www.elsevier.com/locate/apsusc

Applied Surface Science 254 (2008) 1966–1971

Nanocrystalline soft ferromagnetic Ni–Co–P thin film on

Al alloy by low temperature electroless deposition

A. Abdel Aal *, A. Shaaban, Z. Abdel Hamid

Central Metallurgical Research & Development Institute (CMRDI), P.O. 87 Helwan, Cairo, Egypt

Received 24 May 2007; received in revised form 5 August 2007; accepted 5 August 2007

Available online 14 August 2007

Abstract

Soft ferromagnetic ternary Ni–Co–P films were deposited onto Al 6061 alloy from low temperature Ni–Co–P electroless plating bath. The

effect of deposition parameters, such as time and pH, on the plating rate of the deposit were examined. The results showed that the plating rate is a

function of pH bath and the highest coating thickness can be obtained at pH value from 8 to10. The surface morphology, phase structure and the

magnetic properties of the prepared films have been investigated using scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and

vibrating magnetometer device (VMD), respectively. The deposit obtained at optimum conditions showed compact and smooth with nodular grains

structure and exhibited high magnetic moments and low coercivety. Potentiodynamic polarization corrosion tests were used to study the general

corrosion behavior of Al alloys, Ni–P and Ni–Co–P coatings in 3.5% NaCl solution. It was found that Ni–Co–P coated alloy demonstrated higher

corrosion resistance than Ni–P coating containing same percent of P due to the Co addition. The Ni–Co–P coating with a combination of high

corrosion resistance, high hardness and excellent magnetic properties would be expected to enlarge the applications of the aluminum alloys.

# 2007 Published by Elsevier B.V.

Keywords: Electroless Ni–Co–P; Ferromagnetic coatings; Thin films; Al alloy

1. Introduction

The progress in superior soft magnetic materials and films is

the key for making high performance data storage devices. Ni–

Co–P films are generally used as magnetic recording media for

electronic computers and as diffusion barriers [1]. The develop-

ment of soft magnetic materials with improved performance is

considered to be one of the most important features in the field of

information technology. Recently, binary and ternary soft mag-

netic alloys such as Co–Ni, Ni–Fe, Co–B, Co–Fe–B, Co–Ni–P,

are being developed for potential applications in modern high

density recording and data storage discs [2,3]. These soft mag-

netic materials are incorporated in the induction recording heads

for writing bit information onto hard disks for several years [4].

Nowadays, nanostructured soft magnetic films have been

effectively used to construct multilayer structures that exhibit

* Corresponding author at: Surface Protection & Corrosion Control Lab,

Central Metallurgical Research & Development Institute (CMRDI), P.O. 87

Helwan, Cairo, Egypt. Tel.: +20 122690782; fax: +20 225010639.

E-mail address: foralsayed@gmail.com (A.A. Aal).

0169-4332/$ – see front matter # 2007 Published by Elsevier B.V.

doi:10.1016/j.apsusc.2007.08.017

giant magnetoresistive (GMR) behavior [5]. Soft magnetic thin

films can be used in several functional micro-electromechanical

system (MEMS) devices such as, magnetic microactuators,

sensors and micromotors [6].

Magnetic thin films can be deposited by several techniques

such as physical vapor deposition (PVD) processes using dc or

radio frequency sputtering [7,8] and wet methods by

electrochemical or electroless technique [9,10]. However,

electroless deposition method can be considered more suitable

than electrolytic one due to the possibility of achieving uniform

surface coverage and plating micromagnetic patterns on variety

of substrates [11]. The autocatalytic electroless can usually be

achieved at high temperature (60–90 8C) [12]. Besides, it can

be performed at lower temperature (35 � 1 8C) that enables to

reduce the energy requirements and facilitate the controlling of

solution composition and deposition parameters [13,14]. In this

perspective, the present work aims to study the formulation and

development of Ni–Co–P electroless plating bath that operate

at relatively low temperature, the deposition of electroless

Ni–Co–P coatings using such bath and evaluation of coating

characteristics such as corrosion behavior and magnetic

properties.

A.A. Aal et al. / Applied Surface Science 254 (2008) 1966–1971 1967

2. Experimental procedures

2.1. Materials and Ni–Co–P deposition

Al 6061 alloy with chemical composition (1.0% Mg, 0.6%

Si, 0.28% Cu, 0.2% Cr and rest Al) was used as a substrate.

Electroless Ni–Co–P deposition on Al alloy usually requires a

series of pre-treatment steps to ensure good quality deposits.

