Application of Stabilized Silver Nanoparticles as Thin Films as Corrosion Inhibitors for Carbon Steel Alloy in 1 M Hydrochloric Acid

Nanometer scaled materials have attracted tremendous interest as corrosion protective films due to their high ability to form selfassembled films on the metal surfaces. It is well known that the silver nanoparticles have higher reactivity towards aqueous acidic solution. The present work aims to prepare coated silver nanoparticles to protect carbon steel alloys from aqueous acidic corrosive media. In this respect, Ag nanoparticles colloid solutions were produced through reducing AgNO 3 separately with trisodium citrate in an aqueous solution or in the presence of stabilizer such as poly(ethylene glycol)thiol and poly(vinyl pyrrolidone). The morphology of themodified silver nanoparticles was investigated by TEM andDLS. UV-Vis absorption spectrumwas used to study the effect of HCl on the stability of the dispersed silver nanoparticles. The corrosion inhibition efficiency of the poly (ethylene glycol)thiol, the self-assembled monolayers of Ag nanoparticles, was determined by polarization method and electrochemical impedance spectroscopy (EIS). Polarization curves indicated that the coated silver poly (ethylene glycol)thiol acted as a mixed type inhibitor. The data of inhibition efficiencies obtained measured by polarization measurements are in good agreement with those obtained with electrochemical impedance.


Introduction
Due to its particular properties (excellent thermal and electrical conductivity, malleability, ductility, etc.), iron is widely used in industrial production and many other areas which make it very important to research corrosion and protection of iron and its alloy.The high anticorrosion materials for iron are called self-assembled materials (SAMs).The stable SAMs [1] form can prevent corrosive ions from transferring to the metal surface and can protect the metal from corrosion effectively [2,3].There are a lot of materials that have been used to make SAMs as corrosion inhibitors on the copper and iron [4][5][6].Shimura and Aramaki [7][8][9] succeeded to protect iron from corrosion by covering it with ultrathin films.Feng and coworkers investigated self-assembly technique to enhance the iron resistance against environmental corrosion [10,11].
In the past few years, many efforts have been focused on the fabrication of Ag nanoparticles due to their unique chemical and physical properties [12].At present, there are many techniques available for the synthesis of metal nanoparticles, which includes gas reduction process [13], precursor pyrolysis [14], microwave plasma synthesis, hydrodynamic cavitations and liquid chemical reduction [15,16], and laser ablation [17].However, the synthesis of well-dispersed and active Ag nanoparticles was difficult, partially because the surfactant molecules were ineffectively adsorbed on the surface of nanoparticles, resulting in the agglomeration and oxidation propensity of nanoparticles.It was previously [18] reported that the dispersion of Ag nanoparticles can be controlled in the presence of modified organic materials based on polymer and thiol derivatives.On the other hand, organic compounds containing heteroatoms (e.g., N, O, and S atoms) could be used as stabilizer for Ag nanoparticles [19,20].In this respect, this work aims to prepare the highly dispersed and stabilized Ag nanoparticles in both aqueous and acidic media.The corrosion inhibition efficiency of iron was measured in the presence of silver nanoparticles modified with poly (ethylene thiol) aqueous acidic solutions using polarization

Experimental
2.1.Materials.All reagents used in this experiment were analytical grade chemicals.Silver nitrate, trisodium citrate, and poly(vinyl pyrrolidone), PVP molecular weight of 40000 g mol −1 , are delivered from Aldrich Chemicals Co. and used to prepare citrate stabilized silver nanoparticles.P-Toluenesulfonyl chloride, poly(ethylene glycol) monomethyl ether having molecular weight of 550 g/mol (MPEG-550), triethylamine, and thiourea (purchased from Aldrich Chemicals Co.) were used without purification.

