Corrosion Inhibition of Iron in Acidic Solutions by Monoalkyl

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Corrosion inhibition of iron in acidic solutions by monoalkyl phosphate esters with different chain lengths Xiang Gao, Shaotong Liu, Hai-Feng Lu, Feng Gao, and Houyi Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503508h • Publication Date (Web): 30 Jan 2015 Downloaded from http://pubs.acs.org on February 3, 2015

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Industrial & Engineering Chemistry Research

Corrosion inhibition of iron in acidic solutions by monoalkyl phosphate esters with different chain lengths Xiang Gao1, Shaotong Liu1, Haifeng Lu2, Feng Gao3,* and Houyi Ma1,*

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Key Laboratory of Colloid and Interface Chemistry of State Education Ministry, School of

Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China 2

Key Laboratory for Special Functional Aggregate Materials of State Education Ministry,

School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China 3

Applied Physics Department, Shandong Agricultural University, Taian, 271018, China

*Corresponding author. Prof. Houyi Ma Tel: +86-531-88364959; Fax: +86-531-88564464; E-mail: [email protected] Dr. Feng Gao Tel: +86-538-8242488; Fax: +86-538-8249275; E-mail: [email protected]

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Abstract The inhibition effect of three amphiphilic monoalkyl phosphate esters with different chain lengths, mono-n-butyl phosphate ester (BP), mono-n-hexyl phosphate ester (HP) and mono-n-octyl phosphate ester (OP), on the corrosion of iron in 0.5 M H2SO4 solutions was investigated by using electrochemical impedance spectroscopy (EIS) and polarization curve methods. The electrochemical results indicate that, BP, HP and OP all acted as mixed type corrosion inhibitors with dominant cathodic effect. BP shows the lowest inhibition efficiency as compared to HP or OP. However, the inhibition efficiency of HP is almost the same with that of OP under similar conditions since the molecular aggregation state at the iron / solution interface plays a more important role. X-ray photoelectron spectroscopic (XPS) characterization demonstrates that three alkyl phosphate esters adsorbed on the iron surface through the unionized P-OH groups. The adsorption of BP, HP and OP fitted well with the Langmuir model.

Keywords: Alkyl phosphate ester; Iron; Corrosion inhibition; Electrochemical impedance spectroscopy (EIS); X-ray photoelectron spectroscopy (XPS).

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1. Introduction Iron is the most widely used of all the metals. The low cost and high strength make it indispensable in variety of engineering applications, such as the construction of machinery and machine tools, the hulls of large ships, and structural components for buildings. However, when iron and iron-based metal products are used in practice, they have to face all kinds of corrosive environments, including acid corrosion in the processes of acid picking, industrial cleaning and oil well acidification.1-3 Studying the corrosion mechanisms of iron and its alloys in different acidic conditions and developing simple but effective corrosion protection methods are of great scientific significance and practical value. Among a variety of anti-corrosion techniques, the utilization of organic inhibitors has been considered as an efficient and practical method since they can protect metal substrates from being corroded in aqueous solutions by adsorbing themselves on the metal’s surface and forming a protective film. However, most commonly used organic corrosion inhibitors are not only expensive but also dangerous to ecology and environment

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, which has prompted the

search for cheap, readily available and green corrosion inhibitors. Organic inhibitors generally have heteroatoms, such as O, N, S, P atoms. The heteroatoms have higher basicity and electron density and may act as active centers for the process of adsorption on the metal surface.9-15 Because of the low toxicity,16-18 relatively easy synthesis and lower price, alkyl phosphate esters that contain O and P heteroatoms have been used as the film formers to construct barrier layers on the surfaces of metallic aluminium and aluminium oxides, with the purpose of maintaining their original luster.19-21 According to the related studies, the alkyl phosphate esters adsorb onto the metal surface through the specific adsorption of the hydrophilic functional groups and the long alkyl chains interact together through van der Waals interactions to form the stable monolayer film.22 So the corrosion inhibition efficiency of alkyl phosphate esters is affected not only by the nature and surface charge of the metals to be protected, but also by the

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molecular structure of this type of inhibitors. In recent years, much attention has been focused on the application of alkyl phosphate esters in surface treatment of aluminum products, especially the corrosion inhibition of aluminum pigments.23-24 By contrast, there are relatively few reported studies on the corrosion inhibition of iron and its alloys by alkyl phosphate esters. In particular, there is a lack of the research on relationship between molecular structure and inhibition effectiveness for this kind of substances. Alkyl phosphate ester is a type of amphiphilic compound and can be considered as a surfactant when its molecule contains eight or more carbon atoms. Different from common organic inhibitors, the inhibition effects of alkyl phosphate compounds on metals are dependent not only on their molecular structures such as chain length and the number of phosphate groups, but also on the molecular aggregation state at the metal / solution interface. In this paper, choosing three mono-alkyl phosphate esters with different chain lengths, mono-n-butyl phosphate (BP), mono-n-hexyl phosphate (HP) and mono-n-octyl phosphate (OP), as the corrosion inhibitors of iron, we investigated the influence of the length of the alkyl chains on the inhibition effectiveness and interpreted the adsorption mode of alkyl phosphate esters on the iron surface in acidic aqueous solutions. On the other hand, given that HP and OP behave like an anionic surfactant, we also studied the aggregation states of alkyl phosphate ester molecules at different concentrations. The present results are helpful for having a better understanding of how alkyl phosphate esters effectively inhibit the corrosion of iron. 2. Experimental 2.1. Regents and materials The electrolyte of 0.5 M H2SO4 solution was prepared by diluting analytical grade H2SO4 (98%) with ultra pure water (electrical resistivity: ∼18 MΩ cm), which was used as the corrosive

