M. CONRADI, A. KOCIJAN: COMPARISON OF THE SURFACE AND ANTICORROSION PROPERTIES ...
1043–1046
COMPARISON OF THE SURFACE AND ANTICORROSION
PROPERTIES OF SiO2 AND TiO2 NANOPARTICLE EPOXY
COATINGS
PRIMERJAVA POVR[INSKIH IN PROTIKOROZIJSKIH LASTNOSTI
EPOKSIDNIH PREVLEK OBOGATENIH S SiO2 IN TiO2
NANOVKLJU^KI
Marjetka Conradi, Aleksandra Kocijan
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
marjetka.conradi@imt.si
Prejem rokopisa – received: 2017-08-21; sprejem za objavo – accepted for publication: 2017-10-02
doi:10.17222/mit.2017.137
In this article we compare the morphology, wetting and anticorrosion properties of fluorosilane-modified TiO2, FAS-TiO2/epoxy
and SiO2, FAS-SiO2/epoxy coatings. Thirty-nanometre TiO2 and SiO2 nanoparticles were spin coated onto the AISI 316L steel
substrate and covered with a thin epoxy layer for nanoparticle fixation. The morphology of the coatings was analysed with SEM
imaging and the average surface roughness (Sa), and it showed a homogeneous FAS-TiO2 nanoparticle distribution in the
coating, whereas the FAS-SiO2 nanoparticles tended to agglomerate. Static water contact angles were measured to evaluate the
wetting properties, indicating the highly hydrophobic nature of both coatings. Potentiodynamic measurements showed that the
addition of nanoparticles to the epoxy coating significantly improved the corrosion resistance of the AISI 316L stainless steel.
Keywords: TiO2, SiO2, epoxy, coatings, wetting, corrosion
V ~lanku primerjamo morfologijo, omo~itvene lastnosti in antikorozijske lastnosti s fluorosilanom oble~enih TiO2, FAS-TiO2/
epoksi in SiO2, FAS-SiO2/epoxy prevlek. 30-nm TiO2 in SiO2 nanodelce smo na jekleno podlago tipa AISI 316L nanesli s "spin
coaterjem" ter jih prekrili s tanko plastjo epoksidne smole, ki je zagotovila fiksacijo nanodelcev na povr{ini. Morfolo{ke
lastnosti prevlek smo analizirali s SEM mikroskopijo ter z meritvami povpre~ne hrapavosti povr{ine (Sa). Pokazali smo, da so
FAS-TiO2 nanodelci v prevleki enakomerno razporejeni, medtem, ko FAS-SiO2 nanodelci ka`ejo visoko stopnjo aglomeracije.
Omo~itvene lastnosti prevlek smo dolo~ili z meritvami stati~nih kontaktnih kotov, ki so pokazale hidrofobne lastnosti tako
FAS-TiO2/epoksi kot tudi FAS-SiO2/epoksi prevlek. Potenciodinamske meritve potrjujejo, da z dodatkom nanodelcev epoksi,
za{~itnim prevlekam izrazito izbolj{amo korozijsko obstojnost nerjavnega jekla AISI 316L.
Klju~ne besede: TiO2, SiO2, epoksi, prevleke, omo~itvene lastnosti, korozija
1 INTRODUCTION
Designing a solid surface with specific surface
characteristics, such as wetting properties, mechanical
resistance, anticorrosion properties, etc., is challenging
in several applications in aerospace, marine, biomedicine
etc1,2. Therefore, the surface modification of engineering
metallic materials, such as the most commonly used
austenitic stainless steel (AISI),3,4 by various coatings
represents an important subject in the field of enhancing
particular surface properties, mechanical as well as
anticorrosion properties.
Epoxy coatings serve as an excellent physical barrier
for metallic surface protection due to their good
mechanical and electrical insulating properties, chemical
resistance and strong adhesion to different substrates.
However, the highly cross-linked structure of an epoxy
resin often makes epoxy coatings susceptible to the pro-
pagation of cracks and damage by surface abrasion and
wear.5 It has been shown that the implementation of
various nanoparticles like SiO2, TiO2, ZnO, CuO, etc.
additionally improves the performance of coatings.6
Nanoparticles also enhance the corrosion protection
properties of the epoxy coatings by decreasing the
porosities due to the small size and high specific area.