Synthesis of Poly(oxyethylene)thiol Monomethyl Ether. P-
Toluenesulfonyl chloride (0.05 mol, 9.82 g) in 80 mL of methylene chloride was added dropwise to a mixture of 0.05 mol (27.5 g) of poly(ethylene glycol) monomethyl ether having molecular weight of 550 g/mol (MPEG-550) and 0.05 mol (5.06 g, 3.67 mL) of triethylamine over 1 hour at 0 ∘ C. The mixture was then stirred overnight at room temperature.A white triethylamine hydrochloride precipitate was filtered off and washed with 50 mL of methylene chloride.The methylene chloride was removed under reduced pressure to leave pale yellow oil, which was purified by flash chromatography on silica using dichloromethane and acetonitrile (3 : 1, v/v).Solvents were evaporated to give poly(oxyethylene) tosylate monomethyl ether: (65% yield), colorless oil in both cases.
Poly(oxyethylene) tosylate monomethyl ether (0.036 mol) and thiourea (3.1 g) were mixed with 100 mL ofethanol and refluxed for 24 hrs.The mixture was cooled and mixed with solution of (4.1 g) NaOH, water (5 mL), and ethanol (50 mL).The mixture was refluxed for 3 hrs under nitrogen atmosphere.The reaction mixture was cooled and neutralized with 0.1 M of HCl.Solvents evaporated and the residue was dissolved in ether and dried over Na 2 SO 4 .The ether evaporated to give pale yellow oil of poly(oxyethylene) thiol monomethyl ether (PEGSH; 86% yield).

Synthesis of Silver Nanoparticles.
Citrate-stabilized silver nanoparticles were prepared by dissolving 9 mg of silver nitrate in 50 mL of water and bringing it to boiling.A solution of 1% trisodium citrate (1 mL) was added under vigorous stirring.The solution was kept at a boil for 1 h and then allowed to cool at room temperature.The silver nanoparticles were purified by ultracentrifugation (30 min at 30 000 rpm), followed by redispersion in water.The typical yield of citratestabilized silver nanoparticles was around 65% (with respect to silver).
Silver coated nanoparticles were synthesized by adding 1 g of PVP and 0.01 g of PEGSH with silver nitrate solution.The reaction was completed as reported for citrate stabilized silver nanoparticles.

Characterization of AgNP.
Transmission electron microscopy (TEM) micrographs are recorded using a JEOL JEM-2100F.A few drops of silver nanoparticle solution were diluted into 1 mL of ethanol, and the resulting ethanol solution was placed onto a carbon coated copper grid and allowed to evaporate.
Samples for dynamic light scattering (DLS) were prepared by diluting several drops of the silver nanoparticle solution into 2 mL of water under vigorous stirring.The DLS measurements were performed on a Brookhaven Instruments system with a 514.5 nm argon ion laser (model 85 Lexel Laser) as the light source.
Ultraviolet visible (UV-Vis) absorption spectra were obtained with a Techcomp UV2300 spectrophotometer.

Electrochemical Measurements.
EIS impedance measurements were performed using a Solartron 1470E system (potentiostat/galvanostat) with Solartron 1455A as frequency response analyzer.Multistate software was used to run the tests and collect and evaluate the experimental data.Impedance tests were performed in 1 M HCl with and without inhibitor.Ag/AgCl electrode was used as the reference and Pt electrode was used as the counter electrode.The working electrode was prepared from carbon steel with dimensions 1 mm (width) × 10 mm (length).The impedance data were analyzed and fitted with the simulation Z View 3.3c, equivalent circuit software.

