Functionalized multi-walled carbon nanotubes reinforced coating for metal protection

JIANG Yueyue，LI Shouting，F(xiàn)AN Juanjuan，LIU Zequn，GUO Xiaoyu，YING Ye，YANG Haifeng

（College of Chemistry and Materials Science，Shanghai Normal University，Shanghai 200234，China）

Abstract： Polydopamine（PDA）layers were wrapped over functionalized multi-walled carbon nanotubes（PDA＠FMWCNT）by the spontaneous oxidative polymerization of dopamine.PDA＠FMWCNT-based composites film was constructed at the surface of copper（Cu）by taking advantage of the easy attachment capability of PDA.The structure of composites material was characterized by Raman spectroscopy，F(xiàn)ourier transform infrared（FT-IR）spectroscopy，X-ray photoelectron spectroscopy（XPS），and scanning electron microscopy（SEM）.The protection performance of PDA＠FMWCNT for the metal in saline solution was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy（EIS）.Consequently，PDA＠FMWCNT composites film showed excellent corrosion inhibition and the protection mechanism was depicted in detail.

Key words： polydopamine（PDA）；multi-walled carbon nanotubes（MWCNT）；corrosion protection；composites film

1 Introduction

Huge economic losses and major casualties would be caused by metal corrosion，especially the corrosion of copper（Cu）and its alloy.Therefore，corrosion protection methods have been explored［1-2］and the organiccoating-based protection method has attracted great attention［3-4］.

Polydopamine（PDA）which contains plenty of catechol and amino groupscan easily adhere to almost every material［5-6］.Nevertheless，for corrosion inhibition，PDA coating is effective，yet easy to be destroyed since the corrosive media could invade into the coating due to the surface defects［7］.Hence，one of sufficient methods to enhance PDA’s corrosion-proofing ability is to introduce nanomaterials，for instance，Si O2and carbon nanotubes（CNT）［8］.

CNT with lightweight，high specific surface area，good corrosion inhibition ability，good thermal stability，and good mechanical performance［9-10］should be an alternative material to elevate anti-corrosion performance.Previous research showed that corrosion inhibition coating doped with multi-walled carbon nanotubes（MWCNT）exhibited promising corrosion resistance［11-12］.Unfortunately，the insolubility of MWCNT in water limits its wide applications.A possible way to increase its water solubility is the acidification of MWCNT via functionalization and introduction of carboxyl groups and hydroxyl groups（donated as FMWCNT）.Furthermore，PDA can also easily wrap MWCNT to form uniform and high dispersion nanocomposites in water as well as to improve their mechanical properties，along with their resistance to high temperature and different chemical solvents relative to their individual components.

In this work，a simple strategy was adopted to synthesize PDA layers wrapped over functionalized multiwalled carbon nanotubes FMWCNT（PDA＠FMWCNT）for construction of composites coating.In comparison with PDA and PDA wrapped MWCNT（PDA＠MWCNT），PDA＠FMWCNT demonstrated a great inhibition efficiency of corrosion（η=98.9%）in 3.5%（mass fraction）NaCl solution.FMWCNT was observed by X-ray diffraction（XRD），Raman spectroscopy，F(xiàn)ourier transform infrared（FT-IR）spectroscopy，scanning electron microscopy（SEM），transmission electron microscope（TEM），and X-ray photoelectron spectroscopy（XPS）.The corrosion inhibition mechanism of the composites film was studied by electrochemical methods and discussed in detail.

2 Experimental

2.1 Materials

Nitric acid，sulfuric acid，ethanol，sodium chloride（AR），2-amino-2-hydroxymethylpropane-1，3-diol（Tris，pH=8.5，10 mmol·L-1，AR）and dopamine hydrochloride（mass fraction 98%）were obtained from Sigma-Aldrich Corporation.MWCNT（10-20 nm in diameter）was acquired from Shenzhen Nanotech Port.Milli-Q water（18 MΩ·cm）was used to prepare all solutions.

2.2 Electrode pretreatment

The working electrode（a geometric area of 0.031 4 cm2）was constructed by embedding the copper rod（mass fraction 99.999%）into Teflon sheath.First，500-and 1 000-grits papers，and 0.3μm alumina were used to polish the Cu electrode for a mirror-like surface，successively.And to completely remove loose copper rust，the polished electrode was then thoroughly rinsed three times（i.e.Milli-Q water，pure ethanol and Milli-Q water，respectively）.

2.3 Preparation of FMWCNT

The suspension of 100 mL V（H2SO4）∶V（HNO3）=3∶1 containing 1 g MWCNT，was mixed uniformly by stirring for 24 h at room temperature.After pH value was controlled at 7.0，under vacuum，the functionalized MWCNT were dried at 50℃for 12 h.