The hierarchical structures of nanoparticle/epoxy
composites can, on the other hand, also change the
wetting characteristics of the surface. It is well known
that the surface roughness can enhance both hydro-
phobicity and hydrophilicity.7,8 In combination with the
nanoparticle surface chemistry (i.e., functionalization)
we can control the wetting characteristics of epoxy
coatings9 that additionally allows for an improvement of
the anticorrosion properties.
Here we report on a comparison of the surface and
anticorrosion properties of hydrophobic FAS-TiO2/epoxy
and FAS-SiO2/epoxy coatings. The morphology and
wetting properties are characterized as well as the
anticorrosion properties through potentiodynamic
measurements.
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,
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1043–1046 1043
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.1/.2:620.19:532.6 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)1043(2017)
0.021 % C, <0.005 % S in mass fraction) was used as a
substrate.
A biocompatible epoxy EPO-TEK 302-3M (EPOXY
TECHNOLOGY, Inc.) was mixed in the producer-
prescribed two-component w/% ratio 100:45. 30-nm
TiO2 nanoparticles with mean were provided by Cinkar-
na Celje, whereas the 30-nm SiO2 nanoparticles were
supplied by Cab-O-Sil.
Surface functionalization. For hydrophobic effect as
well as for homogenization of nanoparticle distribution,
SiO2 and TiO2 particles were functionalized in 1 % of
volume fractions of ethanolic fluoroalkylsilane or FAS17
(C16H19F17O3Si) solution.
Steel substrate preparation. Prior to the application
of the coating, the steel discs of 25 mm diameter and
with a thickness of 1.5 mm were diamond polished
following a standard mechanical procedure and then
cleaned with ethanol in an ultrasonic bath.
Coating preparation. To improve the SiO2 and TiO2
nanoparticles’ adhesion, the diamond-polished AISI
316L substrate was spin-coated with a thin layer of
epoxy and then cured for 3 h at 65 °C. To ensure good
surface coverage, five drops (20 μL) of 3 % of mass
fractions of TiO2/SiO2 nanoparticle ethanolic solution
were then spin-coated onto an epoxy-coated AISI 316L
substrate and dried in an oven for approximately 20 min
at 100 °C. Finally, the coatings were covered with
another thin layer of epoxy for particle fixation and then
cured for 3 h at 65 °C.
Scanning electron microscopy (SEM). SEM analysis
using FE-SEM Zeiss SUPRA 35VP was employed to
investigate the morphology of the nanoparticle coatings’
surfaces, which were sputtered with gold prior to
imaging.
Contact-angle measurements. The static contact-
angle measurements of water (W) on the nanoparticle
coatings were performed using a surface-energy evalu-
ation system (Advex Instruments s.r.o.). Liquid drops of
5 μL were deposited on different spots of the substrates
to avoid the influence of roughness and gravity on the
shape of the drop. The drop contour was analysed from
the image of the deposited liquid drop on the surface and
the contact angle was determined by using Young-
Laplace fitting. To minimize the errors due to roughness
and heterogeneity, the average values of the contact
angles of the drop were calculated approximately 30 s
after the deposition from at least five measurements on
the studied coated steel. All the contact-angle measure-
ments were carried out at 20 °C and ambient humidity.
Surface roughness. Optical 3D metrology system,
model Alicona Infinite Focus (Alicona Imaging GmbH),
was used for the surface-roughness analysis. At least
three measurements per sample were performed at a
magnification of 20× with a lateral resolution of 0.9 μm
and a vertical resolution of about 50 nm. IF-Measure-
Suite (Version 5.1) software was later on used to
calculate the average surface roughness, Sa, for each
sample, based on the general surface-roughness equation
(Equation (1)):
Sa
L L
z x y x y
x y
LL x
=
<
∫∫
1 1
00
( , ) d d (1)
where Lx and Ly are the acquisition lengths of the sur-
face in the x and y directions and z(x,y) is the height.
The size of the analysed area was 1337×540 μm2.
