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Polyurethane based tri-block-copolymers as corrosion inhibitors for mild steel in 0.5 M H2SO4 Sudershan Kumar, Hemlata Vashisht, Lukman O. Olasunkanmi, Indra Bahadur, Hemant Verma, Madhusudan Goyal, Gurmeet Singh, and Eno E. Ebenso Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03928 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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Industrial & Engineering Chemistry Research

Polyurethane based tri-block-copolymers as corrosion inhibitors for mild steel in 0.5 M H2SO4

Sudershan Kumar,a Hemlata Vashisht,b Lukman O. Olasunkanmi,c,d Indra Bahadur,c Hemant Verma,a Madhusudan Goyal,e Gurmeet Singh,e Eno E. Ebensoc, * a

Department of Chemistry, Hindu College, University of Delhi, Delhi-110007 Department of Chemistry, Kirrori Mal, University of Delhi, Delhi-110007 c Department of Chemistry, North-West University (Mafikeng Campus) and Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa d Department of Chemistry, Obafemi Awolowo University, Ile-Ife 220005, Nigeria. e Department of Chemistry, University of Delhi, Delhi-110007 b

*Corresponding author- e-mail: [email protected] (Eno E. Ebenso) Tel: +27 183892050/2051; Fax: +27183892052

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Abstract Two polyurethane based tri-block-copolymers namely, poly(N-isopropylacrylamide)-bpolyurethane-b-poly(N-isopropylacrylamide) (PIA-PU-PIA) and poly(tert-butylacrylate)-bpolyurethane-b-poly(tert-butylacrylate) (PtBA-PU-PtBA) were synthesized via atom transfer radical polymerization (ATRP) mechanism and characterized. The inhibition potentials of the polymers on mild steel corrosion in 0.5 M H2SO4 were studied using electrochemical, scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The results obtained from potentiodynamic polarization studies showed that the two polymers are mixedtype inhibitors and exhibit passivating activities. The adsorption of PIA-PU-PIA on mild steel surface obeys the Langmuir adsorption isotherm, while that of PtBA-PU-PtBA obeys the ElAwady isotherm. The two polymers adsorb on the mild steel surface via competitive physisorption and chemisorption mechanisms. Both SEM and AFM analyses confirmed the formation of protective layers of the inhibitor molecules on the steel surface. The results btained showed that PIA-PU-PIA exhibited higher inhibition performance than PtBA-PU-PtBA and the trend was corroborated by the results obtained from quantum chemical calculations.

Keywords: Mild steel, Polarization, Polyurethane based copolymers, SEM, AFM, Quantum chemical calculations.

1. Introduction 2


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Corrosion is the destructive attack on a metal or metal alloy by chemical or electrochemical reaction with its environment.1 Corrosion of metal is also considered as extractive metallurgy in reverse.2 Mild steel is an alloy of iron (Fe) with various industrial and structural applicatons. It is relatively cheap and possesses high mechanical strength. Despite its quality properties and wide applications, mild steel is highly susceptible to corrosion in various aggressive environments and as a result, the study of its corrosion/corrosion control in acidic medium is of great economic importance.3 Among several methods of corrosion control, such as cathodic protection, anodic protection, coating & alloying etc., the use of corrosion inhibitors is often considered as one of the cheapest, most effective and practical method of corrosion prevention. Inhibitors can also be added in situ during industrial processes. As a result, corrosion inhibitors are widely used in industries to prevent or reduce the corrosion rate of metallic materials in aggressive environment.4-6 Organic compounds that contain heteroatoms such as nitrogen, oxygen, sulphur and/or electron rich aromatic systems are generally known to exhibit good anticorrosion properties.7-10 Organic compounds such as alkaloids,11 thiourea,12 benzene-thiol derivatives,13 imidazole/azo derivatives,14 quinoline derivatives,15 quarternary ammonium salts,16 aldehydes,17 and Schiff bases18 are among the widely used corrosion inhibitors. Research on the use of polymers as corrosion inhibitors has gained huge attentions in recent years. This is because polymers are generally characterized with large surface area, intrinsic stability and cost effectiveness.19 Many polymers possess atoms and/or functional groups that aid formation of complexes with metal ions or adsorption on metal surface. Metal complexes of polymers are capable of blocking the active sites on metal surface thereby protecting the metal from making direct contact with corrosive environment. The inhibitive power of polymers is often structurally related to the presence of cyclic rings with π-electrons and heteroatoms, which are the major sites for adsorption.20-22 A review of literature has revealed that a number of studies have been conducted in the last decade to demonstrate the potential applications of both conducting and thermoplastic polymers as protective materials for metal corrosion, either as coating formulations or corrosion inhibitors.23-26 Jianguo et al.27 have successfully applied polyvinylpyrrolidone and polyethylenimine as corrosion inhibitors for low carbon steel (LCS) in phosphoric acid. The compounds inhibit both anodic and cathodic 3



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corrosion reactions with predominant anodic inhibition activities. Abd El-Ghaffar et al.28 have reported a series of polymers including poly(o-aminophenol), poly(o-aminothiophenol), poly(manisidine) and polyaniline as efficient corrosion inhibitors for steel. Prakash et al.29 have also demonstrated possible applications of some conducting and electroactive polymer blends, which are composites of polyaniline (PAni) and poly(diallydimethylammoniumchloride) (PDDMAC) as corrosion inhibitors for pure iron in 1 M HCl. The compounds exhibited mixed-type inhibition and a maximum inhibition efficiency of 59 % was reported from polarization measurements by using 150 ppm of the composite that contains 25 % of PDDMAC in 100 ml of PDDMAC/PAni mixture. In the study reported by Bhandari et al.,30 copolymers of aniline and 2-isopropylaniline exhibited about 60 – 80 % inhibition efficiency for the corrosion of iron in 1 N HCl depending on the molar ratio of poly(2-isopropylaniline) in the copolymer feed. In a more recent study,19 a series of sulfonated polyurethane ionomers were reported to exhibit more than 90 % inhibition efficiency for the corrosion of mild steel in 0.5 M H2SO4 and the inhibition potential varies with the ratio of sulfonating agent employed. Most conducting polymers that have been reported in literature exhibit relatively low corrosion inhibition potentials due to their brittleness and poor film forming capability. On the other hand, thermoplastic polymers tend to cover wider surface area and display better adhesion property leading to enhanced corrosion inhibition efficiency. Polyurethanes belong to one of the most important class of thermoplastic elastomers with a wide range of applications as adhesives, coating materials, foams and packaging materials. Polyurethanes also have some biomedical applications due to their excellent biocompatibility.19,31,32 One major factor that limits the use of polyurethane as corrosion inhibitors for metals in aqueous environments is their low solubility in aqueous solutions. To overcome this limiting factor, polymers are often chemically modified by incorporating some hydrophilic units into the polymer chain to enhance their solubility in aqueous systems. Banerjee et al.19 have demonstrated the use of sulfonate functional group to improve the solubility and conductivity of polyurethanes, which also enhanced their corrosion inhibition efficiency.19,33 Introduction of amphiphilic moieties into the polymer chain also appears be a promising option in addressing the solubility problem of polymeric corrosion inhibitors.

