The structure, morphology and electrochemical

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The structure, morphology and electrochemical impedance study of the passivation layers on the surface of the Co-Fe-Si-B-M amorphous metallic alloys

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2007 J. Phys.: Conf. Ser. 79 012033 (http://iopscience.iop.org/1742-6596/79/1/012033) View the table of contents for this issue, or go to the journal homepage for more

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XIII International Seminar on Physics and Chemistry of Solids Journal of Physics: Conference Series 79 (2007) 012033

IOP Publishing doi:10.1088/1742-6596/79/1/012033

The structure, morphology and electrochemical impedance study of the passivation layers on the surface of the Co-Fe-Si-B-M amorphous metallic alloys L. Bednarska, B. Kotur, M. Kovbuz Faculty of Chemistry, Ivan Franko National University of Lviv, Kyryla & Mefodiya Str. 6, UA-79005, Lviv, Ukraine [email protected] A.Budniok, E. Łągiewka Institute of Materials Science, University of Silesia, Bankowa St. 12, 40-007 Katowice, Poland Abstract. The passivation layers on the Co-Fe-Si-B-M amorphous metallic alloys (AMA) surface were synthesized at 64 h contact with 3% NaCl aqueous solution. Potentiometric method was applied to study the dynamics of the passivation of the AMA surface. Electrochemically synthesized oxide-hydroxide passivation films have been studied by X-ray diffraction (XRD) analysis, X-ray microprobe analysis, scanning electron microscopy (SEM), voltammetry and electrochemical impedance spectroscopy (EIS) methods. It was shown that simultaneous presence of Cr, Mn, Ni and Mo as doping elements M makes the quality of the passivation layers worse changing the surface morphology and electrochemical characteristics.

1. Introduction Numerous studies of the amorphous metallic alloys (AMA) show, that the melt preparation parameters and conditions of melt solidification have an essential influence on the technological and chemical properties of solid AMA [1]. The structurized oxide films formed on the surface of doped Co-Fe-Si-B-M (M – doping element) amorphous alloys possess protective and specific optical properties [2]. The spread of the fields of the AMA applications makes the estimate of the influence of the composition of aggressive environment on their anti-corrosion stability inevitable. Complex studies of the chemical activity of the Co-Si-B amorphous alloys with different dopants determines the efficiency of their practical application and attract interest from the fundamental and applied point of view. The Co-Si-B-М AMA are used as biomaterials with good magnetic properties on the one hand, and on the other hand they possess good adhesive properties of the surface layers which absorb medicines [3]. The application of the amorphous metallic alloys as soft magnetic materials in medicine requires thorough studies of the formation of protective oxide-hydroxide coatings on their surface, which can absorb active oligomers forming stable films with high density and antiseptic properties. The aim of this work was to study the influence of Cr, Mn, Ni, Mo as doping element M in Co-Fe-Si-B-M amorphous alloys on the structure and protective properties of the oxidehydroxide passivation layers on the AMA electrodes’ surface in 3% NaCl aqueous solutions. 2. Experimental procedure The Co-Fe-Si-B-M amorphous ribbons of 30 µm thickness were produced by melt spinning with the following compositions: Co75.5Fe4.6Si6.0B16.7 (AMA-1), Co66.5Fe4.0Mo1.5Si16.0B12.0 (AMA-2), Co67.2Fe3.8Сr3.0Si14.0B12.0 (AMA-3), Co73.6Fe3.2Mn3.2Si5.0B15.0 (AMA-4), Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 (AMA 5). The ribbons were produced in the Institute of Metal Physics of the NAS of Ukraine. The oxide films on AMA surface were formed spontaneously after 64 h long immersion of AMA electrodes in 3 % NaCl aqueous solution. The process of the formation of passive layers were performed in Pyrex® (Radiometer No.1734) glass cell. The experiments were carried out

c 2007 IOP Publishing Ltd 

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XIII International Seminar on Physics and Chemistry of Solids IOP Publishing Journal of Physics: Conference Series 79 (2007) 012033 doi:10.1088/1742-6596/79/1/012033 in 3% NaCl aqueous solution at room temperature with platinum as a counter electrode and SCE as a reference electrode. The exposed geometric surface area of working electrode was 1 cm2. Steady-state polarization measurements as well as EIS experiments were conducted using Autolab®/PGSTAT20 with Frequency Response Analyser (FRA) and Differential Electrometer Amplifier (Eco Chemie B.V. the Netherlands), combined with one of the software packages and controlled by IBM compatible PC. The structure details of AMA and phase composition of the surface were determined by X-ray diffraction method (Philips X’Pert diffractometer, Mo-Kα radiation) in the 2θ range 20-90о. Micrographs of the surface were obtained with electron microscope REMMA-102 using various magnifications and microanalysis of the surface was done with X-ray microprobe analyzer. 3. Results and discussion The potentiometric curves of AMA are presented in fig. 1, showing the dynamics of the formation of the oxide films and their protective ability. It is well known [4-6], that the reason for high passivity of AMA is the surface films which are formed spontaneously during AMA synthezys. Thus, the contact of AMA-1 with aggressive environment destroys the passivation film, which loses its protective qualities and the potential is shifted to the cathode side from –0.69 to –0.83 V (fig. 1, curve 1). 4

