A. KOCIJAN et al.: CORROSION RESISTANCE OF SUPERHYDROPHILIC AND SUPERHYDROPHOBIC TiO2/EPOXY ...
383–388
CORROSION RESISTANCE OF SUPERHYDROPHILIC AND
SUPERHYDROPHOBIC TiO
2
/EPOXY COATINGS ON AISI 316L
STAINLESS STEEL
KOROZIJSKA ODPORNOST SUPERHIDROFILNIH IN
SUPERHIDROFOBNIH TiO
2
/EPOKSI PREVLEK NA NERJAVNEM
JEKLU AISI 316L
Aleksandra Kocijan, Marjetka Conradi, ^rtomir Donik
Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Slovenia
aleksandra.kocijan@imt.si
Prejem rokopisa – received: 2017-11-15; sprejem za objavo – accepted for publication: 2018-01-09
doi:10.17222/mit.2017.191
Superhydrophilic and superhydrophobic TiO2/epoxy coatings were prepared with as-received TiO2 and fluoroalkylsilane (FAS)
functionalised TiO2 nanoparticles and successfully applied to the surface of AISI 316L stainless steel. The wetting properties of
the coatings were confirmed with static contact-angle measurements. The corrosion performance of the investigated coatings
was studied by electrochemical impedance spectroscopy (EIS). The open-circuit impedance spectra of the AISI 316L stainless
steel, the epoxy-coated AISI 316L, the as-received TiO2/epoxy coating on AISI 316L and the FAS-TiO2/epoxy coating on AISI
316L were measured in simulated physiological Hank’s solution. Bode plots and Nyquist diagrams were used to evaluate the
corrosion properties of the investigated coatings. The results show the enhanced corrosion resistance of surface-modified stain-
less steel, especially in the case of the superhydrophobic FAS-TiO2/epoxy coating.
Keywords: stainless steel, epoxy coating, TiO
2
, electrochemistry
Superhidrofilne in superhidrofobne TiO2/epoksi prevleke smo pripravili s TiO2 in fluoroalkilsilan (FAS)-TiO2 nanodelci in jih
uspe{no nanesli na povr{ino nerjavnega jekla AISI 316L. Omo~itvene lastnosti prevlek smo potrdili z meritvami kontaktnih
kotov. Korozijske lastnosti pripravljenih prevlek smo raziskovali s pomo~jo elektrokemijske impedan~ne spektroskopije (EIS).
Impedan~ne spektre pri potencialu odprtega kroga smo merili na jeklu, epoksi prevleki ter superhidrofobni in superhidrofilni
TiO2/epoksi prevleki v simulirani fiziolo{ki raztopini (Hankovi raztopini). Korozijske lastnosti omenjenih prevlek smo preu~e-
vali s pomo~jo Bodejevih in Nyquistovih diagramov. Rezultati so pokazali izbolj{ano korozijsko odpornost povr{insko
obdelanega nerjavnega jekla, najbolj izrazito v primeru superhidrofobne FAS-TiO2/epoksi prevleke.
Klju~ne besede: nerjavno jeklo, epoksi prevleka, TiO2, elektrokemija
1 INTRODUCTION
Stainless steels, especially AISI 316L, are commonly
used materials in biomedical applications because of
their superior mechanical properties, high corrosion
resistance and good biocompatibility.
1
However, an
increasing number of clinical procedures are requiring
the development of materials with superior performance
and higher reliability.
2
Polymer coatings are known to
improve the surface characteristics of the metallic sub-
strate. Epoxy resins are used in various applications
including the automotive, aircraft, maritime, flooring,
food and medical industries because of their good che-
mical resistance, mechanical properties, strong adhesion
with the substrate and corrosion protection by providing
an effective physical barrier between the metal and the
biological environment.
3,4
However, to avoid their sus-
ceptibility to abrasion
5
and poor resistance to crack
propagation, due to a highly cross-linked structure,
6
the
basic idea is to enhance the mechanical properties and to
promote the surface compatibility of the coating by
incorporating different nanofillers.
7–16
It is mainly the
favourable effects of the particle size, the volume frac-
tion and the size distribution on the mechanical
properties of composite coatings that have been stu-
died.
17–22
Z. Rubab et al. showed that sub-micron TiO
2
particles significantly enhance the glass-transition
temperature, thermal oxidative stability, tensile strength
and Young’s modulus of epoxy polymers.
12
Hierarchical
structures with a different nanoparticle size distribution
influence the wetting properties of the surface.
