A new strategy for improvement of the corrosion resistance of a green cerium conversion coating through thermal treatment procedure before and after application of epoxy coating

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Applied Surface Science 390 (2016) 623–632
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
A new strategy for improvement of the corrosion resistance of a green
cerium conversion coating through thermal treatment procedure
before and after application of epoxy coating
Z. Mahidashti a , T. Shahrabi a,∗ , B. Ramezanzadeh b,∗
a
b
Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box: 14115-143, Tehran, Iran
Department of Surface Coatings and Corrosion, Institute for Color Science and Technology (ICST), P.O. 16765-654, Tehran, Iran
a r t i c l e
i n f o
Article history:
Received 17 July 2016
Received in revised form 28 August 2016
Accepted 29 August 2016
Available online 31 August 2016
Keywords:
Ce conversion coating
Epoxy coating
Corrosion resistance
Post-heating
SEM/EDS
EIS
a b s t r a c t
The effect of post-heating of CeCC on its surface morphology and chemistry has been studied by scanning
electron microscopy (SEM), energy dispersive spectroscopy (EDS) and contact angle (CA) measurements.
The corrosion protection performance of the coatings was investigated by electrochemical impedance
spectroscopy (EIS). The effect of thermal treatment of CeCC on the corrosion protection performance
of epoxy coating was investigated by EIS. Results showed that the heat treatment of Ce film noticeably
improved its corrosion resistance and adhesion properties compared to that of untreated samples. The
CeCC deposited on the steel substrate at room temperature had a highly cracked structure, while the
amount of micro-cracks significantly reduced after post-heating procedure. Results obtained from EIS
analysis confirmed the effect of post-heating of CeCC on its corrosion protection performance enhancement. The increase of post-heating temperature and time up to 140 ◦ C and 3 h led to better results.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Due to the good mechanical properties, carbon steel has been
used more and more frequently in several industrial applications.
However, the low corrosion resistance of carbon steel in corrosive
environments is still the big concern in many industries. Therefore, various corrosion protection methods have been proposed by
a large number of researchers [1–5] to provide higher resistance
for steel against corrosion. Among these, chemical treatment by
conversion coatings has been introduced as a common approach
for obtaining proper corrosion resistance against corrosive environments [6–9]. In addition, the surface treatment of metals i.e.
steel is a promising strategy for achieving good adhesion of organic
coatings [10–13]. Chromate conversion coatings have been widely
used for chemical treatment of metals [14–16]. However, due to
the presence of toxic and carcinogenic hexavalent chromium compounds in the coating structure, the use of this particular coating
has been strongly restricted in recent years. Therefore, attempts
have been made to replace chromate based conversion coatings
with less toxic and environmentally safe ones [17–24].
∗ Corresponding authors.
E-mail addresses: [email protected] (T. Shahrabi),
[email protected], [email protected] (B. Ramezanzadeh).
http://dx.doi.org/10.1016/j.apsusc.2016.08.160
0169-4332/© 2016 Elsevier B.V. All rights reserved.
In recent years, cerium based conversion coatings have attracted
lots of attention due to theirs bold characteristic of being environmentally friendly [25,26], self-healing ability [27], and acceptable
corrosion performance [3]. Furthermore it has high potential of
improving the adhesion properties of the applied organic coatings
[28]. Unfortunately, the cerium film has a highly cracked morphology reducing its corrosion resistance. In addition, the hydrogen
peroxide, which is added to the solution bath as an accelerator,
causes severe damage of coating morphology due to the fast evolution of H2 gas [29,30]. Therefore, several researchers have focused
on utilizing various methods for modification of CeCC structure.
Ramezanzadeh et al. used zinc phosphate conversion coating as a
sealing agent in order to improve the corrosion resistance of the
CeCC sub-layer. In this way a denser and less cracked coating was
formed on the surface [3]. In another study, they compared the
adhesion properties of the epoxy/polyamide coating applied on the
substrates treated with Zn, Ce and the Ce film post-treated by Zn.
