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A study of corrosion behavior of 3YSZ coating on AZ91D alloy different interlayers

Ayda Shahriari11, Hossein Aghajani 2, Shahin Fazlinejad3

1 Department of Materials Science and Engineering, Faculty of Mechanical Engineering,

University of Tabriz, Iran, AydaShahriari@tabrizu.ac.ir, P.O. Box 51666-16471 2 Department of Materials Science and Engineering, Faculty of Mechanical Engineering,

University of Tabriz, Iran, H_aghajani@Tabrizu.ac.ir 3 Department of Materials Science and Engineering, MalekAshtar University, Tehran, Iran,

shahin1370b@yahoo.com

Abstract The zirconia stabilized by 3wt% Y2O3 (3YSZ) was applied onto the surface of the magnesium

alloy AZ91D using electrophoretic deposition (EPD). Three interlayers of Ni-P, Zinc and aluminum between the substrate and YSZ coating were also prepared by electroless, electroplating and EPD process, respectively. The preparation, microstructure and corrosion resistance of the coatings were investigated. The surface morphologies of the coatings were studied by scanning electron microscopy (SEM) and their compositions were determined by X-ray diffraction (XRD). The corrosion resistance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl neutral solution. However, the results indicate that the YSZ coating improve the corrosion resistance of AZ91D alloy in chloride solution with different interlayers, the aluminum interlayer has a favorable effect on the densification of coating so that it improves significantly the corrosion resistance of AZ91D alloy. The YSZ coating with aluminum interlayer can increase in the charge-transfer resistance of the AZ91D surface in chloride solution and ends to increase the corrosion resistance of Mg alloy.

Keywords: 3YSZ, EPD, Corrosion, interlayers. Introduction Magnesium (Mg) and its alloys have many outstanding properties relative to other engineering materials such as: low density, high strength, great damping capability, excellent fluidity for casting, good electric shielding effect, non-magnetic, satisfactory heat conductivity, low heat capacity, negative electrochemical potential, acceptable recyclability and non-toxicity. These properties make Mg and its alloys attractive to many industries [1]. Also, coatings can protect the surface of metals and alloys by providing a barrier between surface of metal and its environment [2]. When a coating can provide adequate corrosion protection that it be uniform, adhered, pore free [2]. Ceramic coatings on Mg alloy can enhance their corrosion resistance of Mg alloy in corrosive media [3]. Also, among inorganic coatings, yttria-stabilized zirconia (YSZ) is frequently used for these purposes due to its excellent mechanical and chemical properties such as high corrosion resistance in aggressive environment [4]. Electrophoretic coating (E-coat) is a surface coating technology for conductive materials and is applied to magnesium alloys for corrosion protection. The process is also referred to as electrocoat, electrodeposition. The process is based on electrophoretic motion of charged particles suspended in an electrolyte towards an electrode under an applied electrical field [5]. The electrocoating process has many

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advantages compared with other coating processes. First, the electrocoating process produces uniform coatings with very low porosity, and hence provides excellent corrosion protection. Second, the process is capable of coating the surfaces of complicated shapes due to its unique coating formation mechanisms and high throwing power. Third, the process is cost effective for mass production due to its high degree of automation, high throughput and high paint transfer efficiency (95–98%). Furthermore, the process is environmentally friendly. In EPD, the particles that are suspended in an aqueous or non-aqueous solution can accumulate on the surface by applied an electrical filed between two electrodes [6]. The sintering is necessary for increasing the density and the adherence of the coating to substrate [6]. Sintering temperature for ceramic coatings is high and it is a problem to deposit a ceramic material on metal surface because the high sintering temperature is harmful for metal substrate [7]. The previous investigations showed that aluminum and Ni can act as aid- sinter [7], so that, in the presence of these materials, the density of the ceramic coating can improve by reaction bonding in low sintering temperature [7]. The Ni-P coating have a thermal expansion coefficient which is close to the value of Mg alloy and Ni-P electroless interlayer can be well bonded to mg alloy substrate with a blasted surface [8]. The aim of this study is applied an interlayer that help to increase adherence and density of top coating (3YSZ) on AZ91D alloy. In order to this goal, the aluminum, Ni-P and zinc coats were selected as interlayer. Also, the aluminum interlayer was deposited by EPD process, Ni-P interlayer was applied by electroless process and Zinc was applied by electroplating. The effect of these interlayers on quality of YSZ coating was investigated and finally the corrosion resistance of AZ91D alloy with theses coats was studied by electrochemical impedance spectroscopy (EIS).