Results and Discussion
The fundamental application problem of silver NPs is connected with the sufficient stability of their dispersion allowing the prevention of the aggregation process because the generation of spacious aggregates leads to a loss of the silver activity [21].Therefore various surfactants and polymers are commonly applied to stabilize these metal colloids [22].In the previous work [23], a simple method was applied to prepare coated silver nanoparticles based on modified 6propylene-4-nonyl-phenol ethoxy maleate, Noigen R-N10, polymerizable surfactant which was used as dispersing agent for silver nanoparticles.The size and the morphology of the nanoparticles have been examined by conventional and highresolution transmission electron microscopy.The prepared silver nanoparticles have size ranging from 25 to 80 nm.The prepared silver nanoparticles have been employed as corrosion inhibitor for carbon steel in 1 M HCl solution.In the present paper, the work extended to synthesize more stable AgNP based on more stabilized citrate AgNPs.In this respect, poly(ethylene glycol) monomethyl ether was modified to thiol, which can be self-assembled on AgNPs in the presence of PVP as stabilizer.The conversion of MPEG-550 to thiol MPEG-SH was illustrated in Scheme 1.
The chemical structure of the thiol derivatives of PEG was illustrated using 1 HNMR and FTIR spectra.In this respect, the 1 HNMR and FTIR spectra of PEG-SH having molecular weight 750 were selected and represented in Figures 1 and  2, respectively. 1H NMR (CDCl 3 ) illustrated different peaks at  3.76-3.61(m, 14 H, OCH 2 of PEG), 2.73-2.68(m, 2 H, new -CH 2 S), and 1.62 (t, 1 H, new SH group).These peaks indicated the formation of PEG-SH as illustrated in Scheme 3. FTIR spectrum (Figure 2) indicated the formation of thiol Isothiouronium salt Scheme 1: Synthesis of MPEG-SH.derivatives which is confirmed from the appearance of new SH stretching band at 2550 cm −1 .This peak did not appear in spectra of MPEG-550.Considering the adsorption of MPEG-SH and PVP onto AgNPs, Scheme 4 illustrated the formation of self-assembled films on the silver nanoparticles.This model, so-called tail mode, in which the polymer molecule interacts with the solid particle surface only via the end of the polymer chain (Scheme 4), represents a typical interaction mode for the polymer of a high molecular weight having a long linear polymer chain that is not branched.It was a preferable way here using polymer to obtain stable nanoparticles, such as PVP and PEGSH.These polymers have certain polymerization degree and chain length, which can protect the nanoparticles from aggregation during the reduction of silver nitrate [24].The PVP group are bonded stronger on the silver surface through the nitrogen atom in their molecule, while MPEGs are weakly bonded via the oxygen atom.It was previously reported that [25,26] the lower molecular weight polymers such as polyvinyl alcohol and vinyl acetate copolymer with suitable ionic surfactants can synergistically enhance the stability of oil/water emulsion, while longer chain polymers induce bridging flocculation.The polymer is preadsorbed at the colloidal interface, and polymer-surfactant interaction leads to dramatic changes in repulsive forces due to conformational changes of polymers at the interface, enhancing the stability of the colloid considerably [26].In the present work, the studied polymers from the MPEG group accomplish a marginal influence on the aggregation stability of the aqueous silver NP dispersion, possibly excluding MPEG 550.The MPEG 550 thiol shows slightly higher stabilizing effect among the tested MPEG polymers which refereed to the lower molecular weight and therefore can be generated a more compact surface layer on the silver NP surface.In this respect, the optimum concentration of MPEG 550 thiol was (1% w/w).The three kinds of silver nanoparticles (Ag NPs) used in this study were citrate AgNP, MPEGSH coated AgNP, and PVP-PEGSH.
A set of UV-Vis absorption spectra of the dispersed nanoparticles in water is presented in Figure 3.As can be seen from this figure, these nanoparticles are relatively monodispersed in size.The preservation of the nanodimensional character of the system modified by PEGSH, with limited interparticle interactions, is also evident from the UV/Vis absorption spectra (Figure 3) exhibiting relatively intense surface plasmon peak at 405 nm.The particle size of the prepared AgNPs was illustrated by TEM.In this respect, TEM photos were represented in Figure 4.The average particle size of the AgNPs changed from 12 nm to 25 nm and 65 nm by coating AgNPs with PEGSH and PVP/PEGSH, respectively.
The particle size distribution of Ag/PEGSH-550 in the presence of 1 M HCl was estimated using dynamic light scattering (DLS) to evaluate the variation in particle size distribution up to 1 week.Figure 5 represented that the particle size of the AgNP coated with PEGSH-550 ranged between 5 and 20 nm.The particle size was changed to be uniform between 8 and 12 nm after 24 hrs of exposure to 1MHCl (Figure 5(b)).The particle size distribution increased after exposure to 1 M HCl at 7 days (Figure 5(c)).The particle size diameter increased to be 15-30 nm.The higher stability of silver nanoparticles in 1 M HCl recommended application as corrosion inhibitors in acidic aqueous solution.
where  ∘ corr and  corr are the corrosion current densities for carbon steel electrode in the uninhibited and inhibited solutions, respectively.IE% was calculated and quoted in Table 1.It can be seen from the data presented in Table 1 that inhibition efficiency increases with increasing inhibitor concentration.The results may be attributed to more adsorption of inhibitor molecules on carbon steel surface providing wider surface coverage.It is clear from the obtained corrosion data that the Tafel lines shifted towards more negative and more positive potentials during the anodic and cathodic processes,  respectively, relative to the blank curve.It can be concluded that Ag-PVP-PEGSH acted as a mixed type inhibitor.[31,32] as evident from the semicircular appearance shown in Figures 7 and 8.The deviation of semicircles from perfect circular shape is often referred to as the frequency dispersion of interfacial impedance [32][33][34].This behavior is usually attributed to the inhomogeneity of the metal surface arising from surface roughness or interfacial phenomena [35,36].All experimental spectra were fitted with an appropriate equivalent circuit to find the parameters, which describe and are consistent with the experimental data.parallel to Rct.The impedance of a CPE is described by the expression [37]