The results indicated that the lowest adhesion loss of epoxy coating
was obtained on the sample treated by Ce-Zn layer. It was suggested that the Ce-Zn coating reduces the cathodic reaction rate at
the coating/metal interface and prevents the cathodic detachment
of the coating from the steel sub-layer [28]. Yasuyuki and Yutaka
evaluated the effect of SO4 2− addition to the cerium bath on the
corrosion behavior of galvanized steel. The results showed that the
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Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
Table 2
Thermal treatment conditions and nomination for different samples.
Table 1
Chemical composition of steel sheet.
Elements
Fe
C
P
Mn
Si
Cr
Ni
Composition (wt %)
93.9
0.2
1.2
1.1
1
0.7
1.7
SO4 2− acted as a growth inhibitor and/or grain finer and in this way
enhanced the corrosion resistance of cerium layer [31].
As stated above, the cerium conversion coating performance
has been widely considered in previous studies. However, to the
authors’ knowledge, the effect of heat-treatment on the morphological, structural and corrosion resistance of the cerium conversion
coating on the steel substrate has not been reported. This study
investigates the effect of thermal treatment of the cerium conversion coating applied on steel substrate on its surface morphology,
chemistry, surface free energy and corrosion resistance.
Heat treatment
time (h)
Heat treatment
temperature (◦ C)
Samples
designationa
1
2
3
3
3
100
100
100
120
140
S(1 h:100 ◦ C)
S(2 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:120 ◦ C)
S(3 h:140 ◦ C)
a
S(treatment duration (h): treatment temperature (◦ C)), where S is the abbreviated for each sample.
were post-heated for different times and temperatures. The posttreatment conditions and the nomination of the samples are listed
in Table 2. The final samples were kept in desiccator for further
characterization.
2.3. Epoxy coating application
2. Experimental
2.1. Materials
St-12 coupons (dimensions of 20 × 30 × 2 mm) with the composition given in Table 1, were used in this study. Chemical treatment
baths were prepared using cerium nitrate and hydrogen peroxide
which were purchased from Merck Co (Germany). Hydrochloric acid and sodium hydroxide were procured from Mojallali Co
(Iran). Epoxy resin (Araldite GZ 7071 × 75) and polyamide hardener (CRAYAMID 115) were supplied by Saman and Arkema Co,
respectively.
2.2. Surface treatment process
Steel samples were abraded up to #1200 emery paper,
degreased and washed before immersing in the cerium (Ce) chemical bath containing cerium nitrate (2 g/L), hydrogen peroxide
(0.6 mL/L) and hydrochloric acid 37wt % (11.5 mL/L). The pH of the
solution was adjusted at 3 (by addition of NaOH 5wt % solution). The
chemical treatment was carried out for 5 min at ambient temperature (25 ± 5 ◦ C). The prepared samples were washed with distilled
water and dried in air. In the next step, the Ce treated samples
Epoxy coating was prepared through mixing the epoxy resin and
polyamide curing agent with the ratio of 1.3:1 w/w. Then, the coating was applied on the bare steel and Ce treated samples (before
and after post-heating) with a wet film thickness of 120 ␮m using
a film applicator. Finally, the coated samples were kept at ambient
temperature for 24 h and then post-cured at 100 ◦ C for 1 h. The dry
film thickness of the samples was 50 ± 5 ␮m.
2.4. Characterization
2.4.1. Surface characterization techniques
The morphology and composition of the steel samples treated by
Ce film were investigated by scanning electron microscope (SEM)
model Phenom ProX equipped with an energy dispersive spectroscopy (EDS) prior and after post-heating process. Static contact
angles were measured on the surface of different samples by an
OCA 15 plus type contact angle measuring system. Distilled water
was used as probe liquid and the measurements were performed at
temperature and humidity of 25 ± 2 ◦ C and 30 ± 5%, respectively.
For this purpose 1 ␮L distilled water was placed on the samples and
then the shape of the droplets was recorded by a Canon type digital
camera after 20 s.