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solution. Mono-n-butyl phosphate (BP), mono-n-hexyl phosphate (HP) and mono-n-octyl phosphate (OP) were synthesized by the direct esterification reaction of phosphorus pentoxide with 1-butanol, 1-hexanol and 1-octanol. Herein, by taking the synthesis and purification of OP as an example, a detailed procedure was given as follows: 1-octanol (0.135 mol) was heated at 40 ∼ 50 °C, and then P2O5 powders (0.045 mol) were added in three portions and the reaction was maintained at 70 °C for 3 hours. Afterwards deionized water (0.045 mol) was added to the above mixture and stirred at 80°C for another 2 hours. After the esterification reaction finished, the desired product OP was obtained from the mixture through the following three steps: At first, an excessive amount of NaOH solution was added and stirred for at least 30 minutes. The resulting aqueous phase was separated out and extracted by adding diethyl ether. Next, an excessive amount of HCl solution was added to the collected aqueous phase and fully stirred, followed by the extraction of organic phase with diethyl ether. Finally, OP was obtained by the distillation of the collected organic solution. The synthesis and the purification of BP or HP followed the similar procedure. Structural formulae of BP, HP and OP molecules are shown in Figure 1. Unless noted otherwise, the chemicals used in the present study were of analytical reagent grade and purchased from Sinopharm Chemical Reagent Co., Ltd. A 2-mm-diameter iron rod (Alfa Aesar, 99.995%) was used to prepare the working electrodes. Each iron rod specimen was soldered with a copper wire and embedded in a 6101 epoxy resin (average molecular weight: 487.9; obtained from Tianmao Chemical Company) mould, leaving its cross-section only exposed to the corrosive solutions. The purpose that the iron rod was

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sealed with the epoxy resin was to prevent other parts of the working electrode except the cross-section from contacting with the corrosive solutions. A detailed procedure for preparing the working electrode was given as follows: Firstly, the above iron rod specimen was put in an appropriate mould. Secondly, the liquid epoxy resin was mixed with the triethylenetetramine (a kind of low temperature epoxy resin hardener, purchased from Sinopharm Chemical Reagent Co., Ltd.) in a weight ratio of ∼10:1 to form a uniform liquid phase, and then the resulting liquid was poured into the mould, followed by the curing of epoxy resin in an oven at 25 ºC for 24 h. Finally, when the epoxy resin became hard enough, the working electrode (i.e. the iron rod specimen embedded in the cured epoxy resin) was carefully lifted back from the mould. Prior to each experiment, the working electrode was ground with a series of emery papers of decreasing particle sizes (400, 1000, 3000 and 4000) to a mirror-like surface, and then rinsed with 95% ethanol and ultrapure water, and finally blown dry with high purity nitrogen. 2.2. Conductivity measurements Conductivity of a series of BP, HP and OP aqueous solutions was measured at room temperature (∼25 ºC) using a DDS-307A electrical conductivity meter, for the study on the concentration dependence of conductivity for three alkyl phosphate esters, especially the determination of critical micelle concentration (CMC) values of HP and OP. 2.3. Weight loss measurements Iron plate specimens in triplicate were immersed in 50 mL of inhibitor-free and inhibitor-containing 0.5 M H2SO4 solutions at 25 ºC for 24 h. After that, the specimens were removed from the corrosive solutions, rinsed with ultra pure water and absolute ethanol, finally dried and weighted. The average weight loss was used to calculate the mean corrosion rate in

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each case. 2.4. Electrochemical measurements All electrochemical measurements were performed with a three-electrode cell at room temperature (∼25 °C). A saturated calomel electrode (SCE) and a bright platinum plate served as the counter and reference electrodes, respectively. The reference electrode was led to the surface of the working electrode through a Luggin capillary. Herein the potentials were referred to the SCE. Steady-state polarization curves were measured with a CHI 650 electrochemical workstation by scanning the potential at 0.2 mV s-1 going from cathodic side (-0.75 V) to anodic side (-0.25 V). EIS measurements were carried out with an ACM electrochemical workstation in potentiostatic mode at the open circuit potentials (OCPs) in the frequency range from 100 kHz to 50 mHz with eight points per decade under excitation of a sinusoidal wave of ±5 mV amplitude. Before each electrochemical test, the working electrode was immersed in the electrolyte for 30 min to let the electrode system reach the steady-state. The same experiment was done three times at least. 2.5. Surface analyses and characterization The X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Perkin-Elmer PHI5300 system fitted with a microfocused, monochromatic Mg Kα X-ray source (1486.6 eV) and a magnetic lens that increases the electron acceptance angle and hence the sensitivity. The energy scale of the spectrometer was calibrated25 by using an argon-ion etched copper plane and the spectra were referenced using the most intense hydrocarbon C1s photoelectron peak taken at 284.6 eV.

