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Electrosynthesis of zinc phosphate-polypyrrole coatings for improved corrosion resistance of steel
Surfaces and Interfaces 15 (2019) 224–231
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Electrosynthesis of zinc phosphate-polypyrrole coatings for improved corrosion resistance of steel
T
A. El Jaouharia, , A. Chennaha, S. Ben Jaddia, H. Ait Ahsainea, Z. Anfara, Y. Tahiri Alaouib, Y. Naciria, A. Benlhachemia, M. Bazzaouia ⁎
a b
Laboratoire des Matériaux et Environnement, Faculté des Sciences, Département de Chimie, Université Ibn Zohr, BP 8106, Agadir, Morocco Laboratoire d'Analyse et Synthèse de Procédés Industriels (LASPI), Ecole Mohammadia d'Ingénieurs, University Mohammed V in Rabat, BP 765, Rabat, Morocco
ARTICLE INFO
ABSTRACT
Keywords: Polypyrrole Zinc phosphate Corrosion protection Stainless steel Bilayer coating
Zinc Phosphate (ZP), polypyrrole (PPy) and ZP/PPy coatings were electro-electrodeposited on 304 stainless steel (SS) using galvanostatic mode for application as a corrosion protector. The morphology of different coatings on the surface of stainless steel was observed by scanning electron microscopy (SEM). The chemical composition of phase Zn3(PO4)2.6H2O is determined by X-ray diffraction analysis (XRD). The performance of the ZP, PPy and ZP/PPy coating against corrosion of stainless steel in a 3% NaCl solution was measured by electrochemical methods such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PP). These results of the electrochemical analysis were compared with those of the atomic absorption analysis in the same corrosive solution (3% NaCl) for 10 days of immersion. All of these results show that ZP/PPy coatings have an effective resistance against corrosion due to the formation of polypyrrole at sites not coated with ZP and by increasing in the thickness of the ZP/PPy coating.
1. Introduction
organic mixing with other inorganic compounds are increasingly important. These materials can combine the properties of conductive polymers with those of the inorganic compound which gives a significant synergistic effect for the fight against corrosion [18–20]. Chen et al. [21] have synthesized nanocomposite coatings of PPy/ZnO by electrochemical methods. The authors have shown that incorporation of ZnO particles altered the morphology and electrochemical properties of the PPy coating. The nanocomposite coating had a relatively better load capacity, conductivity, and corrosion protection properties than pure PPy. Mondal et al. [22], have deposited by galvanostatic deposition technique a composite of graphene oxide-polypyrrole on a stainlesssteel substrate. The morphology of the composite coating reveals a homogeneous distribution of graphene oxide on the composite with a high surface coverage capability. Anti-corrosion analyzes reflect better and more effective inhibition by improving the corrosion potential, breakdown potential and decrease in corrosion current density. Guixiang [23] reported the electro-synthesis of Polypyrrole-molybdate film on AZ31 Mg Alloy. The authors showed that MoO42− can form a MnO42− complex passive film on the Mg alloy surface before the Pyrrole polymerizes. The PPy/MnO42− composite makes the surface morphology of the film more compact and prevents the penetration of corrosion ions into the surface of the Mg alloy, which can improve the corrosion resistance performance. On the other hand, zinc phosphate
Stainless steel is a very particular metal compared to other materials due to its varied application area and of the use of these alloy in hard functioning conditions in the manufacturing and chemical industries. These steels are generally resistant to dissolution; however, its resistance to the phenomenon of corrosion depends on the composition of the operating environment and the strength of their passive protective films [1,2]. The importance of steel protection process in different fields has required research of corrosion control technique by various scientists of corrosion [3–5]. Conjugated conductive polymers have been widely studied thanks to their non-toxic, environmentally friendly, high stability and ease of synthesis, are the most promising coatings for corrosion protection of industrial metals [6–9]. Among these polymers, polypyrrole remains the best known and the most studied because of its high conductivity, outstanding air stability and special physical, chemical properties compared with other conducting polymers [10–14]. The protective mechanism of this type of polymeric coating is based on a mixed effect of isolation and charge transfer [15,16]. This motivates researchers to find new procedures to correct these defects and improve the effectiveness of polypyrrole against corrosion [17]. Polymeric-inorganic composite materials synthesized by molecular engineering or by
⁎
Corresponding author.
https://doi.org/10.1016/j.surfin.2019.02.011 Received 13 August 2018; Received in revised form 10 February 2019; Accepted 15 February 2019 Available online 18 February 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Chemical composition of stainless-steel working electrode. Element
C
Si
Mn
P
S
Cr
Ni
Fe
mass%
0.05%
0.63%
0.90%
0.032%
0.002%
18.05%
8.10%
balanced
coatings are one of the promising methods to improve the corrosion resistance of metals [24]. It has been generally demonstrated that the addition of metal salts in phosphate coatings can greatly influence the microstructure of the zinc phosphate coating and make coatings denser and thinner [25,26]. The objective of the present work is the study of the anti-corrosion effect of the polypyrrole and zinc phosphate bilayers prepared by electrodeposition on a 304 stainless steel substrate in the saline environment 3% NaCl.
by accelerated electrons at a voltage of 45 kV, the current being 35 mA. This electron beam interacting with the material generates radiation of wavelength λ(Kα1) = 1,5,440,598 Å et λ(Kα2) = 1,544,426 Å. The diffractometer is equipped with a Pixcel-1D detector. The latter is equipped with a nickel filter for eliminating Kβ radiation from copper. Anti-scattering Soller slots eliminate parasitic radiation from the source and the sample. The diffraction patterns are recorded in continuous mode in a 2θ angular range from 5° to 60° with a pitch of 0.00164° and an angular velocity of 0.002°.s−1.
2. Experimental
2.3.2. AAS analysis AA-7000, SHIMADZU atomic absorption spectrometer controlled by Wizard data collection software. The analyses were carried out in 1% HNO3 solutions of 0–5 ppm Fe and Cr, with 0.12 ppm as LDL.