Fig. 1. SEM micrographs and EDS spectra of (a) bare steel and (b) Ce treated sample at room temperature, pH = 3 and t = 5 min.
Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
625
Fig. 2. SEM micrographs and EDS spectra of the Ce treated samples after post-heating treatment: (a) S(1 h:100 ◦ C), (b) S(2 h:100 ◦ C), (c) S(3 h:100 ◦ C), (d) S(3 h:120 ◦ C) and
(e) S(3 h:140 ◦ C).
2.4.2. Corrosion evaluations
The electrochemical experiments were carried out using Autolab PGSTAT 302n electrochemical measurement equipment. A
conventional three electrode cell including treated samples with an
exposure area of 1 cm2 as working electrode, a saturated calomel
electrode (SCE) as reference electrode, and a platinum mesh with a
large surface area as counter electrode. Electrochemical impedance
measurements were performed on the bare steel and Ce treated
Fig. 3. Contact angle values of distilled water droplets placed on different samples: (a) bare steel, (b) Ce treated sample at room temperature, (c) S(3 h:100 ◦ C), (d) S(3 h:120 ◦ C)
and (e) S(3 h:140 ◦ C).
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Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
Table 3
Fe, Ce and O contents detected by EDS analysis on various samples.
3. Results and discussion
Sample
Fe (%)
Ce (%)
O (%)
Bare steel
Ce treated
S(1 h:100 ◦ C)
S(2 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:120 ◦ C)
S(3 h:140 ◦ C)
88.5
68.9
66.3
61
76.7
78.3
78.6
–
1.1
0.9
1.0
0.9
0.8
1.1
5.3
25
27.4
31.5
18.9
16.4
16.7
Table 4
Work of adhesion (WA ) and surface free energy ( sv ) values measured on the samples
treated at different conditions.
Sample
(◦ )
WA (mJ/m2 )
sv (mJ/m2 )
Bare steel
Ce treated
S(3 h:100 ◦ C)
S(3 h:120 ◦ C)
S(3 h:140 ◦ C)
81.0 ± 2
60.0 ± 7
80.0 ± 3
90.0 ± 4
109.0 ± 5
83.0 ± 2
108.0 ± 5
85.0 ± 3
72.0 ± 1
48.0 ± 4
35.0 ± 1
47.0 ± 4
35.0 ± 2
29.0 ± 0.6
17.0 ± 3
samples immersed in 300 mL of 3.5wt % NaCl solution at ambient
temperature as a function of immersion time up to 1 h. In addition, the corrosion protection performance of the epoxy coated
samples was investigated by EIS analysis. For this purpose an artificial scratch (20 mm in width) was produced on the coating using
a sharp surgery knife. The measuring frequency for the EIS analysis was ranged from 100 kHz to 10 mHz and the value of peak to
zero amplitude was ±10 mV. The electrochemical parameters were
obtained using the software package of Autolab workstation (Nova
ver 1.7). The fitted data of the EIS spectra were obtained from Zview
software.
3.1. Surface characterization
3.1.1. SEM/EDS analysis
The effect of thermal treatment of Ce film on its morphology
and composition changes was studied by SEM/EDS analyses. As
depicted in Fig. 1, it is obvious that after the Ce treatment at room
temperature a uniform film composed of cerium oxides/hydroxides
covered the steel surface. However, it contains high level of microcracks which is formed during the drying stage. Along with the
deposition of conversion coating, water molecules trap inside the
coating and eventually evaporate around the boiling point during the drying stage. The main cause of micro-cracks creation in
the cerium conversion coating is the film shrinkage as a result of
stress release during evaporation of the trapped water [32]. This
form of surface morphology has been also reported in the previous researches [33,34]. Despite the physically adsorbed water, the
structural water molecules remain stable up to higher temperatures. However, it is obvious from Fig. 2 that the heat treatment of
Ce film resulted in the decrease of the number of micro-cracks. It
can be seen that the increase of the time and temperature of postheating procedure resulted in significant decrease of the amount
of cracks and the coating structure becomes more compacted.