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Figure 1. The potentiometric curves of AMA in 3% NaCl aqueous solution: 1- Co75.5Fe4.6Si6.0B16.7; 2- Co67.2Fe3.8Сr3.0Si14.0B12.0; 3- Сo66.5Fe4.0Mo1.5Si16.0B12.0; 4- Co73.6Fe3.2Mn3.2Si5,0B15.0 Fig. 2 shows the AMA surface micrographs before and after immersion in 3 % NaCl aqueous solution. Micrographs of AMA-1 surface (fig. 2 a, b) illustrate the presence of spontaneously formed oxide layers (light grey regions). The contact of AMA with 3% NaCl aqueous solution leads to re-oxidation of the surface and formation of the porous oxide films (fig. 2, b). However, the depth of the porosity did not reach the pure metal surface, but just the protective film of spontaneously formed oxides. The additional repeated oxidation of the surface enhances the anti-corrosion stability (table 1). It must be caused by additional isolation of the surface. The thickness (h) of a layered film exceeds considerably the average size (l) of nanocrystalline phase on the surface appeared after it’s 64 h contact with the 3% NaCl aqueous solution. Thus, the multilayer structure with good nanocrystalline quality is formed. The values of corrosion potential (Ecorr.) and especially of corrosion currents (Icorr.) and resistance charge transfer (Rct) of the basic and doped AMA (table 1) indicates on the different nature (composition) of the layered passivation films.

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XIII International Seminar on Physics and Chemistry of Solids Journal of Physics: Conference Series 79 (2007) 012033

IOP Publishing doi:10.1088/1742-6596/79/1/012033

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Figure 2. Micrographs of the AMA surface before (a) and after (b-d) the 64 h contact of samples with the 3 % NaCl aqueous solution: AMA-1 (a, b), AMA-3 (c) and AMA-5 (d) TABLE 1 Dimensional and electrochemical characteristics before and after 64 3% NaCl aqueous solution Before AMA -Ecorr., Icorr.×108, h×10-3, Rct×10-3, l, nm 2 V A/cm Ohm nm 1 0.55 4.26 2.34 11.9 27.0 2 0.56 3.10 32.1 12.0 10.6 3 0.45 3.00 37.2 12.2 35.0 4 0.54 4.10 1.18 14.5 68.0 5 0.56 5.40 13.52 22.8 76.4

h contact of Co-based AMA with After -Ecorr., V 0.48 0.50 0.35 0.53 0.75

Icorr.×108, A/cm2 6.2 3.3 1.2 0.2 12.4

Rct×10-3, Ohm 7.8 1.5 12.4 0.6 0.9

The atomic composition of the alloys of the surface layers (~100 Å) of AMA before and after their contact with the 3% NaCl solution is presented in table 2. Practically in all alloys, the content of metallic elements decreased significantly after 64 h immersion in 3% NaCl aqueous solution (table 2). The content of Si increased in the surface layers of all the alloys except for the AMA-3 Co67.2Fe3.8Сr3.0Si14.0B12.0. Increase of Si content in AMA increases corrosion resistivity of AMA in the 3% NaCl aqueous solution. The content of Mn in the surface layers of AMA-4 remained constant before and after the influence of aggressive medium, what is apparently connected with the formation of strong bonds Mn-metalloid. TABLE 2 Atomic composition of the surface layers before and after 64 h contact of the AMA with 3 % NaCl aqueous solution Before After AMA Elements, at. % Co Fe Cr Mo Mn Si B+O+Е Co Fe Cr Mo Mn Si B+O+Е 1 73.65 5.56 2.58 34.79 40.89 2.79 4,21 52,11 2 66.18 4.97 1.56 6.78 20.51 34.46 1.84 0.70 8,19 54,81 3 63.15 2.74 4.23 10.12 19.76 47.19 2.87 6.45 5,68 37,81 4 67.74 6.25 1.04 4.97 20.00 43.13 6.32 1.06 9,19 40,39 5 70.95 2.43 4.2* 4.41 18.01 29.92 1.21 1.86* 10.01 57.00 * – Total composition of the doping elements Ni, Mo, Mn for АМA-5 E – Other elements, included in the composition of the surface layers

