Experimental and theoretical investigation on corrosion inhibition of AA5052 aluminium alloy by l -cysteine in alkaline solution

Materials Chemistry and Physics xxx (2015) 1e10

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Experimental and theoretical investigation on corrosion inhibition of AA5052 aluminium alloy by L-cysteine in alkaline solution Dapeng Wang a, Lixin Gao a, Daquan Zhang a, *, Dong Yang a, Hongxia Wang b, Tong Lin b a b

School of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, China Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia

h i g h l i g h t s
L-cysteine was used as corrosion inhibitor for Al alloy in alkaline solution.
Adsorption of L-cysteine on Al alloy surface obeyed the amended Langmuir's isotherm.
L-cysteine molecules interacted with the carboxyl groups on the Al alloy surface.
A strong orbital hybridization occurred between the reactive sites in L-cysteine and Al.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 December 2014 Received in revised form 15 May 2015 Accepted 21 November 2015 Available online xxx

The corrosion inhibition of L-cysteine on AA5052 aluminium alloy in 4 mol/L NaOH solution was investigated by hydrogen gas evolution experiment, polarisation curve, galvanostatic discharge, electrochemical impedance spectroscopy measurements and quantum chemical calculations. The adsorption of L-cysteine on aluminium alloy surface obeyed the amended Langmuir's adsorption isotherm. The polarisation curves indicated that L-cysteine acted as a cathodic inhibitor to inhibit cathodic reaction. The inhibition mechanism was dominated by the geometric covering effect. The galvanostatic discharge shows that the additives restrain the hydrogen evolution and increase the anodic utilization rate. Quantum chemical calculations indicated that L-cysteine molecules mainly interacted with on the carboxyl groups on the aluminium alloy surface. A strong hybridization occurred between the s-orbital and p-orbital of reactive sites in the L-cysteine molecule and the sp-orbital of Aluminium. © 2015 Elsevier B.V. All rights reserved.

Keywords: Alloy Corrosion test Adsorption Corrosion

1. Introduction Aluminium and its alloys are widely used in industrial applications and scientific technologies, especially in aerospace, surface coating and aluminium-air batteries. In aluminium-air batteries, aluminium or its alloy is used as anode, and air as cathode. They show advantages in high energy density, high power density and environmental friendliness [1e4]. However, aluminium-air batteries meet problems with substantial self-corrosion [5], hydrogen evolution, and reductive deposition of Al3þ ions on the anode [6]. The first two are more severe when alkaline electrolyte solution is used. Consequently, considerable columbic loss takes place during discharge of the battery [7e11]. It is vital increase the corrosion resistance and the over potential of parasitic hydrogen evolution

* Corresponding author. E-mail address: [email protected] (D. Zhang).

without decreasing the oxidation rate of aluminium. Many efforts have been devoted to improving the anode electrochemical properties. The first major approach is to incorporate the various alloying elements such as Ga, In, Pb, Mg, Sn, Mn and Zn [12e15]. The alloying elements may have high hydrogen overpotential to reduce hydrogen evolution. The second method is to modify the composition of electrolyte by adding corrosion inhibitors [16e24]. Since corrosion inhibitors usually have small amount in dose, they will have little effects on discharge performance of Al-air batteries [25e28]. Among the organic inhibitors, amino acid compounds are attractive owing to their nontoxicity, biodegradability and cost efficiency. However, little work has been reported about using amino acids as a green corrosion inhibitor to prevent aluminium alloy from corrosion in alkaline solutions. As a representative of 5000 series aluminium alloys, AA5052 aluminium alloy has widely range of industrial applications. This multicomponent alloy contains Mg, Mn and Zn, which may have

http://dx.doi.org/10.1016/j.matchemphys.2015.11.041 0254-0584/© 2015 Elsevier B.V. All rights reserved.

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D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

high hydrogen overpotential. In this study, we study the ability of Lcysteine to inhibit AA5052 aluminium alloy corrosion in 4 M NaOH solution. This is the electrolyte usually used for aluminium-air batteries. L-cysteine is selected because it contains a mercapto group, apart from an amino group and a carboxylic group in its molecule (Fig. 1). The inhibition efficiencies were determined by hydrogen gas evolution experiment and weight loss test. Corrosion inhibition was studied by means of polarisation curves and electrochemical impedance spectroscopy (EIS) [29e32]. The inhibition mechanism was investigated by quantum chemical calculation.