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The objective of the present work is to investigate the corrosion inhibition properties of two newly synthesized polyurethane based amphiphilic tri-block copolymers, namely poly(Nisopropylacrylamide)-b-polyurethane-b-poly(N-isopropylacrylamide)

(PIA-PU-PIA)

and

poly(tert-butylacrylate)-b-polyurethane-b-poly(tert-butylacrylate) (PtBA-PU-PtBA) on mild steel in 0.5 M H2SO4. The amphiphilic tri-block copolymers were synthesized using the atom transfer radical polymerization (ATRP) method, which is one of the simplest and well employed methods of polymer synthesis because it does not require harsh conditions and it is adaptable to most monomers. The synthesized polymers were characterized using proton-nuclear magnetic resonance (1H-NMR) spectroscopy and gel permission chromatography (GPC) techniques. The corrosion inhibition potentials and adsorption characteristics of the synthesized PIA-PU-PIA and PtBA-PU-PtBA on mild steel in 0.5 M H2SO4 were studied using electrochemical methods, quantum chemical calculations, scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques.

2. Experimental 2.1 Materials All the chemicals and reagents used for the synthesis of the studied polymers were obtained commercially. The sources and purity of the materials listed somewhere else.34 2.2. Synthesis of PIA-PU-PIA and PtBA-PU-PtBA Sytheses of PIA-PU-PIA and PtBA-PU-PtBA were achieved according to the procedure reported in our recent work34 with little modifications. N-isopropylacrylamide (NIPAAm) was used as the precursor for the synthesis of PIA-PU-PIA, while tert-butylacrylate (tBA) was a starting material in the synthesis of PtBA-PU-PtBA. The overall synthetic route is shown in Figure 1. 2.3. Characterization of the synthesized PIA-PU-PIA and PtBA-PU-PtBA The synthesized polymers were characterized using similar steps reported in our recent work. 34

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2.4. Electrodes and aggressive solutions used for corrosion tests Corrosion tests were carried out on the mild steel rod with the chemical compositions (wt. %) of C (0.15), Si (0.31), S (0.025), P (0.025), Mn (1.02) and Fe (balance). The mild steel working electrodes used for electrochemical corrosion tests were fabricated and pre-treated as previously reported.34 Test solutions of 0.5 M H2SO4 and different concentrations of the synthesized polymers in the acid were prepared using doubly distilled water.

2.5. Electrochemical measurements Electrochemical meaurements were conducted in a three-electrode glass cell comprising mild steel as working electrode (WE), platinum rod as counter electrode (CE) and saturated calomel electrode (SCE) as the reference electrode (RE). Details of electrochemical set-up, equipment and measurements can be found in our recent paper.34 Tafel polarization curves were obtained by sweeping the potential by ±100 mV away from the corrosion potential (Ecorr) at a scan rate of 1 mVs-1. The measurements were carried out in the temperature range of 298-328 K. Potentiostatic polarization curves were recorded at 298 K by sweeping the potential by 2000 mV away from the Ecorr at a scan rate of 1 mVs-1. Impedance spectra were recorded at the Ecorr in the frequency range 10 kHz to 1 Hz with the ac voltage amplitude of 5 mV peak-to-peak. Since the impedance of mild steel in the presence of inhibitor, the data is made to fit with the corresponding impedance values of an equivalent circuit. The process is performed using the software ZSimpWin Version 3.21.

2.6. Surface morphological studies Freshly grounded and polished mild steel specimens were immersed in 0.5 M H2SO4 without and with 1600 ppm each of PIA-PU-PIA and PtBA-PU-PtBA for 24 h at room temperature. The specimens were retrieved after 24 h, dried at room temperature and used for the SEM and AFM analyses. Details of equipment, operating mode and procedures for the surface analyses are reported somewhere else.34

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2.7. Quantum chemical calculations One part of the symmetric monomeric unit was adopted as the representative structure for each of the studied polymers. The representative molecular structures were modeled using GaussView 5.0 from Gaussian Inc. in order to obtain the initial geometries. The initial structures were refined by full optimization at HF/3-21 level of theory, after which higher level calculations were carried out using the density functional theory approach at the B3LYP/6-31G(d) level of theory.35,36 All the calculations were carried out in the gas phase with the aid of Gaussian 09 software suite.37 The frontier molecular orbital (FMO) energies, dipole moment and molar volume were obtained for the optimized structures at B3LYP/6-31G(d). The energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), the energy gap, EL-H (EL-H = ELUMO - EHOMO) and the global electronegativity, χ (χ = -½(EHOMO + ELUMO) were obtained and recorded.

3. Results and discussion 3.1. Characterization of the synthesized polymers The details of the synthesis of the studied polymers are already presented under the experimental section. The molecular weight distributions of the resultant polymers were calculated using the GPC technique and the number average molecular weights,Mn was found to be 10,400 and 13,800 for PIA-PU-PIA and PtBA-PU-PtBA respectively, while their corresponding polydispersity indices (PDIs) 1.6 and 1.38 respectively. The 1H-NMR spectra of the synthesized Br-PU-Br precursor and the final PIA-PU-PIA tri-block copolymers are presented in Figure 2. In the PIA-PU-PIA tri-block copolymer, the -CH protons being at two different chemical environments resonated at 3.10 ppm (close to –CH2) and 3.80 ppm (close to –NH). The –CH2 protons of the PIA unit resonated along with the methyl protons of TDI appearing at 1.90-2.30 ppm. The peaks for the –NH of the amide group and the – CH3 protons present in the PIA unit appeared at 6.78-6.94 ppm and 0.83-1.28 ppm respectively. The 1H-NMR spectra of the synthesized Br-PEOPU-Br precussor and the final PtBA-PU-PtBA tri-block copolymers are shown in Figure 3. The Br-PEOPU-Br showed the peaks for the phenyl and methyl protons present in the TDI unit at 7.03-7.51 ppm and 2.03-2.17 ppm respectively. 7



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The -N-H and the terminal –CH3 protons of 2-methyl-2-bromopropionate groups present in the PEG-PU unit appeared at 8.73-9.56 ppm and 1.83-1.86 ppm respectively. The peaks corresponding to -OCH2 protons of PEG resonated at 3.17-3.25 ppm. The protons of the -OCH2 groups present in HMB and the -OCH2 groups of PEG attached to the urethane group appeared at 4.31-4.34 ppm and 4.06-4.16 ppm respectively. In the 1H-NMR spectrum of the PtBA-PU-PtBA, the –OCH3 protons of PtBA appeared at 1.5 ppm while the –CH2–CH protons resonated at 1.201.36 and 2.37-2.44. The -CH3 and the phenyl protons of the TDI unit (in PtBA-PU-PtBA) appeared at 2.21 ppm and 6.85-7.50 ppm respectively. As the concentrations of -C(CH3)2 groups and -OCH2 groups adjacent to the carboxyl functional group of HMB are low in the tri-block copolymers, the peaks corresponding to these protons were not observed in the 1H-NMR spectrum of the tri-block copolymers. The presence of the peaks corresponding to PU and PIA (in PIA-PU-PIA blocks); and PU and PtBA (in PtBAPU-PtBA blocks) are clear evidences of the formation of the tri-block copolymers.27

3.2 Variation of open circuit potential (OCP) with immersion time It is important to allow an electrochemical system to attain a steady state condition before performing any electrochemical measurements. The mild steel electrode was immersed in each aggressive solution for the period of 4 h in order to ensure a stable OCP, after which the system is electrochemicall perturbed. Both the potentiodynamic polarization and impedance measurements were carried out after OCP stabilization. The plot of OCP against time for the first 2000 s is shown in Figure 4 for the mild steel electrode immersed in the aggressive solutions containing 1600 ppm of each inhibitor in 1 M HCl. The initial OCP was -0.505 V (vs SCE) for PtBA-PU-PtBA and -0.691 V (vs SCE) for PIA-PU-PIA and the values of OCP become almost constant around -0.506 V for PtBA-PU-PtBA and -0.536V for PIA-PU-PIA after 300 s. This revealed that the period of 4 h is more than enough for the attainment of OCP steady state condition. The OCP values varied from a less anodic value to a more anodic/noble potential with increasing immersion time. This kind of variation in OCP values in the presence of the inhibitors can be attributed to the adsorption of the inhibitor molecules on the metal surface, which makes the electrode surface to be more noble and relatively resistant to corrosion.

