4,5-二氨基-2-硫脲嘧啶膜氯化钠溶液中对铜缓蚀性能的研究

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　　摘 要： 通過自组装方法将4，5-二氨基-2-硫脲嘧啶（MPD）分子吸附在铜（Cu）表面，应用电化学极化、电化学阻抗谱（EIS）和拉曼光谱方法，研究其在质量分数为3.5%的氯化钠（NaCl）溶液中的缓蚀能力.在最佳装配条件下，MPD膜的最大缓蚀效率达到98.1%.拉曼光谱研究表明：MPD分子通过N9-H10和S7-H8吸附在Cu表面上.
　　关键词： 铜; 缓蚀; 电化学阻抗谱（EIS）; 极化曲线; 拉曼光谱
　　1 Introduction
　　As one of the most usual metals，copper and its alloys are being widely used in electronic manufacturers，marine industries，power stations，and heat exchangers due to their good corrosion resistance，high electrical and thermal conductivity，and strong malleability[1-3].Copper offers relevant corrosion resistance in the atmosphere and in neutral or alkalescent solutions due to the formation of a passive oxide film or nonconductive layer of corrosion products on its surface[4-5].However，pitting corrosion could occur on the surface when copper is exposed to oxygen or other oxidants，which would therefore cause serious economic loss and casualties[6-7].Hence，using organic and inorganic corrosion inhibitors，one of the most practical and effective methods among different corrosion protection methods，has been widely studied[8-9].The heteroatoms （such as N，O and S） in the organic compounds which acted as the adsorption sites could be adsorbed on the metal surface via π-π and Van der Waals interactions [10-11].
　　Unfortunately，most of the corrosion inhibitors，such as triazines[8]，imidazoles[12]，and benzotriazole[13] are toxic，which hinder their applications in the sea environment[14].Therefore，numerous studies are now focused on eco-friendly drug compounds to reduce the pollution problems[15-16].
　　Thiouracil and its derivatives，potent and safe pharmaceutical intermediates for anti-thyroid drugs and melanoma detection agents，have been widely used in medical and chemical fields.As corrosion inhibitors，they have been proved to have high corrosion inhibition efficiency.For instance，the marine paint formulations based on soluble resin which contain 6-amino-2-thiouracil and their derivatives can protect unprimed steel panels from sea water corrosion for more than two months[17].ISSA et al.[18] calculated the corrosion inhibition efficiency for dithiouracil，thiouracil，uracil and dihydrouracil against the copper corrosion.AL-ANDIS et al.[19] examined thiouracil derivatives on protecting carbon steel corrosion in sulfuric acid using gasometry and potentiometry，whilst HEAKAL et al.[20] measured their impedance data combing with density functional theory （DFT） calculations.
　　In this work，the corrosion inhibition efficiency of one eco-friendly corrosion inhibitor，4，5-diamino-6-hydroxy-2-mercapto-pyrimidine （MPD），against copper corrosion in 3.5% （mass fraction） NaCl solution is carefully studied by potentiodynamic polarization and electrochemical impedance spectroscopy （EIS） techniques and its molecular structure is shown in Figure 1.Besides，Raman spectroscopy was used to verify the adsorption site of MPD on the copper surface. 　　2 Experimental
　　2.1 Materials
　　MPD （85% mass fraction） was purchased from Sigma-Aldrich Corporation.Sulfuric acid，sodium chloride，and ethanol were obtained from Sinopharm Chemical Reagent，Shanghai，China.All chemicals were of analytical grade reagents，and were used without further purification.All solutions were prepared with Milli-Q water （18 MΩ·cm）.
　　2.2 Pretreatment of the copper electrode
　　Teflon sheathed copper rod （99.999% mass fraction，0.031 4 cm2 geometric area） was firstly rubbed with 500- and 1 000-grit papers，then polished by 0.3 μm alumina powder until a shiny mirror-like surface with less oxides and pits was visible.After that，such electrode was rinsed with Milli-Q water，pure ethanol and again with Milli-Q water to entirely remove the alumina particles and loose copper rust.