Table 1 Composition of AA5052 aluminium alloy. Si

Cu

Cr

Mn

Zn

Mg

Fe

Al

0.06

0.10

0.19

0.06

0.10

2.46

0.27

Others

4.0 M NaOH solution was prepared by dissolving AR NaOH (Sinopharm Co.) in distilled water. The working electrode (WE) was made of AA5052 aluminium alloy (Sinopharm Co.). The chemical composition (in wt.%) of AA5052 is given in Table 1.

in the solution, the corrosion potential was stabilized and the electrochemical measurements were performed. All electrochemical measurements were carried out using a Solartron 1287 Electrochemical workstation coupled with a Solartron 1260 Impedance/Gain-Phase Analyzer. The EIS experiments were performed at open circuit potential over a frequency range of 0.01Hze100 kHz. The sinusoidal potential perturbation was 5 mV in amplitude. Tafel polarisation curves were recorded in the potentials region ±300 mV (vs Ecorr). The sweep rate was 1 mV/min. Galvanostatic discharge tests were performed at the current density of 15 mA/cm2 for 2 h. Experiments were carried out at room temperature. Experimental data was recorded in Zplot and Corrware software, fitting by the Zsimp-Win software.

2.2. Gasometry test

2.5. Quantum chemical calculations

2. Experimental 2.1. Materials

AA5052 aluminium alloy samples (2.5 cm  1 cm  0.05 cm) were placed in drainage device (consist of conical flask and gasguide tube). The conical flask contained 4 M NaOH solution (250 mL). The drainage device allowed the volume of evolution hydrogen gas to be measured as a function of time. The reaction rate was calculated from the slope of the straight line in the gasometry plot. The mean corrosion rate (R) over the exposure period was calculated according to the Eq. (1):

R¼

VH2 AT

(1)

where R is in mL/(cm2 min), A is specimen area (in cm2); VH2 is the volume of hydrogen gas collected, T is the immersion period (in min).

A DFT (density functional theory) method [33] was used in this study. All the DFT calculations were performed using the Dmol3 package with numerical atomic orbital basis sets. The exchangecorrelation functional of Perdew, Burke, and Ernzerhof was utilized. The double-numerical plus polarisation (DNP) basis sets were used in the expansion of the Kohn-Sham orbitals and the orbitalconfining cut-off was set as 4.8 Å. ThepAl ffiffiffi (111) surface was simulated by a four-layer rectangular (2  3) slab separated by 30 Å vacuum. 6  6  1 Monkhorst-Pack grid was used for the calculation with s ¼ 0.001 Ha of Fermi smearing. The convergence criteria on the energy, gradient, and displacement were set to 1  105 hartree, 2  103 hartree/Å, and 5  103 Å, respectively. 3. Results and discussion

2.3. Weight-loss measurement

3.1. Gasometry test

The coupons (4 cm  2.5 cm  0.4 cm) were abraded with sandpaper of different grades, rinsed with alcohol and deionized water. The coupons after the surface treatment were then immediately immersed in 4.0 M NaOH solution for 0.5 h at different temperatures.

Fig. 2 shows the volume of the hydrogen gas released for immersing AA5052 aluminium alloy in 4 M NaOH solution at 30  C. The hydrogen gas evolution rate (R) is obtained from the slope of the linear part and shown in Fig. 3.

30

2.4. Electrochemical measurements

C

-2

20 15 10 5 0

H H HS C

25

V (ml cm )

Electrochemical experiments were carried out in the conventional three-electrode cell with a platinum counter electrode (CE), a saturated calomel electrode (SCE) as the reference electrode (RE) and a working electrode (WE). The WE was prepared using an AA5052 cylindrical rod. The periphery of the rod was sealed with epoxy resin so that only the circular cross section (area, 0.5 cm2) was exposed. The WE was abraded with 400, 600 and 1200 grit sandpaper, decreased with AR-grade ethanol and acetone, and rinsed with deionized water prior to test. After 5 min of immersion

0 mmol/L 10 mmol/L 20 mmol/L 30 mmol/L 40 mmol/L

0

COOH

H NH2 Fig. 1. Chemical structure of L-cysteine.

