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dodecyl benzene sulfonic acid nanocomposite as a highly effective anticorrosive coating
Surface & Coatings Technology 307 (2016) 382–391
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Polypyrrole/graphene nanosheets/rare earth ions/dodecyl benzene sulfonic acid nanocomposite as a highly effective anticorrosive coating R. Alam, M. Mobin ⁎, J. Aslam Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India
a r t i c l e
i n f o
Article history: Received 21 June 2016 Revised 3 September 2016 Accepted in revised form 6 September 2016 Available online 13 September 2016 Keywords: Rare earth ions Tem Graphene Nyquist
a b s t r a c t Organic-inorganic nanocomposites (PPy/GNS/RE3+/DBSA) involving pyrrole (Py), graphene nano sheets (GNS), rare earth elements (RE3+ = La3+, Sm3+, Nd3+) and dodecyl benzene sulfonic acid (DBSA) were synthesized via in situ chemical oxidative polymerization using FeCl3 as an oxidant. PPy/GNS/DBSA and PPy/DBSA was also synthesized by following the identical synthesis route. The resultant nanocomposites were characterized by FTIR, XRD, SEM and TEM. The synthesized nanocomposites were chemically deposited on low carbon steel specimens by solvent evaporation method using N-Methyl-2-pyrrolidone (NMP) as solvent and 10% epoxy resin (by weight) as binder. The anticorrosive nature of polymer coatings were studied in 0.1 M HCl solution by subjecting them to various corrosion tests, which includes: EIS, potentiodynamic polarization, free corrosion potential (OCP) measurement and immersion test. The surface morphology of coated samples before and after immersion was evaluated using SEM. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The discovery of conducting polymers such as polyaniline, polypyrrole, polythiophene has shown the way to develop protective coatings, which are non-toxic, eco-friendly, environmentally stable and binds strongly on the metal surface. Among the various conducting polymers polyaniline and polypyrrole based coatings are quite attractive to the researchers due to their good physio-chemical properties [1–4]. The protection ability of these polymer coatings also depend upon the metal substrate on which they are applied. The addition of various organic or inorganic additives or combination of both to the conducting polymers has led to the formation of composites, which have shown superior properties as compared to the polymers itself. It increases the solubility, stability, and adhesion of the polymers to the substrate, and also enhances their anticorrosive nature to a greater extent. The development of polymer composites coating on metal substrate either chemically or electrochemically increase the passivation property of the metal substrate by shifting the corrosion potential towards the direction of more noble metal [5–7]. The composites of graphene with polyaniline and polypyrrole have been used widely for various purposes including protection of metal substrate from degradation [8,9]. Graphene, a two-
dimensional monolayer of sp2 bonded carbon has outstanding mechanical, optical, thermal and electrical properties. It is considered to be effective anticorrosive component in coating system due to its excellent chemical inertness, thermal stability and impermeability. Lanthanides are considered to have low toxicity and their ingestion is not harmful for health. The lanthanide ions form insoluble hydroxides, which suppress the cathodic corrosion reaction and protect the metal from degradation. The surface films generated on the rare earth doped alloys have improved behavior against both high temperature oxidation and aqueous corrosion [10]. In the current work, organic–inorganic nanocomposites of pyrrole, graphene nano sheets and the rare earth elements (La3 +, Sm3 +, Nd3+) along with dodecyl benzene sulfonic acid were synthesized by oxidative polymerization. The synthesized nanocomposites were characterized by FTIR, XRD, SEM and TEM. The composites were applied on carbon steel specimens and their anticorrosive properties were evaluated in 0.1 M HCl using EIS, Potentiodynamic polarization, OCP, immersion test and SEM. 2. Experimental procedure 2.1. Chemicals
⁎ Corresponding author. E-mail address: [email protected] (M. Mobin).
http://dx.doi.org/10.1016/j.surfcoat.2016.09.010 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Pyrrole (Py) and graphite powder were purchased from Sigma Aldrich. Ferric chloride, polyethylene glycol 600, hydrogen peroxide, NMethyl-2-pyrrolidone, ethanol, ortho-dichlorobenzene, p-toluene
R. Alam et al. / Surface & Coatings Technology 307 (2016) 382–391
sulfonic acid, sulfuric acid, and potassium permanganate were of AR grade and supplied by Merck. Nitrate salts of rare earth elements were procured from Alfa Aesar.
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washed with double distilled water and ethanol to remove residual pyrrole, DBSA and oxidant. The filtrate nanocomposite was dried in vacuum for 12 h. PPy/GNS/DBSA and PPy/DBSA was also synthesized following the identical synthesis route.
2.2. Preparation of metal specimen for application of coatings 2.4. Characterization The low carbon steel coupons of dimension 40.0 × 15.0 × 1.3 mm having chemical composition 0.028% P,0.033% V, 0.081% Mo, 0.049% C, 0.723% Mn,0.01% Al, 0.051% Cr and balance Fe were used as substrate for polymer coatings. Prior to application of coatings, the coupons were polished mechanically by series of emery papers, this was followed by rinsing with double distilled water and ultra-sonication with acetone for 5 min. 2.3. Synthesis of GNS, PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/DBSA composites Graphene oxide was synthesized by modified hummer's method as reported earlier [11]. The obtained graphene oxide was transformed into graphene (reduced graphene oxide) by treating with hydrazine, indicated by color change from brown to black, which was subsequently ultra-sonicated for 4 h in ortho-dichlorobenzene to attain graphene nano sheets by liquid-phase exfoliation method [12]. The resulting product was washed with water and ethanol followed by vacuum drying at 50 °C for 24 h. PPy/GNS/RE3+/DBSA was synthesized by chemical oxidative polymerization using FeCl3 as an oxidant and p-toluene sulfonic acid as dopant, illustrated in Scheme 1 [13]. 2 mL pyrrole, the rare earth ions and GNS (both 5% by weight of the amount of pyrrole) along with 0.5 mL of PEG-600 were taken in round bottom flask. PEG600 was used as an anticoagulant agent, which prevents the agglomeration of GNS. Besides, it also acts as a plasticizer to provide flexibility and reduce brittleness of the nanocomposite coatings [14]. 15 mL of ethanol and 1 g of DBSA was added to it and the resultant mixture was ultra-sonicated for 3 h. After sonication the mixture was allowed to cool to 0–5 °C under constant stirring. After 20 min, 0.5 g of p-toluene sulfonic acid was added to the mixture and left for stirring for 30 min. Finally, 4 g of FeCl3 was dissolved in 50 mL of water and added drop wise to the above mixture under constant stirring. After 2 h of stirring the resultant mixture was left at room temperature for 48 h for further polymerization. The obtained mixture was filtered and
The synthesized PPy/GNS/RE3 +/DBSA, PPy/GNS/DBSA and PPy/ DBSA were characterized by FTIR, XRD, SEM and TEM techniques. Xray diffraction (XRD) studies were carried out in the 2θ range of 20°– 80° using Shimadzu 6100X X-ray diffractometer. Fourier Transform Infra-Red (FTIR) spectroscopy (Model: Perkin Elmer) was studied in the frequency range of 500–4000 cm− 1. SEM (Model: JEOL JSM6510LV) and TEM (Model: JEOL JEM-2100) was used to study the morphology of the synthesized polymers. 2.5. Preparation of coating of PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/ DBSA composites on low carbon steel The coatings of PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/DBSA were carried out on low carbon steel by solvent evaporation method using NMP as solvent. Epoxy resin (10% by weight), synthesized by an earlier described method [15], and was used to enhance the binding of nanocomposites to the steel surface. 0.3 g of synthesized nanocomposite was dissolved in 10 mL of NMP and solution was stirred continuously for 24 h. This was followed by the addition of epoxy resin to the above mixture and the content was again stirred for 1 h. The resultant solution was spread on the steel surface with the help of dropper and the solvent was allowed to evaporate at 85–90 °C. After 3 to 4 cycles a strongly adherent coating was obtained. The thickness of coatings was measured using Elcometer (Model: 456) and found to be in the range of 15.72– 17.37 μm. More coated samples were obtained using the identical procedure. 2.6. Assessment of corrosion protection performance of PPy/GNS/RE3 +/ DBSA, PPy/GNS/DBSA and PPy/DBSA composites The anticorrosive properties of polymer composites coated low carbon steel samples in 0.1 M HCl as a corrosive medium at room temperature (30 °C) were evaluated by subjecting them to various corrosion tests, which include: free corrosion potential (OCP) measurements, ac
Scheme 1. Synthesis of PPy/GNS/RE3+/DBSA.
