Accepted Manuscript

Title: Corrosion behavior of magnetic ferrite coating preparedby plasma spraying

Author: Yi Liu Shicheng Wei Hui Tong Haoliang Tian MingLiu Binshi Xu

PII: S0025-5408(14)00508-XDOI: http://dx.doi.org/doi:10.1016/j.materresbull.2014.09.006Reference: MRB 7647

To appear in: MRB

Received date: 10-2-2014Revised date: 14-8-2014Accepted date: 3-9-2014

Please cite this article as: Yi Liu, Shicheng Wei, Hui Tong, Haoliang Tian, Ming Liu,Binshi Xu, Corrosion behavior of magnetic ferrite coating prepared by plasma spraying,Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2014.09.006

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Corrosion behavior of magnetic ferrite coating prepared by plasma spraying

Yi Liu, Shicheng Wei*, Hui Tong, Haoliang Tian, Ming Liu, Binshi Xu

(National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering)

* Corresponding author. Tel: +86 10 66719083; fax: +86 10 66717144. Address: Fengtai district, dujiakan 21th, Beijing

100072, China

E-mail address: wsc33333@163.com

Graphical abstract The saturation magnetization (Ms) of the ferrite coating is 34.417 emu/g while the Ms value of the ferrite powder is 71.916 emu/g. It can be seen that plasma spray process causes deterioration of the room temperature soft magnetic properties.

Highlights

Spinel ferrite coatings have been prepared by plasma spraying.

The coating consists of nanocrystalline grains.

The saturation magnetization of the ferrite coating is 34.417 emu/g.

Corrosion behavior of the ferrite coating was examined in NaCl solution.

Abstract

In this study, spray dried spinel ferrite powders were deposited on the surface of mild steel substrate through plasma

spraying. The structure and morphological studies on the ferrite coatings were carried out using x-ray diffraction, scanning

electron microscope and Raman spectroscopy. It was showed that spray dried process was an effective method to prepare

thermal spraying powders. The coating showed spinel structure with a second phase of LaFeO3. The magnetic property of

the ferrite samples were measured by vibrating sample magnetometer. The saturation magnetization (Ms) of the ferrite

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coating was 34.417 emu/g. The corrosion behavior of coating samples was examined by electrochemical impedance

spectroscopy. EIS diagrams showed three corrosion processes as the coating immersed in 3.5 wt.% NaCl solution. The

results suggested that plasma spraying was a promising technology for the production of magnetic ferrite coatings.

Keywords: Magnetic materials; Magnetic properties; Microstructure

1. Introduction

Recent years, the study of radar absorbing materials (RAM) attracts more and more interest because they can absorb

energy from microwaves. RAM not only can be used in the field of military technology to enhance equipment stealth

ability, but also can be used in civilian technology. Special attention has been focused on the development of new materials

and microwave technology. Spinel ferrites exhibit an interesting behavior, absorbing energy from electromagnetic waves,

and present the best relation between the absorber’s performance and its final thickness [1]. Nature resonance is microwave

absorption mechanism of the ferrite materials [2]. Commercial spinel ferrite absorbers are bulk ceramics, these absorbers

are fitted to components by bonding or brazing, or paints and polymer composites filled with absorbing powder, which

needs a very high thickness in order to be effective [3].

In general, the conventional microwave absorption materials are composed of matrix and absorber, the absorber is

blended with organic binders (such as resin), then the mixtures are directly sprayed or brushed onto the surface of

components to shield target [4, 5]. However, there are some limitations for these RAM because they are easily suffered

deterioration with the change of environment due to their low adhesion and thermal shock resistance. Moreover,

microwave-absorbing coatings usually serve in harsh environment (i.e. marine environment). The corrosion medium must

affect the performance of the coating; nevertheless, the corrosion behavior of RAM has not been studied. A further

disadvantage is the cost intensive procedure to fix the small sintered tile onto large-scale devices. Such limitations can be

overcome by the application of thermal spraying technology for the preparation of microwave absorbing coatings. Thermal

spraying is an important surface modification technique. The principle of the thermal spraying process consists of complete

or partial melting of feedstock material (typically in the form of powder or wire) followed by the acceleration of molten

particles and their subsequent impact onto a coated part, where the particles rapidly solidify and form a lamellar structure