13
Addi-
tionally, the incorporation of inorganic nanoparticles
improves the barrier properties of the coatings by de-
creasing the porosity and increasing the path of aggres-
sive ions and therefore enhancing the corrosion stability
of the metallic substrate.
23
In the present study we prepared superhydrophilic
and superhydrophobic coatings with as-received and
surface-modified TiO
2
nanoparticles, respectively. We
focused on a comparison of the corrosion properties of
epoxy coatings on AISI 316L stainless steel with an
emphasis on tuning the wetting properties between the
two limiting cases, superhydrophobic/superhydrophilic
coatings. The corrosion properties were evaluated by
Materiali in tehnologije / Materials and technology 52 (2018) 4, 383–388 383
UDK 669.018.822:667.648.2:691.714.018.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)383(2018)

electrochemical impedance spectroscopy (EIS) and
compared with the characteristics of bare and epoxy-
coated AISI 316L stainless steel.
2 EXPERIMENTAL PART
Materials. • Austenitic stainless steel AISI 316L
(17 % Cr, 10 % Ni, 2.1 % Mo, 1.4 % Mn, 0.38 % Si,
0.041 % P, 0.021 % C, < 0.005%Sinmass fraction)
was used as a substrate. The steel sheet with a thickness
of 1.5 mm was cut into discs of 25-mm diameter. The
steel discs were grinded mechanically with SiC emery
paper (up to 4000 grit), diamond polished (up to 1 μm),
ultrasonically cleaned with ethanol and dried in warm
air.
A two-component USP Class VI biocompatible
epoxy EPO-TEK 302-3M (EPOXY TECHNOLOGY,
Inc.) was mixed in the w/% ratio 100:45. TiO
2
nano-
particles with mean diameters of 30 nm were provided
by Cinkarna Celje, whereas the 300-nm particles were
supplied by US Research Nanomaterials, Inc. The TiO
2
particles were functionalized in1%ofv olume fraction
of fluoroalkylsilane or FAS17 (C
16
H
19
F
17
O
3
Si) ethanol
solution. The solution was shaken for a few minutes and
left overnight prior to the use in the experiments. Prior to
the TiO
2
nanoparticle adsorption, the diamond-polished
AISI 316L substrate was spin-coated with an epoxy layer
and then cured for 3 h at 65 °C. We decided on a base
epoxy layer to improve the TiO
2
nanoparticle adhesion as
the oxide layer growing on the surface of the clean AISI
316L substrate prevents adhesion and is very difficult to
remove. The nanoparticles were further applied to the
epoxy-coated AISI 316L surface by spin-coating 20 μL
o f3%o fmass fractions of TiO
2
nanoparticle ethanol
solution. We consecutively applied three deposits of
dual-size, dual-layer coating consisting of 30-nm and
300-nm FAS-TiO
2
nanoparticles. The coating was then
dried in an oven for approximately 30 min at 100 °C.
The same procedure was repeated with the as-received,
non-functionalized TiO
2
nanoparticles.
Contact-angle measurements. • The static contact-
angle measurements of water (W) on a clean AISI 316L
diamond-polished sample, on the epoxy-coated AISI
316L substrate, on the TiO
2
/epoxy-coated AISI 316L
substrate and on the FAS-TiO
2
/epoxy-coated AISI 316L
substrate were performed using a surface-energy
evaluation system (Advex Instruments s.r.o.) at 20 °C
and ambient humidity.
Electrochemical measurements. • Electrochemical
measurements were performed on diamond-polished
AISI 316L stainless steel, on the epoxy-coated AISI
316L substrate, on the TiO
2
/epoxy-coated AISI 316L
substrate and on the FAS-TiO
2
/epoxy-coated AISI 316L
substrate. The experiments were carried out in simulated
physiological Hank’s solution at pH = 7.8 and at room
temperature (Table 1). All the chemicals were from
Merck, Darmstadt, Germany. The measurements were
performed using a three-electrode, flat BioLogic corro-
sion cell (volume 0.25 L). The test specimen was
employed as the working electrode (WE), the exposed
area of the sample was 1 cm
2
. The reference electrode
(RE) was a saturated calomel electrode (SCE, 0.242 V
vs. SHE) and the counter electrode (CE) was a platinum
net. Electrochemical measurements were recorded using
a BioLogic Modular Research Grade Potentiostat/Galva-
nostat/FRA Model SP-300 with an EC-Lab Software.