Experimental methods and materials AZ91D alloy (8.3wt% Al, 0.35 wt% Zn, 0.12wt% Mn, 0.03wt% Cu, 0.02 wt% Ni and 0.05wt% Fe) was cut in to 12×12×2 mm specimens and polished using up to 800# SiC paper. Then, specimens were washed with demineralized water and acetone, and then dried in air. In order to coat an aluminum interlayer on AZ91D surface, aluminum powders (Hunan Hongyuan Co., China) with mean particle size of 5µm were used. Also, the electroless plating of Ni-P interlayer was finished by alkaline cleaning, acid pickling, activation, electroless plating. The solution composition and operation conditions of electroless plating of Ni-P interlayer is listed in Table 1.

Table .1. Electroless plating procedure of Ni–P on the Mg alloy substrate.

Procedure Bath Compositions (Per liter) Condition

Acid pickling CrO3=125 g HNO3(70%)=110 mL

Room temperature, stirring for 60 s

Activation HF(40%)=358 mL Room temperature, stiring for 10 min

Ni plating

NiSO4∙6H2O=20 HF (40%)=12 ml C6H8O7·H2O=5 g

NH4F=10 g NaH2PO2·H2O=20g

NH3∙H2O (25%)=30 ml Thiourea=1×10-3

75 °C, pH=6.2

stirring for 2 h

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All chemicals were analytical reagents, and all solutions were prepared with the distilled water. Finally, 3YSZ powders (Sigma-Aldrich, UK) with mean particle size of 50 nm were used to deposit on Mg alloy surface by EPD process. Also, before zinc coating on some specimens, the alkaline cleaning process was carried out to remove soils and greases. The main composition of alkaline cleaning solution including sodium dodecyl sulphate (C12H25NaO4S), Na2CO3 and NaOH. Then, Then the zinc immersion pretreatment process and electroplating was carried out to obtain the Zn undercoating on AZ91D substrate to ensure adequate adhesion of the plated coatings. Table 2. tabulated the copmposition of zinc imersion and zinc plating solution.

Table.2. Zinc plating procedure on the Mg alloy substrate

Procedure Bath Compositions (Per liter) Condition

Zinc immersion ZnSO4=30g

K4P2O7=110g, NaF=5g Na2CO3=5g

Temprature= 70-80°C

Time=1 min

Zinc plating ZnSO4=30g K4P2O7=100g Temprature=60-70°C

Current density=10 mA/Cm2, Time=3min

The impedance spectra of the coated and uncoated specimens were also measured in 3.5 wt% NaCl solution at open circuit potential using a potentiostat/galvanostat (EG & G, PARSTAT 2263, US). A 3-electrode setup was used for the EIS tests. In this setup, the coated and the uncoated AZ91D with coatings and without coatings were used as the working electrode. The auxiliary electrode and the reference electrode in the cell were 316L stainless steel and saturated calomel electrode (SCE), respectively. To perform these tests, a sinusoidal potential wave with a 10 mV amplitude in the frequency range between 100 KHz and 0.01 Hz was applied to the samples. Finally, the surface and cross-section of the samples were studied by scanning electron microscopy (Tescan MIRA3 FEG-SEM, Czech Republic) equipped with energy dispersive X-ray spectrometer (EDS). In addition, the phase analysis of the samples was determined by X-ray diffraction (Equniox, France).

