Synergistic corrosion protection for galvanized steel in 3.0% NaCl solution by sodium gluconate and cationic surfactant

Journal of Molecular Liquids 220 (2016) 549–557

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Synergistic corrosion protection for galvanized steel in 3.0% NaCl solution by sodium gluconate and cationic surfactant M.A. Azaroual a, E.F. El Harrak a, R. Touir a,b,⁎, A. Rochdi a, M. Ebn Touhami a a b

Laboratoire d'Ingénierie des Matériaux et d'Environnement: Modélisation et Application, Faculté des Sciences, Université Ibn Tofail, BP 133, Kénitra 14 000, Morocco Centre Régional des métiers de l'éducation et de la formation (CRMEF), Avenue Allal Al Fassi, Madinat Al Irfane, BP 6210, Rabat, Morocco

a r t i c l e

i n f o

Article history: Received 23 January 2016 Received in revised form 30 March 2016 Accepted 28 April 2016 Available online xxxx Keywords: Galvanized steel Corrosion inhibition Sodium gluconate Surfactant Electrochemical techniques Scanning electron microscopy

a b s t r a c t The influence of Sodium Gluconate (SG), cetyltrimethylammonium bromide (CTAB) and their mixture on galvanized steel corrosion in 3.0% NaCl solution was investigated by using electrochemical measurement coupled with scanning electron microscopy (SEM). The polarization measurements indicated that SG, CTAB and their mixture act as cathodic-type inhibitors. Indeed, a synergistic effect was found between SG and CTAB. In fact, the inhibition efficiency of the mixture reached 94% at 10−3 M of SG with 2.74 × 10−5 M of CTAB. In addition, this inhibition efficiency improved remarkably with immersion time and temperature. The SEM observations and energy dispersive X-ray (EDX) analysis indicated that the surface homogeneity increases with mixture addition. Finally, the thermodynamic parameters were determined and discussed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The galvanized steel industry was large and consumed approximately half of the total zinc produced in the world [1]. So, it is known that the performance of the galvanized steel was provided by the combination of three distinct mechanisms: the barrier action of the zinc layer, the secondary barrier formed by the zinc corrosion products and the cathodic protection [2,3]. Indeed, the behaviour of galvanized steel corrosion consisted of three different stages [4]: Firstly, the dissolution of the formed layer of zinc oxide in the air. Secondary, the surface of the zinc coated layer thickness, and the white rust underlying steel began to corrode. Finally, the amount of red rust on the coating surface increases rapidly and the galvanized steel has almost the same corrosion potential than that of mild steel, while the zinc coating is still covering some parts of the reinforcement steel. The underlying steel corrosion progresses by dissolution of iron and, therefore, at this stage, the zinc coating no longer acts as a sacrificial anode. However, the use of inhibitors for galvanic attack isn't simple because of the high electromotive force and high corrosion rates involved, usually occurring under cathodic rate control. So, the phosphates successfully used as corrosion inhibitor for galvanized steel [5–9]. Indeed, their use caused several problems such as phosphate scale formation, ⁎ Corresponding author at: Laboratoire d'Ingénierie des Matériaux et d'Environnement: Modélisation et Application, Faculté des Sciences, Université Ibn Tofail, BP 133, Kénitra 14 000, Morocco. E-mail addresses: [email protected], [email protected] (R. Touir).

http://dx.doi.org/10.1016/j.molliq.2016.04.117 0167-7322/© 2016 Elsevier B.V. All rights reserved.

fouling of heat exchanger surfaces, eutrophication of seas, rivers… [10]. Consequently, the use of other compounds, such as phosphonates [11–13], anionic polymers [14] and gluconate [15–20] has been reported. In addition, sodium gluconate was an effective non-toxic corrosion and scale inhibitor for low carbon steel in Moroccan simulated cooling water [21–25] and no studies can be found regarding its use as galvanized steel corrosion inhibitor in seawater. In the present work, we have investigated the influence of sodium gluconate (SG), surfactant CTAB and their mixture as corrosion inhibitors for galvanized steel in 3.0% NaCl solution by using electrochemical measurement and SEM/EDX observations. In addition, the effect of immersion time and temperature was also studied. 2. Experimental procedures A conventional three-electrode cell was utilized with a large area Pt counter and saturated calomel (SCE) reference electrodes. The chemical composition of galvanized steel, which used such as working electrode, is summarized in Table 1. The rod was mounted into glass tube of appropriate internal diameter by epoxy resin leaving a front surface area of 0.5 cm2 to contact the test solution. Before each experiment, the surface electrode was abraded using emery paper at different grades (from 100 to 1200), cleaned with acetone, washed with distilled water, and dried at hot air. All potentials were measured and referred to the SCE reference electrode (ESCE = 0.242 mV vs. SHE). The corrosive solutions were 3.0% NaCl, which simulated the seawater, in contact with air without any purging of dissolved oxygen. The temperature was adjusted at