2.4 Preparation of PDA＠MWCNT and PDA＠FMWCNT nanocomposites

Mixture of 400 mg FMWCNT，200 mg dopamine and 100 mL Tris-HCl buffer was magnetically stirred at room temperature for 24 h.The solution color changed to dark brown due to self-oxidation，remarking the formation of PDA.The solution was centrifuged，rinsed with water，and then dried under vacuum at 50℃for 24 h to obtain PDA＠FMWCNT nanocomposites.PDA＠MWCNT nanocomposites was made by the same procedure.

2.5 Modification of copper surface

PDA＠Cu was obtained by immersion of copper in Tris-HCl buffer with 1 mg·mL-1（pH=8.5）dopamine for 2 h.The 1 mg·mL-1PDA＠FMWCNT or PDA＠MWCNT nanocomposites were dipped onto the pretreated Cu surface，respectively.The resultant coating modified coppers were dried in ambient condition，recorded as PDA＠FMWCNT＠Cu and PDA＠MWCNT＠Cu，respectively.The modification process of copper surface was depicted in figure 1.

2.6 Characterization

Fourier transform infrared spectroscopy spectra were acquired with 1 cm-1resolution in transmission mode by FTIR spectroscope（Thermo Fisher Nicolet iS5，USA）with KBr pellets.Raman spectra were recorded with a confocal Raman instrument（Super LabRam II system，Dilor，F(xiàn)rance）.A multichannel air cooled 1 024×800 pixels charge-coupled device（CCD），and a 50×long-working-length objective were employed as a detector，and an objective，respectively.5 mW He-Ne laser was focused on the sample surface through 1 000μm pinhole and 100μm slit.Each Raman spectrum was the average of 10 s integration time by 3 times accumulations，and calibrated using 519 cm-1for silicon.The elementary composition of the nanocomposites were determined using X-ray photoelectron spectroscopy（XPS，PHI 5000 Versa Probe，Japan）.SEM and TEM images were conducted using a Hitachi S-4800 scanning electron microscope，and a 200 kV JEOL JEM-2000 FX with S-3 energy dispersive spectrometer，respectively.

2.7 Electrochemical experiments

A Versa STAT4 electrochemical workstation（AMETEK，princeton applied research）was used to observe the electrochemical behavior of coatings in 3.5%（mass fraction）NaCl aqueous solution at ambient temperature.In a traditional three-electrode cell，a saturated calomel electrode（SCE），a platinum foil and a Cu electrode with or without composites films were acted as reference，counter and working electrodes，respectively.The open circuit potential（OCP vs.SCE）was conducted before every electrochemical measurement.For bare Cu and modified electrodes，the stable OCP could be accomplished under 3 000 s and 10 000 s，respectively，after immersing electrodes in 3.5%（mass fraction）NaCl aqueous solution.Electrochemical impedance spectroscopy（EIS）results，acquired in the frequency range from 100 kHz to 10 MHz，under an OCP with 0.005 V，were fitted with ZSimpW in software.

3 Results and discussion

3.1 Raman spectroscopy

As shown in the figure 2，for Raman spectra of the MWCNT and FMWCNT，the peaks at 1 320 cm-1and 1 570 cm-1belong to D-and G-bands，respectively，which are regarded as the structural defects and graphitic carbon atoms vibration.The appearance of high frequency shoulder of D-band of MWCNT（Dí-band）［13］is a double-resonance band caused by disorder and defects.For IDand IGare the intensity ratios of Dí-band and Gí-band，respectively.The intensity ratios（ID/IG）are 1.74 and 1.83 obtained for MWCNT and FMWCNT，respectively.Therefore，the successful oxidization of MWCNT could be demonstrated due to the increase of ID/IGratio after the functionalization，since higher ID/IGratio value indicates more surface defects and less graphitization degree［14］.

3.2 TEM images

Figure 3（a）is the TEM morphologies of the pure MWCNT，and figure 3（b）and figure 3（c）depicted TEM morphologies of the PDA＠MWCNT and PDA＠FMWCNT.Figure 3（d）is the magnification of PDA＠MWCNT in the figure 3（c）.In comparison with pure MWCNT in figure 3（a），the diameters of PDA＠MWCNT and PDA＠FMWCNT increased，indicating that PDA was successfully wrapped over both CNTs.Obviously，the PDA layers at MWCNT were ragged and partially covered whilst the PDA＠FMWCNT were homogeneous and well dispersed.

