COMPOSITION, STRUCTURE AND SOME CORROSION PROPERTIES OF AS-RECEIVED NdFeB MAGNETIC MATERIAL AND PROTECTIVE ZnAl COATING

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1 COMPOSITION, STRUCTURE AND SOME CORROSION PROPERTIES OF AS-RECEIVED NdFeB MAGNETIC MATERIAL AND PROTECTIVE ZnAl COATING Stanislav LASEK, Miroslav KURSA, Kateřina KONEČNÁ VŠB-TU Ostrava, 17. listopadu 15/2172, Ostrava-Poruba, Česká republika, Abstract Advanced permanent magnet materials provide a higher efficiency and reliability, lower cost, low size and weight for pro many applications, mainly electric motors and renewable energy technologies. The problem of NdFeB type magnets is a lower corrosion resistance in humid environments and lower thermal stability of parameters in comparison with related magnets. The results of ZnAl protective coating study, deposited by special method on NdFeB substrate (produced by powder metallurgy) are presented in this paper. In the metallographic sections a non-uniform thickness and porosity of layer were found out. The base material was formed by sintered (polyhedron) grains and it contains pores and minority phases. Chemical composition of phases in coating and base heterogenic material was determined by EDS microanalysis. In all tested phases and places were detected except Fe and Nd also Co and Dy (composition corresponds to (Nd,Dy) 2 Fe 14 B type magnet) and in some phases O and Si or Al were found out. The tested phases in layer composed of most Zn or Al (above 90% wt.). The fractographic analysis confirmed intercrystalline brittle fracture. On the basis of potentiodynamic polarization method the corrosion parameters of stated materials, commercially zinc-coated steel, zinc and aluminium were determined and compared, especially with respect to pitting corrosion in NaCl water solution. Key words: NdFeB magnet, ZnAl coating, structure, microanalysis, corrosion 1. INTRODUCTION NdFeB base materials have outstanding magnetic properties with high energy products (BH) max (40 MGoe), leading to substantial technical and economic impact on the permanent industry. These magnets have found wide use in automobiles, personal computers and some others commercial products [1]. Due to their composition of highly reactive components (i.e. Nd rich phase) and their complex microstructure, NdFeB magnets exhibit a low corrosion resistance in humid environments. Nd is an active metal with a standard electrochemical potential E o = -2,4 V [2]. It has been also proved that Nd-rich phase and Nd 2 Fe 14 B matrix absorb hydrogen in humid environments leading to decripation. In aqueous solutions, a preferential dissolution of the highly reactive intergranular Nd-rich phase occurs, which is strongly enhanced by the galvanic coupling of the Nd-rich phase to the much more noble ferromagnetic grains. The ferromagnetic grains at the bulk magnet surface loose adhesion and finally detach from the surface [3]. The aim of the contribution is the study of chemical composition, structure and corrosion resistance of asreceived NdFeB type magnet and its protective ZnAl coating. 2. MATERIALS AND EXPERIMENTAL METHODS Selected NdFeB material was used in as-received state before magnetizing. The samples of dimensions 20 x 10 x 4,4 mm were manufactured by powder pressing and sintering. The coating ZnAl on the NdFeB base material were prepared by means of supersonic gas dynamic technique. Two types samples were used for the electrochemical experiments, ZnAl coating as working electrode and sintered NdFeB magnet. This sample was prepared by wet grinding to the depth of 0,15 mm under a larger surface in order to remove ZnAl coating and finished by fine grinding (1200 grit SiC paper), then carefully cleaned in spirit and in demineralised water. A smaller sample (10x7 mm) was prepared for metallographic study by separation from received sample by sawing and brittle fracture. The fractographic analysis and chemical microanalysis was also performed on this fracture surface. The mean boron content and 1