2.1. Electrochemical cell and chemicals Pyrrole monomer (Aldrich) was distilled under nitrogen and all chemicals were obtained from Aldrich Chemical Company with a high analytical purity and used as received. The working electrode was formed from stainless-steel 304 sheets with 2 cm² of area which the following composition (Table 1): Before each use the electrodes were mechanically polished with 800, 1000 and 1200 abrasive paper and degrease with acetone. Following this pretreatment, the working electrode was directly transferred to the electrochemical three-electrode cell (one compartment). The cell is connected to Voltalab PGZ301 potentiostat-galvanostat controlled by VoltaMaster4. The Ag/AgCl (KCl 0.1 M) was used as reference electrode and the auxiliary electrode is a platinum wire. The electrochemical corrosion tests were carried out at room temperature in a 3% NaCl saline solution. The electrochemical impedance spectroscopy (EIS) measurements (Voltalab PGZ301 potentiostat) were carried out to an open circuit potential with a 10 mV in amplitude of the superimposed AC signal, and the applied frequency were between 100 kHz and 10 mHz.
3. Results and discussion 3.1. Electrochemical behavior of electrosynthesis coatings on stainless steel The electrochemical behavior of stainless-steel during electroplating of coatings (PPy, ZP and ZP/PPy) is investigated by the potential polarization of the working electrode in the different electrolytic solution. In order to demonstrate the mechanism of ZP deposition on a stainlesssteel substrate, cyclic voltammetry experiments were carried out in an aqueous solution containing 4 10−2 M Zn(NO3)2.6H2O and 2.67 10−2 M NH4H2PO4 (Fig. 1). The polarization of the electrode between −2 and 1 V was initiated in the negative direction of the potential at a scanning speed of 50 mV/s. The recorded voltammogram shows an increase in cathodic current with a negative sweep of the electrode potential of −0.5 V which could be related to the reduction of nitrate ions [27]. Consequently, no electrochemical reaction takes place under −0.5 V as indicated by different studies [28–30]. During reverse anodic scanning, an anodic peak in the region between −550 and −950 mV probably occurs because the steel surface is exposed to the electrolyte [31]. Its intensity depends on the porosity of the zinc phosphate coating [31]. This peak of porosity is thought to result from the oxidation of Fe, Fe²+ and/or the formation of a complex compound [31]. The disappearance of this peak in the second scanning cycle can be explained by the formation of the first ZP crystals on the surface of the steel and it is an indicator of the protective effect of ZP coating against the dissolution of the steel. The growth mechanism of ZP can be summarized as follows. Under a suitable potential, the reduction of the nitrate occurs near the cathode, which leads to the increase of pH in the vicinity of the electrode which causes the supersaturation of the Zn²+ and PO43− ions after they react with each other for form Zn3(PO4)2 on the working electrode. In the case of a polarized stainless-steel electrode in a solution of 0.1 M Na2C4H4O6 and 0.5 M pyrrole between −0.6 and 1 V with a scanning speed of 50 mV/s (Fig. 1) we notice in the first scan cycle that the voltammogram is characterized by two oxidation peaks which are located respectively at 0 and 0.7 V vs Ag/AgCl. The first oxidation peak is attributed to the oxidation behavior of the SS electrode, followed by a reduction in current density caused by the passive formation of a tartrate/oxide layer [32]. This inhibited the dissolution of the electrode and was followed by the electrochemical polymerization reaction. The second peak is attributed to the oxidation of monomer (pyrrole) to polypyrrole [32]. During the next cycle (2nd scan cycle) the anode oxidation peak disappeared which shows that the PPy coating inhibits the oxidation of the electrode [33]. In the case of deposition of
2.2. Preparation of electrolytic solutions and coatings preparation The electrolyte was prepared by dissolving zinc nitrate hexahydrate Zn(NO3)2•6H2O (4 10−2 M) and ammonium dihydrogen phosphate NH4H2PO4 (2.67 10−2 M). The molar fractions of precursor were taken to ensure that the molar ratios Zn/P were kept constant at approximately 1.5. The initial pH of the electrolyte is equal to 4.05 at 40 °C. The thin films of zinc phosphate were electrodeposited by galvanostatic mode under a current density of −10 mA.cm−2 during 30 min. The polypyrrole coating is electro synthesized also by the galvanostatic mode by applying a current density of 5 mA.cm−2 for 10 min in an electrolytic solution which contains 0.1 M of Na2C4H4O6 and 0.5 M pyrrole at room temperature. The ZP/PPy bilayers electrodeposited one on the other successively. After deposition of ZP the layers are washed with water and dried at room temperature. We also synthesize PPy/ZP layers however these coatings show poor adhesion of polypyrrole (first layer). For this reason, we have not studied the anticorrosive property of these composites. 2.3. Instrumentation 2.3.1. SEM and XRD analysis The morphology was characterized by scanning electron microscopy (SEM) ZEISS SUPRA 40VP COLUMN GEMINI. The microscope chamber was maintained at a pressure between 4 and 10 Pa. For the X-ray diffraction analysis, the apparatus used is a PANALYTICAL EMPYREAN, equipped with a copper anticathode bombarded 225
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Fig. 1. Cyclic voltammograms of ZP, PPy and ZP/PPy coatings growth on stainless steel electrodes (at 50 mV/s scan rate).
electrodeposited coatings of ZP, PPy and ZP/PPy on the stainless-steel substrate. The deconvolution of the graphs is done by X'pert hight score software. The phase compositions of stainless steel coated with normal ZP and ZP/PPy were compared (Fig. 3a and b). It is shown that coatings mainly consisted of Zn3(PO4)2•4H2O (hopeite, JCPD file #37-0465) [36]. The diffraction of PPy coating (Fig. 3c) shows a purely amorphous structure which is normal for organic compounds such as polypyrrole [37]. The deposition of polypyrrole on ZP coating does not affect the position and the nature of the zinc phosphate peaks because of their amorphousness but on the other hand the peak intensity undergoes a big change. The peak intensities of the ZP/PPy coating are lower than those of the normal ZP coatings, which clearly indicate the formation of a PPy layer on the ZP [38].
polypyrrole on stainless steel (Fig. 1), only the pyrrole oxidation wave appears, indicating the formation of PPy on ZP coating. 3.2. Characterization of coatings 3.2.1. Microstructural analysis (SEM) In order to take information on the morphological changes of the different coatings, the scanning electron microscope is performed. Fig. 2 shows the morphologies of different PPy, ZP and ZP/PPy coatings. First, the polypyrrole film (Fig. 2) synthesized on steel by galvanostatic technique (5 mA/cm² for 10 min) has a homogeneous and compact morphology characterized by a globular structure of different sizes between 2 and 10 nm, recalling the appearance and morphology of cauliflower [34,35]. Fig. 2 shows the morphology of ZP thin film, we observe the formation of many clusters of large plate-shaped crystals with uncoated place. The plates have formed uniformly on the surface (preferential orientation), which have an average size of 5–10 μm. This morphology of ZP is widely mentioned in much of the similar work of this coating [36]. ZP/PPy micrographs (Fig. 2) show the formation of polypyrrole spots on uncoated areas of stainless steel. This means that the deposition of PPy on ZP can correct the morphological defects of ZP coating and this is one of the factors that will improve the protection of stainless steel against the penetration of aggressive ions. The improvement of morphological properties remains one of the strong points of organic-inorganic coatings [21].
