Adsorption and Corrosion Inhibition Behavior of Mild Steel by One

Feb 18, 2010 - inhibition of mild steel corrosion in acid solutions by various types of organic ..... by an intramolecular hydrogen bond.44 The proton...
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Ind. Eng. Chem. Res. 2010, 49, 2593–2599

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Adsorption and Corrosion Inhibition Behavior of Mild Steel by One Derivative of Benzoic-Triazole in Acidic Solution Zhihua Tao,† Shengtao Zhang,*,† Weihua Li,‡ and Baorong Hou‡ College of Chemistry and Chemical Engineering, Chongqing UniVersity, Chongqing 400044, China and Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

A newly synthesized benzoic-triazole derivative 3,5-dimethylbenzoic acid [1,2,4]triazol-1-ylmethyl ester (DBT) was investigated as a corrosion inhibitor of mild steel in 1 M HCl solution using weight loss measurements, potentiodynamic polarization, SEM, and EIS methods. The results revealed that DBT was an excellent inhibitor, and the inhibition efficiencies obtained from weight loss and electrochemical experiments were in good agreement. Using the potentiodynamic polarization technique, the inhibitor was proved to have a mixed-type character for mild steel by suppressing both anodic and cathodic reactions on the metal surface. The number of water molecules (X) replaced by a molecule of organic adsorbate was determined from the Flory-Huggins, Dhar-Flory-Huggins, and Bockris-Swinkels substitutional adsorption isotherms applied to the data obtained from the gravimetric experiments performed on a mild steel specimen in 1 M HCl solution at 298 K. 1. Introduction Acids are widely used in industries such as picking, cleaning, descaling, etc.1-3 In efforts to mitigate electrochemical corrosion, the strategy is to isolate the metal from corrosive agents. Use of corrosion inhibitors is the most economical and practical method to achieve this objective.4,5 During the past decade, inhibition of mild steel corrosion in acid solutions by various types of organic inhibitors has attracted much attention.6-8 However, most of the available inhibitors are toxic compounds that should be replaced by new inhibitors for environmental protection. Looking for effective and acceptable corrosion inhibitors in the surroundings instead of toxic inhibitors becomes necessary in the inhibition of mild steel corrosion in acid solutions. These compounds, rich in heteroatoms, such as sulfur, nitrogen, and oxygen, can be used as environmentally friendly inhibitors because of their strong chemical activity and low toxicity.9-14 Examples are triazole-type compounds containing several heterocyclic structures, which have excellent corrosion properties for the corrosion of many metals in various aggressive media.15-18 The objective of the present work was to study the mechanism of corrosion and inhibition of the newly synthesized benzoictriazole derivatives on a metal surface. To the best of our knowledge, no data are available in the literature regarding the behavior of 3,5-dimethylbenzoic acid1,2,4 triazol-1-ylmethyl ester (DBT) as a corrosion inhibitor for metals. Some research methods, such as the weight loss method, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM), were applied to investigate the inhibition performance in acidic medium. 2. Experimental Section 2.1. Materials and Sample Preparation. The benzoictriazole derivative, namely, 3,5-dimethylbenzoic acid [1,2,4]triazol-1-ylmethyl ester (DBT), was synthesized in our laboratory, which was purified and characterized by 1H NMR, IR, and MS. * To whom correspondence should be addressed. Tel.: +86-2365112134. Fax: +86-23-65112134. E-mail: [email protected]. † Chongqing University. ‡ Chinese Academy of Sciences.