Long-term open-circuit potentiostatic electrochemical
impedance spectra (EIS) were obtained for the inves-
tigated samples. The impedance was measured at the
OCP, with a sinus amplitude of 5-mV peak-to-peak and a
frequency range of 65 kHz to 1 mHz, directly after
immersion for (1, 2, 3, 4, 16, 40, 64, 88, 112, 136 and
168) h. All the measurements were made at room tempe-
rature and were repeated at least three times. The data
was collected and analysed by the EC-Lab V10.44
software from Bio-Logic Instruments. The impedance
data are presented in terms of Bode and Nyquist plots.
For the fitting process Zview v3.4d Scribner Associates
software was used.
Table 1: Chemical composition of the simulated physiological Hank’s
solution
Reagent c (g/L)
NaCl 8.0
KCl 0.40
NaHCO 3 0.35
NaH 2
PO
4
·2H
2
O 0.25
Na 2
HPO
4
·2H
2
O 0.06
CaCl 2·2H 2O 0.19
MgCl 2·6H 2O 0.41
MgSO 4
·7H
2
O 0.06
glucose 1.0
3 RESULTS AND DISCUSSION
With the contact-angle measurements we confirmed
the synthesis of the two limiting cases of coatings con-
cerning their wetting characteristics. The FAS-TiO
2
/
epoxy coating was superhydrophobic with a contact
angle above 150° and the as-received TiO
2
/epoxy coating
was superhydrophilic with a contact angle below 3°.
The corrosion performance of the investigated coat-
ings was studied by EIS. The open-circuit impedance
spectra of the AISI 316L stainless steel, the epoxy-
coated AISI 316L, the as-received TiO
2
/epoxy coating on
AISI 316L and the FAS-TiO
2
/epoxy coating on AISI
316L were measured over a 168 h immersion period in
the simulated physiological Hank’s solution.
Figures 1–3 show the EIS Bode plots and Figures 4
to 6 the Nyquist diagrams for all the investigated
coatings after 1 h,1dand7dofimmersion in Hank’s
solution. The results for bare AISI 316L stainless steel
after 1 h,1dand7dofimmersion in Hank’s solution
were used for comparison. In the case of the investigated
A. KOCIJAN et al.: CORROSION RESISTANCE OF SUPERHYDROPHILIC AND SUPERHYDROPHOBIC TiO2/EPOXY ...
384 Materiali in tehnologije / Materials and technology 52 (2018) 4, 383–388

coatings on AISI 316L stainless steel, the first time
constant in the high-frequency range was associated with
the barrier properties of the coatings, the second time
constant in the medium frequency range with the
formation of an oxide layer at the coating/ substrate
interface and the third time constant in the low-frequency
A. KOCIJAN et al.: CORROSION RESISTANCE OF SUPERHYDROPHILIC AND SUPERHYDROPHOBIC TiO2/EPOXY ...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 383–388 385
Figure 3: EIS Bode plot of AISI 316L stainless steel, the epoxy
coating, as-received TiO
2
/epoxy coating and the FAS-TiO
2
/epoxy
coating on the surface of AISI 316L stainless steel in Hank’s solution
after7dofimmersion
Figure 4: Nyquist diagram of AISI 316L stainless steel, the epoxy
coating, as-received TiO
2
/epoxy coating and the FAS-TiO
2
/epoxy
coating on the surface of AISI 316L stainless steel in Hank’s solution
after1ho fimmersion. The inset diagrams represent the high-fre-
quency range of the impedance responses
Figure 2: EIS Bode plot of AISI 316L stainless steel, the epoxy
coating, as-received TiO
2
/epoxy coating and the FAS-TiO
2
/epoxy
coating on the surface of AISI 316L stainless steel in Hank’s solution
after1dofimmersion
Figure 1: EIS Bode plot of AISI 316L stainless steel, the epoxy
coating, as-received TiO
2
/epoxy coating and the FAS-TiO
2
/epoxy
coating on the surface of AISI 316L stainless steel in Hank’s solution
after1hofimmersion

range with the corrosion of the substrate.
24
In the case of
AISI 316L stainless steel, the spectra were characterised
by two time constants. The first time constant in the
high-medium frequency range was assigned to the
resistance due to the ionic paths through the oxide film
and the low-medium-frequency time constant was
correlated with the charge-transfer process.