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Table 1 Chemical composition of galvanized steel in wt.%. Zn

Fe

Cd

Pb

Al

97.44

0.40

1.30

0.63

0.23

Fig. 1. Chemical structure of the inhibitors: (a) sodium gluconate (SG) (b) cetyltrimethylammonium bromide (CTAB).

Fig. 4. Potentiodynamic polarization curves for galvanized steel electrode in 3.0% NaCl at different concentrations of SG at 298 K.

The potentiodynamic polarization experiments were undertaken using a scan rate of 1 mV s−1 to achieve quasi-stationary conditions. The obtained polarization curves were corrected for ohmic drop with the electrolyte resistance determined by electrochemical impedance spectroscopy. The current densities values were determined using the following equation [26]: i ¼ ia þ ib ¼ icorr f exp½ba  ðE−Ecorr Þ− exp½bc  ðE−Ecorr Þg

Fig. 2. Evolution of open circuit potential (OCP) versus time for galvanized steel electrode in 3.0% NaCl in the presence of SG at various concentrations.

25 ± 1 °C except for the study of the temperature effect. All Electrochemical experiments were performed using Potentiostat/ Galvanostat/Voltalab PGZ 100 monitored by a personal computer and Voltalab.4.0 software. The used inhibitors in this study are sodium gluconate (SG) and the cetyltrimethylammonium bromide (CTAB) which are the commercial products and their chemical structures are presented in Fig. 1.

Fig. 3. Evolution of open circuit potential (OCP) versus time for galvanized steel electrode in 3.0% NaCl in the presence of CTAB at various concentrations.

ð1Þ

where icorr is the corrosion current density (A cm−2), ba and bc are the Tafel constant of anodic and cathodic reactions (V− 1), respectively. These constant are linked to the Tafel slope β (V/dec) in usual logarithmic scale by: β¼

ln ð10Þ 2:303 ¼ : b b

ð2Þ

The inhibition efficiency (ηPP) was calculated using the following equation: ηPP ¼

i0corr −icorr i0corr

 100

ð3Þ

Fig. 5. Potentiodynamic polarization curves for galvanized steel electrode in 3.0% NaCl at different concentrations of CTAB at 298 K.

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Table 2 Data obtained from the potentiodynamic polarization curves of galvanized steel in 3.0% NaCl at different concentrations of sodium gluconate (SG) and surfactant CTAB. Solutions

Conc. M

Ecorr mV/SCE

icorr μA cm−2

βa mV/dec

−βc mV/dec

ηPP (%)

3.0% NaCl SG

00 10−4 10−3 10−2 10−1 1.37 × 10−5 2.74 × 10−5 4.11 × 10−5 5.49 × 10−5

−1062 −1046 −1066 −1053 −1100 −1108 −1311 −1345 −1137

20.13 6.44 4.52 6.97 8.96 7.12 4.85 8.02 9.21

496 211 273 296 217 205 198 211 206

90 137 100 112 150 110 120 130 125

– 68 77 65 55 64 76 60 54

CTAB

where i0corr and icorr are the corrosion current density values without and with the inhibitor, respectively. The impedance were recorded at the open circuit potential (OCP) in the frequency domain from 100 KHz down to 0.1 Hz using superimposed ac signal of 10 mV peak to peak. To achieve reproducibility, each experiment was carried out at least twice. The obtained impedance data were analyzed in term of equivalent electrical circuit using Zview program. So, the inhibition efficiency was evaluated from Rct with the relationship: ηEIS % ¼

