The Effect of an Ecological Compound (Cysteine) on the
Behaviour of Copper in Acidic Environment: Electrochemical and Raman Spectroscopy Studies.
Imad Belaida, Hayette Saifia, Achraf Hamrounib,Tayeb Bouarroudjc,d,
aLaboratory of Inorganic Materials Chemistry, Sciences Faculty, Chemistry Departement, Badji-Mokhtar - Annaba University, Box 12, 23000 Annaba, Algeria.
bMetallurgy Materials Laboratory L3M National Hight school of Mines and Metallurgy, Amar Laskri,B.P.233,W129,Sidi Amar, R.P. Annaba 23000 Algeria .
cScientific and Technical Research Center in Physico-Chemical Analysis (CRAPC). BP 384, Bou-Ismail, RP 42415 Tipaza, Algeria
dEnvironmental research center (CRE),BP2024, sidi Amar,RP23005, Annaba, Algeria.
* Corresponding author: E-mail: [email protected] Abstract
The corrosion protection of copper by cysteine in aerated 0.5 M H2SO4 solution at open circuit corrosion potential (Ecorr) was evaluated by electrochemical methods (potentiodynamic polarization and impedance spectroscopy (EIS) measurements). Thethermodynamic parameters were discussed. Cysteine is readily available, environmentally-friendly, non-toxic for human beings and contains many heteroatoms. This molecule is therefore a good candidate as a corrosion inhibitor. The Inhibition efficiencies evaluated using different methods are in a good agreement and were averagely above 97% at10-3 M. The voltammetric measurements indicated that the cysteine acts as mixed type inhibitor, but hinders essentially cathodic reactions. Changes in impedance parameters are indicative of the adsorption of cysteine on the copper surface which obeys Langmuir-type adsorption isotherm. This study was completed by surface characterizations through scanning electron microscopy (SEM) and in-situ Raman spectroscopy.Surface observation shows that the addition of cysteine has improved considerably the surface morphology of the copper electrodeFinally, the in situ Raman spectra corroborate the formation of a complex between copper and cysteine in an acidic medium as deduced from impedance measurements.
Keywords: Adsorption; Cathodic inhibitor; Corrosion mechanism; Complex layer;
Voltammetry
Highlits:
- Chronovoltammetry at open circuit potential noticed that the Cysteine adsorption appears to be a rather slow process since the open circuit potential change takes about 20 minutes to an hour.
- Polarization results indicated that the effect, of Cysteine, was more pronounced on the cathodic rather than anodic on the process.
- Impedance diagrams (EIS) showed that inhibition is accomplished by adsorption of cysteine on the copper surface. Two equivalent circuits matched the experimental data of (EIS).
- In situ Raman spectroscopy showed that the formation of a Cu(I)-cysteine complex, slows down the corrosion process of copper in the corrosive solution which reflects the significant effectiveness of cysteine in a strongly acidic environment.