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The curve fitting of the XPS spectra was carried out using a nonlinear least squares curve-fitting program with a Gaussian / Lorentzian product function. The Gaussian/Lorentzian mixture was taken as 0.5. A nonlinear background was used in the curve-fitting using the previously published procedure.26-28 3. Results and discussion 3.1. Conductivity measurements Monoalkyl phosphate ester molecules are composed of polar hydrophilic moiety (phosphate group) and a nonpolar hydrophobic moiety (alkyl group). The existence of amphiphilic groups makes such substances show some properties of surfactants, such as enrichment at water / air interface, the spontaneous self-assembly at the concentration above critical micelle concentration (CMC), and so on, depending on the length of their alkyl groups. As compared with the adsorption of common inhibitors at the metal / solution interface via electrostatic or covalent bonding between the metal surface atoms and the adsorbates, the adsorption of alkyl phosphate ester molecules on the metal surface is more strongly dependent on the charge of the metal surface and the free energy change of transferring hydrocarbon chains from water to the metal surface. Moreover, it is possible that enough alkyl phosphate ester molecules adsorb onto the metal surface, forming the organized “hemimicelles”, under appropriate conditions. It is also important to note that BP, HP and OP are medium strong acids whose pKa1 values are 1.97, 1.97 and 1.96, respectively. They may ionize partly in water to give 1 or 2 hydrogen ions depending on the pH value of solution. Herein conductivity measurements were carried out to characterize whether three phosphate esters, BP, HP and OP, acted like surfactants. Figure 2 shows the conductivity versus

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concentration curves for BP, HP and OP in pure water at 25 ºC respectively. An apparent turning point (68 ppm) was observed from the curve of the conductivity vs. OP concentration (see Figure 2a), which corresponds to the CMC value of OP. It is of interest that the same was true of HP, as shown by Figure 2b. Because OP is kind of surfactant, the existence of CMC value should be taken for granted. However, the appearance of CMC (166 ppm) implies that HP also behaves like a surfactant to a certain extent despite of the relatively short carbon chain. In contrast to HP and OP, BP (see Figure 2c) does not show any characteristic behavior of a surfactant. Therefore, BP is treated as an organic inhibitor in this paper. In order to better compare the inhibition effects of three alkyl phosphate esters under similar conditions, the maximum concentration of OP or HP to be chosen did not exceed considerably the respective CMC value. 3.2. Weight loss measurements Weight loss measurements were performed to compare the mean corrosion rates (r) of iron in 0.5 M H2SO4 solutions in the absence and presence of different phosphate esters, thereby providing the direct evidence for the evaluation of inhibition performance of three alkyl phosphate esters. The value of r was calculated based on the following equation29: r = (m1 – m2) / S.t

(1)

where m1 and m2 are the mass of an iron sample before and after immersion in a corrosive solution respectively, S is the total area of an iron sample, and t is the immersion time. On that basis, inhibition efficiency (IE) of each phosphate ester was determined as follows: IE % = (r0 – r) / r0 ×100

(2)

where r0 and r stand for the mean corrosion rate of iron in absence and presence of a

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designated inhibitor, respectively. Table 1 shows the values of mean corrosion rates obtained from Equation (1) as well as the inhibition efficiency values calculated using Equation (2). The present results indicate that, three alky phosphate esters can all inhibit the corrosion of iron in H2SO4 solution but the inhibition efficiency of HP or OP is much higher than that of BP. Besides, for every alky phosphate ester, the IE value increased with the increase of its concentration. The inhibition mechanisms of three alky phosphate esters for the iron corrosion were further studied by means of electrochemical methods and XPS analysis. 3.3. Polarization curves Steady-state polarization curves were used to investigate the influence of BP, HP and OP on cathodic and anodic behavior of iron in 0.5 M H2SO4 solution. Figures 3a-c show the polarization curves for iron in 0.5 M H2SO4 in the absence and presence of various concentrations of BP, HP and OP. It is seen from the three figures that the presence of alkyl phosphate esters, regardless of carbon chain lengths, suppressed the cathodic current more significantly than anodic current, making the corrosion potential (Ecorr) move negatively. In general, if the negative shift of Ecorr is greater than 85 mV, the inhibitor is considered to be a cathodic type inhibitor.30 It was observed that the negative shift of Ecorr values ranged from 8 mV to 59 mV in the presence of different concentrations of BP, from 16 mV to 38 mV in the case of HP, from 53 mV to 73 mV in the case of OP, respectively. Thus, BP, HP and OP can all be considered as mixed-type inhibitors. Meanwhile, the cathodic inhibition effectiveness of BP, HP and OP increased obviously with the increase of concentration. Moreover, HP and OP showed the stronger inhibition effect on the cathodic reaction than BP.

