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Corrosion inhibition property of polyester–groundnut shell biodegradable composite
Ecotoxicology and Environmental Safety ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Corrosion inhibition property of polyester–groundnut shell biodegradable composite P. Sounthari, A. Kiruthika, J. Saranya, K. Parameswari, S. Chitra n Department of Chemistry, P.S.G.R. Krishnammal College for Women, Peelamedu, Coimbatore 641004, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2014 Received in revised form 30 July 2015 Accepted 15 August 2015
The use of natural fibers as reinforcing materials in thermoplastics and thermoset matrix composites provide optimistic environmental profits with regard to ultimate disposability and better use of raw materials. The present work is focused on the corrosion inhibition property of a polymer matrix composite produced by the use of groundnut shell (GNS) waste. Polyester (PE) was synthesized by condensation polymerization of symmetrical 1,3,4-oxadiazole and pimelic acid using sodium lauryl sulfate as surfactant. The polyester–groundnut shell composite (PEGNS) was prepared by ultrasonication method. The synthesized polyester–groundnut shell composite was characterized by FT-IR, TGA and XRD analysis. The corrosion inhibitory effect of PEGNS on mild steel in 1 M H2SO4 was investigated using gravimetric method, electrochemical impedance spectroscopy, potentiodynamic polarization, atomic absorption spectroscopy and scanning electron microscopy. The results showed that PEGNS inhibited mild steel corrosion in acid solution and indicated that the inhibition efficiency increased with increasing inhibitor concentration and decrease with increasing temperature. The composite inhibited the corrosion of mild steel through adsorption following the Langmuir adsorption isotherm. Changes in the impedance parameters Rt, Cdl, Icorr, Ecorr, ba and bc suggested the adsorption of PEGNS onto the mild steel surface, leading to the formation of protective film. & 2015 Published by Elsevier Inc.
Keywords: Gravimetric method Impedance spectroscopy Polarization AAS SEM
1. Introduction In spite of much advancement in the field of corrosion science and technology, the phenomenon of corrosion remains a major concern to industries around the world. Though the serious consequences of corrosion can be controlled to a great extent by selection of highly corrosion resistant materials, the cost factor associated with the same, favors the use of cheap metallic materials along with efficient corrosion prevention methods in many industrial applications. Mild steel is the most commonly used engineering material. Often mild steel is exposed to the attack of acid solutions during industrial processes such as acid cleaning, pickling and descaling. This results in easy corrosion of the mild steel, thereby necessitating the use of inhibitor. There are numerous methods for controlling the corrosion of metals but the use of inhibitors is still one of the best methods of protecting metals against corrosion (Viswanathan and Saji, 2010). Many organic compounds are used as corrosion inhibitors for mild steel in acidic environments. Such compounds usually contain nitrogen, oxygen or sulfur in a conjugated system and function via adsorption of the n
Corresponding author. E-mail address: [email protected] (S. Chitra).
molecules on the metal surface, creating a barrier to the corrodant attack (Hari Kumar and Karthikeyan, 2013; Udhayakala et al., 2012). In recent years, polymers have drawn considerable attention as corrosion inhibitors because of their large surface area, inherent stability and cost effectiveness. Polymers exhibit excellent inhibition even at very low concentration when added to the aggressive medium (Banerjee et al., 2011). Several researches have indicated that polymers can be used as corrosion inhibitors because through their functional groups they form complexes with metal ions and on metal surfaces. These complexes occupy a large surface area, thereby blanketing the surface and protect the metal from corrosive agents present in solution (Arthur et al., 2013). The inhibitive power of these polymers is related structurally to the cyclic rings and heteroatoms that are the major active centers of adsorption. The main advantages of polymeric inhibitors are (i) a single polymeric chain displaces many water molecules from the metal surface thus making the process entropically favorable (ii) the presence of multiple bonding sites makes the desorption of polymers a slower process (Amin et al., 2009). Most polymers are not easily biodegradable which allow for their long time storage and usage on corrosion protection of metals and alloys. Mechanism of inhibition is mainly attributed to adsorption and depends on the metal, physicochemical properties of the molecule such as
http://dx.doi.org/10.1016/j.ecoenv.2015.08.014 0147-6513/& 2015 Published by Elsevier Inc.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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functional groups, steric factors, aromaticity at the donor atom and p orbital character of donating electrons, as well as the electronic structure of the molecules. The development in science and technology requires a variety of polymers with good properties and low cost. To improve the inhibition efficiency and to minimize the consumption of polymers, an attempt has been made to incorporate natural filler materials in the polymer matrix. Therefore, polymer composites were considered to be among the more promising approaches to yield new materials and have been investigated extensively. In recent years, many studies have been dedicated to utilize lignocellulosic fillers such as coconut shell, wood, pineapple leaf, palm kernel shell, groundnut shell etc., as fillers in order to replace synthetic fillers through utilization of natural fillers or reinforcement in polymer composites in an attempt to minimize the cost, increase productivity and enhance mechanical properties of product. Due to the toxic nature, high cost and increasing awareness and strict environmental regulations of some of these compounds, the use of natural product as corrosion inhibitor is receiving attention. In the light of the above, the present study is channeled towards producing a composite material using polyester (PE) as the matrix and a groundnut shell (GNS) as the reinforcing filler so as to further reestablish the use of agricultural wastes as reinforcements in polymer matrices. The groundnut shell was collected from local suppliers in and around Coimbatore district. Groundnut botanically known as Arachis hypogeal belongs to Leguminosae family. India is the second largest producer of groundnut after China. Groundnut shell (Arachis hypogea L) is an important oil seed crop of India. The pod or dry pericarp contains about 25–40% shell. Chemical composition of groundnut shell is as follows: 65.7% cellulose, 21.2% carbohydrates, 7.3% protein, 4.5% minerals and 1.2% lipids (Shehu et al., 2014). Inhibitive effects of the polyester composite has been investigated using gravimetric method, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, atomic absorption spectroscopy and scanning electron microscopy.
2. Materials and method 2.1. Electrodes and solutions The mild steel with chemical composition by weight 0.084% C, 0.369% Mn, 0.129% Si, 0.025% P, 0.027% S, 0.022% Cr, 0.011% Mo, 0.013% Ni and balance iron was used for the experiment. Mild steel coupons of size 1 3 0.1 cm3 for weight loss measurements and mild steel rod with an exposed area 0.785 cm2 (cylindrical rod embedded in Teflon with a diameter of 1 cm and area exposed to corrosive medium is 0.785 cm2) for electrochemical measurements were used in the study. Before testing, to remove impurities from the surface, the coupons were abraded with emery sheets (1/ 0, 2/0, 3/0 and 4/0) washed with distilled water followed by degreasing with trichloroethylene. The test solution 1 M H2SO4 was prepared from 36 M H2SO4 analytical grade and double distilled water. The inhibitors concentrations were taken in ppm for all investigation. 2.2. Synthesis of polyester composite The polyester composite (Scheme 1) was prepared as reported earlier (Sava et al., 2003; Sivadhayanithy et al., 2007; GuilbertGarcia et al., 2012). The structure of the synthesized polyester and polyester composite was confirmed by IR spectra using Shimadzu IR Affinity 1. In order to determine the thermal stability of the polymers and its composite, TGA was carried out. The thermo grams were recorded in dynamic nitrogen atmosphere with a heating rate of 10 °C using a Perkin-Elmer (TGS-2 model) thermal analyzer. The crystalline nature of the polyester and polyester composite was confirmed by XRD using Brucker XRD at STIC, CUSAT, Cochin. 2.3. Non-electrochemical techniques 2.3.1. Weight loss measurements The weight loss experiments were carried out by standard procedure (Guilbert-Garcia et al., 2012). The mild steel was immersed in 1 M H2SO4 in presence and absence of various concentrations (500 ppm of PE þ100 ppm/1000 ppm/2000 ppm of
Scheme 1. Synthesis of Polyester composite.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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GNS) of inhibitors for 3 h duration at 303 71 K. The effect of temperature on inhibition of the inhibitors was studied at different temperature (303–333 K) using optimum concentration of inhibitors.