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XIII International Seminar on Physics and Chemistry of Solids Journal of Physics: Conference Series 79 (2007) 012033

IOP Publishing doi:10.1088/1742-6596/79/1/012033

There is a core halo with wide maximum at 2θ = 45-48° on the XRD patterns of all investigated samples of the initial alloys (fig. 3, a). Spontaneously formed films on the AMA surface are thin and, therefore, they cannot be detected by XRD method. Some extra peaks at low angle areas (~ 2θ = 30ο) on all XRD patterns appeared after the 64 h contact of AMA samples with NaCl aqueous solutions (fig. 3, b). Their occurrence is connected with progressive oxidation of metal components and escalation of the layered films on the AMA surface. While contacting with the aggressive medium, the surface of AMA electrodes is covered by oxidehydroxide film [7], the main structure unit of which is a non-stoichiometric oxides Co2-xO3-y. Besides these oxidation products, the AMA surface contains other products, which cannot be easily detected by XRD method. 1400

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Figure 3. XRD patterns of the AMA before (a) and after (b) there 64 h contact with the 3% NaCl aqueous solution: 1- Co75.5Fe4.6Si6.0B16.7; 2- Co67.2Fe3.8Сr3.0Si14.0B12.0; 3-Сo66.5Fe4.0Mo1.5Si16.0B12.0; 4- Co73.6Fe3.2Mn3.2Si5.0B15.0 Fig. 4 presents the values of roughness (Rf) and capacity double layer (Cdl) calculated on the EIS data. Roughness (Rf) and capacity double layer (Cdl) of the surface layers increase for all alloys after their prolonged immersion in 3% NaCl aqueous solution. The increase of Rf and Cdl values is pronounced especially for the AMA–1 Co75.5Fe4.6Si6.0B16.7 (fig. 4). For oxidized AMA5 a reversed dependence is observed. The comparison of micrographs (fig. 2, c, d) shows that the texture of the oxide film of AMA-5 is characterized by higher thickness and roughness than the film on the AMA-3 surface. However, the values of Еcorr., Іcorr., Rct let us assume that the coating on the AMA-5 is not dense enough and its protective ability is significantly lower comparing to the oxide layer on AMA-3. -5

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Figure 4. Comparison of the roughness (Rf) and capacity (Cdl) calculated on the EIS data on the AMA before (a) and after (b) 64 h contact with the 3% NaCl aqueous solutions at the corrosion potentials (values of Ecorr. see table 1).

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XIII International Seminar on Physics and Chemistry of Solids Journal of Physics: Conference Series 79 (2007) 012033

IOP Publishing doi:10.1088/1742-6596/79/1/012033

Conclusions The chemical resistance of Co-Fe-Si-B-M (M=Cr, Mn, Ni, Mo) AMA depends on the nature of M, which can form strong metal-metalloid bonds and make up efficient protective surface films. High protective action was observed in the Co-Fe-Si-B-M amorphous alloys (M=Cr and Mo). Doping by Mn increases the average size and thickness of layered films, what causes decrease of anticorrosive stability of the AMA surface. The changes of the free potential values (Efree) as well as of Ecorr. and Icorr. of doped AMA samples after their 64 h contact with the 3% NaCl aqueous solution indicate on occurring of passivation layers with high protective parameters. Porous oxides films occur of the surface of non-doped AMA Co75.5Fe4.6Si6.0B16.0. Increase of Si content increases corrosion resistivity of AMA to the contact with the 3% NaCl aqueous solution. References [1] Inoue A Shen B Takeuchi A 2006 Mat Sci Eng 441 18 [2] Poperenko L Kravets V Lysenko S Vinnichenko K 2005 J Magn Magn Mat 290-291 640 [3] Pardo A Merino M Otero E López M M’hich A 2006 J Non-Cryst Solids 352 3179 [4] Jung Hundal Alfantazi Akram 2006 Elchem Acta 51 1806 [5] Bednarska L Kovbuz M Nosenko V Mudry S Korolyshyn A 2002 Molecular Phys Rep 36 117 [6] Mudry S Kotur B Bednarska L Kulyk Yu Korolyshyn A Hertsyk O 2004 J Alloys Compd 367 274

[7] Badawy W Al-Khrafi F Al-Ajmi J 2000 J Appl Electrochem 30 673

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