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The electrochemical parameters, including corrosion potentials (Ecorr), corrosion current densities (icorr) and their standard deviations (denoted as Sicorr), and cathodic Tafel slopes (bc), were determined by Tafel extrapolation of cathodic polarization curves to the respective corrosion potentials, and they were all listed in Table 2. The small Sicorr values confirm that the icorr values obtained by Tafel extrapolation of the cathodic branch of polarization curves are reliable and can be used to evaluate the inhibition effectiveness of these inhibitors. The polarization curve of iron in inhibitor-free H2SO4 solution shows that bc = -128.1 mV decade-1, implying that the cathodic hydrogen evolution reaction (HER) occurred through the Volmer-Tafel mechanism controlled by the charge-transfer step.31 In view of low coverage of Hads at the initial stage of iron corrosion, the HER consisted of slow electrochemical adsorption step (Volmer adsorption) and fast chemical desorption step (Tafel desorption). The inhibition action of BP, HP and OP to the corrosion process of iron is dependent on the adsorption of their molecules on the iron surface. As seen in Table 2, the changes of inhibitor concentration did not cause a significant negative shift of Ecorr, so the inhibition effect may be believed to result from geometric blocking mechanism.32, 33 It is well-known that the corrosion process of iron in an acidic aqueous solution involves hydrogen evolution and iron dissolution. The HER involves proton discharge onto surface sites and formation of Hads. In this way, when a part of surface was covered by adsorbed inhibitor molecules, the HER could only occur at uncovered sites on the iron surface. At the same time, the diffusion rate of adsorbed H atoms along the iron surface is greatly hampered, which can be reflected by the increase of cathodic Tafel slopes in the inhibitor-containing solutions, especially in the presence of HP and OP. By contrast, three alkyl phosphate esters showed the lower inhibition effect on the anodic iron

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dissolution than on the HER. Particularly, the anodic curves in the BP-containing solution were almost coincided with that of iron in the blank solution, as indicated in Figure 3a. The main reason is that the BP with the short carbon chain can not form dense barrier layers by the covalent interaction with surface atoms or by reacting with iron ions dissolved. This situation was changed when HP and OP were used as the inhibitors. Figures 3b and c show that HP and OP were able to inhibit the iron dissolution at relatively high concentrations, especially at the concentrations around respective CMC values. It should be noticed that the anodic curves of iron (see Figure 3c) gave a current plateau at the lower anodic potentials when the concentration of OP approached (66 ppm) or exceed (88 ppm) its CMC. OP molecules would aggregate at the electrode / solution interface and form the ordered hemimicelles on the iron surface at the concentration close to or higher than the CMC, which lead to the relatively significant anodic inhibition effect. The rapid rise of anodic current that followed was attributed to the destruction of the hemimicelles because of the iron dissolution driven by the applied anodic potential. Because the formation of hemimicelles affected the inhibition effect more strongly than the length of alkyl groups, there were no significant differences between the inhibition efficiencies of OP and HP under similar conditions. The results of polarization curves demonstrate that the inhibition effectiveness of three alkyl phosphate esters is not only associated with the length of alkyl group but also more related to the aggregation state of inhibitor molecules at the electrode / solution interface. The IE of BP, HP and OP in each case can be calculated using the following equation: IE % = (i0corr – icorr) / i0corr ×100

(3)

where i0corr represents the corrosion current density of iron in the blank solution, and icorr

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represents that obtained in presence of inhibitors. By comparing the IE values of the three phosphates, it is concluded that, for each inhibitor, its IE value increased with increasing the concentration. The highest IE value of BP was 58.1%, much lower than that of HP (82.7%) and that of OP (75.5%).

3.4. EIS spectra Electrochemical impedance spectroscopic (EIS) measurements were performed to better study how three alkyl phosphate esters inhibit the corrosion behavior at their respective corrosion potentials (i.e. open-circuit potentials, OCPs). Figure 4 presents several sets of Nyquist impedance plots (a-c) and Bode impedance plots (d-f) of the iron electrode in 0.5 M H2SO4 solutions without and with different concentrations of alkyl phosphate esters at the respective corrosion potentials. It is clearly observed from Figures 4a-c that, all Nyquist plots display a similar profile in addition to slight differences in low frequency regions. Each spectrum is composed of capacitive loop in high frequency and irregular outline in low frequency. For the corrosion of iron in the blank solution, the high frequency capacitive loop is caused by the relaxation time constant of charge-transfer resistance (Rct) with double-layer capacitance (Cdl) at the electrode / solution interface, and irregular impedance behavior in low frequency (more like a depressed capacitive loop) originates from the relaxation processes of adsorbed species that produced in the processes of the HER or the iron dissolution. The addition of alkyl phosphate esters made the capacitive loop increase significantly in size but kept the shape of the impedance spectrum unchanged. The increase of the capacitive loop in diameter means that the Rct was distinctly enlarged after addition of alkyl phosphate esters in the H2SO4 solutions, even in the level of ppm. More exactly, the corrosion process of iron was strongly inhibited in 13