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Fig. 1. FTIR spectra of (a) PPy/DBSA (b) PPy/GNS/DBSA (c) PPy/GNS/La3+ (d) PPy/GNS/Nd3+/DBSA and (e) PPy/GNS/Sm3+/DBSA.
impedance, potentiodynamic polarization measurements, and gravimetric analysis. SEM images of coated samples were recorded before and after 30 days immersion in 0.1 M HCl. 2.6.1. Free corrosion potential measurements The OCP measurements on coated and uncoated steel samples in 0.1 M HCl solution at room temperature (30 °C) were carried out using saturated calomel electrode (SCE) as a reference electrode. The steel specimens immersed into the test solution were connected to a wire having alligator clip on both the ends. One end of alligator clip was connected to the carbon steel specimen whereas the other end was connected to a multimeter. The change in the voltage against SCE was plotted against time. 2.6.2. AC impedance and potentiodynamic polarization measurements AC impedance and potentiodynamic polarization measurements were carried out using glass cell with steel samples having exposed area of 1 cm2 as working electrode, Pt wire as counter electrode, Ag/
AgCl as reference electrode; Luggin-Haber capillary was used to minimize the IR drop. Prior to each experiment the steel samples were left out in the corrosive solution for 1 h to attain the steady state potential. During the electrochemical analysis the temperature was maintained at 30 ± 2 °C. The EIS measurements were carried out at OCP within the frequency range of 10−2 to 105 Hz with 10 mV perturbation. The values of charge transfer resistance were used to calculate the protection efficiency (%ηR). ηR ð%Þ ¼
Rct ðiÞ −Rct Rct ðiÞ
100
ð1Þ
where, Rct and R(i) ct is the charge transfer resistance in absence and presence of coatings. The polarization studies were carried out by sweeping the potential between −250 to 250 mV with respect to the steady-state potential at a scan rate of 0.001 V/s. The anodic and cathodic curves were extrapolated to obtain the corrosion current densities
Fig. 2. XRD pattern of (a) PPy/DBSA (b) PPy/GNS/DBSA (c) PPy/GNS/La3+ (d) PPy/GNS/Nd3+/DBSA and (e) PPy/GNS/Sm3+/DBSA.
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(a)
(b)
(c)
(d)
(e) Fig. 3. SEM micrographs of (a) PPy/DBSA (b) PPy/GNS/DBSA (c) PPy/GNS/La3/DBSA (d) PPy/GNS/Nd3+/DBSA and (e) PPy/GNS/Sm3+/DBSA.
(icorr) and corrosion potential (Ecorr). The protection efficiency (%ηi) was calculated from the measured icorr values using the following equation: ηi ð%Þ ¼
icorr −iðiÞ corr 100 icorr
ð2Þ
where, icorr and i(i)corr is corrosion current density without and with the coatings, respectively. 2.6.3. Immersion test The coated and uncoated samples were subjected to immersion test in 0.1 M HCl solution for the period of 30 days under unstirred condition at room temperature. The detailed procedure for the immersion test
and calculation of corrosion rates have been reported in our earlier publication [16]. The protection efficiency (%η) was calculated using the following equation: ηð%Þ ¼
νo −ν i 100 νo
ð3Þ
where, νo and νi are the corrosion rates of uncoated and coated sample. 2.7. Surface morphological studies The surface morphology of polymer coatings on low carbon steel specimens was evaluated using scanning electron microscopy
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(a)
(b) Fig. 4. TEM images of (a) GNS and (b) PPy/GNS/Sm3+/DBSA.
(SEM). The coated samples after 30 days immersion in 0.1 M HCl solution were taken out from the test solutions, thoroughly washed with double distilled water and dried in warm air. The dried samples were subjected to SEM and their morphologies were compared with the corresponding samples before immersion in test solution. 3. Result and discussion 3.1. Characterization of PPy/GNS/RE3 +/DBSA, PPy/GNS/DBSA and PPy/ DBSA composites The FTIR spectra of PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/ DBSA nanocomposites are shown in Fig. 1(a–e). In the PPy/DBSA spectrum peak observed at 1453 and 1543 cm− 1 show the stretching of C\\N and C_C, whereas C\\H vibrations are represented by sharp peak at 1312 cm−1, respectively. The presence of polypyrrole ring was shown by in-plane deformation of C\\H bond and N\\H bond predicted
by the peaks at 1053 cm−1. C\\C stretching was attributed by peak at 1162 cm−1. The peak at 966 cm−1 predicts the out of phase C\\C vibration. Comparing the spectra of PPy/GNS/DBSA, PPY/GNS/La+ 3/DBSA, PPy/GNS/Nd+3/DBSA and PPy/GNS/Sm+3/DBSA with PPy/DBSA spectrum, there exist approximately no difference except minor shift in the peaks of PPy/DBSA attributed at 1162 cm−1 due to introduction of GNS [17,18]. Shift of peaks observed at 1196, 1169, 1168 and 1169 cm−1 in the GNS nanocomposites [Fig. 1 (b–e)] is due to π-π stacking between GNS and PPy backbone [19]. The presence of rare earth elements in the nanocomposites was shown by the peaks at 678 (Sm3+), 681 (Nd3+) and 676 (La3+), respectively. The XRD patterns of the synthesized composites are shown in Fig. 2 (a–e). In Fig. 2 (a) the broad peak observed at 2θ = 20°–30° implies the amorphous nature of PPy/DBSA. Generally, PPy shows peak intensity approximately at 2θ = 25° but due to the doping of DBSA in the PPy matrix peak shifts to 2θ = 23° and shows interplanar spacing [20]. In Fig. 2 (b–e) the diffraction peak at 2θ = 32° and 47° implies the presence of graphene in the nanocomposites [21]. The intensity of graphene peaks
Fig. 5. Nyquist plot of (a) uncoated steel (b) PPy/GNS/Sm3+/DBSA (c) PPy/GNS/Nd3+/DBSA (d) PPy/GNS/La3/DBSA (e) PPy/GNS/DBSA (f) PPy/DBSA and Equivalent electrical circuit(inset).
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Table 1 EIS parameters. Sample description
RS (Ω cm2)
Rct (Ω cm2)
Cdl (μF cm−2)
Protection efficiency (%ηR)
Uncoated steel PPy/DBSA PPy/GNS/DBSA PPy/GNS/La3+/DBSA PPy/GNS/Nd3+/DBSA PPy/GNS/Sm3+/DBSA
23.63 42.82 190.50 252.40 266.74 283.73
65.36 1277.20 4952.00 7360.00 8110.20 8543.30
1.00 × 10−4 3.63 × 10−5 2.87 × 10−6 1.45 × 10−6 1.10 × 10−6 0.75 × 10−6
– 94.88 98.68 99.11 99.19 99.23
is very weak due to the wrapping of spherical PPy matrix on the nanosheets. Fig. 3 shows the SEM microstructure of (a) PPy/DBSA, (b) PPY/GNS/ DBSA, (c) PPy/GNS/Sm3+/DBSA, (d) PPy/GNS/Nd3+/DBSA and (e) PPy/ GNS/La3+/DBSA, respectively. The SEM photograph of PPy/DBSA (Fig. 3a) shows the presence of spherical PPy whereas, in nanocomposites the PPy was found to be evenly distributed over the edges of GNS. The
light color spherical PPy can be easily identified on the edges of dark color GNS [22,23]. The TEM technique was also used to characterize the morphology and structure of GNS and PPy/GNS/Sm3 +/DBSA as shown in Fig. 4 (a and b). In GNS the sheet like structure is clearly visible (Fig. 4a), which shows the formation of graphene nano sheets. In Fig. 4(b) the bulk sphere of PPy has been homogeneously surrounded by GNS. The
(a)
(b) Fig. 6. (a) Bode phase and (b) Bode modulus of (a) uncoated steel (b) PPy/GNS/Sm3+/DBSA (c) PPy/GNS/Nd3+/DBSA (d) PPy/GNS/La3+/DBSA (e) PPy/GNS/DBSA and (f) PPy/DBSA.