Here it needs to mention that the Ce conversion coatings contain different forms of Ce compounds such as oxides (i.e. CeO2 ),
hydroxides (i.e. Ce(OH)3 and Ce(OH)4 ) and hydrated form of Ce
oxides (i.e. CeO2 ·nH2 O) which deposit during the coating process
[35–37]. By post-heating of the Ce treated samples, the Ce hydroxides i.e Ce(OH)3 and Ce(OH)4 can undergo a calcination process
[38,39] and converts to oxide species such as Ce2 O3 and CeO2 .
Moreover, the water of hydration, which is weakly bonded to the
oxide compounds, evaporates at higher temperatures. However,
the shrinkage of the healed layer would not be significant during
the evaporation of structural water at high temperatures. Although
Table 5
The electrochemical parameters extracted from impedance plots of the bare steel and Ce treated sample at room temperature immersed in 3.5wt % NaCl solution for 15, 30
and 60 min.
Sample
Time (min)
Rct ( cm2 )
Rp ( cm2 )
CPE
−1
Y0 (␮
Blank
Blank
Blank
Ce
Ce
Ce
893 ± 83
1074 ± 104
1269 ± 128
–
–
–
15
30
60
15
30
60
−2
cm
n
s )
n
838 ± 18
896 ± 31
932 ± 47
–
–
–
0.78 ± 0.02
0.79 ± 0.01
0.80 ± 0.01
–
–
–
–
–
–
1541 ± 87
1910 ± 156
1634 ± 96
CPE
Y0 (␮−1 cm−2 sn )
n
–
–
–
830 ± 121
1014 ± 73
1047 ± 67
–
–
–
0.66 ± 0.03
0.71 ± 0.01
0.69 ± 0.01
Table 6
The electrochemical parameters extracted from impedance plots of variously post-heated samples immersed in 3.5wt % NaCl solution for 15, 30 and 60 min.
Sample
Time (min)
◦
S(1 h:100 C)
S(1 h:100 ◦ C)
S(1 h:100 ◦ C)
S(2 h:100 ◦ C)
S(2 h:100 ◦ C)
S(2 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:120 ◦ C)
S(3 h:120 ◦ C)
S(3 h:120 ◦ C)
S(3 h:140 ◦ C)
S(3 h:140 ◦ C)
S(3 h:140 ◦ C)
15
30
60
15
30
60
15
30
60
15
30
60
15
30
60
Rct ( cm2 )
923 ± 132
1163 ± 145
1618 ± 130
1214 ± 134
1319 ± 125
1484 ± 152
1647 ± 122
1611 ± 163
1645 ± 147
1301 ± 112
1387 ± 157
1847 ± 140
713 ± 94
806 ± 65
1180 ± 87
CPE
Rc
CPE
Rt ( cm2 )
Y0 (␮-1 cm-2 sn )
n
( cm2 )
Y0 (␮-1 cm-2 sn )
n
1263 ± 120
1836 ± 80
1870 ± 95
1698 ± 210
1703 ± 130
1717 ± 95
1361 ± 80
1753 ± 115
2132 ± 140
1951 ± 50
1870 ± 160
1597 ± 135
5073 ± 89
5551 ± 45
4109 ± 75
0.73 ± 0.02
0.73 ± 0.01
0.73 ± 0.01
0.70 ± 0.01
0.72 ± 0.02
0.75 ± 0.01
0.69 ± 0.04
0.73 ± 0.02
0.69 ± 0.02
0.65 ± 0.01
0.65 ± 0.01
0.72 ± 0.02
0.66 ± 0.01
0.68 ± 0.02
0.65 ± 0.02
415 ± 43
358 ± 40
339 ± 19
632 ± 55
615 ± 49
612 ± 31
731 ± 60
728 ± 51
740 ± 26
775 ± 85
777 ± 60
901 ± 64
1548 ± 76
1701 ± 39
2068 ± 80
402 ± 35
511 ± 60
656 ± 20
387 ± 105
485 ± 112
547 ± 133
441 ± 29
611 ± 53
530 ± 81
566 ± 116
513 ± 130
874 ± 156
435 ± 84
432 ± 20
446 ± 98
0.78 ± 0.01
0.77 ± 0.02
0.76 ± 0.01
0.80 ± 0.02
0.80 ± 001
0.79 ± 0.02
0.74 ± 0.01
0.73 ± 0.03
0.76 ± 001
0.74 ± 0.02
0.75 ± 0.02
0.75 ± 0.01
0.83 ± 0.02
0.83 ± 0.02
0.82 ± 0.01
1338 ± 175
1521 ± 185
1957 ± 149
1846 ± 189
1934 ± 174
2096 ± 183
2378 ± 182
2339 ± 214
2385 ± 173
1867 ± 197
2164 ± 217
2748 ± 204
2261 ± 170
2507 ± 104
3248 ± 167
Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
627
Fig. 4. Nyquist and Bode plots of (a1 and a2 ) bare steel and (b1 and b2 ) Ce treated sample at room temperature immersed in 3.5wt % NaCl solution for 15, 30 and 60 min.
