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submitted as paper BG-05 to the 8 th Joint MMM-Intermag Conference, San Antonio, Texas, January 3:1 7-11, 2001 2 2 0 -2 -4 -6 -8 RE = 32 wt.% no Co RE = 30.8 wt. % Co = 3.1 % RE = 31.3 wt.% Co = 3.1 wt.% -10 Fig. 1..Weight change of RE-Fe-Co-Cu-Ga-Al-B magnets in a HAST test (130°C, 95 0 % humidity, no 50 condensation of 100 water vapour) 150 200 exposure time (days) (Nd,Fe)-hydroxides, s. Fig. 1. The corrosion sequence of uncoated RE-Fe-Co-Cu-Al-Ga- B magnets under HAST conditions (no dew) is the following: After some days, a few small chips (50 – 200 µm) will break off from sharp edges. 1. Simultaniously, small, partly removeable stains consisting of RE-rich platelets appear (diameter up to 10 µm, thickness a few µm), there is no substantial weight loss during this period. 2. After 40 – 200 days, material in form of loose powder is lost, initially from the edges. The corrosion rate is accelerated by a factor of about 10 by performing the stress test at 130°C and a humidty of 100 % allowing condensation of water (pressure cooker test). But the sequence is the same as without condensation: after an incubation time of about 10 days with slight weight increase, the material loss again starts at the edges of the magnet. Under salt spray conditions according to DIN 50021 all investigated magnets are covered with red rust after 24 h, independent of wether they contain Co or not. Under such environments Cocontaining magnets must be coated as well. The corrosion behaviour is similar to pure Fe. B. Electrochemical Potentials and Corrosion Mechanism For a certain range of composition, the intergranular constitutents of Nd-Dy-Fe-Co-Cu-Al-Ga-B magnets are only Nd3(Co,Cu), Nd5(Co,Cu,Ga)3, Nd6(Fe,Co)13Ga and unavoidable impurites like RE oxides. In particular, no pure Nd and also no Nd1.1Fe4B4 or Nd2Fe17 is observed, see Fig. 2. Table 1 shows the electrochemical potentials of various Nd- Fe-B magnets, related compounds and of the most important corrosion protective coatings for this material (electronickel, IVD-aluminium) measured in reference to an Ag/AgClelectrode. The pure rare earth metals Nd, Pr, and Dy represent the intergranular phase (> 95 wt.% rare earth) in Co/Cu-free magnets. The three phases with Co, Cu and Ga are the intergranular phases in Co/Cu/Ga-containing materials. Magnets A, B and C represent the matrix phases of different magnet alloy types with and without Co,Cu and Ga. 3:1 5:3 3:1 3:1 6:13:1 Fig. 2. Microstructure of a Nd-Dy-Fe-Co-Cu-Al-Ga-B magnet with a RE content of 37.3 wt.%. The Nd3(Co,Cu), Nd5(Co,Cu,Ga)3 and the Nd6(Fe,Co)13Ga phases are denoted by 3:1, 5:3 and 6:13:1, respectively. TABLE I COMPOSITIONS AND ELECTROCHEMICAL POTENTIALS OF VARIOUS ND-FE-B MAGNETS AND RELATED COMPOUNDS MEASURED IN ARTIFICAL SEA WATER WITH RESPECT TO AN AG/AGCL SATURATED KCL REFERENCE ELECTRODE sample Nd Dy Fe Co Cu Ga potential (all elements in wt.%) (mV) magnet A 30 0.5 bal. / / / -730 magnet B 24.5 8.5 bal. / / / -670 magnet C 22 9 bal. 3 0.2 0.3 -700 Nd 3(Co,Cu) bal. / 1.8 10.5 1.9 / -900 Nd 5(Co,Cu,Ga) 3 bal. / 3.1 4.5 3.3 10.1 -807 Nd 6(Fe,Co) 13Ga bal. 2.5 43.3 2.0 / 3.4 -750 Pr -1689 Nd -1670 Dy -1645 Al -720 Ni -130 The electrochemical potential difference between the individual phases of an alloy is the driving force for corrosion reactions. The bigger the potential difference between the matrix and the intergranular phase of a magnet the higher is the corrosion rate. In humid atmosphere, the following reactions (1) – (3) can occur with Nd-Fe-B magnets. These reactions result in a fast decomposition of the intergranular phase by attack of water vapour at higher temperatures (e.g. autoclave test at 130°C / 100 % humidity, 2.7 bar). 2 Nd + 3 H2O � NdH3 + Nd(OH)3 (1) NdH3 + 3 H2O � Nd(OH)3 + 3 H2 (2) Nd + 3/2 H2 � NdH3 (3) Equation (1) and (2) demonstrate the reaction between the intergranular phase and water. The products of these reactions are Nd-hydroxide and Hydrogen which reacts according to equation (3) with the Nd-rich intergranular phase to form Ndhydride. Nd-hydride is not stable under these conditions and decomposes again to Nd-hydroxide and Hydrogen (2) and the whole sequence starts again. This means every water molecule which reacts with the intergranular phase accelerates the conversion of Neodymium to Nd-hydroxide. The effect of this conversion is a strong volume increase of the intergranular phase what results in cracks along the grainboundaries. A