DBT is a white solid with a yield of 74.0%; mp 105s107 °C. 1 H NMR (CDCl3, 600 MHz) δ: 8.48 (s, 1H, Tr-H), 7.99 (s, 1H, Tr-H), 7.66-7.22 (m, 3H, Ar-H), 6.29 (s, 2H, N-CH2), 2.34 (6H, 2CH3). IR (KBr) ν: 3118 (dC-H), 1721 (CdO), 1515 (CdN), 1108 (C-O-C) cm-1. MS (ESI) m/z: 231, 201, 133, 105, 98, 82, 77, 70, 55. Anal. Calcd for C12H13N3O2: C, 62.42; H, 5.54; N, 18.25. Found: C, 62.31; H, 5.67; N, 18.18. The synthetic route and molecular structures are shown in Figure 1. The mild steel strips having a composition (wt %) of C ) 0.17, P ) 0.0047, S ) 0.017, Mn ) 0.46, Si ) 0.26, Cu ) 0.019, and balance Fe were used for weight loss as well as electrochemical studies. DBT was dissolved in 1 M HCl at different concentrations (from 1 × 10-5 to 1 × 10-3 M). The solution in the absence of DBT was taken as a blank for comparison. 2.2. Electrochemical Techniques. The electrochemical experiments were performed in a classical three-electrode cell assembly with mild steel as the working electrode, platinum foil of 1.5 cm × 1.5 cm as the counter electrode, and a saturated calomel electrode (SCE) provided with a Luggin capillary as the reference electrode. The electrochemical measurements were performed on cuboid mild steel electrodes with a 1 cm2 surface area at 298 K with a PARSTAT 2273 Potentiostat/Galvanostat (Princeton Applied Research) and then the potentiodynamic polarization test. EIS measurement was carried out in the 100 kHz to 10 mHz frequency range at a steady open-circuit potential (OCP) disturbed with an amplitude of 10 mV. The polarization curves were obtained from a cathodic potential of -250 mV vs SCE to an anodic potential of +250 mV vs SCE with respect to the open-circuit potential at a sweep rate of 0.5 mV s-1 in an aerated system to study the effect of inhibitor on mild steel corrosion. The electrochemical experiments data were collected and analyzed by electrochemical software PowerSuite ver.2.58. The inhibition efficiency (IE %) was calculated by eqs 1 and 219,20 IE(i)% )

10.1021/ie901774m  2010 American Chemical Society Published on Web 02/18/2010

° Icorr - Icorr ° Icorr

× 100

(1)

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IE(z)% )

Rct - R°ct × 100 Rct

(2)

where I°corr and Icorr are the corrosion current density in the absence and presence of inhibitors, respectively, and Rct and R°ct are the charge-transfer resistance in the presence and absence of the inhibitors, respectively. 2.3. Weight Loss Experiment. The weight loss study is done on mild steel strips of 3.0 cm × 1.5 cm × 1.5 cm. Mild steel specimens for each inhibitor concentration were immersed in 1 M HCl for 3 h at 298 K. The temperature was controlled by an aqueous thermostat. The inhibition efficiency (IE %) was evaluated by eq 319 IE(w)% )

W0 - W × 100 W0

(3)

where W0 and W are the corrosion rates in the absence and presence of the inhibitors, respectively. Weight loss experiment calculated the mean corrosion rate (W) as expressed in mg cm-2 h-1. 2.4. SEM Analysis. The surface morphology of specimens after immersion in 1 M HCl in the absence and presence of 1.0 × 10-3 M inhibitor (DBT) was performed on a KYKY2800B scanning electron microscope. The accelerating voltage was 25 kV. 3. Results and Discussion 3.1. Electrochemical Experiment. 3.1.1. Polarization Curve. Figure 2 shows the polarization curves of mild steel in 1 M HCl with different concentrations of DBT. The corrosion current density (Icorr), corrosion potential (Ecorr), and anodic (βa) and cathodic (βc) Tafel slopes were obtained by extrapolation of the anodic and cathodic regions of the Tafel plots.17 The electrochemical parameters Ecorr, Icorr, and anodic and cathodic Tafel slopes (βa, βc) obtained from the polarization measurements are listed in Table 1. IE % was calculated by eq 1. In the cathodic domain, it was clear that the cathodic current density decreased with increasing concentration of the inhibitor; this indicated that the DBT were adsorbed on the metal surface and hence inhibition occurs. Thus, addition of the inhibitor hindered acid attack on the mild steel electrode. The parallel cathodic Tafel curves showed that the hydrogen evolution was activation controlled and the reduction mechanism was not affected by the presence of the inhibitor. Compared to the blank sample, the anodic curves of the working electrode

Figure 1. Synthetic route and molecular structure of DBT.

