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Volume 2 Extended Abstract 4

Submitted 26th August 1999

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 Química-CFM, 88040-900, Florianópolis-SC, Brasil
e-mail: ;

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

Keywords: electropolymerization, 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 were related in literature [1] on inert substrates as Pt, C, and TiO. The reductive electropolymerization has been used in our laboratory as an alternative in corrosion protection of active substrates, as Nd-Fe-B magnets. Our laboratory have synthesised and characterised a novel complex with general formula trans-[RuCl2(L)4], where L = vpy = 4-vinyl pyridine, with reductive electropolymerisated sites. In recent reports we demonstrated the feasibility of reductive electropolymerization on inert electrodes of Pt and Pd, as well as in sintered substrates of Fe+5%Ni and Fe+10%Ni alloys, obtained good results with the ligand L = 4-vinylpyridine [2,3]. Results concerning the electrodepositions of this monomer on the stainless steel are in course [4,5].

The Nd-Fe-B permanent magnets shows outstanding magnetic properties. Due 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 thermal stability are the main drawbacks for the use of Nd-Fe-B in some applications. The efforts are concentrate in to increase the magnetic properties of this material, based on evolution of corrosion resistance through the surface coatings [2,7-9]. At this context, poly-{trans-[RuCl2(ypy)4]} films arisen as an alternative surface coating.

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 was 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 at room temperature with non de-aerated system 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.

Electrochemical Impedance Spectroscopy (EIS): EIS were carried out in a Solartron Mod. SI 1255 and a model 273A Potentiostat/Galvanostat controlled by Electrochemical Impedance software model 398. These measurements were carried out at room temperature in a flat cell, Na2SO4 0.5 mol.cm-3 solutions at 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 occur 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 were produced films by cyclic voltammetry (CV) and fixed potential.

The samples coated by films electrodeposited at –2.75V (fixed potential) showed no satisfactory reproducibility and SEM images obtained from these films are suggestive of discontinuity with same parts of the substrate not totally coated (fig.1a). The cyclic voltammetry electrodepositions were more reproducible and the coatings appear more homogeneous (fig.1b). Although the polymerisation only occurs successfully if positive potentials are scanned [14], the poly-{trans-[RuCl2(vpy)4]} film 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 difference was found to films grew in potential range 0.2 or 0.4V vs. SCE.

Figure 1a: poly-{trans-[RuCl2(vpy)4]} film deposited potentiostaticaly in –2.75V vs. SCE and 2400s

Figure 1b: poly-{trans-[RuCl2(vpy)4]} film deposited by CV 0.2 to –2.8V vs. SCE, 30 cycles. Consumption: 20mC.cm-2

The range of potential scanned is so large, then the experiments leads more than one hour per deposition. In order to understand more about work conditions it were carried out electrochemical impedance spectroscopy (EIS) during the film electropolymerization to monitor their growing. The impedance diagrams showed (fig.2) that the film grows until 30 cycles, is visible that the capacitive arc increase progressively until the cycle 29.Probably, after the 29th cycles the film begins to crack or suffer from others kinds of degradation. Based on these results the electrodepositions on Nd-Fe-B by CV were carried out up to 30 cycles between 0.2 and –2.8V vs. SCE.

Figure 2: Nyquist diagram during the polymerisation of trans-[RuCl2(vpy)4] on Nd-Fe-B magnets by CV 0.2 to –2.8V vs. SCE. Applied potential: -2.8V. Range frequency: 100kHz to 5Hz. ( g ) 20 th ; ( 9 ) 30 th and ( W ) 35 th cycles of growing. Non-degassed system was used. The films deposited on these magnetic samples did not show electrochemical response in: TBHP 0.1mol.dm-3in CH3CN, 2-butanone, CH3CN/CH2Cl2 (4/1) and LiClO4 0.1mol.dm-3. Usually, CV experiments detect the ruthenium presence on the film. However, in this case the lack of electroactivity of the films did not help in the characterisation of the coated magnet. The samples coated at the above optimum conditions were analysed by loss mass as a function of immersion time and by EIS, both tests 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.

References

1    A. Deronzier, E. Dominique, P. Jardon, A. Martre, J. Moutet, Journal of Electroanalytical Chemistry. 453 (1998), p. 179-185. 2    O. Shinora, Y.Narumya, Patent Tdk Electronics Co Ltd, Japan. 1994. 3    P. G. Pickup, W. Kutner, C. R. Leidner, R. W. Murray. J. Am. Soc. 106 (1984) 1991-1998 4    A.V.C. Sobral, W. Ristow Jr., S. C. Domenech and C. V. Franco. "Characterization and corrosion behavior of injection molded 17-4 PH steel electrochemically coated with poly- trans- ruthenium dichloro tetra 4-vinyl pyridine". Accepted for publication on the Journal of Solid State Electrochemistry, ref:99-210 (1999). 5    A.V.C. Sobral, I. Costa, S. C. Domenech and C. V. Franco. "Electrochemical characterization and corrosion behavior of injection molded 316L steel electrochemically coated by poly-trans ruthenium dichloro 4-vinylpyridine." Submitted to Corrosion Science. 6    A.S. Kim, F.E. Camp, Journal of Applied Physics. 79(8 part 2A) (1996), p. 5035-5039. 7    T. Nakayama, F. Sato, A. Hanaki, Patent Kobe Steel Ltd, Japan. 1993. 8    G.W. Warren, G.Gao, Q. Li, Journal of Applied Physics. 70(10) (1991), p. 6609-6611. 9    K. Morimoto K. Komada, Patent Mitsubushi Materials Corp, Japan. 1996. 10    C.V. Franco, V.N. de Moraes Jr., F. Mocellin and M.M.S. Paula, Journal of Materials Chemistry, 8 (1998), p. 2049. 11    M.C.E. Bandeira, F.D. Prochnow, I. Costa, C.V. Franco, Brazilian Patent MU 7900518-7 (under request), Brazil. 1999. 12    C.V. Franco, P.B. Prates, V.N.de Moraes Jr., M.M.S. Paula, Synthetic Metals 90 (1997), p. 81-88. 13    M.M.S. Paula and C.V. Franco, unpublished work. 14    M.E.G. Lyons, Electroative Polymer Electrochemistry, part 1: fundamentals, Plenum Press-New York (1994), p. 164-195.

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