Influence of sodium gluconate and cetyltrimethylammonium bromide on the corrosion behavior of duplex (α-β) brass in sulfuric acid solution

Materials Chemistry and Physics 227 (2019) 200–210

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Influence of sodium gluconate and cetyltrimethylammonium bromide on the corrosion behavior of duplex (α-β) brass in sulfuric acid solution

T

Jamila Jennanea, Mohamed Ebn Touhamia, Saman Zehrab, Yacine Baymoua, Seung-Hyun Kimc, Ill-Min Chungc,∗, Hassane Lgazc,∗∗ a

Laboratoire d'ingénierie des Matériaux et d'Environnement: Modélisation et Application, Faculté des Sciences, Université Ibn Tofail, BP 133, Kénitra, 14000, Morocco Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India c Department of Crop Science, College of Sanghur Life Science, Konkuk University, Seoul, 05029, South Korea b

HIGHLIGHTS

and CTAB were used as corrosion inhibitors for Duplex Brass in H SO . • SG adsorption of the inhibitors obeys Langmuir adsorption isotherm. • The and ICPS were used to assess the inhibition performance. • SEM • DFT calculations were used to support the experimental results. 2

4

ARTICLE INFO

ABSTRACT

Keywords: Brass Corrosion inhibition DFT Sodium gluconate Cetyltrimethyl ammonium bromide

The present research focuses on the corrosion inhibition of a two phases (α-β) brass in 0.5 M H2SO4 solution containing sodium gluconate (SG) and cetyltrimethyl ammonium bromide (CTAB) at different temperatures. The inhibition effect was studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), Scanning Electron Microscope with Energy-Dispersive Spectrometer (SEM-EDS), Inductively Coupled Plasma Spectrometry (ICPS) and Density Functional Theory (DFT). Experimental results revealed that the corrosion inhibitors exerted a strong effect on brass corrosion and found to be less effective at a long immersion time (24 h). A concentration of 10−3 M of SG in combination with 5 ppm CTAB (SG/CTAB) can cover up to 89% of the brass’ surface, which provides very good protection against corrosion, reducing the corrosion rate from 49 mA cm−2 to 5.5 mA cm−2. The study also showed that the inhibition efficiency of SG/CTAB was slightly decreased at higher temperatures and their adsorption on brass surface was found to follow the Langmuir adsorption isotherm. The SG/CTAB acted as cathodic type corrosion inhibitors with insignificant changes in the anodic reaction. Electrochemical impedance spectroscopy (EIS) studies revealed that addition of 5 ppm CTAB to different concentrations of SG considerably increases the corrosion resistance of brass. The synergistic effect of SG and CTAB is also discussed. The SEM-EDS and ICPS analyses support the experimental results. Furthermore, quantum chemical calculations were used to understand the electronic properties of SG and CTAB.

1. Introduction The copper and zinc alloy, known as brass is one of the important and essential materials widely used in mechanical engineering, medical and artistic applications, because of its particular properties such as its excellent mechanical properties, machinability, and corrosion resistance [1,2]. However, brasses deployed in service in these environments undergo corrosion destruction either through a simultaneous

∗

dissolution of copper and zinc and/or selective dissolution processes via dezincification of zinc [3]. To overcome this, multiple strategies including micro-additions of other metals have been developed to increase the corrosion resistance of brass [4]. Among these strategies, the use of corrosion inhibitors is one of the practical and effective methods for controlling the corrosion attack of brass alloys [5–7]. The choice of suitable corrosion inhibitor is a key factor in establishing the possibility of success of any protection of the metal. Usually, electron rich organic

Corresponding author. Corresponding author. E-mail addresses: [email protected] (I.-M. Chung), [email protected] (H. Lgaz).

∗∗

https://doi.org/10.1016/j.matchemphys.2019.02.001 Received 25 August 2018; Received in revised form 2 January 2019; Accepted 1 February 2019 Available online 01 February 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.

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compounds that contain heteroatoms, long carbon chain and functional groups inside their frameworks are the most effective in acidic media [8–10]. While a single organic compound can ensure adequate corrosion protection, the importance of mixing more than compound remains a subject of intensive research [11,12]. One of the interesting approaches that have attracted our special attention over the last years is the use of sodium gluconate/Cetyltrimethylammonium bromide mixture. The results were very encouraging and of great practical importance. In this line, a study by Touir et al. [13] on the scale and corrosion inhibition of ordinary mild steel used for cooling water system treatment found that SG/CTAB mixture serves as an effective inhibitor for corrosion and scale with interesting inhibitive properties of SG at higher temperatures. In other work, the same research team examined the influence of SG/CTAB as a non-toxic formulation on the inhibition of corrosion, scale, and biocorrosion of low carbon steel in cooling water system and interesting results have also been obtained [14]. More recently, Azaroual et al. [15] addressed the problem of galvanized steel in 3.0% NaCl solution using SG, CTAB and their mixture. The results showed an enhancement of the corrosion inhibition efficiency from 77% (SG) and 76% (CTAB) to 98% (SG/CTAB) as a direct cause of mixing both compounds. Brass alloys have considerable application prospects, thus studies on their corrosion behavior in different aggressive environments are of great interest to the industrialists, corrosion engineers and academic researchers as a way to create reliable information about this subject [7,16,17]. Although there have been numerous studies investigating the corrosion inhibition of brass, to date there have been limited studies on the brass corrosion in sulfuric acid, even though acid sulfuric solutions have a wide application in the industry [18]. To fill this gap of knowledge, we carried out a corrosion inhibition study on the α-β brass, known as the duplex brass, in sulfuric acid solution using a mixture of SG/CTAB. Duplex brass used in this work is a member of the copperzinc alloys and has a wide range of applications in manufacturing, marine and other diverse areas of industrial and engineering applications [2]. We probed the interaction between SG/CTAB and duplex brass surface in 0.5 M H2SO4 using electrochemical, surface characterization and DFT techniques. To the best of our knowledge, no such research has been conducted and this rarity prompted to report the case. This work is envisaged to have a positive scientific knowledge impact on the further use of this alloy.