Electrochemical measurements. Electrochemical
measurements were performed on the epoxy-coated,
FAS-SiO2/epoxy-coated and FAS-TiO2/epoxy-coated
AISI 316L stainless steel in a simulated physiological
Hank’s solution, containing 8 g/L NaCl, 0.40 g/L KCl,
0.35 g/L NaHCO3, 0.25 g/L NaH2PO4·2H2O, 0.06 g/L
Na2HPO4·2H2O, 0.19 g/L CaCl2·2H2O, 0.41 g/L
MgCl2·6H2O, 0.06 g/L MgSO4·7H2O and 1 g/L glucose,
at pH = 7.8 and 37 °C. All the chemicals were from
Merck, Darmstadt, Germany. The measurements were
performed by using BioLogic Modular Research Grade
Potentiostat/Galvanostat/FRA Model SP-300 with an
EC-Lab Software and a three-electrode flat corrosion
cell, where the working electrode (WE) was the inves-
tigated specimen, the reference electrode (RE) was a
saturated calomel electrode (SCE, 0.242 V vs. SHE) and
the counter electrode (CE) was a platinum net. The po-
tentiodynamic curves were recorded at the open-circuit
potential (OCP), starting the measurement at 250 mV vs.
SCE more negative than the OCP. The potential was
increased using a scan rate of 1 mV s–1.
M. CONRADI, A. KOCIJAN: COMPARISON OF THE SURFACE AND ANTICORROSION PROPERTIES ...
1044 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1043–1046
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: SEM images of the surface morphology of: a) FAS-TiO2/
epoxy and b) FAS-SiO2/epoxy coatings
3 RESULTS AND DISCUSSION
3.1 Surface morphology
Figure 1 compares the morphology of the FAS-TiO2/
epoxy and FAS-SiO2/epoxy coatings. The SEM images
reveal a distinctive morphology between the two coat-
ings that is reflected in the different length scale of the
agglomerate formation, which is also reflected in a dis-
crepancy in the average surface roughness Sa (Table 1).
FAS-TiO2/epoxy coatings (Figure 1a) are characterized
with a more refined structure that is a consequence of the
better FAS-TiO2 nanoparticle dispersion with less
agglomeration on the micrometre scale. FAS-SiO2/epoxy
coatings, in contrast, are characterized by severe agglo-
meration (Figure 1b) and randomly distributed agglo-
merates from nanometres to a few micrometres in
diameter. This suggests that the FAS functionalization
works well with homogenization of TiO2 nanoparticle
distribution, but has no effect on the homogenization of
the SiO2 nanoparticle distribution.
3.2 Wetting properties
To analyse the surface wettability, we performed five
static contact-angle measurements with water (W) on
different spots all over the sample and used them to
determine the average contact-angle values of the coat-
ing with an estimated error in the reading of ±1.0°.
In the first step we prepared the superhydrophobic
FAS-TiO2 and FAS-SiO2 surfaces by spin-coating the
AISI+epoxy substrate with FAS-TiO2 and FAS-SiO2
nanoparticles. The corresponding static water contact
angles are reported in Table 1. Nanoparticle fixation
with a thin layer of hydrophilic epoxy (w = 71.8°) elimi-
nated the superhydrophobic effect; however, a substantial
degree of hydrophobicity that is necessary for desirable
anticorrosion properties10 was retained, as reported in
Table 1. The retained high degree of hydrophobicity is
most probably a combination of surface roughness and
the originally superhydrophobic nature of the FAS-TiO2
and FAS-SiO2 nanoparticles. In addition, the top layer of
epoxy also smoothened the coating, which is reflected in
the reduced average surface roughness, Sa, of the
AISI+epoxy+TiO2/SiO2+epoxy surfaces compared to the
AISI+epoxy+TiO2/SiO2 surfaces. This is most probably
due to the fact that epoxy fills the space between the
nanoparticle agglomerates and reduces the height vari-
ation over the whole surface, and therefore also Sa.