it seems that the heat treated samples at 100, 120 and 140 ◦ C
(for 3 h) have the same and nonporous structures, but their corrosion behaviors may differ, that will be discussed later based on
electrochemical measurements.
The elemental composition of the Ce treated samples was studied before and after different post-heating procedures (Table 3). As
can be seen in Table 3, the Fe content after Ce treatment significantly decreases, which is due to the increase of Ce and O content
as a result of Ce oxides and hydroxides deposition. The O content
would be a proper parameter for describing the changes of Ce film
composition after post-heating procedure. According to Table 3,
the O content initially has a slight increase and finally drops to
a constant value. When the Ce(OH)3 is exposed to air during the
post-heating the following reaction can take place [39]:
4Ce(OH)3 + O2 + 2H2 O → 4Ce(OH)4
(1)
This oxidation reaction results in oxygen rise in the Ce film. However, the intensified evaporation of the hydration water existed in
the Ce film after post-heating at 100 ◦ C for 3 h and also dehydration
reactions can lead to the decrease in the amount of oxygen content.
The XPS analysis performed on the post-heated cerium conversion
coating in previous reports proves the idea that cerium hydroxide
compounds are removed or converted during the heat treatment
[35].
3.1.2. Contact angle measurements
The contact angle () values of water were measured on the steel
substrates with various post-heating conditions. The results are the
average of the values measured at three different points of each
sample. The surface energy and work of adhesion were calculated
according to Nuemann’s (Eq (2)) and Young’s (Eq (3)) equations:
W A = 2( lv . sv )1/2 exp[-ˇ( lv - sv )2 ]
(2)
W A = lv (1 + cos)
(3)
where is the contact angle of water, lv is the surface tension
of water, sv is, the surface free energy of the substrate and ␤ is
Fig. 5. Representative Bode plots fitted with one-time constant equivalent circuit
for the (a) bare steel after 1 h and (b) Ce treated sample after 15 min immersion in
3.5 wt.% NaCl solution.
0.0001247 ± 0.000010 (mJ/m2 )2 . The results obtained are given in
Fig. 3 and Table 4.
It is clear from the results that surface treatment of steel by Ce
film resulted in significant decrease of contact angle and increase
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Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
Fig. 6. Nyquist and Bode plots of (a1 and a2 ) S(1 h:100 ◦ C), (b1 and b2 ) S(2 h:100 ◦ C), (c1 and c2 ) S(3 h:100 ◦ C), (d1 and d2 ) S(3 h:120 ◦ C), (e1 and e2 ) S(3 h:140 ◦ C) samples
immersed in 3.5wt % NaCl solution after 15, 30 and 60 min immersion.
Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
Fig. 7. Representative Bode plot of using two-time constant equivalent circuit to fit
the experimental data of the S(2 h:100 ◦ C) sample immersed in 3.5wt % NaCl solution
after 1 h immersion.
of surface free energy. It can be clearly seen that the contact angle
of the Ce treated samples gradually increases as the temperature
of post-heating rises. Results show that the thermal treatment of
Ce film at 140 ◦ C converts the steel surface nature to hydrophobic entity with a maximum contact angle of approximately 109◦ .