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the presence of tiny amount of BP (HP or OP). The higher the concentration of an alkyl phosphate ester, the larger the diameter of a capacitive loop or the larger the Rct value, which reflects that the increase of alkyl phosphate ester concentration was beneficial to enhancing the inhibition effect on the corrosion of iron. On the other hand, by comparing the size of capacitive loop measured in the solutions containing the similar concentrations of BP, HP and OP, one will draw two conclusions: (i) BP with the shorter carbon chain can not inhibit the corrosion of iron as effectively as HP and OP with the longer carbon chain; (ii) the inhibition effectiveness of HP is roughly equal to that of OP despite of different chain length. Bode magnitude plots (Figure 4(d-f)) clearly illustrate the significant increase of the magnitude of the measured impedance data with increasing the inhibitor concentration, especially in the presence of HP and OP. This is further evidence that BP, HP and OP were able to inhibit the corrosion of iron under acidic condition. At the same time, we noticed that the Bode phase plots obtained when BP served as the inhibitor or the concentration of HP and OP was relatively lower (8 and 40 ppm for HP, and 22 and 44 ppm for OP) only showed one time constant. However, Bode phase plots presented two time constants when the concentrations of HP and OP were around or above their respective CMC values, for example, 80 and 160 ppm for HP, and 66 and 88 ppm for OP. Because HP and OP behave like a surfactant, once their concentration is close to or more than their CMC, HP or OP molecules will aggregate at the electrode / solution interface, forming a certain number of micelles. This is bound to affect the adsorption mode of HP or OP molecules and the ordering of the resulting adsorbed layers on the iron surface, which in turn caused the new time constant in the measured impedance spectra. It is reasonable to assume that the corrosion reaction only occurred at the sites that

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were not effectively covered by the alkyl phosphate ester molecules. And in this study, we mainly focus on the inhibition action of BP, HP or OP on the charge-transfer process at the electrode / solution interface. Then , by ignoring the irregular impedance behavior in the low frequency, which has been known to be associated with the roughness of the electrode surface and the relaxation processes of adsorbed species that produced in the processes of the HER or the iron dissolution, the impedance spectra containing one time constant can be fitted with the equivalent circuit given in Figure 5(a) 34, 35 and the spectra containing two time constants may be fitted with the equivalent circuit shown in Figure 5(b). In the two circuits, Rs represents the solution resistance, Rct the charge-transfer resistance, Ra the resistance to pass through the adsorbed layers, and Qa, Qdl and Q’dl stand for the constant phase elements (CPE: whose admittance is defined as YCPE = Y0(jω)n) that are substituted for a pure capacitor in order to fit more exactly the depressed capacitive loop. In Figure 5(a), the Q’dl element contains the contributions from the double-layer capacitance (Qdl) and the capacitance of the adsorbed layers (Qa). The values of elements of the equivalent circuit obtained by fitting the EIS spectra, together with the standard deviations of Rct (denoted as SRct), were given in Table 3. Small standard deviations of Rct indicate the reliability of the Rct values. Moreover, the value of the double layer capacitance (Cdl) can be calculated by the following equation: Cdl = Y0 (ωmax) n-1

(4)

where ωmax is the frequency at which the imaginary part of impedance has a maximum, and Y0 and n are the magnitude and the exponential term of a CPE respectively. On the basis of Rct values in the absence and presence of inhibitors, IE values of an inhibitor can also be calculated by the equation given below

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IE % = (R’ct – Rct) / R’ct ×100

(5)

where Rct and R’ct are the values of charge-transfer resistance in inhibitor-free and inhibitor-containing H2SO4 solutions. As far as we know, the larger the value of Rct, the better the inhibition effectiveness of an inhibitor. At the same time, the value of Cdl reflects the coverage of adsorbed inhibitor molecules on the metal surface. Accordingly, the smaller the value of Cdl, the higher the coverage of inhibitor; correspondingly, the inhibitor can inhibit the corrosion of iron more effectively. As can be seen from Table 3, the values of Rct are increased and the values of Cdl are decreased as the concentration of any alkyl phosphate ester increased. These changes indicate that the inhibition efficiency of an inhibitor increased with increasing its concentration. Here the highest IE values for BP, HP and OP were 72.4%, 83.6% and 83.4%, respectively. The IE values obtained by EIS method is basically consistent with those obtained by polarization curve method in the range of experimental errors.

