solution.The film resistance value obtained for PoPd/ZnO nanocomposite was relatively low with respect to pure PoPd coating, which might be related to the electronic distribution channels of PoPd/ZnO nanocomposite [23].The charge-transfer resistance value for PoPd/ZnO nanocomposite was lower than that of pure PoPd coating, which was associated with the surface morphology of PoPd/ZnO nanocomposite coating. Introduced ZnO nanoparticles in the polymer matrix lead to the decrease of the resistance for charge transfer during electrochemical reaction and thus charge transfer resistance of the entire electrode system due to the increase of the surface area. The film capacitance of PoPd/ZnO nanocomposite coating was found to be higher than that of pure PoPd. The results showed that PoPd/ZnO nanocomposite coating had better conductivity and charge storage capability [23]. Other authors reported the effect of the nanosize additives to improve the anticorrosion electrochemical barrier layer properties of polymers for diffusion of solvent [40–43]. The increase of PoPd surface area in the reaction medium is one of the advantages of the polymerization of PoPd in the presence of nanoparticles. One of the special properties of electroactive polymers is their ability to interact with the ions liberated during the corrosion reaction of steel in the presence of Cl− ion [44–46]. So, their performance increases with the increase of surface area.


## 3.4.4. The Mechanism of Protection and the Role of ZnO

The electroactive organic coatings could provide anodic protection to stainless steel, by reducing themselves and stabilizing the more protective oxide formation. This behavior must have enhanced the passivation and accelerated the repair of passivity when it is broken [47]. It was shown [48] that the undoping process of conducting polymer works as a cathodic reaction, whereas the metal oxidation acts as an anodic reaction. This process may result in the positive shift of corrosion potential by forming a passive film at the metal-polymer interface.The formed passive film on the surface protects the steel from corrosion and maintains the doping state of the polymer. The conducting polymer can also be reoxidized by atmospheric or dissolved oxygen as the reaction. Therefore, reoxidation of the conducting polymer is necessary for the polymer to continue in its role as electroactive coating(1)poly0+n4O2+nH+⟶polyn++n2H2O
The equation obviously shows the importance of oxygen reduction reaction of the polymer surface. Introducing ZnO in the nanocomposite increases the surface area of nanocomposite and this leads to increasing the surface area of PoPd; this can increase the ability of the PoPd/ZnO nanocomposite coating to interact with the ions liberated during corrosion reaction and/or it causes an increase in the rate probability for the occurrence of cathodic reduction of oxygen on the PoPd and enhances effectively the polymer oxidation and the formation of the oxide film. In addition, the incorporated ZnO nanoparticels also enhance the passivation of the coated SS in acidic solutions.


## 4. Conclusion

Pure PoPd and PoPd/ZnO nanocomposite coating were successfully electrodeposited on 304 SS by potentiostatic method. SEM shows that ZnO incorporated in polymer matrix affect the polymer morphology and increase the surface area of PoPd.The existence of ZnO nanoparticles within the coating increases the anticorrosion effect on metal and this is due to catalyzing oxygen reduction on polymer surface and increases its ability to plug some pores and defects; thus theRpo increases. In other words, the comparison between corrosion performances of pure PoPd and PoPd/ZnO nanocomposite coating has shown that PoPd/ZnO nanocomposite coating could provide much better protection to SS for considerable periods in chloride solution.



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*Source: 540276-2014-04-29.xml*