Investigations of the corrosion behaviour of nanocrystalline Nd–Fe–B hot pressed magnets

doi:10.1016/S0925-8388(00)01112-9

Journal of Alloys and Compounds

Volume 311, Issue 2, 26 October 2000, Pages 299-304

https://doi.org/10.1016/S0925-8388(00)01112-9 Get rights and content

Abstract

The corrosion behaviour of Nd–Fe–B nanocrystalline magnets made from differently processed powders such as melt spun, intensively milled and HDDR (Hydrogenation–Disproportionation–Desorption–Recombination) powders has been investigated in sulphuric acid solution and in air at 25°C. It is observed that the corrosion resistance of the investigated magnets can be correlated with the grain size of the hardmagnetic phase and the distribution and composition of the Nd-rich intergranular phases. The magnet made from HDDR powder exhibits a comparable, if not better, corrosion resistance in acid solution than magnets made from melt spun and intensively milled powders. Small additions of Co, Al and Ga improved the corrosion resistance of the investigated magnets by replacing the high corrosion sensitive intergranular Nd-rich phases with more noble phases. The surface layer formed during anodic polarization of the HDDR magnet was analysed by Auger electron spectroscopy indicating the formation of (Nd,Fe)-oxide with small amounts of Co and Al. The measured electrostatic surface potential differences of magnets containing Co, Al and Ga were lower than those of magnets without alloying additives.

Introduction

Nd–Fe–B-based permanent magnets exhibit excellent magnetic properties, but they suffer strongly from corrosion in many aggressive environments, such as acidic and salt solutions at ambient temperature and humid air [1], [2]. The low corrosion resistance of Nd–Fe–B-based magnets is attributed mainly to the presence of multiple phases in their microstructure and the large electrochemical potential differences between them [3], resulting in the preferential dissolution of the Nd-rich and B-rich intergranular phases and subsequent breaking off of the ferromagnetic grains [4], [5]. The detrimental effect of hydrogen in the corrosion process of Nd–Fe–B magnets has been discussed, i.e. the reactivity of these alloys with hydrogen and its easy absorption, which makes the application of a cathodic protection impossible [6], [7]. The effect of alloying additions on magnetic properties and corrosion behaviour has been the subject of many investigations. Improved corrosion resistance of sintered Nd–Fe–B-based magnets is achieved by small additions of alloying elements such as Al, Co and Cr [8], [9], [10], [11]. According to Fidler [12] there are two types of dopant elements: Type M₁=(Al, Cu, Zn, Ga, Ge, Sn) and M₂=(V, Mo, W, Nb, Ti, Zr); the former forming Nd–M₁ or Nd–Fe–M₁ intergranular phases, the latter forming M₂–B or Fe–M₂–B intergranular phases. Because of their more positive corrosion potentials compared to those of additive-free phases, the new phases which are formed at grain boundaries inhibit dissolution of intergranular regions. However, these alloy modifications improve the corrosion resistance only to a certain degree. Presently, various attempts are directed to optimise the various preparation techniques, such as the powder metallurgical sintering process [13] leading to microcrystalline materials and the melt spinning route [14], the intensive milling process [15], [16] and HDDR (Hydrogenation–Disproportionation–Desorption–Recombination) leading to nanocrystalline materials [17], [18]. The nanoscale powders are of growing importance, especially for the production of bonded and fully dense hot pressed magnets. These magnets have different corrosion behaviour compared to sintered magnets resulting from the differences in their microstructure, i.e. grain sizes and phase distributions.

In the present work the corrosion behaviour of the hot pressed nanocrystalline Nd–Fe–B magnets prepared by HDDR processing, melt spinning and intensive milling as well as the effect of alloying additions such as Co, Al and Ga have been investigated in acid solution.

Section snippets

Experimental

Fully dense, magnetically isotropic, Nd–Fe–B magnets partly with small additions were prepared by hot pressing of powders obtained by different processing routes: intensive milling, melt spinning and HDDR. The preparation parameters are described elsewhere [19]. The chemical compositions of the investigated materials are listed in Table 1. The magnets were cut with a diamond disc into specimens with surface areas of 0.1 cm² for electrochemical tests and 1 cm² for gravimetric tests.

The

Microstructural investigations

The SEM micrographs (backscattered mode) of the Nd–Fe–B nanocrystalline magnets (without additives) are shown in Fig. 1(a). The regions with white contrast were identified by EDX analysis as the Nd-rich phase and the grey regions represent the ferromagnetic phase. The black areas are holes, which appear during surface finishing. The microstructures of the different types of magnets reveal significant differences especially with respect to the distribution of intergranular phases. The Nd-rich

Conclusions

The corrosion behaviour of different nanocrystalline Nd–Fe–B magnets with and without the addition of Co, Al and Ga has been determined using gravimetric and electrochemical techniques, accompanied by SEM, SPM and AES investigations. Based on the results obtained from our present investigation, the following conclusions can be drawn:

1.
The process of preparation of the magnets has a significant influence on the corrosion behaviour. The magnets made from HDDR powder are in terms of their corrosion

Acknowledgements

The authors acknowledge helpful discussions with K. Mummert. The authors would like to thank G. Barkleit, A. Güth and A. John for carrying out SPM, SEM and AES investigations and V. Panchanathan for supplying the melt spun powders.

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Investigations of the corrosion behaviour of nanocrystalline Nd–Fe–B hot pressed magnets

Abstract

Introduction

Section snippets

Experimental

Microstructural investigations

Conclusions

Acknowledgements

J. Magn. Mater.

J. Alloys Comp.

Corros. Sci.

J. Alloys Comp.

Intermetallics

J. Less-Common Met.

J. Magn. Magn. Mater.

Mater. Sci. Eng.

J. Appl. Phys.

Mater. Corr.