Graphic abstract
1. Introduction
Copper is one of the most preferred materials in the industry due to its excellent electricaland thermal conductivities, antifouling, good mechanical workability, and corrosion resistance [1–3]. It is commonly used as a material in heat exchangers. Scale and corrosion products disturb however the heat transfer efficiency, then a periodic de-scaling and cleaning in pickling solutions with sulphuric or hydrochloric acid, are necessary. This intervention constitutes the main maintenance activity the desalination plant[4,5].The mitigation of corrosion not only results in economic gain but also prevents the release of harmful or toxic compounds into the environment. In recent years, the use of organic inhibitors in acidic media has gained popularity. Consequently, corrosion of copper and its inhibition in acid solutions have attracted the attention of several investigators [6,7].Many organic corrosion inhibitors effectively mitigate undesirable destructive effects of aggressive media and slow down the copper dissolution[8–12].The inhibiting action of these inhibitors is usually attributed to their interaction with the copper surface via their adsorption [13].It is widely recognized that organic compounds containing heteroatoms such as oxygen, nitrogen, phosphorus and Sulphur act as a good inhibitor[14,15]. Various investigations showed that the benzotriazole and its derivatives are efficient corrosion inhibitors for the copper and copper alloys, in a large range of temperature and PH [16,17].However, they are irritating, harmful to the environment, toxic and even suspected to be carcinogenic[18], then many investigations were devoted to
discovering non-toxic and ecologically-acceptable inhibitors.Since amino acids are non-toxic, biodegradable, relatively reasonable in cost, and completely soluble in aqueous media, they are interesting potential corrosion inhibitors for copper and its alloys.Consequently, many studies implying the use of amino-acids, were carried out for the prevention of corrosionof copper[19–21], and various other metals, for instance, iron[22], nickel[23], and cupro-nickel alloys[24].In particular, cysteine stands as one of the most promising candidates. Because this molecule contains the amino group [-NH2], carboxyl group [-COOH] and thiol group [-SH]
exhibiting high affinity to the copper[25].The effect of cysteine on the anodic dissolution of copper in sulphuric acid was examined by Matos et al [26] at room temperature using electrochemical methods. The anodic polarization curves showed two different regions; in the low overpotential range, the inhibitory effect of cysteine on the anodic dissolution was verified. On the contrary, at the high overpotential range, they concluded that this amino acid does not influence the anodic dissolution of copper. The inhibiting effect of cysteine on corrosion of copper in 0.1M HCl solution was studied by Ismail[27] using Tafel polarization measurements and electrochemical impedance spectroscopy (EIS). He found a monotonous increase in inhibition efficiency as a function of concentration and reaches a value of 84% at an inhibitor concentration at 18 mM. In this context, only moderate protection efficiency was achieved.In contrast, Silva et al[28]reported completely different results on the effect of cysteine on the corrosion of 304L stainless steel in sulphuric acid. The compound did not affect the corrosion behavior of the steel specimen at low concentrations (10−6 to 10−5 mole L-
1) and at higher concentrations turned the metal surface active, promoting the anodic dissolution reaction. Also the cysteine did not give an interesting inhibitory effect against the corrosion of nickel in 0.5 M H2SO4 media, according to a recent study carried out in our laboratory where the maximum inhibitory efficacy was evaluated at 18% for a concentration of 10-3M[29].
This workaims to study the corrosion inhibition of cysteine on copper in an aerated 0.5 M H2SO4 solution. The assessment of its inhibitor efficiency was performed by electrochemical methods; the evolution of the open circuit corrosion potential, voltammetric curves, and Electrochemical Impedance Spectroscopy (EIS). Morphological changes on the corroding copper surface were visualized by scanning electron microscopy (SEM). Finally, surface characterizations were carried out by the “in-situ” Raman spectroscopy to elucidate, the correlation between inhibition mechanisms and molecular structure of the compound at the copper surface.
2. Experimental procedures 2.1 Electrode and electrolytic cell
A classical three-electrode cell was used for the electrochemical experiments. The counter- electrode was a large platinum grid. The reference electrode was a saturated mercurous sulfate electrode (SSE). The potential will be given with respect to this reference electrode without correction of ohmic drop.
The working electrode was prepared of a cylindrical copper rod of 0.2 cm2 cross-sectional area from Goodfellow (purity 99.999%). The lateral part of the copper rod was firstly protected by a cataphoretic paint layer (PPG; W781-I1292+W975-G292), cured at 150° C for 30 min to avoid the infiltration of corrosive solution in the lateral part of the electrode. Then, the copper cylinder was embedded into an epoxy resin (Buhler, epoxycureTM). Before measurements, the working electrode was mechanically abraded with emery paper up to grade 4000 to yield a smooth and planar surface. Then, sample was thoroughly cleaned using acetone, rinsed with de-ionized water, and dried by argon or nitrogen flow. Subsequently, the sample was transferred into the electrochemical cell.