3.2.3. FTIR analysis To confirm the presence of polypyrrole, the samples ZP, PPy and ZP/PPy are characterized by FTIR (Fig. 4) and Table 2 summarizes the characteristic bands found. The main polypyrrole bands are: 1425 cm−1 attributed to the fundamental vibrations of polypyrrole ring, 1050 cm−1 due to the ]CeH in-plane vibrations, 1166 cm−1 assigned to the CeN stretching vibrations and 1642 cm−1 due to stretching mode of C]C [39, 40]. The appearance of polypyrrole bands on the ZP/PPy spectrum confirms the formation of PPy on the ZP film. 3.3. Evaluation of corrosion resistant property of zinc phosphate/ polypyrrole coatings
3.2.2. XRD analysis The phase compositions in the coatings on the stainless steel were analyzed by XRD. Fig. 3 shows the X-ray diffraction patterns (XRDs) of
The potentiodynamic polarization curves of the various samples provide information on the effectiveness of different coatings against 226
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Fig. 2. SEM micrographs analysis of PPy, ZP and ZP/PPy coatings electrosynthesized galvanostatically.
corrosion in 3% NaCl. Fig. 5 shows the results found and Table 3 summarizes the set of calculated corrosion values estimated by Tafel extrapolation. The results show a shift in potential of coated samples towards more electronegative values and a exchange current density. In a general manner these results confirm a protective effect of the different coating against the dissolution of the stainless steel. More precisely, the Table 3 indicates that the protective effectiveness increases in the following order SS < PPy < ZP < ZP/PPy. Polypyrrole coatings are well known for their ability to protect metals against corrosion in aggressive salt media [13,32,35]. This effect is explained by the energy barrier property of polypyrrole against the penetration of chloride ions responsible for the degradation of the metal [44,45]. In the other hand, the polypyrrole matrix counter-ions act as a reservoir for inhibiting corrosion. Consequently, due its charge and large size these counterions cannot be easily exchanged with the chloride present in the surrounding electrolyte [46,47], this offers protection against corrosion and a decrease in corrosion current. The ZP-coated substrate also shows a decrease in the corrosion current. Indeed, zinc phosphating is an effective method to protect steel against corrosion [36]. It has been shown that the addition of metal salts in the phosphatizing can greatly influence the microstructure of the zinc phosphate coating and make the coatings denser and thinner [48,49]. The depolarization of oxygen is an indication of the course of corrosion. The cathodic current depends mainly on the amount of oxygen reaching the cathode in a certain time interval [50,51]. Oxygen transport to the substrate is impeded by the protective phosphate film between the substrate and the
electrolyte due to the bias current decreases considerably. Also, the polypyrrole in the outer layer of the ZP/PPy coating acts as an oxygen reducing catalyst [52,53]. The ZP/PPy bilayer leads to a greater corrosion current decrease compared to the coatings of PPy and ZP alone. In terms of current density, the presence of the ZP/PPy double layer has practically halted the dissolution of the metal by acting as a barrier against aggressive ions. Zinc phosphate coatings are generally porous, which will promote adhesion of the PPy film to the surface. At the same time, the porosity promotes the diffusion of the electrolyte which will eventually lead to corrosion. The decrease in the corrosion current for coatings developed using the ZP/PPy double layer clearly indicates that the coating is more uniform and less porous than the normal zinc phosphate and polypyrrole coating. The ZP/PPy coatings cover the entire surface of the stainless steel which gives it a more homogeneous coating which leads to a decrease in the corrosion rate of these substrates. To understand these results and to propose a protection mechanism for stainless steel by these layers, we performed electrochemical impedance spectroscopic analysis. The impedance diagrams of the different coatings are shown in Fig. 6 and EIS data are grouped in Table 4. Primarily, Nyquist plots present a capacitive semicircle at high frequencies region and an inclined line assigned to a controlled release behavior at low frequencies. The capacitive behavior of stainless steel coated with PPy is related to the corrosion resistance of the coating. The theoretical simulation of the Nyquist diagrams by specialized software (Zsimpwin software) makes it possible to propose an equivalent electric 227
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Fig. 5. Potentiodynamic polarization curves recorded of ZP, PPy and ZP/PPy coatings electrosynthesized on stainless steel in 3% NaCl solution. Table 3 The potential and current densities of corrosion recorded in 3% NaCl solution.
Fig. 4. FTIR analysis of ZP (a), PPy (b) and ZP/PPy (c) coatings. Table 2 Characteristic FTIR bands of PPy and ZP.
