Silver Nanoparticles Composite: Synthesis

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Carboxymethyl Cellulose/ Silver Nanoparticles Composite: Synthesis, Characterization and Application as a Benign Corrosion Inhibitor for St37 Steel in 15% H2SO4 Medium Moses M Solomon, Husnu Gerengi, and Saviour A. Umoren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14153 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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ACS Applied Materials & Interfaces

Carboxymethyl Cellulose/ Silver Nanoparticles Composite: Synthesis, Characterization and Application as a Benign Corrosion Inhibitor for St37 Steel in 15% H2SO4 Medium

Moses M. Solomona*,Husnu Gerengia, and Saviour A. Umorenb a

Corrosion Research Laboratory, Department of Mechanical Engineering, Faculty of Engineering, Duzce University, 81620, Duzce, Turkey b

Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Abstract This study has been designed to boost the inhibition efficiency and stability of carboxymethyl cellulose (CMC) and this objective has been achieved by incorporating silver nanoparticles (AgNPs) generated in-situ by reduction of AgNO3 using natural honey into CMC matrix. Characterization of CMC/AgNPs composite was done using Transmission Electron Microscope (TEM), Fourier Transform Infrared (FTIR) spectroscopy, Ultraviolet– visible spectroscopy (UV-vis), Scanning Electron Microscope (SEM), and Energy Dispersive X-ray Spectroscopy (EDS). Weight loss, electrochemical (Dynamic Electrochemical Impedance Spectroscopy, Electrochemical impedance Spectroscopy, and Potentiodynamic Polarization) supported by surface assessment (SEM, Atomic Force Microscope, and FTIR) techniques are deployed for the anticorrosion studies of CMC/AgNPs on St37 specimen in 15% H2SO4 medium. CMC/AgNPs performs better than CMC. At 25 oC, optimum inhibition efficiency of 93.94% is afforded by 1000 ppm CMC/AgNPs from DEIS method. Inhibition efficiency of 96.37% has been achieved from weight loss method at 60 oC. CMC/AgNPs is found to retard both the anodic and cathodic reactions and the adsorption is explained using Langmuir adsorption isotherm. AFM and SEM graphics reveal smoother surface for St37 sample in the acid solution containing inhibitor than inthe solution without the inhibiting agent. FTIR and EDS results show that CMC/AgNPs molecules were adsorbed on the metal surface.

Keywords: Carboxymethyl cellulose; Silvernanoparticles; Synthesis; Acid solution; Metals corrosion; Inhibition *

[email protected]

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1. INTRODUCTĐON As the search for metals corrosion inhibitors that can replace the inorganic and organic metals corrosion inhibitors which are found to either have negative influence on the natural ecosystem or too expensive 1,2 continues, search light is beamed on polymers, particularly the natural polymers. The attractive features of polymers are (i) availability, (ii) cost effectiveness, (iii) ecofriendliness, (iv) inherent stability and (iv) presence of multiple adsorption centers

3,4

. The natural polymers have the advantage of being derived from

renewable source as well as being biodegradable 5. Some of the natural polymers tested for corrosion inhibiting property include; starch 5, carboxymethyl cellulose pectin

10, 11

, Gum Arabic

and iota-carrageenan

12, 13

14

6, 7

, chitosan

14

8, 9

,

15, 16

, Gellan gum , hydroxypropyl cellulose , xanthan gum

,

17

. However, the major setback militating against the utilization of

polymers as inhibitor for metals corrosion is the fact that some of them do not dissolve easily in aqueous environment and tend to decompose at high temperature 4. Consequently, most of them exhibit low or moderate inhibiting ability. For instance, Fares et al.

17

noted that, at 30

o

C, 1600 ppm iota-carrageenan could only afford 29.5% protection to Al surface in 2 M HCl

solution. Solomon et al.

7

reported inhibition efficiency of 64.8% for 0.5 g carboxymethyl

cellulose studied as inhibtor for mild steel in sulphuric acid solution. The authors found that, at 60 oC, the protection efficiency of the polymer declined to 60.8%. Rajeswari et al.

14

documented that 500 ppm glucose had 69.5% inhibition efficiency for cast iron in 1 M HCl environment at 28 oC. Umoren et al.

13

equally noted that 0.5 g Gum Arabic could only

suppressed the dissolution of mild steel in 0.1 M M H2SO4 solution at 30oC by 21.9. Corrosion scientists have therefore devised several means in attempt to enhance the inhibitive and stability properties of polymers. Notable among them are; combination with substances that exert synergistic effect

13, 15, 18

, copolymerization 17, blending

16

, cross linking

3, 19

infusion of small amount of metallic substance into the matrix of macromolecules Success seems to have been achieved through compositing. John et al.

21

, and

20, 21

.

reported that

incorporation of ZnO nanoparticles to chitosan matrix brought about remarkable chemical stability, controlled oxidation and as well improved corrosion resistance of mild steel in 0.1 N HCl medium. Hefni et al.

20

recently decumented that infusion of silver nanoparticles into

chitosan-grafted-polyethylene glycol (Ch-g-PEG) backbone stepped up the protection efficacy of Ch-g-PEG from 76.64% to 92.75% for carbon steel in 1 M HCl at 25 oC.