2 concentration of selected elements (Fe, C, N, Co, Nd) was analysed by the glow discharge spectroscopy (GDS, device GDA750). 2.1 Microscopic studies Microstructure was observed on metallographic section after polishing and then after etching. Metallographic study was performed by means of microscope Neophot 2 and stereomicroscope Intraco Micro. Linear method was applied for the determination of secondary phase ratio, porosity, pores size and grain (powder) size. Phase microstructure study and chemical microanalysis were carried out using scanning electron microscope JEOL JSM-6490LV equipped with EDS INCA X ACT probe. The point and surface analysis were performed. The samples were observed before magnetization in the electron microscope. 2.2 Electrochemical testing procedure In this investigation the potentiodynamic cyclic polarization technique was utilized [4]. Experiments were performed using computer-directed PGP-201 Potentiostat/Galvanostat with Voltamaster PC program. Potentiodynamic polarization tests were preceded by a determination of corrosion potential (E cor ). The corrosion rate was determined by the polarization resistance method. The electrochemical cell consisted of the standard three electrodes arrangements, NbFeB or ZnAl working electrode, a reference saturated calomel electrode (SCE), and platinum counter electrode. Polarization curves were obtained in neutral 0,1 M NaCl water solution at room temperature, using a voltage sweep rate of 1,0 mv/s. For comparison purposes the samples of carbon steel, commercial zinc coating, zinc and aluminium were also tested. The exposed surfaces in electrochemical cell had areas 0,5 2 cm RESULTS AND DISCUSSION 3.1 Chemical composition The chemical microanalysis of NdFeB phases was performed on inter-granular brittle fracture surface, Fig. 1. and Table 1. Two basic phases were identified, the grain or powder matrix contains approximately (in at.%) 82% Fe, 11,5% Nd, 2% Co, 2%Al, 1,5% Dy and this corresponds to (Nd,Dy) 2 Fe 14 B magnet type. The intergranular phase, white in Fig. 1, is rich in Nd and contain small amount Co, Fe, Al and Dy. The oxygen content in minority phase (30-50% at.) is probably part of corrosion products of reactive element (Nd(OH) 3 ). The thermal stability of NdFeB is enhanced by Dy [1] and Co improves a corrosion resistance [3]. Table 1. Chemical composition (wt. %) of phases in magnet type NdFeB place Fe Nd Dy Co light light dark dark place Al Si O light light dark dark Fig. 1. Fracture surface of magnet, numbers (1,2) designate selected points of analysis (BEC). Note: Mean boron content - 0,15% wt. 2

3 On the surface of coating ZnAl were also observed two phases, Fig. 2. The light phase contains nearly 90% Zn and 5% Al, a darker structure contains over 90 % Al and 2% Zn (Table 2). The detected oxygen is probably part of corrosion products. Average surface analysis identified around 79% Al, 15% Zn and 15% O. More homogeneous coating can be deposited by classical technologies [5], e. g. electrodeposition. Table 2. Results of surface and point analysis of coating ZnAl wt.% Zn Al Fe O surface surface surface light dark Fig. 2. Surface of coating (BEC), numbers (1,2) designate selected places of analysis. at.% Zn Al Fe O surface surface surface light dark Metallographic study The different phases were found in structure of material under investigation, Fig. 3 and Fig. 4. On the metallographic section, the light phase is rich in Fe, the grey (tint, dark, brown) phases contain more Nd, Small amount of pores of spherical shape (black colour) was observed on the fracture surface and in the metallographic sections. Relatively large dispersion of coating thickness, in the range of m, was identified on sections (Fig. 3). The average porosity ratio of coating (determined by linear method) is around 32±9 %, the mean diameter of pores is 7,5±1,7 m. Possible through pores can be filled with corrosion products and thus decrease or block the corrosion of base material. The minority phases in NdFeB appeared as pores or microcracks at smaller magnification that is difficult to distinguish between these structural parts. Fig. 3. Structure of base material and coating, a higher heterogeneity and porosity of ZnAl coating. Fig. 4. Structure of base material with different phases and pores (etched specimen). Secondary phases (dark) in base material, polished. 3