3.5. XPS analysis The surface compositions of the iron samples treated by immersion in 0.5 M H2SO4 solutions containing different alkyl phosphate esters and the corresponding bonding mode between alkyl phosphate esters and iron substrate were analyzed by core level XPS. Figures 6, 7 and 8 show the most intensive high resolution spectra of major elements, C, P, O and Fe for the iron samples after being immersed in 0.5 M H2SO4 solutions containing BP, HP and OP, respectively. As shown in Figure 6, the P2p core level spectrum is in low intensity, showing that there are few adsorbed BP molecules on the iron surface. P2p region is fitted into a single peak located at 132.5 eV corresponding to P-O in phosphate group of BP. The C1s region can 16


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be fitted into two peaks. The main peak centered at 284.6 eV is related to C-H or C-C bonds and the other peak at 282.8 eV indicates the presence of carbon impurities in the iron sample. The O1s core level spectrum can be fitted into three main peaks. The most intensity peak located at 529.5 eV is related to the oxygen atoms bonded to ferric oxides, such as Fe2O3 and FeOOH.36 The peak centered at 531.5 eV is attributed to O-P in the phosphate group of BP molecule.36-38 The other peak located at 532.1 eV is associated with chemisorbed H2O.36, 39 The high resolution Fe2p spectrum shows one pair of spin orbit doublet peaks with the fixed area ratio 2 : 1 between the Fe2p3/2 and Fe2p1/2 component. The Fe2p3/2 and Fe2p1/2 peaks are located at 711.3eV and 723.4eV, corresponding respectively to the oxidized iron compounds (α-Fe2O3 or α-FeOOH). On the basis of fitted XPS spectra, the peak positions and the corresponding atomic ratios were collected and listed in Table 4. In Table 4, the ratio of O1s (H-O-P / C-O-P) : P2p is 3.0 : 0.7, which equals to 4.2, suggesting that the four P-O- bonds in the BP molecule are similar and there is no Fe-O-P bond between the BP molecule and the iron surface. This indicates that the BP molecules were bonded to the iron surface mainly through the adsorption of unionized molecules and no ferric phosphate compounds formed on the iron surface. And the atomic ratio of O1s (Fe oxide) : Fe2p is 21.8 : 10.2, about 2.1, which suggests that the iron oxide formed on the iron surface is FeOOH. From Figures 7 and 8 we can see that the XPS spectra for iron samples treated by immersion in 0.5 M H2SO4 solutions containing HP and OP are similar to the spectrum for the iron sample which was treated by immersion in 0.5 M H2SO4 solution containing BP. The low intensity P2p core level spectra show that the HP and OP molecules could also adsorb on the iron surface. A

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single peak is curve-fitted located at 132.5 eV corresponding to P atomic in phosphate group of HP and OP. The C1s peak can be fitted into two peaks located at 284.6 eV and 282.8 eV, very similar to that of BP. For the sample which was treated by immersion in 0.5 M H2SO4 solutions containing HP, the O1s spectrum contain three peaks centered at 529.7 eV, 531.4 eV and 532.2 eV, which correspond to oxygen atoms bonded to ferric oxides, O-P in the phosphate group and chemisorbed H2O respectively. The O1s spectrum for the iron sample which was treated by immersion in 0.5 M H2SO4 solutions containing OP is similar to the spectra of BP and HP and can be fitted into three peaks centered at 529.5 eV, 531.4 eV and 532.1 eV, respectively. The Fe2p3/2 and Fe2p1/2 peaks of the iron treated by immersion in 0.5 M H2SO4 solutions containing HP and OP are both located at 711.2eV and 723.4eV, corresponding to the oxidized iron compounds (α-Fe2O3 or α-FeOOH). As shown in Table 4, the specific values of O1s (H-O-P / C-O-P) : P2p are 4.4 and 3.9 for HP and OP, respectively, which indicates that, like BP, HP and OP molecules are also bonded to the iron surface mainly through the adsorption of unionized molecules. The atomic ratio of O1s (Fe oxide) : Fe2p for the iron samples treated by HP and OP is 1.8 and 1.9 respectively, close to 2, suggesting that the iron oxide formed on the iron surface is FeOOH. The similar XPS spectra and the atomic ratios for the iron samples treated by immersion in 0.5 M H2SO4 solutions containing BP, HP and OP indicate that the adsorption behavior of three alkyl phosphate esters is almost identical although there exist small differences in the P atomic ratios (see Table 4). The higher P atomic ratio for HP or OP indicates that the adsorption ability of HP and OP is stronger than that of BP. This may be the reason why the inhibition efficiency of HP or OP is much higher than that of BP.