Figure 2. Polarization curves for mild steel in 1 M HCl with different concentrations of DBT.

in the acidic solution containing the DBT shifted obviously to the direction of current reduction, which implied that the organic compounds can also suppress the anodic reaction and the shift in the anodic Tafel slope βa may be due to the chloride ions or inhibitor modules adsorb onto the steel surface.21 Differently, for the anodic polarization curves of DBT (in Figure 2), at concentrations of 3.2 × 10-4 M or higher, it seemed that the working electrode potential was higher than -330 mV/ SCE; the presence of the inhibitor did not change the current versus potential characteristics. This potential can be defined as the desorption potential.22 The phenomenon might be due to the obvious metal dissolution, leading to a desorption of the DBT molecule from the electrode surface; in this case the desorption rate of the inhibitor was higher than its adsorption rate, so the corrosion current increases more obviously with increasing potential.23 According to Riggs and others,24 the classification of a compound as an anodic or cathodic inhibitor is feasible when the OCP displacement is at least 85 mV in relation to that measured for the blank solution. Figure 3 shows the steady-state open-circuit potential for mild steel in 1 M HCl containing different concentrations of DBT. From Figure 3 it can be seen that the displacement is at most 20 mV with respect to E°corr (the open circuit potential of l M HCl without DBT). Therefore, DBT might act as a mixed-type inhibitor. 3.1.2. Electrochemical Impedance Spectroscopy (EIS). Nyquist plots of mild steel in 1 M HCl solution in the absence and presence of various concentrations of DBT are given in Figure 4. The shapes of these impedance spectra did not present

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Table 1. Electrochemical Parameters Calculated from Polarization Measurements on the Steel Electrode in 1 M HCl + x M Inhibitor at 298 K after 0.5 h of Its Immersion in the Corresponding Solution under Open-Circuit Conditions inhibitor

concentration x (M)

Ecorr (mV vs SCE)

βc (mV dec-1)

βa (mV dec-1)

Icorr (µA cm-2)

IE(i) %

DBT

0 1.0 × 10-5 3.2 × 10-5 1.0 × 10-4 3.2 × 10-4 1.0 × 10-3

-486.2 -467.9 -474.9 -486.9 -479.6 -471.6

139.3 117.3 115.2 113.5 136.6 194.0

81.7 42.4 41.3 58.7 88.1 93.8

869. 93.1 55.7 32.6 12.7 9.2

89.3 93.6 96.3 98.5 98.9

perfect semicircles. The Nyquist curve contains a single capacitive semicircle for mild steel electrode in 1 M HCl, 1 M HCl + 3.2 × 10-4 M DBT, and 1 M HCl + 10-3 M DBT. The standard Randles’ circuit model was used and is shown in Figure 5a. However, for 1 M HCl + 10-5 M DBT, 1 M HCl + 3.2 × 10-5 M DBT, and 1 M HCl + 10-4 M DBT, the Nyquist plots were composed of two parts: one depressed a capacitive loop at the higher frequency range (HF) and an inductive loop that is observed in the lower frequency region (LF). In this case, the related electrochemical equivalent circuit model is shown in Figure 5b. The solution resistance Rs, charge-transfer resistance Rct, constant phase element (CPE), and inductive elements of L and RL were fitted, and their values are listed in Table 2. It is seen that addition of inhibitor increases the values of Rct and reduces the Cdl. At the highest concentration of 10-3 mol/L, the inhibition effect of DBT increased markedly and Rct has reached the highest value of 3468 Ω/cm2. The increase in