Fig. 1. Schematic representation Cetyltrimethylammonium bromide.

of

the

sodium

gluconate

and

effects and at 303, 313, 323 and 333 K for temperature effect. At first, in order to be in equilibrium with the environment, the duplex brass samples were dipped in the test solution for 30 min, at the open circuit potential. Measurement of the impedance spectra was obtained in the frequency range of 10 mHz–100 kHz and an AC amplitude of 10 mV. The Tafel polarization measurements were recorded at a scanning rate of 1 mV s−1 in the range from 200 to −800 mV vs. SCE relative to the corrosion potential. Using the following equations, we calculated the inhibition efficiency from electrochemical impedance spectroscopy (EIS) (Eq. (1)) and potentiodynamic polarization (PDP) (Eq. (2)) data [19]: EIS (%)

PDP (%)

=[

Rsum(inh) Rsum ] × 100 Rsum(inh) icorr ] × 100 icorr

= [1

(1) (2)

In these equations, icorr and Rsum correspond to the corrosion current density and sum of resistances under uninhibited solutions, respectively whereas icorr and Rsum(inh) are the corrosion current density and sum of resistances in presence of SG/CTAB. The effect of the immersion time on corrosion inhibition of brass in H2SO4 solution was investigated in the time interval from 30 min to 24 h. 2.3. SEM-EDS and inductively coupled plasma spectrometry Duplex brass samples were scanned using a Hitachi TM-1000 SEM after 12 h of immersion in sulfuric acid solution without and with SG, CTAB and SG/CTAB and the results were compared with SEM image of polished duplex brass. Using inductively coupled plasma spectrometry, the duplex brass samples, i.e. uninhibited, with SG and SG/CTAB, immersed in sulfuric acid solution in sterile tubes, were analyzed after the immersion period (12 h). The ICP-OES experiments were carried out at 303K.

2. Experimental section 2.1. Materials and solutions The chemical composition of duplex brass samples was: 61.08 wt% Cu, 2.03 wt% Pb, 0.11 wt Sn, 0.12 wt% Fe, 0.03 wt% Ni, 0.09 wt% As, 0.02 wt% Al and balance Zn. Cetyltrimethylammonium bromide (CTAB), sodium gluconate (SG), and sulfuric acid (H2SO4) (98%) were purchased from Sigma-Aldrich. All chemicals are of analytical grade. The test solution (H2SO4 of 0.5 M concentration) was prepared from the commercially obtained 98% H2SO4, by diluting with aid of distilled water. The range of concentrations of employed inhibitors was kept in between 1 × 10−6 to 1 × 10−3 M for sodium gluconate and 1–7 ppm for CTAB. No significant results were found at higher or lower concentrations. The molecular structures of the above-mentioned compounds are illustrated in Fig. 1.

2.4. DFT calculations All quantum chemical calculations were performed by using DMol3 module implemented in Materials Studio [20,21]. The generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) formula was used for the electronic exchange-correlation potential [22], and all the electron calculations were performed with doublenumeric basis set (DNP 4.4) [22] and COSMO [23] solvation model. The Fukui functions and dual descriptor were calculated based on Hirshfeld population analysis and proposed as follows [24]:

2.2. Electrochemical measurements

f k+ = qk (N + 1)

All electrochemical experiments were performed with the potentiostat/galvanostat PGZ100 electrochemical station. A standard three-electrode cell system was used. The three-electrode system consisted of a working electrode (duplex brass), Pt as an auxiliary electrode alongside a saturated calomel reference electrode. The experimental temperature was fixed at 303K for concentration and immersion time

f k = qk (N ) f (k ) = f k+

qk (N )

qk (N

1)

fk

In the above equations, qk (N ) , qk (N + 1) and qk (N 201

(3) (4) (5)

1) are the

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Fig. 2. Potentiodynamic polarization curves of duplex brass in 0.5 M H2SO4 at various concentrations of CTAB at 303K.