submitted as paper BG-05 to the 8 th Joint MMM-Intermag Conference, San Antonio, Texas, January 7-11, 2001 3 complete decomposition to a magnet powder (”white corrosion”) occurs. The starting reaction (1) is controlled by the electrochemical potential of the intergranular phase. The matrix phase which is more noble does not react significantly with water under these conditions. Therefore, the way to avoid this type of corrosion is to shift the electrochemical potential of the intergranuler phase close to that of the matrix phase. This can be achieved by adding more noble metals to the intergranular phase, such as Co or Cu. By these additions, the electrochemical potential increases from about – 1600 mV to – 800 mV. The result is that the corrosion rate of the magnet material in the autoclave test decreases by a factor of 1000. Selective intergranular corrosion and powder decomposition is strongly slowed down. A similar behaviour is observed with coated magnets. The corrosion rates in all type of tests drop significantly. C. Magnetic Properties Additionally, the remanence as well as the coercivity of the new Co-containing magnets were increased compared to conventional sintered Nd-Fe-B magnets. Due to the cobalt content of about 3 wt.% and an optimized microstructure, the temperature coefficients of the remanence and of the coercivity were improved by about 20 % and 10 %, respectively. Tailoring the Dy-content, typical energy densities, remanences and coercivities from (BH)max = 44 – 28 MGOe, Br = 1,35 – 1,08 T and HcJ = 18 – 33 kOe are achieved in mass production. IV. CONCLUSIONS The corrosion mechanism in hot and humid climates for conventional magnets as well as for magnets containing Co, Cu, Al and Ga is determined by the formation of Ndhydroxide and free hydrogen. The hydrogen embrittles the remaining intergranular phase and together with the volume expansion caused by the formation of the Nd-hydroxide, matrix grains will peel off. This reaction is much slower in magnets containing Co, Cu, Al and Ga. In these magnets the intergranular constituents consists of Nd3(Co,Cu), Nd5(Co,Cu,Ga)3 and Nd6(Fe,Co)13Ga compounds, all of them are much more noble compared to the pure Nd present in standard Nd-Fe-B magnets. Therefore the elcectrochemical driving force to form Nd-hydroxide is smaller in the new magnets, resulting in a reduction of the corrosion rate by a factor of about 1000. In water and particularly under salt spray conditions, the matrix grains themselves are attacked to form red rust. Here, the better corrosion stability of the intergranular constituents has no influence on the overall corrosionstability. Under such environments all Nd-Fe-B magnets must be coated irrespective of wether they contain Co or not. Besides the improved corrosion resistance, the magnetic properties were also enhancend, especially at elevated temperatures. Therefore, the new Nd-Fe-B magnets containing Co, Cu, Al and Ga are suited particularly for motor applications. ACKNOWLEDGMENT The authors greatly acknowledge microstructural investigations by P. Schrey and electrochemical measurements and corrosion tests which have been performed by G. Rodner. Many thanks for R. Mottram for proof-reading the manuscript. REFERENCES [1] A. S. Kim and F. E. Camp, ”High performance NdFeB magnets,” J. Appl. Phys. vol. 79, 1996, pp. 5035-5039. [2] T. Kajitani, K. Nagayama, and T. Umeda, ”Microstructure of Cu-added Pr-Fe-B magnets: crystallization of antiferromagnetic Pr6Fe13Cu in the boundary region,” J. Magn. Magn. Mat, vol. 117, 1992, p. 379-86. [3] M. Tokunaga, ”Magnetic properties and thermal stability of Nd-Fe-B sintered magnets and their microstructure,” Proc. of the MRS International Meeting on Advanced Materials, vol. 11, Mater. Res. Soc: Pittsburgh, 1989, p.53-67. [4] W. Fernengel, M. Velicescu, W. Rodewald, P. Schrey, and B. Wall, ”Magnetic properties and microstructure of high-energy sintered (Nd,Dy)-(Fe,Co,Cu)-B magnets,” Proc. 3 rd ISPMM Conference, Seoul, 1995, p. 748. [5] K. Ohashi, Y. Tawara, T. Yokoyama, and N. Kobayashi, ”Corrosion resistance of cobalt-containing Nd-Fe-Co-B magnets,” Proc. 9 th Int. Workshop on Rare Earth Magnets and their Applications, 1987, p. 355- 61. [6] B. Grieb, ”New corrosion resistant materials based on neodym-ironboron,” IEEE Trans. Magn., vol. 33, 1997, p. 3904-6. [7] M. Katter, ”Sintered NdFeB magnets with improved temperature and corrosion stability,” The U.K. Magnetics Society, one-day seminar on Recent Advances in Permanent Magnets: Processing and Applications, 6. Okt. 1999, University of Birmingham. [8] S. Hirosawa, S. Mino, and H. Tomizawa, ”Improved corrosion resistance and magnetic properties of Nd-Fe-B-type sintered magnets with Mo and Co,” J. Appl. Phys., vol. 69, 1991, p 5844-6. [9] G.W. Warren, G. Gao, and Q. Li, ”Corrosion of NdFeB permanent magnet materials,” J. Appl. Phys., vol. 70, 1991, p. 6609-11. [10] A. M. El-Aziz, G. Barkleit, M. Herrich, K. Mummert, L. Schultz, and B. Grieb, ”Corrosion behaviour of Nd-Fe-B permanent magnetic alloys,” Proc. 15 th Intern. Workshop on Rare Earth Magnets and their Applications, 1998, p. 905-13. [11] K. Mummert, A.M. El-Aziz, G. Barkleit, W. Rodewald, and L. Schultz, ”Corrosion behaviour of Nd-Fe-B permanent magnets,” Materials and Corrosion, vol. 51, 2000, p. 13-19.
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