Inhibition efficiency (%)
=
Weight loss without inhibitor− Weight loss with inhibitor Weight loss without inhibitor (1)
× 100 Corrosion rate (g cm−2h−1) =
534 × weight loss in grams Density × Area in cm2 × time in hours
(2)
3
2.5. Surface analysis 2.5.1. Scanning electron microscope-energy dispersive X-ray spectroscopy Mild steel specimens of size 1 3 0.1 cm3 were immersed in 1 M H2SO4 in the absence and presence of inhibitors with optimum concentrations for 3 h. Then the specimens were removed and washed with distilled water and dried at ambient temperature. The chemical composition of the corrosion products was recorded with an EDS detector coupled with the SEM using Medzer biomedical research microscope (Mumbai, India). The following two case were selected for SEM-EDS study (a) mild steel coupon in 1 M H2SO4 solution with no inhibitors (b) with optimum concentration of 500 ppm of PE þ2000 ppm of GNS after 3 h immersion.
Surface coverage(θ ) 3. Result and discussion
(Weight loss without inhibitor − Weight loss with inhibitor) = Weight loss without inhibitor (3) 2.3.2. Atomic absorption spectroscopy (AAS) Atomic absorption spectrophotometer (Model GBC 908, Australia) was used for estimating the amount of dissolved iron in the corrodent solution containing 500 ppm of PE and 500 ppm þ100 ppm/1000 ppm/2000 ppm of GNS in 1 M H2SO4 after exposing the mild steel specimen for 3 h. From the amount of dissolved iron, the inhibitor efficiency was calculated.
Inhibition efficiency (%) =
(B−A) × 100 B
(4)
where A and B are the amount of dissolved iron in the presence and absence of inhibitors. 2.4. Electro chemical techniques Electrochemical measurements were conducted using IVIUM Compactstat Potentiostat/Galvanostat. A conventional three electrode cell setup with mild steel as working electrode, platinum foil as auxiliary electrode and a saturated calomel electrode as reference electrode was used for the measurements. The open circuit potential was obtained by immersing the working electrode in the test solution 1 M H2SO4 for 30 min. EIS measurements were carried out at corrosion potentials across the frequency range 10– 0.01 Hz with 10 mV amplitude of wave form. For potentiodynamic polarization measurements, potential was scanned in the range 200 mV to þ 200 mV at a scan rate of 1 mV/s. The electrode was allowed to corrode freely prior to EIS and polarization measurements. During this time OCP was recorded for 30 min to obtain a steady state value representing the corrosion potential (Ecorr) of the working electrode. All data for electrochemical measurements were analyzed using IVIUM Soft software.
I. E (%) =
I. E (%) =
(R t*−R t ) × 100 R t* (Icorr−Icorr (inh)) Icorr *
× 100
(5)
(6)
where, Rt and Rt are the charge transfer resistance obtained in the absence and presence of the inhibitors. Icorr and Icorr(inh) signifies the corrosion current density in the absence and presence of inhibitors.
3.1. Spectral studies 3.1.1. IR studies The FT-IR spectrum of the groundnut shell, polyester and groundnut shell composite were compared. The IR spectrum of groundnut shell (GNS) showed a number of adsorption peaks which indicates its complex nature. The bands were observed at 3300–3357 cm 1 characteristic of –NH and –OH groups. The peaks at 1737 cm 1, 1619 cm 1 and 1532 cm 1 are due to the presence of carboxyl, ester and carbonyl groups. GNS is mainly composed of polymeric OH groups, 4CH2, COOH and OH groups of poly saccharides (Shehu et al., 2014; Burham et al., 2014; Malik et al., 2006). In the IR spectrum of the polyester (PE) shows bands at 3020 cm 1, 1679 cm 11,600 cm 1 and 1078 cm 1 characteristic of –NH, 4C ¼O, C ¼N and C–O–C groups respectively (Sava et al., 2003; Sivadhayanithy et al., 2007). The IR spectrum of the composite PES-GNS shows peaks at 3364 cm 1, 1723 cm 1, 1607 cm 1, 1465 cm 1 and 1079 cm 1 characteristic of –OH, 4C ¼O, 4C ¼N, N ¼N–C– and –C–O–C– groups respectively. The results confirm that the IR spectrum of the PES-GNS composite is a combination of the IR spectrum of GNS and PE, a shift in frequency is observed for some functional groups. It suggests physical interaction between polyester and groundnut shell, this confirms the formation of the composite. 3.1.2. Thermal stability studies According to Chukwuemeka et al., (Azubuike et al., 2012) the thermal stability of the powdered groundnut shell is about 305 °C. The results of TGA of polyester and polyester composite reveal that the thermal degradation of the polyester takes place in a single step and decomposition temperature is about 270–320 °C which may be attributed to the main pyrolitic rupture of –N ¼N–, commencing at about 300 °C with the evolution of aromatics from the degradation of the oxadiazole backbone. But TGA of the composite shows a different trend. The thermal stability of the polymer composite is higher than the thermal stability of polymer and filler material. At 600 °C the char residue of the composite is 30%. 3.1.3. XRD studies The XRD pattern of the groundnut shell infers its semi crystalline nature whereas the polymer is crystalline in nature. In the composite the addition of the filler material to the polymer matrix shows the disappearance of peaks and confirms the high crystalline nature of the material and the formation of the composite (Azubuike et al., 2012).
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Table 1 Inhibition efficiencies of PE/PEGNS composite for corrosion of mild steel in 1 M H2SO4 obtained by weight loss measurement at 303 7 1 K. Inhibitors
Concentration of PE (ppm)
Weight of GNS (ppm)
Weight loss (g)
IE (%)
Surface coverage (θ)
CR (g cm 2 h 1)
Blank PE PEGNS composite
– 500 500
– – 100 1000 2000
0.2209 0.1356 0.0870 0.0424 0.0298
– 38.6 60.6 80.8 86.5
– 0.3862 0.6062 0.8081 0.8651
14.31 8.79 5.64 2.75 1.93
desorption of PEGNS from the metal surface and exposure of more area of the metal surface to acidic medium. The decrease in the adsorption at higher temperature indicates physical adsorption of the inhibitor. Table 2 shows that the values of Ea for inhibited solution were higher than those for uninhibited solution. The higher values of Ea indicates that the dissolution of mild steel is slow in the presence of PEGNS. Rise in activation energy in the presence of inhibitor is explained in different ways in the literature. The decrease in apparent activation energy at higher levels of inhibition arises from a shift of the net corrosion reaction, from uncovered surface to directly involving the adsorbed sites. The increase in activation energy could be attributed to an appreciable decrease in the adsorption of the inhibitor on the mild steel surface with increase in temperature (Szauer and Brand, 1981). The free energy of adsorption (ΔG°ads) was calculated from the equilibrium constant of adsorption at different temperatures using the following equation:
3.2. Evaluation of inhibition efficiency 3.2.1. Weight loss method The values of percentage inhibition efficiency (%) and corrosion rate obtained from weight loss method at 500 ppm concentration of PE and different concentrations filler material (GNS) in polyester composite at 303 7 1 K are summarized in Table 1. The results shows that the corrosion rate decreases noticeably with an increase in the concentration of the filler (GNS) amount from 5.64 to 1.93 g cm 2 h 1 at 100 ppm, 1000 ppm and 2000 ppm of groundnut shell respectively, i.e., the corrosion inhibition enhances with the inhibitor concentration. This behavior is due to the fact that the adsorption coverage of inhibitor on mild steel surface increases with the inhibitor concentration. From Table 1, it was found that, the polyester shows only 38.6% inhibition efficiency at 500 ppm. But in the polyester composite, the inhibition efficiency increased upto 86.5%, while increasing the amount of groundnut shell. The polyester composite exhibited the greatest inhibiting effect than the polyester separately. This is mainly due to the surface area of groundnut shell is very high, which shows its high adsorption capacity. The results show a significant decrease in weight loss with various concentrations of additive (GNS) because the composite helps to enhance hydrophobic character and thus improve the interfacial compatibility between filler and polyester. Hence the mechanical properties of the composite are enhanced and this facilitates corrosion inhibition (Malik et al., 2006).