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3.6. Adsorption isotherms The adsorption mode of BP, HP and OP can be derived from the curves of surface coverage against inhibitor concentration. The most frequently used adsorption isotherms are Langmuir, Temkin, Frumkin and Freundluich isotherms. After several adsorption isotherms were tested, we found that the Langmuir isotherm provided the best description of the adsorption behavior. C / θ =1 / Kads + C

(6)

where C is the concentration of an inhibitor and Kads is the adsorption equilibrium constant, the plot C / θ versus C will be a straight line with a slop of unity. Here surface coverage (θ) may be considered to be approximately equal to IE value since inhibitor inhibits the corrosion of iron through geometric blocking effect in acidic solutions. IE Data calculated from polarization curves results were used for the plot of C / θ versus C. Figure 9 shows that there is a good linear relationship between C / θ and C, with the regression coefficient of 0.9979, 0.9997 and 0.9979 for BP, HP and OP, respectively. This suggests that the adsorption isotherm of BP, HP and OP belongs to Langmuir Type. And moreover, the values of ∆G0ads for the BP, HP and OP are obtained from the following equation40: ln Kads = ln (1 / 55.5) –∆G0ads / RT

(7)

where 55.5 (mol L-1) is the molar concentration of pure water, R (8.314 J K-1 mol-1) the gas constant, and T (K) the absolute temperature. The thermodynamic parameters derived from the Langmuir adsorption isotherms of BP, HP and OP are shown in Table 5. The negative values of ∆G0ads suggest that the adsorption of BP, HP and OP on the iron surface in 0.5 M H2SO4 is a spontaneous process. Generally, if the value

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of ∆G0ads is up to -20 kJ mol-1, this type of adsorption is considered as physisorption, and the adsorption process is attributes to the electrostatic interaction between the charged molecules and the charged metal surface. Furthermore, if the value of ∆G0ads is close to or much lower than -40 kJ mol-1, the possible adsorption type is considered as the chemisorption involving the charge sharing or the formation of a coordinate covalent bond by the transfer from organic molecules to the metal surface.41-44 The values of ∆G0ads for BP, HP and OP clearly indicate that the adsorption of these compounds is predominantly controlled by chemisorption. One can see from Table 5 that the values of ∆G0ads for HP and OP are much lower than the value of ∆G0ads for BP, meanwhile the values of K for HP and OP are much higher the that of BP. The lower values of ∆G0ads along with higher K values for HP and OP demonstrate that HP and OP molecules adsorb on the iron surface more easily than BP molecules.

4. Conclusion Carbon chain length of alkyl phosphate esters and molecular aggregation state of these substances at the iron / solution interface both affect their inhibition effectiveness to the corrosion of iron in acidic solutions. Although BP, HP and OP all act as a mixed type inhibitor and inhibit the corrosion of iron very effectively, the inhibition efficiency of BP with the shorter alkyl chain is much lower that that of HP or OP with the longer alkyl chain under similar condition. By contrast, the inhibition efficiency of HP is appropriately equal to that of OP since HP and OP behave like a surfactant and are able to form ordered molecular aggregates at the iron / solution interface. The XPS results give evidence that HP and OP molecules adsorb on the iron surface more easily than BP molecules. The adsorption of BP, HP and OP all obey the Langmuir adsorption isotherm and the adsorption type belongs to 20


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chemisorption. The values of adsorption free energy are -30.40 KJ mol-1, -35.78 KJ mol-1 and -33.43 KJ mol-1 for BP, HP and OP, respectively.

Acknowledgment This work was supported by the National Natural Science Foundation of China (21373129) and the Fundamental Research Funds of Shandong University (2014YQ004).

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Figure 1. Chemical structures of BP (a), HP (b) and OP (c).

22


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Figure 2. Curves of electrical conductivity against the OP (a), HP (b) and BP (c) concentration at room temperature (25 ºC).

23



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Figure 3. Polarization curves of iron in 0.5 M H2SO4 solutions in the absence and presence of BP (a), HP (b) and OP (c). Before each test, the working electrode was placed in the electrolyte for 30 min.

24


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Figure 4. Nyquist (a, c, e) and Bode (b, d, e) impedance plots of iron electrode in 0.5 M H2SO4 solutions in the absence and presence of BP (a, d), HP (b, e) and OP (c, f).

25



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Figure 5. Equivalent circuits used to fit the EIS data of iron electrode in 0.5 M H2SO4 solutions in the absence and presence of BP, HP (8 and 40 ppm) and OP (22 and 44 ppm) (a), HP (80 and 160 ppm) and OP (66 and 88 ppm) (b).

26


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Figure 6. High resolution spectra of the major elements on the iron surface after immersion in 0.5 M H2SO4 solution containing BP.

27



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Figure 7. High resolution spectra of the major elements on the iron surface after immersion in 0.5 M H2SO4 solution containing HP.

28


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Figure 8. High resolution spectra of the major elements on the iron surface after immersion in 0.5 M H2SO4 solution containing OP.

29



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Figure 9. Linearized form of the Langmuir adsorption isotherms for the adsorption of BP (a), HP (b) and OP (c) on the iron surface.