Rct −R0ct  100 Rct

ð4Þ

where, R0ct and Rct are the charge transfer resistance values in the absence and presence of inhibitor. Examination of galvanized steel specimens after exposure into the 3.0% NaCl solution, without and with mixture after two days of immersion time, was carried out using scanning electron microscopy (SEM; JOEL JSM-5500).

with a formation of corrosion products. So, in the presence of inhibitors the same tendency was observed. 3.1.2. Potentiodynamic polarization curves Figs. 4 and 5 show the potentiodynamic polarization curves for galvanized steel, in 3.0% NaCl solution, in the absence and presence of different concentrations of SG and CTAB, separately. It is know that the electrochemical reactions of the galvanized steel in corrosive medium, in the absence of inhibitors, is the anodic dissolution reaction of zinc and cathodic reactions related to the oxygen and proton reduction: Zn↔Zn2þ þ 2e−

ð5Þ

O2 þ 2 H2 O þ 4 e‐ ↔4HO−

ð6Þ

2 Hþ þ 2 e‐ ↔H2 :

ð7Þ

These reactions were accompanied hydrolysis and hydroxide precipitation of zinc reactions:

3. Results and discussions

Zn2þ þ 2 HO‐ ↔ZnðOHÞ2

ð8Þ

3.1. Inhibitive effect of SG and CTAB

Zn2þ þ 2 H2 O↔ZnðOHÞþ þ Hþ :

ð9Þ

3.1.1. Evolution of open circuit potential (OCP) versus time Figs. 2 and 3 show the open circuit potential (OCP) versus time at different inhibitor concentration of SG and CTAB separately. It is noted that the potential for free-inhibitor solution decreases gradually with time and stabilizes at value of −1089 mV/SCE after 22 min of immersion. This phenomenon characterizes the galvanized steel corrosion

However, in the presence of inhibitors, an evident effect was observed on the cathodic parts of the polarization curves with a shift of the corrosion potential Ecorr towards more negative values for both compounds. These findings indicate that the used inhibitors can be classified as cathodic-type inhibitors. The associated electrochemical parameters and the inhibition effecency values are listed in Table 2. It is

Fig. 6. Nyquist diagrams for galvanized steel in 3.0% NaCl, at different concentrations of SG at Ecorr and 298 K.

Fig. 7. Nyquist diagrams for galvanized steel in 3.0% NaCl, at different concentrations of the CATB at Ecorr and 298 K.

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Constant phase elements have widely been used to account for deviations brought about by surface roughness [34,35]. The impedance of CPE is given by equation: Z CPE ðωÞ ¼ Q −1 ðjωÞ−n

Fig. 8. Equivalent circuit proposed for fitting the impedance spectra obtained on galvanized steel surface (a) blank solution (b) in the presence of SG or CTAB.

remarked that the inhibition efficiency values increase with concentrations and reaches 77% and 76% at 10−3 M of SG and 2.74 × 10−5 M of CATB, respectively. In addition, it is noted that the inhibition of SG decreases gradually with concentration up to critical concentration value (10−3 M). This suggests the formation of soluble SG complex [27,28]. The same trend was observed in the case of CTAB beyond 2.74 × 10− 5 M. This behaviour may be due to the saturation of the adsorbed surfactant layer and formation of free micelles which decreased the inhibitor molecules numbers transported to the metallic surface causing a decrease in inhibition efficiency [29,30]. However, the cathodic (βc) and anodic (βa) Tafel slopes change with inhibitors addition indicating that the SG and CTAB change the oxygen and hydrogen reduction and zinc dissolution reactions mechanisms.

3.1.3. Electrochemical impedance spectroscopy The Nyquist diagrams obtained for the galvanized steel in 3.0% NaCl solution with and without different concentrations of SG and CTAB, separately, are presented in Figs. 6 and 7. These diagrams are significantly changed with inhibitors addition suggesting a change in the corrosion process of the galvanized steel/electrolyte interface. So, for the blank solution, the diagram was composed of two separated loops. The first was attributed to the layer of the formed products and the second can be related to the charge transfer resistance. In the presence of each compounds, the diagrams were composed of three loops relatively well separated. At high frequencies, the first loop might be attributed to the layer of formed products, and that at the medium frequencies can be related to the relaxation of the double layer in parallel with the charge transfer resistance. At the low frequencies, the observed loop was generally assigned to a slow process [31]. Indeed, the diffusion process may be due to either the transportation of corrosive ions and soluble corrosion products at the metal/solution or the dissolved oxygen diffusion to the surface [32] where the last process is more probable [33]. A simple electrical equivalent circuit (Figs. 8a and b) has been proposed to model the experimental data. Where, Rs represents the resistance electrolyte; Rct is the charge transfer resistance; Rad is the resistance of the formed products layer and Zw is Warburg impedance. The Qad and Qdl are the constant phase elements used to replace the formed products layer and the double layer capacitances, respectively.