2.2 Corrosion test solutions
The electrolyte used was 0.5 M H2SO4 at room temperature prepared with doubly distilled water in thermodynamic equilibrium to air; neither stirring nor bulling. The electrode was set near the center of a cylindrical cell without rotation. The inhibitor used was L-cysteine (HSCH2CHNH2COOH) at concentrations ranging from 10-6 to 10-3 mole L-1. The molar mass of this compound is 121.16.
2.3 Electrochemical devices and measurement procedures
Collections of experimental data were started after one hour of immersion at open circuit conditions in corrosion test solution to achieve stable potential. Potentiodynamic experiments were performed using a galvanostat-potentiostat (AUTO LAB PGSTAT 20) monitored by GPES software with a scan rate of 0.5 mV s-1. Two independent measurements were carried out; one the potential scan towards more negative direction from ca. 10 mV more positive from the open circuit potential up to -1.1 VSSE. Another one was the anodic potential scan from 10 mV more negative to the open circuit potential up to 0 VSSE.
Electrochemical Impedance Spectroscopy (EIS) measurements were carried out using a potentiostat (SOTELEM PG-STAT.Z1) at the open circuit potential (Ecorr) coupled with a
frequency response analyzer (SOLARTRON model SI 1255). For all EIS measurements perturbing signal was the sine-wave signal of which amplitude was 10 mVrms. The frequency range examined was from 63 kHz to 10 mHz. The impedance spectra were represented by an electrical equivalent circuit, and the values of these elements were determined by the home- made simplex algorithm.
2.4 SEM observations and in-situ micro-Raman analyses
For morphological studies, the samples were prepared and treated as described above and immersed in the test solution during 24 h. The surface state was observed using scanning electronic microscope LEICA STEREOSCAN 440 coupled with EDS elemental semi- quantitative analyses (Princeton Gamma-Tech) at 20 keV.
The in-situ µ-Raman spectroscopy measurements were carried out with a Labram–Jobin–
Yvon spectrometer. The samples were left in the test solutions with inhibitor, and irradiated with a helium neon laser at λ = 632.8 nm. The laser power was varied between 0.1 and 1 mW to avoid any thermal effect on sample during the analyses. A confocal microscope was used.
3. Results and discussion
Electrochemical data will be presented first. The corrosion rates evaluated by electrochemical measurements.Then, the morphology and Raman spectroscopic characterization of the surface film formed with cysteine will be described.
3.1. Chronovoltammetry at open circuit potential
The time evolution of the open circuit corrosion potential (Ecorr) for copper in 0.5 M H2SO4 in the absence and presence of different concentrations of cysteine (10-6- 10-3 M) during one hour is illustrated in Figure 1. The potential range observed corresponds to the area where Cu+ is stable according to theE-pH diagram. The corrosion of copper in acid solution takes thus, as largely recognized, by formation of cuprous species. The higher concentration of cysteine is, the more negative the values of Ecorr for all through the immersion period.
Figure 1: Effect of cysteine concentration on the corrosion potential in aerated 0.5M H2SO4
At low cysteine concentrations, the corrosion potential (Ecorr) decreases during the first three to five minutes according to the cysteine concentration, then shifts progressively towards more positive direction. This variation may be explained by an initial decrease of anodic current due to the formation of cuprous species covering the electrode surface, then that of the cathodic current. The adsorption of cysteine seems to be rather a slow process since the open circuit potential variation lasts about 20 minutes to one hour. Such potential shift may correspond to a significant decrease of cathodic current by adsorption of cysteine molecules on the copper surface. This point will be verified by voltammetry measurements presented below.
3.2. Voltammetry experiments
The cathodic and anodic polarization curves of the copper in 0.5 M H2SO4 with various cysteine concentrations (between 10-6 and 10-3M) are presented in Fig. 2 and 3, respectively.
