effect of preparation method on the anti-corrosive properties of nanocrystalline zn–coo ceramic...

Materials and Corrosion 2011, 62, No. 5 DOI: 10.1002/maco.201005758 405

Effect of preparation method on the anti-corrosiveproperties of nanocrystalline Zn–CoO ceramic pigments

S. Rasouli* and I. Danaee

Zn–CoO green ceramic pigments were synthesized by two different methods;

high energy ball milling and solution combustion, with two different fuels; citric

acid and glycine. Products were characterized by X-ray diffraction and scanning–

transmission electron microscopy (TEM). The anti-corrosive properties of the

obtained pigments were investigated by electrochemical impedance

spectroscopy (EIS) techniques. Results have shown that either by solid state

reaction or combustion by citric acid, a calcination step was needed to obtain

the desired phase whereas by glycine fuel, pure ZnO phase was obtained

directly. TEM showed particles with mean particle size of about 70, 150, and

180nm for glycine, citric acid, and solid state reaction samples, respectively.The

corrosion performance of the coating in 3% w/v NaCl solution was evaluated by

EIS and polarization measurements. According to the measurements of EIS and

electrochemical polarization, the coatings with glycine-based pigment showed

the highest corrosion resistance among the prepared coatings.

1 Introduction

The anti-corrosive properties of ceramic pigments based on ZnO

doped with cobalt have been investigated [1] and it is shown that

the combination of cobalt and zinc oxides improve the anti-

corrosive properties of the obtained pigment. Furthermore, the

green colored oxides of ZnO–Co system, with low Co content,

have been interested as new environmental friendly colored

pigments and they can be used as substitution of chromium

oxides [2, 3].

Corrosion of iron and mild steel is a fundamental academic

and industrial concern that has received a considerable amount of

attention [4]. The use of inhibitors is one of the most practical

methods for protection against corrosion, especially in acidic

media [5]. The progress in this field has been phenomenal in

recent years and is borne out by the output of literature [6]. Acid

and NaCl solutions are widely used in industry, the most

important fields of application being acid pickling, industrial acid

cleaning, acid descaling, and oil well acidizing. Because of the

general aggressive nature of acid solutions, inhibitors are

S. Rasouli

Department of Nanotechnology, Institute for Color Science and

Technology (ICST), 55 Vafamanesh Ave., HosseinAbad Square, Pasdaran

St., 1668814811 Tehran (Iran)

E-mail: [email protected]

I. Danaee

Abadan Faculty of Petroleum, Petroleum University of Technology

(PUT), Abadan (Iran)

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commonly used to reduce the corrosive attack on metallic

materials.

Most of the well-known inhibitors are organic compounds

containing nitrogen, sulfur, and oxygen atoms. The influence of

organic compounds containing nitrogen, such as amines and

heterocyclic compounds, on the corrosion of steel in acidic and

NaCl solutions has been investigated by several works [7–9]. Most

of corrosion inhibitors were based on ceramic pigments, which

have good adsorption on the steel surface.

The application of electrochemical impedance spectroscopy

(EIS) technique as a new tool in corrosion research has resulted in

a wealth of information concerning the methods of corrosion

protection which were difficult to study with traditional dc

techniques. This includes corrosion protection by conversion

coatings [10], polymer coating, and anodic films [11]. EIS has also

provided information concerning corrosion protection by

inhibitors [12]. In addition to specification of the physical

properties of the system, the technique leads to important

mechanistic and kinetic information [13]. Potentiodynamic

polarization and EIS techniques are applied to study the ability

of ceramic pigments as a non-ionic surfactant (NS) to inhibit the

corrosion of mild steel in 3% w/v NaCl solution.

Zn–CoO pigments can be synthesized via different methods,

such as high energy ball milling [3, 14], wet chemical [15], and

combustion [16, 17] methods. High energy ball milling (solid

state reaction) is the easiest method for industrial applications [8].