PPy
ZP
Reference
599 and 670 CeH deformation 1050 ]CeH in-plane vibrations 1166 CeN stretching vibrations 1425 and 1642 Stretching mode of C]C and pyrrole ring 576 and 640 OePeO bending 944 OePeO asymmetric bending 1015 υs (PeO) PO4 symetric stretching 1068 and 1109 υas (PeO) PO4 asymetric stretching
[39,40]
Ecorr (mV)
Jcorr (µA/cm²)
βa
βc
E(%)
SS SS/PPy SS/ZP SS/ZP/PPy
−313.30 −366.46 −403.12 −403.20
30.19 8.63 4.89 1.99
−81.95 −57.95 −61.63 −98.95
102.81 92.71 78.46 69.40
– 71.4 ± 0.06 83.8 ± 0.03 93.4 ± 0.08
circuit of the stainless-steel protection system by the PPy, ZP and ZP/ PPy coatings. Fig. 7 illustrates the equivalent circuits found; in these circuits the polarization resistance (Rp,c) of the ZP/PPy-coated steel (Fig. 7b) is the diameter of the semicircle, which is the sum of the coating pore (Rpore) and charge transfer (Rct) resistances, CPEc is the constant phase element for coatings, CPEdl is the constant phase element for double layer of the coating/solution interface and ZW is Warburg impedance. In the case of polypyrrole coating (Fig. 7a) and due to the size of the counter-ion, the low frequency scattering behavior indicates the movement of these ions through the polypyrrole backbone [32]. For the other two samples of steel coated with ZP and ZP/PPy the same behavior of a capacitive loop in the high frequency and the inclined line (ZW) in the low frequencies is observed with an increase in resistance to the penetration of aggressive ions. The capacitive loop in the high frequency reflecting the mechanism of charge transfer and the presence of an inclined line indicating in the low-frequencies reflecting that the mechanism of corrosion is under the control of a diffusion process. Impedance studies have confirmed that the corrosion behavior of ZP/PPy layers is a much more diffusion-controlled process and thus offers higher corrosion resistance than the phosphate and polypyrrole coating. As previously discussed in the SEM part, double ZP/PPy coatings increase the coverage area, uniformity and thickness of the coating and decrease the porosity which improves the corrosion resistance of the coatings. These results of the electrochemical methods have to be compared with those of a non-electrochemical technique. The atomic absorption spectroscopy analysis makes it possible to follow the dissolution of the electrode in the corrosive medium by the qualitative and quantitative identification of the metal ions in the solution. In this part we used this technique to follow the evolution of the iron and chromium ions released by stainless steel in the corrosive solution at 3% NaCl for 10 days of immersion time (Fig. 8). The coating of PPy and ZP shows protection of the stainless steel against corrosion, however these coatings show structural defects over time under the effect of the aggressive ions of the saline medium which decreases the power of these protectors. Several
Fig. 3. XRD pattern of ZP (a), PPy (b) and ZP/PPy (c) coatings.
Wavenumber (cm−1) Assignments
Samples
[41–43]
228
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400
(a)
50
40
10 mHz
-Zi [kohm.cm²]
30
20
300
10
SS/ ZP/ PPy
0
-Zi [kohm.cm²]
0
10
20
30
CPEdl
Rpore
Rct
e
40
(b)
Zr [kohm.cm²]
SS/ZP
CPEc Rs
100 kHz
200
CPEc Rs
CPEdl Rpore
SS/PPy
e
100
Rct
W ZW
Fig. 7. Equivalent circuit models of: (a) SS/PPy and (b) ZP and ZP/PPy coated SS.
SS 0 0
100
200
300
400
500
600
Zr [kohm.cm²] 6
Log Zr [ohm]
5 SS/PPy SS/ZP
4
3 SS/PPy/ZP
SS
2
1 1
2
3 Log Frenquency [Hz]
4
Fig. 8. Atomic absorption spectroscopy analysis of electrosynthesized coating on stainless steel.
5
studies have shown that the ZP films have porosity in the coating surface [31,54] which has agreed with the SEM images of this work which decreases the resistance of ZP coatings to corrosion. The ZP/PPy bilayer can correct these defects by the formation of the polypyrrole layer on the sites of stainless steel not coated with ZP. The presence of dense and adherent ZP/PPy coating on stainless steel surface provides an organicinorganic layer that acts as an effective barrier against the attack from the corrosive environment [55,56]. On the other hand, ZP/PPy coatings offer an inhibitor of steel corrosion by the reduction of oxygen and also the anions released by the reduction of the polypyrrole, or by a concentration gradient, could act as inhibitors. These huge doping anions such as the tartrate ion are difficult to release in the polymer network and the insertion of cations into the polymer backbone occurs to compensate for the negative charge [57]. The coating thickness is an essential factor that can explain the improvement in the anti-corrosion properties of ZP/PPy. The thickness of the coatings electrodeposited on stainless steel was measured by a VEECO Dektak 6 M profilometer: the average thickness of the layer was 10 ± 1 μm. The thicknesses of ZP and ZP/PPy films can be approximated by the depth of X-ray penetration during analyses. For ZP the thickness value found is 0.2 μm and the ZP/PPy has a value of 0.26 μm. This increase therefore explains the improved anti-corrosion properties of ZP/PPy as a barrier against the penetration of corrosive ions in 3%NaCl medium.
SS/ZP
80
SS/PPy/ZP
Phase [deg]
60
40
SS/PPy SS/ZP
20
0
-20 1
2
3 4 Log Frenquency [hz]
5
Fig. 6. Electrochemical impedance data recorded of ZP, PPy and ZP/PPy coatings in 3% NaCl solution. Table 4 The EIS data recorded in 3% NaCl solution. Samples
Rs (ohm.cm−2)
Rpore (ohm.cm−2)
SS SS/PPy SS/ZP SS/ZP/PPy
37.57 3202 1656 622
1.29 3.03 1.95 4.47
108 105 105 105
nf
Rct (ohm.cm−2)
– 0.707 0.779 0.725
9.47 4.56 8.95 2.05
104 104 104 105
R²
4. Conclusion
0.899 0.932 0.876 0.931
In this work the ZP, PPy and ZP/PPy coatings are electrodeposited on stainless steel successfully. The ZP electrodeposition mechanism is initiated by the reduction of nitrate ions on the cathode which 229
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liberation of Zn+2 ions. These ions react with PO34− ions to form ZP. The electropolymerization of PPy is ensured by the passivation of the surface of the steel by the formation of a tartrate protective layer. The morphology of ZP is in the form of large crystal clusters; however, the morphology is more porous on this surface which decreases the resistance of the coating to corrosion. Polypyrrole shows a globular morphology of different sizes. The polypyrrole deposit on the ZP coating shows that the PPy is deposited on the surface areas of the uncoated ZP steel, which leads to the formation of a homogenous and compact deposit and consequently an increase in the protective properties of ZP/PPy films. The electrochemical analysis (EIS and potentiodynamic polarization) and also that of atomic absorption actually shows a reduction of the corrosion rate by the increasing of ZP/PPy thickness and also the formation of PPy on the surfaces not coated with ZP.