For the cathodic polarization curves, Fig.2, in the absence and with the lowest concentration of cysteine examined, a small hump is observed at potential 30 to 60 mV more negative values than Ecorr. This phenomenon was also observed by other researchers, and was allocated to the reduction of corrosion deposit accumulated at the electrode surface during the potential stabilization period[30,31].
Figure. 2: Cathodic polarization curves of Copper in 0.5M H2SO4 for different cysteine concentrations; scan rate 0.5 mV s-1.
The nature of the corrosion products will be analyzed below by Raman spectroscopy. The charge involved this current peak is evaluated close to 1.6 and 1 Mc.cm-2 respectively in absence and in presence of 1 µM Cysteine. For an ideally flat surface, the electrical charge of single molecular layer Q is 0.30 Mc.cm-2. Therefore, the charge accumulated on the electrode surface corresponds to 5 and 3 molecular layers respectively in the absence and in presence of 1 µM Cys. In other terms, the corrosion products formed during the potential stabilization period are thin.
During cathodic polarization, a current plateau of ca. 10 µA cm-2 was observed in the potential range between -0.7 and -0.95 VSSEfor these two cases. This current plateau will be ascribed to the reduction of dissolved oxygen, and can be expressed as:
O2 + 4 H+ + 4 e- 2 H2O (1)
For potential more negative than -1 VSSE, the current density increases (in absolute value) almost linearly with the potential in the semi-logarithmic plan. This process corresponds to the hydrogen evolution reaction, and can be represented by the reaction:
2 H3O+ + 2 e- H2 + 2 H2O (2)
Both processes are strongly hindered by the presence of cysteine in 0.5 M H2SO4. The adsorption of cysteine at the electrode surface slows down therefore the reduction process of the dissolved oxygen, as well as that of the hydrogen evolution reaction. The higher the
cysteine concentration is, the slower the both cathodic reactions are.The voltammograms collected during the anodic scan are presented in Fig.3.
Figure 3: Effect of cysteine concentration on the anodic polarization curves of copper electrode in 0.5M H2SO4; scan rate 0.5mVs−1
In this figure, one can notice that the current density increases steeply beyond -0.45 VSSE.This current increase appears to a potential even more anodic when the cysteine concentrations were increased. This change in the Tafel slope may be allocated to the modification of the dissolution mechanism; for lower anodic over-potential, the copper dissolves as cuprous ions whereas at higher over-potential, the cupric species will be formed in agreement with the E- pH diagram. Besides, above ca. -0.3 VSSE, all polarization curves overlap themselves, that is, no effect of cysteine can be observed for the dissolution of cupper as Cu(II) as stated by Matos in HCl medium[21]. At high over-potential, the copper dissolution takes place in two consecutive steps[32] to form cupric ions in sulphuric acid solution:
Cu Cu+(ads) + e- (fast) (3)
and Cu+(ads) Cu2+ + e- (slow) (4)
For low over-potential, the reaction will be stopped at reaction step (3) to form Cu+ in the electrolyte. The reaction mechanism for lower over-potential will be discussed later with the results obtained by EIS techniques (cf. §3.4).
3.3. Inhibiting efficiency
Close to the corrosion potential Ecorr, the Stern-Geary relationship[33] may be applied, provided that both anodic and cathodic processes, are purely additive, and that each processes
can be expressed by an exponential law in the narrow potential examined. Then, the overall current density I followed by the next relationship:
a corr c corr
corr b E E b E E
I
I exp exp (5)
Where Icorr is corrosion current density (A cm−2); ba and bc are respectively the anodic and cathodic Tafel constants (V−1). It will be worth to mention that sometimes, the Tafel constant is allocated to the slope of polarization curves in log(I) - E scale, i.e. Tafel slope in unit of V dec-1. Icorr, Ecorr, ba and bc, were evaluated from the experimental results using a user defined function of “Non-linear least squares curve fit” of graphic software used (Origin, OriginLab).