8,25,26
Magnitude plots revealed that the total impedance for all
three investigated coatings increased compared to the
bare alloy, the values increased from 10
6
 cm
2
for AISI
316L to 10
7
 cm
2
for the epoxy-coated AISI 316L, 10
8
 cm
2
for the as-received TiO
2
/epoxy-coated AISI 316L
and 10
9
 cm
2
for the FAS-TiO
2
/ epoxy-coated AISI
316L. This indicated the higher corrosion stability of the
coated samples, especially with the addition of TiO
2
nanoparticles. Furthermore, a high-frequency time
constant was clearly observed for all the investigated
coatings compared to the bare AISI 316L, thus
indicating the barrier ability of the coatings. This effect
was more pronounced for the FAS-TiO
2
/epoxy coating
with the phase angle approaching –90°, which is typical
of the capacitor and is ascribed to the capacitive
behaviour of the coating.
Figure 7 presents the equivalent circuits that were
used to fit the experimental data to the theoretical
impedance data. The EIS technique enables us to match
the electrochemical behaviour of the investigated system
with model circuits consisting of specific components
equivalent to the parameters of the electrochemical
system.
27
We selected a model that provided the results
with the smallest errors of fit. In the equivalent circuit
applied for the evaluation of the investigated coatings R
s
refers to the solution resistance. High-frequency resistance
(R
pore
) was associated with the electrolyte in the pores of
the coating, medium-frequency resistance (R
ox
)w a s
related to the interfacial oxide and low-frequency resis-
tance (R
sub
) reflected the corrosion process (Figure 7a).
Each resistance was coupled with the CPE element,
expressing the coating capacitance (CPE
pore
), capacitance
of the oxide layer (CPE
ox
) and capacitance of the double
layer (CPE
sub
). The circuit was simplified to the one with
two time constants up to 24 h of immersion due to the
lower fitting errors (Figure 7b). In the case of AISI 316L
stainless steel (Figure 7c), the spectra were characterised
by two time constants, the first time constant in
high-medium frequency range was simulated by the
resistance due to the ionic paths through the oxide film
(R
ox
) and was coupled with a capacitive behaviour of the
oxide film (CPE
ox
). The low-medium-frequency time
constant was correlated with the charge-transfer process,
which was composed of charge-transfer resistance (R
sub
)
in parallel with the double-layer capacitance
(CPE
sub
).
8,24,26
The evolution of the pore resistance (R
pore
) revealed
the barrier properties of the coatings (Figure 8a). We
A. KOCIJAN et al.: CORROSION RESISTANCE OF SUPERHYDROPHILIC AND SUPERHYDROPHOBIC TiO2/EPOXY ...
386 Materiali in tehnologije / Materials and technology 52 (2018) 4, 383–388
Figure 5: Nyquist diagram of AISI 316L stainless steel, the epoxy
coating, as-received TiO
2
/epoxy coating and the FAS-TiO
2
/epoxy
coating on the surface of AISI 316L stainless steel in Hank’s solution
after1do fimmersion; the inset diagrams represent the high-fre-
quency range of impedance responses
Figure 7: Equivalent electrical circuits for modelling the EIS data
Figure 6: Nyquist diagram of AISI 316L stainless steel, the epoxy
coating, as-received TiO
2
/epoxy coating and the FAS-TiO
2
/epoxy
coating on the surface of AISI 316L stainless steel in Hank’s solution
after7do fimmersion; the inset diagrams represent the high-fre-
quency range of impedance responses

observed that the R
pore
values were the highest for the
FAS-TiO
2
/epoxy coating and the TiO
2
/epoxy coating
compared to the pure epoxy coating, indicating a better
corrosion performance. Additionally, the R
pore
of epoxy
coating slightly decreased with the immersion time,
probably due to the easier penetration of the electrolyte.
The addition of TiO
2
nanoparticles to the epoxy coating
prolongs the diffusion path of the ionic species through
the coating.
8
The admittance of the CPE (Table 2) was in
the range of 10
–7
 –1
cm
–2
s
n
for the epoxy coating and
the TiO
2
/epoxy coating, and in the range of 10
–8
 –1
cm
–2
s
n
for the FAS-TiO
2
/epoxy coating. The values for all three
coatings remained almost constant at different immer-
sion times. The CPE exponent was above 0.88 for the
FAS-TiO
2
/epoxy coating, 0.84 for the TiO
2
/epoxy coat-
ing, and 0.75 for the epoxy coating.