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3.3. Tafel polarization studies Tafel polarization curves for mild steel in 0.5 M H2SO4 solution without and with different concentrations of the inhibitors, at different temperatures are shown in Figures 5 and 6 for PIAPU-PIA and PtBA-PU-PtBA respectively. Tafel curve for mild steel in 0.5 M H2SO4 in the absence of the inhibitors shows intial sharp increase in both anodic and cathodic current densities attributed to the anodic partial reaction involving the early dissolution of mild steel and cathodic hydrogen reduction reaction. The later decrease and relatively constant current densities as reflected by the near horizontal shape of the polarization curves over the applied potential range could be due to the formation of corrosion products around the active sites on the mild steel.38 The polarization curves in the presence of the inhibitors show that both PIA-PU-PIA and PtBAPU-PtBA reduce the cathodic corrosion current density much more than the anodic one, especially at lower temperatures. The anodic arms of the polarization curves in the presence of both PIA-PU-PIA and PtBA-PU-PtBA exhibit initial sharp increase in corrosion current density, characteristic of early anodic mild steel dissolution. The relatively constant or slow increase in anodic current densities observed as a plateau after about 100 mV above Ecorr in the presence of the inhibitors could be as a result of adsorbed inhibitor molecules or intermediate products such as Fe/Inhibitor complexes.38 The limiting current plateau observed at the anodic arms of the polarization curves in the presence of PIA-PU-PIA appeared to be more pronounced at 308 and 318 K compared to 298 K, and disappeared at 328 K. The same feature of the curves observed in the presence of PtBA-PU-PtBA seemed to be more pronounced at 298 K, and only apparent at 1600 ppm concentration at 308 and 318 K, but disappeared at 328 K. This suggests that the formation/adsorption of intermediate corrosion products or Fe/inhibitor complexes on the anodic sites of the mild steel surface is affected by change is temperature as well as change in concentration of the inhibitors. Electrochemical kinetic parameters such as corrosion current density (Icorr), corrosion potential (Ecorr) and anodic and cathodic Tafel slopes (ba and bc respectively) were obtained by extrapolating the linear Tafel regions to the Ecorr. The values of corrosion current density in the absence (Icorr) and presence (I’corr) of the inhibitors were used to determine the inhibition efficiency (IE) as according to the equation:

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IE % =

I corr − I ' corr × 100 I corr

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(1)

The results of electrochemical kinetic parameters, calculated IE and surface coverage (θ = IE/100) values at 298 K and 328 K are tabulated in Table 1. The results for the entire temperature range (298, 308, 318 and 328 K) for both inhibitors are contained in Table S1. It is difficult to generalize the trend of the change in Ecorr with change in concentration of the inhibitors and/or change in temperature. However, the Ecorr of the mild steel in 0.5 M blank acid system becomes more negative with increase in temperature, indicating increase aggressiveness of the electrochemical reaction with increasing temperature. The magnitude of the relative shift in Ecorr after the addition of an inhibitor compared to the blank is often used to characterize an inhibitor an anodic, cathodic or mixed-type. A shift in Ecorr > 85 mV is generally attributed to an inhibitor of anodic or cathodic mitigating effect, while a shift in Ecorr < 85 mV implies that the inhibitor is of mixed effect, that is, it inhibits both anodic metal dissolution and cathodic reduction reaction.39,40 The results in Tables 1 and S1 showed that for the studied tri-block copolymers, the shifts in Ecorr over the whole concentration and temperature ranges are less than 85 mV, which implies that both PIA-PU-PIA and PtBA-PU-PtBA are mixed-type inhibitors. The values of the anodic (ba) and cathodic (bc) Tafel slopes change with change in concentration and temperature. This suggests that the inhibitive actions of the studied compounds are both concentration and temperature dependent. The change in ba values with change in concentration are generally more pronounced than change in bc values. This suggests that the inhibitors reduce the rate of the cathodic partial reaction by blocking the active sites (on the mild steel surface) involved in the cathodic hydrogen evolution without altering the mechanism of the cathodic reaction, while on the other hand, the anodic mild steel dissolution might have witnessed some change in mechanism as informed by the large change in ba values, which is also inferrable from the appearance of the anodic limiting current plateau in the polarization curves. The values of icorr in Table 1 (and Table S1) decrease with increasing concentration of both PIA-PU-PIA and PtBA-PU-PtBA. In other words, the IE increases with increase in concentration of the studied inhibitors. This implies that as the concentration of the studied polymers increases, more molecules of the inhibitors or Fe/inhibitor complexes adsorb on the mild steel surface thereby increasing the surface coverage and also the inhibition efficiency. This 10


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observation is a direct indication of the corrosion inhibition characteristics of the studied polymers. At a particular concentration of the inhibitors, the inhibition efficiency decreases with increase in temperature. This may be attributed to increase in the solubility of the protective films and/or any reaction products precipitated on the surface of the mild steel that may otherwise inhibit the corrosion reaction rate. This also suggests possible desorption of some of the adsorbed inhibitor molecules from the mild steel surface at higher temperatures.41 A comparason of IE values of both PIA-PU-PIA and PtBA-PU-PtBA as shown in Table 1 (and Table S1) revealed that PIA-PU-PIA exhibits better corrosion inhibition potentials than PtBAPU-PtBA over the concentration and temperature ranges considered in this study. Since the two studied polymers differ in the length of the linking alkyl group “a” (a = 4 in PIA-PU-PIA; a = 1 in PtBA-PU-PtBA), and also in the R-group (R = -CH2NHC(CH3)2 in PIA-PU-PIA; R = -CH2-OC(CH3)3 in PtBA-PU-PtBA) as shown in scheme 1, it can be inferred that the higher inhibition efficiency of PIA-PU-PIA compared to PtBA-PU-PtBA is as a result of the increased alkyl chain length and the presence of additional N-atom in PIA-PU-PIA, unlike PtBA-PU-PtBA with additional O-atom. Longer alkyl chain has been reported to favour higher IE due to increased adsorption at surface layers,42 while N-containing inhibitors have been reported to generally exhibit better inhibition potentials than O-containing inhibitors due to better electron-donating tendency of N-atom.43

3.4. Adsorption isotherms and thermodynamic parameters The experimental data for the adsorption of PIA-PU-PIA and PtBA-PU-PtBA were subjected to various adsorption isothersm including the Langmuir, Temkin, Frumkin, Freundlich, El-Awady and Flory-Huggins isotherms. It was infered from the correlation coefficient (R2) values that the adsorption of PIA-PU-PIA on mild steel in 0.5 M H2SO4 obeys the Langmuir adsorption isotherm, while the adsorption of PtBA-PU-PtBA on mild steel in 0.5 M H2SO4 follows the El-Awady adsorption isotherm. The linear forms of the Langmuir and El-Awady isotherms respectively are expressed as: C

θ ln

=

1 +C K ads

θ 1−θ

(2)