ð10Þ

where Q is the constant phase element (CPE), ω is the sine wave modulation angular frequency (in rad s−1), j2 = −1 is the imaginary number and n is the CPE exponent. According to n value, the CPE can be resistance [ZCPE = R, n = 0], pure capacity [ZCPE = C, n = 1], inductance [ZCPE = L, n = −1] or Warburg impedance (n = 0.5) [36]. The most important parameters derived from the fitting of the impedance diagrams are presented in Table 3. It is seen that that the inhibition efficiency increases with inhibitors concentration reaching a maximum 82% and 80% for 10−3 M of SG and 2.74 × 10−5 M of CTAB, respectively. These results are in good agreement with those obtained from potentiodynamic polarization curves. Additionally, the charge transfer resistance Rct and the electrolytic resistance Rs values increase while the double layer Cdl and the products layer Cad capacities decrease with increasing of inhibitors concentration until an optimum concentration. These can be explained by the increasing portion of the blocking electrode surface due to the inhibitor molecules adsorption [37]. However, it is noted that the nad and ndl values are well below 1 and reflect the very flat shape of the impedance diagrams. So, the increase nad and ndl with inhibitor concentration reflects the reduction of heterogeneity of galvanized steel surface which is the result of the adsorption of SG and CTAB molecules on the metallic surface [30]. 3.2. Synergistic effect 3.2.1. Evolution of open circuit potential (OCP) versus time Fig. 9 shows the open circuit potential (OCP) versus time at different inhibitor concentration of SG, CTAB and their mixture. It is noted that the potential for free-inhibitor, in the presence of CTAB and SG separately, decreases gradually with time and stabilizes after 22 min of immersion. So, in the presence of mixture the potential stabilized after the first minutes of immersion. This phenomenon characterizes the formation of protective layer at the galvanized steel surface. 3.2.2. Potentiodynamic polarization curves Fig. 10 presents the potentiodynamic polarization curves for galvanized steel in 3.0% NaCl solution without and with the mixture (10−3 M of SG + 2.74 × 10−5 M of CTAB). It can be seen that the mixture addition causes a very strong decrease in the cathodic current branches and the corrosion potential Ecorr values shift in to more negative values in comparison with that obtained with the addition of inhibitors solely and blank solution. It is seen also that corrosion current densities are reduced significantly with mixture addition and became only 1.21 μA cm−2 (Table 4). Therefore, the inhibition efficiency increases from 77% and 76% for SG and CTAB, respectively to 94% for

Table 3 Electrochemical impedance parameters for galvanized steel in 3.0% NaCl at different concentrations of SG and CTAB. Solutions

Conc. M

Rs Ω cm2

Rad KΩ cm2

Cad μF cm−2

nad

Rct KΩ cm2

Cdl μF cm−2

ndl

ηEIS %

3.0% NaCl SG

00 10−4 10−3 10−2 10−1 1.37 × 10−5 2.74 × 10−5 4.11 × 10−5 5.49 × 10−5

10 32 40 21 20 46 34 28 16

0.390 0.550 1.200 0.980 0.975 0.520 1.250 0.880 0.720

60 5 2 10 15 8 2.5 15 25

0.82 0.83 0.88 0.87 0.88 0.82 0.85 0.86 0.89

0.5 2.01 2.30 1.55 1.48 1.79 1.98 1.85 1.30

639 6.87 2.87 16.87 27 14.36 2.47 16.27 20

0.79 0.81 0.84 0.82 0.83 0.80 0.86 0.85 0.85

– 75 78 68 66 72 74 72 61

CTAB

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Table 4 Data obtained from the potentiodynamic polarization curves of galvanized steel in 3.0% NaCl in presence of sodium gluconate (SG), surfactant (CTAB) and their mixture.

