Self-healing corrosion protection by phosphate-doped enamel coatings on steel in... cement pore fluid

Self-healing corrosion protection by phosphate-doped enamel coatings on steel in simulated
cement pore fluid
Xiaoming Cheng1,a
1
Department of Materials Science and Engineering
2
Department of Civil, Architectural, and Environmental Engineering
3
Department of Mechanical & Aerospace Engineering
Missouri University of Science and Technology, Rolla, MO 65409
a
[email protected];
ABSTRACT
electrolyte and corrosive species, slowing the
corrosion rate.
Phosphate was incorporated into enamel coatings by
doping sodium borosilicate glasses with 7 mol%
P2O5. Corrosion resistance of phosphate-doped (7P)
and base enamels (0P) in simulated cement pore fluid
(Lawrence solution) was evaluated, with the focus of
studying the effect of artificial damage on the
protection property of the coating. It reveals that after
three days immersion in Lawrence solution (LS) 7P
coating developed a precipitate layer on the defect,
whereas 0P shows initialization of pitting corrosion
in the exposed substrate at the defect. The precipitate
layer was characterized by Micro-Raman and X-ray
diffraction, suggesting formation of hydroxyapatite.
Linear polarization of damaged coatings shows active
corrosion for 0P through all immersed times.
However, there is a passivation gradually developed
for 7P coatings with increasing immersion time.
Electrochemical impedance spectroscopy was also
used to study the corrosion resistance of damaged
coating. By equivalent circuit simulation, it reveals
that the charge transfer resistance of 7P coating
increased as a function of immersion time and is two
orders of magnitude higher than 0P coating after
three day immersion. It suggests that 7P coating
exhibit active corrosion protection, which is not only
protect the substrate with a nonconductive barrier but
provide continued protection to the exposed substrate
after partial damage of the coating. The self-healing
property is believed to be attributed to the Ca-P
precipitates which impede the movement of
1. Introduction
In view of coatings for corrosion protection of
metallic substrates, organic coating, metallic coating
and porcelain enamels are the most common coatings
used in practice. To achieve long-term corrosion
protection, the following requirements must be met
for the coatings: a) high mechanical resistance and
adhesion, especially during transport and installation;
b) chemical stability under service conditions
(aging); c) sufficiently low permeability for corrosive
component under service conditions; d) sufficient
stability under electrochemical influences, especially
with electrochemical protection measures. Organic
coatings show varying degree of solubility and
permeability for components of the corrosive
medium. In addition, organic coatings in general
contain many polar groups that promote adhesion or
pretreatment on substrate (conversion coatings) to
achieve necessary bond strength against peeling.
Once defect is created on organic coatings, however,
permeation of oxygen and corrosive species through
the coatings initialize the cathodic reaction and
complete the electrical cell, while the exposed
substrate at the defect oxidized as the anodic reaction,
which leads to disbonding of the coating and
catastrophic corrosion. With regard to metallic
coating, it has the advantage of providing
electrochemical protection by preferential oxidizing
the metal coating instead of substrate metal due to
galvanic corrosion. The intermetallic layer developed
1
during the heat treatment between the metallic
coating and substrate promotes the bonding and
prevents delamination. However, as sacrificial
coatings, metal coatings will ultimately corrode away
and exposing the underneath substrate and also direct
electrical contact between uncoated and coated parts
has to be avoided for the same reason.
the role of phosphate incorporated in the glass to
provide active protection of the steel.
2. Experimental
2.1. Glass preparation
Table 1 shows the composition of a typical,
commercially-available borosilicate enamel that was
used on reinforcing steel [2]. This composition has
the thermal properties (coefficient of thermal
expansion CTE, softening temperature T s) to form a
mechanically strong bond to steel at a relatively low
(~800 °C) temperature. For the present study,
compositions from Na2O ∙ B2O3 ∙ SiO2 systems were
prepared, with and without P 2O5. Table 2 shows the
compositions of glasses in this study.
Porcelain enamel displays an exceptional ability
to withstand harsh environments. It is now widely
used on ovens, clothes washers, and bathtubs.