= ln K '+ y ln C

(3) 11



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where θ is the surface coverage, and C is the concentration of the inhibitor. The constant Kads in the Langumuir isotherm (Equation 2) is the adsorption equilibrium constant, while K’ in the ElAwady isotherm (Equation 3) is related to the adsorption equilibrium constant as Kads= (K')1/y, where y is the slope of the isotherm such that, 1/y is the number of inhibitor molecules occupying one active site on the metal surface. The representative isotherm plots at 298 K are shown in Figure 7. The values of Kads were caculated for both PIA-PU-PIA and PtBA-PU-PtBA and the change in Gibb’s free energy of adsorption was calculated for both compounds using the relation: o ∆G ads = − RT ln(55.5 K ads )

(4)

where R is the gas constant and 55.5 is the molar concentration of water in aqueous solution. The values of Kads and are ∆Goads for the studied compounds are reported in Table 2. Large values of Kads connote better adsorption due to strong electrostactic interactions or enhanced charge trasfer between the inhibitor molecules and the metal. Small values of Kads on the other hand imply weak interactions between the inhibitor molecules and the metal surface. It can be deduced from the results in Table 2 that the values of Kads decrease with increase in temperature, which implies that the adsorption equilibrium shifts to the left, favouring desorption process as the temperature is increased. The negative Gibb’s free energy of adsorption (∆Goads) obtained for the studied compounds implies that the adsorption of the inhibitors on mild steel surface in 0.5 M H2SO4 is spontaneous. Generally, adsorption of inhibitors on metal surface is classified as a physisorption mechanism, involving electrostatic interactions between the charged inhibitor molecules and the charged metal surface, when the value of ∆Gads is around -20 kJ/mol or less negative. A value of ∆Gads around -40 kJ/mol or more negative suggests that the adsorption of an inhibitor on metal surface features chemisorption mechanism, involving charge sharing or charge transfer from organic inhibitor molecule to the metal surface to form a coordinate type of bond.44,45 In the present study, the values of ∆Goads obtained for both PIA-PU-PIA and PtBA-PU-PtBA over the range of temperatures studied lie between the threshold values for pure physisorption and chemisorption mechanisms, which suggests that the adsorption of these polymers on mild steel in 0.5 M H2SO4 involves competitive physisorption and chemisorption mechanisms. Furthermore, the values of ∆Goads for PIA-PU-PIA at 298 – 318 K revealed essentially chemisorption mechanism. The values of 1/y obtained from the El-Awady isotherm plot of PtBA-PU-PtBA 12


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could not produce a simple generalized trend with increasing temperature. However, the number of inhibitor molecules occupying one active site on the steel surface is highest at the highest concentration (1600 ppm). By assuming a direct relationship between the corrosion current density and the rate constant for the corrosion reaction, the values of the activation energy Ea were calculated from the plots of lnicorr against 1/T according to the Arrehnius type equation of the form:

log icorr = log A −

Ea 2.303RT

(5)

where A is the pre-exponential factor, R is the gas constant and T is the absolute temperature. The Arrhenius plots for mild steel in 0.5 M H2SO4 containing various concentrations of PIA-PUPIA and PtBA-PU-PtBA are shown in Figure 8. Other activation thermodynamic parameters including the enthalpy and entropy of activation were calculated from the plots of log icorr/T against 1/T according to the transition state equation: ∆H *  i    R    ∆S *   − log corr  =  log   +   T    Nh    2.303 R  2.303RT

(6)

where is h the Planck’s constant, N is is the Avogadro’s number, R and T retain their definitions as in Equation 5 above. The transition state plots for mild steel in 0.5 M H2SO4 containing various concentrations of PIA-PU-PIA and PtBA-PU-PtBA are shown in Figure 9. The results of Ea, ∆H* and ∆S* are reported in Table 3. The higher values of Ea in the presence of the inhibitors is an indication of the increased energy barrier for the corrosion reaction due to the inhibitive actions of the studied polymers. Though the values of Ea does not follow a simple trend with increasing concentrations of the polymers, the maximum energy barrier for the corrosion reaction was observed at 1600 ppm for both PIA-PU-PIA and PtBA-PU-PtBA, which implies that the polymers show highest inhibition strength for the corrosion process at concentration of the studied inhibitors.46-49 This suggests that the corrosion reaction will be forced to move towards the sites on the mild steel surface that are characterized by progressively higher values of Ea as the concentration of the inhibitors is increased.50 The higher values of Ea in the presence of PIA-PU-PIA compared to PtBA-PU-PtBA also corroborate the higher inhibition potentials of the former as observed from the polarization measurements. The results in Table 3 also show that the values of ∆H* and ∆S* are higher in the presence of the studied polymers than in the blank 13



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acid system and appear to increase with increase in temperature. The positive values of ∆H* suggest that the formation of activated complex is an endothermic process. The positive values of ∆S* could imply that the rate determining step represents a dissociative step involving the dissociation of water molecules from the steel surface. The increasing value of ∆S* with increase in concentration in some cases could mean desorption of large fraction of more disordered water molecules from the metal surface and replacement with more ordered inhibitor molecules.

3.5. Potentiostatic polarization studies The potentiostastic polarization curves for mild steel in 0.5 M H2SO4 without and with various concentrations of PIA-PU-PIA and PtBA-PU-PtBA as inhibitors are presented in Figure 10. The curve for mild steel in 0.5 M H2SO4 without the inhibitors shows the formation of passive layer, which is often made up of various corrosion products. The passive potential range was observed to be 1.3034 – 1.5330 V. As described by Geana et al.,51,52 a broad peak exhibited by mild steel in the current-potential profile may be due to the occurrence transitions from active through pre-passive to passive state. The passivating regions are wider in the presence of the studied inhibitors except at 400 ppm of PIA-PU-PIA at which no sign of passivation was observed (Figure 10). This implies that the studied inhibitors might act as passivating agents for mild steel corrosion in 0.5 M H2SO4 and the degree of passivating ability increases with increasing concentration of the inhibitors. Passivation is often attributed to the formation of some metal-inhibitor complexes such as [M-Inh-OH], [M-OH-Inh], [M-Inh-X] etc., where X can be the other anions present in the solution.53-57 Electrochemical parameters such as the critical current (ic), passivation current (ip) and passivation potential range (EPP) were obtained and reported in Table 4. The values of the ic in the presence of the studied polymers are lower than that of the acid blank, which suggests that the adsorption of the inhibitor molecules on mild steel surface thereby lowering the maximum current. The Eppvalues in the presence of the inhibitors show wider range compared to the blank acid system, which again confirms the formation of passive films of the inhibitors or some Fe-inhibitor complexes on the mild steel surface thereby making the surface relatively inactive for corrosion process.