Damontea et al. [18] used the mechanical alloying, via high energy

ball milling method, for cobalt incorporation into the wurtzite

structure. Solution combustion method is one of the most

interesting synthesis routes for production of homogenous

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

406 Rasouli and Danaee Materials and Corrosion 2011, 62, No. 5

nanocrystalline single and multicomponent oxides. The ability to

achieve high purity single andmultiphase complex oxide powders

is the great advantage of this technique for preparing ceramic

materials. Sulcova and Trojan [3] have reported the synthesis of

ZnO–Co pigment. They reported that an intense green hue can be

obtained by Zn0.9Co0.1O compared to other compositions of

Zn–CoO.

The preparation methods of the Zn–CoO pigments can

influence the characteristics of the obtained particles such as the

particle size and shape. In the present work, two preparation

methods, mechanical (high energy milling) and chemical

(solution combustion using two different fuels; glycine and

citric acid) were used to prepare Zn–CoO pigments. The anti-

corrosive properties of the obtained pigments have been

investigated by EIS technique.

2 Materials and methods

2.1 Preparation of pigments

Experimental conditions are shown in Table 1 and are detailed as

follows.

2.1.1 High energy milling

Solid solution of Zn–CoO was prepared by using analytical grade

of ZnO (99.9% Merck) and cobalt oxide pure Co3O4 (99.9%

Merck) as starting materials. A complete solid solution of Zn–

CoO, can be achieved by addition of 10% of Co3O4 [18]. All

samples were wet milled in a planetary mono mill (pulverisette 6,

Fritsch, Germany) with zircon grinding bowls (volume of 500ml)

and grinding balls (20 balls, 20mmdiameter) at 300 rpm in air for

120min. After milling the slurry was dried in an oven at 110 -Cfor 2 h to remove residual water. Then powders were calcined in a

chamber furnace at 1000 -C with a heating rate of 10 -C/min.

2.1.2 Solution combustion

Zn(NO3)2 � 6H2O (Merck) and Co(NO3)2 � 6H2O (Merck) of

analytical grade were used as an oxidant and citric acid and

glycine as fuel and complexing agent.

In the case of citric acid, the propellant chemistry for

synthesis of Zn0.9Co0.1O can be written by following equation

[19]:

0:9ZnðNO3Þ2 � 6H2Oþ 0:1CoðNO3Þ2 � 6H2O

þ 5

9C6H8O7 ! Zn0:9Co0:1Oþ CO2 þH2Oþ N2

(1)

The exothermic redox reaction between zinc nitrate, cobalt

nitrate, and glycine to achieve Zn0.9Co0.1O composition can be

Table 1. Colorimetric data of Zn0.9Co0.1O powders prepared by solid state r

(W) as fuel

Samples Preparation route

1 (S) Solid state reaction Zn

2 (C) Combustion with citric acid Zn(NO3)

3 (W) Combustion with glycine Zn(NO3)


expressed as follows [20]:

0:9ZnðNO3Þ2 � 6H2Oþ 0:1CoðNO3Þ2 � 6H2O

þcNH2CH2COOH ! Zn0:9Co0:1O

þ2cCO2 þ 7:9cH2Oþ 0:95cN2

(2)

where the value C¼ 10:9 represents the equivalent ratio for the

present work (the ratio at which the value of oxygen balance is

equal to zero).

Metal nitrates and fuels were dissolved in distilled water to

form a solution in which Zn2R and Co2R concentration meet the

formula of Zn0.9Co0.1O.

After adding metal nitrates and fuel (citric acid or glycine) to

the deionized water in the beaker vessel, the obtained solution

was heated on a hotplate (80–90 -C) until the excess water was

removed and a highly viscous precursor gel was gained. The

beaker vessel was transferred into a microwave oven (Samsung,

Korea, 900W, 2.45GHz frequency) to complete the combustion

reaction. All experiments were performed at maximum power of

microwave for 50 s. The obtained powder with citric acid was

calcined at 1000 -C in a chamber furnace.

2.2 Corrosion tests

As-prepared pigments were incorporated in an alkyl-based

coating system in 2wt%. The dispersion was done using a

mechanical stirrer followed by ultrasonication. The nanocompo-

site coating was applied on pre-treated steel test panels

(76.2mm� 152.4mm� 0.8mm) by an appropriate applicator.

