Impeding corrosion of sintered NdFeB magnets with titanium nitride coating

130 DOI: 10.1002/maco.200905265 Materials and Corrosion 2010, 61, No. 2

Impeding corrosion of sintered NdFeB magnets withtitanium nitride coating

A. Ali*, A. Ahmad and K. M. Deen

Sintered NdFeB magnets have poor corrosion resistance that renders them

susceptible to corrosion in industrial and marine environments. This paper

evaluates the properties of cathodic arc physical vapour deposited (CAPVD)

titanium nitride coating for corrosion protection of sintered NdFeB permanent

magnets.The performance of titanium nitride coating has been compared to the

electrodeposited nickel–copper–nickel multilayer coating. The rates of coatings

degradation in simulated marine environment were estimated with

electrochemical impedance spectroscopy (EIS). Cyclic polarization was carried

out to assess the pitting potential. The surface chemistry and coating

morphologies were studied with scanning electron microscope (SEM). X-ray

diffraction (XRD) was used for qualitative phase analyses of coatings and the

substrate. It was figured out that the charge transfer resistance of CAPVD

titanium nitride coating increased with exposure time. The negative rate of Rp-

degradation for titanium nitride coating compared to the nickel–copper–nickel

multilayer for equivalent exposure time is a unique and valuable result.

Polarization results showed that ‘pits re-passivation’ of titanium nitride coating

could be responsible for the extended corrosion protection of the NdFeB

substrate. The magnetic properties remained comparable for both types of

coatings.

1 Introduction

The NdFeB permanent magnets were discovered in 1983 and

emerged as important technological materials due to their

excellent magnetic properties [1–3]. These magnets have the

highest energy product at ambient temperature and can use the

electrical energy very efficiently. Presently, these magnets occupy

a leading position amongst the strong permanent magnetic

materials for a variety of engineering applications including

computer peripherals, magnetic resonance, acoustics, biomedi-

cal, automation and consumer electronics [4, 5]. However, the

sintered NdFeB magnets have poor corrosion resistance in

various environments [6, 7] due to their complex microstructure

[8] thus declining in efficiency as they corrode.

Two-dimensional efforts have been made to control the

corrosion of sintered NdFeB magnets, that is through alloying to

A. Ali, A. Ahmad

Department of Metallurgical & Materials Engineering, University of

Engineering & Technology, Lahore 54890 (Pakistan)

E-mail: [email protected]

K. M. Deen

Department of Metallurgy and Materials Engineering, CEET, University

of Punjab, Lahore, 54590 (Pakistan)

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

modify the electrochemical potential of active phases in the

microstructure [9–12] and through surface engineering by

applying sacrificial or barrier coatings like metallic or epoxy

coatings [13–15]. Recently, an attempt has also been made to

establish the ceramic coatings by cathodic arc physical vapour

deposition (CAPVD) for corrosion protection of sintered NdFeB

magnets [16]. The CAPVD process involves the ejection of metal

vapours from the solid metal at ambient temperature by plasma

assisted high energy cathodic arc in an evacuated chamber. The

vapour flux consisting of metallic ions accelerates towards the

substrate with kinetic energies ranging from 10 to 100 eV, under

applied potential gradient. Methane or nitrogen is introduced in

the chamber that reacts with the metallic ions to form carbides or

nitrides. The vapour-deposited transition metal nitride coatings

normally havemicroscopic defects such as pores, pinholes and/or

voids [17, 18] thereby enabling the environmental species to

access the substrate surface. The density of such permeable

defects can be reduced by depositing thicker coatings [19, 20], by

interrupting columnar growth with multilayered coatings [21], by

incorporating noble and dense interlayer [22] or by post-

deposition sealing of defects with polymeric layer [23].

The present work aims at improving the performance of

CAPVD titanium nitride coating for corrosion protection of

NdFeB permanent magnets. It also aims at understanding the

degradation mechanism of the coating/substrate system during

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Materials and Corrosion 2010, 61, No. 2 Impeding corrosion of NdFeB magnets 131

exposure to simulated marine environment by electrochemical

impedance spectroscopy (EIS). The properties of the CAPVD

titanium-nitride/NdFeB-magnet system were compared to elec-

trodeposited nickel–copper–-nickel/NdFeB-magnet system.

2 Experimental

2.1 Material

Powder metallurgically produced commercial sintered NdFeB

magnets were used as substrate material in this work. The

hydrostatic density of the sintered NdFeBmagnets was 7.58 g/cm3

whereas their theoretical density is 7.60 g/cm3. Table 1 gives the

chemical composition of the magnets, determined through wet

analysis as EDS cannot detect the low energies associated with the

electronic transitions of low atomic number elements like boron.