atomic charges of the systems with N, N+1 and N−1 electrons, respectively. 3. Results and discussion 3.1. Potentiodynamic polarization The PDP curves of the duplex brass electrode surface before and after adding SG, CTAB and SG/CTAB are shown in Figs. 2 and 3. In sulfuric acid solution, the first corrosion product formed on the brass surface is copper, it follows that the corrosion layer has been formed by the solution of copper and zinc and the redeposition of copper, followed by the formation of either redeposited or residual layers of β- and γbrass [25]. It can be seen from the figures that the addition of SG/CTAB significantly reduces the current density of the cathode while an insignificant effect was observed on the anode branch, which indicates that the adsorption of the corrosion inhibitors on the electrode surface mainly prevents it from the acid attack. At the same time, as the concentration of corrosion inhibitor increases, the current density gradually decreases. Obviously, the addition of corrosion inhibitors mainly inhibits the cathodic reaction without modification of the corrosion process but suppresses the active corrosion sites. As the zinc content is relatively small in the used brass, the corrosion inhibitors may be acted selectively as copper complexing agents and leave the exposed zinc relatively unaffected. Further inspection of results reveals that the cathodic parts of the polarization curves represent a limiting current associated with the oxygen reduction process. This means that the cathodic process is controlled by diffusion of oxygen gas from the bulk solution to the metal surface rather than activation polarization, which prevented a clear linear extrapolation of the cathodic curves. Thus, it was not possible to extract the cathodic Tafel slopes by linear extrapolation to Ecorr [26]. This behavior is similar to that obtained by other authors, who studied the corrosion and corrosion inhibition of brass in H2SO4 solutions [27–29]. At a more negative potential, an apparent increase of current density was observed which is probably due to hydrogen evolution [26]. Corrosion electrochemical parameters were obtained by fitting the cathodic polarization curves in Figs. 2 and 3. The results are shown in Tables 1 and 2. Table 1 shows that the corrosion current density (icorr) is inversely proportional to the addition of inhibitors’ concentration, that is, the corrosion inhibition performance is proportional to the concentration of the inhibitor. In addition, the effect of CTAB was barely observed except 5 ppm concentration but as noticed above, the

Fig. 3. Potentiodynamic polarization curves of duplex brass in 0.5 M H2SO4 at (a) various concentrations of SG and (b) SG with 5 ppm CTAB at 303K. Table 1 The PDP parameters for the corrosion of duplex brass in 0.5 M H2SO4 in the absence and presence of different concentrations of CTAB at 303K. [CTAB] ppm

Ecorr (mV/SCE)

βa (mV/dec)

icorr (mA cm−2)

η (%)

0.5 M H2SO4 1 3 5 7

−79 −45 −70 −91 −66

43 55 66 48 67

49 65 28 20 59

– −32 42 60 −20

corrosion current density was significantly decreased in presence of different concentration of SG with 5 ppm CTAB. Thus, this mixture was chosen to carry out all the next experiments. From the results in Tables 1 and 2, it is apparent that the anodic Tafel slope of brass in solutions containing CTAB, SG and SG/CTAB remains almost unchanged. Moreover, corrosion potential values of brass in inhibited solutions are slightly shifted toward the cathodic region, which confirm the major effect of the cathodic process. On the other hand, the inhibition efficiency increases exponentially with increasing SG/5 ppm CTAB concentration, which indicates the effective role of used formulation in protecting the duplex brass surface from corrosion. The interaction of inhibitor molecules can be described by a parameter called synergism parameter, which is calculated by the 202

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Table 2 The PDP parameters for the corrosion of duplex brass in 0.5 M H2SO4 in the absence and presence of different concentrations of SG and SG with 5 ppm CTAB at 303K. [SG] M

[CTAB] ppm

Ecorr (mV/ SCE)

βa (mV/ dec)

icorr (mA cm−2)

η (%)

Sa

0.5 M H2SO4 10–6 10–5 10–4 10–3

– – – – –

−79 −88 −85 −78 −133

43 44 46 39 37

49 87 32 22 14

– −77 34 55 71

– – – – –

10–6 10–5 10–4 10–3

5 5 5 5

−112 −118 −108 −109

47 36 41 43

20 14 10 5.5

60 71 80 89

−0.30 1.32 1.44 1.47

a

S is the synergism parameter.

relationship given by Aramaki and Hackermann [30]:

SI =

1 1

I1 + 2 I1 + 2

(6)

where I1 + 2 = I1 + I2 , I1 and I2 are the inhibition efficiencies of the CTAB and the SG, respectively. I1 + 2 is the inhibition efficiency for the SG in combination with the CTAB [31]. When S approaches 1 there is no interaction between inhibitor compounds, while S > 1 implies that a synergistic effect exists. The result may suggest an antagonistic interaction between inhibitors in the case of S < 1, which may be attributed to competitive adsorption [31]. The S values in Table 2 are more than unity for higher concentrations while the S value for 10−6 M of SG/5 ppm CTAB is lower than unity. These results suggest that the enhanced inhibition efficiency caused by the addition of CTAB to SG is only due to the synergistic effect [32]. A possible explanation of the synergistic effect is the formation of an intermediate at the alloy surface by the interaction with SG. The lower synergism value at the lower concentration of SG indicates that the formed intermediate is soluble. With the increase of SG concentration, the formed intermediate becomes insoluble, thus leading to higher inhibition efficiencies.