The voltammograms were collected from Fig.2 or 3 at 10 mV more positive or negative from Ecorr respectively. Then, this narrow potential domain induces a large uncertainty to the Tafel constant. Therefore, bc was then determined from the cathodic scan and ba from an anodic scan. Fig.4 illustrates the results of the regression calculation for two concentrations, uninhibited and 10 µM Cys of anodic or cathodic potential scan. In all cases, the correlation factor R2 is greater than 0.99 except for 1 mM Cys where a marked current scattering was observed as can be seen in Fig.2 and 3. Except for these two cases, fitted data are therefore reliable.
Figure 4: Comparison between experimental and calculated I-E curves. Copper in 0.5 M H2SO4 without or with 10−5 M cysteine by cathodic or anodic potential scan.
Table 1: Corrosion parameters determined by nonlinear regression calculation according to Equation 5 for experimental results
In these calculations, the potential domain is limited to -40 to 10 mV with respect to Ecorr or -10 to 40 mV respectively for cathodic or anodic potential scan. It can be seen in this figure a good agreement between the calculated and the fitted polarization data. The results of regression calculation are presented in Table 1. There are two corrosion current density Icorr and two inhibitive efficiencies IE evaluated from cathodic or anodic potential scan data.
As can be noticed readily from Fig. 4, Ecorrfor both voltammogram data are close for two series of experiments. It can be noticed also that, the corrosion current densities determined from two data are close to each other for a given cysteine concentration. Icorr evaluated from cathodic Icorr,cath and anodic Icorr,anod scans in absence of inhibitor are close to that of the diffusion current plateau observed in Fig.2. The corrosion rate is thus essentially determined by the reduction of the dissolved oxygen. In contrast, though the diffusion limited plateau is also observed in presence of 1 µM cysteine, the corrosion current density is not entirely determined by the diffusion process. The values of bc are small namely in absence of cysteine, that is the Tafel slope varies from -344 to -77 mV dec-1. The cathodic reduction reaction is therefore the mixed kinetics of the reaction (1) presented above.This table shows also that ba
becomes slightly smaller when the cysteine concentration increases, but remains essentially close to 60 V-1, that is ca.40 mV dec-1.
The inhibiting efficiency (IE) in percent was evaluated from the following relationship[34]:
corr corr corr
I I IE I
0 0
100
(6)
[Cys]
µM
Ecorr
VSSE
Icorr,Cath
µA cm-2
Icorr,Anod
µA cm-2
ba
V-1
bc
V-1
B mV
IEcath
%
IEanod
%
0 -0.452 11.7 9.37 64.3 -6.70 13.9 0 0
1 -0.457 3.99 3.95 63.2 -19.7 12.1 76.1 57.8
10 -0.494 0.654 0.226 56.7 -26.7 12.0 97.6 97.5
100 -0.506 0.0275 0.146 53.7 -28.9 12.1 98.7 98.4
1000 -0.549 0.0298 0.0655 53.1 -29.8 12.1 99.7 99.3
where I0corr and Icorr denote, respectively the corrosion currents densities in absence and in presence of inhibitor. The current density decreases when the concentration of inhibitor increases. This confirms the inhibitory effect of cysteine in acidic media. The inhibition efficiencies determined from both cathodic and anodic voltammograms are presented in the last two columns, and they reach a value of 97% at a concentration of 10-5 M Cysteine. This molecule is thus highly efficient inhibitor for the corrosion protection of copper.
3.4. Electrochemical impedance spectroscopy
Another way to evaluate the protecting effect of the cysteine is the application of the electrochemical impedance spectroscopy (EIS), since it is recognized as a powerful tool in the investigation of electrode kinetics[35–38]. The impedance spectra of the copper electrode were recorded in the presence of different concentrations (10-6 to 10-3 M) of cysteine in 0.5 M H2SO4, after one hour immersion at the open-circuit. The measurements were carried out under potential regulation at Ecorr determined just before the beginning of the experiments.