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3.6. Electrochemical impedance spectroscopy (EIS) measurements The Nyquist and Bode phase angle plots of mild steel in 0.5 M H2SO4 without and with various concentrations of the studied polymers are presented in Figures 11 and 12 respectively. The Nyquist plots shown in Figure 11 exhibit depressed semicircles with only one capacitive loop over the frequency range stuidied and the shapes of the plots both in the absence and presence of the inhibitors are similar. This suggests that the dissolution of mild steel in the studied aggressive media is controlled by charge tranfer reaction,58 and that the inhibitors only retard the corrosion rate by blocking the active sites on the mild steel surface without altering the mechanism of the corrosion process.59 The depressed semicircular feature of the Nyquist plots is the characteristic of solid electrodes, and it is often regarded as frequency dispersion due to different possible forms of inhomogeneity.60-64 There is apparent increase in the diameter of the Nyquist semicircles in the presence of the inhibitors as compared to that without the inhibitors. This also increases with increase in concentration of the inhibitors. This suggests that the studied polymers change the impedance response of mild steel in 0.5 M H2SO4 possibly due to adsorption of the polymer molecules on the steel surface and the degree of surface coverage by the adsorbed molecules increases with increase in number of polymer molecules (polymer concentration) in solution. On the other hand, the Bode phase angle plots reflect apparently higher values at intermediate frequencies in the presence of the inhibitors. This observation can be attributed to the formation of protective films of the inhibitor molecules on the steel surface, which makes it to exhibit relatively closer resemblance to an ideal capacitor than the mild steel surface in the acid blank system.58 One other overt behaviour of the the Bode plots in Figure 12 especially in the presence of the inhibitors, is the appearance of the second time-constant at higher frequency, which suggests the formation and/or adsorption of intermediate corrosion products and/or mild steel -inhibitor complexes. This attribute is not evident at lower concentrations (400, 800 ppm) of PIA-PU-PIA, indicating that its obviousness depends on the concentration of the inhibitor. The EIS spectral of mild steel in 0.5 M H2SO4 in the absence and presence of the inhibitors are fitted and simulated by the equivalent circuit shown in Figure 13. The adopted equivalent circuit satisfactorily reproduced the experimental curves as shown in Figures 13a and 13b. The χ2 values resulting from the statistical comparisons of experimental and fitted 15



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data are in the forth order magnitude (10– 4). This is an indication that the equivalent circuit is suitable for fitting the experimental EIS spectral. The electrochemical kinetic parameters including the Rct, double layer capacitance (Cdl), and the percentage inhibition efficiency (IE) values are reported in Table 5. The IE was calculated from the Rct values as:

IE % =

Rct − R' ct × 100 Rct

(7)

where Rct and R`ct are the charge tranfer resistances with and without inhibitors respectively. The Cdl values were obtained from the equation:

Cdl = Y0 (2πf max )

n −1

(8)

where Y0 is the CPE constant, n is the CPE exponent, and f(-Z”max) is the maximum frequency on the imaginary impedance axis. The values of f(-Z”max) are also reported in Table 5. The Rct values in the presence of the inhibitors are higher than the one in the absence of the inhibitors, which implies that the inhibitors reduce the rate of charge transfer process associated with the corrosion reaction, and it is indicative of the corrosion inhibition characteristics of the studied polymers.65-69 More so, the Cdl values in the presence of the inhibitors are generally lower than that of the acid blank, suggesting the formation of the protective layer of the inhibitor molecules on the steel surface. The Rct value increases with increasing concentration of the studied polymers leading to increase in IE. The trend of the IE obtained from the EIS study is similar to that acquired from the polarization experiment, which again confirms the higher inhibition potentials of PIA-PU-PIA than PtBAPU-PtBA.

3.7. SEM The surface morphologies of freshly polished mild steel surface and mild steel surface immersed in the corrosive solutions for 12 h in the absence and presence of 1600 ppm of the studied inhibitors are shown in Figure 14. The freshly polished mild steel shows smooth, noncorroded surface with polishing scratches. The mild steel retrieved from 0.5 M H2SO4 without the inhibitors exhibits strongly damaged and corroded surface due to direct acid attack in the 16


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uninhibited corrosive environment. It is obvious from Figure 14 (c and d) the mild steel specimens immersed in the aggressive solutions containing the inhibitors are not as badly damaged as the one recovered from the acid blank solution. This is an indication of the formation of protective film of the polymer molecules on the mild steel surface thereby shielding it from direct acid attack. The better inhibitive potential of PIA-PU-PIA compared to PtBA-PtBA is also reflected in Figure 14 as the surface of the mild steel in Figure 14c shows closer resemblance to the freshly polished mild steel surface.

3.8. Atomic Force Microscopy The topographical surfaces of the plain mild steel specimen and mild steel specimens immersed in 0.5 M H2SO4 without and with 1600 ppm of the studied polymers were characterized by the AFM images as shown in Figure 15. The AFM micrographs of plain mild steel shows surface morphology indicative of only few scratches from mechanical polishing. The mild steel surface retrieved from the uninhibited 0.5 M H2SO4 show noticeable roughness with deep holes and pits associated with corrosive attack. The mild steel specimens retrieved from the inhibited aggressive media however exhibit relatively smooth surfaces compared to the one from the acid blank. This again confirms the formation of protective layer of the inhibitor molecules on the steel surface. The surface roughness of each of the micrographs in Figure 15 was quantitavely evaluated by measuring the change in the surface roughness (RMS). The higher the value of RMS the more the extent of corrosion on the surface. The results of the RMS recorded for the mild steel specimen in 0.5 M H2SO4, as well as the mild steel specimens in 0.5 M H2SO4 containing 400 and 1600 ppm of the two studied polymers are listed in Table S2. The RMS value of mild steel dipped in 0.5 M H2SO4 without the inhibitors is 503.2 nm, which is significantly higher than the values obtained for the mild steel specimens in the presence of the inhibitors. The trend of the results of RMS shown in Table S2 for both PIA-PUPIA (188.4 nm, 111.1 nm) and PtBA-PU-PtBA (308.2 nm, 196.0 nm) at 400 and 1600 ppm respectively also confirms the increased inhibition potentials of the polymers with increase in concentration, and the better inhibitive strength of PIA-PU-PIA compared to PtBA-PU-PtBA.

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3.9. Quantum chemical calculations The gas phase optimized structures of the polymers are shown in Figure 16. The electron density distributions of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are shown in Figure 17. The HOMO provides information about the sites of an inhibitor molecule from where electrons are likely to be donated to the appropriate vacant orbitals of the metal atom in a bid to adsorb on metal surface and inhibit metal corrosion. The LUMO on the other hand gives an insight into the sites of the inhibitor molecule that are most likely to receive electrons from the suitable occupied orbitals of the metal atom in a retro-donation step. The HOMO and LUMO of the studied polymers (Figure 17) show similar features with electron densities essentially distributed over the aromatic benzene ring and the heteroatoms around its neighbourhood. The straight-chain units of the molecules and the bromine atoms are not involved in the HOMO and LUMO. The values of some quantum chemical parameters of the optimized structures of the studied polymers and listed in Table 6. Generally, a higher value of EHOMO implies a better tendency of the molecule to donate its HOMO electrons to the suitable vacant orbitals of an accepting molecule or atom, while a lower value of ELUMO suggests a better chance of the molecule to accept electrons into its LUMO from the appropriate occupied orbitals of a donor molecule or atom.70-72 The results in Table 6 show that PIA-PU-PIA has higher EHOMO than PtBA-PU-PtBA, which suggests the greater tendency of the former to donate electrons to the vacant d-orbitals of Fe and hence its better adsorption onto mild steel surface and higher corrosion inhibition potential than the latter. This is in agreement with the observed experimental results. The results of the ELUMO do not agree with the experimental trend of inhibition efficiencies. The value of the ∆E gives information about the relative stability or reactivity of an inhibitor molecule. A molecule with small value of ∆E is generally considered more reactive and often exhibits higher inhibition efficiency and vice versa. The trend of ∆E values in Table 6 is not in agreement with the experimental IE %. The non-correlation of the values ELUMO and ∆E with experimental results suggests that the relative strengths of corrosion inhibition of the studied compounds are not (or less) dependent on the ability of the molecules to accept electrons from the occupied orbitals of the metal for back-bonding formation. The global electronegativity (χ) measures the 18