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Fig. 7. Tafel curves of (a) uncoated steel (b) PPy/GNS/Sm3+/DBSA (c) PPy/GNS/Nd3+/ DBSA (d) PPy/GNS/La3/DBSA (e) PPy/GNS/DBSA and (f) PPy/DBSA.
size of PPy/GNS/DBSA or PPy/GNS/RE3 +/DBSA nanocomposites is smaller than the PPy/DBSA as shown in Fig. 4(a) suggesting the π-π stacking between GNS and PPy backbone [13].
3.2. Corrosion protection performance of PPy/GNS/RE3+/DBSA, PPy/GNS/ DBSA and PPy/DBSA composites 3.2.1. AC impedance and potentiodynamic polarization measurements The evaluation of corrosion protection ability of the resultant polymer coatings i.e. PPy/DBSA, PPy/GNS/DBSA, PPy/GNS/Sm3 +/DBSA, PPy/GNS/Nd3 +/DBSA and PPy/GNS/La3 +/DBSA on carbon steel was studied in 0.1 M HCl by using EIS technique. This technique allows us to understand the mode of protection of steel by the nanocomposite coatings along with the electrochemical processes taking place at the steel/coating/electrolyte junction. Fig. 5 shows the electrochemical impedance analysis in the form of Nyquist plots of uncoated and coated samples in the medium under investigation. Nyquist plots show depressed semicircle for coated and uncoated mild steel specimens, probably due to the presence of corrosion product or surface heterogeneity on mild steel substrate, and the diameter of the semicircle increased in presence of polymer coatings. The diameter of the Nyquist plots increased in the order of Uncoated b PPy/DBSA b PPy/GNS/DBSA b PPy/ GNS/La3+/DBSA b PPy/GNS/Nd3+/DBSA b PPy/GNS/Sm3+/DBSA showing better corrosion protection performance of the nanocomposite coatings containing rare earth elements. The EIS results was fitted by using the equivalent circuit shown in Fig. 5 (inset), which comprised of Rs i.e. resistance of electrolyte, Cdl representing an electric double layer capacitance and Rct, the charge transfer resistance. The obtained EIS
parameters are listed in Table 1. From the Table 1 it is apparent that the values of Rct increases, whereas the values of Cdl decreases in the presence of polymer coatings as compared to the uncoated mild steel sample. The increase in the Rct values or decrease in Cdl values in presence of the graphene nano sheets in the polymer nanocomposite coatings is attributed to the barrier effect of coatings along with the development of protective passive oxide film on mild steel substrate and the reduction in local dielectric constant and/or increase in thickness of double layer, respectively [24,25]. The increase in Rct and decrease in Cdl value is more in nanocomposite coatings containing rare earth elements as compared to others. Within the rare earth elements presence of Sm3+ offers maximum corrosion protection as compared to Nd3+ and La3+. The superior protection effect of PPy coating containing GNS or both GNS and RE elements is attributed to homogeneously dispersed graphene nano particles in the coating matrix, which helped in the formation of a uniform passive film on the carbon steel surface. Considering the results of corresponding Bode plots (Fig. 6 (a) and (b), in presence of polymer nanocomposite coatings containing rare earth the highest increase in the value of the absolute impedance at low frequencies or more negative value of phase angle at higher frequencies confirmed its superior protection behavior in comparison to PPy/DBSA or PPY/GNS/DBSA coatings. The results as obtained from EIS are consistent with the results obtained from potentiodynamic polarization measurements. The corrosion protection behavior of PPy/DBSA, PPY/GNS/DBSA, PPy/GNS/RE3 +/DBSA coated low carbon steel in 0.1 M HCl was also studied using the potentiodynamic polarization measurements. The Tafel plots of coated and uncoated steel obtained from potentiodynamic polarization technique are illustrated in Fig. 7. The corrosion kinetics parameters obtained from Tafel extrapolation, e.g., corrosion potential (Ecorr), and corrosion current density (icorr) are listed in Table 2. Polarization resistance Rp is calculated from Tafel plots by using Stern-Geary equation [26]: Rp ¼
ð4Þ
ba bc 2:303ðba þbc Þicorr
where, icorr is determined by the intersection of the linear portions of the anodic and cathodic curves, and ba and bc are the anodic and cathodic slopes, respectively. There is positive (noble) shift in Ecorr, reduction of icorr and increase in Rp values of nanocomposite coatings as compared to the bare steel sample, which is suggestive of better protection performance of nanocomposite coatings. The noble shift of Ecorr is more in case of rare earth containing nanocomposites as compared to PPy/DBSA and PPy/ GNS/DBSA indicating better protection performance of rare earth nanocomposites. The decrease in icorr and increase in Rp values of coatings is due to the presence of dense coating on the steel surface. Graphene and rare earth elements present in the coating matrix increases the density of coating material, therefore retarding the seepage of electrolyte. The addition of rare earth elements to nanocomposite causes a displacement of the cathodic branch towards the negative values and a reduction in the corrosion current density by blocking of cathodic sites. The
Table 2 Potentiodynamic polarization measurements. Sample description Uncoated steel PPy/DBSA PPy/GNS/DBSA PPy/GNS/La3+/DBSA PPy/GNS/Nd3+/DBSA PPy/GNS/Sm3+/DBSA
Ecorr (mv) −483.650 −442.280 −374.870 −229.840 −0.196 −0.162
icorr (μA/cm2) 149.030 3.260 0.328 0.052 0.028 0.013
RP (Ω/cm2) 2
1.28 × 10 2.19 × 104 3.70 × 105 3.14 × 106 3.41 × 106 4.77 × 106
CR (mpy)
Porosity (P)
Protection efficiency (%ηi)
68.414 1.450 0.149 0.024 0.0120 0.006
− 3.01 × 10−3 6.00 × 10−5 6.91 × 10−7 1.67 × 10−8 1.19 × 10−8
− 97.88 99.77 99.96 99.98 99.99
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Potential (mV vs SCE)
0
Uncoated
PPy/DBSA
PPy/GNS/DBSA
PPy/GNS/La3+/DBSA
PPy/GNS/Nd3+/DBSA
PPy/GNS/Sm3+/DBSA
389
-100 -200 -300 -400 -500 -600 0
50
100
150
200
Fig. 8. Open circuit potential (OCP) vs. time plots.