A steel sheet was used as working electrode. The exposed

surface area of each electrode was 1 cm2. Prior to the coating of

the surface, the surface pre-treatment of the working electrode

was performed by mechanical polishing of the electrode surface

with successive grades of emery papers down to 1200 grit up to a

mirror finish. The electrode was then rinsed with acetone and

distilled water before film formation. The experiments were

performed in a 100 cm3 volume cell at 20 8C, using a platinum

and calomel as auxiliary and reference electrodes, respectively.

The experiments were carried out in 3% w/v NaCl solution. All

solutions were freshly prepared from analytical grade chemical

reagents (Merck) using doubly distilled water and were used

without further purification. For each run, a freshly prepared

solution as well as a new sample of electrodes was used.

2.3 Characterization techniques

A D-500 diffractometer (Siemens, Karlsruhe, Germany) was used

for XRD analysis. Morphology analysis was performed using LEO

eaction (S) and combustion method by using citric acid (C), and glycine

Raw materials Calcination temperature (8C)

O, Co(NO3)2 � 6H2O 1000

2 � 6H2O, Co(NO3)2 � 6H2O 900

2 � 6H2O, Co(NO3)2 � 6H2O

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Materials and Corrosion 2011, 62, No. 5 Anti-corrosive properties of nanocrystalline Zn–CoO ceramic pigments 407

1455VP scanning electron microscope (Oxford, UK). The IR

spectra of the dried gel and calcined powder were given by FT-IR

(Spectrum one, FT-IR spectrometer, Perkin Elmer).

Electrochemical studies were carried out in a conventional

three electrode cell powered by an electrochemical system

comprising of EG&G model 273 potentiostat/galvanostat and

Solartron model 1255 frequency response analyzer. The system is

run by a PC through M270 and M398 commercial software via a

GPIB interface.

3 Results and discussion

3.1 Pigments synthesis

Three samples were prepared according to the experimental

conditions shown in Table 1 and they were investigated by X-ray

diffractometer. Figure 1a–c shows the obtained XRD patterns for

the samples. In the case of the high energy milling method a

calcination step has been performed at 1000 8C [21]. In the

combustion method by using citric acid as fuel the obtained

powder was in an amorphous state and a calcination step was

performed at 900 8C [3]. The XRD patterns of the glycine-based

sample presented on Fig. 1c shows that the well-crystalline single

phase ZnO was obtained directly during the combustion process

and no calcination step was needed. According to the P63mcstructure (JCPDS 5-664) no peaks attributable to other phases

were observed, indicating the single phase of zinc oxide and

complete entrance of Co into the ZnO structure.

The average crystallite size (D) of the samples has been

calculated by Debye–Scherrer’s equation on the XRD patterns

according to Equation (3):

D ¼ 0:9l

b cos u(3)

where l is the wavelength of incident X-ray, b the half width of

diffracted peak, and u is the diffracted angle. The average

crystallite size was 30, 37, and 65 nm for glycine, citric acid, and

solid state reaction samples, respectively.

Figure 2 shows SEM images of Zn0.9Co0.1O prepared by

using (a) glycine, (b) citric acid after calcination at 900 8C, and(c) solid state reaction after calcination at 1000 8C.

Figure 1. XRD pattern of Zn–CoO prepared by (a) high energy milling,

(b) citric acid, and (c) glycine

Figure 2. SEM images of Zn–CoO powder prepared by (a) glycine,

(b) citric acid, and (c) high energy milling

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Figure 2a shows the as-prepared Zn0.9Co0.1O sample is

spongy with porous agglomerates with pore size about 0.1–5mm.

During the combustion process, gases were given out instantly,

leading to the formation of the pores. From Fig. 2b it seems that

particles were partially sintered and grain growth occurred during

calcination. Figure 2c reveals quasi-spherical particles with large

size distribution between 0.2 and 0.8mm.

Figure 3 shows TEM images of Zn0.9Co0.1O solid solution

prepared by (a) glycine, (b) citric acid after calcination at 900 8C,and (c) solid state reaction after calcination at 1000 8C.