Table 2: Fitting parameters (admittances) for the epoxy coating,
as-received TiO
2
/epoxy coating and the FAS-TiO
2
/epoxy coating on
the surface of AISI 316L stainless steel after 24 h and 168 h of
immersion in Hank’s solution
t
(h)
CPE
pore
( –1
cm
–2
s
n
)
CPE
ox
( –1
cm
–2
s
n
)
CPE
sub
( –1
cm
–2
s
n
)
Epoxy coated AISI 316L
24 5,1 × 10
–7
2,1×10
–7
6,5×10
–7
168 5,6 × 10
–7
2,0×10
–7
1,9×10
–7
TiO
2
/epoxy coated AISI 316L
24 2,6 × 10
–7
2,9×10
–7
2,4×10
–7
168 2,1 × 10
–7
1,1×10
–7
1,0×10
–7
FAS-TiO
2
/epoxy coated AISI 316L
24 2,9 × 10
–8
5,0×10
–8
1,4×10
–8
168 2,3 × 10
–8
9,4×10
–9
1,3×10
–8
The resistance associated with the medium-frequency
time constant (R
ox
)( Figure 8b) had higher values, espe-
cially for the FAS-TiO
2
/epoxy coating. The values for
the TiO
2
/epoxy coating were comparable to the pure
epoxy coating. However, a slight increase for the
TiO
2
/epoxy coating compared to a slight decrease for
pure epoxy coating was recorded, demonstrating im-
proved protective interfacial properties by the addition of
TiO
2
nanoparticles. The admittance of the CPE for the
epoxy coating was approximately 10
–7
 –1
cm
–2
s
n
and
remained constant for different immersion times. The
admittance of the CPE (Table 2) for the as-received
TiO
2
/epoxy coating was similar as for the epoxy coating,
but slightly decreased with the immersion time. In the
case of the FAS-TiO
2
/epoxy coating, the admittance of
the CPE decreased from5×10
–8
 –1
cm
–2
s
n
to9×10
–9
 –1
cm
–2
s
n
with the immersion time. The CPE exponent
was in the range 0.75–0.80 for the FAS-TiO
2
/epoxy coat-
ing, above 0.83 for the TiO
2
/epoxy coating, and 0.80 for
the epoxy coating.
The evolution of the low-frequency resistance (R
sub
)
is shown in Figure 8c. The low-frequency resistance was
the highest for the FAS-TiO
2
/epoxy and the TiO
2
/epoxy
coatings, indicating that the addition of nanoparticles,
especially hydrophobic ones, increased the corrosion sta-
bility of the substrate. The values of R
sub
slightly in-
creased with the immersion time for the FAS-TiO
2
/epoxy
and the TiO
2
/epoxy-coated samples compared to the pure
epoxy coating. The admittance of the CPE (Table 2) for
the epoxy coating was in the range of6×10
–7
 –1
cm
–2
s
n
and for the TiO
2
/epoxy coating in the range of2×10
–7
 –1
cm
–2
s
n
. The admittance of the CPE for the FAS-
TiO
2
/epoxy coating was approximately 10
–8
 –1
cm
–2
s
n
.
All the values slightly decreased with the immersion
A. KOCIJAN et al.: CORROSION RESISTANCE OF SUPERHYDROPHILIC AND SUPERHYDROPHOBIC TiO2/EPOXY ...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 383–388 387
Figure 8: Evolution of resistance for bare AISI 316L stainless steel,
the epoxy coating, as-received TiO
2
/epoxy coating and the FAS-
TiO
2
/epoxy coating on AISI 316L stainless steel during immersion in
Hank’s solution

time. The CPE exponent was approximately 0.59 for the
FAS-TiO
2
/epoxy coating, 0.68 for the TiO
2
/epoxy coat-
ing, and approximately 0.58 for the epoxy coating.
4 CONCLUSIONS
In this paper we presented the preparation of super-
hydrophobic FAS-TiO
2
/epoxy and superhydrophilic
as-received TiO
2
/epoxy coatings on the surface of AISI
316L stainless steel. The electrochemical study was
applied to study the barrier coatings of the prepared coat-
ings in a simulated physiological solution. The obtained
results show a significant improvement in terms of the
corrosion stability of the epoxy coatings with the addi-
tion of TiO
2
nanoparticles compared to the pure epoxy
coating, especially in the case of the surface-modified
superhydrophobic TiO
2
, manifesting in higher R values
and lower CPE values. The addition of nanoparticles pro-
vides better barrier properties due to the prolonged
penetration of the electrolyte through the coating.
Acknowledgments
The authors acknowledge the project (Antibacterial
Nanostructured Surfaces for Biological Applications, ID
J2-7196) was financially supported by the Slovenian
Research Agency.
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