　　2.3 Assembling MPD layer
　　The pretreated copper electrodes were immersed immediately into the MPD aqueous solutions with different molar concentrations （i.e.5×10-5，1×10-4，5×10-4，1×10-3 mol·L-1），with different assembly times （1，3，5，8，12 h），respectively.Each MPD solution was firstly deoxygenated via purging nitrogen for 20 min before being used.After then，the MPD modified electrodes were taken out，rinsed with Milli-Q water，and dried under flowing nitrogen gas.
　　2.4 Electrochemical experiments
　　CHI750C electrochemistry workstation （CH Instruments，Inc.） was used to determine the electrochemical behavior of a traditional three-electrode cell （a saturated calomel electrode （SCE），a platinum foil electrode，and a copper electrode were used as reference，counter，and working electrode，respectively） in 3.5% （mass fraction），NaCl aqueous solution.Prior to every test，the copper electrodes with and without MPD modification were immersed in 3.5% NaCl aqueous solution for 3 000 s until a stable open circuit potential（OCP） （OCP vs.SCE） was achieved.EIS results were acquired under OCP with a 5 mV amplitude perturbing signal in the frequency range from 100 kHz to 10 mHz.The EIS data were then analyzed using Zsimpwin software.The electrochemical polarization curves were recorded with a voltage range from -0.4 to -0.05 V vs.SCE，a scan rate of 1 mV·s-1，and a sensitivity of 10-3.
　　2.5 Surface characterization
　　2.5.1 Scanning electron microscopy （SEM） test
　　SEM images were taken by scanning electron microscopy （SEM，Hitachi S-4800 scanning electron microscope）.For SEM test，the copper surfaces with or without MPD modification were examined before and after immersed in 3.5% NaCl aqueous solution for 5 h （corrosion time）. 　　2.5.2 Raman spectroscopy
　　The pretreated copper electrode was roughened by the oxidation-reduction cycle （ORC） method[21] to obtain the surface enhance Raman scatting （SERS） active surface.Generally，the electrode was cycled in 2 mol·L-1 H2SO4 solution from -0.55 to +0.45 V vs.SCE （initial from -0.55 V） at 20 mV·s-1 for 10 scans，then washed thoroughly with Milli-Q water.
　　The Raman equipment was a confocal micro-Raman spectrometer （Super LabRam II system，Dilor，France）.A multichannel air cooled 1024 pixel ×800 pixel charge-coupled device was employed as a detector.The objective was with 50× long-working-length，and the laser power for 632.8 nm He-Ne laser was 5 mW.The pinhole and slit were 1000 μm and 100 μm，respectively.Each spectrum was average of 3 scans，and each scan time was 8 s.All spectra were calibrated with silicon at 519 cm-1.