ΔDG°ads = −RT ln (55. 5K)
(7)
The negative values of ΔG°ads (Table 2) are consistent with the spontaneity of the adsorption process and the stability of the adsorbed layer on the mild steel surface. Generally values of ΔG°ads up to 40 kJ/mol are consistent with electrostatic interaction between charged molecule and a charged metal which indicates physisorption while those more negative than 40 kJ/mol involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond which indicates chemisorption (Chitra et al., 2011). The values of ΔG°ads in our measurements ranged from 30.5 to 27.4 kJ/mol. Thus the adsorption of the polyester and polyester composite on mild steel in 1 M H2SO4 is physisorption. The ΔG°ads values for the composite is slightly higher than the polyester which confirms that the composite is more spontaneously adsorbed on mild steel surface compared to polyester. An alternative formulation of Arrhenius equation is
3.2.2. Effect of temperature The effect of rising temperature on the corrosion rate of mild steel in 1 M H2SO4 containing 500 ppm of polyester and 500 ppm þ100 ppm/1000 ppm/2000 ppm of the polyesterþ groundnut shell composites was studied in temperature range from 303 K, 313 K, 323 K and 333 K by mass loss method. The effect of temperature on the inhibited acid–metal reaction is very complex, because many changes occur on the metal surface such as rapid etching, desorption of inhibitor and the inhibitor itself may undergo decomposition. In general IE increases with the inhibitor concentration (Li et al., 2009). But in acidic media, corrosion of metal is accompanied with evolution of H2 gas, rise in temperature accelerates corrosion rate which results in a higher dissolution rate of metal. It has been observed IE decreased with increase in temperature. The decrease in IE with temperature might be attributed to desorption of the inhibitor molecule from the metal surface at higher temperatures (Shukla and Quraishi, 2010). When temperature increases, CR increases which is due to
CR =
⎛ ΔS ° ⎞ ⎛ −ΔH ° ⎞ RT ⎟ ⎟exp⎜ exp⎜ ⎝ RT ⎠ ⎝ R ⎠ Nh
(8)
where, h is Planck's constant, N is Avogadro number, T is absolute temperature, R is universal gas constant, ΔH° is the enthalpy of activation and ΔS° is the entropy. The negative values of ΔH° indicate that the adsorption of the inhibitor is an exothermic process which is confirmed by a decrease in the inhibition efficiency with increasing temperature.
Table 2 Kinetic/Thermodynamic Parameters of mild steel corrosion in 1 M H2SO4. Name of the inhibitor
Blank Polyester (500 ppm) Composite (500 ppm PE þ2000 ppm GNS)
Ea (kJ)
37.36 46.28 56.47
ΔG°ads (kJ/mol) 303 K
313 K
323 K
333 K
– 28.1 29.91
– 27.93 30.28
– 27.89 30.35
– 27.4 30.57
ΔH °kJ/mol
ΔS °kJ/mol/K
– 40.94 39.77
– 0.669 0.368
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Arrhenius plot
1.8 1.7 1.5
2
R = 0.9991
1.4
2500
1.3 1.2 1.1 1.0
C/θ
log (corrosion rate)
Langmuir plot
3000
Blank PEGNS PE
1.6
5
0.9
2000
0.8
1500
0.7 0.6 0.5 0.4
1000
0.3 0.2 3.00
3.05
3.10
3.15
3.20
3.25
3.30
1000 / T (K)
500
1000
1500
2000
2500
Concentration (ppm)
Fig. 1. Arrhenius and Langmuir plots for corrosion of mild steel in 1 M H2SO4 solution in the absence and presence of inhibitors.
The values of ΔH° obtained in this study for polyester and polyester composite are 40.9 kJ/mol and 39.7 kJ/mol (Table 2 and Fig. 1) confirming that both polyester and polyester composite are physically adsorbed on to the mild steel surface, since ΔH° values lower than 40 kJ/mol indicate physical adsorption and values approaching 100 kJ/mol indicate chemical adsorption (Hassan Refat and Zaafarany Ishaq, 2013). The negative values of ΔS° (Table 2) can be explained in the following way: before the adsorption of inhibitors onto the metal surface, inhibitor molecules freely move in the bulk solution (the inhibitor molecules are chaotic), but with the progress in the adsorption, inhibitor molecules are orderly adsorbed on to the steel surface, as a result a decrease in entropy results. 3.2.3. Adsorption isotherm In order to understand the mechanism of corrosion inhibition, the adsorption behavior of the inhibitor on the metal surface has to be known. The data were tested graphically by fitting to various isotherms including Langmuir, Frumkin, Flory–Huggins and Temkin. However, the best fit is obtained from Langmuir isotherm (Fig. 1). The values of regression coefficients (R2) confirmed the validity of this approach. Though the linearity of the Langmuir plot (Fig. 1) may be taken to suggest that the adsorption of inhibitor follows the Langmuir isotherm. A straight line obtained was suggesting
that the adsorbed inhibitor molecules form monolayer on the mild steel surface and there is no interaction among the adsorbed inhibitor molecules. 3.3. Electrochemical measurements 3.3.1. Polarization studies Fig. 2 is a collection of cathodic and anodic polarization curves for mild steel in 1 M H2SO4 solution in the presence and absence of the PE and PEGNS inhibitors at room temperature. The corresponding electrochemical parameters such as corrosion potential (Ecorr), corrosion current (Icorr), cathodic Tafel slope (bc), anodic Tafel slope (ba) and corrosion rate (CR) derived by extrapolation of Tafel lines are summarized in Table 3. Inspection of Table 3 reveals that there is a definite decrease in corrosion current for inhibited solutions signifying the effective surface coverage of the inhibitor on metal surface. When the concentration of the inhibitor is increased, there is a shift of corrosion potential to noble directions. When the difference in the Ecorr values between inhibited and uninhibited solutions are more than 85 mV then the inhibitor can be differentiated as cathodic or anodic type (Shorky et al., 1998). But in this case the difference in Ecorr values is only less than 40 mV proving that the inhibitor is of mixed-type. The variation of both the Tafel slopes with respect to the uninhibited solution is not very high indicating that the inhibitor has not altered the
Fig. 2. Polarization curves and Nyquist diagram for mild steel recorded in 1 M H2SO4 of selected concentrations of the inhibitor (polyester composite).