Figure 1 Synthesis procedures for construction of（a）PDA＠FMWCNT＠Cu and（b）PDA＠MWCNT＠Cu

Figure 2 Raman spectra of FMWCNT and MWCNT

Figure 3 TEM micrographs of（a）MWCNT，（b）PDA＠MWCNT and（c）PDA＠FMWCNT，（d）is the magnification of（c）

Figure 4 FT-IR spectra of（a，b）FMWCNT，（c，d）PDA，and（e，f）PDA＠FMWCNT

Figure 5 XPS spectra of（a）PDA＠FMWCNT＠Cu，（b）C 1s spectra peak，（c）N 1s spectra peak，（d）O 1s spectra peak，（e）Cu 2p3 spectra peak and（f）the deconvolution of C 1s spectrum

Figure 6 Polarization curves of different copper samples recorded in 3.5%（mass fraction）NaCl aqueous solution

Figure 7（a）Nyquist plots and（b）phase angle plots for different copper samples recorded in 3.5%（mass fraction）NaCl aqueous solution

Figure 8 Electrochemical equivalent circuits simulated for the impedance of（a）bare copper，（b）PDA＠Cu，（c）PDA＠MWCNT＠Cu and PDA＠FMWCNT＠Cu

Figure 9 SEM images of the（a，b）bare copper，（c，d）PDA＠Cu，（e，f）PDA＠MWCNT＠Cu and（g，h）PDA＠FMCNTs＠Cu before and after immersed in 3.5%（mass fraction）NaCl aqueous solution for 7 d

3.3 FT-IR characterization

FT-IR spectra of FMWCNT，PDA，and PDA＠FMWCNT were acquired in figure 4.As presented in figure 4（a）and figure 4（b），stretching vibrations of C-H bonds and-OH groups arecontributed to the peaks at 2 917 cm-1and 2 849 cm-1，and intensive bands near 3 418 cm-1，respectively.The peaks at 1 649 cm-1and 1 260 cm-1belong to the C=O and C-O in FMWCNT，respectively，validating the successful oxidation of the MWCNT.In case of FT-IR spectrum of PDAin figure 4（c）and figure 4（d），a broad band at 3 356 cm-1and a strong peak at 1 575 cm-1are respectively attributed to catechol-OH groups and aromatic rings［15］.In FT-IR spectrum of PDA＠FMWCNT compositesin figure 4（e）and figure 4（f），the spectral profile was almost the mixed characteristic peaks of PDA and FMWCNT.It should be noted that with comparison of figures 4（a）and 4（b），the-OH and C=O vibrations red-shifted to lower wavenumbers，which could be attributed to the hydrogen bond formation of either-OH between PDA and FMWCNT or the-NH2in PDA and-COOH in the FMWCNT［16］.It should reinforce the coating of PDAby FMWCNT，which was beneficial to improve the PDAcapability against salt corrosion.

3.4 XPS characterization

In figure 5（a），the carbon（284.6 eV），nitrogen（400.1 eV），oxygen（531.6 eV），and copper（932.6 eV）elements could be visible in the scanning bonding energy range from 0-1 200 eV，validating the successful modification of the PDA＠FMWCNT film on the copper surface.The scaled XPS spectra of C，N，O，Cu elements were shown in figures 5（b）-5（e），respectively.The correlated atomic contents were 59.37%，7.89%，26.05%，and 6.69%for C，N，O，and Cu elements，respectively.As shown in figure 5（f），the high resolution C1s XPS spectra of PDA＠FMWCNT film could be fitted to five component peaks which are 284.3 eV（sp2C=C/sp3C-C），285.3 eV（C-N），286.3 eV（C-O），287.6 eV（C=O），and 288.8 eV（O-C=O），respectively.Clearly，the groups of O-C=O were introduced by the FMWCNT preparation process，which could increase the hydrophilicity as well as prohibit the smooth coating of the PDA layers via decreasing the conjugate structure of MWCNT［17］.The presence of C-N peak indicated covalent binding of PDA molecules onto the surface of FMWCNT.

3.5 Electrochemical measurements

The corrosion properties of copper without and with different composite films were examined in 3.5%（massfraction）NaCl solution by potentidynamic polarization.By the Tafel extrapolation in figure 6，the corrosion potential（Ecorr），corrosion current density（Icorr），and the anodic and cathodic Tafel slopes（βaand βc），were tabulated in table 1.Clearly，the Icorrdecreased after modification of surface by coatings.Lower current densities，in both cathodic and anodic directions，could be observed in all of the modified electrodes，compared to bare ones.In addition，PDA＠FMWCNT＠Cu presented the lowest Icorrvalue.It is worth mentioning that no pitting corrosion happened in case of PDA＠FMWCNT＠Cu，indicating that the relatively perfect coating for protection of copper from corrosion effectively delay the reaction of anode dissolution and cathodic hydrogen evolution by forming a more compact membrane on the copper surface.It is further shown that functionalized MWCNT can overcome the shortcomings of ordinary materials and have a stronger interaction with PDA.