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There are several techniques available for the synthesis of polymer nanocomposite. This include wet chemical reduction 22, sonochemical method 23, microemulsion method 24, photo chemical reduction25, laser mediated technique assisted technique

28

as well as biogenic method

26

, hydrothermal method

27

,microwave

29, 30

. Some of these methods uses chemicals

like hydrazine, sodium sulphide, sodium borohydride, trisodium citrate, and dimethyl formamide as the reducing agents which has negative influence on the natural environment. Besides, most of these techniques are not cost effective and involves complex synthesis route; hence the biogenic method has enjoyed greater patronage in recent times. This technique uses plant extract 31, 32, microorganisms 33, fungi 34, and natural honey 29, 30 as the reducing agent. The use of natural honey is particularly beneficial in that it can perform dual functions of reducing and stabilizing the metal particles. This work was designed specifically to enhance the inhibitve ability of carboxymethyl cellulose (CMC) (the most abundant natural polymer) for St37 steel in 15% H2SO4 solution. This has been achieved by infusing AgNPs into CMC matrix using natural honey as the capping and reducing agent. CMC/AgNPs was characterized using FTIR, EDS, SEM, TEM, and UV-vis while chemical and electrochemical (EIS, PDP, and DEIS) methods supported with surface morphological analysis (SEM, AFM, and EDS) were used for corrosion studies. 2.

EXPERĐMENTAL SECTĐON

2.1

Materials and Chemicals St37-2 (St37 is used, for convenience, in the text) steel was procured from Erdemir

Steel Company, Turkey. The chemical composition (wt.%) of the metal is as listed in Hassan et al. 35. The metal samples were prepared following the method previously described by us 29, 30

. Carboxymethyl cellulose (Mw = ~ 90,000g/mol) (Fig. 1), AgNO3 and H2SO4 were

supplied by Sigma-Aldrich while the reducing agent (mad honey) was gotten from Duzce University Bee Keeping Research, Development and Application Centre in Yigilca. 2.2. Preparation of CMC/AgNPs We replicated the procedure previously reported by us

29, 30

in the preparation of

CMC/AgNPs composite. Firstly, 1000 ppm of CMC was prepared in 15% H2SO4 solution. Secondly, 1 mM AgNO3 solution was prepared using the CMC solution. Thirdly, natural honey (5 cm3) was introduced to every 100 cm3 of the CMC-AgNO3 solution. A change in colour after 96 h was assumed to indicate the formation of CMC/AgNPs composite and this was verified chemically using NaCl solution. The prepared composite solution was then 3 ACS Paragon Plus Environment


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diluted with 15% H2SO4 solution to other concentrations (50, 100, 500, and 750 ppm) for corrosion inhibition studies. 2.3

CMC/AgNPs Characterization

2.3.1. FTIR Studies The CMC, honey, and CMC/AgNPs FTIR spectra were recorded in a FTIR spectrometer (Agilent Technologies Cary 630) with 4000 to 450 cm−1 as the wavenumber and 4 cm−1 as the resolution accuracy.

2.3.2 EDS Studies The preparation of CMC/AgNPs composite sample for EDS analysis can be found in Solomon and Umoren

29

. An energy dispersive X-ray spectroscopy coupled to the SEM J.

Quanta FEG 250 model (FEI, Holland) was used for elemental composition determination.

2.3.3 TEM Studies A JEM-2100F transmission electron microscopy was used for the study of the size and morphology of the synthesized composite. Detail description on the preparation of sample for TEM analysis can be seen in Solomon and Umoren 29.

2.3.4 Uv-visible Studies This study was undertaken using JASCO770- UV–Vis spectrophotometer (200 to 800 nm) operated at a resolution of 1 nm with a scan rate of 100 nm/min at ordinary temperature.

2.4

Corrosion Measurements

2.4.1 EIS and PDP These experiments were done using a Gamry instrument, Reference 600. The instrument has software EIS300 for EIS and DC105 for PDP experiments. The working electrode was St37 sample with exposed area of 0.75 cm2; Silver/Silver chloride was deployed as the reference electrode, while Pt plate was used as the counter electrode. The working electrode was held in the electrolyte solution for an hour for the purpose of achieving a steady-state potential. In order to obtain a reproducible results, experiments were repeated for four times under the same experimental conditions.


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For polarization experiments, the working electrode was scanned from cathodic to anodic direction at a rate of 1 mV/s and with potential in the range −250 mV to +250 mV with respect to Ecorr. The values of corrosion current density (icorr) were obtained following the Tafel extrapolation method described by Hefni et al.

20

. The range of frequency which

EIS experiments were performed was 100 kHz to 10 mHz and amplitude signal of 10 mV peak-to-peak was used. Echem 6.32 program was used for the analysis of EIS data.

2.4.2 DEIS A frequency response analyzer (FRA) was used to measure the instantaneous changes on the St37 surface. The generation of perturbation current and the frequency range of measurement are the same as stated in Gerengi et al. 36. The experiments were performed for 15 h. 2.4.1

Chemical Measurements Here, the prepared metal samples, in trıplicate, were suspended, freely in 200 ml

glass vessels containing 150 ml of the considered solutions (Blank and Blank + different concentrations of CMC/AgNPs composite) at 25–60oC in a thermostated bath. The specimens were removed after 10 h and treated following the method described in Umoren Solomon et al. 7. The corrosion rate (mpy) and the

3

and

inhibition efficiency ( %η ) were

computed using Equations given elsewhere 37. 2.4.4 Surface Morphological Screening The St37 surface morphologies after exposure to 15% H2SO4 solutions devoid of and containing CMC/AgNPs composite for 15 h were observed using a J Quanta FEG 250 model (FEI, Holland) scanning electron microscope and a Park Systems XE-100E model atomic force microscope. The elemental constituents of the metal samples before and after experiments were recorded using a EDS detector. Spectra of the film from St37 steel surface exposed to 15% H2SO4 solution containing CMC/AgNPs composite was obtained using Fourier Transform Infrared (Agilent Technologies Cary 630 FTIR) spectrometer.

3.