[21] [22] [23] [24] [25] [26] [27]
Compliance with ethical standards
[28]
The authors declare that they have no conflict of interest.
[29]
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Electrosynthesis of zinc phosphate-polypyrrole coatings for improved corrosion resistance of steel
T
A. El Jaouharia, , A. Chennaha, S. Ben Jaddia, H. Ait Ahsainea, Z. Anfara, Y. Tahiri Alaouib, Y. Naciria, A. Benlhachemia, M. Bazzaouia ⁎
a b
Laboratoire des Matériaux et Environnement, Faculté des Sciences, Département de Chimie, Université Ibn Zohr, BP 8106, Agadir, Morocco Laboratoire d'Analyse et Synthèse de Procédés Industriels (LASPI), Ecole Mohammadia d'Ingénieurs, University Mohammed V in Rabat, BP 765, Rabat, Morocco
ARTICLE INFO
ABSTRACT
Keywords: Polypyrrole Zinc phosphate Corrosion protection Stainless steel Bilayer coating
Zinc Phosphate (ZP), polypyrrole (PPy) and ZP/PPy coatings were electro-electrodeposited on 304 stainless steel (SS) using galvanostatic mode for application as a corrosion protector. The morphology of different coatings on the surface of stainless steel was observed by scanning electron microscopy (SEM). The chemical composition of phase Zn3(PO4)2.6H2O is determined by X-ray diffraction analysis (XRD). The performance of the ZP, PPy and ZP/PPy coating against corrosion of stainless steel in a 3% NaCl solution was measured by electrochemical methods such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PP). These results of the electrochemical analysis were compared with those of the atomic absorption analysis in the same corrosive solution (3% NaCl) for 10 days of immersion. All of these results show that ZP/PPy coatings have an effective resistance against corrosion due to the formation of polypyrrole at sites not coated with ZP and by increasing in the thickness of the ZP/PPy coating.
1. Introduction
organic mixing with other inorganic compounds are increasingly important. These materials can combine the properties of conductive polymers with those of the inorganic compound which gives a significant synergistic effect for the fight against corrosion [18–20]. Chen et al. [21] have synthesized nanocomposite coatings of PPy/ZnO by electrochemical methods. The authors have shown that incorporation of ZnO particles altered the morphology and electrochemical properties of the PPy coating. The nanocomposite coating had a relatively better load capacity, conductivity, and corrosion protection properties than pure PPy. Mondal et al. [22], have deposited by galvanostatic deposition technique a composite of graphene oxide-polypyrrole on a stainlesssteel substrate. The morphology of the composite coating reveals a homogeneous distribution of graphene oxide on the composite with a high surface coverage capability. Anti-corrosion analyzes reflect better and more effective inhibition by improving the corrosion potential, breakdown potential and decrease in corrosion current density. Guixiang [23] reported the electro-synthesis of Polypyrrole-molybdate film on AZ31 Mg Alloy. The authors showed that MoO42− can form a MnO42− complex passive film on the Mg alloy surface before the Pyrrole polymerizes. The PPy/MnO42− composite makes the surface morphology of the film more compact and prevents the penetration of corrosion ions into the surface of the Mg alloy, which can improve the corrosion resistance performance. On the other hand, zinc phosphate
Stainless steel is a very particular metal compared to other materials due to its varied application area and of the use of these alloy in hard functioning conditions in the manufacturing and chemical industries. These steels are generally resistant to dissolution; however, its resistance to the phenomenon of corrosion depends on the composition of the operating environment and the strength of their passive protective films [1,2]. The importance of steel protection process in different fields has required research of corrosion control technique by various scientists of corrosion [3–5]. Conjugated conductive polymers have been widely studied thanks to their non-toxic, environmentally friendly, high stability and ease of synthesis, are the most promising coatings for corrosion protection of industrial metals [6–9]. Among these polymers, polypyrrole remains the best known and the most studied because of its high conductivity, outstanding air stability and special physical, chemical properties compared with other conducting polymers [10–14]. The protective mechanism of this type of polymeric coating is based on a mixed effect of isolation and charge transfer [15,16]. This motivates researchers to find new procedures to correct these defects and improve the effectiveness of polypyrrole against corrosion [17]. Polymeric-inorganic composite materials synthesized by molecular engineering or by
⁎
Corresponding author.
https://doi.org/10.1016/j.surfin.2019.02.011 Received 13 August 2018; Received in revised form 10 February 2019; Accepted 15 February 2019 Available online 18 February 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Chemical composition of stainless-steel working electrode. Element
C
Si
Mn
P
S
Cr
Ni
Fe
mass%
0.05%
0.63%
0.90%
0.032%
0.002%
18.05%
8.10%
balanced
coatings are one of the promising methods to improve the corrosion resistance of metals [24]. It has been generally demonstrated that the addition of metal salts in phosphate coatings can greatly influence the microstructure of the zinc phosphate coating and make coatings denser and thinner [25,26]. The objective of the present work is the study of the anti-corrosion effect of the polypyrrole and zinc phosphate bilayers prepared by electrodeposition on a 304 stainless steel substrate in the saline environment 3% NaCl.
by accelerated electrons at a voltage of 45 kV, the current being 35 mA. This electron beam interacting with the material generates radiation of wavelength λ(Kα1) = 1,5,440,598 Å et λ(Kα2) = 1,544,426 Å. The diffractometer is equipped with a Pixcel-1D detector. The latter is equipped with a nickel filter for eliminating Kβ radiation from copper. Anti-scattering Soller slots eliminate parasitic radiation from the source and the sample. The diffraction patterns are recorded in continuous mode in a 2θ angular range from 5° to 60° with a pitch of 0.00164° and an angular velocity of 0.002°.s−1.
2. Experimental
2.3.2. AAS analysis AA-7000, SHIMADZU atomic absorption spectrometer controlled by Wizard data collection software. The analyses were carried out in 1% HNO3 solutions of 0–5 ppm Fe and Cr, with 0.12 ppm as LDL.