5

10

15

20

25

30

t (min) Fig. 2. Hydrogen gas evolution volume vs time for immersing AA5052 aluminium alloy in testing solutions at 30  C.

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D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

The linear variation of the hydrogen gas evolution volume with AA5052 immersion time in the alkaline solution is observed. Visual observation shows that the hydrogen gas evolution decreases upon the addition of L-cysteine into the NaOH solution. Fig. 2 indicates that the hydrogen gas evolution rate decreases with the increase of the concentration of L-cysteine in 4 M NaOH. The slope value has no significant changes with the concentration of L-cysteine above 30 mmol/L. This indicates that inhibition effect reaches a maximum value at the concentration around 30 mmol/L.

0.04

WL ¼ kt

-2

WL (g cm )

0.02

0.01

0.00 0

10

20

30

40

50

60

time (min) Fig. 4. The weight loss with time for the corrosion of AA5052 aluminium alloy in different solution at 30  C.

20°C

(2)

0.24 0.20

c/θ (mol L )

30°C 40°C 50°C

-1

k in the Eq. (2) is the zero order rate constant. The difference in the k value confirms that the presence of L-cysteine in 4 M NaOH solution considerable affected corrosion behaviour of aluminium. The inhibition saturation at 30 mmol/L can be explained by the adsorption of L-cysteine on the aluminium alloy surface. With the increasing of its concentration, more L-cysteine molecules are adsorbed on Al surface. When the concentration reaches 30 mmol/ L, the coverage of the L-cysteine on the aluminium alloy surface is saturated.

−4

0 mmol/L k= 5.867×10 −4 10 mmol/L k= 4.724×10 −4 20 mmol/L k= 4.137×10 −4 30 mmol/L k= 3.911×10 −4 40 mmol/L k= 4.119×10

0.03

3.2. Weight-loss measurement The AA5052 aluminium alloy lost weight during immersion in 4 M NaOH solution. The weight loss (g cm2) at different time points is shown in Fig. 4. The presence of L-cysteine in the solution reduces the weight loss of AA5052 alloy, and weight loss rate decreases with increasing the L-cysteine concentration. When the concentration reaches 30 mmol/L, the weight loss rate became constant, which is similar to the result obtained from gasometry test. Based on Fig. 4, the weight loss (WL) followed a zero order reaction mechanism [34,35].

3

0.16 0.12 0.08 0.04

3.3. Amended adsorption isotherm

0.00

0.01

0.02

0.03

0.04

-1

c (mol L ) Fig. 5. Plotting c/q against c for AA5052 aluminium alloy in different solutions.

c H ¼ þ Hc q K I¼

Ru  Rp  100 Ru

(4)

0.85

q¼

I 100

(5)

-2

(3)

0.80

-1

Adsorption isotherm provides information on the interaction between a corrosion inhibitor and metal surface. From adsorption isotherm, the surface coverage (q) of the inhibitor on metal surface was obtained. Fig. 5 shows c/q ~ c plot and the corresponding linear regression parameters are listed in Table 2. Attempts are made to fit experimental data to various isotherms including Langmuir, Freundlich, Frumkin, Temkin, Bockris-Swinkels and FloryeHuggins isotherms. The result is for to best fit with an amended Langmuir adsorption isotherm [36]:

0.75

where c is the concentration of inhibitor, K is the adsorptive equilibrium constant, H is the correction coefficient, I is the values of inhibition efficiency, Ru is the corrosion rate in the absence of inhibitor, Rp is the corrosion rate in the presence of inhibitor. It is evident that the linear correlation coefficient (R) values are

R (mL min cm )

0.90

0.70 0.65

Table 2 Corresponding linear regression parameters by fitting c/q and c.

0.60 0.55 -10

0

10

20

30

40

50

-1

C (mmolL ) Fig. 3. Variation of hydrogen gas evolution rate with the concentration of L-cysteine.