RESULTS AND DĐSCUSSĐON

3.1

CMC/AgNPs Composite Characterization

3.1.1 FTIR Measurements



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FTIR is an important characterization technique and its usage has increased in recent times as it can provide an easy way of identifying functional groups in a molecule. Previous reports have shown that natural honey is capable of performing the function of a reducing and capping agent in nanoparticles synthesis

29, 30, 38, 39

. Honey is a complex compound with

fructose, glucose, sucrose, proteins, minerals, and vitamins as the essential ingredients

39

.

Enzymes such as diastase, invertase, glucose oxidase, catalase, and acid phosphatase have also been reported to be present in honey 40. These biomolecules as well as the enzymes can play a part in the conversion reaction of metal ions to the atomic form. To identify the honey component that predominantly influenced the reduction of Ag+ to Ag0 and the capping of Ag0, FTIR experiment was performed on mad honey which was used for the synthesis of CMC/AgNPs composite. The obtained spectrum is compared with the FTIR spectra of CMC and CMC/AgNPs composite (Fig. 2). In the FTIR spectrum of honey (Fig. 2(a)), sharp and intense peak is seen at 1025.0 cm–1 and is typical of the carbon-oxygen-carbon (C-O-C) symmetric stretching and carbon-oxygen-hydrogen (C-O-H) bending vibrations of protein 38. The characteristic primary and secondary amide bands arising from the stretching of carboxyl groups and the deformation vibration of N-H in the amide linkages of the honey proteins can be visibly seen at 1653.5 cm–1 and 1374.5 cm–1 38, 39 respectively. The strong and broad peak at 3331.8 cm–1 and the weak peak at 2927.8 cm–1 are unequivocally assigned to hydrogen bonded O-H groups and C-H stretching respectively. The CMC spectrum (Fig. 2(b)) is characterized by adsorption peaks at 1030.0, 1323.2, 1415.2, and 2887.4 cm–1. The medium peak at 1030.0 cm–1 and the weak peak at 2887.4 cm–1 arose due to CH2 twisting and stretching vibrations 41. The assymetrical deformation bands of CH2 and C-OH respectively appear at 1323.2 and 1415.5 cm–1. A comparison of the CMC and honey spectra with that of the CMC/AgNPs (Fig. 2(c)) spectrum reveals some strikely differences. The C-H stretching band at 2927.8 cm–1 in the honey spectrum and at 2887.4 cm–1 in the CMC spectrum becomes broad in the CMC/AgNPs spectrum. The O-H and Amide II stretching bands at 3331.8 cm–1 and 1374.5 cm–1 respectively in the honey spectrum almost completely dissappeared in the CMC/AgNPs spectrum. The difference in the CMC and CMC/AgNPs spectra suggest modification of CMC by AgNPs. The disaapearance of the O-H and secondary amide stretching peaks indicate the involvement of honey enzymes as well as proteins in the reduction of Ag+ to Ag0; possibly by the oxidation of the biomolecules into their corresponding acids 42. It has, however, been reported

38, 39, 42

that through free -NH2 groups

–

or –COO ions of protein’s amino acid residue, metals nanoparticles can be stabilized. Either case can be recognized using the presence or absence of C=O stretch band at about 1700 cm– 6 ACS Paragon Plus Environment

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1

. Phillip

38,39

had reported the capping of Au nanoparticles by proteins through free amine

groups and in the reports, C=O appeared at 1721 cm–1 and 1714 cm–1 respectively. A close inspection of CMC/AgNPs spectrum in Fig. 2(c) reveals that the C=O band is absent; hence we submit that the stabilization of AgNPs is through carboxylate ions of the honey amino acid residue.

3.1.2 EDS Studies The formation of AgNPs can be ascertained with the aid of EDS. Due to the so-called surface plasmon resonance, silver nanoparticles generally give signal at apporximately 3 keV in EDS analysis

29, 30, 43

. Fig. 3 presents the EDS spectrum of the prepared CMC/AgNPs

composite. As could be clearly seen in the figure, intense Ag peak appears at 3 keV. Peaks from CMC component elements (i.e C and O) are also seen in the spectrum. The Si, P, Mn, and Fe peaks also seen in Fig. 3 arose due to the Fe sample holder used during analysis while S peak emanated from sulphuric acid solution used as the solvent during synthesis.

3.1.3 UV-Vis and TEM Studies Fig. 4 shows the UV-vis spectrum of the prepared CMC/AgNPs composite. A sharp and narrow absorption band which is a common feature of monodispersed spherical nanoparticles

39

can be visibly seen at about 348 nm in the figure. The structure and size of

the nanoparticles were observed using TEM. Fig. 5 shows the TEM pictures of the synthesized AgNPs. Clearly, it is seen that the prepared AgNPs are of various sizes and 29, 30 39, 44

spherical in shape; agreeing with the UV-vis result. Earlier reports

had shown that

metals nanopaticles of varying sizes can be obtained using bio-reducing agent. The differences in the size of the nanoparticles may be due to the fact that they are formed at different times. The nanoparticles, however, appear in aggregated form. This has been attributed to high surface activity and large specific surface area 45.