Fig. 9. Evolution of open circuit potential (OCP) versus time for galvanized steel electrode in 3.0% NaCl in the absence and presence of SG, CTAB and their mixture (10−3 M of SG + 2.74 × 10−5 M of CTAB) at 298 K.

their mixture. This behaviour was attributed to the synergistic effect between these compounds. Similar behaviours were obtained in our pervious study for the ordinary steel in simulated cooling water [22,24,25].

3.2.3. Electrochemical impedance spectroscopy To confirm the synergistic effect obtained by potentiodynamic polarization curves and more informations about this effect, the impedance diagrams for galvanized steel in 3.0% NaCl solution without and with mixture were carried out at open-circuit potential after one hour immersion (Fig. 11). Their experimental data were extracted using the same electrical equivalent circuit, presented in Fig. 8b, and are given in Table 5. It is revealed that the mixture addition in 3.0% NaCl solution increases the charge transfer resistance Rct and decreases the double layer capacitance Cdl values. These evolutions may be due to the increase of the adsorbed inhibitors molecules quantities at the metallic surface [38–40]. However, it is noted that the nad and ndl increase with inhibitor addition indicating a reduction of heterogeneity of galvanized steel surface such as mentioned above. However, Aramaki et al. [41] have proposed two modes of adsorption mechanisms, namely competitive and cooperative, to explain the synergistic effect. In competitive adsorption, the two compounds are

Fig. 10. Potentiodynamic polarization curves for galvanized steel electrode in 3.0% NaCl in the absence and presence of SG, CTAB and their mixture (10−3 M of SG + 2.74 × 10−5 M of CTAB) at 298 K.

Solutions

Ecorr mV/SCE

icorr μA cm−2

βa mV/dec

- βc mV/dec

ηPP (%)

3.0% NaCl 10−3 M of SG 2.74 × 10−5 M of CTAB Mixture

−1062 −1066 −1311 −1368

20.13 4.52 4.85 1.21

496 273 178 261

90 100 120 134

– 77 76 94

adsorbed at different sites on the metallic surface, while in cooperative adsorption; one is chemisorbed on the surface when the other comes to physisorbe thereon. Then, the synergism parameter, S, was calculated from the impedance data according to the equations described in literature [42]. It is found that this value is greater than unity (Table 5) indicating a cooperative adsorption which explained by the synergetic effect between SG and CTAB [40]. In addition, in the later of this work, we have investigated the influence of temperature and immersion time on the inhibition of mixture.

3.3. Parameters effect on the mixture inhibition 3.3.1. Influence of immersion time Fig. 12 shows the Nyquist diagrams of galvanized steel in 3.0% NaCl solution in the presence of mixture at different immersion time. The obtained plots consisted of tow loops, the first at high frequencies might be attributed to the formed products layer, and the second can be related to the charge transfer resistance. The disappearance of the Warburg impedance with immersion time indicates that the products prevented the diffusion of O2 to the galvanized steel substrate and/or suppressed the cathodic reduction of corrosion products [43]. The same electrical equivalent circuit (Fig. 8a) has been proposed to model the experimental data. The obtained parameters are summarized in Table 6. It is seen that the Rct and Rs values increased with immersion time to achieve a higher values after 36 h of immersion. This increase illustrates a formation of stable protective layer on galvanized steel surface reinforcing with immersion time. In addition, it is remarked that the Cdl decreased with immersion time indicating a gradual replacement of water molecule by the chloride anions and/or the adsorption of the organic molecules on the metal surface, decreasing the dissolution reaction extent [21].

Fig. 11. Nyquist diagrams for galvanized steel in 3.0% NaCl in the absence and the presence of mixture: (10−3 M of SG + 2.74 × 10−5 M of CTAB) at 298 K.

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Table 5 Data obtained from the impedance diagrams of galvanized steel in 3.0% NaCl in presence of SG, CTAB and their mixture. Solutions

Rs Ω cm2

Rad KΩ cm2

Cad μF cm−2

nad

Rct KΩ cm2

Cdl μF cm−2

ndl

ηEIS %

S

3.0% NaCl 10−3M SG 2.74 × 10−5 M of CTAB Mixture

10 40 34 268

0.390 1.200 1.250 5.340

60 2 2.5 0.5

0.82 0.88 0.85 0.90

0.500 2.300 1.980 22.507

639 2.87 2.47 0.88

0.79 0.84 0.86 0.88

– 78.26 74.74 97.77

– – – 2.47

Fig. 12. Nyquist diagrams for galvanized steel in 3.0% NaCl in the presence of mixture at different immersion time.