　　3 Results and discussion
　　3.1 EIS measurements
　　EIS，as an effective and nondestructive testing technique，was used to investigate the inhibition efficiency of MPD at the copper surface without destroying the protection layer.Figure 2 and Figure 3 are the Nyquist，Bode and phase angle plots of the copper electrodes modified with different MPD of molar concentrations and with different assembly time after corroded in 3.5% NaCl aqueous solution，respectively.In Fig.2（b），a straight line at low frequency range in the Nyquist plot of bare copper indicated the Warburg impedance and was due to either the soluble copper species （CuCl-2，CuCl-4） or the copper oxides diffused from the copper surface to the bulk solution or the dissolved oxygen transported to the copper surface[22].A small semicircle at high frequencies was owing to the surface inhomogeneity.Compared with bare copper，MPD covered copper electrodes [Fig.2（a）] showed much larger semicircles in the Nyquist plots.With the increasing MPD molar concentration，the diameter of the capacitance loops increased sharply at the beginning and reached its maximum value when the MPD molar concentration was 5×10-4 mol·L-1.However，as the molar concentration of MPD was higher than 5×10-4 mol·L-1，the semicircle of impedance value became smaller.The results suggested that the corrosion inhibition performance of MPD layer on the copper surface was molar concentration-dependent.Increasing its molar concentration could cause more MPD molecules to be adsorbed on the copper surface.When the molar concentration reached 5×10-4 mol·L-1，MPD molecules adsorbed on the copper surface were dense and compact，preventing the corrosion media to attack the copper interface and thus providing good corrosion inhibition efficiency.Nevertheless，further raising the MPD molar concentration would result in the accumulation of MPD molecules on the copper surface，leading to rough surfaces or shedding of the protection，hence causing intensified corrosion. 　　concentrations in 3.5% mass fraction of NaCl aqueous solution，（b） is the magnification Nyquist plot of bare copper
　　EIS measurements of 5×10-4 mol·L-1 MPD modified copper with different assembly times were then conducted.In Fig.3（a），diameters of the semicircles in the Nyquist plots gave similar trend：increased and then decreased with the increasing assembly time.The optimized assembly time was 8 h.It indicated that MPD molecules adsorbed on the copper surface were not dense enough if the assembly time was less than 8 h，while if the assembly time was more than 8 h，the accumulation of MPD would occur on the copper surface，leading to defects which hindered the corrosion inhibition effect.
　　According to the Bode plots [shown in Fig.2（c） and Fig.3（b）]，the same trend could also be found：the logZ values rose and then fell with the increasing MPD molar concentration and assembly time.The peak value was obtained with a MPD molar concentration of 5×10-4 mol·L-1 and an assembly time of 8 h.
　　In addition，as shown in Fig.2（d） and Fig.3（c），the phase angle values were up and then down with the increasing MPD molar concentration and assembly time，and the maximum phase angle was ca.80° referring to the optimal assembly condition.
　　The EIS data were then fitted by Zsimpwin software for a more detailed analysis.The main criterion for best fitting model selection is least error and chi-square value （χ2）.As shown in Fig.4，R（Q（RW）） is the equivalent circuit mode for the Nyquist plots of bare copper，while for MPD modified copper electrode，R（Q（RW））（QR） was more suitable.In which，Rs，Rf，and Rct represent the solution resistance，the resistance of MPD film formed on the copper surface，and the charge transfer resistance，respectively.W is the Warburg impedance.Q is the constant phase elements （CPE），where Qf，Qdl are the film capacitance and double layer capacitance，respectively [23].
　　The corresponding impedance parameters for R（Q（RW）） and R（Q（RW））（QR） are listed in Table 1 and Table 2，respectively.And Q can be described as below[24]：
　　Potentiodynamic polarization curves of the copper electrodes with and without MPD modifications recorded in 3.5% NaCl aqueous solution are shown in Fig.5 and Fig.6，respectively.And the related electrochemical parameters obtained from the extrapolation of the Tafel curves，such as cathodic and anodic Tafel slopes （βc and βa），corrosion potential （Ecorr），and corrosion current density （jcorr） are listed in Table 3 and Table 4，respectively. 　　As presented in Table 3 and Table 4，both cathodic and anodic Tafel slopes shifted to much lower current density values after the MPD modifications，compared with bare electrode.Besides，cathodic potion shifted more，indicating that MPD layer acted as a cathode-dominated mixed inhibitor on the copper surface.Moreover，the jcorr value decreased and the Ecorr value increased with MPD assembly on the copper surface.Furthermore，the lowest jcorr value was obtained in optimized coating condition.