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tendency of a molecule to retain its own electrons during molecular interactions. The higher value of χ observed for PtBA-PU-PtBA suggests that it has relatively lesser tendency to donate its electrons to the metal during the donor-acceptor interactions between the inhibitor molecules and the metal. In other words, lower value of χ for PIA-PU-PIA favours electron donation to the metal and enhances its inhibition efficiency. The inhibition efficiency of an inhibitor molecule is also related to the surface coverage, which in turn may depend on the molar volume of the molecule. The molar volume of PIA-PU-PIA is greater than that of PtBA-PU-PtBA (as shown in Table 6), which supports the observed trend of inhibition efficiencies. Though there are dissenting opinions on the use of dipole moments as molecular parameter to explain relative inhibition performances of corrosion inhibitors, but the trend of the dipole moments of the studied compounds (PIA-PU-PIA < PtBA-PU-PtBA) in the present study suggests that the lower dipole moment of PIA-PU-PIA might favour its enhanced inhibition efficiency. This is in accordance with the opinion that low values of dipole moment favour accumulation of inhibitor molecules in the surface layer.73

4. Conclusions Two polyurethane copolymers, PIA-PU-PIA and PtBA-PU-PtBA were synthesized, characterized and applied as inhibitors of mild steel corrosion in 0.5 M H2SO4. The following conclusions can be drawn from the results.

GPC and 1H-NMR results confirmed that PIA-PU-PIA and PtBA-PU-PtBA were successfully synthesized.

Both PIA-PU-PIA and PtBA-PU-PtBA inhibit mild steel corrosion in 0.5 M H2SO4 showing the IE as high as 99 % at 1600 ppm and 298 K, and the IE increases with increasing concentration of the inhibitors.

Potentiodynamic polarization measurements revealed that the studied polymers are mixed-type corrosion inhibitors and show passivating characteristics.

The EIS measurements confirmed that the polymers impede electrochemical corrosion in the studied aggressive medium and the charge transfer resistance increases with increasing inhibitor concentration.

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SEM and AFM analyses confirmed the formation of protective film of the inhibitor molecules on the steel surface thereby preventing it from direct acid attack.

The results of quantum chemical calculations revealed that the relative strength of inhibition performance of the studied polymers depends on their tendency to donate electrons to the metal and also on their molar volume.

The results obtained from both experimental and theoretical studies indicated that the trend of the inhibition efficiency potentials of the two polymers is: PIA-PU-PIA > PtBAPU-PtBA.

Acknowledgements Authors are grateful to the University of Delhi, India and North-West University, South Africa for granting them funding to carry out this work. L. O. Olasunkanmi thanks NorthWest University for granting him Post-Doctoral fellowship. I. Bahadur, L. O. Olasunkanmi and E. E. Ebenso thank the National Research Foundation (NRF) of South Africa for funding.

Supporting Information: Table S1. Tafel parameters for mild steel in 0.5 M H2SO4 without and with different concentrations of PIA-PU-PIA and PtBA-PU-PtBA at different temperatures.

Table S2. Roughness data from AFM measurements for mild steel surface in 0.5 M H2SO4 without and with 400 ppm and 1600 ppm of PIA-PU-PIA and PtBA-PU-PtBA.

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(56) Yagan, A.; Pekmez, N. O.; Yildiz, A. Electropolymerization of poly(N-ethyl aniline) on mild steel: synthesis, characterization and corrosion protection. Electrochim. Acta 2006, 51, 2949-2955. (57) Kanojia, R. Singh, G. An interesting and efficient organic corrosion inhibitor for mild steel in acidic medium. Surf. Eng. 2005, 21, 180-186. (58) Chandrasekaran, V.; Kannan, K. Thiocarbamide as a corrosion inhibitor for mild steel in phosphoric acid medium. Bullet. Electrochem. 2004, 20, 471-479. (59) Sudheer; Quraishi, M. A. 2-Amino-3,5-dicarbonitrile-6-thio-pyridines: new and effective corrosion inhibitors for mild steel in 1 M HCl. Ind. Eng. Chem. Res. 2014, 53, 2851-2859. (60) Sasikumar, Y.; Adekunle, A. S.; Olasunkanmi, L. O.; Bahadur, I.; Baskar, R.; Kabanda, M. M.; Obot, I. B.; Ebenso, E. E. Experimental, quantum chemical and Monte Carlo simulation studies on the corrosion inhibition of some alkyl imidazolium ionic liquids containing tetrafluoroborate anion on mild steel in acidic medium. J. Mol. Liq. 2015, 211, 105-118. (61) Chauhan, L. R.; Gunasekaran, G. Corrosion inhibition of mild steel by plant extract in dilute HCl medium. Corros. Sci. 2007, 49, 1143-1161. (62) Yagan, A.; Pekmez, N. O.; Yildiz, A. Corrosion inhibition by poly(Nethylaniline) coatings of mild steel in aqueous acidic solutions. Prog. Org. Coat. 2006, 57, 314-318. Dhayabaran, V. V.; Vasudevan, T.; Rajendran, A.; Kumar, A. A. Effects of slurry (63) flow rate and pad conditioning temperature on dishing, erosion and metal loss during copper CMP. Bullet. Electrochem. 2006, 22, 117-121. (64) Dhayabaran, V. V.; Rajendran, A.; Suganya, P.; Inhibitive action of Tryptophan on the corrosion of commercial mild steel in 1.0 M HCl. Bullet. Electrochem. 2006, 22, 43-48. (65) Tueken, T.; Yazici, B.; Erbil, M.; The corrosion behaviour of polypyrrole coating synthesized in phenylphosphonic acid solution. Appl. Surf. Sci. 2006, 252, 2311-2318. (66) Tao, Z.; Zhang, S.; Li, W.; Hou, B. Adsorption and inhibitory mechanism of 1H1,2,4-triazol-l-yl-methyl-2-(4-chlorophenoxy) acetate on corrosion of mild steel in acidic solution, Ind. Eng. Chem. Res. 2011, 50, 6082-6088. (67) Atta, A. M.; El-Azabawy, O. E.; Ismail, H. S.; Hegazy, M. A. The corrosion behaviour of polypyrrole coating synthesized in phenylphosphonic acid solution, Corros. Sci. 2011, 53, 1680-1689. (68) Ayati, N. S.; Khandandel, S.; Momeni, M.; Moayed, M. H.; Davoodi, A.; Rahimizadeh, M.; Inhibitive effect of synthesized 2-(3-pyridyl)-3,4-dihydro-4quinazolinone as a corrosion inhibitor for mild steel in hydrochloric acid. Mater. Chem. Phys. 2011, 126, 873-879.

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(69) Karpakam, V.; Kamaraj, K.; Sathiyanarayanan, S.; Venkatachari, G.; Ramu, S. Electrosynthesis of polyaniline–molybdate coating on steel and its corrosion protection performance. Electrochim. Acta 2011, 56, 2165-2173. (70) Hegazy, M. A.; Ahmed, H. M.; El-Tabei, A. S. Investigation of the inhibitive effect of p-substituted 4-(N,N,N-dimethyldodecylammonium bromide)benzylidenebenzene-2-yl-amine on corrosion of carbon steel pipelines in acidic medium. Corros. Sci. 2011, 53, 671-678. (71) Fang, J.; Li, J.; Quantum chemistry study on the relationship between molecular structure and corrosion inhibition efficiency of amides. J. Mol. Struct. 2002, 593, 179185. (72) Bouklah, M.; Hammouti, B.; Benkaddour, M.; Benhadda, T. Thiophene derivatives as effective inhibitors for the corrosion of steel in 0.5 M H2SO4. J. Appl. Electrochem. 2005, 35, 1095-1101. (73) Olasunkanmi, L. O.; Obot, I. B.; Kabanda, M. M.; Ebenso, E. E. Some quinoxalin-6-yl derivatives as corrosion inhibitors for mild steel in hydrochloric acid: Experimental and theoretical studies. J. Phys. Chem. C 2015, 119, 16004−16019.