3.3.2. Effect of solution temperature Temperature has a great effect on the metal corrosion rate and its variation is a very useful tool for studying and clarifying the adsorption mechanism of inhibitor [44]. Generally, the adsorption of the organic compound can be described by two main types of interactions: physical adsorption and chemisorption. They are influenced by the metal charge, the chemical structure of inhibitor and the electrolyte type. To clarify the adsorption nature, the influence of temperature at the range from 298 K to 328 K on the corrosion rate of galvanized steel in 3.0% NaCl without and with mixture was investigated using potentiodynamic polarization curves. The obtained results are given in Figs. 13 and 14 and their corresponding data are summarized in Table 7. It is seen that the corrosion current density values increase with temperature for both in uninhibited and inhibited solutions. In addition, the inhibition efficiency decreases slightly from 93.98% to 95.35% when the temperature increases from 298 K to 328 K. These results confirm that mixture acts as an efficient inhibitor at the high temperature.

Fig. 13. Potentiodynamic polarization curves for galvanized steel electrode in 3.0% NaCl at different temperature.

However, the effect of temperature on the corrosion parameter can be deduced by comparing the activation energy with and without mixture. The Arrhenius plots were used to determine the activation energy (Ea) values according to the equation [45]:   −Ea icorr ¼ A exp RT

ð11Þ

where icorr is corrosion current density, A is the pr-exponential constant, Ea is the activation energy of the metal dissolution reaction, R is the gas constant and T is the absolute temperature. Fig. 15 presents the Arrhenius plots obtained for galvanized steel in 3.0% NaCl without and with mixture addition. The activation energy values were found to be

Table 6 Data obtained from the impedance diagrams of galvanized steel in 3.0% NaCl in presence of mixture at different immersion time. Rad nad Immersion Rs KΩ cm2 KΩ cm2 time (h)

Cad Rct Cdl ndl μF cm−2 KΩ cm2 μF cm−2

ηEIS %

1 2 4 8 12 16 20 24 32 36

0.5 0.48 0.45 0.42 0.38 0.39 0.36 0.34 0.31 0.25

97.77 97.94 98.03 98.85 98.64 98.71 98.77 98.82 99.01 99.02

0.268 1.441 1.496 2.173 2.974 2.945 2.954 3.055 3.125 3.455

5.340 5.540 5.621 5.712 6.450 6.490 6.589 7.111 8.320 9.320

0.90 0.91 0.90 0.92 0.93 0.94 0.93 0.95 0.95 0.96

22.507 24.280 25.440 35.587 36.967 38.934 40.466 42.496 50.924 51.377

0.88 0.22 0.33 0.29 0.28 0.21 0.33 0.27 0.22 0.19

0.88 0.89 0.87 0.87 0.90 0.89 0.91 0.92 0.93 0.93

Fig. 14. Potentiodynamic polarization curves for galvanized steel electrode in 3.0% NaCl in presence of mixture at different temperature.

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Table 7 Data obtained from the polarization curves of galvanized steel in 3.0% NaCl in presence of mixture at different temperature. Solution

T K

Ecorr mV/SCE

icorr μA cm−2

ηPP %

Blank solution

298 308 318 328 298 308 318 328

−1062 −1060 −1014 −1009 −1368 −1060 −1314 −1309

20.13 22.10 25.01 28.85 1.21 1.23 1.27 1.34

– – – – 93.98 94.43 94.92 95.35

Mixture

9.75 KJ mol− 1 and 2.73 KJ mol−1 without and with mixture, respectively. This decrease in Ea value after mixture addition probably was attributed to chemisorption of SG and CTAB molecules on galvanized steel surface [46,47]. In this context, Singh et al. have considered that the increase in temperature produces an increase in the electron density around the adsorption centers, which enhances the inhibition efficiency [48]. In addition, Ivanov considered that the increase of inhibition efficiency with temperature as the change in the adsorption mode; the inhibitor is physisorbed at low temperatures, while the chemisorption is enhanced at high temperature [49]. Other kinetic data are accessible using the alternative formulation of the transition state Eq. (12) Arrhenius [50]: Ln