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Table 3 Corrosion parameters for corrosion of mild steel with selected concentration of. the inhibitors in 1 M H2SO4 by potentiodynamic polarization, AC-impedance and AAS method. Inhibitors
Conc. of PE (ppm)
Weight of GNS (ppm)
Polarization method Tafel slopes (mV/dec)
Blank PE PE-GNS
– 500 500
– – 100 1000 2000
ba
bc
73 63 75 77 53
152 149 143 125 148
mechanism of the corrosion inhibition. Increase in the inhibition efficiency with respect to concentration is due to the adsorption of inhibitor on the metal surface. Various types of adsorptions that take place at metal–solution interface include (Ferreira et al., 2004): i. Electrostatic interactions between charged molecules and charged metal. ii. Interaction of uncharged electron pairs in the molecule with the metal. iii. Interaction of pi electrons with the metal. vi. Combination of 1and 2. In this study, the uncharged electron pairs present in the hetero atoms (O and N) and the presence of different groups like lignin, hydroxyl groups etc., in PEGNS could have contributed to the effective adsorption. 3.3.2. Electrochemical impedance spectroscopy The corrosion behavior of mild steel in acidic solution in the presence of polyester and polyester composite were investigated by EIS at 303 71 K. Nyquist plots of mild steel in uninhibited and inhibited acidic solutions containing various concentrations of polyester composite displayed one capacitive arc (Fig. 2). The impedance parameters derived from the Nyquist plots are given in the Table 3. From the table, it is clear that as concentration of the inhibitor increases charge transfer resistance Rct increases
AC-impedance parameters
AAS
Ecorr (mV)
Icorr (μA/cm2)
IE (%)
Rt (ohm cm2)
Cdl (lF/cm2)
IE (%)
IE (%)
463.4 484.6 488.4 509.1 477.4
1212 795.1 684.9 499.1 388.1
– 34.4 43.5 58.8 67.9
11.06 20.8 26.08 35.49 50.82
27.8 25.2 25.2 23.3 21.3
– 46.8 57.6 68.8 78.2
– 36.5 67.3 85.0 85.6
and double layer capacitance Cdl decreases. Decrease in Cdl is due to an increase in the thickness of the electrical double layer. The differences in the corrosion resistances observed under each test condition can be interpreted as a reflection of the morphological changes of adsorbed structure which in turn alters the surface coverage by inhibitor. The EIS results support the finding that a strong correlation exists between the corrosion rate and the adsorbed structure of inhibitor film formed at the surface (Pandarinathan et al., 2014). 3.4. Atomic absorption spectrophotometric studies (AAS) The inhibitor efficiency (%) of the polyester and polyester composite were calculated from the percentage of Fe dissolved obtained from AAS (Table 3). The inhibitor efficiency (%) obtained by this technique was found to be in good agreement with that obtained from the conventional weight loss method. 3.5. Surface morphology 3.5.1. SEM images The polished mild steel specimen, mild steel immersed in the
Fig. 3. (a–d): Scanning electron microscopy images in the absence and presence of inhibitors (a) polished mild steel (b) blank (c) PE (d) PEGNS.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Fig. 4. (a) SEM image with EDS for blank. (b) SEM image with EDS for the inhibitor polymer composite.
blank acid (1 M H2SO4), mild steel immersed in the acid containing 500 ppm PE and inhibited acid (1 M H2SO4 þ500 ppm of PE þ2000 ppm GNS) were observed under a scanning electron microscope and the SEM images are shown in Fig. 3a–d respectively. The SEM images show that mild steel is heavily corroded in 1 M H2SO4 (Fig. 3b) where as in the presence of inhibitor the surface condition is comparatively better Fig. 3d. This suggests the presence of protective adsorbed layer of inhibitor molecules on the mild steel surface which reduces the corrosion rate of the metal appreciably. 3.5.2. Energy Dispersion Spectroscopy (EDS) studies The EDS spectra was used to determine the elements present on the surface of mild steel after 3 h of exposure in the uninhibited and inhibited 1 M H2SO4. Fig. 4a portrays the EDS analysis of mild steel in 1 M H2SO4 which indicates only the presence of Fe and O. This confirms that the passive film contained only Fe2O3. The EDS of polymer composite Fig. 4b in 1 M H2SO4 shows the presence of additional lines due to Fe, C, N and O. This may be attributed to the presence of polymer composite on mild steel which protects the steel surface from corrosion. 3.6. Mechanism of inhibition Adsorption of inhibitor molecules on the metal surface is either physical or chemical adsorption. For the physical adsorption (physisorption) of inhibitors on the metal surface, there must be interaction between metal surface and the molecules via relatively weak interactions, for example dipole–dipole interactions. On the other hand, chemisorption occurs by the sharing of electrons in inhibitor molecule and metals. The presence of N-atom in the polymers form pπ–dπ bonds resulting in transfer of 3d electrons from Fe atom to the vacant 2p orbital of N-atom, which enhances adsorption process on metal surface due to strong back bonding. The studied polymers have hetero atoms, aromatic rings, long chain, which get adsorbed on metal surface and form a layer to prevent direct contact of metal with aggressive media and slow down corrosion rate of metal (Ahamad et al., 2010). The polyester might adsorb on the iron surface as suggested below: (i) by sharing of electrons between the nitrogen atom and iron. (ii) Through π electron cloud interactions between the benzene ring of the molecule and the metal surface. (iii) Through the cationic form with positively charged part of the molecule oriented towards the negatively charged iron surface (i.e., inhibitors function by
electrostatic adsorption between the positively charged nitrogen atom (N þ) and the negatively charged metal surface to prevent a further Hþ getting nearer to the metal surface). In addition, the adsorption of the polyesters could be due to the formation of links between the d orbitals of iron atoms, involving the displacement of water molecules from the metal surface and the lone sp2 electron pairs present in the N and O atoms of both the oxadiazole monomers and ester group (Ren et al., 2008). The adsorption capacity of the polyester (PE) is enhanced by the addition of the filler material (GNS). The chemical components in groundnut shell are lignin, hemicellulose, pentosans and ash etc. Lignin contains hydroxyl, carboxyl, benzyl alcohol, methoxyl, aldehydic and phenolic functional groups. The functional group in the groundnut shell facilitates the greater adsorption of the composite on the mild steel surface by forming a barrier between the metal and corrodent solution (Malik et al., 2006). Further lignin in groundnut shell has primary hydroxyl groups present in the chain which further enhances the adhesion of the composite to the substrate.
4. Conclusion From the above studies, it can be concluded that
The synthesized polyester shows inhibition efficiency of 38.6%
at 500 ppm and the inhibition efficiency increased from 38.6% to 60%, 80% and 86% with increase in the weight of GNS 100 ppm, 1000 ppm and 2000 ppm respectively. The adsorption of inhibitors on the mild steel surface obey Langmuir adsorption isotherm and the activation energy (Ea) is higher for inhibited acids than for uninhibited acids showing the temperature dependence of inhibition efficiency. The less negative values of ΔG°ads indicate the spontaneous adsorption of the inhibitors on the metal surface. Electrochemical impedance spectroscopy experiments have shown that an increase in inhibitor concentration causes an increase in charge transfer resistance Rt and a decrease in Cdl value, owing to the increased thickness of the adsorbed layer. The Tafel slopes obtained from potentiodynamic polarization curves indicate that all the inhibitors behave as mixed type. The inhibition efficiency obtained from atomic absorption spectrophotometric studies was found to be in good agreement with that obtained from the conventional weight loss method.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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aluminum in acidic media by water-soluble natural polymeric pectates as anionic polyelectrolyte inhibitors. Materials 6, 2436–2451. Li, X., Deng, S., Fu, H., Mu, G., 2009. Inhibition effect of 6- benzylaminopurine on the corrosion of cold rolled steel in H2SO4 solution. Corros. Sci. 51, 620–634. Malik, R., Ramteke, D.S., Wate, S.R., 2006. Physico-chemical and surface characterization of adsorbent prepared from groundnut shell by ZnCl2 activation and its ability to adsorb colour. Indian J. Chem. Technol. 