Temperature (ºC)

R

Slope

K (L mol1)

20 30 40 50

0.9935 0.9967 0.9901 0.9904

4.53 4.78 4.19 3.07

61.78 288.37 1217.37 1906.84

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D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

almost equal to 1, indicating the adsorption of L-cysteine on aluminium surface obeys amended Langmuir adsorption isotherm. The adsorptive equilibrium constant (K) value increases with the increase of temperature. This suggests that L-cysteine molecules are more easily adsorbed on the aluminium surface at a relatively higher temperature. The adsorptive equilibrium constant (K) is related to the standard adsorption free energy (DGq):

K¼

  1 DGq exp 55:5 RT

(6)

where R is the gas constant (8.314 J mol1 K1), T absolute temperature (K), and the values 55.5 is the concentration of water in solution expressed in mol/L. The adsorption enthalpy (DHq) is calculated on the basis of Van't Hoff equation:

dln K DHq ¼ dT RT 2

(7)

Table 3 Thermodynamic parameters of L-cysteine adsorption on AA5052 alloy surface. Temperature (ºC)

DHq (kJ mol1)

DGq (kJ mol1)

DSq (J mol1 k1)

20 30 40 50

92.58 92.58 92.58 92.58

19.8 24.4 28.9 31.1

37.71 37.98 38.20 37.71

with increasing concentration of L-cysteine. In general, the value of

DGq about 20 kJmol1 is consistent with physisorption, while

those below 40 kJ mol1 correspond to chemisorption. So, the adsorption of L-cysteine on AA5052 alloy surface belongs to physical and chemical adsorption. The positive values of DS q could be explained by that the adsorption of L-cysteine is accompanied by desorption of water molecules from the AA5052 alloy surface. The positive DSq values suggest that disorder in inhibitor molecule arrangement in the metal/solution interface increases, which is the driving force for the adsorption of inhibitor from the AA5052 alloy surface [37,38].

This equation can also be rearranged as the following equation: 3.4. Electrochemical measurements

DH q þ ln A RT

ln K ¼

(8)

where lnA is integration constant. Fig. 6 shows lnK ~1/T. From the slope (-DHq/R), DHq is calculated, being 92.58 kJ mol1. With the obtained both parameters of DGq and DHq, the standard adsorption entropy (DSq) can be obtained using the following thermodynamic basic equation:

DSq ¼

DHq  DGq T

(9)

The corresponding thermodynamic parameters are shown in Table 3. The values of DGq are negative and lies between 19.8 and 31.1 kJ mol1. The negative value indicates that the adsorption of the L-cysteine molecules on the aluminium surface is a spontaneous process. The values are more negative with an increase in the L-cysteine concentrations, indicating that the adsorption of L-cysteine on the aluminium surface is favourable

3.4.1. Open circuit potential The open circuit potential (EOCP) of AA5052 electrode in 4 mol/L NaOH solution changes with time, the value is affected by Lcysteine and its concentration (Fig. 7). At initial 150 s, EOCP increases toward positive value with prolonging the immersion time. After about 200 s, EOCP reaches a steady state regardless of the L-cysteine concentration. The presence of L-cysteine in the solution led to shift of the curve to the negative potential direction, indicating that the cathodic corrosion is affected prominently by L-cysteine additive. 3.4.2. Polarisation curve measurement Fig. 8 shows the polarisation curve for aluminium in 4 M NaOH solution with and without L-cysteine at 30  C. Table 4 is the electrochemical polarisation parameters, corrosion potential (Ecorr), cathodic Tafel constant (bc) and corrosion current density (icorr). The values of electrochemical polarisation parameters are obtained by the extrapolation of linear Tafel segments of the cathodic curve

8

LnK

7

6

5

4 3.1

3.2

3.3 -3

3.4

-1

1/T (10 K ) Fig. 6. lnK ~1/T (The linear correlation coefficient was 0.9844).

Fig. 7. EOCP ~ time curves for AA5052 electrode in 4 mol/L NaOH solution containing different concentrations of L-cysteine (temperature 30  C).