Results and discussion In order to deposit aluminum particles on the AZ91D surface, a suspension of these particles were prepared. The aluminum powders (10 g/L) were dispersed in the ethanol solution under stirring and then a small quantity of AlCl3.6H2O (0.6 mM) was added to the suspension. The AlCl3.6H2O acts as an electrolyte to control the conductivity and enhances the dispersion of suspension. Subsequently, the slurry was ultrasonically vibrated for 30 min to prepare a well-dispersed stable suspension. The EPD process of the aluminum powders was done at applied voltage 30 V and deposition time of 1 min. A cell of two electrodes was used. The cathode was the magnesium alloy substrate, while the stainless steel 316L was selected as the anode. The equivalent potential was produced by a set of two parallel-plane electrodes separated by a distance of 1 cm. Fig. 1 shows the surface morphology of aluminum particles that are deposited on the mg alloy substrate. So, it is seen from Fig.1 that the aluminum coating on AZ91D alloy is a porous layer with mean size particles of 20µm. Also, the particles are flake form and empty space between of these particles is seen well. The empty space between of the aluminum particles can be suitable place for non-size YSZ particles, so that the YSZ particles can deposit in these empty spaces.

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Fig.1. SEM micrograph of surface morphology of aluminum coating deposited on AZ91D, (a) the

low magnification image and (b) the enlarged image.

In addition, the Fig. 2 shows the cross section of these coating. The thickness of aluminum layer is about 11µm.

Fig.2. SEM image shows the cross section of the aluminum layer on AZ91D alloy.

Then, the YSZ suspension was prepared by acetylacetone- ethanol solvent by ratio volume 1:1, the YSZ powders (10 g/l) were dispersed in acetylaceton-ethanol solution and I2 was added to this suspension as an additive for charging the particles in the suspension and the slurry was ultrasonically vibrated for 10 min. Again, the magnesium alloy with the aluminum interlayer was selected as the cathode and the stainless steel 316L as the anode in the cell of EPD. The distance of the electrodes was fixed at 1 cm during deposition. An applied voltage of 40 V was used for the deposition of 3YSZ particles in a period of 3 min. Also, the samples were heat treated at 400±10 ºC in air for 60 min. The surface morphology of YSZ coating on AZ91D alloy with aluminum layer is shown in Fig. 3. The particle size of YSZ coating on AZ91D alloy with aluminum layer is about 0.3µm and cracks are observed from Fig.3 that they are related to lose of water and drying process.

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Fig.3. Surface morphology of the YSZ coating on AZ91D alloy, (a) and (b) with aluminum

interlayer by low and high magnification respectively.

This is expected that the aluminum particles end to roughen of the surface and the area place for depositing of the YSZ particles increase, therefore, the agglomerate particles in these condition is less than YSZ coating on AZ91D alloy without aluminum layer, so that is shown in Fig. 4. Also, the YSZ particles can place between of the aluminum particles and end to increase density of the coating.

Fig.4. SEM micrograph of YSZ coating on AZ91D alloy: (a) without aluminum interlayer, and

(b) with aluminum interlayer.

Therefore, it can be expected that the YSZ coating applied on AZ91D alloy with aluminum interlayer can fill the empty space between the aluminum particles. Also, the heat treatment after coating can end to form oxide phase of aluminum, so the X-Ray diffraction of coating showed the presence of Al2O3 phase in surface coating (Fig. 5). Therefore, the oxidation of Al to Al2O3 can help to increase density of the ceramic coating on the AZ91D alloy.

The Ni-P electrolessn coating on the AZ91D alloy was applied using an acidic bath that the condition of this bath is shown in Table. 1. After applied the Ni-P coating on AZ91D alloy, the heat treat ment was done at 400ºC for 1h under a controlled atmosphere.

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Fig.5. the X-Ray pattern of YSZ coating applied on AZ91D alloy with aluminum layer.

Finally, 3YSZ particles were applied onto the surface by EPD process. The acetylacetone- ethanol by ratio volume 1:1 was prepared as solvent for EPD process and then the YSZ powders (10 g/l) was added to this solvent with I2 (0.5 g/l) as discernment and stirred for 1 hour and then were vibrated ultrasonically for 30min. Therefore, the AZ91D alloy with Ni-P layer was cathode of the cell in a suspension of YSZ particles and the stainless steel 316L was selected as anode. Then the 3YSZ coating was applied in 40 V and for 3 min on mg alloy surface with Ni-P layer. Fig. 6 shows the surface morphology of Ni-P layer on AZ91D alloy. It can be seen that the coat has fully covered substrate surface. The Ni–P layer is formed of fine nodules that are “cauliflower” shaped.