Figure 3. The photos by TEM of the samples (a) glycine, (b) citric acid

calcined at 900 8C, and (c) high energy milling

Figure 4. Typical potentiodynamic anodic and cathodic polarization

curves recorded for steel in 3% w/v NaCl solution in the absence and

presence of the inhibitor on the surface of the electrode. (W, the

pigment from glycine; C, the pigment from citric acid; S, the pigment

from solid state reaction; and A, film without any pigment)

Table 2. Polarization parameters for the corrosion of steel electrode in 3%

Sample Ecorr (mV) Icorr (mA cm�2) Tafel slop

A �657 15.8

S �465 2.11

C �525 1.05

W �375 0.5


Figure 3a shows quasi-spherical particles with mean particle

size of about 70 nm. From Fig. 3b, the rod-like particles with

mean particle size of 150 nm are observed whereas particles of

about 180 nm are illustrated on Fig. 3c. It seems that the

calcination step has caused the growth in the particle size in the

case of citric acid and solid state reaction.

3.2 Electrochemical investigation

In order to study the influence of the inhibitors on the corrosion

resistance of steel panels, a potentiodynamic study has been

performed on steel in NaCl solutions for pigments prepared by

glycine, citric acid, and solid state reaction and the results were

compared to bare electrode without any pigment. Figure 4 shows

typical potentiodynamic polarization curves recorded for steel

electrode in solution containing 3% w/v NaCl for different

prepared samples with and without pigments: W, the pigment

from glycine; C, the pigment from citric acid; S, the pigment from

solid state reaction; and A, film without any pigment. The

polarization curves show Tafel type behavior of these samples.

Tafel calculations are listed in Table 2, where Ecorr, Icorr, CR, ba, bc,and IE (IE¼ (Iblank� Iinhi)/Iblank) [22] are the corrosion potential,

corrosion current density, corrosion rate, anode Tafel constant,

cathode Tafel constant, and inhibition efficiency, respectively.

The open circuit corrosion potentials, Ecorr, are drifted to

more positive values and a significant decrease of the corrosion

current was observed on the steel substrate in presence of C, W,

and S. These results indicate that these inhibitors act generally as

anodic inhibitor. According to the data of Table 2, it is obvious that

w/v NaCl solution in the absence and presence of inhibitors at 25 8C

e ba (mVdec�1) Tafel slope bc (mVdec�1) IE (%)

181 640 �99 370 0.94

412 523 0.93

287 275 0.96

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Materials and Corrosion 2011, 62, No. 5 Anti-corrosive properties of nanocrystalline Zn–CoO ceramic pigments 409

Figure 5. Typical potentiodynamic anodic and cathodic polarization

curves recorded for steel after 7 days immersion of the electrode in 3%

w/v NaCl solution in the absence and presence of the inhibitor on the

surface of the electrode.(W, the pigment from glycine; C, the pigment

from citric acid; S, the pigment from solid state reaction; and A, film

without any pigment)

Figure 6. (a) Nyquist plots for steel sample in 3% w/v NaCl solution at

OCP and at 25 8C in absence and presence of inhibitors. (b) Bode plots

for steel sample in the same solution with inhibitors. (W, the pigment

from glycine; C, the pigment from citric acid; S; the pigment from solid

state reaction; and A, film without any pigment)

in the presence of the inhibitor, the corrosion rate of the samples

decreases in the order of A> S>C>W. So the pigment from

glycine has the highest corrosion resistance and also higher

inhibition efficiency in NaCl solution.

Figure 5 shows typical potentiodynamic polarization curves

recorded for steel electrode in solution containing 3%w/v NaCl in

the absence and presence of S, C, and W after 7 days immersion.

As can be seen, after seven days the protective effect of inhibitor

layer of C and W was not obviously changed. Tafel calculations

after 7 days immersion are listed in Table 3.

It seems that quasi-spherical particles withmean particle size

of 70 nmhave better corrosion inhibition surface than the rod-like

ones with mean particle size of 150–180 nm. This can be due to

the better adhesion and compression of quasi-spherical particles

with mean particle size of 70 nm.