13, 319–328. Pandarinathan, Vedapriya, Lepkova, Katerina, Bailey, Stuart I., Becker, Thomas, Gubner, Rolf, 2014. Adsorption of corrosion inhibitor 1‑dodecylpyridinium chloride on carbon steel studied by in situ afm and electrochemical methods. Ind. Eng. Chem. Res. 53, 5858–5865. Ren, Yiwei, Luo, Yi, Zhang, Kaisong, Zhu, Gefu, Tan, Xiaolin, 2008. Lignin terpolymer for corrosion inhibition of mild steel in 10% hydrochloric acid medium. Corros. Sci. 50 (11), 3147–3153. Shehu, U., Aponbiede, O., Ause, T., Obiodunukwe, E.F., 2014. Effect of particle size on the properties of Polyester/Palm Kernel Shell (PKS) Particulate Composites. J. Mater. Environ. Sci. 5 (2), 366–373. Sava, I., Iosip, M.D., Corneliu Hamciuc, M.B., Robison, J., Okrasa, L., Pakula, T., 2003. Aromatic polyamides with pentent acetoxy benzamide groups and thin films made there from. Eur. Poly. J. 39, 725–738. Sivadhayanithy, M., Ravikumar, L., Ramachandran, T., 2007. Thermally stable polymers containing phenylthiourea, azo and ether groups in the backbone. Int. J. Appl. Chem. 3, 245–260. Shukla, S.K., Quraishi, M.A., 2010. The effects of pharmaceutically active compound doxycycline on the corrosion of mild steel in hydrochloric acid solution. Corros. Sci. 52, 314–321. Szauer, T., Brand, A., 1981. Adsorption of oleates of various amines on iron in acidic solution. Electrochim. Acta 26, 1253–1256. Shorky, H., Yuasa, M., Sekine, I., Issa, R.M., El-Baradie, H.Y., Gomma, G.K., 1998. Corrosion inhibition of mild steel by Schiff base compounds in various aqueous solutions. Corros. Sci. 40 (12), 2173–2186. Udhayakala, P., Rajendiran, T.V., Gunasekaran, S., 2012. Theoretical evaluation of corrosion inhibition performance of some triazole derivatives. J. Adv. Sci. Res. 3 (2), 71–77. Viswanathan, S., Saji, A., 2010. Review on recent patents in corrosion inhibitors. Recent Pat. Corros. Sci. 2, 6–12.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Corrosion inhibition property of polyester–groundnut shell biodegradable composite P. Sounthari, A. Kiruthika, J. Saranya, K. Parameswari, S. Chitra n Department of Chemistry, P.S.G.R. Krishnammal College for Women, Peelamedu, Coimbatore 641004, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2014 Received in revised form 30 July 2015 Accepted 15 August 2015
The use of natural fibers as reinforcing materials in thermoplastics and thermoset matrix composites provide optimistic environmental profits with regard to ultimate disposability and better use of raw materials. The present work is focused on the corrosion inhibition property of a polymer matrix composite produced by the use of groundnut shell (GNS) waste. Polyester (PE) was synthesized by condensation polymerization of symmetrical 1,3,4-oxadiazole and pimelic acid using sodium lauryl sulfate as surfactant. The polyester–groundnut shell composite (PEGNS) was prepared by ultrasonication method. The synthesized polyester–groundnut shell composite was characterized by FT-IR, TGA and XRD analysis. The corrosion inhibitory effect of PEGNS on mild steel in 1 M H2SO4 was investigated using gravimetric method, electrochemical impedance spectroscopy, potentiodynamic polarization, atomic absorption spectroscopy and scanning electron microscopy. The results showed that PEGNS inhibited mild steel corrosion in acid solution and indicated that the inhibition efficiency increased with increasing inhibitor concentration and decrease with increasing temperature. The composite inhibited the corrosion of mild steel through adsorption following the Langmuir adsorption isotherm. Changes in the impedance parameters Rt, Cdl, Icorr, Ecorr, ba and bc suggested the adsorption of PEGNS onto the mild steel surface, leading to the formation of protective film. & 2015 Published by Elsevier Inc.
Keywords: Gravimetric method Impedance spectroscopy Polarization AAS SEM
1. Introduction In spite of much advancement in the field of corrosion science and technology, the phenomenon of corrosion remains a major concern to industries around the world. Though the serious consequences of corrosion can be controlled to a great extent by selection of highly corrosion resistant materials, the cost factor associated with the same, favors the use of cheap metallic materials along with efficient corrosion prevention methods in many industrial applications. Mild steel is the most commonly used engineering material. Often mild steel is exposed to the attack of acid solutions during industrial processes such as acid cleaning, pickling and descaling. This results in easy corrosion of the mild steel, thereby necessitating the use of inhibitor. There are numerous methods for controlling the corrosion of metals but the use of inhibitors is still one of the best methods of protecting metals against corrosion (Viswanathan and Saji, 2010). Many organic compounds are used as corrosion inhibitors for mild steel in acidic environments. Such compounds usually contain nitrogen, oxygen or sulfur in a conjugated system and function via adsorption of the n
Corresponding author. E-mail address: [email protected] (S. Chitra).
molecules on the metal surface, creating a barrier to the corrodant attack (Hari Kumar and Karthikeyan, 2013; Udhayakala et al., 2012). In recent years, polymers have drawn considerable attention as corrosion inhibitors because of their large surface area, inherent stability and cost effectiveness. Polymers exhibit excellent inhibition even at very low concentration when added to the aggressive medium (Banerjee et al., 2011). Several researches have indicated that polymers can be used as corrosion inhibitors because through their functional groups they form complexes with metal ions and on metal surfaces. These complexes occupy a large surface area, thereby blanketing the surface and protect the metal from corrosive agents present in solution (Arthur et al., 2013). The inhibitive power of these polymers is related structurally to the cyclic rings and heteroatoms that are the major active centers of adsorption. The main advantages of polymeric inhibitors are (i) a single polymeric chain displaces many water molecules from the metal surface thus making the process entropically favorable (ii) the presence of multiple bonding sites makes the desorption of polymers a slower process (Amin et al., 2009). Most polymers are not easily biodegradable which allow for their long time storage and usage on corrosion protection of metals and alloys. Mechanism of inhibition is mainly attributed to adsorption and depends on the metal, physicochemical properties of the molecule such as
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Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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functional groups, steric factors, aromaticity at the donor atom and p orbital character of donating electrons, as well as the electronic structure of the molecules. The development in science and technology requires a variety of polymers with good properties and low cost. To improve the inhibition efficiency and to minimize the consumption of polymers, an attempt has been made to incorporate natural filler materials in the polymer matrix. Therefore, polymer composites were considered to be among the more promising approaches to yield new materials and have been investigated extensively. In recent years, many studies have been dedicated to utilize lignocellulosic fillers such as coconut shell, wood, pineapple leaf, palm kernel shell, groundnut shell etc., as fillers in order to replace synthetic fillers through utilization of natural fillers or reinforcement in polymer composites in an attempt to minimize the cost, increase productivity and enhance mechanical properties of product. Due to the toxic nature, high cost and increasing awareness and strict environmental regulations of some of these compounds, the use of natural product as corrosion inhibitor is receiving attention. In the light of the above, the present study is channeled towards producing a composite material using polyester (PE) as the matrix and a groundnut shell (GNS) as the reinforcing filler so as to further reestablish the use of agricultural wastes as reinforcements in polymer matrices. The groundnut shell was collected from local suppliers in and around Coimbatore district. Groundnut botanically known as Arachis hypogeal belongs to Leguminosae family. India is the second largest producer of groundnut after China. Groundnut shell (Arachis hypogea L) is an important oil seed crop of India. The pod or dry pericarp contains about 25–40% shell. Chemical composition of groundnut shell is as follows: 65.7% cellulose, 21.2% carbohydrates, 7.3% protein, 4.5% minerals and 1.2% lipids (Shehu et al., 2014). Inhibitive effects of the polyester composite has been investigated using gravimetric method, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, atomic absorption spectroscopy and scanning electron microscopy.