However, not all porcelain technology is aimed at the
appliance industry. Vitreous enamel has been applied
on reinforcing steel rebar to protect the steel from
corrosive environment. Recent studies conducted at
Missouri S&T [1] compared the corrosion resistance
of pure enamel on steel rebar with ECR. It was found
that pure enamel coatings were significantly
outperformed by the intact epoxy coating. However,
in the presence of defects on coatings, pitted
corrosion was initiated at the location of damaged
pure enamel coatings but restrained locally due to
well-adhered glassy layers on rebar surface. As a
result, epoxy coatings exhibited significantly
degraded corrosion performance with local damage
due to the well-known under-film corrosion
mechanism. Porcelain enamel coating holds great
promise in corrosion protection and extending the
service life of reinforcing steel in concrete structures.
The superior corrosion resistance can only be
maintained when the enamel coating remains intact.
Coatings lose the protection property when their
thickness is significantly reduced or cracks/defects
form. Therefore, active corrosion protection based on
self-healing of defects in coatings is a vital issue for
development of new advanced corrosion protection
coating system.
Considering the exceptional corrosion resistance
of vitreous enamel coating on metal surface and the
loss of this protection property with the presence of
defects, the present work is intended to investigate
the corrosion resistance of phosphate-doped enamel
coatings on steel rebar in simulated concrete pore
solution, using electrochemical methods in an attempt
to shed light on active corrosion protection of
inorganic enamel coating systems. At the same time,
it is very important to understand how enamels
dissolve under such environments and to understand
Table 1 Chemical composition of a commercial borosilicate
enamel [2].
Oxides
SiO2
B2O3
Na2O
K2O
CaO
CaF2
Al2O3
ZrO2
MnO2
NiO
CoO
Total
Concentration (mol%)
49
19
17
2
0
4
3
3
1
1
1
100
Table 2 Nominal glass compositions xP2O5 ∙ (100x)(25Na2O ∙ 25B2O3 ∙ 50SiO2) in mol%.
SBNxP
0P (x=0)
7P (x=7)
Na2O
25
23.25
B2O3
25
23.25
SiO2
50
46.5
P2O5
7
The sodium borosilicate and sodium phosphoborosilicate glasses were prepared using reagent
grade SiO2, H3BO3, Na2CO3 and (NaPO3)n. The
initial batches were melted in alumina crucibles for 1
h at 1200 °C, and then the melts were quenched in a
steel mold. The resulting glasses were annealed at
around 530 °C for 3 hr.
2.2. Enamel preparation
Glass frits were crushed using shatter box to
reduce the particle size to less than m. To create
2
defects is between 0.006 cm2 – 0.02 cm2. The
damaged coating was immersed in simulated pore
water (composition shown in Table. 3) for 3 days. All
tests were carried out on both phosphate-doped (7P)
and phosphate-free enamels (0P). Samples immersed
for various times would be pulled out of solution to
examine by optical microscope and corrosion
resistance by electrochemical tests.
Table 3 Composition of Lawrence solution (LS).
a solid suspension, 20 g of glass powders were mixed
with 150 ml of ethanol and the mixture was stirred at
450 rpm with a stirring bar. Discs of 1.7 cm in
diameter and 5 mm in thickness were cut from steel
rebar and polished to 600 grit. These discs were
loaded by a copper wire basket into the suspension.
When the stirring stopped, glass powders were
deposited on steel surface uniformly by gravity. The
deposition set-up is shown in Fig. 1.
During the firing process, coated steel pieces
were slowly heated at 10 °C/min to 200°C and held
for 1hr to burn off the ethanol. Then the temperature
was ramped up to 800°C at 40 °C/min and held for
10min before the samples were cooled down to room
temperature with fast cooling. Figure. 2 shows the
appearance of the coating before and after firing.
KOH
NaOH
Ca(OH)2
CaCl2
pH at
25°C
LS [3]
0.061
0.022
0.0065
--
12.95
2.4. Electrochemical tests
In the electrochemical tests, 3.5 wt.% NaCl
solution was used as the test electrolyte and a typical
three-electrode set-up was used, including a 25.4 mm
x 25.4 mm x 0.254 mm platinum sheet as the counter
electrode, a saturated calomel electrode (SCE) as a
reference electrode, and the testing sample as the
working electrode. Three electrodes were connected
to a Gamry, Reference 600 potentiostat/galvanostat
for data acquisition. Open circuit potentials (Eocp)
were stabilized for 2500 seconds immediately after
the sample was immersed in the electrolyte.