2.1. Electrochemical cell and chemicals Pyrrole monomer (Aldrich) was distilled under nitrogen and all chemicals were obtained from Aldrich Chemical Company with a high analytical purity and used as received. The working electrode was formed from stainless-steel 304 sheets with 2 cm² of area which the following composition (Table 1): Before each use the electrodes were mechanically polished with 800, 1000 and 1200 abrasive paper and degrease with acetone. Following this pretreatment, the working electrode was directly transferred to the electrochemical three-electrode cell (one compartment). The cell is connected to Voltalab PGZ301 potentiostat-galvanostat controlled by VoltaMaster4. The Ag/AgCl (KCl 0.1 M) was used as reference electrode and the auxiliary electrode is a platinum wire. The electrochemical corrosion tests were carried out at room temperature in a 3% NaCl saline solution. The electrochemical impedance spectroscopy (EIS) measurements (Voltalab PGZ301 potentiostat) were carried out to an open circuit potential with a 10 mV in amplitude of the superimposed AC signal, and the applied frequency were between 100 kHz and 10 mHz.
3. Results and discussion 3.1. Electrochemical behavior of electrosynthesis coatings on stainless steel The electrochemical behavior of stainless-steel during electroplating of coatings (PPy, ZP and ZP/PPy) is investigated by the potential polarization of the working electrode in the different electrolytic solution. In order to demonstrate the mechanism of ZP deposition on a stainlesssteel substrate, cyclic voltammetry experiments were carried out in an aqueous solution containing 4 10−2 M Zn(NO3)2.6H2O and 2.67 10−2 M NH4H2PO4 (Fig. 1). The polarization of the electrode between −2 and 1 V was initiated in the negative direction of the potential at a scanning speed of 50 mV/s. The recorded voltammogram shows an increase in cathodic current with a negative sweep of the electrode potential of −0.5 V which could be related to the reduction of nitrate ions [27]. Consequently, no electrochemical reaction takes place under −0.5 V as indicated by different studies [28–30]. During reverse anodic scanning, an anodic peak in the region between −550 and −950 mV probably occurs because the steel surface is exposed to the electrolyte [31]. Its intensity depends on the porosity of the zinc phosphate coating [31]. This peak of porosity is thought to result from the oxidation of Fe, Fe²+ and/or the formation of a complex compound [31]. The disappearance of this peak in the second scanning cycle can be explained by the formation of the first ZP crystals on the surface of the steel and it is an indicator of the protective effect of ZP coating against the dissolution of the steel. The growth mechanism of ZP can be summarized as follows. Under a suitable potential, the reduction of the nitrate occurs near the cathode, which leads to the increase of pH in the vicinity of the electrode which causes the supersaturation of the Zn²+ and PO43− ions after they react with each other for form Zn3(PO4)2 on the working electrode. In the case of a polarized stainless-steel electrode in a solution of 0.1 M Na2C4H4O6 and 0.5 M pyrrole between −0.6 and 1 V with a scanning speed of 50 mV/s (Fig. 1) we notice in the first scan cycle that the voltammogram is characterized by two oxidation peaks which are located respectively at 0 and 0.7 V vs Ag/AgCl. The first oxidation peak is attributed to the oxidation behavior of the SS electrode, followed by a reduction in current density caused by the passive formation of a tartrate/oxide layer [32]. This inhibited the dissolution of the electrode and was followed by the electrochemical polymerization reaction. The second peak is attributed to the oxidation of monomer (pyrrole) to polypyrrole [32]. During the next cycle (2nd scan cycle) the anode oxidation peak disappeared which shows that the PPy coating inhibits the oxidation of the electrode [33]. In the case of deposition of
2.2. Preparation of electrolytic solutions and coatings preparation The electrolyte was prepared by dissolving zinc nitrate hexahydrate Zn(NO3)2•6H2O (4 10−2 M) and ammonium dihydrogen phosphate NH4H2PO4 (2.67 10−2 M). The molar fractions of precursor were taken to ensure that the molar ratios Zn/P were kept constant at approximately 1.5. The initial pH of the electrolyte is equal to 4.05 at 40 °C. The thin films of zinc phosphate were electrodeposited by galvanostatic mode under a current density of −10 mA.cm−2 during 30 min. The polypyrrole coating is electro synthesized also by the galvanostatic mode by applying a current density of 5 mA.cm−2 for 10 min in an electrolytic solution which contains 0.1 M of Na2C4H4O6 and 0.5 M pyrrole at room temperature. The ZP/PPy bilayers electrodeposited one on the other successively. After deposition of ZP the layers are washed with water and dried at room temperature. We also synthesize PPy/ZP layers however these coatings show poor adhesion of polypyrrole (first layer). For this reason, we have not studied the anticorrosive property of these composites. 2.3. Instrumentation 2.3.1. SEM and XRD analysis The morphology was characterized by scanning electron microscopy (SEM) ZEISS SUPRA 40VP COLUMN GEMINI. The microscope chamber was maintained at a pressure between 4 and 10 Pa. For the X-ray diffraction analysis, the apparatus used is a PANALYTICAL EMPYREAN, equipped with a copper anticathode bombarded 225
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Fig. 1. Cyclic voltammograms of ZP, PPy and ZP/PPy coatings growth on stainless steel electrodes (at 50 mV/s scan rate).
electrodeposited coatings of ZP, PPy and ZP/PPy on the stainless-steel substrate. The deconvolution of the graphs is done by X'pert hight score software. The phase compositions of stainless steel coated with normal ZP and ZP/PPy were compared (Fig. 3a and b). It is shown that coatings mainly consisted of Zn3(PO4)2•4H2O (hopeite, JCPD file #37-0465) [36]. The diffraction of PPy coating (Fig. 3c) shows a purely amorphous structure which is normal for organic compounds such as polypyrrole [37]. The deposition of polypyrrole on ZP coating does not affect the position and the nature of the zinc phosphate peaks because of their amorphousness but on the other hand the peak intensity undergoes a big change. The peak intensities of the ZP/PPy coating are lower than those of the normal ZP coatings, which clearly indicate the formation of a PPy layer on the ZP [38].
polypyrrole on stainless steel (Fig. 1), only the pyrrole oxidation wave appears, indicating the formation of PPy on ZP coating. 3.2. Characterization of coatings 3.2.1. Microstructural analysis (SEM) In order to take information on the morphological changes of the different coatings, the scanning electron microscope is performed. Fig. 2 shows the morphologies of different PPy, ZP and ZP/PPy coatings. First, the polypyrrole film (Fig. 2) synthesized on steel by galvanostatic technique (5 mA/cm² for 10 min) has a homogeneous and compact morphology characterized by a globular structure of different sizes between 2 and 10 nm, recalling the appearance and morphology of cauliflower [34,35]. Fig. 2 shows the morphology of ZP thin film, we observe the formation of many clusters of large plate-shaped crystals with uncoated place. The plates have formed uniformly on the surface (preferential orientation), which have an average size of 5–10 μm. This morphology of ZP is widely mentioned in much of the similar work of this coating [36]. ZP/PPy micrographs (Fig. 2) show the formation of polypyrrole spots on uncoated areas of stainless steel. This means that the deposition of PPy on ZP can correct the morphological defects of ZP coating and this is one of the factors that will improve the protection of stainless steel against the penetration of aggressive ions. The improvement of morphological properties remains one of the strong points of organic-inorganic coatings [21].