The pre-treatment series starts by degreasing in order to remove

soaks, lubricants and fingerprints followed by acidic etching.

Then the main step is the zincate treatment which applied to

remove the oxides and deposits a protective zinc layer. It is

usually necessary to repeat this process to obtain the best

quality converging [15].

For electroless of Ni–Co–P deposition bath, NiSO4�6H2O

and CoSO4�6H2O were used as the source of nickel and cobalt,

respectively. NaH2PO2�H2O was used as a reducing agent,

which also forms the source of phosphorus in the deposit.

Na3C6H5O7 was used as the complexing agent to control the

rate of release of free metal ions for the reduction reaction. In

addition to other constituents, ammonia solution was added to

control the bath pH and the bath was operated at a constant

temperature 35 � 1 8C during the deposition process. The pre-

treatments, the bath composition and operating conditions

employed for preparation are given in Table 1.

2.2. Analysis

The electrolessed Ni–Co–P deposit was stripped in 10%

HNO3 and the dissolved metal (Co, P and Ni) was determined

Table 1

The bath composition and operating conditions of electrolessed Ni–Co–P film for

Process Composition

1 Mechanical polishing 600# emery paper then,

380# alumina sand paper

2 Alkaline degreaser Na2CO3

Na3PO4

NaOH

3 Acidic etching HNO3:HF:H2O 9:2:1, v/v

4 Zincate solution NaOH

ZnO

FeCl3KNa-tartrate

Na3N

5 Zincate removal HNO3

6 Re-zincate solution NaOH

ZnO

FeCl3KNa-tartrate

Na3N

7 Electroless Ni–Co–P NiSO4�6H2O

CoSO4�6H2O

NaH2PO2�H2O

Na3C6H5O7

H3BO3

by atomic absorption technique using Perkin-Elmer, Atomic

Absorption Model, A Analyst 200. The morphology was

observed using scanning electron microscopy (JEOL-JSM-

5410) before and after heat treatment, respectively. Vicker’s

microhardness measurements were carried out using a tukon

series B200 microhardness tester with loads of 100 g and

indentation time 10 s. The hardness values were measured as

average along the specimen surface. The mean hardness of five

readings was calculated for every specimen.

Potentiodynamic polarization corrosion tests were used to

study the general corrosion resistance of the samples (Al

alloy, as-deposited Ni–Co–P) at room temperature using an

Auto lab1 Pgstate 30 with corrware software. The

electrochemical measurements were made in a conventional

three-electrode cell using a saturated calomel electrode

(SCE) as a reference electrode and a platinum rod as a

counter electrode. After the electrochemical testing system

was stable, the measurements were carried out in 3.5% NaCl

solution. Potentiodynamic polarization curves of the tested

samples were obtained and corrosion rates were determined

for comparison. For comparison, the corrosion tests have

been applied for Ni–P coatings containing the same percent

of P.

The magnetic properties of electroless Ni–Co–P ternary

alloy deposits were studied using a vibrating magnetometer

device (9600-VSM) at an applied field of 5 KOe. The magnetic

properties, viz. saturation magnetization Ms, remanence Mr,

coercivety Hc and the squareness S derived from the hysteresis

loop perpendicular to the film surface with 5 kOe magnetic

field.

mation onto aluminum alloys sequences

Operating conditions

–

32 g/l Time, 2 min, temperature, 50 8C32 g/l

1.5 g/l

Time, 1 min room temperature

525 g/l Time, 1–3 min room temperature

10 g/l

10 g/l

10 g/l

1 g/l

50%, v/v Time, 0.5 min room temperature

525 g/l Time, 0.5 min room temperature

10 g/l

10 g/l

10 g/l

1 g/l

7 g/l pH 8–11

22 g/l Time, 15–65 min

25 g/l Temperature, 35 � 1 8C50 g/l

30 g/l

Fig. 1. Effect of deposition time and pH on the coating thickness of electroless Ni–Co–P ternary film deposition.

Fig. 2. XRD patterns of (a) coated Ni–Co–P and (b) annealed Ni–Co–P films.