[6-9]. With the advantages of low substrate temperature processing, bulk production capability and cost efficiency, plasma

spraying offers a promising route for the synthesis of ferrite coatings using in industry [10]. However, despite the wide

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industrial application of thermal spraying for the preparation of a large variety of coatings, their uses for the deposition of

electromagnetic coatings have been scarce. Since the disadvantage of high deposition, temperatures and thermal stress limit the

applicability of ferrite coatings through plasma spraying; only a few studies have been reported up to now [11-14]. Among the

few examples, there is a great interest to investigate thermal spraying process for preparing hexagonal ferrite absorbers;

therefore, a further research needs to be done in order to verify the possibility to use plasma spraying for the preparation of

spinel ferrite coatings and discuss the corrosion behavior of the microwave absorbers. The aim of this research is to investigate

the deposition of spinel ferrite coatings by plasma spraying using spray-dried agglomerates of micrometric particles and

evaluate the magnetic property as well as corrosion behavior of the ferrite coatings.

2. Experimental

2.1 Powder production

The feedstock powder consisted of spray-dried spherical agglomerates of micrometric spinel ferrite particles with

composition of Ni0.5Zn0.4Mg0.1La0.05Fe1.95O4. Stoichiometric amounts of MgO, ZnO, NiO, La2O3 and Fe2O3 as starting

materials were mixed and homogenized in a ball mill. The particles were synthesized by solid-state reaction. These

reactants were calcined at 1300 ℃ for 2 h and then were broken down to powders again, after this process; these powders

were calcined at 900 ℃ for 1 h and cooled down to room temperature naturally. However, the powders were not suitable for

plasma spraying due to their poor flow ability. In order to increase the flow ability, spray-drying process was selected to

produce the plasma spraying materials. The preparation process was as follows: the ferrite powders were used as raw

materials and polyvinyl alcohol as binder. The particles were agglomerated into spherical granules, using a spray-drier

attached to a cyclone; the entry and exit temperature were 240 ℃ and 140 ℃, respectively. The spray-dried powders were

sieved between 200 screen mesh and 400 mesh and thus separated from the finer ones.

2.2 Coating samples preparation

Mild steel substrate was used in this study. The substrate was ground with abrasive papers up to 600 grit, cleaned with

acetone, and then sandblasted using emery grit prior to plasma spraying. Ultrasonic plasma spraying equipment was

applied to prepare the ferrite coatings [15]. Detailed operating parameters were listed in Table 1. Besides, in order to

measure the magnetic properties of the ferrite coating, aluminum foil was selected as a substrate because it allowed simple

preparation and had a small paramagnetic moment that had no significantly interfere with the magnetic signal of the ferrite

coating.

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2.3 Characterization

The phase structure of the ferrite powder and coating were studied by D8 advance x-ray diffraction (XRD) with

λ=0.154nm Cu-Kα radiation. The cross sectional samples were prepared by metallographic cutting, hot-mounting in resin,

grinding with abrasive papers (up to 2000 mesh) and polishing with diamond slurry. The morphology patterns of ferrite

powders and the coating surface were observed using a scanning electron microscope (SEM, Quanta 200, FEI). The cross

section morphology of the ferrite coating was analyzed using field emission scanning electron microscope (FEI, Navo

NanoSEM 450). The composition analysis of the ferrite coating was performed by energy dispersive spectra (EDS,

OXFORD Feature Max). The microstructure of the ferrite coating nanocrystalline is studied by a JEOL JEM 2000FX

transmission electron microscope (TEM). The polished cross-section of the coating was also studied by micro-Raman

spectroscopy (HR800, Jobin-Yvon, France; laser wavelength: 532 nm). The magnetic properties of the ferrite samples were

measured under a static magnetic field using a Lake Shore 7410 vibrating sample magnetometer (VSM) with a maximum

applied magnetic field of 20 kOe. In order to accurately access the magnetic property of the ferrite feedstock, the ferrite

powders were compacted into a block with a dimension of 3 mm×3 mm×4 mm for the magnetic measurement. For

accurately measuring magnetic property of the ferrite coating, the coating was prepared on aluminum substrate with the

same plasma spraying parameter. Then the ferrite coating foil was cut from the aluminum substrate with a linear cutting

machine. The foil was ground to 1000 mesh for the magnetic measurement.