On the other hand, this phenomenon can be explained easily

by filling free spaces between particles of smaller sizes. At larger

particles, the filling of pores by means of oxide is incomplete and

the leakages can lead to more easy liquid and gas penetrations

through the paint film. With particles of rod-like type, the sealing

of rather voluminous pores in the coating is difficult, and the

coatings exhibit a lower anti-corrosion efficiency.

In order to get more information about the corrosion

inhibition phenomenon, solution resistance (Rs), charge-transfer

resistance (Rp), double layer capacitance (Cdl) of steel electrode, and

impedance measurements have been carried out in chloride

solutions in the absence and presence of the inhibitors at the OCP.

Table 3. Polarization parameters for the corrosion of steel electrode after 7

inhibitors at 25 8C

Sample Ecorr

(mV)Icorr

(mA cm�2)

A �625 19.9

S �542 3.9

C �589 7.9

W �575 1.25

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The Nyquist plot for steel sample in 0.2M NaCl solution at

OCP and at 25 8C is shown in Fig. 6a. The data reveal that the

impedance diagram consists of a capacitive loop due to charge

transfer resistance and double layer capacitance. Bode phase plots

for the same sample is shown in Fig. 6b. Distinguishable peaks

are observed in the Bode plots corresponding to semi-circles in

the Nyquist plot. The equivalent circuit compatible with the

Nyquist diagram recorded in the presence of alcohols was

depicted in Fig. 7. To obtain a satisfactory impedance simulation

of corrosion, it is necessary to replace the capacitor C with a

constant phase element (CPE),Q, in the equivalent circuit. In this

days immersion in 3%w/v NaCl solution in the absence and presence of

Tafel slopeba (mVdec�1)

Tafel slopebc (mVdec�1)

IE (%)

101 629 �311 328 0.81

350 401 0.6

125 205 0.93



Figure 7. The equivalent circuit model used to analyze the

experimental data presented in Fig. 6

Table 4. Equivalent circuit parameters for the corrosion of steel elec-

trode in 3%w/v NaCl solution in the absence and presence of inhibitors

at 25 8C

Sample Rs (V) Cdl� 105 (F) Rct (V) n

A 42 6.1 5152 0.85

S 43 1.05 1 2550 0.82

C 44 2.2 1 7223 0.8

W 45 0.91 4 1325 0.83

electrical equivalent circuit, Rs, CPEdl, and Rct represent solution

resistance, a constant phase element corresponding to the double

layer capacitance and the charge transfer resistance.

To corroborate the equivalent circuit, the experimental data

are fitted to equivalent circuit and the circuit elements are

obtained. Table 4 illustrates the equivalent circuit parameters for

the impedance spectra of corrosion of steel in NaCl solution. As

can be seen from Table 2, in the presence of inhibitors, the

diameters of semi-circle in Nyquist diagrams increase. Therefore

higher charge transfer resistance and also lower corrosion current

density are observed in the presence of the film with the pigment

from glycine.

4 Conclusions

In this investigation, Zn0.9Co0.1O green ceramic pigments were

successfully synthesized by microwave-assisted solution combus-

tion by two fuels and high energy ball milling methods. X-ray

results have shown a calcination step was needed to obtain the

desired phase when citric acid was used as fuel whereas by

glycine, pure ZnO phase was obtained directly. Transmission

electronmicroscopy (TEM) demonstrated the quasi-spherical and

rod-like particles with mean particle size of 70, 150, and 180 nm


for combustion with glycine and citric acid and solid state

reaction, respectively.

Investigations of the corrosion inhibition of steel electrode in

solution containing chloride ions through electrochemical

methods demonstrated that the coatings with glycine-based

pigment showed better corrosion resistance and lower corrosion

current. It was revealed that this coating was more compact and

uniform rather than the coatings with citric acid-based and solid

sate reaction-based pigments.

5 References

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683.

(Received: April 24, 2010)

(Accepted: June 16, 2010)

W5758

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effect of preparation method on the anti-corrosive properties of nanocrystalline zn–coo ceramic...

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