Electrochemical impedance spectroscopy (EIS) was
conducted at five points per decade around Eocp with
a sinusoidal potential wave of 10 mV in amplitude
and frequency ranging from 0.01 Hz to 10 6 Hz. The
sample
was
subsequently
tested
using
potentiodynamic polarization method from 300 mV
below Eocp to 1500 mV above Eocp with a scanning
rate of 1 mV/s. Triplicate samples for each condition
were tested and the representative results are shown
below.
Figure 1 Deposition set-up to apply thin film of glass
powders uniformly on steel surface. The steel discs were
loaded in the middle of copper wire basket and immersed in
the glass-ethanol suspension.
3 cm
Conc.
(mol/L)
3. Results and Discussion
3 cm
3.1. Immersion and Optical Observation
Optical microscope (OM) images of the
damaged samples before and after immersion in LS
are shown in Fig. 3. P-free enamel (0P) shows signs
of corrosion of exposed steel after immersing in LS
for 3 days, whereas P-doped enamel (7P) forms white
film of precipitates above the defect and no corrosion
products of steel were observed beneath the layer.
Figure 2 Enamel (0P) coated steel samples before (a)
and after (b) firing.
2.3. Immersion tests
In order to evaluate the corrosion resistance of
these coatings through electrochemical methods, the
coatings were deliberately damaged using a drill with
1/16 inch drill bit to expose the substrate. The area of
3
(a) 0P-as damaged
(b) 0P-LS-3d
rust
(d) 7P-LS-3d
(d) 7P-as damaged
Figure 3 OM images of 0P enamel coated on steel discs: as damaged (a), after immersion LS for 3 days (b); 7P enamel coatings
as damaged (c), after immersion in LS for 3 days (d), where red dashed circle indicates the defect location.
3.2. Characterization of precipitates
Energy dispersive spectroscopy (EDS) showed a
Ca-rich phase in the precipitates formed on enamel
surface (shown in Tab. 4). Micro-Raman
spectroscopy and X-ray diffraction (XRD) results
(shown in Fig. 4) indicate that this phase is a Ca-P
rich phase which is most likely the hydroxyapatite
(HA).
Hydroxyapatite-Fisher
Table 4 Elemental composition of enamel bulk (probe #1)
and the precipitate layer (probe #2) from EDS result.
Atomic %
Probe #1
Probe #2
O
48
26
Na
9
--
Si
34
8
P
10
19
Cl
-0.5
200
400
600
800
Ca
-46
o Hydroxyapatite
o
3.3. Potentiodynamic polarization test
Potentiodynamic polarization test (PD) is a good
means to compare different coatings by accelerating
the corrosion process. Representative PD data for
each condition are shown in Fig. 5.
o
o
o
o
o
o
oo
o
o
0
Corrosion current for both 0P
close with 7P slightly higher.
region was observed with 7P
immersed for 3d and
1000 1200 1400 1600 1800 2000
Raman shift (cm-1)
and 7P remain very
However, a passive
(600mV for sample
400mV for 2d
10
20
30
40
50
60
70
80
2
Figure 4 Micro-Raman spectroscopy (top) and XRD
(bottom) on precipitaes of 7P enamel coatings immersed in
LS for 3 days.
4
0P-as defect
0P-def-LS1d
0P-def-LS2d
0P-def-LS3d
1.0
1.0
0.5
E vs.SCE (V)
E vs.SCE (V)
0.5
Active corrosion
0.0
-0.5
Passive region
0.0
-0.5
-1.0
-1.0
-1.5
-10
10
7P-as defect
7P-def-LS1d
7P-def-LS2d
7P-def-LS3d
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
-1.5
-10
10
0
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
-3
10
10
-2
10
-1
10
0
10
2
2
I (Amps/cm )
I (Amps/cm )
Figure 5 Potentiodynamic polarization of enamel coated steel discs immersed in simulated pore water for different amount of
time: (a) phosphorus-free enamel 0P; (b) 7 mol% P-doped enamel coating 7P.