3.2.3. FTIR analysis To confirm the presence of polypyrrole, the samples ZP, PPy and ZP/PPy are characterized by FTIR (Fig. 4) and Table 2 summarizes the characteristic bands found. The main polypyrrole bands are: 1425 cm−1 attributed to the fundamental vibrations of polypyrrole ring, 1050 cm−1 due to the ]CeH in-plane vibrations, 1166 cm−1 assigned to the CeN stretching vibrations and 1642 cm−1 due to stretching mode of C]C [39, 40]. The appearance of polypyrrole bands on the ZP/PPy spectrum confirms the formation of PPy on the ZP film. 3.3. Evaluation of corrosion resistant property of zinc phosphate/ polypyrrole coatings
3.2.2. XRD analysis The phase compositions in the coatings on the stainless steel were analyzed by XRD. Fig. 3 shows the X-ray diffraction patterns (XRDs) of
The potentiodynamic polarization curves of the various samples provide information on the effectiveness of different coatings against 226
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Fig. 2. SEM micrographs analysis of PPy, ZP and ZP/PPy coatings electrosynthesized galvanostatically.
corrosion in 3% NaCl. Fig. 5 shows the results found and Table 3 summarizes the set of calculated corrosion values estimated by Tafel extrapolation. The results show a shift in potential of coated samples towards more electronegative values and a exchange current density. In a general manner these results confirm a protective effect of the different coating against the dissolution of the stainless steel. More precisely, the Table 3 indicates that the protective effectiveness increases in the following order SS < PPy < ZP < ZP/PPy. Polypyrrole coatings are well known for their ability to protect metals against corrosion in aggressive salt media [13,32,35]. This effect is explained by the energy barrier property of polypyrrole against the penetration of chloride ions responsible for the degradation of the metal [44,45]. In the other hand, the polypyrrole matrix counter-ions act as a reservoir for inhibiting corrosion. Consequently, due its charge and large size these counterions cannot be easily exchanged with the chloride present in the surrounding electrolyte [46,47], this offers protection against corrosion and a decrease in corrosion current. The ZP-coated substrate also shows a decrease in the corrosion current. Indeed, zinc phosphating is an effective method to protect steel against corrosion [36]. It has been shown that the addition of metal salts in the phosphatizing can greatly influence the microstructure of the zinc phosphate coating and make the coatings denser and thinner [48,49]. The depolarization of oxygen is an indication of the course of corrosion. The cathodic current depends mainly on the amount of oxygen reaching the cathode in a certain time interval [50,51]. Oxygen transport to the substrate is impeded by the protective phosphate film between the substrate and the
electrolyte due to the bias current decreases considerably. Also, the polypyrrole in the outer layer of the ZP/PPy coating acts as an oxygen reducing catalyst [52,53]. The ZP/PPy bilayer leads to a greater corrosion current decrease compared to the coatings of PPy and ZP alone. In terms of current density, the presence of the ZP/PPy double layer has practically halted the dissolution of the metal by acting as a barrier against aggressive ions. Zinc phosphate coatings are generally porous, which will promote adhesion of the PPy film to the surface. At the same time, the porosity promotes the diffusion of the electrolyte which will eventually lead to corrosion. The decrease in the corrosion current for coatings developed using the ZP/PPy double layer clearly indicates that the coating is more uniform and less porous than the normal zinc phosphate and polypyrrole coating. The ZP/PPy coatings cover the entire surface of the stainless steel which gives it a more homogeneous coating which leads to a decrease in the corrosion rate of these substrates. To understand these results and to propose a protection mechanism for stainless steel by these layers, we performed electrochemical impedance spectroscopic analysis. The impedance diagrams of the different coatings are shown in Fig. 6 and EIS data are grouped in Table 4. Primarily, Nyquist plots present a capacitive semicircle at high frequencies region and an inclined line assigned to a controlled release behavior at low frequencies. The capacitive behavior of stainless steel coated with PPy is related to the corrosion resistance of the coating. The theoretical simulation of the Nyquist diagrams by specialized software (Zsimpwin software) makes it possible to propose an equivalent electric 227
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Fig. 5. Potentiodynamic polarization curves recorded of ZP, PPy and ZP/PPy coatings electrosynthesized on stainless steel in 3% NaCl solution. Table 3 The potential and current densities of corrosion recorded in 3% NaCl solution.
Fig. 4. FTIR analysis of ZP (a), PPy (b) and ZP/PPy (c) coatings. Table 2 Characteristic FTIR bands of PPy and ZP.
