Journal of Magnetism and Magnetic Materials 248 (2002) 121–133 Corrosion behaviour of hot-pressed and die-upset nanocrystalline NdFeB-based magnets A.A. El-Moneim a,c , O. Gutfleisch a , A. Plotnikov b , A. Gebert a, * a Institute for Solid State and Materials Research, IFW Dresden, P.O. Box 270016, D-01171 Dresden, Germany b Institute for Solid State Analysis and Structural Research, IFW Dresden, Germany c Physical Chemistry Department, National Research Center, Cairo, Egypt Received 13 December 2001; received in revised form 14 March 2002 Abstract Isotropic and anisotropic nanocrystalline Nd 14 Fe 80 B 6 and Nd 12 Dy 2 Fe 73.2 Co 6.6 Ga 0.6 B 5.6 magnets have been produced from melt-spun materials by hot pressing and subsequent die-upsetting. The microstructure has been characterized using XRD, scanning electron microscope and energy dispersive X-ray analysis. The corrosion behaviour of die-upset NdFeB-based magnets has been studied in 0.1M H 2 SO 4 by inductively coupled plasma solution analysis and electrochemical polarization techniques and compared with their hot-pressed counterparts. Texturing of hot-pressed (isotropic) NdFeB-based magnets via die-upsetting significantly modifies their corrosion performance. Textured Nd 12 Dy 2 Fe 73.2 Co 6.6 Ga 0.6 B 5.6 magnets exhibit the highest corrosion resistance in this study. The low effective diffusivity of corrosion hydrogen inside the bulk magnet and the reduction in the strength of galvanic coupling between magnet phases are the main reasons for the observed improvement in the corrosion resistance. The corrosion behaviour of the magnets in relation to their phase composition and phase distribution is discussed in terms of dissolution, hydrogenation and pulverization. Pulverization trends are correlated with hydrides formation and hydrogen-trapping sites using thermal desorption analysis. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.50.W; 82.30.R; 81.65.K; 82.45 Keywords: Nanocrystalline magnet; NdFeB; Die-upsetting; Hydrogen; Corrosion 1. Introduction At present, NdFeB alloys are commercially used for permanent magnetic materials exhibiting the highest maximum energy density. In order to exploit the excellent potential of the Nd 2 Fe 14 B compound (hard magnetic phase), c-axis align- ment (i.e. texturing) is needed. The two most common processes used for producing anisotropic or textured NdFeB magnets with high energy density are: (a) conventional powder metallurgy route (sintering) [1] and (b) hot pressing with subsequent hot deformation of nanocrystalline powders [2]. In the former method, crystal align- ment is achieved by applying a magnetic field during the cold-pressing stage. The latter employs fine-grained powders produced by using non- equilibrium alloying techniques such as melt *Corresponding author. Tel.: +49-351-4659-275; fax: +49- 351-4659-320. E-mail address: gebert@ifw-dresden.de (A. Gebert). 0304-8853/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0304-8853(02)00286-X