2.4 Electrochemical experiments

The electrochemical impedance spectroscopy (EIS) was used to measure the corrosion behavior of the ferrite coatings

immersed in 3.5 wt.% NaCl solution for 24 h，96 h，168 h，264 h and 408 h. The tests started by recording the electrode

potential with time. When the corrosion potential remained stable, a sinusoidal AC signal of 5 mV (rms) amplitude at the

open circuit potential (OCP) was applied to the electrode over the frequency ranged from 100 kHz to 10 mHz. The

measurements were carried out using a ZAHNER IM6 electrochemical working station. The impedance data were

displayed as Nyquist and Bode plots. The acquired data were fitted and analyzed using Zsimpwin software. All

electrochemical measurements were conducted in a conventional three-electrode electrolyte cell with the coated sample as

the working electrode, a platinum plate as the counter electrode and a saturated calomel electrode (SCE) as the reference

one. All experiments were carried out at room temperature. The specimens for electrochemical measurements were ground

up to 1000 grit by SiC papers and mounted in epoxy resin with only a square area about 1 cm2 exposed to NaCl solution.

The experiments were monitored using the software of Thales.

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3. Results and discussion

3.1 Characterization of the feedstock powders

The various plasma spraying process parameters and the properties of powders influences the characteristics of deposited

coatings strongly. Fig. 1(a) shows the micrographs of sintered ferrite. It can be seen that the particles are agglomerate together

with a size of 4～10 μm. These particles are not suitable for plasma spraying due to poor flow ability and small particle size. The

SEM images of spray dried ferrite powder are showed in Fig. 1(b). It can be see that the rough shape of the spray-dried particles

is nearly spherical with the particles size ranging from 50 to 85 μm. The spray-dried powders have excellent flow ability, which

is appropriate for plasma spraying. Moreover, the spray-dried processes do not alter the original phase structure and the

magnetic properties of the powders.

3.2 Characterization of the ferrite coating

The XRD patterns of the sintered ferrite powder and the ferrite coatings samples are presented in Fig. 2. The sharp and

strong peak around 35° and the well resolved broad diffraction peaks corresponding to (220), (311), (222), (400), (422), (511)

and (440) reflections planes confirm the formation of cubic spinel phase structure in the prepared samples. The XRD patterns

prove the success of preparing spinel ferrite coating through plasma spraying. In addition, a secondary phase of LaFeO3 can be

found. The secondary phase is often exhibited in the ferrite when substituted with La3+ [16]. It has been established that the

LaFeO3 is formed upon La substitution for Fe. The presence of LaFeO3 phase along with the major ferrite indicates that La does

not form a solid solution with spinel ferrite or La3+ has limited solid solubility [17]. However, there is some difference for the

sintered ferrite and the coating. The diffraction peaks of the coating become broader than the ferrite powders. The background

noise and broadness of the peaks are characteristic of particles with nanometer dimensions. There are insufficient diffraction

centers for the nano-sized particles, which cause the line broadening [18]. In addition, residual stress also have an effect on

XRD peak broadening and signal/background ratio, particularly when only a low proportion of particles are melted [19].

Moreover, in some literature, wustite FeO can be found in coatings under conditions of oxygen loss during spraying process

[10]. However, FeO is not found in our study, It may be due to the spraying power is larger and the carrier gas is nitrogen rather

than air in our study.

A thermal spraying coating is produced by the deposition of numerous consequent layers formed by flattening and

solidification of molten particles impacting at high velocity on the substrate or on previously deposited layers [20]. In some

cases, some particular compounds and phase decompose during spraying process. The cross section image of the ferrite coating

is showed in Fig. 3(a). Micro sized lamellar structure can be seen in the as-sprayed coatings, which is a clear sign of a high

degree of melting feedstock. In addition, there is an obvious transition region at the interface of the coating and the substrate.