2. Materials and method 2.1. Electrodes and solutions The mild steel with chemical composition by weight 0.084% C, 0.369% Mn, 0.129% Si, 0.025% P, 0.027% S, 0.022% Cr, 0.011% Mo, 0.013% Ni and balance iron was used for the experiment. Mild steel coupons of size 1 3 0.1 cm3 for weight loss measurements and mild steel rod with an exposed area 0.785 cm2 (cylindrical rod embedded in Teflon with a diameter of 1 cm and area exposed to corrosive medium is 0.785 cm2) for electrochemical measurements were used in the study. Before testing, to remove impurities from the surface, the coupons were abraded with emery sheets (1/ 0, 2/0, 3/0 and 4/0) washed with distilled water followed by degreasing with trichloroethylene. The test solution 1 M H2SO4 was prepared from 36 M H2SO4 analytical grade and double distilled water. The inhibitors concentrations were taken in ppm for all investigation. 2.2. Synthesis of polyester composite The polyester composite (Scheme 1) was prepared as reported earlier (Sava et al., 2003; Sivadhayanithy et al., 2007; GuilbertGarcia et al., 2012). The structure of the synthesized polyester and polyester composite was confirmed by IR spectra using Shimadzu IR Affinity 1. In order to determine the thermal stability of the polymers and its composite, TGA was carried out. The thermo grams were recorded in dynamic nitrogen atmosphere with a heating rate of 10 °C using a Perkin-Elmer (TGS-2 model) thermal analyzer. The crystalline nature of the polyester and polyester composite was confirmed by XRD using Brucker XRD at STIC, CUSAT, Cochin. 2.3. Non-electrochemical techniques 2.3.1. Weight loss measurements The weight loss experiments were carried out by standard procedure (Guilbert-Garcia et al., 2012). The mild steel was immersed in 1 M H2SO4 in presence and absence of various concentrations (500 ppm of PE þ100 ppm/1000 ppm/2000 ppm of
Scheme 1. Synthesis of Polyester composite.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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GNS) of inhibitors for 3 h duration at 303 71 K. The effect of temperature on inhibition of the inhibitors was studied at different temperature (303–333 K) using optimum concentration of inhibitors.
Inhibition efficiency (%)
=
Weight loss without inhibitor− Weight loss with inhibitor Weight loss without inhibitor (1)
× 100 Corrosion rate (g cm−2h−1) =
534 × weight loss in grams Density × Area in cm2 × time in hours
(2)
3
2.5. Surface analysis 2.5.1. Scanning electron microscope-energy dispersive X-ray spectroscopy Mild steel specimens of size 1 3 0.1 cm3 were immersed in 1 M H2SO4 in the absence and presence of inhibitors with optimum concentrations for 3 h. Then the specimens were removed and washed with distilled water and dried at ambient temperature. The chemical composition of the corrosion products was recorded with an EDS detector coupled with the SEM using Medzer biomedical research microscope (Mumbai, India). The following two case were selected for SEM-EDS study (a) mild steel coupon in 1 M H2SO4 solution with no inhibitors (b) with optimum concentration of 500 ppm of PE þ2000 ppm of GNS after 3 h immersion.
Surface coverage(θ ) 3. Result and discussion
(Weight loss without inhibitor − Weight loss with inhibitor) = Weight loss without inhibitor (3) 2.3.2. Atomic absorption spectroscopy (AAS) Atomic absorption spectrophotometer (Model GBC 908, Australia) was used for estimating the amount of dissolved iron in the corrodent solution containing 500 ppm of PE and 500 ppm þ100 ppm/1000 ppm/2000 ppm of GNS in 1 M H2SO4 after exposing the mild steel specimen for 3 h. From the amount of dissolved iron, the inhibitor efficiency was calculated.
Inhibition efficiency (%) =
(B−A) × 100 B
(4)
where A and B are the amount of dissolved iron in the presence and absence of inhibitors. 2.4. Electro chemical techniques Electrochemical measurements were conducted using IVIUM Compactstat Potentiostat/Galvanostat. A conventional three electrode cell setup with mild steel as working electrode, platinum foil as auxiliary electrode and a saturated calomel electrode as reference electrode was used for the measurements. The open circuit potential was obtained by immersing the working electrode in the test solution 1 M H2SO4 for 30 min. EIS measurements were carried out at corrosion potentials across the frequency range 10– 0.01 Hz with 10 mV amplitude of wave form. For potentiodynamic polarization measurements, potential was scanned in the range 200 mV to þ 200 mV at a scan rate of 1 mV/s. The electrode was allowed to corrode freely prior to EIS and polarization measurements. During this time OCP was recorded for 30 min to obtain a steady state value representing the corrosion potential (Ecorr) of the working electrode. All data for electrochemical measurements were analyzed using IVIUM Soft software.
I. E (%) =
I. E (%) =
(R t*−R t ) × 100 R t* (Icorr−Icorr (inh)) Icorr *
× 100
(5)
(6)
where, Rt and Rt are the charge transfer resistance obtained in the absence and presence of the inhibitors. Icorr and Icorr(inh) signifies the corrosion current density in the absence and presence of inhibitors.
3.1. Spectral studies 3.1.1. IR studies The FT-IR spectrum of the groundnut shell, polyester and groundnut shell composite were compared. The IR spectrum of groundnut shell (GNS) showed a number of adsorption peaks which indicates its complex nature. The bands were observed at 3300–3357 cm 1 characteristic of –NH and –OH groups. The peaks at 1737 cm 1, 1619 cm 1 and 1532 cm 1 are due to the presence of carboxyl, ester and carbonyl groups. GNS is mainly composed of polymeric OH groups, 4CH2, COOH and OH groups of poly saccharides (Shehu et al., 2014; Burham et al., 2014; Malik et al., 2006). In the IR spectrum of the polyester (PE) shows bands at 3020 cm 1, 1679 cm 11,600 cm 1 and 1078 cm 1 characteristic of –NH, 4C ¼O, C ¼N and C–O–C groups respectively (Sava et al., 2003; Sivadhayanithy et al., 2007). The IR spectrum of the composite PES-GNS shows peaks at 3364 cm 1, 1723 cm 1, 1607 cm 1, 1465 cm 1 and 1079 cm 1 characteristic of –OH, 4C ¼O, 4C ¼N, N ¼N–C– and –C–O–C– groups respectively. The results confirm that the IR spectrum of the PES-GNS composite is a combination of the IR spectrum of GNS and PE, a shift in frequency is observed for some functional groups. It suggests physical interaction between polyester and groundnut shell, this confirms the formation of the composite. 3.1.2. Thermal stability studies According to Chukwuemeka et al., (Azubuike et al., 2012) the thermal stability of the powdered groundnut shell is about 305 °C. The results of TGA of polyester and polyester composite reveal that the thermal degradation of the polyester takes place in a single step and decomposition temperature is about 270–320 °C which may be attributed to the main pyrolitic rupture of –N ¼N–, commencing at about 300 °C with the evolution of aromatics from the degradation of the oxadiazole backbone. But TGA of the composite shows a different trend. The thermal stability of the polymer composite is higher than the thermal stability of polymer and filler material. At 600 °C the char residue of the composite is 30%. 3.1.3. XRD studies The XRD pattern of the groundnut shell infers its semi crystalline nature whereas the polymer is crystalline in nature. In the composite the addition of the filler material to the polymer matrix shows the disappearance of peaks and confirms the high crystalline nature of the material and the formation of the composite (Azubuike et al., 2012).