There is, however, a noticeable difference in the static
water contact angles and a difference by a factor of 10 in
the value of the average surface roughness, Sa, As re-
ported in Table 1, the FAS-SiO2/epoxy coating is rougher
and more hydrophobic compared to the FAS-TiO2/epoxy
coating. The difference in the average surface roughness
was already expected from the surface morphology
analysis (Figure 1) due to the severe SiO2 nanoparticle
agglomeration compared to the TiO2 coated surface. The
increased hydrophobicity of the FAS-SiO2/epoxy coating
can, on the other hand, be attributed to the more
pronounced micro- to nanoparticle-textured surface with
a refined roughness structure.9
Table 1: Comparison of the static water contact angles (W) and the
average surface roughness (Sa) of the FAS-TiO2/epoxy and FAS-SiO2/
epoxy coatings
Substrate Contact angle 
w
(°)
Roughness Sa
(μm)
epoxy 71.8 0.02
AISI+epoxy+TiO2 150.1 0.41
AISI+epoxy+SiO2 155.4 3.69
AISI+epoxy+TiO2+epoxy 121.1 0.23
AISI+epoxy+SiO2+epoxy 130.6 2.34
3.3 Potentiodynamic measurements
Figure 2 shows the potentiodynamic behaviour of the
FAS-TiO2/epoxy-coated, FAS-SiO2/epoxy-coated and
epoxy-coated AISI 316L stainless steel in a simulated
physiological Hank’s solution. The polarization and the
passivation behaviour of the tested material after the
surface modification was studied. The corrosion poten-
tial (Ecorr) for the epoxy coating in the Hank’s solution
was approximately –256 mV vs. SCE, for the FAS-
TiO2/epoxy-coated AISI 316L it was –335 mV vs. SCE
and for FAS-SiO2/epoxy-coated AISI 316L it was –407
mV vs. SCE. After the Tafel region, the investigated
sample exhibited a broad passive range followed by the
breakdown potential (Eb). The passivation range of the
FAS-TiO2/epoxy-coated and FAS-SiO2/epoxy-coated
AISI 316L specimen was moved to the significantly
lower corrosion-current densities compared to the pure
epoxy-coated AISI 316L. We established that the addi-
tion of nanoparticles to the epoxy coating significantly
M. CONRADI, A. KOCIJAN: COMPARISON OF THE SURFACE AND ANTICORROSION PROPERTIES ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1043–1046 1045
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Potentiodynamic curves for FAS-TiO2/epoxy-coated,
FAS-SiO2/epoxy-coated and epoxy-coated AISI 316L substrate in a
simulated physiological Hank’s solution
0.021 % C, <0.005 % S in mass fraction) was used as a
substrate.
A biocompatible epoxy EPO-TEK 302-3M (EPOXY
TECHNOLOGY, Inc.) was mixed in the producer-
prescribed two-component w/% ratio 100:45. 30-nm
TiO2 nanoparticles with mean were provided by Cinkar-
na Celje, whereas the 30-nm SiO2 nanoparticles were
supplied by Cab-O-Sil.
Surface functionalization. For hydrophobic effect as
well as for homogenization of nanoparticle distribution,
SiO2 and TiO2 particles were functionalized in 1 % of
volume fractions of ethanolic fluoroalkylsilane or FAS17
(C16H19F17O3Si) solution.
Steel substrate preparation. Prior to the application
of the coating, the steel discs of 25 mm diameter and
with a thickness of 1.5 mm were diamond polished
following a standard mechanical procedure and then
cleaned with ethanol in an ultrasonic bath.
Coating preparation. To improve the SiO2 and TiO2
nanoparticles’ adhesion, the diamond-polished AISI
316L substrate was spin-coated with a thin layer of
epoxy and then cured for 3 h at 65 °C. To ensure good
surface coverage, five drops (20 μL) of 3 % of mass
fractions of TiO2/SiO2 nanoparticle ethanolic solution
were then spin-coated onto an epoxy-coated AISI 316L
substrate and dried in an oven for approximately 20 min
at 100 °C. Finally, the coatings were covered with
another thin layer of epoxy for particle fixation and then
cured for 3 h at 65 °C.
Scanning electron microscopy (SEM). SEM analysis
using FE-SEM Zeiss SUPRA 35VP was employed to
investigate the morphology of the nanoparticle coatings’
surfaces, which were sputtered with gold prior to
imaging.
Contact-angle measurements. The static contact-
angle measurements of water (W) on the nanoparticle
coatings were performed using a surface-energy evalu-
ation system (Advex Instruments s.r.o.). Liquid drops of
5 μL were deposited on different spots of the substrates
to avoid the influence of roughness and gravity on the
shape of the drop. The drop contour was analysed from
the image of the deposited liquid drop on the surface and
the contact angle was determined by using Young-
Laplace fitting. To minimize the errors due to roughness
and heterogeneity, the average values of the contact
angles of the drop were calculated approximately 30 s
after the deposition from at least five measurements on
the studied coated steel. All the contact-angle measure-
ments were carried out at 20 °C and ambient humidity.
Surface roughness. Optical 3D metrology system,
model Alicona Infinite Focus (Alicona Imaging GmbH),
was used for the surface-roughness analysis. At least
three measurements per sample were performed at a
magnification of 20× with a lateral resolution of 0.9 μm
and a vertical resolution of about 50 nm. IF-Measure-
Suite (Version 5.1) software was later on used to
calculate the average surface roughness, Sa, for each
sample, based on the general surface-roughness equation
(Equation (1)):
Sa
L L
z x y x y
x y
LL x
=
<
∫∫
1 1
00
( , ) d d (1)
where Lx and Ly are the acquisition lengths of the sur-
face in the x and y directions and z(x,y) is the height.