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D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

as described in Fig. 9. The presence of inhibitor shifts the corrosion potential to negative values and decreases current densities. The cathodic part of the polarisation curves is suppressed evidently in the presence of L-cysteine. This could be the reason for the inhibition effect of Lcysteine against hydrogen gas evolution. Since the cathodic polarisation curves are almost in parallel, the presence of L-cysteine should not alter the mechanism of hydrogen reaction.

-0.5

-2

logi (A·cm )

-1.0 -1.5 -2.0 0mmol/L 10mmol/L 20mmol/L 30mmol/L 40mmol/L

-2.5 -3.0 -3.5 -2.0

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

The polarisation curve has a limiting current, which indicates that the anodic process is controlled by diffusion process. Aluminium in alkaline solution is usually considered to carry out the following corrosion reaction [39,40]:

Al þ OH ¼ AlðOHÞads þ e

(10)

AlðOHÞads þ OH ¼ AlðOHÞ2;ads þ e

(11)

AlðOHÞ2;ads þ OH ¼ AlðOHÞ3;ads þ e

(12)

AlðOHÞ3;ads þ OH ¼ AlðOHÞ4;ads

(13)

The diffusion of Al(OH)1e4, ads ions has an important influence on the anode dissolution. The limiting current is an important parameter to characterize mass transport rate in electrochemical systems. When an electrochemical system occurs under limiting current condition, the reaction proceeds at the maximum rate [41]. The electrochemical polarisation for AA5052 in 4 M NaOH solution containing different concentrations of L-cysteine shows the same limiting max current. This indicates that L-cysteine has little effect on the activity of the anode. Drazic et al. [31] suggested that the corrosion of aluminium occurs through ionic migration through the

-1.2

0.6

0.4

2

-Z''(Ω cm )

E(V vs.SCE) Fig. 8. Polarisation curves of AA5052 aluminium alloy in 4 mol/L NaOH solution containing different concentrations of L-cysteine at 30  C.

5

0 mmol/L 1580Hz 10 mmol/L 20 mmol/L 2511Hz 1584Hz 30 mmol/L 40 mmol/L 10Hz

0.2

Table 4 Electrochemical polarisation parameters of AA5052 aluminium alloy in 4 mol/L NaOH solution. C (mmol L1)

-Ecorr (V)

-bc (mVdec1)

icorr (mAcm2)

0 10 20 30 40

1.561 1.627 1.626 1.634 1.637

444.7 356.9 383.6 381.7 347.5

70.02 47.62 42.87 39.98 35.74

15Hz 1995Hz 12Hz

0.0 125Hz

0.8

1.0

1.2

125Hz

158Hz 1Hz 12Hz

1.4

1.6

2

1.8

1Hz

2.0

z' (Ω cm ) Fig. 10. Nyquist plots of AA5052 aluminium alloy in 4 mol/L NaOH solution containing different concentrations of L-cysteine at 30  C.

0.0 -0.5 E = -1.626 V -2

logi (A·cm )

-1.0 -1.5 -2.0 -2.5

E = -1.626 V i = 42.87 mA cm-2

logi = -3.217- 1.137E -1

-3.0

20mmolL L-C E

-3.5

logi vs E

-2.0

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

E ( V vs. SCE) Fig. 9. Polarisation curves of AA5052 aluminium alloy in 4 M NaOH solution containing 20 mM of the L-cysteine represent the calculation of corrosion parameters.

Fig. 11. Equivalent circuit of electrochemical impedance spectra: (a) blank and (b) with additive.

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D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

0.5

Msd. Cal.

-Z'' / Ω⋅cm

2

0.4 0.3 0.2 0.1 0.0 -0.1 1.0

1.2

1.4

1.6

1.8

2

z' / Ω⋅cm

Fig. 12. Experimental and computer fit results of Nyquist plot for AA5052 aluminium alloy in 4 M NaOH solution.