A.A. Aal et al. / Applied Surface Science 254 (2008) 1966–19711968

3. Results and discussion

3.1. Effect of pH and deposition time on the coating

thickness

The effect of the bath pH on the coating thickness of

electroless Ni–Co–P alloys deposit is depicted in Fig. 1a. It is

evident that at lower pH values (pH<8), the deposition process

is characterized by instability of the bath producing irregular

and poor deposits. An increase in the bath pH from 8 to 10 was

found to increase the coating thickness with formation of bright

and adherent deposits. However, at higher pH values (pH>10),

the bath decomposes and the coating thickness drastically

decreases. It was reported previously that OH� ions

concentration increases with pH increasing in range of 8–10

and the hypophosphite oxidation is the dominant factor in the

electroless process [16]. In alkaline media, the oxidation of

hypophosphite reaction occurs as follows:

H2PO2� þ 3OH� $ H2PO3

�2þ 2H2O þ 2e� (1)

From the above chemical reaction, an increase in bath pH

value leads to increasing in the oxidation rate of hypophosphite

which in turn accelerates the plating rate. However, the

solutions become unstable at higher pH, especially above pH

11.0 where nickel and cobalt hydroxides will precipitate during

the deposition process. Hence, higher values of pH were not

recommended because of hydroxides precipitation and also

because of the microelectronic industry requirements for circuit

preservation [17]. In conclusion, it is preferable to use the bath

approximately pH 9.5, where there is a good coating thickness

as well as the stability of the bath.

The variation of coating thickness with the deposition time is

illustrated in Fig. 1b. It can be seen that the coating thickness

increases with the processing time. This can be ascribed to the

autocatalytic nature of the electroless plating process. However,

the relationship between time and coating thickness is not linear

due to the build-up of orthophosphite ions by oxidation of

sodium hypophosphite in Eq. (1). The quantitative analysis of

Ni–Co–P deposit showed that the contents of Ni, Co and P in

the film are 60, 15 and 25 wt.% for, respectively.

3.2. Phase structure and surface morphology

Fig. 2 shows X-ray diffraction patterns of electrolessed Ni–

Co–P deposits before and after annealing at 400 8C for 2 h.

XRD pattern of Ni–Co–P as-deposited (without annealing)

shows peaks of hcp-Co, fcc-Ni and metastable bcc-Ni7P3

phases. The appearance of Ni7P3 is attributed to the higher

phosphorous segregation in grain boundary regions. On the

other hand, for the annealed samples, new peaks of stable bct-

Ni3P phase are observed at 36.58, 41.78 and 46.68 in addition to

hcp-Co and fcc-Ni peaks. The higher intensity for annealed

samples pattern indicates to the higher degree of crystallinity in

these samples. During the annealing process, the higher

phosphorous regions were supposed to be further increased by

A.A. Aal et al. / Applied Surface Science 254 (2008) 1966–1971 1969

extraction of P dissolved in nickel grains which in turn gave rise

to precipitation of the hard inter-metallic Ni3P phase. These

transformations support the earlier observation obtained by

Sankara et al. [18] and Hur et al. [19].

The surface morphology of the Ni-Co–P layer before and

after annealing is shown in Fig. 3. The deposit obtained at

optimum conditions (pH 9.5 and deposition time 15 min) was

compact and smooth with nodular grains as shown in Fig. 3a.

The observed randomness in distribution of spherical nano-

particles with size in the range 150–250 nm is due to the

partially amorphous structure in Ni–Co–P film grown in

perpendicular direction to the substrate surface. Fig. 3b shows

SEM of Ni–Co–P coating after annealing at 400 8C for 2 h. It

can be observed that there is no significant change in the

crystallite size after annealing process. However, the crystal

structure and the phase were converted from bcc-Ni7P3 into bct-

Ni3P as shown in the XRD pattern (Fig. 2b). This transforma-

tion in crystal structure is attributed to two reasons. Firstly, the

deposit components and substrate, which have different thermal

expansion coefficients, undergo different volume changes due

to a temperature change by annealing. The evolution of texture

during annealing of electroless Ni–Co–P has been studied

previously by Lee and Hur [20]. The strains and stresses due to

the different thermal expansion coefficients are calculated by

the integral equation:

eth ¼Z T

T0

ðas � afÞ dT (I)

Fig. 3. SEM images of Ni–Co–P films (pH 9.5 and deposition time 15 min).

where T and a are temperature and thermal expansion coeffi-

cient, respectively. Subscripts 0, s and f indicate initial state,

substrate and film, respectively. Setting T = 400 8C, T0 = 35 8C,

as = aAl = 25.8 � 10�6 K�1, af = aNi = 13.3 � 10�6 K�1 and

the eth = 12.5 � 10�6. For more accuracy, the thermal expan-

sion coefficients of fcc-Ni–Co solid solutions should be used.