3.2

Anticorrosion Investigations

3.2.1 EIS Measurements Fig. 6 presents the EIS results recorded for St37 steel samples in 15% H2SO4 solution without and with diverse concentrations of CMC/AgNPs composite in (a) Nyquist, (b) Bode modulus and (c) Phase angle representations. Clearly, single capacitive loop is seen at high frequency in all the Nyquist diagrams. This may mean that the corrosion of St37 steel in the studied environment is controlled by a charge transfer process

19

. The semicircles are not 7

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perfect; a behaviour which has been associated with the presence of micro-roughness and heterogeneity of the working electrode

19, 46

. Compared to the blank, the semicircles of the

presented Nyquist diagrams of the inhibited systems are larger in size. This effect which is a function of concentration is also seen in the Bode modulus and Phase angle plots (Fig. 6(b) and (c)). That is, the impedance and Phase angle are shifted toward higher values in the inhibited systems. This shows that CMC/AgNPs composite inhibited St37 steel corrosion in the studied corrosive environment. CMC/AgNPs molecules may have adsorbed onto the St37 surface and cover reaction sites such that the rate of charge transfer was reduced. In the Bode modulus and Phase angle diagrams (Fig. 6(b)and (c)), three distinctive regions are observed. First, in the high frequency, log /Z/ is low and appears near constant as log /f/ increases while the Phase angle steadily falls toward 0o. Authors have documented similar observation in the literature 46, 47 and is a response typical of a resistor. It corresponds to the uncompensated solution resistance (Rs)

46, 47

. The second region is in the middle

frequency where log/Z/ varies linearly with log/f/ and the Phase angle approaches 80o. This behaviour is typical of a capacitor 46, 47. The third region is in the low frequency where log/Z/ is seen to be independent of log/f/ while the Phase angle, again goes toward 0o. This surface behaviour seems to suggest two relaxation processes: first describing the properties of the CMC/AgNPs adsorbed layer and the second the bare St37 metal. It should be noted that the Phase angles do not tend toward – 45o and the Nyquist spectra have not deviated from the xaxis as such, diffusion process is ruled out. A R(QR)(QR) equivalent circuit (EC) presented in Fig. 7 was therefore selected for the analysis of the Nyquist spectra. A good fit was gotten with this circuit as could be seen in the fitting diagrams (Fig. S1 of the supplementary information) as well as the low values of chi square (Table 1). In the equivalent circuit, Rs = the uncompensated solution resistance, Rct = the charge transfer resistance, Rf = the film resistance and CPE = the constant phase element which replaced the capacitive element for the purpose of getting a more accurate fit. Eq. 1 defines the CPE 46: QCPE = Yo−1 ( jω )

−n

(1)

where = CPE constant, n = CPE exponent; = (−1) / which is an imaginary number and ω = angular frequency in rad/s. The double layer capacitance ( ) values were calculated from Eq. 2 48: C dl =

Yω n −1 sin( n(π / 2))

(2)


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where ω = angular frequency (ω = 2πfmax); fmax = frequency at which the imaginary component of the impedance is maximum. Depending on the value of , the CPE can represents a pure resistor ( = 0), an inductor ( = −1), and a pure capacitor ( = +1) 47. All the parameters gotten from EIS studies are presented in Table 1. The equation used for the calculation of the inhibition efficiency also given in Table 1 can be seen in Finˇsgar et al. 47

.

From the table, it could be seen that the values of Rs, Rct, and Rf of the CMC/AgNPs inhibited systems are bigger while that of Cdl is smaller compared to those of the corrodent. This means that the metal was more stable in 15% H2SO4 soultion containing CMC/AgNPs composite than in the blank solution. Rp value is found to increase while Cdl value decrease with increasing composite concentration. This variation is seen to have a direct effect on the protection efficiency of CMC/AgNPs. For example, in acid solution containing 50 ppm CMC/AgNPs, Rp value increased from 47.05 Ω cm2 and Cdl decreased from 960.09 µF cm–2 to 209.13 Ω cm2 and 252.15 µFcm–2 respectively. The corresponding η is 77.50%. As the concentration was raised to 1000 ppm, the Rp value increase to 407.76 Ω cm2 and Cdl value decline to 160.89 µFcm–2 with 88.46% being the corresponding inhibition efficiency. This observation can be explained using the Helmholtz model 47:

δ

ads

=

ε εo A

(3)

C dl

where δ ads = thickness of CMC/AgNPs adsorbed layer, = permittivity of air, = local dielectric constant, and = surface area of St37 steel electrode. By this equation, for Cdl to decrease, δ ads must increase or decrease or both. It can be argued that the decrease in the Cdl value in the present case was due to the growth CMC/AgNPs adsorbed film as the

concentration was increased. As the protective film grows, the rate of charge transfer becomes slower due to greater resistance as indicated by Rct and η values. It is also seen in Table 1 that the ndl and nf values are near unity pointing to a capacitive interface 49.

3.2.2 PDP Measurements Fig. 8 shows the polarization graphs obtained for St37 steel in 15% H2SO4 solution in the absence and presence of varying amounts of CMC/AgNPs composite. The polarization branches are projected up to their point of interception to get the corrosion current density (Icorr) and corrosion potential (Ecorr)

50

. These parameters, and (anodic and cathodic

Tafel slopes), as well as corrosion rate (CR) derived from the polarization curves are 9 ACS Paragon Plus Environment


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presented in Table 2. The protection efficiency from this technique also given in Table 2 was calculated from equation given elsewhere 50. Worthy of note in Fig. 8 is the fact that, the presence of CMC/AgNPs composite in the studied corrosive solution causes both Ia and Ic (anodic and cathodic current densities) to be shifted to lower values and the corrosion potential slightly displaced towards nobler direction relative to the blank. This behaviour is typical of a mixed type corrosion inhibitor 51, 52

. To class an inhibitor as cathodic or anodic type demands that the difference between the

Ecorr of the uninhibited and inhibited systems be up to ± 85 mV

29, 30, 37

, otherwise the

inhibitor is regarded as mixed type. The Ecorr values in Table 2 reveals that CMC/AgNPs composite is in the category of a mixed type inhibitor. It is obvious from Table 2 that the Icorr and CR values gotten for the metal sample in 15% H2SO4 solution containing the composite are smaller than those without. Interestingly, there is significant reduction in the value of these parameters by even the least studied concentration of CMC/AgNPs composite. For instance, the presence of 50 ppm CMC/AgNPs caused Icorr and CR values to decrease from 395.0 µA cm–2 and 74.13 mpy respectively to 89.3 µA cm–2 and 17.02 mpy. This shows the effectiveness of the composite in disrupting the corrosion process. The small Icorr values recorded in CMC/AgNPs inhibited systems indicate that, it took a longer time for the corrosion circle to be completed in the inhibited systems than in the unihibited. Further examination of the table discloses that Icorr and CR values vary inversely with composite concentration while inhibition efficiency varies directly. Optimum inhibition efficiency of 89.97% is obtained for 1000 ppm CMC/AgNPs composite from this method. As could be seen in the table, no specific pattern is follwed by the and values as the concentration of the inhibitor increases suggesting unmodification of reaction mechanisms by the presence of CMC/AgNPs composite 46.

