Cold-Sprayed Al Coating for Corrosion Protection of Sintered NdFeB
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JTTEE5 23:456–462 DOI: 10.1007/s11666-013-9994-8 1059-9630/$19.00 Ó ASM International
Cold-Sprayed Al Coating for Corrosion Protection of Sintered NdFeB Chunchun Ma, Xiaofang Liu, and Chungen Zhou (Submitted May 20, 2013; in revised form August 27, 2013) A protective Al coating was achieved on the sintered NdFeB magnet by cold spray. The sprayed Al particles generate plastic deformation and hang together. The thickness of the coating is about 170 lm. The corrosion currents of Al coating and NdFeB without immersion tested by potentiodynamic polarization in 3.5 wt.% NaCl solutions are 1.350 3 1026 and 4.361 3 1026 A/cm2, respectively. X-ray photoelectron spectrometry results confirm that the oxide film is Al2O3 and the corrosion process can be derived into two different stages. The Al coating can provide long-term protection for NdFeB effectively.
Keywords
cold spray, EIS, metal coatings, passive films, pitting corrosion
1. Introduction As a new generation of permanent magnetic material with excellent properties, the sintered neodymium-ironboron (NdFeB) has been widely used in various fields such as electronics, acoustics, automation, communications, and magnetic resonance imaging (Ref 1). However, the presence of the rare earth Nd in the magnet sharply decreases its corrosion resistance, which has seriously hindered and limited its applications in humid conditions (Ref 2, 3). Therefore, it is very significant to improve the corrosion resistance of the NdFeB magnet. In order to improve the corrosion resistance of the permanent magnets, numerous attempts have been employed, such as the addition of alloying elements (Ref 4, 5) and surface coating (Ref 6-8). Previous works (Ref 9-11) have demonstrated that alloying elements such as Co, Cu, Al, Dy, Nb can improve the corrosion resistance of NdFeB, but deteriorate the magnetic properties of the NdFeB magnet. At present, the surface coating technologies are industrially used by the process of electroplating for its good performance and low processing costs (Ref 2, 12, 13), including Ni, Ni-Cu-Ni, Ni/Al, Ni-Co-TiO2, etc. However, the electroplating method may lead to some environment problems (Ref 14). Al coating is one of the potential candidates for protection because of its friendly price and good corrosion resistance (Ref 15). In industry, Al coatings deposited by evaporation (Ref 16) and ion vapor deposition (IVD) (Ref 6) have been applied for the protection of NdFeB. However, the Al coatings prepared Chunchun Ma, Xiaofang Liu, and Chungen Zhou, School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China. Contact e-mail: [email protected].
456—Volume 23(3) February 2014
by evaporation and IVD always present a columnar structure with a high concentration of inter-column defects, which could result in premature failure of the coatings (Ref 17). As an emerging technique for depositing coatings in the solid state, the cold spray (CS) process has attracted more and more attention. Compared with thermal spray
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