The thermal expansion coefficient of cobalt (hcp) is

13.8 � 10�6 K�1 [21], which is very close to that of nickel.

Therefore, the closed thermal expansions of these components

explain why the coating is not exposed to the delamination

during the annealing process. In addition, the formation of

nickel phosphide could lead to volume changes [20].

Secondly, Ni–Co–P film tensile stress (which attributed to

the high phosphorus content) is relieved after annealing

process, so the lattice parameter of bcc is contract in one

dimension along (c-axis) from 8.643 to 4.386 A and the other

lattice parameters a, b almost still constant to form bct crystal

structure.

3.3. Hardness measurements

The microhardness of Al alloy, as-deposited film and

annealed one has been measured by Vicker’s method. The

hardness of Al alloy increased from 66 � 5 to 287 � 5 HV100 g

after coating. It was found that the microhardness of Ni–Co–P

deposits enhanced after annealing (400 8C for 2 h) up to

520 � 5 VH100 g. Such improvement of the microhardness after

annealing can be attributed to the formation of Ni3P inter-

metallic phase which generated the effect of precipitation

hardening [22].

3.4. Corrosion and polarization measurements

Electrochemical potentiodynamic polarization experiment

was applied in a 3.5% NaCl solution in order to measure the

corrosion resistances of Al alloy and as-deposited Ni–Co–P

coating. For comparison, the corrosion properties of Al alloy

coated by Ni–P, deposited from standard bath and containing

the same content of P, have been studied (Fig. 4). The

Fig. 4. Potentiodynamic curves for (a) uncoated Al alloy and coated by (b) Ni–

P and (c) Ni–Co–P tested in 3.5% NaCl.

Table 2

Electrochemical corrosion data related to polarization curves of uncoated Al alloy and coated by Ni–P and Ni–Co–P tested in 3.5% NaCl solution

Sample ba bc Rp (V/cm2) Icorr (mA/cm2) Ecorr (V) Cr (mpy)

Uncoated alloy 0.062 0.032 129 71.1 �0.733 30.81

Ni–P coating 0.031 0.042 287 27.2 �0.657 11.52

Ni–Co–P coating 0.005 0.077 370 5.5 �0.595 2.38

A.A. Aal et al. / Applied Surface Science 254 (2008) 1966–19711970

electrochemical parameters obtained from the polarization

curves for uncoated Al alloy and the coated alloy by Ni–Co–P

and Ni–P are displayed in Table 2.

The corrosion potential of the Al alloy is �0.733 V, which

is higher than that of Ni–P (�0.657 V) and Ni–Co–P coated

alloy (�0.593 V). On the contrary, the corrosion current

densities of Ni–P and Ni–Co–P coated Al alloys are 5.5 and

27.2 mA/cm2, which are somewhat lower than that of

uncoated Al alloy 71.1 mA/cm2. The polarization resistance

(Rp) of Al, Ni–P and Ni–Co–P coated alloys are 129, 287 and

370 V/cm2, respectively. It is evident that the Ni–Co–P

coating layer provides effective protection for Al alloy than

Ni–P coating. Therefore, the corrosion current Icorr values can

be calculated by using the Stearn–Geary equation with

Ozyilmaz et al. [23].

Icorr ¼b

RP

(II)

where b is constant value calculated by the following equation:

b ¼ babc

2:3ðba þ bcÞ(III)

where ba and bc are anodic and cathodic Tafel slopes that were

obtained from polarization curves. The value of b were 0.009,

0.007, 0.002 V for Al alloy, Ni–P and Ni–Co–P coated alloys.

Corrosion potential Ecorr is obtained using Tafel extrapolation

method and corrosion rate is calculated from the following

equation:

Corrosion rate ðmpyÞ ¼ 0:13Icorrðeq: wt:Þd

(IV)

where (eq. wt.) is the equivalent weight in gram and d is the

density in g/cm3 of Al alloy (for uncoated samples) or of Ni (for

coated samples). Therefore, the corrosion rates of uncoated,

Fig. 5. The hysteretic behavior of (a) coate

Ni–P and Ni–Co–P coated Al alloys in 3.5% NaCl solution can

be estimated from Icorr and the Faraday’s law, which are 30.81,

11.52 and 2.38 mpy, respectively.