3.2.3 DEIS Measurements The DEIS technique has advantage over the conventional EIS method in that it can accurately track changes on metal surface in a changing environment. The ideal EIS requires three strict conditions, namely, linearity, casuality, and a stationary state to be made before measurement

52

. In practice, these conditions are technically impossible to achieve. Fig. 9

presents the DEIS spectra recorded for St37 steel in (a, d) 15% H2SO4 solution, (b, e) 15% sulphuric acid solution containing 50 ppm CMC/AgNPs, and (c, f) 15% acid solution + 1000 ppm CMC/AgNPs composite after 2 h and 15 h of measurements respectively at room temperature. In all cases, net-like semicircle is seen implying that the charge transfer process 10 ACS Paragon Plus Environment

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is continuous. The capacitive loops in both the uninhibited and inhibited systems are similar; meaning that the presence of CMC/AgNPs composite in the sulphuric acid solution did not alter the mechanism of St37 dissolution. However, the influence of the presence of the composite on the corrosion process can be visibly seen. In Fig. 9(a), the tail of the capacitive loop terminates at Re/Z/ value of 100/Ω cm2 whereas in Fig. 9(b & c), the tails end at 200 and 300/Ω cm2 respectively. This may be reflective of inhibition by the composite. A comparison of Fig. 9(a, b, & c) with Fig. 9(d, e, f) reveals that immersion time has effect on the deterioration of St37 steel as well as on the protection efficacy of CMC/AgNPs composite. For instance, the capacitive loop in Fig. 9(d) projects and terminates at Re/Z/ value of 60/Ω cm2 as against Re/Z/ value of 100/Ω cm2 in Fig. 9(a). It appears, at 2 h of immersion, corrosion products deposited on the St37 specimen and in a way, protected the metal surface such that a bigger value of corrosion resistance was recorded. As time prolonged, this products which are unstable, decompose and expose the surface to further attack. The DEIS spectra recorded for inhibited systems for 15 h (Fig. 9(e & f) are more compact than the ones recorded for 2 h (Fig. 9(b & c)) and the capacitive loop tails terminate in a higher values of Re/Z/. According to some authors

46, 52

, this signifies stability and better inhibition at longer

immersion time. The electrochemical parameters derived from the DEIS spectra using the EC in Fig. 7 are summarized in Table 3. The derived parameters vary in like manner as those presented in Table 1. That is, Rs, Rct, Rf, and Rp of the CMC/AgNPs containing systems are bigger compare to those of the blank. The n values again reflect an interface that behaved nearly capacitive. The results in the table clearly shows that exposure duration has significant effect on the composite inhibiting ability. For instance, at 2 h, the values of Rp for 50 and 1000 ppm CMC/AgNPs are 266.58 and 457.85 Ωcm2 respectively corresponding to inhibition efficiency of 84.43 and 90.49% respectively. The Rp value increased to 352.66 and 718.97 Ωcm2 for 50 and 1000 ppm CMC/AgNPs respectively at 15 h with 87.65 and 93.94% as the inhibition efficiency. This suggests that, at longer immersion time, greater amount of CMC/AgNPs molecules were adsorbed on the surface of the working electrode and cover larger area such that the section available for corrosive attack was reduced. The inhibition efficiency obtained from EIS (Table 1), PDP (Table 2) and DEIS at 2 h (Table 3) are comparable.

3.2.4 Weight loss (WL) Assessments WL method was also used to study the influence of CMC/AgNPs on the dissolution pattern of St37 steel in 15% H2SO4 solution. The computed values of WL, corrosion rate (CR) 11 ACS Paragon Plus Environment


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, surface coverage (θ), and η gotten from this technique at 25 oC are summarized in Table 4. From the results presented in the table, WL and CR are found to decrease while θ and η increase as inhibitor concentration increases. The optimum η gotten from this method is 92.12% (offered by the highest studied concentration). By comparing the value of inhibition efficiency obtained from WL method with those obtained from electrochemical methods (Table 1-3), it is found that the values from WL are bigger. This discrepancy may not be far from the fact that WL measures the average CR whereas the electrochemical techniques records the instantaneous CR 50. To examine the influence of temperature on the inhibitive performance of CMC/AgNPs composite, WL experiments were undertaken at 25, 40, 50, and 60 oC. The results obtained are presented in Fig. 10. It is observed from the figure that η increases gradually with temperature. For the highest studied concentration, η increased from 92.12% at 25 oC to 96.37% at 60 oC. This results suggest that CMC/AgNPs species are deposited on St37 steel surface in 15% H2SO4 solution via chemical adsorption mechanism7. This could be possible through covalent bonding between the lone pair of electrons from the oxygen heteroatoms in CMC molecule (Fig. 1) and vacant d-orbital of Fe or direct interaction between AgNPs and the metal surface or both.