Fig. 4. Nyquist diagrams of duplex brass in 0.5 M H2SO4 at (a) various concentrations of SG and (b) SG with 5 ppm CTAB at 303K.

3.2. Electrochemical impedance spectroscopy 3.2.1. Effect of inhibitor concentrations Figs. 4 and 5(a) show Nyquist and Bode plots for the duplex brass electrode in 0.5 M H2SO4 medium at 303K. From the EIS results, it can be concluded that the Nyquist spectra before and after the addition of SG, CTAB and SG/CTAB were mainly characterized by two semicircles, an adsorbed inhibitor layer at higher frequencies and the second loop at low frequencies can be attributed to surface double layer [29]. In addition, the EIS semicircle is a flattened capacitive reactance arc, indicating the existence of a frequency dispersion effect caused by uneven surface roughness of the electrode [33]. After adding the SG, the diameter of the impedance arc moderately increased with respect to the blank solution while a considerable increase was observed after the addition of 5 ppm CTAB to different SG concentrations. The electrochemical impedance spectra of duplex brass in H2SO4 with and without inhibitors were analyzed by an equivalent circuit (Fig. 5(b)). In this equivalent circuits, Rs is the solution resistance, CPE is the constant phase element (CPEdl related to double layer and CPEL to the adsorbed inhibitor layer), Rp is the polarization resistance and RL is the inhibitor layer resistance. Generally, a slight inclination of the impedance plots is frequently observed and it is related to the surface heterogeneity attributed to roughness and inhomogeneities of solid surface [34]. To account for this effect, we replace the capacitance of the electric double-layer by a constant phase element (CPE) in the electrical circuit

Fig. 5. (a) Bode plots and (b) equivalent circuit model of duplex brass in 0.5 M H2SO4 at various concentrations of SG with 5 ppm CTAB at 303K.

model used to fit the EIS data. A close-up look in Fig. 4 shows that a good fit of the impedance spectra has been obtained. Impedance of the CPE (ZCPE) is defined by the following equation [35]:

ZCPE = Q 1 (i )

n

(7)

where, Q is a proportionality coefficient, ω is the angular frequency for which imaginary part of the impedance reaches its maximum (Eq. (8)) [36], i is the imaginary unit and n is a measure of surface irregularity. For whole number of n = −1, 0, 1, CPE represent inductance (L), resistance (R) and capacitance (C), respectively. 203

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Table 3 Impedance parameters recorded for duplex brass electrode in 0.5 M H2SO4 solution in the absence and presence of SG and SG with 5 ppm CTAB. [SG] M

0.5H2SO4 10–6 10–5 10–4 10–3 10–6 10–5 10–4 10–3

=(

[CTAB] ppm

– – – – – 5 5 5 5

Rs (Ω cm2)

1.18 2.4 2.32 2.13 2.00 2.1 2.3 2.31 2.29

1 1 )n Rsum Y0

RL (Ω cm2)

2.60 2.3 6 26 76.3 0.5 0.4 292 9.8

Rp (Ω cm2)

CPEL 2

nL

Q (μF/cm )

0.99 0.97 0.99 0.99 0.96 0.98 1.00 0.95 0.99

34 27 31 22 24 18 25 21 29

425 252 637 994 1102 848 1197 1711 2678

CPEct

Cdl (μF/cm2)

Rsum

η%

143 189 36 91 47 49 102 15 30

428 156 645 1022 1180 850 1199 2005 2690

– −69 33 58 63 49 64 78 84

2

nct

Q (μF/cm )

0.82 0.88 0.81 0.85 0.69 0.88 0.67 0.85 0.83

237 273 75 131 118 72 205 26 47

(8)

As an example, for a circuit including a CPEdl, the double layer capacitance (Cdl) values are obtained using the following relationship [37]:

Cdl =

n

Q × R p1

n

(9)

In this study, Rsum was used to discuss the inhibition performance, (Rsum = Rs + Rp + RL). The Rsum along with electrochemical parameters obtained by fitting are shown in Table 3. It can be seen from Table 3 that in the range of inhibitor concentration, as the concentration of the inhibitor molecule increases, the Rsum value increases, indicating the strong inhibition effect of inhibitor molecules on duplex brass corrosion. However, we found no increasing or decreasing trend in the Cdl values. Typically, it is frequently observed that the inhibitor molecules change the double layer behavior of steels and clear trends in increasing the Cdl was widely observed with increasing inhibitor's concentration [35,38,39]. However, the surface condition of brass alloys is not as simple as steels. Based on this, it can be claimed that various surface conditions can be produced by different inhibitor concentrations because of unrelated values of the surface film thickness and the local dielectric constant [29]. The values of the phase shift nct remain generally unchanged and no increasing or decreasing trend could be identified from the data, which is in contrast with the more typical situation in which the phase shift value increases after the addition of inhibitors [40]. Indeed, this is consistent with the modification of the chemical composition of the surface double layer in combination with its thickness as suggested by the Q value. This result has appeared in a previous similar publication [41]. On the other hand, the values of nL were found in the 0.97–1.00 interval, revealing that the adsorbed film was relatively homogeneous and probably acts as an insulator [42,43]. In accordance with the results from PDP, the inhibition efficiency values are found to be higher for SG concentrations with 5 ppm CTAB than that of SG alone. These data once again confirm the importance of using CTAB with sodium gluconate in the protection of duplex brass surface.