The impedance spectra obtained in the Nyquist plot are presented in Fig. 5.
Figure.5: Electrochemical impedance spectra in Nyquist plots for Cu in 0.5 M H2S04 with different cysteine concentrations. Some frequencies at which these data were collected are
shown close to the impedance spectra
The high frequency part of the impedance is displayed with an enlarged scale in the insert to illustrate better the results concerning the absence and presence of 1 µM cysteine. It can be noticed that the impedance modulus increased in presence of inhibitor.
Though not clearly seen, the impedance diagram obtained in presence of cysteine concentration higher than 10 µM; badly separated two time capacitive loops should be taken into consideration. This phenomenon is attested by an asymmetry of the loop. The impedance contains therefore two time constants. As for the impedance spectra collected in the absence and 1 µM of cysteine, an additional tail at low frequency can be seen. As it was illustrated in Figure 2, the cathodic reaction at Ecorr is mixed kinetics, activation and diffusion. Therefore, the diffusion impedance should be added in presence of an inhibitor.The Bode and phase angle plots for copper electrode in 0.5 M H2SO4 solution with different Cysteine concentrations are depicted in Fig. 6(a) and (b), respectively, are consistent with the Nyquist diagrams (Fig. 5).
Figure.6: Bode plot of copper in 0.5 M H2SO4 in the absence and presence of different concentrations of cysteine, a) Module representation VS. frequency, b) Phase
representation VS. frequency
It can be seen that the impedance modulus Fig. 6(a), at low frequency, increases with increasing inhibitor concentration, showing that cysteine adsorption improves the corrosion resistance of copper by reducing the surface exposed to acid attack. Two time constants are clearly detected at the lowest concentration of inhibitor and without inhibitor Fig. 6 (b), this behavior reveals the presence of two different contributions for the corrosion process at the
copper- solution interface, as already mentioned in the previous paragraph. Also, it can be observed that in the presence of cysteine on the graph-phase angle against the frequency logarithm presents a maximum very well established assigned at a phase angle of proximate the 80°. This is attributed to a higher capacitive behavior in accordance with Nyquist diagrams. An equivalent circuit model was therefore proposed in the frequency range developed in order to adjust and analyze the EIS data obtained.
The impedance diagram exhibited a depressed feature, therefore a model using Cole-Cole dispersion of time constant was applied. For a single depressed capacitive loop can then be expressed by
R C
nZ R
j 1
(7)
In contrast to the CPE approach, Cole-Cole type distribution does not privilege the distribution of time constant merely to a capacitive behaviour, but both resistive and capacitive behavior. The dimension of C is actually that of capacitance, i.e. F cm-2.The impedance spectra in presence of inhibitor are therefore analyzed with two R-C ladder circuits illustrated in Fig.7a for high cysteine concentration and that in Figure 7b in the absence and presence of 1 µM cysteine
Re
Rt
RF Cd,nd
CF,nF
Figure.7: Equivalent circuit used in the fitting data of Cu at various cysteine concentrations.Symbols used are presented below
The origin of these circuit elements are as follows:
Re: Electrolyte resistance (F cm2), Cd: Double layer capacitance (F cm-2), Rt: Charge transfer resistance (F cm2),
CF: Faradaic capacitance due to the redox process with corrosion products (F cm-2),
RF: Faradaic resistance associated with the stability of corrosion product (F cm2), Rd: Resistance of bounded diffusion impedance (F cm2),
τD: Diffusion time constant (s)
nd, nF: Coefficient describing a depressed feature of impedance in Nyquist plot.
For regression calculations, house-made software was used which allowed expressing directly the mathematic equations of any model. Table 2 summarizes the results of regression calculations.
Table 2: Results of regression calculations of impedance spectra of Cu electrode in 0.5 M H2SO4 with circuit presented in Figures 7a and b.