16

0.0

14 12 10

|Z| Msd |Z| Calc Phase Msd Phase Calc

Log |Z|

Phase (deg)

-0.1

8 6 4

-0.2

2 0 -2

-0.3 -1

0

1

2

3

4

Log f (HZ) Fig. 13. Experimental and computer fit results of Bode plots (impedance and q) for AA5052 aluminium alloy in 4 M NaOH solution.

oxide film followed by dissolution at the oxide-electrolyte interface. On the other hand, Macdonald et al. [32] proposed a stepwise addition of OH species to the surface aluminium atoms leading to the formation of Al(OH) 4 [36,42]. As indicated in Table 4, L-cysteine concentration has an effect on the corrosion current. This could be attributed to the adsorption of the L-cysteine active ingredient [38] on the aluminium surface, leading to decrease of the exposed area necessary for aluminium dissolution and hydrogen evolution.

3.4.3. Electrochemical impedance spectroscope (EIS) Fig. 10 shows Nyquist plots for aluminium in 4 M NaOH

solutions at 30  C. Without L-cysteine, the impedance spectrum consists of three loops, (i) a large capacitive loop at HFs, (ii) a small inductive loop at medium frequencies (MFs), and (iii) a second capacitive loop at LFs. However, in the presence of L-cysteine in the solution, the impedance spectra consisted of four loops. Apart from the loops (i) ~ (iii), a second small inductive loop occurred at LFs. The whole diagram appears an approximate elliptic shape. Similar impedance results for aluminium in alkaline solutions have been reported by other researchers [32,43]. By fitting the experimental data with an EIS model, the reaction mechanism is studied. The proposed equivalent circuit in Fig. 11 is used to analyse the impedance spectra of aluminium in alkaline solutions. The experimental and computer fit results of Nyquist plot and Bode plots (impedance and q) for AA5052 aluminium alloy in 4 M NaOH solution containing 20 mmol/L of the L-cysteine is demonstrated in Fig. 12 and Fig. 13. It is found that the fit results are consistent with the experimental data within 10% of errors. The corresponding fitting parameters are shown in Table 5. The first part in the equivalent circuit model includes a series combination of resistance, R1, and inductance, L1, in parallel with charge transfer resistance (Rct)1, and the constant phase element (CPE1). The first capacitive semicircle at higher frequencies is attributed to the redox Al4Alþ reaction since it is the ratedetermining step in the charge transfer process. Therefore, the resistance value obtained from the intercepts of the first capacitive semicircle with the real axis corresponds to the AleAlþ charge transfer resistance. The first part inductive loop is explained by the occurrence of adsorbed intermediates on the surface in the aluminium dissolution process. Adsorbed intermediate species such as Al(OH)1,ads, Al(OH)2,ads, Al(OH)3,ads and Al(OH)-4,ads might be involved in the aluminium dissolution process. The second part in the equivalent circuit model includes a series combination of resistance, R2, and inductance, L2, in parallel with charge transfer resistance (Rct)2, and the constant phase element (CPE2). The second capacitive semicircle could be attributed to the fast complementary redox AlþeAl3þ reaction. However, in the alkaline solutions, Al3þ exist as Al(OH)1-4, ads ions. Due to the larger size, it is hard for the Al(OH)1e4, ads ions to diffuse in the alkaline solution. Compared with the blank solution, the second small inductive loop at LFs could be attributed to the coordination bond, which is formed between aluminium ions and L-cysteine. The adsorption of L-cysteine molecules on the aluminium surface to form larger molecules could be the reason to prevent the hydrogen release. Besides, the curves show that increasing the concentration of Lcysteine in the alkaline solutions lead to an increase the size of the capacitive semicircles, an indication of increase of charge transfer resistances but decrease in corrosion rate.

3.4.4. Galvanostatic discharge The galvanostatic discharge of AA5052 aluminium in 4 M NaOH solution without and with inhibitor is shown in Fig. 14. The weight of the anodes is measured both before and after discharge. The

Table 5 Nyquist plots parameters of AA5052 alloy in 4 mol/L NaOH solution containing different concentration of L-cysteine at 30  C. C (mmolL1)

Rs (Ucm2)

0 10 20 30 40

0.46 0.51 0.48 0.49 0.52

(Rct)1 (Ucm2)

CPE1 n

Y0 (Ss cm 0.18 0.09 0.07 0.09 0.07

2

)

L1 (Hcm2)

