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Volume 2 Paper 4


Electroreductive polymerisation and corrosion resistance of trans-[RuCl2(vpy)4] on Nd-Fe-B magnets in Na2SO4 solution.

Bandeira, M.C.E.1; Prochnow, F. D. 1; Costa, I. 2; Franco, C.V. 1

1 Universidade Federal de Santa Catarina UFSC-Campus Trindade, LEC/LABMAT,
Depto. de Qu�mica-CFM, 88040-900, Florian�polis-SC, Brasil
e-mail: ;

2 Instituto de Pesquisas Energeticas e Nucleares, IPEN/CNEN-SP
Caixa Postal 11049, CEP 05422-970, S�o Paulo-SP
e-mail :

Abstract

This work reports the reductive electrochemical polymerisation of the trans-[RuCl2(vpy)4] monomer, where vpy = 4-vinylpyridine. Its potentiostatic and potentiodynamic (cyclic voltammetry) electropolymerisation produced strongly adhesive and electroactive films on platinum electrodes. However the films deposited on Nd-Fe-B permanent magnets did not show electroactivity in several solvents/electrolytes tested. The efficiency of the film formation indicated the existence of a strong correlation between the upper-limit value of the potential range applied and the charge density consumed in the polymerisation. Electrochemical Impedance Spectroscopy studies in Na2SO4 0,5 mol.dm-3 solution showed that the charge transfer resistance (RCT) associated to the Nd-Fe-B magnets coated with poly-trans-[RuCl2(vpy)4], was four times higher than that of uncoated magnets. Gravimetric tests indicated no significant weight losses of the coated magnets. Morphology and thickness measurement studies were performed by Scanning Electron Microscopy (SEM). Energy Dispersive Spectroscopy (EDS) confirmed the presence of Ru in the polymeric film.

Keywords: electropolymerisation, Nd-Fe-B magnets, corrosion.

Introduction

The electrocoating from monomers with redox centres constitute an alternative way of application in electrocatalysis and corrosion protection, partially due to the low oxidation-state of immobilised transition metals in the polymeric matrix. Organic coatings catodically electrodeposited on inert substrates, such as Pt, C and TiO, have been reported in the literature [1]. Reductive electropolymerisation has been used in our laboratory as an alternative in the corrosion protection of active substrates, such as Nd-Fe-B magnets. Our laboratory has synthesised and characterised a novel complex with general formula trans-[RuCl2(L)4], where L = vpy = 4-vinyl pyridine, with reductive electropolymerised sites. In recent reports the feasibility of reductive electropolymerisation with the ligand L = 4-vinylpyridine on inert electrodes of Pt and Pd, as well as on sintered substrates of Fe+5%Ni and Fe+10%Ni alloys, has been demonstrated [2,3]. Results concerning the electrodeposition of this monomer on stainless steels are underway [4,5].

Nd-Fe-B permanent magnets shows outstanding magnetic properties. Due to their high-energy product, applications of this material have increased in the last decade. The main applications include consumer electronics, computer peripherals, acoustics, office automation, and magnetic resonance image [6]. Nevertheless, poor corrosion resistance and low thermal stability are the main drawbacks for the use of Nd-Fe-B in some applications. Efforts have been concentrated in increasing the magnetic properties of this kind of material, based on evolution of corrosion resistance through the application of surface coatings [2,7-9]. Poly-{trans-[RuCl2(ypy)4]} films are under investigation as an alternative surface coating for NdFeB magnets.

Experimental

Reagents and Synthesis: Commercially available reagents and solvents with analytical grade were employed throughout this work. RuCl3.3H2O (Jonhson-Matthey) and 4-vinylpyridine (Aldrich) were used without further purification. The synthesis of Ruthenium blue solution and trans-[RuCl2(vpy)4] was carried out employing a method described in the literature [10].

Preparation of samples: Nd-Fe-B magnets produced by Sumitomo were prepared for coating by polishing on 500 grit emery paper followed by an ethanol wash. The exposed sample area to coating was 0.95 cm2.