Rct values is attributed to formation of the barrier film which prevented acidic medium from attacking the metal surface.25 The presence of the LF inductive RL-L loop in the lower concentration range of DBT (10-5-10-4 M) might be attributed to the relaxation process obtained by adsorption species as Hads+ and Clads- on the electrode surface.26 It might be also attributed to the redissolution of the passivated surface at low frequencies.27 However, at a relatively high inhibitor concentration (3.2 × 10-4 to 10-3 M) the adsorption of DBT at a metal/solution interface could be represented as a substitution adsorption process between the inhibitor molecules in acidic solution and the adsorption species such as Hads+ and Clads-on the metallic surface. In other words, at a concentration of 3.2 × 10-4 M or more, it could be ignored for the relaxation process obtained by adsorption species as Hads+ and Clads- on the electrode surface. Thus, a similar structure model of the interface was proposed (Figure 5a). The capacitive loop was related to charge transfer in the corrosion process, whereas the depressed form of the higher frequency loop reflected the surface inhomogeneity of structural or interfacial origin, such as those found in adsorption processes.28 CPE is most often used to describe the frequency dependence of nonideal capacitive behavior. The impedance of the CPE is expressed as ZCPE ) Y -1(jω)-n

Figure 3. Steady-state open-circuit potential-t curve for mild steel in 1 M HCl containing different concentrations of DBT.

where Y is the magnitude of the CPE, -1 e n e 1, j equals -1, ω ) 2πf, and n is the fractional exponent which for the solid electrode/solution interfaces. The higher frequency range loops have a depressed semicircular appearance, 0.5 e n e 1, which is often referred to as the frequency dispersion as a result of the nonhomogeneity or roughness of the solid surface.29,30 The capacitance values can be calculated from CPE parameter values Y and n using the expression31 Cdl )

Figure 4. Nyquist diagrams for mild steel in 1 M HCl containing different concentrations of DBT.

(4)

Yωn-1 sin(nπ/2)

(5)

Figure 5. Equivalent circuits used to fit the EIS data of steel in 1 M HCl + x M inhibitor without (a) and with (b) an inductive loop.

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Table 2. Electrochemical Parameters Calculated from EIS Measurements on the Steel Electrode in 1 M HCl+ x M inhibitor at 298 K after 0.5 h of Its Immersion in the Corresponding Solution under Open-Circuit Conditions CPE 2

2

inhibitor

concentration (M)

Rs (Ω/cm )

Y (uF/cm )

n

Rct (Ω/cm2)

DBT

0 1.0 × 10-5 3.2 × 10-5 1.0 × 10-4 3.2 × 10-4 1.0 × 10-3

0.58 1.21 0.75 0.86 1.12 0.74

184.2 66.6 47.3 27.8 33.2 23.4

0.908 0.951 0.931 0.917 0.879 0.852

29.1 106 236.3 514.9 1682 3468

The addition of DBT provides lower Y values, probably as a consequence of replacement of water molecules by inhibitor at the electrode surface. In addition, the n value has a tendency to decrease with increasing inhibitor concentration interpreted as evidence for steel surface adsorption of molecules.32 The thickness of the protective layer (d) is related to Cdl according to the expression of the layer capacitance presented in the Helmholtz model33 Cdl )

(6)

where d is the thickness of the film, S the surface of the electrode, ε° the permittivity of the air, and ε the local dielectric constant. The value of Cdl is always smaller in the presence of the inhibitor than in its absence as a result of the effective adsorption of DBT. The results obtained from EIS measurements are in good agreement with that obtained from potentiodynamic polarization and weight loss measurements. 3.2. Weight Loss Measurements and Substitutional Adsorption Isotherms. IE(w) % obtained from weight loss measurements for different concentrations of DBT in 1 M HCl are given in Figure 6. This result suggested that the inhibition efficiency increases with increasing concentration of DBT. As the concentration reached 1.0 × 10-3 M, the inhibition efficiency of DBT obtained a high value of 90.8%, which represented excellent inhibitive ability. Adsorption of benzoic-triazole derivatives can be explained on the basis that adsorption of the inhibitor was mainly via the nitrogen atoms in the triazole ring in addition to the availability of π electrons (by resonance structures) in the aromatic system.34 Basic information on the interaction between the inhibitor molecules and metal surfaces could be provided from the

Figure 6. Inhibition efficiency (IE(w) %) obtained from weight loss measurements for mild steel in 1 M HCl solution in the presence of DBT at different concentrations.