R1 (Ucm2)

n

n 0.91 0.94 0.88 0.91 0.82

CPE2 Y0 (Ss cm

0.31 0.45 0.64 0.78 0.84

0.006 0.049 0.042 0.041 0.081

0.50 0.55 0.58 0.64 0.72

4.0 2.5 2.5 2.4 2.3

    

2

104 104 104 104 104

)

(Rct)2 (Ucm2)

L2 (Hcm2)

R2 (Ucm2)

0.21 0.39 0.42 0.41 0.43

e 3.1 2.8 2.9 3.9

e 1.433 1.971 1.843 2.152

n 0.95 0.87 0.91 0.84 0.92

   

103 103 103 103

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D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

the mercapto group. The initial structure for DFT modelling is built on the surface of Al (1 1 1) by linking the adsorption sites of the inhibitor molecules with the metal surface [45,46]. Several different adsorption modes of L-cysteine molecules on Al (1 1 1) are probed, and the most stable modes are presented in Fig. 15. Fig. 15(a) is the optimized structure of L-cysteine molecule adsorbed on Al(1 1 1) via N and O atoms. Fig. 15(b) is the optimized structure adsorbed on Al(1 1 1) via O atoms. Fig. 15(c) is the optimized structure adsorbed on Al(1 1 1) via S atom. Fig. 15(d) gives the optimized structure adsorbed on Al(1 1 1) via S and O atoms. Fig. 15(e) showed the optimized structure of water molecules adsorbed on Al(1 1 1). The adsorption energy was calculated as [47]:

-1.40

15 mAcm-2

E (V VS.SCE) OCP

-1.45

Blank 30mmol/L

-1.50 -1.55 -1.60 -1.65 0

1000 2000 3000 4000 5000 6000 7000 8000

Time (sec) Fig. 14. Galvanostatic discharge of AA5052 aluminium alloy in 4 M sodium hydroxide solution without and with 30 mmol/L of the L-cysteine.

anode utilization (h), capacity density and energy density are calculated using the following formulas [44]:

h¼

100It DmF=9

(14) Ih Dm

(15)

U,Ih Dm

(16)

Capacity density ¼

Energy density ¼

7

where h is the anode utilization (in %); I is the current (in A); Dm is the weight loss (in g); F is the Faraday constant; and t is the time (in s); U is the average voltage (in V); h is the time (in h). The aluminium alloy anode in the 4 M sodium hydroxide solution with 30 mmol/L L-cysteine presents a high electrode potential at the current density of 15 mAcm2. From the Table 6, compared with the blank solution (1.545 V), the electrode potential is 1.531 V in the 4 M sodium hydroxide solution with 30 mmol/L L-cysteine. However, the capacity density increases from 2710 mAh g1 to 2778 mAh g1, and the energy density improves from 4186 Wh kg1 to 4253 Wh kg1. Although larger electrode polarisation is still observed, the AA5052 alloy electrode presents good discharge performance in our experimental. After adding L-cysteine in 4 M sodium hydroxide solution, the anode utilization (h) increases from 91.1% to 93.3%. This suggests that additives restrain hydrogen evolution and increase anodic utilization rate.

3.5. Quantum chemical calculations The reactive sites of L-cysteine molecule are the O atom in the carboxylic group, N atom in the amino group and S atom in

DEads ¼ Etotal e (Esurf þ Emol)

(17)

where Etotal, Esurf, and Emol are the total energies of the adsorption system, the clean surface of aluminium, and the isolated Lcysteine molecules, respectively. Adsorption energy of different adsorption modes are presented in Fig. 15. The negative values suggest that the adsorption of L-cysteine is in a stable state. The greater absolute values of adsorption energy, the stronger ability of inhibitor adsorbed on Al(1 1 1). The adsorption energy decreases in the order, (e) ＜ (a) ＜ (d) ＜ (c) ＜ (b), which indicates that inhibitor molecules adsorbes on the surface of aluminium instead of water molecules and the adsorption is mainly based on the reactive groups. To further understand the adsorption of L-cysteine molecule on Al surface, we calculate the charge density difference, Dr. Charge density difference is calculated as [48]:

Dr ¼ rmol/sur(r)rsur(r)rmol(r)