Fig.6. SEM micrograph of Ni-P coating on AZ91D alloy: (a) with low magnification and (b)

high magnification.

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Also, micro-pores are distributed on the surface of nodules. Analyzed EDS (Fig. 7) of nodules showed that the compositions of the layer were identified as 0.79 wt. % phosphorus and 75.14 wt. % nickel.

Fig. 7. The EDS analyze of Ni-P coating on AZ91D alloy.

Furthermore, the morphology of the YSZ coating on AZ91 with the Ni-P interlayer and without the Ni-P interlayer is shown in Fig. 8, so that, it is seen, the pores in coating without the Ni-P interlayer is more than the YSZ coating with the Ni-P interlayer. The roughness of the surface is increased by the Ni-P interlayer that it ends to increase the suitable positions of surface in order to deposit of the YSZ particles. Therefore, with the Ni-P interlayer, it is expected that density of the top coat is increased. In addition, the YSZ particles can deposit in pinholes that are formed between the nodules of the Ni-P interlayer, so a composite coating can be formed. On the other hand, the results of the XRD analyze is shown in Fig.9. From Fig.9, it is seen that in addition of YSZ phase, NiP2 phase is seen.

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Fig. 8. Surface morphology of the YSZ coating on AZ91D alloy, (a) without Ni-P interlayer and (b) with Ni-P interlayer.

The Ni-P interlayer can possesses the high strength bonding with substrate and this layer can improve the adherence of top coat to mg alloy. In addition, the thermal expansion coefficient of Ni-P is close to AZ91D alloy, therefore, it can end to decrease the cracks that produce due to difference of thermal expansion coefficient.

Fig. 9. The XRD pattern of YSZ coating on AZ91D alloy with Ni-P interlayer.

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Also, Fig. 10 shows the morphology surface and cross section of AZ91D with zinc and YSZ coating. Similar to Ni-P and aluminum interlayer, the zinc plated coating was selected as a interlayer between the YSZ and AZ91D alloy. After the zinc was applied to surface of AZ91D the YSZ coating was deposited by EPD process.

Fig.10. SEM image of YSZ coating with zinc interlayer on AZ91D alloy, (a) surface morphology,

and (b) cross-section morphology.

Also, according to Figures 4, 8 and 10, the micro-cracks are observed on the YSZ coating with interlayers, but it seems that the agglomerate particles on the YSZ coating with aluminum layer is less than the YSZ coating with Ni-P layer and zinc layer. The Ni-P layer on the AZ91D alloy are nodular shape and therefore the rougher than the aluminum particles on the AZ91D alloy. So, the YSZ particles on the Ni-P layer deposit more heterogeneous and the possibility of agglomeration of particles increases. Also, the micro pores between the aluminum particles that are flake shape is more than the Ni-P nodular layer, therefore, the YSZ Nano-particles can place between the aluminum particles and fill the pores and empty space between aluminum particles. Also, the during the heat treatment the zinc layer can be evaporated and therefore it can help to produce cracks on the surface of YSZ coating.

EIS analyze coatings The EIS tests were done on the coated and uncoated samples. So, the Nyquist diagrams of the samples are shown in Fig. 11. The Nyquist diagram of the AZ91D alloy without coating, AZ91D alloy with zinc and aluminum interlayer and YSZ coating show an inductive behavior that a potential- dependent adsorption of this part of the impedance plot seems to be rather complicated and is immaterial in studies of electrochemical corrosion, in which the kinetic information is estimated from the capacitive arcs [9]. In addition, the Nyquist curves of coated samples show two semicircles which the semicircle at high frequencies can be related to the presence coatings on the magnesium alloy. Also, the semicircles at low frequencies are attributed to. oxide/redox reactions on the mg alloy substrate according to Eq. (1)-(3). Mg = Mg + 2e

(1)

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2H O + 2e = H + 2OH

(2) Mg + 2H O = Mg(OH) + H

(3)

The equivalent circuits used to explain the corrosion behavior of the AZ91D with coatings and without coatings are shown in Fig. (12). The suitable equivalent circuit for analyzing the corrosion behavior of the AZ91D without coatings and with zinc interlayer and YSZ coating is presented in Fig. 12 (a). Conversely, the circuit model for the AZ91D with aluminum and YSZ coating is presented in Fig. 12(b) and Fig. 12(c) shows the circuit model for YSZ and Ni-P coating on the mg alloy.