　　3.3 SERS analysis
　　SERS was used to investigate the molecular surface interaction due to its high sensitivity.Figure 7（a） and 7（b） displayed the normal Raman spectrum of MPD powder and SERS spectrum of MPD modified copper formed under optimized condition.For better understanding of the spectral information，the vibrational assignments which were calculated from density functional theory （DFT） calculations based on UB3LYP/LANL2DZ were summarized in Table 5.According to Figure 7（a） and Table 5，the strongest peak at 1639 cm-1 and 488 cm-1 belonged to C4-C5 rocking vibration and N12-H13 stretching vibration，respectively.Peak at 1101 cm-1 was assigned to C4-C5 in-plane bending，while band at 886 cm-1 was S7-H8 rocking vibration.Based on the surface selection rule[26]，SERS signal would be enhanced when the vibration mode was perpendicular to or getting close to the metal surface.Otherwise，if the vibration was parallel or away from the surface，SERS signal would be weakened.Therefore，it could be concluded that the MPD molecule was physisorbed on the copper surface via N9-H10 and S7-H8.Besides，with high MPD molar concentration，the vertical adsorbed MPD molecules would form π-π interaction between heterocyclic rings in pyrimidine molecules.Such π-π interaction would ensure that the coating formed on the copper surface would be dense and compact，providing excellent corrosion inhibition ability.The suggested adsorption fashion for MPD on the copper surface was displayed in Figure 8.
　　3.4 Adsorption isotherm
　　To further verify the MPD adsorption fashion on the copper surface，adsorption isotherm plot along with the standard Gibbs free energy was determined.θ （the degree of surface coverage） at different MPD molar concentrations in 3.5% NaCl solutions was obtained from EIS measurement according to：
　　Assuming that the adsorption of MPD molecule on the copper surface obeys Langmuir adsorption isothermal，then the correlation between θ and c can be  represented as： 　　where c is MPD molar concentration，and Kads is the equilibrium constant [27].The plot of c/θ against c gave a straight line （y=1.025x+0.007） as shown in Figure 9.Both the linear correlation coefficient （R2=0.9993） and the slope （value is 1.025） are close to 1，indicating the adsorption of MPD molecules on the copper surface in NaCl solution obeys the Langmuir adsorption isotherm.
　　Generally，if ΔG0ads value is above -20 kJ·mol-1，adsorption behavior is assumed to be physisorption，dominated via electrostatic interactions between inhibitor molecules and the charged metal surface，whilst if ΔG0ads value is lower than 40 kJ·mol-1，chemisorption controlled where coordination bond is formed based on charge sharing or transfer from organic molecules to the metal surface[29].The calculated ΔG0adsvalue was -22.25 kJ·mol-1，suggesting that the adsorption mechanism of MPD molecule on the copper surface is mainly physisorption.
　　SEM images of copper electrodes without and with optimized MPD modifications were observed before and after 5 h （corrosion time） immersion in 3.5% NaCl solution.Figure 10（a） and 10（b） exhibited the surface morphologies of the bare copper before and after immersion in NaCl solution，respectively，while Fig.10（c） and 10（d） demonstrated the MPD modified copper surface before and after immersion in NaCl solution，respectively.Obviously，as shown in Fig.10（b），seriously corrosion occurred on the bare copper surface after immersion in NaCl solution for 5 h.In contrast，in the presence of MPD [Fig.10（d）]，the copper surface corroded barely.
　　4 Conclusion
　　In this work，MPD was prepared for the corrosion inhibition of copper in 3.5% mass fraction of NaCl aqueous solution.Under optimal assembly condition，5×10-4 mol·L-1 MPD assembly for 8 h，the MPD modified copper surface exhibited the greatest inhibition efficiency，98.1%.SERS spectrum indicated that MPD molecule was adsorbed on copper surface with N9-H10 and S7-H8.
　　Acknowledgement
　　This work is supported by International Joint Laboratory on Resource Chemistry （IJLRC），Shanghai Key Laboratory of Rare Earth Functional Materials and Shanghai Municipal Education Committee Key Laboratory of Molecular Imaging Probes and Sensors.
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