Table 5. Capacitance values obtained from experimental data for undamaged enamels
f = 999.95 Hz,
r = 1.81×106 Ω
f = 100.00 Hz,
r = 1.66×107 Ω
f = 9.9987 Hz,
r = 1.41×108 Ω
0P enamel (F cm-2)
8.78 × 10-11
f = 999.95 Hz,
r = 1.74×106 Ω
f = 100.00 Hz,
r = 1.51×107 Ω
f = 9.9987 Hz,
r = 9.99×107 Ω
9.61 × 10-11
1.13 × 10-10
7P enamel (F cm-2)
9.14 × 10-11
1.05 × 10-10
1.59 × 10-10
immersion) but not with 0P (active corrosion).
Therefore, higher potential is required for 7P enamel
coating to break the passive layer, where the potential
plateau is right after the passive region. This
potential increased with increasing immersion time in
simulated pore water.
Therefore, the protection resulted from the
passivation could be related with the reaction
products between P-doped enamel and Lawrence
solution.
respectively, through the frequency range between
105 to 10 Hz. The phase angle describes an angle near
90° at the corresponding frequencies, revealing a
purely capacitive behavior of enamel coatings [5, 6].
Such responses confirm that both enamels
behave as an insulating and protective coating on
steel. The capacitance values associated with coatings
are determined based on the equation:
3.4. Electrochemical impedance
spectroscopy
Electrochemical impedance spectroscopy (EIS)
has been widely used to understand the corrosion
mechanisms especially in coating system [4, 5].
where fi and ri are coordinates of any point on Bode
modulus line [7]. When the slope is -1, capacitance
obtained is the same irrespective of which point it is
calculated. Whereas, when the slope slightly differs
from -1, the capacitance value obtained may vary
with different frequencies. Table 5 shows
capacitances determined from three different points
on 0P and 7P Bode plot, respectively. In each case all
of values are of the order of 10-10 Fcm-2, suggesting
a very low capacitance that is typical of nonconductive coatings. 0P and 7P enamel coatings
exhibit the similar capacitive property as black
enamel reported elsewhere [5]. It also reveals that in
lower frequency (10-2 ~ 10 Hz) horizontal section
3.4.1.
𝐶=
Characterization of intact enamel
coatings
Firstly, undamaged vitreous enamel coatings are
characterized by electrochemical measurement.
Figure 6 shows the EIS Bode plots obtained on 0P
and 7P enamel coatings, respectively. The responses
from both coatings are characterized by a straight line
with a slope of -0.96, and -0.94 for 0P and 7P,
5
1
2𝜋𝑓𝑖 𝑟𝑖
(1)
(A) 0P
(B) 7P
-80
Phase angle
-60
-40
/Z/ (ohm.cm2)
-2011
10
/Z/ (ohm cm2)
Phase angle
-80
109
107
slope= -0.96
105
103
10-2
10-1
100
101
102
103
104
105
-60
-40
-20
107
slope= -0.94
105
10-2
10-1
100
Figure
6.
EIS
Bode
plot
for
undamaged
vitreous
103
enamel
coatings:
(A)
80
40
Phase angle
cal-as def
cal-3d
msd-as def
msd-3d
20
0
104
105
0P,
(B)
.
7P.
cal-as def
cal-3d
msd-as def
msd-3d
60
40
20
0
106
105
/Z/ (ohm.cm2)
Phase angle
102
(B) 7P
(A) 0P
60
/Z/ (ohm.cm2)
101
Freq (Hz)
Freq (Hz)
104
103
2
10
10-2
10-1
100
101
102
103
104
105
Freq (Hz)
105
104
103
102
10-2
10-1
100
101
102
103
104
105
Freq (Hz)
Figure 7 Experimental measured (msd) and model calculated (cal) Bode plots for enamels as defected and immersed
in LS for 3 days: (A) 0P, (B) 7P.
was observed on 7P enamel which indicates resistive
control of the response. Such response is associated
with metallic substrate which may be exposed due to
the pores and defects in the coating
3 days immersion (Fig. 7-A). Impedance modulus
characterizes a slope of -0.69 followed by a
horizontal section, which indicates a resistive
behavior under charge transfer control. /Z/10 mHz
reaches a value of 105 Ω, five orders of magnitude
lower than that of undamaged sample. Bode phase
curve appears a wide maximum around 10 Hz.