In these circuit models, Rs represents the solution resistance, Ccoat is the capacitance of the constant phase element (an indicator of the capacity of the coat), and n1 is the power associated with it. In addition, Rcoat is the coat resistance, Cdl is the double layer constant phase element (an indicator of the double layer capacitance between the AZ91D alloy surface and the solution), and ndl is the constant phase element power associated with it, Rt is charge-transfer resistance of the corrosion reaction, Ladsorp is the inductance, and Radsorp represents the inductance real resistance. The impedance parameters based on the proposed circuit model were calculated by use of Zview software, in other words using the software’s numerical methods, appropriate values are fitted on the measured data and these values are shown in Tables 3 with the sum of weighted squares.

Also, the Eq.4 shows the formula for calculating the capacitance of the constant phase element impedance. In this equation, ZCPE is the impedance of the constant phase element, C represents the capacitance, ω stands for the frequency, n is the constant phase element exponent, and i is the imaginary unit. It is also worth noting that n varies between 0 and 1 [10]. = [ ( ) ] (4)

Table. 3. Circuit element values calculated by fitting the model presented in Fig. 12.

Sample RS

(Ω.cm2)

Cdl

(µF/cm-

2) ndl

Rt

(Ω.cm2)

Ccoat

(µF/cm-

2) ncoat

Rcoat

(kΩ.cm2) Ladsorp

(kH/cm2) Radsorp,

(kΩ.cm2)

Sum of weighted squares

AZ91 1.87 3.9 0.76 1470.2 --- --- --- 4.52 4.9 0.06

YSZ/Zinc coating 1.87 1.3 0.56 115.2 3.24 3.1 0.04

YSZ/Al coatings 1.81 2.1 0.81 5267.5 2.4 0.9 1361.1 1.86 1.4 0.08

YSZ/Ni-P coatings 1.80 2.6 0.92 5032 1.1 0.85 532.4 --- --- 0.05

According to Eq. (4), by decreasing the capacitance, the capacitive impedance increases. Therefore, in the corrosion process, if the charge- transfer capacitance decreases, it will be expected that the charge- transfer resistance increases. The YSZ coating with aluminum interlayer and also Ni-P interlayer show the charge- transfer capacitance lower than the AZ91D

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alloy without coating, therefore, it expected that the charge- transfer resistance of YSZ with interlayers coatings be higher than mg alloy without coating, so that it is seen from Table. 3 and also Fig. 11. On the other hand, the YSZ coating with interlayers can act as an effective barrier opposite of NaCl solution, and it ends to increase the corrosion resistance mg alloy substrate with coatings. Also, the YSZ- aluminum coating shows a charge-transfer and coating resistance higher than YSZ-Ni-P coatings on AZ91D alloy. It can be related to the morphology of YSZ coating on the aluminum interlayer and Ni-P interlayer, so according to Fig. 3, the cracks and pits on the YSZ coating with aluminum layer is smaller than YSZ coating with Ni-P layer, and also, the agglomerate particles of YSZ coating with aluminum layer is lower than YSZ with Ni-P layer, therefore, the ways for the diffusion of aggressive species of solution is less for YSZ coating with aluminum interlayer. In addition, the aluminum particles are fake shape, so that, the YSZ nano particles can fill the space between these particles, therefore, it is expected that the space between the aluminum particles are blocked and therefore, these coating can provide a suitable protective of the mg alloy surface. On the other hand, AZ91D alloy with zinc interlayer and YSZ coating shows the lowest corrosion resistance. The corrosion resistanc of AZ91D alloy with zinc and YSZ coating is lower than AZ91D alloy without coating. It can be related to peresne of pits and crakcs on Zinc-YSZ coating that they can be formed because evaporating the zinc interlayer during the heat treatment, the evaporation of the zinc ends to delamination of YSZ coating, therefore, the micro pills are formed in this coating that can end to the drastic corrosion.