Therefore, the thermal conditions have a considerable influence on
the surface characteristics of the conversion coating. As the temperature rises, the amount of oxides compounds increases. The lower
capability of Ce oxides compared with Ce hydroxides for providing polar components and making hydrogen bonding with water
molecules is responsible for the higher contact angles. The decrease
in film porosity is another reason responsible for the lower diffusion
of water molecules into the film leading to the increase of contact
angle.
3.2. Corrosion performance characterization
3.2.1. Electrochemical impedance spectroscopy measurements
3.2.1.1. Bare steel and Ce treated sample evaluations. The corrosion
performance of bare steel and Ce treated samples (at room temperature) was studied by EIS analysis. The corresponding impedance
spectra are given in Fig. 4. EIS experiments were conducted for an
exposure time of 1 h at open circuit potential (OCP).
To model the EIS data, two different electrical equivalent circuits (EEC) were utilized [40]. In both models, the constant phase
element (CPE) was used instead of ideal capacitance (C) because of
the porous nature and inhomogeneity of the formed layer. In the
proposed EEC, Rs is the solution resistance, Rct is the charge transfer
resistance, and CPEdl is the non-ideal capacitance of double layer.
CPEdl consists of Y0 and n which are the admittance and exponent
629
of CPE, respectively. In the case of Ce treated samples, the polarization resistance (Rp ) was defined as the sum of coating resistance
(Rc ) and charge-transfer resistance (Rct ) i.e. Rp = Rc + Rct . The typical Bode diagrams of the experimental and fitted data are given in
Fig. 5. The electrochemical parameters extracted from each model
are listed in Table 5.
According to Table 5, the Rct of the bare sample slightly increased
with the raise of immersion time. This behavior can be attributed to
the relative barrier properties of corrosion products formed on the
electrode surface [15]. Distinguishing a new time constant at high
frequency domain of impedance spectra, corresponding to the Ce
layer of the treated samples, is somehow controversial, indicating
the poor corrosion protection performance of the Ce film formed at
room temperature. This behavior may be attributed to the presence
of several micro-cracks in the coating matrix that are pathways for
diffusion of corrosive electrolyte and/or the hydrophilic nature of
the Ce film [3]. However, the Rp values of the Ce treated sample
are higher than Rct values of the bare steel, revealing the significant
improvement of corrosion resistance. This can be attributed to the
barrier effect and non-conductive nature of the Ce film which limits
the areas of anodic and cathodic zones. According to Table 5, it is
shown that the Rp of Ce treated sample increases with the increase
of immersion time from 15 to 30 min. This could be resulted from
the active inhibition characteristic of the Ce film due to the presence
of high amount of Ce4+ cations in the final film [41].
3.2.1.2. Post-heated samples evaluations. Fig. 6 shows the
impedance spectra of post-heated samples at various immersion times. The plots, exhibited two time constants. In other word,
the semicircle of the coating film at high frequency was clearly
obvious from the Bode plots of these samples despite those treated
by Ce at room temperature. This observation explicitly shows
the effect of post-heating on the Ce film corrosion protection
properties improvement. Therefore, the impedance data were
fitted by proper EECs as shown in Fig. 7 and the data extracted are
reported in Table 6.
According to the data extracted it is clear that the Ce coating
resistance, i.e. Rc , increases by increasing the time and temperature of heat treatment, so that the sample post-heated at 140 ◦ C
for 3 h has the highest coating resistance On the other hand, the
double layer resistance, i.e. Rct generally increases with increasing
the immersion time. The increase of Rct is a sing of the active inhibition behavior of Ce film. In contrary to the Ce treated sample at
room temperature, no significant drop is observed in the double
layer resistance of post-heated samples, which resulted from the
improved performance of Ce film, restricting the access of corrosive
species to the substrate. For the sake of comparison, Rt = (Rc + Rct )
was calculated for all of the samples. It can be seen from the results
that by increasing the time of post-heating at 100 ◦ C, Rt is increased
and the sample heated for 3 h showed the highest and the most sta-
Fig. 8. Bode plots of (a) bare steel and (b) Ce treated sample coated with epoxy coating with a scratch immersed in 3.5wt % NaCl solution for 3, 6, 24 and 72 h.