Fig. 6. Nyquist diagrams of duplex brass in 0.5 M H2SO4 in (a) absence and (b) presence of 10−3 M SG with 5 ppm CTAB at different immersion time at 303K.

3.2.2. Effect of immersion time Fig. 6 and Table 4 represent the electrochemical impedance plots and the electrochemical parameters of the duplex brass electrode in 0.5 M H2SO4 in presence of 10−3 M SG with 5 ppm CTAB at different immersion times. On the close inspection of the results of Fig. 6, we find that the EIS plots at different immersion time exhibit the same behavior as that of the inhibitor concentration effect. Results in Table 4 reveal that there is no appreciable increase in the inhibition efficiency with the increase of the immersion time. However, the formulation still efficient at relatively long immersion time. The decrease in the inhibition efficiency is mainly due to desorption of the inhibitor molecules from the duplex brass surface with increasing immersion time.

3.3. Temperature effect and kinetic parameters To gain a better understanding of the influence of the temperature on the metal-inhibitor interaction and the mode of inhibition of the duplex brass corrosion, PDP measurements were performed at different temperatures i.e. 303,313, 323 and 333 K without and with SG/CTAB (10−3 M SG with 5 ppm CTAB). The corresponding plots and the computed parameters are represented in Fig. 7 and Table 5, respectively. Results from Table 5 revealed a decrease in the η(%) with corresponding increase in the corrosion current density (icorr) values which 204

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Table 4 Impedance parameters recorded for duplex brass electrode in 0.5 M H2SO4 solution in the absence and presence of 10−3 M SG with 5 ppm CTAB at different immersion time. Medium

Blank

SG/CTAB

Time (h)

0.5 2 8 16 24 0.5 2 8 16 24

Rs (Ω cm2)

1.18 1.95 1.78 1.61 1.49 2.29 1.52 1.48 1.47 1.43

RL (Ω cm2)

2.60 36.6 36 32 28 9.8 11 26 19 18

Rp (Ω cm2)

CPEL nL

Q (μF/cm2)

0.99 0.97 0.95 0.99 0.93 0.99 0.92 0.91 0.94 0.97

34 13 21 19 15 29 12 23 14 22

CPEct

425 345 318 282 265 2678 1686 1315 1220 1093

nct

Q (μF/cm2)

0.82 0.78 0.83 0.89 0.74 0.83 0.87 0.89 0.79 0.88

237 186 131 109 211 47 38 67 89 99

Cdl (μF/cm2)

Rsum

η%

143 85 68 70 76 30 25 49 49 73

428 383 355 315 294 2690 1698 1342 1240 1112

– – – – – 84 77 73 74 73

Table 5 The PDP parameters for the corrosion of duplex brass in 0.5 M H2SO4 in the absence and presence of 10−3 M SG with 5 ppm CTAB at different temperatures. Inhibitor

Temperature (K)

Ecorr (mV/SCE)

icorr (mA cm−2)

η (%)

0.5 M H2SO4

303 313 323 333 303 313 323 333

−79 −81 −90 −73 −109 −76 172 −122

49 66 102 148 5.5 12 26 39

– – – – 89 81 74 68

SG/CTAB

brass in 0.5 M H2SO4 solution without or with inhibitors were calculated by employing the Arrhenius equation [45]:

icorr = k exp(

Ea ) RT

(10)

Where R = the universal gas constant, and k = the Arrhenius pre-exponential factor. Fig. 8 illustrated a straight line obtained for the variation of the ln icorr versus 1/T for the inhibited and uninhibited duplex brass specimens. The slope obtained from the straight line gives the apparent activation energy using the relation [46]:

Slope =

Ea R

(11)

The Ea obtained from the plot are listed in Table 6. It can easily be concluded from the data that the value of Ea is high in the presence 10−3 M SG with 5 ppm CTAB as compared to that for the uninhibited solution indicating the decreased corrosion of duplex brass due to the formation of a barrier film by adsorbed SG and CTAB molecules. The activation parameters like enthalpy (ΔHa) and entropy of activation (ΔSa), were computed by employing Arrhenius’ transition equation [47]:

icorr =

RT S Ha exp( a )exp( ) Nh R RT

(12)

Where h is the Planck's constant, and N is the Avogadro's number. From the data, it can be concluded that the corrosion of duplex brass reflects the endothermic nature in the presence of SG/CTAB and can be proven by its positive sign. The values of both ΔHa and Ea follow the similar trends which permit to verify the known thermodynamic equation between the Ea and ΔHa:

Fig. 7. Potentiodynamic polarization curves of duplex brass in 0.5 M H2SO4 at different temperatures: (a) blank and (b) 10−3 M SG with 5 ppm CTAB.