3.2.4 Influence of AgNPs on the Protection Efficiency of CMC/Corrosion Inhibition Mechanism by CMC/AgNPs composite We had earlier reported

7, 18

that CMC had moderate corrosion inhibiting effect on

mild steel in 2 M H2SO4 medium. In the report, 500 ppm CMC (the highest concentration studied) had inhibition efficiency of 64.8% at 30 oC from WL method. The inhibition efficiency decreased to 61.0%, 60.9%, and 60.8% on increasing the system temperature to 40, 50, and 60 oC respectively. The variation of η with temperature and ∆ value (standard

free energy of adsorption) indicated that CMC molecules were physically adsorbed onto the metal surface in the acid medium. We pointed out that CMC molecules were predominantly in cationic form in H2SO4 solution and that steel surface acquired net positive charge such 7, 18 that the surface was hydrated with sulphate ions (SO . Adsorption of protonated CMC )

species was on SO layer. But because of the bulkiness of SO , it could not appreciably

replenished the charged steel surface to have allowed adequate amount of the protonated CMC molecules to be adsorbed. On the observed physisorption mechanism, we explained that the replacement process in CMC is somewhat cooperative than random such that the


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unsubstituted and trisubstituted areas are higher than expected

7, 53

. As such, CMC molecule

exists in rod-like and most extended form at low temperatures but overlap, coil up, and entangle at higher temperatures to become thermoreversible gel which could not appreciably cover the metal surface to protect it from corrosive attack. The results we obtained in the present investigation indicate that the η and stability of CMC have been significantly boosted by the incorporation of AgNPs into CMC matrix. For instance, 500 ppm CMC/AgNPs composite at 40 oC has inhibition efficiency of 86. 4% (from WL method) in 15% H2SO4 solution (acid concentration remarkably higher than that of previous study). By increasing the system temperature to 50 and 60 oC, the inhibition efficiency increases to 88.5 and 89.1% respectively. Going by the way η varies with temperature, the mechanism of adsorption of CMC/AgNPs onto the St37 surface is chemisorption. It seems, when the St37 sample was exposed to 15% H2SO4 solution containing CMC/AgNPs composite, the composite species in protonated form first adsorb onto the metal surface through electrostatic interaction between the sulphate ions hydrated surface and the protonated CMC/AgNPs species. On the surface, interactions occur between AgNPs and the Fe atoms owing to the active properties of the AgNPs

20

. This decreases the opposing force on the St37 surface, allowing more of the

CMC/AgNPs molecules to be adsorbed. Lone pairs of electrons from the oxygen heteroatoms in CMC can be transfer to the vacant d-orbital of Fe. As it is known, increase in temperature favours this type of adsorption

18

. The presence of AgNPs in CMC matrix (Fig. 3(b)) may

have also prevented the coiling up of the polymer at higher temperature.

3.3 Adsorption Assessments Information regarding the adsorption of CMC/AgNPs molecules onto St37 surface in 15% H2SO4 solution can be derived from adsorption isotherm. To select the adsorption isotherm with the best fit, the θ values (θ = η/100) were fitted into different adsorption isotherms. The best fit for the studied adsorption process was found for Langmuir adsorption isotherm given as 7: C inh

θ

=

1 K ads

(4)

+ C inh

where Cinh = CMC/AgNPs concentration and Kads = equilibrium constant of the process. Fig. 11 shows the linear graph obtained for St37 in 15% H2SO4 solution containing diverse concentrations of CMC/AgNPs composite by plotting Cinh/θ against Cinh (a) using θ values gotten from different techniques at 25 oC and (b) using θ values gotten for WL at various 13 ACS Paragon Plus Environment


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temperatures. In all cases, the R2 values are close to one (Table 5) implying that the adsorption of CMC/AgNPs molecules onto the metal surface obeys Langmuir isotherm. However, the slopes deviate slightly from unity expected by a perfect Langmuir adsorption equation7. This might be due to the interplay of adsorbed CMC/AgNPs molecules with one another. Langmuir equation had been derived on the assumption that no interaction exist among adsorbed inhibitor molecules. This is false as many studies 7, 29, 50 have shown that large organic molecules having polar groups can interplay by common repulsion or attraction. From the intercepts of the graphs, Kads values were computed and are presented in Table 5. The Kads value was used to calculate the value of the ∆ according to Eq. 5 29: 0 ∆Gads = − RT ln(1 × 106 K ads )

(5)

where, 1 × 10! = amount of water molecules expressed in ppm , " = universal gas constant and # = absolute temperature. ∆ is connected to ∆$ (standard enthalpy) and ∆%

(entropy) of adsorption thus 50: 0

0

0

(6)

∆G ads = ∆H ads − T∆S ads Combining Eqs. 5 and 6 and rearranging gives 0

ln K ads = − ∆H ads + RT

0

∆S ads − ln(1 × 106 ) R

(7)

A plot of ln K ads versus 1/T yields − ∆H 0ads R as the slope and ∆S 0ads R − ln (1 × 106 ) as the intercept from which ∆$ and entropy ∆% can be deduced from. Fig. 12 shows the graph

of ln Kads versus 1/T for St37 in 15% H2SO4 solution containing various concentration of CMC/AgNPs. All the adsorption parameters deduced for this process are given in Table 5. It values are negative and in the range of could be seen, from the results in the table, that ∆ 26.43 – 30.27 kJ/mol. Generally, ∆ ≤ – 20 kJ/mol is linked to physisorption while that ≥ – 40 kJ/mol points to chemisorption 7, 54. In most recent reports 55, 56, ∆ value in the range

of – 28 to – 38 kJ/mol is interpreted to be reflective of mixed adsorption (i.e both physisorption and chemisorption). The values obtained in the present investigation fall into the category of mixed adsorption type. It should, however, be noted that the value of ∆

becomes more negative at elevated temperatures suggesting that chemical adsorption was the dorminant adsorption mechanism at higher temperature. This assertion is also supported by