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The as-sprayed ferrite coatings have characteristics of pores, voids and micro-cracks in the process of thermal spraying. The

defects lead to decrease in the compactness of the coating, but it is difficult to find an open pore from the cross-section

observation. The presence of void and pores can be attributed to random deposition and lack of complete over lap of adjacent

splats. During plasma spraying, the latter splat impacts onto the former splat and produces plastic strain, transferring heat into

the former splat, and pores are produced if the adjacent splats do not overlap fully [21]. On the one hand, the pores and voids

provide penetration channel for the corrosive medium and lead to deterioration of the corrosion resistance of the coating, on the

other hand, the presence of an appropriate number of pores will increase the reflections of electromagnetic wave within the

coating, which is beneficial to the electromagnetic wave loss. Fig. 3(b) demonstrates the composition profile from an EDS

linear analysis in the cross section of the coating. The substitution element of Ni, Zn and Mg are homogeneous distributed

along the cross section of the coating. However, EDS line scan shows that the content of La is higher in some position,

according to the XRD analysis, the secondary phase LaFeO3 can be found in the coating, Thus the position where the

content of La is higher perhaps is the LaFeO3 phase.

It can be concluded that the ferrite coating is deposited on the substrate and the elements distribute homogeneously except

La. The thermal spray coating is often made up of molten lamellae and shows different composition in details. Therefore, a high

magnification view of the cross section image is necessary to investigate the deposition property of the ferrite coating. Fig. 3(c)

shows the amplified microstructure of the ferrite coating. The lamellar microstructure shows various shades of grey.

According to the EDS analysis (Fig. 3d), the grey region and dark region exhibit different compositions. The dark region

(area 1) shows higher content of Zn and Ni than the grey region (area 2). The chemical content of dark region is

approximately close to the nominal composition. However, the bright area is typical of zinc loss area. The composition

analysis of the ferrite coating is also investigated. It can be seen that the main element Fe, O, Ni, Zn, Mg and La are all

detected with an EDS spectrum in the rectangle area. The details element contents are listed in Table 2. The mole ratio of Ni:

Mg: La: Fe is approximately 5: 1: 0.5: 20, which is well correspondence to the nominal composition. However, the mole

ration of Ni: Zn is about 2:1, which shows that the content of zinc is less than the nominal composition, indicating zinc loss

in the process of thermal spray. Zinc has a higher vapor pressure compared to Ni, Mg, Fe and preferentially evaporates

during flight. Yan et al have investigated zinc loss of spinel MnZn ferrite prepared through plasma spraying. They use

single splats to evaluate the effect of zinc loss in plasma spraying and find a variation in zinc content within splats of

different sizes after plasma spraying, even though the powder has the same staring stoichiometry. The smaller particles

have a higher zinc evaporation rate during the flight time [10].

The surface morphology of the coating is shown in Fig 4. It can be seen that some particles are well melted and some

particles remain with spherical shape. The coating actually contains many unmelted agglomerates, where the individual

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micrometric particles are still clearly recognizable. In the process of plasma spraying, very large quenching stresses arise in

the stacking of well-bonded lamellae, which causes extensive micro cracks in the coating. Plasma spraying has the high

temperature characteristics, resulting in the burning loss of low melting metal ingredient (such as zinc) and leading to the

variation of composition. In order to decrease zinc loss, we reduced the energy of plasma spraying, as a result, some

unmelted particles appeared in the coating. Fig. 5 represents the TEM image of the as-sprayed coating samples. The coating

consists of nanocrystalline grains with the grain size of 100-200 nm, which confirms the XRD peak broadening.

In order to verify the deposition of soft magnetic spinel ferrite coating after the high temperature through plasma

spraying, Micro-Raman spectroscopy is introduced in this study. Raman spectroscopy is a powerful and sensitive tool for

characterization of the cation distribution. This technique is useful in understanding the microstructure of the materials.