2.2 Sample preparation

Disc-shaped NdFeB magnets with 12 mm diameter and 2.5 mm

thickness were metallographically polished with 1.0 mm alumina

suspension and ultrasonically cleaned in ultrasonic soap solution

at ambient temperature. For metallographic examination the

samples were etched in 2% Nital.

2.3 Set up for CAPVD coating

The polished and cleaned substrates were mounted on a rotary

sample holder at an angle of 308 with normal to the cathode. The

coating set up consisted of a disc-shaped titanium target (cathode)

mountedonwater-cooledcopper stage.Thedisc-shaped anodewas

fixed at the top of the target with a perpendicular distance of

300 mm. The whole assembly was enclosed in a double walled

stainless steel chamber.After evacuation, the chamberwas cleaned

with thehollowcathodeargonplasmadischarge.Groundedcopper

wire was used to trigger the 1.6 kVA arc. The arc was operated

continuously for 30 min. Nitrogen was then introduced in the

chamber with controlled partial pressure through automatic

microprocessor controlled feeding system. Different combina-

tions of coating time and bias voltage were used to improve the

coating thickness and minimize the permeable defects density

while other parameters persisted as reported earlier [16].

2.4 Electrodeposition of Ni–Cu–Ni multilayer

The Watt’s solution and copper sulphate bath was used for the

electrolytic deposition of nickel and copper, respectively. The bath

composition and coating parameters have already been

reported [16]. Initially a nickel strike layer was deposited, then

the copper interlayer followed finally by nickel layer. The surfaces

were buffed before intermediate shifting between the baths, i.e.

Watt’s solution to copper sulphate bath and again to Watt’s

solution.

Table 1. Chemical composition of sintered NdFeB magnets

Element B Nd Fe

Wt% 1.26� 0.14 32.90� 0.35 Bal

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2.5 Characterization

The coated samples were gently cleaned with muslin cloth and

fixed in the specimen holder for exposure to aerated 3.5% NaCl

aqueous solution. The working electrode was NdFeB substrate

whereas a graphite rod was used as a counter electrode and

Ag/AgCl as a reference electrode. Gamry Potentiostat and Echem

Analyst software was used to measure and plot the ac impedance

values of the coatings. The EG&G Potentiostat 273 A was

employed for dc cyclic polarization measurements. The qualita-

tive phase analyses were carried out with Siemens D-500 X-ray

diffractometer using Fe filtered Co-ka radiations and Origin-5

graphic software. Jeol scanning electron microscope (SEM)

equipped with EDS analyser was used to study the back-scattered

electron images. The Riken Denshi B-H curve tracer was used to

measure the magnetic properties.

3 Results and discussion

In order to deposit a dense and thick titanium nitride coating with

minimum density of permeable defects, the bias voltage and

coating durations were varied systematically at a constant

nitrogen partial pressure. Figure 1 shows two curves of coating

thicknesses: one as a function of bias voltage and other as a

function of coating time. Maximum coating thicknesses were

obtained with 145 V bias voltage and 150 min coating time. The

intersection point of two curves corresponds to 155 V bias voltage

and 110 min coating time to get titanium nitride coating with

mean thickness ranging from 1.5 to 1.63 mm. Figure 2 shows the

X-ray diffraction patterns of samples with different coating

durations at a constant bias voltage and nitrogen partial pressure.

It is clear that 120 min coating time provides maximum surface

coverage as it has the lowest intensities of the substrate peaks and

maximum intensity of titanium nitride peak. These results

indicate that the coating time has a major role in completely

covering the substrate surface whereas the bias voltage has a

supporting role. Therefore, the optimum coating duration for

CAPVD titanium nitride coating on NdFeB magnets is 110–

120 min with bias voltage 120–140 V. The surface morphology of

Figure 1. Plot of coating thicknesses as a function of coating time and

coating bias voltage


132 Ali, Ahmad and Deen Materials and Corrosion 2010, 61, No. 2

Figure 2. X-ray diffraction patterns showing effect of coating duration on the peak intensities of sintered NdFeB magnet substrate and titanium

nitride coating

the coating and cross-sectional view for coating thickness

estimation in comparison with the Ni–Cu–Ni multilayer and

the un-coated NdFeB magnet are shown in Fig. 3. The mean

coating thickness values are 1.6 and 17 mm for titanium nitride

and nickel–copper–nickel multilayer respectively. The surface

morphology shows nodular as well as flaky deposits of titanium

nitride with voids and craters. The craters are most likely formed

as the nodules fall out due to stresses as they grow in size with the

passage of coating time. These voids and craters are permeable

defects in the coating that provide access to the substrate surface.