PPy
ZP
Reference
599 and 670 CeH deformation 1050 ]CeH in-plane vibrations 1166 CeN stretching vibrations 1425 and 1642 Stretching mode of C]C and pyrrole ring 576 and 640 OePeO bending 944 OePeO asymmetric bending 1015 υs (PeO) PO4 symetric stretching 1068 and 1109 υas (PeO) PO4 asymetric stretching
[39,40]
Ecorr (mV)
Jcorr (µA/cm²)
βa
βc
E(%)
SS SS/PPy SS/ZP SS/ZP/PPy
−313.30 −366.46 −403.12 −403.20
30.19 8.63 4.89 1.99
−81.95 −57.95 −61.63 −98.95
102.81 92.71 78.46 69.40
– 71.4 ± 0.06 83.8 ± 0.03 93.4 ± 0.08
circuit of the stainless-steel protection system by the PPy, ZP and ZP/ PPy coatings. Fig. 7 illustrates the equivalent circuits found; in these circuits the polarization resistance (Rp,c) of the ZP/PPy-coated steel (Fig. 7b) is the diameter of the semicircle, which is the sum of the coating pore (Rpore) and charge transfer (Rct) resistances, CPEc is the constant phase element for coatings, CPEdl is the constant phase element for double layer of the coating/solution interface and ZW is Warburg impedance. In the case of polypyrrole coating (Fig. 7a) and due to the size of the counter-ion, the low frequency scattering behavior indicates the movement of these ions through the polypyrrole backbone [32]. For the other two samples of steel coated with ZP and ZP/PPy the same behavior of a capacitive loop in the high frequency and the inclined line (ZW) in the low frequencies is observed with an increase in resistance to the penetration of aggressive ions. The capacitive loop in the high frequency reflecting the mechanism of charge transfer and the presence of an inclined line indicating in the low-frequencies reflecting that the mechanism of corrosion is under the control of a diffusion process. Impedance studies have confirmed that the corrosion behavior of ZP/PPy layers is a much more diffusion-controlled process and thus offers higher corrosion resistance than the phosphate and polypyrrole coating. As previously discussed in the SEM part, double ZP/PPy coatings increase the coverage area, uniformity and thickness of the coating and decrease the porosity which improves the corrosion resistance of the coatings. These results of the electrochemical methods have to be compared with those of a non-electrochemical technique. The atomic absorption spectroscopy analysis makes it possible to follow the dissolution of the electrode in the corrosive medium by the qualitative and quantitative identification of the metal ions in the solution. In this part we used this technique to follow the evolution of the iron and chromium ions released by stainless steel in the corrosive solution at 3% NaCl for 10 days of immersion time (Fig. 8). The coating of PPy and ZP shows protection of the stainless steel against corrosion, however these coatings show structural defects over time under the effect of the aggressive ions of the saline medium which decreases the power of these protectors. Several
Fig. 3. XRD pattern of ZP (a), PPy (b) and ZP/PPy (c) coatings.
Wavenumber (cm−1) Assignments
Samples
[41–43]
228
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400
(a)
50
40
10 mHz
-Zi [kohm.cm²]
30
20
300
10
SS/ ZP/ PPy
0
-Zi [kohm.cm²]
0
10
20
30
CPEdl
Rpore
Rct
e
40
(b)
Zr [kohm.cm²]
SS/ZP
CPEc Rs
100 kHz
200
CPEc Rs
CPEdl Rpore
SS/PPy
e
100
Rct
W ZW
Fig. 7. Equivalent circuit models of: (a) SS/PPy and (b) ZP and ZP/PPy coated SS.
SS 0 0
100
200
300
400
500
600
Zr [kohm.cm²] 6
Log Zr [ohm]
5 SS/PPy SS/ZP
4
3 SS/PPy/ZP
SS
2
1 1
2
3 Log Frenquency [Hz]
4
Fig. 8. Atomic absorption spectroscopy analysis of electrosynthesized coating on stainless steel.
5
studies have shown that the ZP films have porosity in the coating surface [31,54] which has agreed with the SEM images of this work which decreases the resistance of ZP coatings to corrosion. The ZP/PPy bilayer can correct these defects by the formation of the polypyrrole layer on the sites of stainless steel not coated with ZP. The presence of dense and adherent ZP/PPy coating on stainless steel surface provides an organicinorganic layer that acts as an effective barrier against the attack from the corrosive environment [55,56]. On the other hand, ZP/PPy coatings offer an inhibitor of steel corrosion by the reduction of oxygen and also the anions released by the reduction of the polypyrrole, or by a concentration gradient, could act as inhibitors. These huge doping anions such as the tartrate ion are difficult to release in the polymer network and the insertion of cations into the polymer backbone occurs to compensate for the negative charge [57]. The coating thickness is an essential factor that can explain the improvement in the anti-corrosion properties of ZP/PPy. The thickness of the coatings electrodeposited on stainless steel was measured by a VEECO Dektak 6 M profilometer: the average thickness of the layer was 10 ± 1 μm. The thicknesses of ZP and ZP/PPy films can be approximated by the depth of X-ray penetration during analyses. For ZP the thickness value found is 0.2 μm and the ZP/PPy has a value of 0.26 μm. This increase therefore explains the improved anti-corrosion properties of ZP/PPy as a barrier against the penetration of corrosive ions in 3%NaCl medium.
SS/ZP
80
SS/PPy/ZP
Phase [deg]
60
40
SS/PPy SS/ZP
20
0
-20 1
2
3 4 Log Frenquency [hz]
5
Fig. 6. Electrochemical impedance data recorded of ZP, PPy and ZP/PPy coatings in 3% NaCl solution. Table 4 The EIS data recorded in 3% NaCl solution. Samples
Rs (ohm.cm−2)
Rpore (ohm.cm−2)
SS SS/PPy SS/ZP SS/ZP/PPy
37.57 3202 1656 622
1.29 3.03 1.95 4.47
108 105 105 105
nf
Rct (ohm.cm−2)
– 0.707 0.779 0.725
9.47 4.56 8.95 2.05
104 104 104 105
R²
4. Conclusion
0.899 0.932 0.876 0.931
In this work the ZP, PPy and ZP/PPy coatings are electrodeposited on stainless steel successfully. The ZP electrodeposition mechanism is initiated by the reduction of nitrate ions on the cathode which 229
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liberation of Zn+2 ions. These ions react with PO34− ions to form ZP. The electropolymerization of PPy is ensured by the passivation of the surface of the steel by the formation of a tartrate protective layer. The morphology of ZP is in the form of large crystal clusters; however, the morphology is more porous on this surface which decreases the resistance of the coating to corrosion. Polypyrrole shows a globular morphology of different sizes. The polypyrrole deposit on the ZP coating shows that the PPy is deposited on the surface areas of the uncoated ZP steel, which leads to the formation of a homogenous and compact deposit and consequently an increase in the protective properties of ZP/PPy films. The electrochemical analysis (EIS and potentiodynamic polarization) and also that of atomic absorption actually shows a reduction of the corrosion rate by the increasing of ZP/PPy thickness and also the formation of PPy on the surfaces not coated with ZP.
[21] [22] [23] [24] [25] [26] [27]
Compliance with ethical standards
[28]
The authors declare that they have no conflict of interest.
[29]
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