White and DeAngelis first give a symmetry-based normal mode assignment for cubic spinel [22]. There are five photon

mode for Raman active (A1g+Eg+3T2g), cation redistribution in the tetrahedral and octahedral site alter the symmetry of the

crystal structure. The measured Raman spectrum of the coating is presented in Fig. 6. The samples show four peaks at 323

cm-1, 475 cm-1, 563 cm-1 and 686 cm-1, respectively. The Raman spectrum is similar to the spinel structure of the ferrite and

matches with the early study of NiFe2O4 [23]. Moreover, it is noticed that different modes exhibits different sensitivity

towards irradiation, which is in agreement with previous reports [24]. The mode of A1g seems to be sensitive towards

irradiation compared to other modes. Raman peaks over the region of 670～710 cm-1 represent the mode of tetrahedral in

the ferrite, the strong peak is a common feature of A1g band, which involves the symmetric stretching of oxygen atom with

respect to metal ion in tetrahedral. The peaks at 563 cm-1 represents the T2g (1) mode while the peaks at 475 cm-1 represents

the T2g (2) mode. The peaks at 323 cm-1 represents the Eg mode. In some literature, there is a peak around 220 cm-1, which

represents the T2g (3) mode [25]. However, in this work, the T2g (3) peak is so weak that it nearly cannot be detected. The

low frequency photo peaks (Eg and 3T2g) correspond to the lattice vibrations of Fe3+ and O2- in octahedral site. The Raman

spectrum of the ferrite coating matches almost perfectly with that of spinel ferrite, indicating that plasma spraying is a

promising technique for the preparation of ferrite coating..

3.3 Magnetic properties of the ferrite samples

In order to understand the magnetic properties of the plasma spraying ferrite coating, the field dependent magnetization

of the ferrite powder and coating are measured using a vibrating sample magnetometer with the applied field of 20 kOe at

300 K. Fig. 7 shows the hysteresis loops of the ferrite samples. It can be seen that the ferrites are well saturated at the

applied field. The minimal hysteresis area indicates that the ferrite is a soft magnetic material. The magnetic parameters are

showed in Table 3. The Ms value of the ferrite coating is about 34.417 emu/g. As Mg2+ substitution is introduced into the

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sublattice in the NiZn ferrite, it may occupy the octahedral sites, replacing Zn2+ or Ni2+ ions due to their nearly the same

radii. The strength of the super exchange interaction is reduced whereas the magnetic moments are improved, resulting in

an increase in the spontaneous magnetization [26]. The Ms value of the ferrite powder reaches 71.916 emu/g, the reason for

the different magnetization of the powder and the coating is that the ferrite powder is calcined at high temperature for 2 h,

thus increases the grain size of the ferrite. The magnetization increases with the crystalline size. Secondly, the saturation

magnetization depends on the chemical composition, in spinel ferrite, since Zn2+ has a strong site preference for the

tetrahedral coordination of the A sites over other ions because of its electronic configuration and crystal field effects [27].

Zn2+ ions occupy A sites and force Fe3+ ions to move to B sites, which increases the total magnetic moment per ferrite

formula and exhibits high magnetic permeability. However, zinc loss during the high temperature spray process is found to

create an inhomogeneous composition profile. In the case of zinc loss, Fe3+ ions can go onto the A sites where they replace

the missing Zn2+ ions. Thus, the A-B exchange energy becomes weaker and the magnetic moment reduces, leading to a

decrease in the saturation magnetization. The remanent magnetization (Mr) of the ferrite coating is 8.011 emu/g while the

remanent magnetization of the ferrite powder is 2.006 emu/g. The difference may be due to the variation of the particle size.