Table 3 Immersion test results. Sample description
Immersion period (days)
Corrosion rate (mpy)
Protection efficiency (%η)
Uncoated steel PPy/DBSA PPy/GNS/DBSA PPy/GNS/La3+/DBSA PPy/GNS/Nd3+/DBSA PPy/GNS/Sm3+/DBSA
30 30 30 30 30 30
23.67 1.78 0.99 0.62 0.48 0.43
− 92.47 95.78 97.37 97.93 98.15
porosity of the nanocomposite coatings on steel was determined from potentiodynamic polarization measurements by the following relationship [27]: P ¼
Rp ðuncoatedÞ ðΔEcorr Þ 10 ba Rp ðcoatedÞ
ð5Þ
where, P is the total porosity, Rp (uncoated) and Rp (coated) is the polarization resistance of uncoated and coated carbon steel, respectively, ΔEcorr is the difference between the corrosion potential and ba is the anodic Tafel slope for uncoated low carbon steel. The porosity values are listed in Table 2. The porosity of rare earth nanocomposites is much lower as compared to the other coatings, this is suggestive of their better corrosion protection ability as they did not allow the electrolyte to diffuse to the steel surface. The corrosion rate listed in Table 2 are in the order of uncoated N PPy/DBSA N PPY/GNS/DBSA N PPy/GNS/La3 +/ DBSA N PPy/GNS/Nd3+/DBSA N PPy/GNS/Sm3+/DBSA. 3.2.2. Open circuit potential measurements Fig. 8 shows the OCP vs time plots of uncoated and polymer coated low carbon steel samples in 0.1 M HCl. It is evident from the OCP plots
that the potential of polymer coated steel shifts to nobler values as compared to the uncoated steel sample under the same condition and remained nobler (positive) till the end of the immersion. The noble shift in potential for rare earth containing nanocomposite coatings is more noticeable than PPy/DBSA or PPY/GNS/DBSA nanocomposite coatings. The noble shift in OCP is indicative of redox reaction induced passivation of steel surface. As the immersion is continued there is a little reduction in the noble OCP values due to the initiation of corrosion at the steel surface as a result of the seepage of electrolytic solution through the pore in the coating but the final OCP is still quite nobler than the OCP of bare steel under the same condition. However, the reduction in OCP values for nanocomposites containing rare earth elements is very small compared to PPy/ DBSA and PPy/GNS/DBSA coatings. The graphene present in the nanocomposites provides barrier to the electrolyte and generate passivation to the underlying steel surface. The π-π stacking of graphene and PPy backbone made the coating denser to electrolyte to penetrate. In case of rare earth containing nanocomposites the change in potential of Sm3+ containing nanocomposite is found to be very minimal as compared to that of nanocomposites containing Nd3+ and La3+. The OCP of Sm3+ containing nanocomposite is almost constant throughout the immersion period. The addition of rare earth elements might be causing formation of rare earth hydroxides, which hinders both anodic and cathodic processes by forming passive layer on the steel surface. Initially, after immersion the corrosion process generates hydroxyl ions at cathodic site and the rare earth elements reacts with these hydroxyl ions and forms partially insoluble rare earth hydroxides. More the generation of hydroxyl ions more the formation of rare earth hydroxides. RE3þ þ 3OH− →REðOHÞ3
ð6Þ
Theses partially insoluble rare earth hydroxides deposited on the steel surface and form passive layer, which protect the underlying
Fig. 9. Coating containing DBSA showing bio mimic effect.
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 10. SEM images of (a) and (b) PPy/DBSA coated before and after 30 days immersion; (c) and (d) PPy/GNS/DBSA coated before and after 30 days immersion; (e) and (f) PPy/GNS/Sm3+/ DBSA coated before and after 30 days immersion.
steel from further corrosion. Some of the rare earth hydroxides undergo dehydration and result in the formation of rare earth oxides, which also forms passive layer and protect the underlying metal. 2REðOHÞ3 →RE2 O3 þ 3H2 O
ð7Þ
3.2.3. Immersion test Table 3 shows the immersion test results of the nanocomposite coated steel samples as well as uncoated samples in 0.1 M HCl solution. The immersion tests were performed under unstirred condition for 30 days
at room temperature. Considering the protection efficiency of the various polymer coatings as obtained by immersion tests, the rare earth containing nanocomposite coatings showed the highest protection and followed the order: PPy/GNS/Sm3 +/DBSA N PPy/GNS/Nd3 +/ DBSA N PPy/GNS/La3+/DBSA N PPY/GNS/DBSA N PPy/DBSA N uncoated steel. The presence of graphene in the polymer matrix results in the π-π stacking between the graphene and PPy backbone, therefore providing the barrier effect to the electrolyte to reach to the steel surface. Thus graphene present in the nanocomposite increases tortuosity of the diffusion pathway of the electrolyte resulting in extended corrosion
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protection by the nanocomposites. The rare earth elements present in the coating matrix reacts with hydroxyl ion to form rare earth hydroxides, which covers the underlying metal surface and forms a passive layer to reduce the corrosion rate. The hydroxides of rare earth elements may react with the test solution and decrease the concentration of Cl− ions in the solution by forming chloride salts but the rate of backward reaction is more as compared to forward reaction implying the stability of hydroxides in acidic condition. REðOHÞ3 þ 3HCl↔2RECl3 þ 3H2 O
ð8Þ
The stability of hydroxides of rare earth elements are in the order of Sm(OH)3 N Nd(OH)3 N La(OH)3. More the stability of rare earth oxide greater is the passivation of steel surface. PPy/GNS/Sm3+/DBSA shows minimum corrosion as compared to others due to the presence of graphene and more stability of its hydroxide. The DBSA present in the nanocomposites, apart from making the nanocomposites soluble in organic solvents, also reduces the corrosion rate by acting as an inhibitor as shown in Fig. 9. If somehow the electrolyte seeps through the coatings, the DBSA present in the coatings releases DBSA− anions, which form a passive layer by reacting with the cation of iron (Fe+2) and reduces the corrosion rate to a greater extent [28]. The effect of varying concentration of rare earth elements and GNS in the nanocomposite needs further study. 3.3. Surface morphological studies The surface morphology of the coated samples prior to and after 1 month immersion in test solution (0.1 M HCl) is shown in Fig. 10 (a–f). The sample coated with PPy/DBSA in Fig. 10 (b) show fine cracks in the coating after immersion resulting in the degradation of metal. In Fig. 10 (d) presence of GNS in the coating system does not allow formation of cracks by maintaining the integrity of coating on the metal surface. Although some corrosion products are visible in Fig. 10 (d), which was completely diminished by the addition of rare earth elements as represented in Fig. 10 (f) showing no cracks as well as corrosion product. 4. Conclusion The soluble organic-inorganic nanocomposites of pyrrole containing rare earth ions and dodecyl benzene sulfonic acid embedded on the surface of graphene nano sheets were successfully synthesized. The formation of PPy/GNS/DBSA and PPy/GNS/RE3+/DBSA nanocomposites were confirmed using FTIR, XRD, SEM and TEM. The EIS measurements exhibited highest charge transfer resistance (Rct) and lowest double layer capacitance (Cdl) for PPy/GNS/RE3 +/DBSA compared to PPy/DBSA and PPy/GNS/DBSA coated steel. Potentiodynamic polarization studies showed substantial reduction in icorr, noble shift (positive shift) in Ecorr, decrease in porosity (P) and increase of Rp values of the low carbon steel in presence of nanocomposite coatings. The results of OCP measurements show much nobler and stable potential for PPy/GNS/RE3+/ DBSA than PPy/DBSA and PPy/GNS/DBSA coated steel. The results of electrochemical tests find adequate support from immersion tests. The morphology of the nanocomposite films deposited on steel specimen was examined by SEM to support the results obtained from electrochemical studies. The nanocomposite coatings were observed to exhibit both barrier and passivation behavior. Considering the excellent corrosion protection of nanocomposite coatings the studied compounds can be used for future industrial assessments. Acknowledgements The financial support from CSIR, New Delhi, India, through the major research Project No. 01/(2746)/13/EMR-II is gratefully acknowledged.