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Table 1 Inhibition efficiencies of PE/PEGNS composite for corrosion of mild steel in 1 M H2SO4 obtained by weight loss measurement at 303 7 1 K. Inhibitors
Concentration of PE (ppm)
Weight of GNS (ppm)
Weight loss (g)
IE (%)
Surface coverage (θ)
CR (g cm 2 h 1)
Blank PE PEGNS composite
– 500 500
– – 100 1000 2000
0.2209 0.1356 0.0870 0.0424 0.0298
– 38.6 60.6 80.8 86.5
– 0.3862 0.6062 0.8081 0.8651
14.31 8.79 5.64 2.75 1.93
desorption of PEGNS from the metal surface and exposure of more area of the metal surface to acidic medium. The decrease in the adsorption at higher temperature indicates physical adsorption of the inhibitor. Table 2 shows that the values of Ea for inhibited solution were higher than those for uninhibited solution. The higher values of Ea indicates that the dissolution of mild steel is slow in the presence of PEGNS. Rise in activation energy in the presence of inhibitor is explained in different ways in the literature. The decrease in apparent activation energy at higher levels of inhibition arises from a shift of the net corrosion reaction, from uncovered surface to directly involving the adsorbed sites. The increase in activation energy could be attributed to an appreciable decrease in the adsorption of the inhibitor on the mild steel surface with increase in temperature (Szauer and Brand, 1981). The free energy of adsorption (ΔG°ads) was calculated from the equilibrium constant of adsorption at different temperatures using the following equation:
3.2. Evaluation of inhibition efficiency 3.2.1. Weight loss method The values of percentage inhibition efficiency (%) and corrosion rate obtained from weight loss method at 500 ppm concentration of PE and different concentrations filler material (GNS) in polyester composite at 303 7 1 K are summarized in Table 1. The results shows that the corrosion rate decreases noticeably with an increase in the concentration of the filler (GNS) amount from 5.64 to 1.93 g cm 2 h 1 at 100 ppm, 1000 ppm and 2000 ppm of groundnut shell respectively, i.e., the corrosion inhibition enhances with the inhibitor concentration. This behavior is due to the fact that the adsorption coverage of inhibitor on mild steel surface increases with the inhibitor concentration. From Table 1, it was found that, the polyester shows only 38.6% inhibition efficiency at 500 ppm. But in the polyester composite, the inhibition efficiency increased upto 86.5%, while increasing the amount of groundnut shell. The polyester composite exhibited the greatest inhibiting effect than the polyester separately. This is mainly due to the surface area of groundnut shell is very high, which shows its high adsorption capacity. The results show a significant decrease in weight loss with various concentrations of additive (GNS) because the composite helps to enhance hydrophobic character and thus improve the interfacial compatibility between filler and polyester. Hence the mechanical properties of the composite are enhanced and this facilitates corrosion inhibition (Malik et al., 2006).
ΔDG°ads = −RT ln (55. 5K)
(7)
The negative values of ΔG°ads (Table 2) are consistent with the spontaneity of the adsorption process and the stability of the adsorbed layer on the mild steel surface. Generally values of ΔG°ads up to 40 kJ/mol are consistent with electrostatic interaction between charged molecule and a charged metal which indicates physisorption while those more negative than 40 kJ/mol involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond which indicates chemisorption (Chitra et al., 2011). The values of ΔG°ads in our measurements ranged from 30.5 to 27.4 kJ/mol. Thus the adsorption of the polyester and polyester composite on mild steel in 1 M H2SO4 is physisorption. The ΔG°ads values for the composite is slightly higher than the polyester which confirms that the composite is more spontaneously adsorbed on mild steel surface compared to polyester. An alternative formulation of Arrhenius equation is
3.2.2. Effect of temperature The effect of rising temperature on the corrosion rate of mild steel in 1 M H2SO4 containing 500 ppm of polyester and 500 ppm þ100 ppm/1000 ppm/2000 ppm of the polyesterþ groundnut shell composites was studied in temperature range from 303 K, 313 K, 323 K and 333 K by mass loss method. The effect of temperature on the inhibited acid–metal reaction is very complex, because many changes occur on the metal surface such as rapid etching, desorption of inhibitor and the inhibitor itself may undergo decomposition. In general IE increases with the inhibitor concentration (Li et al., 2009). But in acidic media, corrosion of metal is accompanied with evolution of H2 gas, rise in temperature accelerates corrosion rate which results in a higher dissolution rate of metal. It has been observed IE decreased with increase in temperature. The decrease in IE with temperature might be attributed to desorption of the inhibitor molecule from the metal surface at higher temperatures (Shukla and Quraishi, 2010). When temperature increases, CR increases which is due to
CR =
⎛ ΔS ° ⎞ ⎛ −ΔH ° ⎞ RT ⎟ ⎟exp⎜ exp⎜ ⎝ RT ⎠ ⎝ R ⎠ Nh
(8)
where, h is Planck's constant, N is Avogadro number, T is absolute temperature, R is universal gas constant, ΔH° is the enthalpy of activation and ΔS° is the entropy. The negative values of ΔH° indicate that the adsorption of the inhibitor is an exothermic process which is confirmed by a decrease in the inhibition efficiency with increasing temperature.
Table 2 Kinetic/Thermodynamic Parameters of mild steel corrosion in 1 M H2SO4. Name of the inhibitor
Blank Polyester (500 ppm) Composite (500 ppm PE þ2000 ppm GNS)
Ea (kJ)
37.36 46.28 56.47
ΔG°ads (kJ/mol) 303 K
313 K
323 K
333 K
– 28.1 29.91
– 27.93 30.28
– 27.89 30.35
– 27.4 30.57
ΔH °kJ/mol
ΔS °kJ/mol/K
– 40.94 39.77
– 0.669 0.368
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Arrhenius plot
1.8 1.7 1.5
2
R = 0.9991
1.4
2500
1.3 1.2 1.1 1.0
C/θ
log (corrosion rate)
Langmuir plot
3000
Blank PEGNS PE
1.6
5
0.9
2000
0.8
1500
0.7 0.6 0.5 0.4
1000
0.3 0.2 3.00
3.05
3.10
3.15
3.20
3.25
3.30
1000 / T (K)
500
1000
1500
2000
2500
Concentration (ppm)
Fig. 1. Arrhenius and Langmuir plots for corrosion of mild steel in 1 M H2SO4 solution in the absence and presence of inhibitors.
The values of ΔH° obtained in this study for polyester and polyester composite are 40.9 kJ/mol and 39.7 kJ/mol (Table 2 and Fig. 1) confirming that both polyester and polyester composite are physically adsorbed on to the mild steel surface, since ΔH° values lower than 40 kJ/mol indicate physical adsorption and values approaching 100 kJ/mol indicate chemical adsorption (Hassan Refat and Zaafarany Ishaq, 2013). The negative values of ΔS° (Table 2) can be explained in the following way: before the adsorption of inhibitors onto the metal surface, inhibitor molecules freely move in the bulk solution (the inhibitor molecules are chaotic), but with the progress in the adsorption, inhibitor molecules are orderly adsorbed on to the steel surface, as a result a decrease in entropy results. 3.2.3. Adsorption isotherm In order to understand the mechanism of corrosion inhibition, the adsorption behavior of the inhibitor on the metal surface has to be known. The data were tested graphically by fitting to various isotherms including Langmuir, Frumkin, Flory–Huggins and Temkin. However, the best fit is obtained from Langmuir isotherm (Fig. 1). The values of regression coefficients (R2) confirmed the validity of this approach. Though the linearity of the Langmuir plot (Fig. 1) may be taken to suggest that the adsorption of inhibitor follows the Langmuir isotherm. A straight line obtained was suggesting
that the adsorbed inhibitor molecules form monolayer on the mild steel surface and there is no interaction among the adsorbed inhibitor molecules. 3.3. Electrochemical measurements 3.3.1. Polarization studies Fig. 2 is a collection of cathodic and anodic polarization curves for mild steel in 1 M H2SO4 solution in the presence and absence of the PE and PEGNS inhibitors at room temperature. The corresponding electrochemical parameters such as corrosion potential (Ecorr), corrosion current (Icorr), cathodic Tafel slope (bc), anodic Tafel slope (ba) and corrosion rate (CR) derived by extrapolation of Tafel lines are summarized in Table 3. Inspection of Table 3 reveals that there is a definite decrease in corrosion current for inhibited solutions signifying the effective surface coverage of the inhibitor on metal surface. When the concentration of the inhibitor is increased, there is a shift of corrosion potential to noble directions. When the difference in the Ecorr values between inhibited and uninhibited solutions are more than 85 mV then the inhibitor can be differentiated as cathodic or anodic type (Shorky et al., 1998). But in this case the difference in Ecorr values is only less than 40 mV proving that the inhibitor is of mixed-type. The variation of both the Tafel slopes with respect to the uninhibited solution is not very high indicating that the inhibitor has not altered the
Fig. 2. Polarization curves and Nyquist diagram for mild steel recorded in 1 M H2SO4 of selected concentrations of the inhibitor (polyester composite).