With regards to 7P enamels, similar responses in
Bode impedance curve except for 3d /Z/10 mHz reaches
a value of 106 Ω versus 4.95×104 Ω for as defected
7P enamel. In the phase curve, the phase angle
maximum for 3d sample shifts to lower frequency
than both as defected 7P and 0P samples. Bode phase
curve for as defected 7P describes two maximum at 1
and 102 Hz, respectively. In the literature, the
presence of the phase angle maximum in the
frequency range (0.010-10 Hz) is associated with a
However, after the coating was intentionally
damaged and immersed in simulated pore water the
impedance response differed significantly to that of
the intact coating, as will be discussed in greater
detail below.
3.4.2.
Deterioration of damaged enamel
coating under immersion condition
Figure 7 shows Bode curves for as defected
coating and after immersion in LS for 3 days. For 0P
enamels, the response differs drastically to that
described above and almost remains constant through
6
corrosion process that occurs in the metallic
substrate, while the maximum located in the range
(102-104 Hz) is related with the response of the pores
and defects in the coatings [8]. Therefore, the
impedance response of as defected 7P samples may
be comprised by both the exposed substrate due to
the damage and pores and defects present within the
coating.
In such cases, it is possible to determine the
capacitance value associated with the system using
the expression by Walter [7]:
𝐶=
1
occurs under diffusive control (Fig. 8-A, RQ). When
defect was created in the coating either by artificial
defect or degradation of coating, corrosion happens
under charge transfer control with the R as the charge
transfer resistance (Fig. 8-B, R(QR)). The capacitor
is represented by a constant phase element Q, which
is defined by the parameters Yo and n to take into
account the nonhomogeneity of the coating surface.
When n = 1, Q is a pure capacitor and the Nyquist
curve describes a semicircle. However, most of the
time the value of n is less than unity, reflected by a
depression in the arc as a consequence of the lack of
homogeneity caused by deterioration of the coating.
Figure 7 shows the experimental data for as
defect coating and coating immersed in LS for 3
days, together with simulated results based on R(QR)
model. Even though the model does not produce the
best concordance between the experimental and
simulated results for some samples (especially as
defect-7P), the relative error associated with each
element comprising the circuit are always below 5%
(shown in Table 7). The two maximums in phase
angle of 7P-as defect strongly indicates two time
constants, which corresponds to the case in Fig. 8-C
(R(Q(R(QR))). Applying this simulation to the
experimental data yields excellent concordance.
However, the error associated with some of the
elements is very high (58%), which leads to distrust
of their values and validity of the model.
(2)
2𝜋𝑓𝜃𝑚𝑎𝑥 𝑟𝜃𝑚𝑎𝑥
where fmax is the frequency at which the bode phase
angle reaches its maximum and rmax is the value of
impedance modulus at that corresponding frequency.
This expression fits the situation better than Eq. (1)
since the slope greatly differs from -1. Thus,
capacitance values obtained, shown in Table 6, are of
the order of 10-6 F cm-2. This is a very high value to
be considered for a protective coating. However, it
falls in the range of values associated with an
electrochemical
double
layer,
indicating
electrochemical double layer was obtained in both
damaged and immersion cases.
Table 6 Capacitance values obtained for artificially
damaged coating and defected coatings immersed in
LS for 3days.
as defect
LS-3d
as defect
LS-3d
3.4.3.
fmax (Hz)
7.8939
9.9987
fmax (Hz)
1.9949
1.0001
rmax (Ω)
3000.3
2424.4
rmax (Ω)
5947.7
33563.5
Corrosion evaluation
circuit modeling
by
RQ
0P (F cm-2)
6.72 × 10-6
6.57 × 10-6
7P (F cm-2)
1.34 × 10-5
4.74 × 10-6
equivalent
R(QR)
The impedance responses can be studied in
greater details by means of equivalent circuit
simulation. The equivalent circuit used in this study
is that broadly described in the literature for defective
coating [5, 6, 9, 10]. Figure 8. shows the equivalent
circuit model along with schematic representation of
coating systems for different conditions. Intact
coating behaves as a capacitor which isolates the
substrate from the electrolyte only allowing corrosion
R(Q(R(QR)))
Figure 8. Equivalent circuit model used for numerical
simulation of EIS data. (A) intact coating; (B) coating
presenting defect and pathways for electrolyte uptake; (C)
development of anodic activity at the steel/coating interface
[6].