The enhancement in corrosion resistance by Ni–P and

Ni–Co–P deposits is due to the formation of an adsorbed

layer of hypophosphite anion H2PO2� on the surface of

coatings could provide passivity in aqueous environment

[24]. Flis and Duquette reported that phosphorous is enriched

on the surface layer by the preferential dissolution of nickel

[25]. Then, the passive barrier layer of H2PO2� is formed

by reaction of the enriched P with water. By this way,

H2PO2� layer will block the supply of water to the electrode

surface and prevent the hydration of nickel, which is

considered to be the first step to form either soluble Ni+2

species or a passive nickel film. Generally, it can be

concluded that the improved corrosion resistance obtained

for electroless Ni–Co-P and Ni–P coatings is due to the

enrichment of phosphorous on the electrode surface [26].

Therefore, a conclusion can be drawn that the presence of Co

in Ni–P amorphous deposits improves their corrosion

resistance as obtained previously by Parent et al. [27]. Such

corrosion resistance improvement can be attributed to the

more compact passive film in Ni–Co–P coatings based on

anodic polarization and impedance tests [27].

3.5. Magnetic properties

The magnetic properties of as-plated electroless Ni–Co–P

before and after annealing have been studied using a vibrating

magnetometer device by applying a magnetic field of 5 KOe

perpendicular to the sample surface. The hysteresis loops of

investigated samples are shown in Fig. 5. It can be seen that

the hysteresis loops shapes for both of coated and annealed

samples are narrow and curved. This shape seems to be very

d Ni–Co–P and (b) annealed Ni–Co–P.

Table 3

Magnetic parameters of electroless Ni–Co–P ternary alloy deposits

Sample Mr (emu/g) Ms (emu/g) Hc (Oe) HK (Oe) S

Coated alloy 2.15 25.8 36.5 1194 0.08

Annealed alloy 3.24 23.2 96.3 1196 0.14

A.A. Aal et al. / Applied Surface Sc

similar to that exhibited by partially or totally amorphous

materials and by an amorphous Ni–Co–P film [28,29]. It can

be mentioned that the prepared electroless Ni–Co–P thin film

of the present study have an amorphous structure with soft

magnetic characteristics. The coercivety Hc of annealed Ni–

Co–P film is affected not only by the grain size but also by

impurities and variation of magnetic anisotropy energy

governed by shape, film stress and crystalline anisotropy.

Although SEM images showed no significant change in grain

size of Ni–Co–P film before and after annealing, the

coercivety was higher for the annealed ones. This behavior

can be attributed to the increasing of the magnetoelastic

anisotropy in the film after stress relief due to the annealing

process [30].

The magnetic properties, viz. saturation magnetization

Ms, remanence Mr, coercivety Hc and the squareness S of

the Ni–Co–P thin film were derived from the hysteresis

loops. The data are summarized in Table 3. It was noticed

that both of coated and annealed samples exhibited soft

ferromagnetic behavior. Besides, there is no appreciable

change in either magnetic properties or the hysteretic

behavior by annealing except the increasing in coercivety

Hc and the squareness S from 36.5 to 96.3 Oe and from 0.08

to 0.14, respectively.

4. Conclusions

- Electrolessed Ni–Co–P films have been deposited onto Al

alloy with nanocrystallite sizes in 150–250 nm range.

- T

he coating thickness is dependent of plating parameters (pH

and deposition time).

- T

he optimum bath pH and deposition time are of 9.5

and 15 min, respectively, where there are the highest

coating thickness (0.92 mm) as well as the stability of the

bath.

- T

he Ni–Co–P films showed short-range order without grain

boundaries forming an amorphous phase. However, after

annealing (at 400 8C for 2 h), the degree of crystallinity

increases and the microhardeness is improved.

- T

he presence of Co in Ni–P film improves the corrosion

resistance of Al alloy.

- T

he Ni–Co–P coating with a combination of high corrosion

resistance and high hardness would be expected to enlarge the

applications of the aluminum alloys.

- T

he coated and annealed Ni–Co–P films deposited onto

Al alloys exhibit excellent magnetic recording media

properties.

Acknowledgements

Help received from both of Dr. W. Agami (Ain Shams

University) and F. Abdel Mouez (CMRDI) during the course of

this work is greatly acknowledged.

References

[1] W.L. Liu, W.J. Chen, T.K. Tsai, S.H. Hsieh, S.Y. Chang, Appl. Surf. Sci.

253 (2007) 3843.

[2] B. Lochel, A. Maciossek, Electrodeposited magnetic alloys for surface

micromachining, Electrochem. Soc. 143 (1996) 3343.