(18)

where rmol/sur(r), rsur(r), and rmol(r) are the total density difference of the system, the clean surface of aluminium, and the isolated Lcystenine molecules, respectively. As shown in Fig. 16, the red area represents charge accumulation regions, while the blue area indicates charge deficit regions. The charge density difference visually shows the interaction between inhibitor molecules and Al surface. The area and intensity of reddish regions are around the atoms, N, O and S, the atoms of which can provide lone pair electrons. The area and intensity of blue regions are around the Al atoms which have empty orbital. The adsorption of the inhibitor molecules on aluminium surface comes from the formation of coordination bonds between the atoms N, O and S in L-cysteine and Al atoms. Projected density of state (PDOS) provides evidence of molecule-surface bonding. The PDOS to the s-orbital and p-orbital of the different reactive sites and the sp-orbital of Al is shown in Fig. 17. The s-orbital and p-orbital of the different reactive sites and the sp-orbital of Al have the corresponding peak values at some same energy level. This indicates that a strong hybridization occurs between the s-orbital, p-orbital of reactive sites and the sp-orbital of Al.

Table 6 Batteries performance parameters in 4 M sodium hydroxide solution without and with 30 mmol/L of the L-cysteine. Additive (mmolL1)

Dm (g)

Current density (mA cm2)

Average discharge voltage (V)

Capacity density (mAh g1)

Energy density (Wh kg1)

h％

0 30

0.0110 0.0108

15 15

1.545 1.531

2710 2778

4186 4253

91.1 93.3

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8

D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

Fig. 15. Optimized structure and adsorption energy of different adsorption modes of L-cysteine molecules on Al(1 1 1).

Fig. 16. Charge density difference of different adsorption modes of L-cysteine molecules on Al (1 1 1).

4. Conclusion In this study, the corrosion inhibition ability of L-cysteine against the corrosion of aluminium alloy in 4 M NaOH solution has been demonstrated. The presence of L-cysteine 4 M NaOH solution show noticeable inhibition of Al to release hydrogen gas and weight loss. The L-cysteine gives a maximal inhibition effect at 30 mmol/L concentration. The adsorption of L-cysteine on aluminium surface obeys amended Langmuir adsorption isotherm. The polarisation curves indicate that the L-cysteine acts as a cathodic inhibitor to

inhibit cathodic reaction, and the inhibition mechanism is dominated by geometric covering effect. The galvanostatic discharge shows that the additives restrain the hydrogen evolution and increase the anodic utilization rate. Quantum chemical calculation suggests that the carboxyl group, thiol and amino are mainly active sites absorbed on the surface of aluminium, and carboxyl group shows the largest absorption ability. A strong hybridization occurs between the s-orbital, p-orbital of in the L-cysteine molecule and the sp-orbital of aluminium atom.

Please cite this article in press as: D. Wang, et al., Experimental and theoretical investigation on corrosion inhibition of AA5052 aluminium alloy by L-cysteine in alkaline solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.041

D. Wang et al. / Materials Chemistry and Physics xxx (2015) 1e10

PDOS (electron/eV)

8

10 Al Mol(s-orbital) Mol(p-orbital)

6

4

6

4

2

2

0 -12

Al Mol(s-orbital) Mol(p-orbital)

8 PDOS (electron/eV)

10

-10

-8

-6

-4

-2

0

2

0 -12

4

-10

-8

-6

8

Al Mol(s-orbital) Mol(p-orbital)

10

PDOS (electron/eV)

PDOS (electron/eV)

-2

0

2

4

0

2

4

Al Mol(s-orbital) Mol(p-orbital)

8

6

4

2

0 -12

-4

E-Ef (eV)

E-Ef (eV)

10

9

6

4

2

-10

-8

-6

-4

-2

0

2

4

0 -12

E-Ef (eV)

-10

-8

-6

-4

-2

E-Ef (eV)

Fig. 17. Projected Density of States (PDOS) to the different reactive sites and the metal atoms.

Acknowledgments We are grateful to the support from Shanghai Science & Technology Committee Project (11JC1404400) and Shanghai EnergySaving Center of Heat-Exchange-System (Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power).

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