Download - VACUUMSCHMELZE GmbH & Co. KG

submitted as paper BG-05 to the 8 th Joint MMM-Intermag Conference, San Antonio, Texas, January 7-11, 2001 1
Corrosion Mechanism of
RE-Fe-Co-Cu-Ga-Al-B Magnets
Abstract—The corrosion stability of Nd-Fe-B magnets in hot
and humid climates was increased by about three orders of
magnitude by simultaneous additions of Co, Cu, Al and Ga. Such
magnets show almost no weight loss in a highly accelerated stress
test at 130°C (95 % humidity) even after 200 days of exposure. In
these magnets the very reactive Nd-rich intergranular
constituents are replaced by Nd 3(Co,Cu), Nd 5(Co,Cu,Ga) 3 and
Nd 6(Fe,Co) 13Ga compounds which are much more noble. The
free electrochemical potentials of almost single phase samples of
these compounds were determined to be about 800 mV higher
compared to pure Nd. Therefore, the formation of hydroxides
and thus the corrosion rate is much slower.
Index Terms—corrosion, electrochemical potentials, highly
accelerated stress test, Nd-Fe-B magnets
P
I. INTRODUCTION
ERMANENT magnets based on Nd-Fe-B are widely used for
actuators, motors, MRI systems, sensors etc. Due to their
high energy densities and lower costs, they replaced Sm-Co
magnets in many applications. However, in hot and humid
climates – typical operating conditions for motors – they
suffer from irreversible flux losses and their poor corrosion
resistance [1]. The temperature stability of Nd-Fe-B magnets
can be improved by partial substitution of Nd by Dy, which
raises the coercive field strength, and by the addition of Co,
leading to an increase of the Curie temperature. On the other
hand Co decreases the coercivity, which can be compensated
by the simultaneous addition of Al, Cu and Ga [2,3]. Due to
these additions, the conventional Nd-rich intergranular phase
is replaced by Nd3(Co,Cu), Nd5(Co,Cu,Ga)3 or
Nd6(Fe,Co)13Ga compounds [3-5]. It has been demonstrated
that such magnets have a much better corrosion stability in hot
and humid climates [6,7].
The corrosion mechanism in hot and humid climates is
determined by the reaction of the Nd-rich phase with H2O
forming Nd(OH)3, causing decohesion of the magnetic grains.
The speed of this reaction, which will be determined by the
electrochemical properties, is much lower in the case of
Nd3Co [8]. To date, there have not been many investigations
on the electrochemical behaviour of Nd-Fe-B magnets [9,10].
In particular, information on the electrochemical properties of
the individual phases is rare [11]. The phenomenological
corrosion behaviour of commercial Nd-Dy-Fe-Co-Cu-Al-Ga-
B magnets in various climates has been investigated. In
Manuscript received Oct. 11, 2000.
All authors are with Vacuumschmelze GmbH & Co KG, 63412 Hanau,
Germany (telephone: +49 6181 38 2083, e-mail:
Matthias.Katter@Vacuumschmelze.com).
M. Katter, L. Zapf, R. Blank, W. Fernengel, W. Rodewald
addition, the electrochemical potentials of almost single phase
samples of the Nd3(Co,Cu),
Nd5(Co,Cu,Ga)3 and
Nd6(Fe,Co)13Ga phase will be reported.
II. EXPERIMENTAL
Commercial Nd-Dy-Fe-Co-Cu-Al-Ga-B magnets with a
total RE content of 30.5 to 32 wt.% have been produced by
the standard powder metallurgical process. The corrosion
behaviour of these magnets has been tested in an autoclave at
130°C at either 95 % rel. humidity (without condensation of
water vapour) or at 100 % humidity (in water under sand).
Some magnets have also been exposed to a salt spray test.
The actual compositions of the intergranular phases were
determined by EDX on RE-rich magnets sintered at 980°C
(composition of the magnet: 37.3 wt.% RE, 3.5 wt.% Co, 0.33
wt.% Cu, 0.5 wt.% Ga and 0.73 wt.% B). Three ingots with
the compositions of the Nd6(Fe,Co)13Ga, Nd5(Co,Cu,Ga)3 and
the Nd3(Co,Cu) phase were prepared by induction melting.
These were subsequently homogenized at 700°C, 500°C and
460°C for 50 h, respectively. Standard metallography revealed
that the ingots consist of at least 85 % of the target phase.
The free electrochemical potential for these samples, as
well as for some reference elements and magnets, were
measured with a potentiostat, model Radiometer PGP 201. All
experiments were performed in an electrochemical cell
consisting of a working electrode, represented by the sample
(rotating disc electrode /1000 rpm/ A=0.28cm²), an Ag/AgCl
saturated KCl reference electrode with a potential value of
198.9 mV measured versus the Normal Hydrogen Electrode at
a temperature of 25°C, and a platinium counter electrode. A
synthetic sea water solution according to DIN 50900 was used
as the electrolyte at a temperature of 25°C. The potentials
were monitored until a constant value was achieved (at the
longest 10 minutes).
III. RESULTS AND DISCUSSION
A. Corrosion Behaviour of Commercial Magnets
In hot and humid climates, conventional Nd-Fe-B magnets
without Co will corrode heavily. In a highly accelerated stress
test (HAST) at 130°C and 95% humidity, such magnets will
loose more than 10 mg/cm 2 after a few days, see Fig. 1. For
magnets with about 3 wt.% Co and some Cu, Ga, Al this
weight loss is strongly reduced. Depending on the RE content,
these magnets show no weight loss at all after a period 200
days. The weight of these magnets even increases sligthly due
to the formation of a passivating surface layer, probably some

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.