4 The secondary phases and partly pores in base material occurred at approximately 27±3 % and their average size 10,2±1,1 m. Grains with polyhedral equiaxed shape have mean diameter d g = 23±2 m. 3.3 Potentiodynamic experiments Typical polarization curves of samples obtained during the potentiodynamic measurements are shown in Fig. 5. On exposed surfaces of tested materials and samples were observed pits and/or spots. Values of corrosion potential (E cor ) of tested materials and layer can provide information on their tendency to corrode, the differences in corrosion potentials establish possible galvanic couples in a specific environment, Table 3 and Fig. 5. The values of depassivation (E d ) and repassivation (E r ) potential have been determined for conventional current densities (100 A/cm 2 ; 10 A/cm 2 ). The lower values of depassivation and repassivation potentials, the lower resistance to pitting.the polarization resistance (R p ) and corrosion rate (r c ) were measured and calculated approximately by Stern method [6] before and after pitting (in parenthesis). Zn Fe ZnAl FeNd (E cor) Fig. 5. Comparison of typical cyclic polarization curves for materials tested in 0,1 M NaCl solution. Coordinates: potential E [mv] SCE log i, i [A/cm 2 ]. Table 3. Results of potentiodynamic polarization measurements; 0,1 mol/l NaCl water solution. material cycle E cor E d E r E v R p r c notes sample No. mv mv mv mv k.cm 2 mm/a ZnAl/FeNd ,7 0,024 0,5 cm 2 delivered (1,12) (0,122) SVUOM cell ZnAl/FeNd ,01 0,147 1,5 cm 2 delivered (1,0) (0,26) immersion FeNd ,3 0,15 0,5 cm 2 deliv. ground (0,97) (0,26) Zn/Fe ,35 0,047 0,5 cm 2 reference (1,86) (0,167) steel C ,4 0,21 1,0 cm 2 reference (1,2) (0,37) Zn ,8 cm 2 reference Avesta cell Al ,8 cm 2 reference Avesta cell 4

5 The possibility of cathode protection of NdFeB by sacrificial anode coating ZnAl is given by difference of corrosion potential around 0,3 V. It is also confirmed that addition of Nd decreases the corrosion resistance of Fe (Fig. 5.) According to potentiodynamic test, the corrosion resistance of ZnAl coating is similar to commercial Zn/Fe one in diluted NaCl water solution. The further corrosion tests for magnet materials and coatings under investigation are desirable. 4. CONCLUSION The composition, microstructure and selected corrosion properties of advanced NdFeB magnet with ZnAl coating has been determined. Chemical microanalysis has shown that composition of sintered magnet material correspond to the type (Nd,Dy) 2 Fe 14 B with secondary Nd-rich phases distributed preferably at grain boundaries. Heterogeneous structure and relatively higher porosity was observed in ZnAl coating, deposited by mean of special technology using Al and Zn alloy powders. The brittle intercrystalline mechanism of fracture was prevailing with microscopic dimples. Reasonable values of corrosion and pitting potentials as well as corrosion rates were found out for the magnet material, coating and reference metals (Fe, Zn, Al) in NaCl water solution on the base of potentiodynamic polarization measurements. REFERENCES [1] GUTFLEISC,H O., WILLARD, M. A., BRUCK, E. et.al. Magnetic Materials and Devices for the 21 st Century: Stronger, Lighter, and More Energy Efficient. Advanced Materials, 2010, vol. 20, p [2] ARENAS, M., WARREN, G. W., Aqueous Corrosion Study of Melt-Spun NdFeB Ribbons with TiC Additions. TMS, Alabama, 1999, 10 p. [3] El-MONEIM A. A., GEBERT A., UHLEMANN M. et. al., The influence of Co and Ga on the corrosion behavior of nanocrystalline NdFeB magnets. Corrosion Science, 2002, vol. 44, p [4] ASTM G 61: Standard Test Method for Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys. Annual Book of ASTM Standards, Vol , Metal Corrosion, 2001, pp [5] KREJCIK, V.: Surface treatment of metals I, II (in Czech), 1. ed. Prague, SNTL, 1998, 480 p. [6] ASTM G 59: Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements. Annual Book of ASTM Standards, Vol , Metal Corrosion, 2001, pp Acknowledgements Authors acknowledge the aid of project RMTVC No.Cz 1.05/2.1.00/