Figure 3. SEM back-scattered electron images of (a) sintered NdFeB subs

view of titanium nitride coating on sintered NdFeB magnet, (d) cross-sec


These craters are covered with the thinner layer of titanium

nitride and the environmental species can penetrate through

these defects thereby corroding the surface. It is important to

mention that the magnetic properties of sintered NdFeBmagnets

remained unaffected by the CAPVD titanium nitride coating, see

Table 2.

The NdFeBmagnets with optimized titanium nitride coating

were then subjected to EIS. The ac impedance measurements

were made at systematic time intervals for both types of coatings

to assess their rates of degradation in simulated marine

trate, (b) CAPVD titanium nitride deposited surface, (c) cross-sectional

tional view of Ni–Cu–Ni multilayer deposit on sintered NdFeB magnet

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Table 2. Magnetic properties of coated NdFeB magnets

Coating Br (Gauss) iHC (kOe) BHmax (MGOe)

Ti2N 1.36T 104 15.22 47.60

Ni-Cu-Ni 1.29T 104 15.30 42.18

environment. Figure 4 shows the Nyquist plot for NdFeB

magnets coated with Ni–Cu–NI multilayer and titanium nitride.

None of the curves intersected theZreal axis at the lower frequency

side in the measured frequency range. For Ni–Cu–Ni multilayer,

the size of the parabolic curves decreased with exposure time

indicating the decline of coating impedance with the exposure

Figure 4. Nyquist plot showing electrochemical impedances of two types

environment

Figure 5. Rate of change of polarization resistance as a function of expo

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time. On the other hand, the height of ac impedance arcs of

titanium nitride coating increased with exposure time, which is a

unique and extraordinary result. It means that the impedance of

the titanium nitride coating on NdFeB magnets in simulated

marine environment increased with exposure time and thus has a

negative rate of degradation. Figure 5 shows that the Rp or the

charge transfer resistance of Ni–Cu–Ni multilayer decreased

from 4.28 to 2.59 kohm cm2 during 24 h exposure. While the Rp

of titanium nitride coating increased from 1.68 to 3.18 kohm cm2

for the same duration of exposure. This exceptional behaviour

could be related to the formation/growth of a passive film or re-

passivation of pits/voids in the vapour deposited coating. To

support the evidence and understand the mechanism involved

of coatings at systematic durations of exposure in simulated marine

sure time for two types of coatings


134 Ali, Ahmad and Deen Materials and Corrosion 2010, 61, No. 2

Figure 6. Cyclic polarization plot for titanium nitride coated sintered NdFeB magnet in 3.5% NaCl solution at ambient temperature

the coated samples were subjected to cyclic polarization test. The

results showed that during forward potential scan, the anode

appeared to passivate initially but pitting started around�738mV

(EPit) and the current density increased to 70 mA/cm2 with a

slight increase in the potential, see Fig. 6. The reversed potential

scan showed re-passivation of pits around �778 mV. The narrow

hysteresis loop for the forwards and reversed potential scans and

EProt above the ECorr indicated that the vapour deposited coating

has the tendency of ‘pits re-passivation’. So this re-passivation of

coating defects appears to be the reason behind the exceptional

negative rate of degradation of the vapour deposited coating. It

means that the optimized coating parameters reduced the density

of defects in the vapour deposited titanium nitride coating but

retained some permeability in the form of craters and pits.

Corrosion initiates through these craters and pits that tend to re-

passivate during exposure to the marine environment and thus

improve the coating impedance with exposure time. It might also

be due to the clogging of voids/pits by the corrosion product.

Therefore, a post-deposition sealing treatment is being suggested

for further improvement of the corrosion resistance of CAPVD

titanium nitride coated sintered NdFeB magnets. The results of

post-deposition treatment shall be published separately.

4 Conclusions

The process parameters for CAPVD technique were optimized

systematically to deposit thick and dense titanium nitride coating

with reduced density of permeable defects on sintered NdFeB

magnets.

The CAPVD titanium nitride coating for sintered NdFeB

magnets enhanced the corrosion resistance significantly without


affecting the magnetic properties thereby ensuring maximum

exploitation of the magnetic energy.

However, a post-deposition sealing treatment is suggested

for further improvement of corrosion protection characteristics of

the CAPVD titanium nitride coating.

Acknowledgements: The authors would like to thank Mr. AzmatHussain, Mr. Irfan Ahmad and Mr. Khawar Shoaib for their

assistance in coating deposition and characterization.

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(Received: February 4, 2009)

(Accepted: February 18, 2009)

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Impeding corrosion of sintered NdFeB magnets with titanium nitride coating