The ratio of remanent magnetization to the saturation magnetization (Mr/Ms) is called the remanence ratio [28]. High

remanence ratio indicates the material moving to the nearest easy axis magnetization direction after the magnetic field

removed. The lower remanence ratio is indication of isotropic nature of material [29]. The remanence ratio of the ferrite

coating is 0.233 while the remanence ratio of the ferrite powder is 0.028. It can be seen that thermal spraying process of the

ferrite coating causes deterioration of the room temperature soft magnetic properties compared with the ferrite powder. The

extreme high temperature and rapid quenching conditions in the plasma spraying process allow retention to room

temperature of the random cation distribution, which affects the growth of the crystalline grain and composition

distribution [10]. In some literatures, researchers investigated the effect of annealing temperature on the magnetic

properties of the ferrite coating. Lisjak prepared the barium hexaferrite coatings through plasma spraying and found the

maximum magnetization increasing significantly when the deposited coating was annealed over 800 ℃ [30]. The

researchers attributed this phenomenon to the crystallization of the hexaferrite phase.

3.4 Corrosion behavior of the ferrite coating

The corrosion resistance of the ferrite coating in 3.5 wt. % NaCl solution is evaluated by EIS. Fig. 8(a) shows a variation of

the capacitive semicircles depending on immersion time in Nyquist diagram. All experimental plots have a depressed

semicircular shape in the complex impedance plane, with the center under the real axis, which is a typical behavior of solid

metal electrodes that show frequency dispersion of the impedance data [31]. The capacitive semicircle is attributed to the

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double layer capacitance and the charge transfer resistance. Furthermore, the diameter of the capacitive semicircle in

Nyquist plots represent the corrosion rate, The Nyquist diagrams indicate that the corrosion behavior is controlled by the

charge transfer reaction with an increase of the polarization resistance during immersion. The capacitive semicircles

gradually change to a straight line at low frequencies due to the development of a Warburg resistance. The straight line

indicates that a mass transfer step appeared in the corrosion process. The mass transfer step involves the diffusion of

reactants and corrosion products, such as oxygen and chloride ion. After 96 h, the diameter of the capacitive semicircles

shifts to a higher value, which indicates that the polarization resistance Rp of the ferrite coating increases in this period. As

the coating immersed in 3.5 wt. % NaCl solution for a longer time, the diameter of the capacitive semicircles decreases

sharply compared with the earlier stage. This is indicative of a significant dissolution of the ferrite coating and the corrosion

of substrate.

Fig. 8(b) presents the phase angle of Bode diagram obtained of the ferrite coating immersed in NaCl solution. The

diagram shows a peak during immersion, which exhibits one time constant. The peak in the intermediate range indicates

better inhibitive property of the coating for 96 h immersion. Furthermore, a flattening of the maximum value in the φ-f

curves also provides evidence for the better corrosion resistance of the ferrite coating. The peak position with a maximum

phase angle shifts to low frequency after 96 h, which suggests that the dielectric property of the coatings may be influenced

due to the exposure. In addition, the decrease of peaks height with longer immersion time indicates that the response

becomes less capacitive; indeed, as immersion time increases, the pores permit solution to penetrate into the substrate,

causing local corrosion [32, 33].

Fig. 8(c) presents the impedance modulus of bode diagram obtained of the ferrite coating immersed in 3.5 wt. % NaCl

solution. The value of the impedance modulus at low frequency (0.01 Hz) is a simple parameter, which is used to compare

the protection ability provided by different system against corrosion [34]. It can be seen that the impedance modulus

increases to the highest value with the immersion up to 96 h and then gradually decreases. As the coating immersed in NaCl

solution for 408 h, the impedance modulus at the lowest frequency is less than 1000 Ω.cm2, which suggests the inferior

protection of the ferrite coating. The low resistance value obtained from the ferrite coating could be due to the existence of

micro pores in the coating through which the aggressive ions penetrate and attack the substrate layer.