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We also would like to acknowledge USIF and Department of Applied Physics, Aligarh Muslim University, Aligarh for SEM and XRD. References [1] G. Ruhi, O.P. Modi, S.K. Dhawan, Chitosan-polypyrrole-SiO2 composite coatings with advanced anticorrosive properties, Synth. Met. 200 (2015) 24–39. [2] L.H.E. Santos, J.S.C. Branco, I.S. Guimaraes, A.J. Motheo, Synthesis in phytic acid medium and application as anticorrosive coatings of polyaniline-based materials, Surf. Coat. Technol. 275 (2015) 26–31. [3] B.N. Grgura, A.R. Elkais, M.M. Gvozdenovic, S.Z. Drmanic, T.L. Trisovic, B.Z. Jugovic, Corrosion of mild steel with composite polyaniline coatings using different formulations, Prog. Org. Coat. 79 (2015) 17–24. [4] M.B. Gonzalez, S.B. Saidman, Electrodeposition of bilayered polypyrrole on 316 L stainless steel for corrosion prevention, Prog. Org. Coat. 78 (2015) 21–27. [5] A. Olad, R. Nosrati, Preparation and corrosion resistance of nanostructured PVC/ ZnO–polyaniline hybrid coating, Prog. 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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Polypyrrole/graphene nanosheets/rare earth ions/dodecyl benzene sulfonic acid nanocomposite as a highly effective anticorrosive coating R. Alam, M. Mobin ⁎, J. Aslam Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India
a r t i c l e
i n f o
Article history: Received 21 June 2016 Revised 3 September 2016 Accepted in revised form 6 September 2016 Available online 13 September 2016 Keywords: Rare earth ions Tem Graphene Nyquist
a b s t r a c t Organic-inorganic nanocomposites (PPy/GNS/RE3+/DBSA) involving pyrrole (Py), graphene nano sheets (GNS), rare earth elements (RE3+ = La3+, Sm3+, Nd3+) and dodecyl benzene sulfonic acid (DBSA) were synthesized via in situ chemical oxidative polymerization using FeCl3 as an oxidant. PPy/GNS/DBSA and PPy/DBSA was also synthesized by following the identical synthesis route. The resultant nanocomposites were characterized by FTIR, XRD, SEM and TEM. The synthesized nanocomposites were chemically deposited on low carbon steel specimens by solvent evaporation method using N-Methyl-2-pyrrolidone (NMP) as solvent and 10% epoxy resin (by weight) as binder. The anticorrosive nature of polymer coatings were studied in 0.1 M HCl solution by subjecting them to various corrosion tests, which includes: EIS, potentiodynamic polarization, free corrosion potential (OCP) measurement and immersion test. The surface morphology of coated samples before and after immersion was evaluated using SEM. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The discovery of conducting polymers such as polyaniline, polypyrrole, polythiophene has shown the way to develop protective coatings, which are non-toxic, eco-friendly, environmentally stable and binds strongly on the metal surface. Among the various conducting polymers polyaniline and polypyrrole based coatings are quite attractive to the researchers due to their good physio-chemical properties [1–4]. The protection ability of these polymer coatings also depend upon the metal substrate on which they are applied. The addition of various organic or inorganic additives or combination of both to the conducting polymers has led to the formation of composites, which have shown superior properties as compared to the polymers itself. It increases the solubility, stability, and adhesion of the polymers to the substrate, and also enhances their anticorrosive nature to a greater extent. The development of polymer composites coating on metal substrate either chemically or electrochemically increase the passivation property of the metal substrate by shifting the corrosion potential towards the direction of more noble metal [5–7]. The composites of graphene with polyaniline and polypyrrole have been used widely for various purposes including protection of metal substrate from degradation [8,9]. Graphene, a two-
dimensional monolayer of sp2 bonded carbon has outstanding mechanical, optical, thermal and electrical properties. It is considered to be effective anticorrosive component in coating system due to its excellent chemical inertness, thermal stability and impermeability. Lanthanides are considered to have low toxicity and their ingestion is not harmful for health. The lanthanide ions form insoluble hydroxides, which suppress the cathodic corrosion reaction and protect the metal from degradation. The surface films generated on the rare earth doped alloys have improved behavior against both high temperature oxidation and aqueous corrosion [10]. In the current work, organic–inorganic nanocomposites of pyrrole, graphene nano sheets and the rare earth elements (La3 +, Sm3 +, Nd3+) along with dodecyl benzene sulfonic acid were synthesized by oxidative polymerization. The synthesized nanocomposites were characterized by FTIR, XRD, SEM and TEM. The composites were applied on carbon steel specimens and their anticorrosive properties were evaluated in 0.1 M HCl using EIS, Potentiodynamic polarization, OCP, immersion test and SEM. 2. Experimental procedure 2.1. Chemicals
⁎ Corresponding author. E-mail address: [email protected] (M. Mobin).
http://dx.doi.org/10.1016/j.surfcoat.2016.09.010 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Pyrrole (Py) and graphite powder were purchased from Sigma Aldrich. Ferric chloride, polyethylene glycol 600, hydrogen peroxide, NMethyl-2-pyrrolidone, ethanol, ortho-dichlorobenzene, p-toluene
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sulfonic acid, sulfuric acid, and potassium permanganate were of AR grade and supplied by Merck. Nitrate salts of rare earth elements were procured from Alfa Aesar.
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washed with double distilled water and ethanol to remove residual pyrrole, DBSA and oxidant. The filtrate nanocomposite was dried in vacuum for 12 h. PPy/GNS/DBSA and PPy/DBSA was also synthesized following the identical synthesis route.
2.2. Preparation of metal specimen for application of coatings 2.4. Characterization The low carbon steel coupons of dimension 40.0 × 15.0 × 1.3 mm having chemical composition 0.028% P,0.033% V, 0.081% Mo, 0.049% C, 0.723% Mn,0.01% Al, 0.051% Cr and balance Fe were used as substrate for polymer coatings. Prior to application of coatings, the coupons were polished mechanically by series of emery papers, this was followed by rinsing with double distilled water and ultra-sonication with acetone for 5 min. 2.3. Synthesis of GNS, PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/DBSA composites Graphene oxide was synthesized by modified hummer's method as reported earlier [11]. The obtained graphene oxide was transformed into graphene (reduced graphene oxide) by treating with hydrazine, indicated by color change from brown to black, which was subsequently ultra-sonicated for 4 h in ortho-dichlorobenzene to attain graphene nano sheets by liquid-phase exfoliation method [12]. The resulting product was washed with water and ethanol followed by vacuum drying at 50 °C for 24 h. PPy/GNS/RE3+/DBSA was synthesized by chemical oxidative polymerization using FeCl3 as an oxidant and p-toluene sulfonic acid as dopant, illustrated in Scheme 1 [13]. 2 mL pyrrole, the rare earth ions and GNS (both 5% by weight of the amount of pyrrole) along with 0.5 mL of PEG-600 were taken in round bottom flask. PEG600 was used as an anticoagulant agent, which prevents the agglomeration of GNS. Besides, it also acts as a plasticizer to provide flexibility and reduce brittleness of the nanocomposite coatings [14]. 15 mL of ethanol and 1 g of DBSA was added to it and the resultant mixture was ultra-sonicated for 3 h. After sonication the mixture was allowed to cool to 0–5 °C under constant stirring. After 20 min, 0.5 g of p-toluene sulfonic acid was added to the mixture and left for stirring for 30 min. Finally, 4 g of FeCl3 was dissolved in 50 mL of water and added drop wise to the above mixture under constant stirring. After 2 h of stirring the resultant mixture was left at room temperature for 48 h for further polymerization. The obtained mixture was filtered and
The synthesized PPy/GNS/RE3 +/DBSA, PPy/GNS/DBSA and PPy/ DBSA were characterized by FTIR, XRD, SEM and TEM techniques. Xray diffraction (XRD) studies were carried out in the 2θ range of 20°– 80° using Shimadzu 6100X X-ray diffractometer. Fourier Transform Infra-Red (FTIR) spectroscopy (Model: Perkin Elmer) was studied in the frequency range of 500–4000 cm− 1. SEM (Model: JEOL JSM6510LV) and TEM (Model: JEOL JEM-2100) was used to study the morphology of the synthesized polymers. 2.5. Preparation of coating of PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/ DBSA composites on low carbon steel The coatings of PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/DBSA were carried out on low carbon steel by solvent evaporation method using NMP as solvent. Epoxy resin (10% by weight), synthesized by an earlier described method [15], and was used to enhance the binding of nanocomposites to the steel surface. 0.3 g of synthesized nanocomposite was dissolved in 10 mL of NMP and solution was stirred continuously for 24 h. This was followed by the addition of epoxy resin to the above mixture and the content was again stirred for 1 h. The resultant solution was spread on the steel surface with the help of dropper and the solvent was allowed to evaporate at 85–90 °C. After 3 to 4 cycles a strongly adherent coating was obtained. The thickness of coatings was measured using Elcometer (Model: 456) and found to be in the range of 15.72– 17.37 μm. More coated samples were obtained using the identical procedure. 2.6. Assessment of corrosion protection performance of PPy/GNS/RE3 +/ DBSA, PPy/GNS/DBSA and PPy/DBSA composites The anticorrosive properties of polymer composites coated low carbon steel samples in 0.1 M HCl as a corrosive medium at room temperature (30 °C) were evaluated by subjecting them to various corrosion tests, which include: free corrosion potential (OCP) measurements, ac
Scheme 1. Synthesis of PPy/GNS/RE3+/DBSA.