7
4
(A)
-3.5x10
4
-3.0x10
7P-as defect
7P-def-LS1d
7P-def-LS2d
7P-def-LS3d
(B)
0P-as defect
0P-def-LS1d
0P-def-LS2d
0P-def-LS3d
5
-4x10
4
Zimg (ohm)
Zimg (ohm)
-2.5x10
4
-2.0x10
4
-1.5x10
5
-3x10
5
-2x10
4
-1.0x10
5
-1x10
3
-5.0x10
0.0
0
4
4
4
4
4
4
0
4
1x10 2x10 3x10 4x10 5x10 6x10 7x10
0
5
5
5
5
5
5
5
5
1x10 2x10 3x10 4x10 5x10 6x10 7x10 8x10
Zreal (ohm)
Zreal (ohm)
Figure 9 EIS curves for enamels vs. immersion time in LS: (a) 0P enamel; (b) 7P enamel.
Table 7. Values obtained for each element parameters for
7P-as defect sample based on R(RQ) model.
Values
Rs
123.3
error %
2.42
Yo
2.58 ×
10-5
3.15
n
0.684
0.94
associated with an electrical double layer at the
interface of the substrate and the electrolyte.
Capacitance values for as defect 0P and 7P coatings
stay very close (1.66 × 10-5 F cm-2 vs. 1.24 × 10-5 F
cm-2), in agreement with the capacitance values
calculated earlier (Table 6). As immersion time
elapses, Yo slightly decreases for both coatings with
0P having values 2 times higher than 7P. Generally,
coating systems with lower capacitance and higher
resistance always shows slower corrosion rate and
higher corrosion resistance.
Charge transfer resistances for as defect 0P and
7P coatings are identical (8.3 × 104 Ω cm2). During
immersion, the resistance of 0P coating stays around
105 Ω cm2. However, resistance of 7P coating
increases as a function of immersion time in LS.
After 3 days immersion, the resistance of 7P coating
is two orders of magnitude higher than that of 0P,
suggesting slower corrosion rate and higher corrosion
resistance of defected 7P enamel coatings after
immersion.
It reveals that base enamel coatings (0P) can
protect the substrate metal when they are intact as a
nonconductive barrier. It implies the protection
mechanism persist until defects appear in the coating.
Such defected coatings will allow the electrolyte and
corrosive species reach the substrate and corrosion
starts to develop, as can be seen in the significant
drop of impedance at lower frequency and five orders
of magnitude increase in capacitance. The corrosion
R
5.75 ×
104
4.00
Therefore, R(QR) model was used to simulate all
experimental data of defected coatings and the
parameters extracted from the model were used to
quantitatively study the protective property of the
coating system. Figure 9 shows the corrosion
evolution of coatings that have been artificially
damaged and immersed in LS for various time spans
(as defect, 1d, 2d and 3d). The capacitance (Yo)
associated with the constant phase element and
charge transfer resistance (R) were plotted against
immersion time, as shown in Fig. 10. The overall
capacitance values are at the order of 10-5, which are
too high to be considered as protective coatings due
to the relative large area of exposed substrate by
artificial defect. Although there is only one time
constant (arc) appear drawn (Fig. 9), there may be
more time constants involved. The time constant
associated with the coating would be located at
higher frequencies and would not be in the spectrum
due to the limitations of the equipment [5, 8].
Therefore, in this study, the EIS responses
correspond to the attack occurring in the metallic
substrate at the artificial defect and the capacitance is
8
10-2
108
0P
7P
107
R (Ohm cm2)
10-3
Yo (Fcm-2)
0P
7P
10-4
10-5
106
105
104
10-6
0
1
2
0
3
1
2
3
Time (d)
Time (d)
Figure 10 Corrosion evolution for defected 0P and 7P enamel coatings: (A) capacitance Y o associated with the
constant phase element, (B) charge transfer resistance R, as a function of immersion time in LS
resistance of the coating does not change with further
immersion in LS and the corrosion is mainly due to
attacking the metallic substrate exposed at the defect.