Ea

can be associated to desorption of some adsorbed inhibitor molecules [44]. However, the formulation provided a good inhibition efficiency at higher temperatures, mainly due to the existence of some chemical interactions. The values of the activation energy (Ea) for the corrosion of duplex

Ha = RT

(13)

The close inspection of the data reveals that the formation of activated complex in the rate determining step is association step, as the values of ΔSa is negative. This refers towards more ordering in transformation from reactants to activated complexes due to desorption of 205

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Fig. 9. Plot of the Langmuir adsorption isotherm of duplex brass in 0.5 M H2SO4 containing various concentrations of SG with 5 ppm CTAB at 303K.

Table 7 The adsorption parameters for the corrosion of duplex brass in 0.5H2SO4 with SG/CTAB at 303 K. Inhibitor

Slope

Kads (M−1)

R2

SG/CTAB

1.06

190147

0.9999

Cinh

=

Table 6 Corrosion kinetic parameters for duplex brass in 0.5 M H2SO4 in the presence and absence of 10−3 M SG with 5 ppm CTAB at different temperatures. Ea (kJ/mol)

ΔHa (kJ/mol)

ΔSa (J mol−1 K−1)

Ea - ΔHa

Blank SG/CTAB

30.21 60.01

27.57 57.37

−121 −41

2.64 2.64

1 + Cinh K ads

−44.1

(14)

where Cinh = the inhibitor concentration, and Kads = the equilibrium constant for adsorption process. The investigated inhibitors i.e. SG/CTAB molecules got adsorbed on the surface of the duplex brass specimens in the monolayers, and this fact is confirmed by the applicability of the Langmuir's model of adsorption isotherm. Which means, the adsorbed molecules of SG and CTAB occupy only one site, preventing interactions between the adsorbed molecules and the duplex brass surface. The θ values were calculated from the data of polarization measurement using equation:

Fig. 8. (a) Arrhenius and (b) transition state plots for corrosion inhibition of duplex brass in absence and presence of 10−3 M SG with 5 ppm CTAB in 0.5 M H2SO4.

Inhibitor

=

° (kJ/mol) Gads

(%) 100

(15)

Fig. 9 represents the linear fitting of θ values obtained for the duplex brass in 0.5H2SO4 solution in the presence of SG/CTAB to Langmuir isotherm model at 303 K. The value of linear correlation coefficients (r2) are close to 1.0 (0.9999), indicating the adsorption of SG/CTAB on the duplex brass surface at 303 K obeyed the proposed model. The value of Kads interpreted from the intercept of the straight line of Fig. 9 is given in Table 7. The high value of calculated Kads represents the high stability and strong adsorption of the SG/CTAB on the duplex brass surface. The Gibbs free energy of adsorption, ΔG°ads, value was calculated using Kads using equation [38]:

water molecules from the duplex brass specimen's surface followed by the adsorption of SG/CTAB molecules [48]. 3.4. Adsorption isotherm study

ΔG°ads = -RTln(55.5 Kads)

In order to understand the inhibition mechanism of inhibitor molecules, understanding of their mode of adsorption onto the duplex brass surface is the key step. For this, the surface coverage values calculated from PDP tests were allowed to fit into several isotherm models. However, the best agreement was found in case of Langmuir Isotherm model, which can be represented by the following equation [49]:

(16)

The negative values of ΔG°ads indicate the spontaneous adsorption of SG/CTAB on the brass surface. Generally, if the values of ΔG°ads are in the range up to −20 kJ mol/ 1, they are consistent with physisorption of the inhibitor on the metal surface, while those more negative than about −40 kJ/mol involve chemisorption [50]. In present investigation, the value of ΔG°ads was 206

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Table 8 Weight% analysis of Fig. 10. Points

Cu

Zn

C

O

N

S

Pb

Polished Brass Blank 5 ppm CTAB 10−3 M SG SG/CTAB

62.81 50.10 51.62 48.88 49.88

36.36 31.35 27.67 27.78 28.73

– 16.75 19.98 17.04

– 12.90 1.73 3.37 2.05

– – 2.23 – 2.31

– 5.65 – – –

0.83 – – –

aggressive solution and the surface showed very rough morphology with many cavities. The presence of an additional peak of O and S in the respective EDS spectrum is suggestive of the formation of corrosion products. In contrast, the surface heterogeneity of the duplex brass samples is markedly decreased in the presence of SG and CTAB, this improvement in surface morphology is mainly attributable to the inhibiting effect of the added inhibitors to the aggressive environment. The corresponding EDS spectra revealed the presence of the O and N peaks in addition to Cu and Zn, indicating towards the existence of the adsorbed CTAB and SG molecules, respectively, on the duplex brass surface. The surface smoothness of the duplex brass specimens retrieved from SG/CTAB containing corrosive media is further improved suggesting its superior inhibition effect. There is also suppression in the Cu and Zn peaks when compared with the EDS spectrum of duplex brass in absence of inhibitors. This is suggestive for the adsorption of the CTAB/ SG molecules onto the duplex brass surface, thus indicative of the successful prevention of the surface from the direct attack by the corrosive media.