Based on the above-mentioned analysis of the EIS spectrum, equivalent circuits (EC) are used for fitting the EIS data, as

showed in Fig. 9, Rs corresponds to the solution resistance; Q1 is the constant phase element which represents the

capacitance of double layer, R1 is the charge transfer resistance, W is the Warburg resistance. In the EC circuits, the constant

phase elements Q is introduced to represent the capacitances of double layer and corrosion scale to account for the

deviation from the ideal capacitive behavior due to surface inhomogeneity, roughness and absorption effects [35]. The EC

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in Fig. 9(a) is used for fitting the EIS data immersion for 24 h and 408 h, while the EC in Fig. 9(b) is used for fitting the EIS

data immersion from 96 h to 264 h. According to the EIS diagrams, the corrosion behavior of the ferrite coating can be

divided into three processes, the first process relates to the charge transfer behavior existed on the coating pores of the

electrode at the beginning of immersion (24 h) [36]. The second process is the precipitation of corrosion products and the

mass transfer step at the intermediate stage of immersion (96～264 h). The third process is the development of localized

corrosion of substrate at the immersion of 408 h.

Conclusions

In the present work, spinel ferrite powders are prepared by solid-state reaction synthesis from the constituent oxide

powders with appropriate molar ratios. Spray drying treatment of fabricated powders results in the formation of spherical

and smooth powder particles necessary for plasma spraying. Spinel ferrite coatings are deposited on the mild steel substrate

through plasma spraying. The as-sprayed coating is composed of double structure, i.e. completely melted lamellar and

partially melted structure. The sintered ferrite and the coating show a cubic spinel structure with a secondary phase of

LaFeO3. The recorded Raman spectrum displays four photon mode of Raman active (A1g+Eg+2T2g). The ferrite powder and

coating exhibit soft magnetic properties. The value of saturation magnetization (Ms) of the ferrite coating is 34.417 emu/g.

The corrosion resistance of the ferrite coating is evaluated by EIS. The measured impedance spectroscopy is associated

with the charge-transfer reaction of the coating. The impedance measurements show that the ferrite coatings have superior

corrosion resistance when it is immersed in NaCl solution for 96 h. We have shown that plasma spraying is a promising

technology for the production of magnetic ferrite coatings.

Acknowledgements

The paper is financially supported by National Natural Science Foundation of China (No. 51222510) and 973 Project

(2011CB013403). The authors gratefully extend thank to the support of all members of the project working group.

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Figure captions

Fig. 1 SEM image of the ferrite powders (a) sintered ferrite (b) spray dried ferrite

Fig. 2 X-ray diffraction pattern of ferrite samples (a) sintered ferrite powders (b) ferrite coating

Fig. 3 Cross sectional image of the ferrite coating (a) lower magnification (b) EDS linear analysis on cross-section (c)

higher magnification (d) EDS analysis of the selected regions.

Fig. 4 Surface morphology of the ferrite coating.

Fig. 5 TEM image of the ferrite coating sample.

Fig. 6 Raman spectra of the ferrite coating

Fig. 7 Hysteresis loops of the spinel ferrite samples

Fig. 8 Nyquist plots and Bode plots of the ferrite coatings after immersion for 24 h, 96 h, 168 h, 264 h, 408 h in 3.5

wt.% NaCl solution. (a) Nyquist diagrams (b) Bode diagrams of phase angle (c) Bode diagrams of ︱Z︱

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Fig. 9 The equivalent circuit of EIS plots. Rs is the solution resistance; R1 is charge transfer resistant, Q1 is the double layer capacitance; W is Warburg impedance of ion diffusion in the coating. (a) equivalent circuit 1 (b) equivalent circuit 2

Table 1 Plasma spraying parameters

Current

(A)

Voltage

(V)

Ar

(m3/h)

H2

(m3/h)

Powder feeding rate

(g/min)

Spray distance

(mm)

380 120 3.2 0.4 30 100

Table 2 Composition of the cross section of the ferrite coating

Element Weight% Atomic%

C 4.43 10.74 O 30.98 56.45 Mg 1.04 1.25 Fe 44.65 23.31 Ni 10.17 5.05 Zn 5.78 2.58 La 2.95 0.62 Totals 100.00

Table 3 Static magnetic properties of the ferrite powder and the ferrite coating

Sample label Ms(emu/g) Hc(Oe) Mr(emu/g)

Ferrite powder 71.916 25.028 2.006

Ferrite coating 34.417 170.230 8.011

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

Fig. 6

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Fig. 7

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Fig. 8

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(a) (b)

Fig. 9

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