Electrocoating: Nd-Fe-B magnets were coated with poly-{trans-[RuCl2(vpy)4]} films, electrodeposited by Cyclic voltammetry (CV) and potentiostatic technique. The monomer concentration was 5 mmol.cm-3 in CH3CN/CH2Cl2 (4/1) and the electrolyte used was TBHP (tetrabutilamoniun hexafluorphosfate) 0.1 mol.cm-3 [11]. Electrodepositions were carried out in a electrochemical cell with one compartment using a 273A Princeton Applied Research (PARC) Potentiostat/Galvanostat, interfaced with a DOS-compatible computer through a National Instrument General Purpose Interface Board (GPIB). Instrumental control, data acquisition, and processing were performed by a 270 EG&G Research Electrochemistry Software. All the electrodepositions were performed at room temperature and the electrolyte was quiescent.

Electrochemical Impedance Spectroscopy (EIS): EIS tests were carried out with a frequency response analyser (Solartron SI 1255) and a potentiostat ( EG&G 273A) controlled by an electrochemical impedance software. These measurements were carried out at room temperature in a flat cell, in Na2SO4 0.5 mol.cm-3 solutions at the open circuit potential. Measurements on Nd-Fe-B magnets without poly-{trans-[RuCl2(vpy)4]} films were performed for the sake of comparison.

Results

Several experiments aimed at obtaining electropolymer films from the monomer trans-[RuCl2(vpy)4] were conducted using electrodes of different materials [4,5,10-12]. The film formation on Nd-Fe-B magnets occurs almost at the same range of applied potential (–2.6 and –2.8V vs. SCE) found by Paula et al. [13] to electropolymerise successfully trans-[RuCl2(vpy)4] on Fe-5%Ni and Fe-10%Ni sintered alloys [10]. However, electrodepositions on Nd-Fe-B occur preferentially at –2.75 and –2.8V. In this work, electropolymerised films were produced by either cyclic voltammetry (CV) or at fixed potentials.

The samples coated by films electrodeposited at –2.75V (fixed potential) showed no satisfactory reproducibility. SEM images obtained from these films are suggestive of discontinuities and consequently some parts of the substrate were not totally coated (fig.1a). The cyclic voltammetry electrodepositions were more reproducible and the coatings more homogeneous (fig.1b), although the polymerisation only occurred successfully when positive potentials were scanned The redox polymer poly(vinyl ferrocene) exhibits a maximum of conductivity when the concentration of oxidised and reduced sites in the polymer matrix matches. This occurs at the standard potential (E0) of the redox centres in the polymer [14]. In agreement with the studies of poly(vinyl ferrocene), the electropolymerisation of trans-[RuCl2(vpy)4] on Nd-Fe-B magnets are successfully when the range of applied potential reaches the region of the standard potential of the redox centres, that is, nearly 0.2V vs. SCE for poly-{trans-[RuCl2(vpy)4]} on Pt electrodes.

Poly-{trans-[RuCl2(vpy)4]} films could be formed at the following conditions: 0.2 to –2.8V and 0.4 to –2.8V at 50 mV.s-1, 30 to 50 cycles. No significant differences were found in the films polymerised in either potential range, 0.2 to –2.8V or 0.4 to –2.8V vs. SCE, apparently because E0 of Ru2+/3+ on Nd-Fe-B magnets, occurs before 0.2 V.

Figure 1a: poly-{trans-[RuCl2(vpy)4]} film deposited potentiostaticaly at –2.75V vs. SCE up to the elapsed time of 2400s Figure 1b: poly-{trans-[RuCl2(vpy)4]} film deposited by CV at potentials ranging from 0.2 to –2.8V vs. SCE, 30 cycles. Consumption: 20mC.cm-2

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Since the range of potential scanned was too large, the experiments took more than one hour per deposition. In order to gather information on the working conditions, electrochemical impedance spectroscopy (EIS) was carried out during the film electropolymerisation, to monitor the film growth. The impedance diagrams (fig.2) showed that the film growth reaches a maximum after 30 cycles. The diagrams corresponding to the 2nd, 5th and 12th cycles suggested a diffusion control, related to monomer mass transport from the bulk solution to the magnet sample surface. For a higher number of cycles, a capacitive arc starts to delineate, and a complete arc could be firstly seen at the 15th cycle. The capacitive arc increased progressively for further cycles up to the 30th cycle. For additional cycles, the film resistance decreased, as indicated in fig.2, probably due to cracks or other kinds of degradation.