However, phosphate-doped enamel coatings (7P)
exhibit enhanced corrosion resistance during
immersion with presence of defect, indicating active
corrosion protection. Active corrosion protection
implies not only mechanical covering of the protected
surface with a dense barrier coating (the case of intact
coating), but also provides self-healing properties
which allow continued protection even after partial
damage of the coating [11]. The active corrosion
protection shown in 7P coatings is most likely related
to the Ca-P precipitates forming on the defect, which
is not observed in 0P coating. The self-healing is the
consequence of the release of the PO43- from the
enamel coating, promoted by alkalinity solution
followed by fast precipitation of phosphate inhibitor
with calcium ions present in the surrounding
environment. The precipitates effectively impede the
movement of electrolyte and ionic species, increasing
the charge transfer resistance.
exposed substrate metal at the defect. Linear
polarization of the damaged coating shows active
corrosion of the exposed substrate, whereas the
phosphate-doped coating exhibit wide passivation
range on corrosion potential (600 mV) for samples
after 3 day immersion in LS, which indicates after
immersion 7P enamel coating passivates the defect
and slows the corrosion rate.
Unlike the polarization technique, by using EIS
it is possible not only to evaluate the surface state of
the coating, in situ, without the need to induce the
polarization of the systems and damage the material
applying direct current, but also to establish
hypothesis about the mechanism responsible for the
corrosion process and carry out simulations which
allow to evaluate the validity of the hypothesis. In
this study, the response of defected coating is
attributed to the attacking in the metallic substrate,
which is described by a phase angle maximum at
lower frequency. Consequently the corrosion process
changes from diffusive control to resistive control,
which is characterized by a horizontal section in the
Bode impedance spectrum. The corrosion of defected
coatings was evaluated quantitatively by capacitance
and charge transfer resistance extracted from model
simulation. It reveals that the corrosion resistance of
0P enamel is independent on immersion time and it
almost stays identical after immersion. However, the
corrosion resistance of 7P enamel increased with
immersion time and reaches a value of two orders of
magnitude higher than as that of defect 7P and 0P
enamels. The increased corrosion resistance of
defected 7P after immersion suggests active
4. Conclusions
It can be concluded that the protective property
of both 0P and 7P enamel coatings diminishes after
the coating is damaged. The undamaged coatings
behave as a barrier layer which isolates the metallic
substrate from the aggressive medium. However,
after the artificial defect was created the system’s
responses changes drastically as a result of the
9
corrosion protection which is a continued protection
of the substrate even after partial damage of the
coating.
Either the passivation or active corrosion
protection could be attributed to the self-healing
property of the phosphate-doped coating. Phosphate
released from the glass fast precipitates with the
calcium ion present in LS, which is enhanced by the
high alkalinity nature of the cement environment.
The precipitates form a layer on top of the defect as a
barrier to the electrolyte medium and impeding the
movement of the corrosive species. Whereas the base
enamel creates direct pathways for electrolyte to
reach the metallic substrate after the coating is
damaged. Therefore, phosphate-doped enamel
coating outperforms the base enamel by providing
active corrosion protection and passivates the defect.
5.
6.
Trabelsi, W., et al., The use of pretreatments based on doped silane solutions for
improved corrosion resistance of galvanised steel
substrates. Surface and Coatings Technology, 2006.
200(14-15): p. 4240-4250.
7.
Walter, G.W., A review of impedance plot
methods used for corrosion performance analysis of
painted metals. Corrosion Science, 1986. 26(9): p.
681-703.
8.
Kouloumbi, N., et al., Evaluation of the
behaviour of particulate polymeric coatings in a
corrosive environment. Influence of the concentration
of metal particles. Progress in Organic Coatings,
1996. 28(2): p. 117-124.
9.
Feliu, S., J.C. Galván, and M. Morcillo, The
charge transfer reaction in Nyquist diagrams of
painted steel. Corrosion Science, 1990. 30(10): p.
989-998.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial
support provided by Intelligent Systems Center and
the U.S. National Science Foundation under Award
No. CMMI-0900159.
10.
Trabelsi, W., et al., Electrochemical
assessment of the self-healing properties of Ce-doped
silane solutions for the pre-treatment of galvanised
steel substrates. Progress in Organic Coatings, 2005.
54(4): p. 276-284.
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1.
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