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FIGURE CAPTIONS Figure 1. Schematic diagram for the synthesis of PIA-PU-PIA and PtBA-PU-PtBA. Figure 2. 1H-NMR spectra of (a) Br-PU-Br and (b) PIA-PU-PIA. Figure 3. 1H NMR of (i) Br-PEOPU-Br and (ii) PtBA-PU-PtBA.

Figure 4. Variation of open circuit potentials with time for mild steel in 0.5 M H2SO4 in the presence of 1600 ppm of PIA-PU-PIA and PtBA-PU-PtBA.

Figure 5. Potentiodynamic polarization curves for mild steel in 0.5 M H2SO4 without and with different concentrations of PIA-PU-PIA at different temperatures.

Figure 6. Potentiodynamic polarization curves for mild steel in 0.5 M H2SO4 without and with different concentrations of PtBA-PU-PtBA at different temperatures.

Figure 7. Representative adsorption isotherms for mild steel in 0.5 M H2SO4 containing different concentrations of (a) PIA-PU-PIA: Langmuir isotherm; and (b) PtBA-PU-PtBA: El-Awady isotherm at 298 K.

Figure 8. Arrhenius plots for mild steel in 0.5 M H2SO4 without and with different concentrations of (a) PIA-PU-PIA and (b) PtBA-PU-PtBA.

Figure 9. Transition state plots for mild steel in 0.5 M H2SO4 without and with different concentrations of (a) PIA-PU-PIA and (b) PtBA-PU-PtBA.

Figure 10. Potentiostatic polarization curves for mild steel in 0.5 M H2SO4 without and with different concentrations PIA-PU-PIA and PtBA-PU-PtBA at 298 K.

Figure 11. Nyquist plots for mild steel in 0.5 M H2SO4 without and with different concentrations of PIA-PU-PIA and PtBA-PU-PtBA at 298 K. 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 12. Bode phase angle plots for mild steel in 0.5 M H2SO4 without and with different concentrations of PIA-PU-PIA and PtBA-PU-PtBA at 298 K.

Figure 13. Equivalent electrical circuit used for the fitting of impedance spectra (a), and the screeshots of the representative fittings for mild steel in 0.5 M H2SO4 containing 1600 ppm of PIA-PU-PIA (b) and PtBA-PU-PtBA (c) at 298 K.

Figure 14. SEM images of (a) plain mild steel surface, (b) mild steel in 0.5 M H2SO4, (c) mild steel in 0.5 M H2SO4 + 1600 ppm PIA-PU-PIA (d) mild steel in 0.5 M H2SO4 + 1600 ppm PtBA-PU-PtBA.

Figure 15. Atomic force micrograph of(a) plain mild steel surface, (b) mild steel in 0.5 M H2SO4, (c) mild steel in 0.5 M H2SO4 + 1600 ppm PIA-PU-PIA (d) mild steel in 0.5 M H2SO4 + 1600 ppm PtBA-PU-PtBA.

Figure 16. Optimized molecular structures of PIA-PU-PIA and PtBA-PU-PtBA obtained at B3LYP/6-31(G) level of theory.

Figure 17. HOMO and LUMO electron density surfaces of PIA-PU-PIA and PtBA-PU-PtBA obtained at B3LYP/6-31(G) level of theory.

28


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Page 29 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1

NCO + HO

2

H

a Ox

NCO TDI 65 °C O

OCN N H

a

O

NCO

H N

O x

n

O

Br-PEO-PU-Br

Br O

2 HO O O

O

Br

O

H N

O

O

N H

O

a

H N

H N

O x

n

O

O O

O

Br

O

80 0C DMF

Br m

O

O O

H O N O

O N H

R

PIA - Pu - PIA = R =

NH

PtBA - Pu - PtBA = R =

m

O R

H N O a O x O

m

H N O O n

and a = 4

O

and a = 1

29


O O

Br m

O R


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 52

Figure 2

(b)

(a)

ppm

10

8

6

4

30


2

0

Page 31 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3

(a)

(b)

ppm

10

8

6

4

31


2

0


Figure 4

-7.50E-01 -7.00E-01 -6.50E-01

E Vs SCE (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PIA-PU-PIA

-6.00E-01 -5.50E-01 -5.00E-01 -4.50E-01 -4.00E-01 -3.50E-01 -3.00E-01 0.00E+00 5.00E+02 1.00E+03 1.50E+03 2.00E+03

t/s

32


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Page 33 of 52

Figure 5 0

0

-1

-1 -2 log I (A /cm 2 )

log I (A/cm 2 )

-2 -3

-3

At 298 K

-4

1600 ppm -5

1200 ppm

-6

800 ppm

-4

-7

-5

0

-1

-1 log I (A/cm 2 )

0

-2

At 318 K 1600 ppm

BLANK

-2 -3

At 328 K 1600 ppm

-4

1200 ppm

-5

800 ppm

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E vs SCE (V)

E vs SCE (V)

-4

1600 ppm

400 ppm -6

BLANK

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

-3

At 308 K 1200 ppm

400 ppm

log I (A /cm 2 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800 ppm

1200 ppm 800 ppm

-5

400 ppm BLANK

400 ppm BLANK

-6 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E vs SCE (V)

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E vs SCE (V)

33



Figure 6

-1

-1

-2

-2 log I (A/cm 2 )

0

log I (A/cm 2 )

0

-3

-3

At 298 K

-4 -5

1600 ppm

-6

At 308 K 1600 ppm

-4

1200 ppm 800 ppm

1200 ppm 800 ppm

-5

400 ppm

400 ppm -6

BLANK

BLANK

-7 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E vs SCE (V)

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

0

0

E vs SCE (V)

-1

-1 log I (A/cm 2 )

-2 log I (A/cm 2 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 52

-2

-3 -4

At 318 K 1600 ppm

-3

1200 ppm -5 -6

800 ppm

-4

At 328 K 1600 ppm 1200 ppm 800 ppm 400 ppm

400 ppm

BLANK

BLANK

-5 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E vs SCE (V)

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E vs SCE (V)

34


Page 35 of 52

Figure 7

2.2

1800

(b)

(a) 1600

2.0

2

R =1.0000

1.8

2

R =0.9452

1400 1.6

log(θ/(1-θ))

1200

C/θ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000 800

1.4 1.2 1.0 0.8

600

0.6

400 0.4

400

600

800

1000

1200

1400

2.6

1600

2.7

2.8

2.9 log C

C

35


3.0

3.1

3.2

3.3


Figure 8

1.0

1.0

0.5

0.5

(a)

-0.5

(b)

0.0 log i

0.0

log i

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 52

-0.5

2

-1.0 -1.5 3.00

0.5 M H2SO4; R =0.9326

-1.0

2

400 ppm; R =0.9959 2 800 ppm; R =0.9745 2 1200 ppm; R =0.9736 2 1600 ppm; R =0.9640 3.05

3.10

3.15

3.20

-1.5

3.25

3.30

3.35

3.40

3.00

2

400 ppm; R =0.9286 2 800 ppm; R =0.8952 2 1200 ppm; R 2=0.9842 1600 ppm; R =0.9918 3.05