Based on these results the electrodepositions on Nd-Fe-B were carried out by CV up to 30 cycles in the potential range between 0.2 and –2.8V vs. SCE.

Figure 2: Nyquist diagrams obtained during the polymerisation of trans-[RuCl2(vpy)4] on Nd-Fe-B magnets by CV from 0.2 to –2.8V vs. SCE. Applied potential: -2.8V.  Frequency range: 100 kHz to 5 Hz. ( g ) 20 th ; ( 9 ) 30 th and ( W ) 35 th cycle.

The films deposited on the specimens of NdFeB magnet did not show any electrochemical response in: TBHP 0.1mol.dm-3 in CH3CN, 2-butanone, CH3CN/CH2Cl2 (4/1) and LiClO4 0.1 mol.dm-3 solutions. Usually, CV experiments detect the ruthenium presence in the film. However, in this particular case, the lack of electroactivity of the films did not permit the coated magnets to be adequately characterised.

The corrosion resistance of the coated samples, coated by using the optimum conditions stablished above, was analysed by gravimetric tests as a function of immersion time and by EIS in Na2SO4 0.5 mol.dm-3 solution. Comparative studies were made between poly-{trans-[RuCl2(vpy)4]} coated Nd-Fe-B magnets and uncoated magnets.

Figure 3 shows the impedance diagrams corresponding to the coated and uncoated Nd-Fe-B magnets investigated. The RCT of the coated magnet after 2 hours in the sulphate solution (ca. 2.5 kW .cm2 ) was approximately 4 times higher than that of the uncoated magnet (ca. 0.6 kW .cm2 ) in the same solution. The double layer capacitances associated to the coated (2 hours of immersion) and to the uncoated magnets, were around 930 and 2200 m F.cm-2, respectively. These results show that the electrodeposited film provides temporary protection to the substrate. The protective characteristics of the film could decrease with the immersion time. Comparing the weight losses and EIS results, this behaviour could be attributed to the permeation of the thin film by aggressive species of the electrolyte.

The solution resistance (RS) estimated from the electrochemical response of the uncoated NdFeB magnet in the Na2SO4 electrolyte was approximately 15 W .cm2. However, higher RS was found (44 W .cm2) when the magnet was tested in the same electrolyte, after been coated. That behaviour can be rationalised in terms of the electronic resistance of the film (electrons transport between the redox centres Ru2+/3+) which are associated in series with RS, producing that ohmic drop.

Figure 3: Nyquist diagrams of poly-{trans-[RuCl2(vpy)4]}coated (g ) and uncoated Nd-Fe-B magnets (9 ) in Na2SO4 0.5mol.dm-3 solution at open circuit potential. Frequency range: 100kHz to 10mHz.

Figure 4 shows that coated and uncoated samples had similar behaviour when immersed in the sulphate medium. The poly-{trans-[RuCl2(vpy)4]} film did not improve considerably the corrosion resistance of the Nd-Fe-B magnets. This was likely due to the low thickness of the polymeric film associated to its high permeability to ions from the solution.

Figure 4: Gravimetric tests in Na2SO4 0.5 mol.dm-3 solution.

Conclusions

The conditions of electroreductive polymerisation of trans-[RuCl2(vpy)4] on Nd-Fe-B magnets associated to improved reproducibility and to the formation of homogeneous film were CV at potentials ranging from 0.2 to –2.8V vs. SCE.

EIS experiments performed during the film growth indicated a decrease in the resistance RCT after the 30th cycle, which could be due to the degradation of the polymeric film.

The impedance diagrams showed that the polymeric film offers a temporary protection to the corrosion process.

The gravimetric tests in Na2SO4 solution indicated that the poly-{trans-[RuCl2(vpy)4]} coating was not highly effective, probably due to its low thickness and high permeability.

References

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