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the increase in Kads value with rise in temperature. Kads value, most often, is used to describe the stamina of the bond between adsorbent and adsorbate 56. ∆$ value can provide vital information pertaining adsorption mechanism of an inhibitor. For an endothermic adsorption process, ∆$ > 0 reflects chemical adsorption whereas ∆$ < 0 signifies physical adsorption

57

. Similarly, in exothermic adsorption,

∆$ < 40 kJ/mol is indicative of physisorption while ∆$ value approaching 100 kJ/mol

is consistent with chemical adsorption

50

. In the present study, ∆$ value is positive

implying endothermic adsorption. Similar result had been reported by Hoseinzadeh et al.57. value is greater than zero and thus verifies the chemical As could be seen in Table 5, ∆$

adsorption mechanism suggested by the variation of η with temperature (Fig. 10). The calculated value of ∆% is positive and this has been associated with rise in the solvent

energy and greater positive H2O desorption entropy

57

. Noor and Al-Moubaraki

58

had also

associated the positive ∆% value with substitution of more water molecules by one

inhibitor molecule.

3.4

Analysis of Corrosion Kinetics The contribution of temperature to the dissolution of St37 specimen in 15% H2SO4

solution devoid of and with CMC/AgNPs composite has been examined at 25 – 60 oC. Consequently, the corrosion kinetic parameters, namely Ea (activation energy), ∆$ ∗ (enthalpy of activation), and ∆% ∗ (entropy of activation) are calculated from the Arrehenius and Transition State equations respectively given elsewhere 29, 30. A graph of log CR as a function of 1/T and log( "/#) versus 1/T for St37 steel in 15% H2SO4 solution with and without CMC/AgNPs composite is shown in Fig. S2 of the supplementary information. From the slopes of Fig. S2 (a) and (b), . and ∆$∗ values respectively were computed. ∆% ∗ values are deduced from the intercept of the plots in Fig. S2(b). The computed values of ∆$∗ , Ea, and ∆% ∗ are presented in Table 6. Clearly, it is seen in Table 6 that Ea and ∆$∗ values in CMC/AgNPs containing systems are smaller than those in acid solution without composite. Increase in CMC/AgNPs concentration causes further decrease in Ea and ∆$∗ values. This trend further supports the chemisorption mechanism proposed for the adsorption of CMC/AgNPs molecules onto St37 surface in 15% H2SO4 solution. Authors

30, 59

have given similar interpretation in the corrosion literature. As 15 ACS Paragon Plus Environment


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required by the famous thermodynamic equation: . − ∆$∗ = "#, the values of ∆$∗ are smaller compared to those of Ea. The ∆% ∗ values are big and negative. The ∆% ∗ value becomes more negative with increasing composite concentration indicating a decline in the level of perturbation of the system on moving from reactants to the activated complex 7.

3.5

Surface Morphological Examination Fig. 13 presents the SEM pictures and EDS spectra for St37 sample after (a, b)

abrasion, (c, d) immersion in 15% H2SO4 solution, and (e, f) exposure to 15% H2SO4 solution containing 1000 ppm CMC/AgNPs composite for 15 h at 25 oC. The surface in Fig. 13(a) is relatively smooth and grove lines arising from mechanical abrasion is seen. The corresponding EDS spectrum (Fig. 13(b)) displays intense Fe peak. It is certain from Fig. 13(c) that the metal surface was severely destroyed in 15% H2SO4 solution. As could be seen, the surface outer layer is dislodged. The EDS spectrum in Fig. 13(d) reveals that the metal sample deteriorated losing some of its constituent elements to corrosion. For instance, the intensity of Fe peak in Fig. 13(d) diminish in Fig. 13(b) and Si, Mn, and Mg peaks completely dissappear. However, prominent peaks of S and O showed up in Fig. 13(d) and thus provide an experimental evidence to the claim that the surface of St37 steel sample was hydrated with sulphate ions in 15% H2SO4 solution. By comparing Fig. 13(c) with Fig. 13(e) and Fig. 13(d) with Fig. 13(f), it can be said that the availability of CMC/AgNPs composite in 15% H2SO4 environment prevented the corrosion of St37 steel. The surface in Fig. 13(e) is smoother compare to Fig. 13(c) and the Fe peak in Fig. 13(f) prominent than the one in Fig. 13(d). Again, Mn and Si peaks reappear in Fig. 13(f) while S peak becomes less intense. The presence of Ag peak in Fig. 13(f) also provide evidence to the adsorption of CMC/AgNPs molecules on the St37 surface. Fig. 14 shows the 2-dimensional and 3-dimensional AFM photographs of St37 sample after submerged in (a, b) 15% H2SO4 solution, and (c, d) 15% H2SO4 + 1000 ppm CMC/AgNPs composite for 15 h at 25 oC. To provide quantitative information on the surface roughness, the 2D surface was analyzed along the sections shown with green and red lines in Fig. 14(a) and (c). For the surface in Fig. 14(a), Ra is found to be in the range 127.072 nm – 196.45 nm while Rz is between 639.34 nm – 802.000 nm. For the surface exposed to the CMC/AgNPs inhibited solution (Fig. 14(c)), Ra value is in the range of 105.863 nm – 176.856 nm and Rz range between 510.852 nm – 580.900 nm. Ra gives the mean of a set of 16 ACS Paragon Plus Environment

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individual measurement of surface bands and valleys while Rz gives the RMS (root mean square average) of profile height deviations from mean line

47

. Going by this results, St37

steel surface in CMC/AgNPs containing solution is less damaged than in solution devoid of the composite. This again shows the effectiveness of CMC/AgNPs composite in retarding the deterioration of St37 sample in the studied environment. The 3D images in Fig. 14(b) and (d) are in support of the numerical values as a smoother surface is seen in Fig. 14(d) compare to Fig. 14(b). To further verify the adsorption of CMC/AgNPs molecules on St37 steel surface in 15% H2SO4 solution, the FTIR analysis of the film extracted from St37 steel sample immersed in 15% H2SO4 solution containing 1000 ppm CMC/AgNPs composite for 15 h at 25 oC was recorded. The obtained spectrum is compared with the FTIR spectrum of CMC/AgNPs composite (Fig. 15). As could be seen in the figure, the two spectra are similar. However, the peak arising from OH stretching at 3331.8 cm–1 in the CMC/AgNPs spectrum shifted to 3458.4 cm–1 in the film spectrum. This is reflective of the involvement of oxygen heteroatom in the adsorption process.