30


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Table 1 The calculated inhibition efficiencies of BP, HP and OP for the iron corrosion in 0.5 M H2SO4 based on the weight loss tests at 25 ºC. Concentration

r

(ppm)

(mg cm-2 h-1)

Blank

4.69

-

50

2.52

46.3

100

2.09

55.4

200

2.15

54.2

400

2.03

56.7

8

2.57

45.2

40

1.04

77.8

80

0.78

83.4

160

0.17

96.4

22

2.18

53.5

44

2.07

55.9

66

1.40

70.1

88

0.94

80.0

BP

HP

OP

IE%

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Table 2 Polarization parameters for iron electrode in 0.5 M H2SO4 solutions in the absence and presence of BP, HP and OP

Concentration

bc

icorr

(ppm)

(mV dec-1)

(A cm-2)

Blank

- 128.1

2.30×10-4 1.58×10-6 - 491

50

- 133.3

1.44×10-4 1.58×10-6 - 499 37.4

100

- 125.3

1.02×10-4 1.00×10-7 - 520 55.6

200

- 132.3

1.08×10-4 2.64×10-7 - 547 53.0

400

- 126.5

9.63×10-5 2.12×10-7 - 550 58.1

8

- 136.2

9.22×10-5 3.46×10-7 - 509 59.9

40

- 136.6

6.21×10-5 1.73×10-7 - 507 73.0

80

- 142.0

4.18×10-5 2.60×10-7 - 507 81.8

160

- 136.8

3.98×10-5 1.58×10-7 - 529 82.7

22

- 137.9

1.02×10-4 2.34×10-7 - 544 55.6

44

- 134.2

8.38×10-5 2.73×10-7 - 564 63.6

66

- 134.2

6.90×10-5 3.60×10-7 - 564 70.0

88

- 137.4

5.64×10-5 4.06×10-7 - 564 75.5

BP

OP

HP

Sicorr

32


Ecorr

IE

(mV)

(%) -

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Table 3 Values of the elements of the equivalent circuit in Figure 5 obtained by fitting the EIS spectra of iron electrode in 0.5 M H2SO4 solutions in the absence and presence of BP, HP and OP.

Concentration

Rs

Ra

Rct

Qa

SRct

(ppm)

(Ω cm2)

Y0

(Ω cm2) -1

n

(Ω cm )

Y0

-2 n

-1

(Ω cm s )

HP

OP

n

(uF cm-2)

IE (%)

-2 n

(Ω cm s )

0.51

-

-

-

34.96

0.432

2.56 × 10-4

0.80

100

-

50

0.27

-

-

-

73.53

1.54

1.76 × 10-4

0.81

95.4

52.4

100

0.43

-

-

-

111.0

1.73

1.29 × 10-4

0.81

66.1

68.5

200

0.44

-

-

-

102.0

1.22

1.60 × 10-4

0.80

84.0

65.7

400

0.44

-

-

-

126.7

2.36

1.37 × 10-4

0.80

71.9

72.4

8

0.55

-

-

-

86.11

1.23

1.56 × 10-4

0.74

79.9

59.4

40

0.37

-

-

-

113.2

2.04

1.33× 10-4

0.69

57.5

69.1

80

0.32

3.12

5.96× 10-6

1

136.5

2.57

1.16 × 10-4

0.69

42.7

74.4

160

0.22

3.96

4.37× 10-6

1

212.8

4.46

8.20 × 10-5

0.78

40.4

83.6

22

0.45

-

-

-

112.7

2.18

1.52 × 10-4

0.81

82.4

69.0

44

0.28

-

-

-

159.2

4.01

1.59 × 10-4

0.74

68.8

78.0

66

0.42

2.45

8.05× 10-6

1

158.0

2.97

1.16 × 10-4

0.76

53.5

77.9

88

0.25

4.05

5.22× 10-6

1

210.6

3.12

1.13 × 10-4

0.72

45.8

83.4

Blank BP

Cdl

Qdl

2

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Table 4 The positions, peak areas and atomic ratios of the most intense peaks in the core XPS region of the iron sample after immersion in 0.5 M H2SO4 containing BP, HP and OP.

Element Peak position (eV) Atomic ratio (%) BP P 132.4 0.7 C 284.6 37.9 282.8 13.8 O 529.5 21.8 531.5 3.0 532.1 12.6 Fe 711.3 / 723.4 10.2 HP

P

132.4

1.0

C

284.6 282.8 529.7 531.4 532.2 711.2 / 723.2

45.1 10.5 18.2 4.4 10.6 10.2

133.0 284.6 282.8 529.5 531.4 532.1 711.2 / 723.2

1.2 47.7 12.7 15.7 4.7 9.7 8.3

O

Fe OP

P C O

Fe

34


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Table 5 Thermodynamic parameters for the adsorption of BP, HP and OP in 0.5 M H2SO4 solutions on the surface of iron electrode at 25 ºC

R2 K ads (M-1) Inhibitor ∆G0ads (KJ mol-1) BP - 30.40 0.9958 3876.0 HP - 35.78 0.9994 33932.8 OP - 33.43 0.9958 13157.9

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Corrosion Inhibition of Iron in Acidic Solutions by Monoalkyl

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