Fig. 10. SEM images of duplex brass: (a) exposed to 0.5 M H2SO4, (b), (c) and (d) exposed to 0.5 M H2SO4 with CTAB, SG and 10−3 M SG/5 ppm CTAB respectively, after 12 h of immersion time at 303 K.

found to be −44.1, which indicates the tendency for chemisorption. However, bearing in mind the decrease in the inhibition efficiency at higher temperatures, the adsorption of inhibitor molecules is expected to occur via a combination of chemical and physical interactions with chemical interactions are the predominant one. 3.5. SEM-EDS analysis

3.6. Inductively coupled plasma spectrometry

Surface morphological study and elemental studies of the duplex brass surface retracted from immersion in 0.5 M H2SO4 without and with SG and CTAB alone and in combination after 12 h are illustrated in Figs. 10 and 11 respectively. The percentage elemental composition is depicted in Table 8. It can be seen, the SEM image without CTAB and SG, the duplex brass specimen surface is attacked markedly by the

The concentration of copper and zinc ions in blank and inhibited solutions was analyzed using a high resolution inductively coupled plasma mass spectrometry. This is particularly important because controlling and blocking released ions is the key role involved in corrosion inhibition process, having most likely a pivotal role in reaching a better protection. Fig. 12 represents the concentration of copper and zinc ions in 0.5 M H2SO4 alone and in the presence of CTAB, SG and SG/ CTAB after 12 h immersion at 303K. According to the results demonstrated in Fig. 12, one can see that the concentration of zinc ions in blank solution is larger than that of copper ions, which seems consistent with the fact that the copper is nobler than zinc and due to a higher

Fig. 12. ICPS of duplex brass immersed in 0.5 M H2SO4 and in presence of CTAB, SG and SG/CTAB after 12 h immersion time at 303K.

Fig. 11. EDS analysis of duplex brass sample derived from SEM tests. 207

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Fig. 13. Frontier molecular orbitals of SG, SGH (protonated form) and CTAB calculated using DFT method, (a) Optimized molecular structure, (b) HOMO and (c) LUMO.

percentage of zinc in duplex brass. In addition, the effect of CTAB is greater than that of SG. This is may be due to a possible complexation between CTAB and released ions. However, a better control of the ions release was obtained in the solution containing SG/CTAB, confirming the paramount role of this formulation in the corrosion inhibition of duplex brass in 0.5 M H2SO4 solution.

SG molecule. Its EHOMO is shifted to more negative value compared to that of the neutral form, which suggests that after protonation, the electron donating capability of the SG was decreased. The results also denote an increase in the energy gap and a decrease in the ELUMO upon the protonation of SG molecule. Therefore, the anionic form of SG presumably has a stronger affinity for the brass surface and can be considered the most reactive compared to the protonated form. Though important, frontier molecular orbitals (FMOs) i.e. HOMO and LUMO, do not successfully address the local reactivity of an inhibitor molecule. Thus, extending these calculations by incorporating other prediction parameters, like Fukui functions is important. The Fukui functions are widely used to predict the susceptibility of atoms of the molecule to an electrophilic attack or a nucleophilic attack [35,55]. Furthermore, the difference between the nucleophilic and electrophilic Fukui function is known as the dual descriptor and it is a very important tool to characterize the reactivity of a site within a molecule towards a nucleophilic or an electrophilic attack. The Fukui functions indices along with the dual descriptor are listed in Table 10. By analyzing the Fukui functions

3.7. Quantum chemical calculations Available knowledge about electronic properties of corrosion inhibitors can be obtained from the theoretical calculations, especially the density functional theory (DFT) [10,51]. The adsorption of inhibitor molecules arises from the donor-acceptor interactions between the free electron pairs of heteroatoms and π-electrons as well as other functional groups and the vacant metal orbitals. On the one hand, the propriety of an inhibitor molecule to donate its electron is associated to its electron donating power, while on the other hand, its tendency to accept electron follows its electron accepting ability [52,53]. The electron donating ability is often related to the highest occupied molecular orbital energy EHOMO of the inhibitor molecule while the electron accepting is related to the lowest-lying unoccupied molecular orbital (LUMO). Fig. 13 represents the optimized molecular structures, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of the sodium gluconate (neutral and protonated forms) and CTAB molecules. Table 9 lists some quantum chemical parameters derived from DFT calculations. As it is seen from Fig. 13, the HOMO and LUMO electrons are mainly delocalized on the oxygen atoms of SG while in CTAB, the HOMO electron density is delocalized on the hydrophobic part and the LUMO density is mostly distributed on the head group. The pivotal role of a long carbon chain in the corrosion inhibition process is well demonstrated [54]. The reactive regions in HOMO and LUMO could be the possible sites for the nucleophilic and electrophilic interactions with the metal surface. In the protonated form, both HOMO and LUMO orbitals moved to the opposite side of the

Table 10 Fukui functions and dual descriptor of the SG and CTAB molecules calculated at DFT/GGA/DNP. CTAB