-1

3.10

3.15

3.20

3.25 -1

1000/T (K )

1000/T (K )

36


3.30

3.35

3.40

Page 37 of 52

Figure 9

(b)

-1.5 -1.5 -2.0 -2.0

(a) log (i/T)

log (i/T)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2.5 -3.0

-4.0 3.00

-3.0 2

2

-3.5

-2.5

0.5 M H2SO4; R =0.9146

-3.5

2


3.10

3.15

3.20

-4.0 3.00 3.25

3.30

3.35


3.40

-1

1000/T (K )

37


3.10

3.15

3.20

3.25 -1

1000/T (K )

3.30

3.35

3.40


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10

38


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Page 39 of 52

Figure 11 250

At 298 K 1600 ppm

200

1600 ppm

200

1200 ppm

1200 ppm

800 ppm

150

400 ppm BLANK

100

400 ppm

0

0

100

150

200

BLANK

100 50

50

800 ppm

150

50

0

PtBA-PU-PtBA

At 298 K

PIA-PU-PIA

Z ''/ Ω .cm 2

250

Z ''/ Ω .c m 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

250

50

100

150

Z'/Ω.cm2

Z'/Ω.cm2

39


200

250


Figure 12

80 PIA-PU-PIA -Phase Angle (Degree)

60 40 20

At 298K 1600 ppm 1200 ppm 800 ppm 400 ppm BLANK

0 -20 -40 -1

0

1

2 log f (Hz)

3

4

5

4

5

60 PtBA-PU-PtBA -Phase Angle (Degree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 20

At 298K 1600 ppm 1200 ppm 800 ppm 400 ppm BLANK

0

-20

-1

0

1

2 log f (Hz)

3

40


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Page 41 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13

(a)

(b)

(c)

41



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 14

42


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 15

43



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 16

PIA-PU-PIA

PtBA-PU-PtBA

44


Page 44 of 52

Page 45 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 17

PIA-PU-PIA

LUMO

HOMO

PtBA-PU-PtBA LUMO

HOMO

45



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 52

Table 1. Tafel parameters for mild steel in 0.5 M H2SO4 without and with different concentrations of PIA-PU-PIA and PtBA-PU-PtBA at 298 K and 328 K. Temp. (K) 298

328

298

328

Conc. (ppm) Blank 400 800 1200 1600

Icorr -Ecorr vs SCE ba (mA/cm2) (mV) (mV/dec.) PIA-PU-PIA 9.679 465 141.66 0.102 451 36.61 0.065 470 30.49 0.049 440 30.99 0.034 438 25.43

Blank 400 800 1200 1600

22.09 7.461 5.771 4.363 4.285

Blank 400 800 1200 1600

9.679 2.070 0.121 0.131 0.035

Blank 400 800 1200 1600

22.09 15.27 10.78 8.682 7.679

bc (mV/dec.)

IE%

θ

164.25 112.99 119.14 111.15 111.89

98.93 99.32 99.48 99.64

0.98 0.99 0.99 0.99

172.41 111.82 96.87 81.32 78.68

212.53 184.26 174.21 168.74 171.02

66.22 78.40 80.24 80.60

0.66 0.78 0.80 0.80

PtBA-PU-PtBA 465 141.66 470 86.87 486 33.69 492 130.17 472 25.67

164.25 145.17 131.04 129.38 133.31

78.61 98.74 98.64 99.60

0.78 0.98 0.98 0.99

212.53 202.63 191.86 187.05 177.14

30.50 51.10 60.60 65.20

0.30 0.51 0.60 0.65

490 458 467 459 464

490 497 495 440 446

172.41 198.29 205.33 143.04 134.13

46


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Adsorption parameters for mild steel in 0.5 M H2SO4 containing PIA-PU-PIA and PtBA-PU-PtBA at different temperatures. Polymer PIA-PU-PIA(a)

PtBA-PU-PtBA(b)

(a)

T (K) 298 308 318 328 298 308 318 328

K (x10-4) 252.30 80.69 16.40 8.75 6.84 3.28 2.19 1.64

∆Gads (kJ mol-1) -43.67 -42.70 -40.28 -38.59 -37.53 -36.90 -37.04 -37.41

1/y 0.4332 0.3638 0.3259 0.9357

parameters derived from Langmuir isotherm. (b) parameters derived from El-Awady

47



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 52

Table 3. Activation thermodynamic parameters for mild steel in 0.5 M H2SO4 in the presence of different concentrations of PIA-PU-PIA and PtBA-PU-PtBA. Compound H2SO4 PIA-PU-PIA

PtBA-PU-PtBA

Concentration (ppm) 0.5 M 400 800 1200 1600 400 800 1200 1600

Ea (kJ/mol) 21.42 118.27 114.66 113.00 121.35 53.84 115.83 107.34 142.67

∆H* (kJmol-1) 18.82 115.68 112.06 110.41 118.75 51.24 113.23 104.74 140.07

48


∆S* (J/mol.K) 1108.64 1393.53 1381.64 1371.40 1395.29 1206.08 1394.85 1360.82 1466.50

Page 49 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Potentiostatic electrochemical parameters for the anodic dissolution of mild steel in 0.5 M H2SO4 without and with different concentrations of PIA-PU-PIA and PtBA-PU-PtBA at 298 K.

Solutions H2SO4 PIA-PU-PIA

PtBA-PU-PtBA

Concentration (ppm) 0.5 M 400 800 1200 1600 400 800 1200 1600

ic (A/cm2) 0.3869 0.0468 0.0359 0.0224 0.0060 0.4338 0.3448 0.3194 0.2383

ip (A/cm2) 0.0470 0.3576 0.2721 0.2372 0.0385 0.0178 0.0075

49


Epp Range (V) 1.3034-1.5330 0.8198-1.5328 0.5638-1.5708 0.5327-1.3865 0.8471-1.5878 0.6240-1.4547 0.6121-1.4782


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 52

Table 5. Electrochemical parameters from the EIS measurements for mild steel in 0.5 M H2SO4 without and with different concentrations of PIA-PU-PIA and PtBA-PU-PtBA at 298 K. Compound

χ2 (x10-4)

Conc. (ppm)

Rct (Ωcm2)

f(-Z’’max) (Hz)

Cdl (µF/cm2)

IE (%)

0.5 M H2SO4 PIA-PU-PIA

0 400 800 1200 1600

3.4 104.7 206.8 221.7 239.2

1.9 5.5 4.6 6.6 8.1

136.9 93.14 88.21 80.91 72.73

96.75 98.35 98.46 98.57

29.7 35.8 38.0 37.5

7.71 2.15 2.66 3.76

PtBA-PU-PtBA

400 800 1200 1600

12.8 119.9 221.2 245.7

2.1 3.1 5.4 7.6

97.62 86.82 76.08 69.68

73.43 97.16 98.46 98.61

7.7 26.8 25.1 29.5

5.58 5.41 5.29 2.98

|α| (degree)

50


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 6. Quantum chemical parameters for PIA-PU-PIA and PtBA-PU-PtBA obtained at B3LYP/6-31G(d) level of theory Parameters →

EHOMO

ELUMO

∆EL-H

χ

Dipole moment

Molar Volume

Inhibitors ↓

(eV)

(eV)

(eV)

(eV)

(Debye)

(cm3/mol)

PIA-PU-PIA

-6.161

-0.798

5.363

2.682

3.342

447.479

PtBA-PU-PtBA

-6.190

-1.558

4.632

3.874

4.112

316.732

51



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