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Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
Fig. 9. Bode plots of (a) S(3 h:100 ◦ C), (b) S(3 h:120 ◦ C) and (c) S(3 h:140 ◦ C) samples coated with epoxy coating with a scratch immersed in 3.5wt % NaCl solution for 3, 6, 24
and 72 h.
Table 7
The electrochemical parameters extracted from impedance plots of the epoxy coatings (with an artificial defect) applied on the steel substrates without and with Ce treatment,
before and after post-heating at various temperatures and times, immersed in 3.5wt % NaCl solution at different period of immersion times (Rt = Rct + Rc ).
Sample
Time (h)
Rct ( cm2 )
Rc ( cm2 )
CPE
−1
Y0 (␮
Blank
Blank
Blank
Blank
Ce
Ce
Ce
Ce
S(3 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:100 ◦ C)
S(3 h:120 ◦ C)
S(3 h:120 ◦ C)
S(3 h:120 ◦ C)
S(3 h:120 ◦ C)
S(3 h:140 ◦ C)
S(3 h:140 ◦ C)
S(3 h:140 ◦ C)
S(3 h:140 ◦ C)
3
6
24
72
3
6
24
72
3
6
24
72
3
6
24
72
3
6
24
72
5571 ± 110
5686 ± 124
6103 ± 219
1550 ± 234
12103 ± 459
6591 ± 224
3950 ± 343
1691 ± 257
12471 ± 643
10001 ± 563
8065 ± 456
2734 ± 404
28576 ± 578
15299 ± 986
12033 ± 456
2680 ± 347
28566 ± 434
22250 ± 578
19631 ± 421
4084 ± 265
49 ± 34
77 ± 65
34 ± 24
651 ± 86
257 ± 23
107 ± 12
105 ± 7
103 ± 3
72 ± 54
132 ± 78
105 ± 34
552 ± 86
27 ± 45
16 ± 27
85 ± 34
72 ± 86
18 ± 24
18 ± 19
26 ± 30
22 ± 45
−2
cm
n
s )
−1
n
Y0 (␮
0.78 ± 0.02
0.81 ± 0.02
0.78 ± 0.03
0.55 ± 0.02
0.72 ± 0.01
0.34 ± 0.01
0.46 ± 0.02
0.48 ± 0.02
0.67 ± 0.01
0.65 ± 0.01
0.51 ± 0.02
0.53 ± 0.01
0.88 ± 0.02
0.88 ± 0.01
0.63 ± 0.02
0.80 ± 0.01
0.83 ± 0.01
0.83 ± 0.01
0.71 ± 0.02
0.53 ± 0.04
ble corrosion resistance. By increasing the temperature from 100 ◦ C
to 140 ◦ C, corrosion protection performance of Ce film was further
improved. The higher Rc , i.e. Ce coating resistance, and Rt values
of the samples post-heated at 140 ◦ C than other samples indicate
that post-heating of Ce film could noticeably improve the coating
protection performance.
