JOURNAL OF APPLIED ELECTROCHEMISTRY 23 (1993) 1017-1024 Effect of impurities on the corrosion behaviour of neodymium H. BALA*, S. SZYMURA *, G. PAWLOWSKA* Department of Chemistry* and Institute of Physics ~, Polytechnic of Czestochowa, Al.Armii Krajowej 19, PL 42-200 Czestochowa, Poland YU. M. RABINOVICH Research and Technology Enterprise MIRT, Gospitalnyi Val 5-18 kv.172 105094 Moscow, Russia Received 13 April 1992; revised 2 February 1993 The corrosion behaviour of neodymium contaminated with up to 0.9% C, up to 15% Fe and about 16% of Tb or Dy was examined in various aggressive environments. It was found that in acid media the C and Fe additions accelerated the corrosion process of the alloys. The Nd-base alloys were fairly resistant to corrosion in atmospheric conditions, however, the impurities worsened the corrosion behaviour in this environment as compared with pure Nd. 1. Introduction In the early 1980s wide application of new high-per- formance RE-Fe-B type magnets (where RE = Nd, more rarely Pr or Dy or their compositions) began in many countries. The magnets exhibit excellent mag- netic properties, however, they suffer strongly from great sensitivity to corrosion in many aggressive environments. This poor resistance to corrosion results from the presence of relatively high amounts of rare earth elements (30-35wt %). The rare earth elements are among the most electrochemically active metals, the standard potentials for RE/RE 3+ systems being -2.6 to -2.0V [1]. The corrosion mechanism of Nd-Fe-B type magnets consists of the preferential oxidation (dis- solution) of a Nd-rich phase (chemical formula Nd4Fe), situated between the grains of a ferro- magnetic Nd2Fel4B phase. This causes surface degradation, a worsening of magnetic properties, and sometimes reduces to pulverization. The corrosion mechanism of RE-containing permanent magnets is discussed in [2-6]. The susceptibility of the Nd-rich phase to corrosion can be reduced by such alloying additions as Co (about 5 at %) [4, 7] or 1 at % of Cr, A1, Ti and Zr [6, 7]; these additions also improve the magnetic properties of the alloys. In permanent magnet technology a high grade purity neodymium is used, which increases the price of the magnet [8]. The separation and purification of RE elements is a complex process. Especially un- desirable impurities of RE elements are considered to be mainly O and C [9] because these impurities degrade the coercive force of the RE-Fe-B mag- nets. On the other hand, the presence of decimal percentages of carbon in the alloy causes a strong enrichment of the Nd-rich phase (during sintering inclusions of NdC2 carbide appear [9]), which can have significant consequences for the corrosion behaviour. Taking into account that the corrosion resistance of sintered Nd-Fe-B type magnets is determined mainly by the electrochemical activity of the Nd-rich phase (containing 75-85wt% Nd), it is important to know the effect of technological impurities on the corrosion behaviour of RE alloys. There is little literature concerned with the prob- lem of corrosion behaviour of pure RE metals and their alloys. In [10, 11] the corrosion characteris- tics of pure (99.9%) RE metals (apart from Pm) are presented, however, they refer mainly to the metal's behaviour in atmospheric conditions and at elevated temperatures. Rybakov et al. [12, 13] noticed the important role of air pollutants (SO2, H20, CO2) on the corrosion of pure metals: La (99.9%) and Ce (99.8%). All cited authors believe that the corrosiveness of air rises sharply with an increase in relative humidity. Lee and Greene [11] consider RE-oxides to have protective properties whereas hydroxides (formed on the surface in the presence of humidity in the air) do not protect the surface of the RE-metal against further corrosion attack. To characterize the corrosion behaviour of RE metals in acid media, it is important to consider the possibility of the creation of RE hydrides as products of the corrosion process. The great affinity of RE metals to hydrogen is commonly known. Gaseous hydrogen reacts with RE elements at room tempera- ture. In the case of Nd-Fe-B magnets, the creation of hydrides in both the ferromagnetic and Nd-rich phases was found. As a result, non-stoichiometric products, with formulae Nd2FelaBH3. 3 and NdH28, respectively, are formed [14]. Absorption of gaseous 0021-891X © 1993 Chapman & Hall 1017