Electrochemical Impedance Spectroscopy (EIS)
where A is the CPE constant,  is the angular frequency ( = 2f, where  is the AC frequency), and j is the imaginary unit and n is CPE exponent which can be used as a gauge of the heterogeneity or roughness of the surface [38].The inhibition efficiency (IE) is calculated from where  * ct and  ct are the charge transfer resistances with and without inhibitors, respectively.IE% was calculated and presented in Table 1.It can be concluded that the  * ct values increased with inhibitor concentration increment.This may be due to any increase in the surface coverage by the inhibitor, which led to an increase in the inhibition efficiency.The inhibition efficiencies calculated from EIS are in good agreement with those obtained from potentiodynamic polarization curves.

Conclusions
New highly dispersed silver coated nanoparticles were prepared and the results of TEM and DLS indicated that nanoparticles are relatively monodispersed in size.The average particle size of the AgNPs changed from 12 nm to 25 nm and 65 nm by coating AgNPs with PEGSH and PVP/PEGSH, respectively.Polarization data reveal that Ag-poly(ethylene glycol) thiol behaves as a mixed type inhibitor.The inhibition efficiencies obtained by polarization measurements are in good agreement with those obtained with electrochemical impedance spectroscopy for increasing the efficiency of the inhibitor with inhibitor concentration.

7 Scheme 2 :
Scheme 2: Reduction of AgNO 3 in the presence of MPEG-SH and sodium citrate solution.

3. 1 .Scheme 4 :
Scheme 4: Drafts of possible modes of how polymers can be bonded on the silver NP surface.

Figure 6 :
Figure 6: Polarization curves for carbon steel in 1 M HCl solution containing different inhibitor concentrations.

3. 2 .
Polarization Measurements.The influence of various concentrations (0.075-0.22 mmol/L) of AgNP, MPEGSH coated AgNP, and PVP-PEGSH on the polarization behavior of carbon steel (CS) in 1 M HCl solution was shown in Figure 6.The presence of Ag-PVP-PEGSH shifts the anodic curves to more positive potentials and the cathodic curves to more negative potentials and lowers the values of current densities.The data reveals that both anodic and cathodic reactions are drastically inhibited.The results can be attributed to the adsorption of Ag-PVP-PEGSH on both anodic and cathodic reactive sites, which inhibited the anodic and cathodic reactions of carbon steel corrosion.The inhibition efficiencies for different inhibitor concentrations were calculated from the following equation [27-30]:

Figure 7 :
Figure 7: Nyquist diagram for carbon steel in 1 M HCl solution.Open square and closed circle show the experimental and fitted data.

Figures 7 and 8
show the Nyquist diagrams of CS in 1 M HCl without and with different inhibitor concentrations (0.075-0.22 mmol/L) of Ag-PVP-PEGSH, respectively.All the impedance spectra exhibit one single semicircle.The diameter of semicircle increases with the increase of Ag-PVP-PEGSH concentration.It is clear that the corrosion of CS is controlled by the charge transfer and the addition of Ag-PVP-PEGSH to HCl solution does not change the mechanism of CS dissolution

Figure 8 :Figure 9 :
Figure 8: Nyquist diagram for carbon steel in 1 M HCl containing different concentration of inhibitors.

Table 1 :
Inhibition efficiency values for carbon steel in 1 M HCl with different concentrations of cationic calculated by polarization method and EIS method.