L (H/cm2)

IE(z) %

9.1 30.2 55.2

138.8 131.8 442.5

72.55 87.69 94.35 98.27 99.16

adsorption isotherms. The values of surface coverage (θ) are defined as IE %/100 and obtained from weight loss measurements at 298 K. The adsorption of an organic adsorbate at the metal/solution interface can be presented as a substitution adsorption process between the organic molecules in aqueous solution (Orgaq) and the water molecules on the metallic surface (H2Oads)35 Orgaq + X · H2Oads T Orgads + X · H2Oaq

◦

εε S d

RL (Ω/cm2)

(7)

where X, the size ratio, is the number of water molecules displaced by one molecule of organic inhibitor. X is assumed to be independent of coverage or charge on the electrode.36 The following equations are the most commonly used substitutional isotherms of Flory-Huggins37 θ X(1 - θ)X

(8)

θ exp(X - 1)(1 - θ)X

(9)

KC ) Dhar-Flory-Huggins37 KC )

and Bockris-Swinkels38 KC )

[θ + X(1 - θ)](X-1) θ (1 - θ)X XX

(10)

where K is the equilibrium constant of the inhibitor adsorption process and C is the inhibitor concentration. Equations 8-10 may be written as ln[f(θ, X)] ) ln C + ln K

(11)

where f(θ,X) is the configurational term on which it depends. The isotherm parameters K and X shown in Table 3 were deduced by application of a nonlinear least-squares fitting procedure. Isotherm {ln C,ln[f(θ,X)]} points, shown in Figure 7, were computed from the experimental {C,θ} points using the value of X obtained from the nonlinear fitting procedure.36 A plot of ln[f(θ,X)] versus ln C is a straight line with a slope approximately equal to unity. All correlation coefficients exceeded 0.977, indicating that the inhibition of mild steel by DBT was attributed to adsorption of the inhibitor on the metal surface. From Table 3 it could be found that the three isotherms yield similar results but the best approach is obtained by the Dhar-Flory-Huggins isotherm in which X ) 2.75. However,

Table 3. Parameters of the Flory-Huggins, Dhar-Flory-Huggins, and Bockris-Swinkels Model Parameters K and X, Slopes of the Straight Lines Depicting the Linear Behavior of the Configurational Function with Respect to the Logarithm of Concentration and Corresponding Coefficients of Correlation, and the Fee Energy of Adsorption ∆Gads° of DBT on Mild Steel in 1 M HCl at 298 K Adsorption isotherm

K (mol/L)

X

correlation coefficient

slope ln[f(θ, X)] vs ln c

∆Gads° (kJ mol-1)

Flory-Huggins Dhar-Flory-Huggins Bockris-Swinkels

161 873.4 77 522.2 23 603.9

2.75 2.75 3.09

0.978 0.979 0.977

0.97 0.97 0.89

-39.7 -37.8 -34.9

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inhibition. The free energy of inhibitor adsorption shown in Table 3 was calculated by the following equation41 K)

Figure 7. Flory-Huggins (9), Dhar-Flory-Huggins (b), and BockrisSwinkels (2) isotherms for mild steel in 1 M HCl solution at 298 K.

Figure 8. Neutral or protonated molecule form of the benzoic-triazole derivative in acidic medium.

the slopes of the ln[f(θ,X)]versus ln C plots show a little deviation from unity, which means nonideal simulating, which is unexpected from the substitutional adsorption isotherms. They might be the result from interactions between the adsorbed species on a mild steel surface.39,40 The values of the free energy parameter (∆Gads°) could provided valuable information about the mechanism of corrosion

1 ° exp(-∆Gads /RT) 55.5

(12)

where the value 55.5 in the above equation is the molar concentration of water in solution in mol/L.41 The negative values of ∆Gads° indicated that the adsorption of DBT molecule was a spontaneous process. Generally, values of ∆Gads° up to -20 kJ mol-1, the types of adsorption were regarded as physisorption, the inhibition acts due to the electrostatic interactions between the charged molecules and the charged metal, while values around -40 kJ mol-1 or smaller are associated with chemisorption as a result of sharing or transfer of electrons from organic molecules to the metal surface to form a coordinate type of bond (chemisorption).42,43 The values of ∆Gads° in our measurements range from -34.9 to -39.7 kJ mol-1 (in Table 3); it was suggested that the adsorption of DBT on a mild steel surface involves two types of interaction, chemisorption and physisorption. In aqueous acidic media, the trizaole derivatives exist either as neutral species or in the form of cations which are stabilized by an intramolecular hydrogen bond.44 The protonated benzoic-triazole derivative in acid solution is shown in Figure 8. The charge of the metal surface can be determined from the potential of zero charge (PZC) on the correlative scale (φc) by the equation45 φc ) Ecorr - Eq)0