[Cys]
µM Rs
cm2 Rt
cm2
Cd µF cm-2
ad RF
k cm2
CF µF cm-2
aF Rd
cm2
D
s 0 0.832 20.0 30.3 0.971 1.00 22.6 0.610 226 32.6 1 0.890 38.1 17.8 0.954 1.27 21.2 0.627 2500 100
10 0.859 166 23.3 1.00 8.90 9.69 0.645 - -
100 0.919 76.9 30.3 0.988 16.6 17.5 0.633 - -
1000 0.940 33.2 19.5 1.00 22.3 31.0 0.717 - -
The impedance spectra obtained by regression calculations are overlaid on the experimental data on Fig.5. A reasonable agreement with experimental (symbol alone) and calculated date (symbol+line) can be verifier.
On this table, it can be seen that the value of Cd is 30 µF cm-2 in absence and 17 to 30 µF cm-2 in presence of the cysteine. These values are in good agreement with the capacitance of the double layer. The inhibitive effect of cysteine leads to only a slight decrease of the double layer capacitance. The cysteine being small and polar, the adsorption of this species at the copper surface may induce only a small decrease of the double layer capacitance.
The presence of CF attests to the existence of reversible surface species even though the current hump is no longer observed at higher cysteine concentration. The value of the Faradaic capacitance CF is rather small in agreement with a small quantity of surface product
accumulated at the electrode surface. The resistance RF is more narrowly correlated with the reactivity. The cysteine stabilizes markedly corrosion products covering the copper surface.
The copper is dissolving as cuprous species and the electrode surface is also covered with monovalent copper species. If the dissolution of copper takes place by dissolution of the surface species, then this reaction is chemical, without charge transfer. Hence this reaction will be likely independent of the electrode potential, in contradiction the experimental data (Fig.2 and 3). Therefore two reactions, dissolution and formation of cuprous corrosion product will occur in parallel.
Dissolution: Cu Cu+(sol) + e- (8)
Surface product: Cu Cu(I)(surf) + e- (9)
Reaction 8 will take place at uncovered surface area of copper.
According to our previous work, in this situation, in the presence of redox process, the polarization resistance Rp is more closely correlated to the corrosion rate even in presence of diffusion process than the charge transfer resistance [35–37].
Rp = Rt + RF (10)
Stern and Geary [34] formulated the following equation for the evaluation of corrosion current:
p c a p
corr R
B b b
I R
1 1
(11)
B value is evaluated above from cathodic and anodic voltammograms and presented in Table 1. Table 3 presents the evaluation of the corrosion current density and the inhibiting efficiency by addition of cysteine in 0.5 M H2SO4.
Table 3: The corrosion current density evaluated from EIS measurements
[Cys]
µM
Rp k cm2
B (mV)
Icorr µA cm-2
IE
%
0 1.02 13.9 13.6 -
1 1.31 12.1 0.924 32.2
10 9.07 12.0 0.132 90.3
The corrosion current density in absence of inhibitor is only slightly higher than that determined by voltammograms. The inhibiting efficiencies of cysteine calculated by EIS data are lower than those evaluated above (cf. Table 1), but reasonably close.
3.5. In situ Raman spectroscopy Characterization
In situ Raman spectroscopy has been used to characterize chemical species on the surface of the copper electrode during the corrosion process. The Raman spectra of the molecule in its crystalline form, in solution of water (0.25M pH 4.5), in solution (0.25M) of 0.5M H2SO4, and in 0.5M H2SO4 solution are given from bottom to top on Figure 8. The vibrations of the cysteine molecule are attributed in the literature[39].