  icorr R ΔS ΔH a ¼ ln þ a− T R RT Nh

ð12Þ

where h is Plank's constant, N is Avogadro's number, ΔSa⁎ and ΔHa⁎ are the entropy and enthalpy of activation, respectively. Plot of ln (icorr/T) versus the reciprocal of temperature in the absence and presence of mixture are presented in Fig. 16. The straight lines are obtained with a slope of (−ΔHa⁎ / R) and an intercept of (ln R / Nh + ΔSa⁎ / R). The values of ΔHa⁎ and ΔSa⁎ are calculated and are listed in Table 8. It is shown that ΔHa⁎ and Ea deceases with the mixture addition. This finding suggested that the mixture's presence further supports the proposed chemisorption mechanism. Unchanged or lower values of Ea in inhibited systems compared to the blank to be indicative of chemisorption mechanism, while higher values of Ea suggest a physical adsorption mechanism. This type of inhibitors retards the corrosion process [51,52]. In the other, literature postulates that the positive sign of the enthalpy (ΔHa⁎) is an endothermic nature of the galvanized steel dissolution process

Fig. 16. Arrhenius plots of Ln (icorr/T) versus 1/T for galvanized steel in 3.0% NaCl (a) blank solution and (b) with mixture.

[53]. The entropy of activation ΔSa⁎ in the absence and presence of mixture is negative and this value increases negatively with mixture addition. The negative sign of ΔSa⁎ can be explained by the formation of a stable layer on metallic surface [54,55]. 3.4. Surface analysis The scanning electronic microscopy (SEM) images of galvanized steel surface immersed in the corrosion solution without and with mixture are displayed in Fig. 17. It can be observed that the galvanized steel surface was damaged in the absence of mixture (Fig. 17a). In fact, this image shows a heterogeneous surface of corrosion products layer. In addition, the EDX analysis (Fig. 18a) of the rust layer on the galvanized steel showed the presence of Fe and Cl elements, indicating that the zinc coating was damaged under Cl− erosion. So in the presence of mixture (Fig. 17b), the layer properties of the electrode surface was markedly modified. In fact, the compact and more homogeneous layer was observed compared to free solution. This is could be due to the involvement of the inhibitors molecules in the interaction with the reaction sites of galvanized steel surface. In addition, the EDX analysis (Fig. 18b) of the formed layer at the surface electrode showed the presence of a great picks of O and N elements, indicating that SG/CTAB molecules were adsorbed on the galvanized steel surface. These results show that the mixture plays an important role in the corrosion protection of the galvanized steel and they are in accordance with the inhibition efficiency values obtained by electrochemical measurements. 4. Conclusion Sodium gluconate (SG), surfactant CTAB and their mixture were investigated as corrosion inhibitors for galvanized steel in 3.0% NaCl solution using electrochemical techniques and SEM/EDX analysis. Potentiodynamic polarization curves showed that the SG and CTAB reduced cathodic reactions and thus act as cathodic-type inhibitors. Relatively low inhibitions were obtained with SG (77%) and CTAB (76%). Table 8 Values of activation parameters Ea, ΔHa* and ΔSa* for galvanized steel in 3.0% NaCl in presence of mixture.

Fig. 15. Arrhenius slopes for galvanized steel in 3.0% NaCl (a) blank solution and (b) with mixture.

Solution

Ea (kJ mol−1)

ΔH⁎a (kJ mol−1)

ΔS⁎a (J K−1 mol−1)

Blank Mixture

9.75 2.73

7.15 0.13

−196.22 −243.14

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Fig. 17. SEM images for the galvanized steel surface after two days of immersion in 3.0% NaCl (a) blank solution (b) with mixture.

However, this inhibition efficiency was improved in the presence of their mixture and reached 98%. In addition, the inhibition efficiency of the mixture was found to enhance remarkably with immersion time and temperature and it was 95% at 328 K. The activation

thermodynamic parameters showed that the adsorption process is endothermic and accompanied with a decrease in entropy. The SEM/EDX analysis of the galvanized steel surface showed that a layer of inhibitors was formed on its surface.

Fig. 18. EDX analysis of galvanized steel electrode surface after two days of immersion in 3.0% NaCl (a) blank solution and (b) with mixture.

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