(13)

where Eq)0 is the potential of zero charge and φc is referred to as Antropov’s rational potential or potential on the correlative scale.46 If the metal surface is positively charged (φc > 0) with respect to the potential of zero charge (PZC), the negatively

Figure 9. SEM micrographs of mild steel samples: (a) only surface polishing, (b) after immersion in 1 M HCl solution without inhibitor, and (c) after immersion in 1 M HCl solution in the presence of 1.0 × 10-3 M DBT.

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charged Cl- in 1 M HCl solution with a different concentration of DBT will first adsorb on the metal surface; then the adsorbed Cl- and protonated triazole molecules could attach by means of electrostatic interaction.47 On the contrary, if the mild steel surface is negatively charged (φc < 0) with respect to the potential of PZC, the protonated triazoles indirectly adsorb on a mild steel surface through the electrostatic interaction. On the other hand, chemisorption of the molecular form of the inhibitor could occur through donor/acceptor interactions between the π electrons of sCdN, phenyl, triazolyl groups and the vacant d orbitals of the iron surface atoms. In addition, the contribution from the chemisorption process could also be described as follows: these compounds adsorb chemically in the form of neutral molecules involving displacement of adsorbed water molecules from the iron surface by the organic molecule. In view of the above, two kinds of adsorption can be acting on the steel surface:32 (I) physisorption, the electrostatic attraction between the protonated inhibitor molecules and the charged metal, and (II) chemisorption, the interaction between the π electrons of sCdN, phenyl, triazolyl groups, and the vacant d orbitals of the iron surface atoms. 3.3. SEM Analyses. In order to further confirm the corrosion inhibition ability of the benzoic-triazole derivative, scanning electron microscopy was applied here. Figure 9a shows the micrograph of the unattacked steel sample in the as-received state, so that a comparison can be drawn with the morphology after exposure to the acid media (Figure 9b). After specimen immersion in 1 M HCl solution without inhibitor for 3 h, as expected, the resulting morphology is shown in Figure 9b, which is typical of a generalized corrosive attack on a surface with diverse features and rugosity apparent. Figure 9c is the morphology that resulted after testing in 1 M HCl in the presence of 10-3 M inhibitor; the result proved that the inhibitor can protect iron from corrosion effectively, and thus, the benzoictriazole derivative can be regarded as a good inhibitor for iron corrosion in HCl solution. This conclusion is consistent with the results obtained by our electrochemistry experiments. 4. Conclusion All measurements showed that DBT has excellent inhibition properties for the corrosion of mild steel in 1 M HCl at 298 K, and the inhibition efficiency increases with increasing concentration of the inhibitor. Polarization curves proved that DBT was a mixed-type inhibitor, which can suppress anodic and cathodic reactions at the same time. EIS plots indicated that the inhibitor increases the charge-transfer resistances and showed that the inhibitive performance depends on adsorption of the molecule on the metal surface, and the inhibitor efficiencies determined by polarization, EIS, and weight loss methods were in good agreement. According to Flory-Huggins, Dhar-Flory-Huggins, and Bockris-Swinkels substitutional adsorption isotherms, one DBT inhibitor molecule replaces about three water molecules in the process of adsorption. The inhibition behavior of DBT involved two types of interaction, chemisorption and physisorption. Acknowledgment The author gratefully acknowledges the support of National Key Technology R&D Program (Grant No. 2007BAB27B03) and Postdoctoral Scientific Research Project of ShanDong Province China. (Grant No. N84072603).

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ReceiVed for reView November 11, 2009 ReVised manuscript receiVed January 4, 2010 Accepted February 7, 2010 IE901774M