3.2.2. The effect of post-heated Ce film on the epoxy coating
performance
Another important feature is the effect of post-heating of the
Ce film on the adhesion properties of an epoxy coating in a corrosive media. Therefore, EIS analysis was performed on the scratched
Rt ( cm2 )
CPE
22 ± 8
67 ± 12
387 ± 19
51 ± 20
294 ± 45
355 ± 65
565 ± 73
100 ± 23
15 ± 10
29 ± 18
660 ± 34
180 ± 67
513 ± 24
243 ± 75
212 ± 34
415 ± 85
913 ± 39
1009 ± 84
440 ± 53
416 ± 40
64 ± 12
94 ± 15
46 ± 22
47 ± 16
67 ± 17
43 ± 10
48 ± 9
50 ± 7
51 ± 4
44 ± 8
96 ± 17
421 ± 56
65 ± 14
106 ± 19
10 ± 6
211 ± 33
61 ± 9
63 ± 14
52 ± 8
202 ± 23
−2
cm
n
s )
n
0.80 ± 0.02
0.81 ± 0.02
0.76 ± 0.03
0.83 ± 0.01
0.72 ± 0.03
0.74 ± 0.03
0.92 ± 0.01
0.92 ± 0.01
0.69 ± 0.01
0.65 ± 0.04
0.64 ± 0.03
0.52 ± 0.01
0.78 ± 0.03
0.71 ± 0.01
0.82 ± 0.02
0.59 ± 0.01
0.72 ± 0.01
0.71 ± 0.01
0.69 ± 0.01
0.58 ± 0.02
5593 ± 118
5753 ± 178
6490 ± 238
1601 ± 254
12397 ± 504
6946 ± 289
4515 ± 416
1791 ± 280
12486 ± 653
10030 ± 585
8725 ± 490
2914 ± 471
29089 ± 602
15405 ± 1061
12245 ± 490
3095 ± 432
29479 ± 473
23259 ± 662
20071 ± 474
4500 ± 305
epoxy coatings in order to examine the effect of sub-layer on the
adhesion properties of epoxy coating. This experiment was performed after 3, 6, 24 and 72 h.
EIS analysis was performed for better investigation of the effect
of post-heating of Ce film on the epoxy coating performance when
there is a defect in the coating structure. The Bode plots, typical plots of fitting impedance data with electrical circuits and the
extracted data are given in Figs. 8 and 9, and Table 7, respectively.
From Table 7 it can be seen that the Rc, i.e. the resistance of
oxide layer, and Rct of blank sample increases with increasing the
immersion time up to 24 h. At the beginning of exposure time,
the corrosion products fill the scratch and thus hinder the access
Z. Mahidashti et al. / Applied Surface Science 390 (2016) 623–632
of electrolyte to the sub-layer, leading to the rise of Rc and Rct .
However, both Rc and Rct significantly decrease after 72 h immersion. This shows that the corrosion products deterioration at longer
immersion times reduces their barrier role. As a result the electrolyte diffuses beneath the coating through defected site and
results in the coating delamination and corrosion products development beneath the coating. These are responsible for the coating
adhesion loss and disbondment. There is direct correlation between
the coating disbondment and decrease of Rct . The decrease of Rct
means that the corrosion process occurred at the steel/epoxy coating interface in the defective areas and the electrolyte had reached
beneath the coating, leading to the coating delamination and the
substrate began to corrode. It can be seen from Table 7 that surface treatment of steel substrate by Ce film significantly increased
the Rct and Rc values compared with blank sample. In addition, the
increase of post-heating temperature resulted in the increase of
Rt (Rct + Rc ) at all immersion times. This can be attributed to the
active corrosion inhibition behavior of Ce film at defected site and
its effect on the adhesion properties of epoxy coating on the steel
substrate. The Ce layer increases the surface free energy and nano
scale roughness, resulting in the epoxy coating adhesion properties enhancement. So the coating delamination when there is a
scratch in the coating occurs in lower extent. Moreover, the Ce layer
decreases the cathodic activity of steel substrate through blocking
the corrosive species access to the active cathodic sites, leading to
lower hydroxyl ions creation beneath the coating. Therefore, the
increase of pH in this case would be much lower than blank sample. As a result, the coating delamination occurs in lower extent
when the immersion time increases.
4. Conclusion
Results revealed that by increasing the post-heating time and
temperature the number of micro-cracks decreased and the coating compactness was improved. The sample heated at 140 ◦ C for
3 h showed the highest contact angle and the largest value of
impedance, suggesting the better corrosion protection of steel.
Moreover, post-heating of Ce film promoted the adhesion properties of the epoxy coating even when there was a defect in the
coating structure.
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