Table 1 Corrosion parameters obtained from potentiodynamic polarization curves for different samples in 3.5%（mass fraction）NaCl aqueous solution

As shown in figure 7（a），for Nyquist plots of each copper surface，a depressed semicircle at high frequencies exhibited the resistance of charge transfer.The occurrence of the Warburg impedance（straight line）in lower frequencies for bare Cu and PDA＠Cu electrode was associated with the diffusion of CuCl2［18］.After formation of composite films at the copper surface，the Warburg impedance disappeared and the radius of the capacitive loops increased，suggesting that the bare copper was readily corroded，while the surface modification could enhance the corrosion resistance.In case of the phase angle plots in figure 7（b），the bare Cu showed only a time constant，whilst the modified coppers presented two ones.The PDA＠FMWCNT nanocomposites coating presented the maximum phase angle，which was in good consistency with that of potentiodynamic polarization analysis.

The electrical equivalent circuit（EEC）models，demonstrated in figure 8，were fitted using Z SimpWin software.The fitting evaluation was depended on the standard deviations（χ2），which ought to be smaller than 10-3［19］.Rsrepresented the solution resistance，W indicated the diffusion process due to the corrosive reactants or corrosion products diffusing to the bulk solution.The two main parameters in such time constant were the resistance and the constant phase element（CPE）of the inhibitor protection layer，which were due to Rfand Q，respectively.Time constant included two parts，Rct（charge transfer resistance）and capacitance of the double-charge layer（Cdl）which related with the charge-transfer at the copper/corrosive media interface.And Q1，Q2correspond to two time constants，respectively in figure 8（c）.The time constant（Q）involved either the Rf（film resistance）.The mathematic description of CPE was［20］：

where Y0was the magnitude of CPE；j was the imaginary number；ωwas the angular frequency；n was the CPE exponent，depending on the unevenness of the surface.The higher n value indicated the smoother metal surface and less corrosion occurrence［21］.Generally，the n valued from 0 to 1，and for some special cases，i.e.，the n value was-1，0，and 1，n was an inductance，a resistance and a capacitance，respectively.The fitted results of electrochemical parameters were summarized in table 2.The Rctcould be estimated from the diameter of a semicircle at high frequencies，and the higher Rctvalue，the higher corrosion protection ability［22］.The Rctof PDA＠FMWCNT＠Cu rapidly increased up to 1521Ω·cm2，indicating the improvement of inhibition.Besides，the following equation could be used to calculate the inhibition efficiency（IE）：

where R0pand Rp，the resistance of the bare copper and total resistance，respectively.Therefore，the R0pis the sum of Rctand Rfafter neglecting the solution resistance（since it was relatively small）［23］.A high IE of 98.8%could be achieved for PDA＠FMWCNT＠Cu as expected.

Table 2 Electrochemical parameters calculated from EIS measurements for different samples in 3.5%（mass fraction）NaCl aqueous solutions

3.6 SEM observation

After the Cu species were immersed in 3.5%（mass fraction）NaCl solution for 7 d，their morphologies were investigated by SEM.With PDA modification（as comparison of figures 9（a）and 9（b）），the Cu surface became rougher along with the polished traces.Obviously，the serious corrosion occurred on the bare Cu after salty immersion for 7 d.With the modification with only PDA and PDA＠MWCNT，the degree of Cu corrosion decreased，but the pitting corrosion still occurred in severe corrosive media.Clearly，for PDA＠FMWCNT coating，no remarkable change appeared on the surface and even no pitting corrosion was found，which agreed with the above results.

4 Conclusions

PDA＠FMWCNT nanocomposites（an optimal protection film for Cu from corrosion）through oxidation polymerization and functionalization methods were characterized via TEM，SEM，F(xiàn)T-IR，XPS，and XRD.Potentiodynamic polarization and EIS studies revealed that PDA＠FMWCNT-based composites coating could efficiently protect Cu against NaCl corrosion with a high inhibition efficiency of 98.8%.The great protection ability of the PDA＠FMWCNT coating was probably due to strong hydrogen bond interaction between PDA and FMWCNT to enforce passive coating with uniform and dense structure.