Table 9 The quantum chemical parameters of SG and CTAB calculated using DFT/GGA/ DNP. Inhibitor

EHOMO (eV)

ELUMO (eV)

ΔEgap (eV)

SG SGH CTAB

−0.169 −6.125 −6.810

3.425 −1.704 0.628

3.594 4.421 7.438

208

SG

Atoms

f k+

fk

C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) C(16) N(17) C(18) C(19) C(20)

0.000 −0.000 −0.000 −0.000 0.000 −0.000 −0.000 −0.000 0.000 0.000 0.001 0.001 0.004 0.012 0.016 0.056 0.031 0.063 0.080 0.063

0.018 0.020 0.028 0.031 0.035 0.043 0.046 0.038 0.033 0.030 0.020 0.016 0.013 0.009 0.001 0.000 0.001 0.000 0.002 0.000

fk −0.018 −0.020 −0.028 −0.031 −0.035 −0.043 −0.046 −0.038 −0.033 −0.030 −0.019 −0.015 −0.009 0.003 0.015 0.056 0.030 0.063 0.078 0.063

Atoms

f k+

fk

O(1) C(2) C(3) C(4) C(5) C(6) C(7) O(8) O(9) O(10) O(11) O(12) O(13)

0.149 0.056 0.021 0.010 0.003 0.003 0.002 0.019 0.057 0.008 0.022 0.009 0.012

0.017 0.005 0.002 0.004 0.007 0.040 0.100 0.156 0.004 0.016 0.022 0.081 0.384

fk 0.132 0.051 0.019 0.006 −0.004 −0.037 −0.098 −0.137 0.053 −0.008 0 −0.072 −0.372

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and dual descriptor indices, the results reveal that all the electrophilic sites are located on the hydrophobic part of CTAB in agreement with HOMO density distribution while the nucleophilic sites are located on the carbon atoms of the head group. For SG, the favorable centres for electron acceptance i.e., nucleophilic attacks are O(1), C(2) and O(9) atoms having the higher f k+ values and a positive dual descriptor. On the other hand, the most susceptible sites for electrophilic attacks i.e., electron donation are C(7), O(8), O(12) and O(13) atoms having the higher f k values and a negative dual descriptor. Similarly, for CTAB, the sites for electron acceptance as reflected by the highest f k+ values and the positive dual descriptor are C(16), N(17), C(18) and C(19) atoms whereas, the most electron donating centres correspond to C(5), C(6), C(7) and C(8) atoms for their highest f k values and their negative dual descriptor. These results provide further confirmation on the participation of the heteroatoms in the interaction with the metal surface and are consistent with the FMOs distribution.

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3.8. Proposed corrosion inhibition mechanism The results of the present study raise important questions regarding the exact mechanism of action of the investigated compounds on duplex brass corrosion. To tackle some of these questions, it is crucial to elucidate the nature of inhibitor-alloy interactions. According to previous results [16,56], in 0.5 M H2SO4, the brass surface is positively charged at the corrosion potential. This means that for electrostatic attraction, positively charged inhibitors are unlikely to directly adsorb on the brass surface. In our results, we found that the addition of CTAB to SG has a considerable effect on the inhibition process. The most possible explanation for this finding is that the presence of bromide ions in sulfuric acid solutions can play a role in this process. These intermediates species could be easily adsorbed into the brass/solution interface due to the excess of positive charge. Therefore, the alloy surface will become negative, thus the C16H33N(CH3)3+ can be electrostatically adsorbed on the alloy surface [56]. Furthermore, the great inhibitive effect of CTAB could also be explained by the fact that the anionic form of SG can also be an intermediate, thus a significant amount of C16H33N(CH3)3+ can be attracted. Along with the electrostatic adsorption, the chemisorption of n-cetyl and SG on the alloy surface must also be considered, and we believe that it takes place simultaneously [56]. Quantum chemical calculations are an interesting tool to identify the atomic sites that could be involved in inhibitor-alloy interactions. From Fukui function and frontier orbitals results, it is clear that almost all n-cetyl carbons have an electron donating character, which confirms their tendency to participate in the chemisorption process. The same can be said about SG, which also has a high number of electron donating sites. Together, these results highlight the paramount role of the synergistic effect in limiting the corrosion process of the duplex brass. 4. Conclusion The corrosion inhibition of duplex brass has been investigated by using Sodium Gluconate (SG) and Cetyltrimethylammonium bromide (CTAB) in 0.5 M H2SO4. The results show that SG/CTAB can act as effective inhibitors and provide a significant improvement in corrosion resistance of duplex brass. The electrochemical results illustrate that the SG/CTAB mainly behaves as cathodic type inhibitors; leading to a high increase of the corrosion resistance and a lower corrosion current density. The adsorption of SG/CTAB follows Langmuir adsorption model. SEM-EDS and ICPS studies confirm the presence of strong inhibitor-duplex brass interactions and support the effective adsorption of inhibitors. Quantum chemical calculations extend the understanding of the experimental findings and show that the oxygen atoms play a crucial role in adsorption process. These insights seem very important and encourage researchers to pay more attention to corrosion of brass in acidic medium. 209

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