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Table 3 Corrosion parameters for corrosion of mild steel with selected concentration of. the inhibitors in 1 M H2SO4 by potentiodynamic polarization, AC-impedance and AAS method. Inhibitors
Conc. of PE (ppm)
Weight of GNS (ppm)
Polarization method Tafel slopes (mV/dec)
Blank PE PE-GNS
– 500 500
– – 100 1000 2000
ba
bc
73 63 75 77 53
152 149 143 125 148
mechanism of the corrosion inhibition. Increase in the inhibition efficiency with respect to concentration is due to the adsorption of inhibitor on the metal surface. Various types of adsorptions that take place at metal–solution interface include (Ferreira et al., 2004): i. Electrostatic interactions between charged molecules and charged metal. ii. Interaction of uncharged electron pairs in the molecule with the metal. iii. Interaction of pi electrons with the metal. vi. Combination of 1and 2. In this study, the uncharged electron pairs present in the hetero atoms (O and N) and the presence of different groups like lignin, hydroxyl groups etc., in PEGNS could have contributed to the effective adsorption. 3.3.2. Electrochemical impedance spectroscopy The corrosion behavior of mild steel in acidic solution in the presence of polyester and polyester composite were investigated by EIS at 303 71 K. Nyquist plots of mild steel in uninhibited and inhibited acidic solutions containing various concentrations of polyester composite displayed one capacitive arc (Fig. 2). The impedance parameters derived from the Nyquist plots are given in the Table 3. From the table, it is clear that as concentration of the inhibitor increases charge transfer resistance Rct increases
AC-impedance parameters
AAS
Ecorr (mV)
Icorr (μA/cm2)
IE (%)
Rt (ohm cm2)
Cdl (lF/cm2)
IE (%)
IE (%)
463.4 484.6 488.4 509.1 477.4
1212 795.1 684.9 499.1 388.1
– 34.4 43.5 58.8 67.9
11.06 20.8 26.08 35.49 50.82
27.8 25.2 25.2 23.3 21.3
– 46.8 57.6 68.8 78.2
– 36.5 67.3 85.0 85.6
and double layer capacitance Cdl decreases. Decrease in Cdl is due to an increase in the thickness of the electrical double layer. The differences in the corrosion resistances observed under each test condition can be interpreted as a reflection of the morphological changes of adsorbed structure which in turn alters the surface coverage by inhibitor. The EIS results support the finding that a strong correlation exists between the corrosion rate and the adsorbed structure of inhibitor film formed at the surface (Pandarinathan et al., 2014). 3.4. Atomic absorption spectrophotometric studies (AAS) The inhibitor efficiency (%) of the polyester and polyester composite were calculated from the percentage of Fe dissolved obtained from AAS (Table 3). The inhibitor efficiency (%) obtained by this technique was found to be in good agreement with that obtained from the conventional weight loss method. 3.5. Surface morphology 3.5.1. SEM images The polished mild steel specimen, mild steel immersed in the
Fig. 3. (a–d): Scanning electron microscopy images in the absence and presence of inhibitors (a) polished mild steel (b) blank (c) PE (d) PEGNS.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Fig. 4. (a) SEM image with EDS for blank. (b) SEM image with EDS for the inhibitor polymer composite.
blank acid (1 M H2SO4), mild steel immersed in the acid containing 500 ppm PE and inhibited acid (1 M H2SO4 þ500 ppm of PE þ2000 ppm GNS) were observed under a scanning electron microscope and the SEM images are shown in Fig. 3a–d respectively. The SEM images show that mild steel is heavily corroded in 1 M H2SO4 (Fig. 3b) where as in the presence of inhibitor the surface condition is comparatively better Fig. 3d. This suggests the presence of protective adsorbed layer of inhibitor molecules on the mild steel surface which reduces the corrosion rate of the metal appreciably. 3.5.2. Energy Dispersion Spectroscopy (EDS) studies The EDS spectra was used to determine the elements present on the surface of mild steel after 3 h of exposure in the uninhibited and inhibited 1 M H2SO4. Fig. 4a portrays the EDS analysis of mild steel in 1 M H2SO4 which indicates only the presence of Fe and O. This confirms that the passive film contained only Fe2O3. The EDS of polymer composite Fig. 4b in 1 M H2SO4 shows the presence of additional lines due to Fe, C, N and O. This may be attributed to the presence of polymer composite on mild steel which protects the steel surface from corrosion. 3.6. Mechanism of inhibition Adsorption of inhibitor molecules on the metal surface is either physical or chemical adsorption. For the physical adsorption (physisorption) of inhibitors on the metal surface, there must be interaction between metal surface and the molecules via relatively weak interactions, for example dipole–dipole interactions. On the other hand, chemisorption occurs by the sharing of electrons in inhibitor molecule and metals. The presence of N-atom in the polymers form pπ–dπ bonds resulting in transfer of 3d electrons from Fe atom to the vacant 2p orbital of N-atom, which enhances adsorption process on metal surface due to strong back bonding. The studied polymers have hetero atoms, aromatic rings, long chain, which get adsorbed on metal surface and form a layer to prevent direct contact of metal with aggressive media and slow down corrosion rate of metal (Ahamad et al., 2010). The polyester might adsorb on the iron surface as suggested below: (i) by sharing of electrons between the nitrogen atom and iron. (ii) Through π electron cloud interactions between the benzene ring of the molecule and the metal surface. (iii) Through the cationic form with positively charged part of the molecule oriented towards the negatively charged iron surface (i.e., inhibitors function by
electrostatic adsorption between the positively charged nitrogen atom (N þ) and the negatively charged metal surface to prevent a further Hþ getting nearer to the metal surface). In addition, the adsorption of the polyesters could be due to the formation of links between the d orbitals of iron atoms, involving the displacement of water molecules from the metal surface and the lone sp2 electron pairs present in the N and O atoms of both the oxadiazole monomers and ester group (Ren et al., 2008). The adsorption capacity of the polyester (PE) is enhanced by the addition of the filler material (GNS). The chemical components in groundnut shell are lignin, hemicellulose, pentosans and ash etc. Lignin contains hydroxyl, carboxyl, benzyl alcohol, methoxyl, aldehydic and phenolic functional groups. The functional group in the groundnut shell facilitates the greater adsorption of the composite on the mild steel surface by forming a barrier between the metal and corrodent solution (Malik et al., 2006). Further lignin in groundnut shell has primary hydroxyl groups present in the chain which further enhances the adhesion of the composite to the substrate.
4. Conclusion From the above studies, it can be concluded that
The synthesized polyester shows inhibition efficiency of 38.6%
at 500 ppm and the inhibition efficiency increased from 38.6% to 60%, 80% and 86% with increase in the weight of GNS 100 ppm, 1000 ppm and 2000 ppm respectively. The adsorption of inhibitors on the mild steel surface obey Langmuir adsorption isotherm and the activation energy (Ea) is higher for inhibited acids than for uninhibited acids showing the temperature dependence of inhibition efficiency. The less negative values of ΔG°ads indicate the spontaneous adsorption of the inhibitors on the metal surface. Electrochemical impedance spectroscopy experiments have shown that an increase in inhibitor concentration causes an increase in charge transfer resistance Rt and a decrease in Cdl value, owing to the increased thickness of the adsorbed layer. The Tafel slopes obtained from potentiodynamic polarization curves indicate that all the inhibitors behave as mixed type. The inhibition efficiency obtained from atomic absorption spectrophotometric studies was found to be in good agreement with that obtained from the conventional weight loss method.
Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i
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Please cite this article as: Sounthari, P., et al., Corrosion inhibition property of polyester–groundnut shell biodegradable composite. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.08.014i