4. CONCLUSĐON (1) CMC/AgNPs composite has been synthesized using natural honey and characterized with FTIR, EDS, SEM, and UV-Vis. (2) Infusion of AgNPs into CMC menbrance benefits the inhibiting ability and stability of the polymer at elevated temperature. (3) CMC/AgNPs effectively suppressed the St37 oxidation and hydrogen ions reduction reactions at the anodic and cathodic sites respectively. (4) DEIS results reveal that CMC/AgNPs composite is more effective inhibitor for St37 steel in 15% H2SO4 environment at longer immersion time. Weight loss results reveal higher inhibition efficiency at elevated temperatures. (5) CMC/AgNPs molecules are endothermically chemisorbed on St37 surface and the adsorption can be explained by Langmuir adsorption isotherm. (6) SEM and AFM images reveal smoother surface of St37 steel sample exposed to CMC/AgNPs inhibited solution than in uninhibited system. FTIR and EDS results support the adsorption mechanism proposed from experimental data.

5. ACKNOWLEDGEMENTS 17 ACS Paragon Plus Environment


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Moses M. Solomon is thankful to The Scientific and Technological Research Council of Turkey (TÜBITAK) for financing the research work under the TÜBITAK 2216 – Postdoctoral Research Fellowship

(TUBITAK 21514107-115.02-56312) and Duzce

Unıversity, Turkey for making available the facilities.

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(35) Hassan, S. A.; Hadi, A. K. Sudan III as corrosion ınhıbıtor for carbon steel St37-2 in H2SO4 solutıons. International Journal of Recent Scientific Research 2015, 6, 5445-5453 Gerengi, H.; Bereket, G.; Kurtay, M. A Morphological and Electrochemical (36) Comparison of the Corrosion Process of Aluminum Alloys under Simulated Acid Rain Conditions. Journal of Taiwan Institute of Chemical Engineers 2016, 58, 509516. (37) Solomon, M. M.; Umoren, S. A.; Israel, A. U.; Ebenso, E. E. Polypropylene Glycol-Silver Nanoparticle Composites:A Novel Anticorrosion Material for Aluminum in Acid Medium, Journal of Materials Engineering and Performance 2015, 24, 4206–4218 (38) Philip, D.Honey Mediated Green Synthesis of Silver Nanoparticles. Spectrochimica Acta PartA 2010, 75, 1078–1081 (39) Philip, D. Honey Mediated Green Synthesis of Gold Nanoparticles. Spectrochimica Acta Part A 2009, 73, 650–653 (40) White Jr. J. W.; Kushnir, I. The Enzymes of Honey: Examination by Ionexchange Chromatography, Gel Filtration, and Starch-Gel Electrophoresis. Journal of Apicultural Research 1967, 6,69-89 (41) Wang J.; Somasundaran, P. Adsorption and Conformation of Carboxymethyl Cellulose at Solid–Liquid Interfaces using Spectroscopic, AFM and Allied Techniques. Journal of Colloid and Interface Science 2005, 291, 75–83 (42) Smitha, S. L.; Philip, D.; Gopchandran, K. G. Green Synthesis of Gold Nanoparticles Using Cinnamomum zeylanicumLeaf Broth. Spectrochimica Acta Part A 2009, 74, 735–739 (43) Umoren, S. A.; Obot, I. B.; Gasem, Z. M. Green Synthesis and Characterization of Silver Nanoparticles Using Red Apple (Malus domestica) Fruit Extract at Room Temperature. Journal of Materials and Environmental Science 2014, 5, 907-914 (44) Kasthuri, J.; Veerapandian, S.; Rajendiran, N.Biological Synthesis of Silver and Gold Nanoparticles using Apiin as Reducing Agent. Colloids and Surfaces B: Biointerfaces 2009, 68, 55–60 (45) Mohammadi, S.; Pourseyedi, S.; Amini, A. Green Synthesis of Silver Nanoparticles with a Long Lasting Stability using Colloidal Solution of Cowpea Seeds (Vigna sp. L). Journal of Environmental Chemical Engineering 2016,4, 2023– 2032 (46) Gerengi, H.; Mielniczek, M.; Gece, G.; Solomon, M. M. Experimental and Quantum Chemical Evaluation of 8‑Hydroxyquinoline as a Corrosion Inhibitor for 21 ACS Paragon Plus Environment


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Table 1: Electrochemical impedance parameters for St37 steel in 15% H2SO4 solution in the absence and presence of different concentrations of CMC/AgNPs composite at 25 oC.

Concentration (ppm) 0 50 100 500 750 1000

/0 (Ω234 )

0.6528 0.7283 0.8704 0.7803 0.7502 0.9552

56789 (: 0 234 ) ; 4

1.515 2.075 2.642 2.208 1.927 2.800

(Ω234 ) (:; 04 =@4) 41.99 199.80 249.30 333.80 375.60 395.00

6.573 4.647 7.330 5.759 5.805 13.620

Silver Nanoparticles Composite: Synthesis

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