[3] G. Herzer, in: A. Hemando (Ed.), Nanomagnetism, Kluwer Academic

Publishers, Dordrecht, The Netherlands, 1993, p. 1ll.

[4] P.C. Andricacos, N. Robertson, IBM. J. Res. Dev. 42 (1998) 671.

[5] H.A.M. Berg, in: U. Haitman (Ed.), Magnetic Multilayers and Gaint

Magnetoresistance: Fundamentals and Industrial Applications, Springer,

Berlin, 1999.

[6] C.H. Ahn, M.G. Alien, IEEE Trans. Ind. Electron. 45 (1998) 866.

[7] S. Tunashima, Y. Maehata, S. Uchiyama, IEEE Trans. Magn., MAG-17

(1981) 3073.

[8] K. Hironaka, S. Uedaira, IEEE Tram. Magn., MAG-26 (1990) 2421.

[9] J.M. Riveiro, M.C. Sanchez-Trujillo, IEEE Tram. Magn., MAG-16 (1980)

1426.

[10] K. Ohashi, M. Ito, T. Maruyama, Magnetic materials, processes, and

devices, in: L.T. Romankiw, D.A. Herman, Jr. (Eds.), PV 90-8, The

Electrochemical Society Proceedings Series, Pennington, NJ, 1990, p. 247.

[11] K. Yoshino, Electrochemically deposited thin films, in: M. Paunovic, I.

Ohno, Y. Migoshi (Eds.), PV 93-26, The Electrochemical Society Pro-

ceedings Series, Pennington, NJ, 1993, p. 370.

[12] F.A. Lowenheim, in: Mordechey Schlesinger, Milan Paunovic (Eds.),

Modern Electroplating, John Wiley & Sons, New York, 2000, p. 676.

[13] G.Z. Zou, M.S. Cao, L. Zhang, J.G. Li, H. Xu, Y.J. Chen, Surf. Coat.

Technol. 201 (2006) 108.

[14] I. Baskaran, R.S. Kumar, T.S.N. Sankara Narayanan, A. Stephen, Surf.

Coat. Technol. 200 (2006) 6888.

[15] M.T. Abou El-Khair, A. Abdel Aal, J. Mater. Sci. A 454 (2007) 156.

[16] M. Bouanani, F. Cherkaoui, M. Cherkaoui, S. Belcadi, R. Fratesi, G.

Roventi, J. Appl. Electrochem. 29 (1999) 1171.

[17] L. Magagnin, V. Sirtori, S. Seregni, A. Origo, P.L. Cavallotti, Electrochim.

Acta 50 (2005) 4621.

[18] T.S.N. Sankara Narayanan, S. Selvakumar, A. Stephen, Surf. Coat.

Technol. 17 (2003) 298.

[19] K.H. Hur, J.H. Jeong, D.N. Lee, J. Mater. Sci. 26 (1991) 2037.

[20] D.N. Lee, K.H. Hur, Scripta Metall. Mater. 40 (1999) 1333.

[21] G.T. Murray, ninth ed., Metals Handbook, vol. 2, ASM, Metals Park, OH,

1979, p. 725.

[22] O.M. Glenn, B.H. Juan, Electroless plating: Fundamentals and Applica-

tions, AESF, Florida, 1990, p. 111.

[23] A.T. Ozyilmaz, G. Kardas, M. Erbil, B. Yazici, Appl. Surf. Sci. 242 (2005)

97.

[24] H. Ashassi-Sorkhabi, S.H. Rafizadeh, Surf. Coat. Technol. 176 (2004)

318.

[25] J. Flis, D.J. Duquette, Corrosion 41 (1985) 700.

[26] R.B. Diegle, N.R. Sorensen, C.R. Clayton, M.A. Helfand, Y.C. Yu, J.

Electrochem. Soc. 135 (1988) 2113.

[27] M.M.V. Parent, O.R. Mattos, S.L. Diaz, P.L. Neto, F.J. Fabri Miranda, J.

Appl. Electrochem. 31 (2001) 67.

[28] M.R. Khan, E.L. Nicholsan, J. Magn. Magn. Mater. 54 (1986) 1654.

[29] N. Fenineche, A.M. Chaze, C. Coddet, Surf. Coat. Technol. 88 (1996)

264.

[30] B.Y. Yoo, S.C. Hernandez, D.-Y. Park, N.V. Myung, Electrochim. Acta 51

(2006) 6346.

ience 254 (2008) 1966–1971 1971