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Fig. 1. FTIR spectra of (a) PPy/DBSA (b) PPy/GNS/DBSA (c) PPy/GNS/La3+ (d) PPy/GNS/Nd3+/DBSA and (e) PPy/GNS/Sm3+/DBSA.
impedance, potentiodynamic polarization measurements, and gravimetric analysis. SEM images of coated samples were recorded before and after 30 days immersion in 0.1 M HCl. 2.6.1. Free corrosion potential measurements The OCP measurements on coated and uncoated steel samples in 0.1 M HCl solution at room temperature (30 °C) were carried out using saturated calomel electrode (SCE) as a reference electrode. The steel specimens immersed into the test solution were connected to a wire having alligator clip on both the ends. One end of alligator clip was connected to the carbon steel specimen whereas the other end was connected to a multimeter. The change in the voltage against SCE was plotted against time. 2.6.2. AC impedance and potentiodynamic polarization measurements AC impedance and potentiodynamic polarization measurements were carried out using glass cell with steel samples having exposed area of 1 cm2 as working electrode, Pt wire as counter electrode, Ag/
AgCl as reference electrode; Luggin-Haber capillary was used to minimize the IR drop. Prior to each experiment the steel samples were left out in the corrosive solution for 1 h to attain the steady state potential. During the electrochemical analysis the temperature was maintained at 30 ± 2 °C. The EIS measurements were carried out at OCP within the frequency range of 10−2 to 105 Hz with 10 mV perturbation. The values of charge transfer resistance were used to calculate the protection efficiency (%ηR). ηR ð%Þ ¼
Rct ðiÞ −Rct Rct ðiÞ
100
ð1Þ
where, Rct and R(i) ct is the charge transfer resistance in absence and presence of coatings. The polarization studies were carried out by sweeping the potential between −250 to 250 mV with respect to the steady-state potential at a scan rate of 0.001 V/s. The anodic and cathodic curves were extrapolated to obtain the corrosion current densities
Fig. 2. XRD pattern of (a) PPy/DBSA (b) PPy/GNS/DBSA (c) PPy/GNS/La3+ (d) PPy/GNS/Nd3+/DBSA and (e) PPy/GNS/Sm3+/DBSA.
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385
(a)
(b)
(c)
(d)
(e) Fig. 3. SEM micrographs of (a) PPy/DBSA (b) PPy/GNS/DBSA (c) PPy/GNS/La3/DBSA (d) PPy/GNS/Nd3+/DBSA and (e) PPy/GNS/Sm3+/DBSA.
(icorr) and corrosion potential (Ecorr). The protection efficiency (%ηi) was calculated from the measured icorr values using the following equation: ηi ð%Þ ¼
icorr −iðiÞ corr 100 icorr
ð2Þ
where, icorr and i(i)corr is corrosion current density without and with the coatings, respectively. 2.6.3. Immersion test The coated and uncoated samples were subjected to immersion test in 0.1 M HCl solution for the period of 30 days under unstirred condition at room temperature. The detailed procedure for the immersion test
and calculation of corrosion rates have been reported in our earlier publication [16]. The protection efficiency (%η) was calculated using the following equation: ηð%Þ ¼
νo −ν i 100 νo
ð3Þ
where, νo and νi are the corrosion rates of uncoated and coated sample. 2.7. Surface morphological studies The surface morphology of polymer coatings on low carbon steel specimens was evaluated using scanning electron microscopy
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(a)
(b) Fig. 4. TEM images of (a) GNS and (b) PPy/GNS/Sm3+/DBSA.
(SEM). The coated samples after 30 days immersion in 0.1 M HCl solution were taken out from the test solutions, thoroughly washed with double distilled water and dried in warm air. The dried samples were subjected to SEM and their morphologies were compared with the corresponding samples before immersion in test solution. 3. Result and discussion 3.1. Characterization of PPy/GNS/RE3 +/DBSA, PPy/GNS/DBSA and PPy/ DBSA composites The FTIR spectra of PPy/GNS/RE3+/DBSA, PPy/GNS/DBSA and PPy/ DBSA nanocomposites are shown in Fig. 1(a–e). In the PPy/DBSA spectrum peak observed at 1453 and 1543 cm− 1 show the stretching of C\\N and C_C, whereas C\\H vibrations are represented by sharp peak at 1312 cm−1, respectively. The presence of polypyrrole ring was shown by in-plane deformation of C\\H bond and N\\H bond predicted
by the peaks at 1053 cm−1. C\\C stretching was attributed by peak at 1162 cm−1. The peak at 966 cm−1 predicts the out of phase C\\C vibration. Comparing the spectra of PPy/GNS/DBSA, PPY/GNS/La+ 3/DBSA, PPy/GNS/Nd+3/DBSA and PPy/GNS/Sm+3/DBSA with PPy/DBSA spectrum, there exist approximately no difference except minor shift in the peaks of PPy/DBSA attributed at 1162 cm−1 due to introduction of GNS [17,18]. Shift of peaks observed at 1196, 1169, 1168 and 1169 cm−1 in the GNS nanocomposites [Fig. 1 (b–e)] is due to π-π stacking between GNS and PPy backbone [19]. The presence of rare earth elements in the nanocomposites was shown by the peaks at 678 (Sm3+), 681 (Nd3+) and 676 (La3+), respectively. The XRD patterns of the synthesized composites are shown in Fig. 2 (a–e). In Fig. 2 (a) the broad peak observed at 2θ = 20°–30° implies the amorphous nature of PPy/DBSA. Generally, PPy shows peak intensity approximately at 2θ = 25° but due to the doping of DBSA in the PPy matrix peak shifts to 2θ = 23° and shows interplanar spacing [20]. In Fig. 2 (b–e) the diffraction peak at 2θ = 32° and 47° implies the presence of graphene in the nanocomposites [21]. The intensity of graphene peaks
Fig. 5. Nyquist plot of (a) uncoated steel (b) PPy/GNS/Sm3+/DBSA (c) PPy/GNS/Nd3+/DBSA (d) PPy/GNS/La3/DBSA (e) PPy/GNS/DBSA (f) PPy/DBSA and Equivalent electrical circuit(inset).