The C carbon is the carrier of the amino substituent. In its crystalline form and in solution of water, cysteine is in the zwitterionic form SH-CH2-CH(NH3)+(COO)-as evidenced by the existence of COO- vibrations in the spectra (1400 cm-1 O-C-O elongation, 877 cm- 1 elongation C-COO-),on the other hand in sulfuric medium the appearance of the vibration C=O at 1743 cm-1 shows that the molecule is in its acid form SH-CH2-CH(NH3)+COOH even if the spectrum of the H2SO4 solution due to the ion (HSO4)- is intense and masks many vibrations of the molecule.
Figure 8: (a) [Cys] in crystalline form, (b) [Cys] (0.25M) in aqueous solution, (c) [Cys]
(0.25M) in 0.5M H2SO4 solution, (d) 0.5M H2SO4 M
100 16.7 12.1 0.0725 94.7
1000 22.3 12.1 0.0543 96.0
We were particularly interested in whether cysteine adsorbs to the surface of copper, orwhether cysteine forms a complex with copper ions. An in situ Raman analysis was performed on the surface of a copper electrode immersed in 0.5 M H2SO4 medium with the addition of 10-3 M cysteine at the open circuit potential. The Raman spectrum obtained after 3 hours of immersion is shown in Fig. 9(a).The spectrum of Fig.9 (a) was compared with the Raman spectrum of cysteine in the solid state shown in Fig.9 (b), as well as the Raman spectrum of a 0.25 M cysteine solution in 0.5 M H2SO4 medium shown in Fig.9 (c). It should be noted here that at the concentration of cysteine used, we do not see the characteristic bands of cysteine in solution, so the Raman spectrum observed in Fig.9 (a) corresponds to species on the copper surface. It is clearly seen that the SH band observed at 2554 cm-1 in the spectrum of cysteine in solid state and at 2585 cm-1 in the spectrum of cysteine solution has disappeared. This indicates that the group is ionized at the cooper surface. The band at 681cm-
1 in the solution spectrum or at 693 cm-1 in the solid cysteine spectrum attributed to νC-S [40]is shifted to the lower wave numbers in the case of the spectrum observed on the surface metallic. This can be explained by the electron donor action from the S atom to the Cu atoms.The absence of the SH band and the displacement of νC-S indicate that the sulfur atom is bonded to copper.
Figure 9: (a) Raman spectrum obtained on the copper electrode immersed in 0.5 M H2SO4
+ 10-3 M Cys, at open circuit potential -0.534 V / ESS. (b) Raman spectrum of L-Cys in solid state. (c) Raman spectrum of a solution of L-Cys in H2SO4
In order to verify the hypothesis according to which a complex between cysteine and copper
synthesized such a complex from cysteine and copper sulfate according to the procedure described in[42]. Figure 9 (d) represents the Raman spectrum of this complex.A number of similarities have been identified by comparing this spectrum with that of Figure 9 (a).
However the complexity of the massif in the region of νC-S suggests that there may be both adsorption of cysteine and formation of a complex with copperas shown in the fig.10.
Figure. 10. Suggested mechanism of the inhibitor on cooper.
4. Conclusion
The main conclusions of the present study can be summarized as follows:
Tafel polarization studies have shown that the adsorption of cysteine at the electrode surface hinders the reduction process of the dissolved oxygen, as well as that of the hydrogen evolution reaction. The cysteine can therefore be considered as a cathodic-type inhibitor. The inhibition efficiency tends to increase by increasing the inhibitor concentration and reaches a value of 97 % at a concentration of 10-5 M of cysteine. The results of EIS indicate that the existence of reversible surface species. In the absence and presence of the lowest cysteine concentration examined here, the bounded diffusion impedance was observed in agreement with the corrosion process completely or partly controlled by the diffusion of dissolved oxygen. In contrast to many studies on organic inhibitor, the double layer capacitance changes little by addition of cysteine. This observation was allocated to a small size and polarized structure of the molecule.Characterization by in situ Raman spectroscopy has shown that the formation of a Cu(I)-cysteine complex, slows down the corrosion process of copper in the corrosive solution, which reflects the significant effectiveness of the inhibitor in acidic environments.
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