Conclusion The conclusions of this study can be summarized as follows: The applied of Ni-P interlayer or aluminum interlayer can improve the quality of the YSZ coating on the AZ91D alloy, so that, the micro cracks and agglomerate of YSZ particles on the YSZ coating with interlayers is lower YSZ coating without interlayers. The aluminum and Ni-P interlayer end to decrease the solvent of the mg surface in acidic suspension (acetylaceton- ethanol solution) that it can help to stability of the YSZ particles in the suspension because the mg ions can increase the conductivity of the suspension and finally effect on the stability of suspension. The YSZ coating with aluminum interlayer is denser than YSZ coating with Ni-P interlayer that it can be related to the morphology of the Ni-P interlayer which is nodular shape and it ends to create hills on the surface so that, this kind of the surface increases the probability of porosity and cracks in coating. Also, the aluminum particles are fake shape and YSZ particles can fill the space between the aluminum particles and the aluminum oxide can be formed during the heat treatment that it can help to increase the density of the coating because the volume expansion from changing of aluminum to aluminum oxide (Al2O3). On the other hand, the YSZ coatings with aluminum interlayer or Ni-P layer on AZ91D alloy show corrosion resistance higher than AZ91D alloy without and AZ91D alloy with zinc and YSZ coating. The charge-transfer resistance the YSZ coating with aluminum and Ni-P interlayers is increased, so that, the YSZ coating and interlayers can act as a barrier opposite the corrosive media and finally increase the charge-transfer resistance double layer that formation between the mg alloy surface and corrosive solution. The YSZ coating with aluminum layer shows corrosion resistance better than YSZ coating with Ni-P layer, that it can be attributed to morphology of the YSZ coating with aluminum layer that are denser.

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Fig. 11. The Nyquist curves coated and uncoated samples.

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Fig.12. Circuit models proposed for the electrochemical impedance data measured (a) related to uncoated sample and coated sample with zinc and YSZ coating, (b) Coated sample with YSZ and

aluminum coating, and (c) Coated sample with YSZ and Ni-P coating.

Acknowledgments

We gratefully acknowledge of the advanced material laboratory in the Central laboratory in the University of Tabriz that provided the valuable services for this research.

References

1. GL. Song, Corrosion of magnesium alloys, Woodhead Publishing Limited, 2011

2. J.E. Gray, B. Luan, J. Alloys Compd., 336 (2002) 88.

3. R. Rojaeea, M. h. Fathia, and K. Raeissic, Appl. Surf. Sci., 285 (2013) 664.

4. R.G. Hua, S. Zhanga, J. F.Bua, C.J. Lina, and G.L. Songb, Prog. Org. Coat., 73 (2012) 129.

5. Z. Yao, W. H. Gao, Z. Jiang, and F. Wang, J. Am. Ceram. Soc., 91 (2008) 555.

6. M. Federica De Riccardis, Ceramic Coatings Obtained by Electrophoretic Deposition: Fundamentals, Models, Post-Deposition Processes and Applications, Ceramic Coatings - Applications in Engineering, Prof. Feng Shi, ISBN: 978-953-51-0083-6, InTech, 2012, p 43-68.

7. L. Besra, M. Liu, Prog. Mater. Sci., 52 (2007) 1.

8. A.R. Boccaccini, I. Zhitomirsky, Curr. Opin. Solid State Mater. Sci., 6 (2002) 251.

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9. A. Pardoa, M.C. Merinoa, A.E. Coya, F. Viejob, R. Arrabalb, S. Feliú Jr., Electrochim. Acta., 53 (2008) 7890.

10. E. Barsoukov , J. Ross Macdonald, Impedance Spectroscopy Theory, Experiment and Applications, 2nd ed., John Wiley, New Jersey, 2005.