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Table 1 EIS parameters. Sample description
RS (Ω cm2)
Rct (Ω cm2)
Cdl (μF cm−2)
Protection efficiency (%ηR)
Uncoated steel PPy/DBSA PPy/GNS/DBSA PPy/GNS/La3+/DBSA PPy/GNS/Nd3+/DBSA PPy/GNS/Sm3+/DBSA
23.63 42.82 190.50 252.40 266.74 283.73
65.36 1277.20 4952.00 7360.00 8110.20 8543.30
1.00 × 10−4 3.63 × 10−5 2.87 × 10−6 1.45 × 10−6 1.10 × 10−6 0.75 × 10−6
– 94.88 98.68 99.11 99.19 99.23
is very weak due to the wrapping of spherical PPy matrix on the nanosheets. Fig. 3 shows the SEM microstructure of (a) PPy/DBSA, (b) PPY/GNS/ DBSA, (c) PPy/GNS/Sm3+/DBSA, (d) PPy/GNS/Nd3+/DBSA and (e) PPy/ GNS/La3+/DBSA, respectively. The SEM photograph of PPy/DBSA (Fig. 3a) shows the presence of spherical PPy whereas, in nanocomposites the PPy was found to be evenly distributed over the edges of GNS. The
light color spherical PPy can be easily identified on the edges of dark color GNS [22,23]. The TEM technique was also used to characterize the morphology and structure of GNS and PPy/GNS/Sm3 +/DBSA as shown in Fig. 4 (a and b). In GNS the sheet like structure is clearly visible (Fig. 4a), which shows the formation of graphene nano sheets. In Fig. 4(b) the bulk sphere of PPy has been homogeneously surrounded by GNS. The
(a)
(b) Fig. 6. (a) Bode phase and (b) Bode modulus of (a) uncoated steel (b) PPy/GNS/Sm3+/DBSA (c) PPy/GNS/Nd3+/DBSA (d) PPy/GNS/La3+/DBSA (e) PPy/GNS/DBSA and (f) PPy/DBSA.
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Fig. 7. Tafel curves of (a) uncoated steel (b) PPy/GNS/Sm3+/DBSA (c) PPy/GNS/Nd3+/ DBSA (d) PPy/GNS/La3/DBSA (e) PPy/GNS/DBSA and (f) PPy/DBSA.
size of PPy/GNS/DBSA or PPy/GNS/RE3 +/DBSA nanocomposites is smaller than the PPy/DBSA as shown in Fig. 4(a) suggesting the π-π stacking between GNS and PPy backbone [13].
3.2. Corrosion protection performance of PPy/GNS/RE3+/DBSA, PPy/GNS/ DBSA and PPy/DBSA composites 3.2.1. AC impedance and potentiodynamic polarization measurements The evaluation of corrosion protection ability of the resultant polymer coatings i.e. PPy/DBSA, PPy/GNS/DBSA, PPy/GNS/Sm3 +/DBSA, PPy/GNS/Nd3 +/DBSA and PPy/GNS/La3 +/DBSA on carbon steel was studied in 0.1 M HCl by using EIS technique. This technique allows us to understand the mode of protection of steel by the nanocomposite coatings along with the electrochemical processes taking place at the steel/coating/electrolyte junction. Fig. 5 shows the electrochemical impedance analysis in the form of Nyquist plots of uncoated and coated samples in the medium under investigation. Nyquist plots show depressed semicircle for coated and uncoated mild steel specimens, probably due to the presence of corrosion product or surface heterogeneity on mild steel substrate, and the diameter of the semicircle increased in presence of polymer coatings. The diameter of the Nyquist plots increased in the order of Uncoated b PPy/DBSA b PPy/GNS/DBSA b PPy/ GNS/La3+/DBSA b PPy/GNS/Nd3+/DBSA b PPy/GNS/Sm3+/DBSA showing better corrosion protection performance of the nanocomposite coatings containing rare earth elements. The EIS results was fitted by using the equivalent circuit shown in Fig. 5 (inset), which comprised of Rs i.e. resistance of electrolyte, Cdl representing an electric double layer capacitance and Rct, the charge transfer resistance. The obtained EIS
parameters are listed in Table 1. From the Table 1 it is apparent that the values of Rct increases, whereas the values of Cdl decreases in the presence of polymer coatings as compared to the uncoated mild steel sample. The increase in the Rct values or decrease in Cdl values in presence of the graphene nano sheets in the polymer nanocomposite coatings is attributed to the barrier effect of coatings along with the development of protective passive oxide film on mild steel substrate and the reduction in local dielectric constant and/or increase in thickness of double layer, respectively [24,25]. The increase in Rct and decrease in Cdl value is more in nanocomposite coatings containing rare earth elements as compared to others. Within the rare earth elements presence of Sm3+ offers maximum corrosion protection as compared to Nd3+ and La3+. The superior protection effect of PPy coating containing GNS or both GNS and RE elements is attributed to homogeneously dispersed graphene nano particles in the coating matrix, which helped in the formation of a uniform passive film on the carbon steel surface. Considering the results of corresponding Bode plots (Fig. 6 (a) and (b), in presence of polymer nanocomposite coatings containing rare earth the highest increase in the value of the absolute impedance at low frequencies or more negative value of phase angle at higher frequencies confirmed its superior protection behavior in comparison to PPy/DBSA or PPY/GNS/DBSA coatings. The results as obtained from EIS are consistent with the results obtained from potentiodynamic polarization measurements. The corrosion protection behavior of PPy/DBSA, PPY/GNS/DBSA, PPy/GNS/RE3 +/DBSA coated low carbon steel in 0.1 M HCl was also studied using the potentiodynamic polarization measurements. The Tafel plots of coated and uncoated steel obtained from potentiodynamic polarization technique are illustrated in Fig. 7. The corrosion kinetics parameters obtained from Tafel extrapolation, e.g., corrosion potential (Ecorr), and corrosion current density (icorr) are listed in Table 2. Polarization resistance Rp is calculated from Tafel plots by using Stern-Geary equation [26]: Rp ¼
ð4Þ
ba bc 2:303ðba þbc Þicorr
where, icorr is determined by the intersection of the linear portions of the anodic and cathodic curves, and ba and bc are the anodic and cathodic slopes, respectively. There is positive (noble) shift in Ecorr, reduction of icorr and increase in Rp values of nanocomposite coatings as compared to the bare steel sample, which is suggestive of better protection performance of nanocomposite coatings. The noble shift of Ecorr is more in case of rare earth containing nanocomposites as compared to PPy/DBSA and PPy/ GNS/DBSA indicating better protection performance of rare earth nanocomposites. The decrease in icorr and increase in Rp values of coatings is due to the presence of dense coating on the steel surface. Graphene and rare earth elements present in the coating matrix increases the density of coating material, therefore retarding the seepage of electrolyte. The addition of rare earth elements to nanocomposite causes a displacement of the cathodic branch towards the negative values and a reduction in the corrosion current density by blocking of cathodic sites. The
Table 2 Potentiodynamic polarization measurements. Sample description Uncoated steel PPy/DBSA PPy/GNS/DBSA PPy/GNS/La3+/DBSA PPy/GNS/Nd3+/DBSA PPy/GNS/Sm3+/DBSA
Ecorr (mv) −483.650 −442.280 −374.870 −229.840 −0.196 −0.162
icorr (μA/cm2) 149.030 3.260 0.328 0.052 0.028 0.013
RP (Ω/cm2) 2
1.28 × 10 2.19 × 104 3.70 × 105 3.14 × 106 3.41 × 106 4.77 × 106
CR (mpy)
Porosity (P)
Protection efficiency (%ηi)
68.414 1.450 0.149 0.024 0.0120 0.006
− 3.01 × 10−3 6.00 × 10−5 6.91 × 10−7 1.67 × 10−8 1.19 × 10−8
− 97.88 99.77 99.96 99.98 99.99
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Potential (mV vs SCE)
0
Uncoated
PPy/DBSA
PPy/GNS/DBSA
PPy/GNS/La3+/DBSA
PPy/GNS/Nd3+/DBSA
PPy/GNS/Sm3+/DBSA
389
-100 -200 -300 -400 -500 -600 0
50
100
150
200
Fig. 8. Open circuit potential (OCP) vs. time plots.
Table 3 Immersion test results. Sample description
Immersion period (days)
Corrosion rate (mpy)
Protection efficiency (%η)
Uncoated steel PPy/DBSA PPy/GNS/DBSA PPy/GNS/La3+/DBSA PPy/GNS/Nd3+/DBSA PPy/GNS/Sm3+/DBSA
30 30 30 30 30 30
23.67 1.78 0.99 0.62 0.48 0.43
− 92.47 95.78 97.37 97.93 98.15
porosity of the nanocomposite coatings on steel was determined from potentiodynamic polarization measurements by the following relationship [27]: P ¼
Rp ðuncoatedÞ ðΔEcorr Þ 10 ba Rp ðcoatedÞ
ð5Þ
where, P is the total porosity, Rp (uncoated) and Rp (coated) is the polarization resistance of uncoated and coated carbon steel, respectively, ΔEcorr is the difference between the corrosion potential and ba is the anodic Tafel slope for uncoated low carbon steel. The porosity values are listed in Table 2. The porosity of rare earth nanocomposites is much lower as compared to the other coatings, this is suggestive of their better corrosion protection ability as they did not allow the electrolyte to diffuse to the steel surface. The corrosion rate listed in Table 2 are in the order of uncoated N PPy/DBSA N PPY/GNS/DBSA N PPy/GNS/La3 +/ DBSA N PPy/GNS/Nd3+/DBSA N PPy/GNS/Sm3+/DBSA. 3.2.2. Open circuit potential measurements Fig. 8 shows the OCP vs time plots of uncoated and polymer coated low carbon steel samples in 0.1 M HCl. It is evident from the OCP plots
that the potential of polymer coated steel shifts to nobler values as compared to the uncoated steel sample under the same condition and remained nobler (positive) till the end of the immersion. The noble shift in potential for rare earth containing nanocomposite coatings is more noticeable than PPy/DBSA or PPY/GNS/DBSA nanocomposite coatings. The noble shift in OCP is indicative of redox reaction induced passivation of steel surface. As the immersion is continued there is a little reduction in the noble OCP values due to the initiation of corrosion at the steel surface as a result of the seepage of electrolytic solution through the pore in the coating but the final OCP is still quite nobler than the OCP of bare steel under the same condition. However, the reduction in OCP values for nanocomposites containing rare earth elements is very small compared to PPy/ DBSA and PPy/GNS/DBSA coatings. The graphene present in the nanocomposites provides barrier to the electrolyte and generate passivation to the underlying steel surface. The π-π stacking of graphene and PPy backbone made the coating denser to electrolyte to penetrate. In case of rare earth containing nanocomposites the change in potential of Sm3+ containing nanocomposite is found to be very minimal as compared to that of nanocomposites containing Nd3+ and La3+. The OCP of Sm3+ containing nanocomposite is almost constant throughout the immersion period. The addition of rare earth elements might be causing formation of rare earth hydroxides, which hinders both anodic and cathodic processes by forming passive layer on the steel surface. Initially, after immersion the corrosion process generates hydroxyl ions at cathodic site and the rare earth elements reacts with these hydroxyl ions and forms partially insoluble rare earth hydroxides. More the generation of hydroxyl ions more the formation of rare earth hydroxides. RE3þ þ 3OH− →REðOHÞ3
ð6Þ
Theses partially insoluble rare earth hydroxides deposited on the steel surface and form passive layer, which protect the underlying
Fig. 9. Coating containing DBSA showing bio mimic effect.
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 10. SEM images of (a) and (b) PPy/DBSA coated before and after 30 days immersion; (c) and (d) PPy/GNS/DBSA coated before and after 30 days immersion; (e) and (f) PPy/GNS/Sm3+/ DBSA coated before and after 30 days immersion.
steel from further corrosion. Some of the rare earth hydroxides undergo dehydration and result in the formation of rare earth oxides, which also forms passive layer and protect the underlying metal. 2REðOHÞ3 →RE2 O3 þ 3H2 O
ð7Þ
3.2.3. Immersion test Table 3 shows the immersion test results of the nanocomposite coated steel samples as well as uncoated samples in 0.1 M HCl solution. The immersion tests were performed under unstirred condition for 30 days
at room temperature. Considering the protection efficiency of the various polymer coatings as obtained by immersion tests, the rare earth containing nanocomposite coatings showed the highest protection and followed the order: PPy/GNS/Sm3 +/DBSA N PPy/GNS/Nd3 +/ DBSA N PPy/GNS/La3+/DBSA N PPY/GNS/DBSA N PPy/DBSA N uncoated steel. The presence of graphene in the polymer matrix results in the π-π stacking between the graphene and PPy backbone, therefore providing the barrier effect to the electrolyte to reach to the steel surface. Thus graphene present in the nanocomposite increases tortuosity of the diffusion pathway of the electrolyte resulting in extended corrosion
R. Alam et al. / Surface & Coatings Technology 307 (2016) 382–391
protection by the nanocomposites. The rare earth elements present in the coating matrix reacts with hydroxyl ion to form rare earth hydroxides, which covers the underlying metal surface and forms a passive layer to reduce the corrosion rate. The hydroxides of rare earth elements may react with the test solution and decrease the concentration of Cl− ions in the solution by forming chloride salts but the rate of backward reaction is more as compared to forward reaction implying the stability of hydroxides in acidic condition. REðOHÞ3 þ 3HCl↔2RECl3 þ 3H2 O
ð8Þ
The stability of hydroxides of rare earth elements are in the order of Sm(OH)3 N Nd(OH)3 N La(OH)3. More the stability of rare earth oxide greater is the passivation of steel surface. PPy/GNS/Sm3+/DBSA shows minimum corrosion as compared to others due to the presence of graphene and more stability of its hydroxide. The DBSA present in the nanocomposites, apart from making the nanocomposites soluble in organic solvents, also reduces the corrosion rate by acting as an inhibitor as shown in Fig. 9. If somehow the electrolyte seeps through the coatings, the DBSA present in the coatings releases DBSA− anions, which form a passive layer by reacting with the cation of iron (Fe+2) and reduces the corrosion rate to a greater extent [28]. The effect of varying concentration of rare earth elements and GNS in the nanocomposite needs further study. 3.3. Surface morphological studies The surface morphology of the coated samples prior to and after 1 month immersion in test solution (0.1 M HCl) is shown in Fig. 10 (a–f). The sample coated with PPy/DBSA in Fig. 10 (b) show fine cracks in the coating after immersion resulting in the degradation of metal. In Fig. 10 (d) presence of GNS in the coating system does not allow formation of cracks by maintaining the integrity of coating on the metal surface. Although some corrosion products are visible in Fig. 10 (d), which was completely diminished by the addition of rare earth elements as represented in Fig. 10 (f) showing no cracks as well as corrosion product. 4. Conclusion The soluble organic-inorganic nanocomposites of pyrrole containing rare earth ions and dodecyl benzene sulfonic acid embedded on the surface of graphene nano sheets were successfully synthesized. The formation of PPy/GNS/DBSA and PPy/GNS/RE3+/DBSA nanocomposites were confirmed using FTIR, XRD, SEM and TEM. The EIS measurements exhibited highest charge transfer resistance (Rct) and lowest double layer capacitance (Cdl) for PPy/GNS/RE3 +/DBSA compared to PPy/DBSA and PPy/GNS/DBSA coated steel. Potentiodynamic polarization studies showed substantial reduction in icorr, noble shift (positive shift) in Ecorr, decrease in porosity (P) and increase of Rp values of the low carbon steel in presence of nanocomposite coatings. The results of OCP measurements show much nobler and stable potential for PPy/GNS/RE3+/ DBSA than PPy/DBSA and PPy/GNS/DBSA coated steel. The results of electrochemical tests find adequate support from immersion tests. The morphology of the nanocomposite films deposited on steel specimen was examined by SEM to support the results obtained from electrochemical studies. The nanocomposite coatings were observed to exhibit both barrier and passivation behavior. Considering the excellent corrosion protection of nanocomposite coatings the studied compounds can be used for future industrial assessments. Acknowledgements The financial support from CSIR, New Delhi, India, through the major research Project No. 01/(2746)/13/EMR-II is gratefully acknowledged.
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