The size of the analysed area was 1337×540 μm2.
Electrochemical measurements. Electrochemical
measurements were performed on the epoxy-coated,
FAS-SiO2/epoxy-coated and FAS-TiO2/epoxy-coated
AISI 316L stainless steel in a simulated physiological
Hank’s solution, containing 8 g/L NaCl, 0.40 g/L KCl,
0.35 g/L NaHCO3, 0.25 g/L NaH2PO4·2H2O, 0.06 g/L
Na2HPO4·2H2O, 0.19 g/L CaCl2·2H2O, 0.41 g/L
MgCl2·6H2O, 0.06 g/L MgSO4·7H2O and 1 g/L glucose,
at pH = 7.8 and 37 °C. All the chemicals were from
Merck, Darmstadt, Germany. The measurements were
performed by using BioLogic Modular Research Grade
Potentiostat/Galvanostat/FRA Model SP-300 with an
EC-Lab Software and a three-electrode flat corrosion
cell, where the working electrode (WE) was the inves-
tigated specimen, the reference electrode (RE) was a
saturated calomel electrode (SCE, 0.242 V vs. SHE) and
the counter electrode (CE) was a platinum net. The po-
tentiodynamic curves were recorded at the open-circuit
potential (OCP), starting the measurement at 250 mV vs.
SCE more negative than the OCP. The potential was
increased using a scan rate of 1 mV s–1.
M. CONRADI, A. KOCIJAN: COMPARISON OF THE SURFACE AND ANTICORROSION PROPERTIES ...
1044 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1043–1046
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: SEM images of the surface morphology of: a) FAS-TiO2/
epoxy and b) FAS-SiO2/epoxy coatings
3 RESULTS AND DISCUSSION
3.1 Surface morphology
Figure 1 compares the morphology of the FAS-TiO2/
epoxy and FAS-SiO2/epoxy coatings. The SEM images
reveal a distinctive morphology between the two coat-
ings that is reflected in the different length scale of the
agglomerate formation, which is also reflected in a dis-
crepancy in the average surface roughness Sa (Table 1).
FAS-TiO2/epoxy coatings (Figure 1a) are characterized
with a more refined structure that is a consequence of the
better FAS-TiO2 nanoparticle dispersion with less
agglomeration on the micrometre scale. FAS-SiO2/epoxy
coatings, in contrast, are characterized by severe agglo-
meration (Figure 1b) and randomly distributed agglo-
merates from nanometres to a few micrometres in
diameter. This suggests that the FAS functionalization
works well with homogenization of TiO2 nanoparticle
distribution, but has no effect on the homogenization of
the SiO2 nanoparticle distribution.
3.2 Wetting properties
To analyse the surface wettability, we performed five
static contact-angle measurements with water (W) on
different spots all over the sample and used them to
determine the average contact-angle values of the coat-
ing with an estimated error in the reading of ±1.0°.
In the first step we prepared the superhydrophobic
FAS-TiO2 and FAS-SiO2 surfaces by spin-coating the
AISI+epoxy substrate with FAS-TiO2 and FAS-SiO2
nanoparticles. The corresponding static water contact
angles are reported in Table 1. Nanoparticle fixation
with a thin layer of hydrophilic epoxy (w = 71.8°) elimi-
nated the superhydrophobic effect; however, a substantial
degree of hydrophobicity that is necessary for desirable
anticorrosion properties10 was retained, as reported in
Table 1. The retained high degree of hydrophobicity is
most probably a combination of surface roughness and
the originally superhydrophobic nature of the FAS-TiO2
and FAS-SiO2 nanoparticles. In addition, the top layer of
epoxy also smoothened the coating, which is reflected in
the reduced average surface roughness, Sa, of the
AISI+epoxy+TiO2/SiO2+epoxy surfaces compared to the
AISI+epoxy+TiO2/SiO2 surfaces. This is most probably
due to the fact that epoxy fills the space between the
nanoparticle agglomerates and reduces the height vari-
ation over the whole surface, and therefore also Sa.
There is, however, a noticeable difference in the static
water contact angles and a difference by a factor of 10 in
the value of the average surface roughness, Sa, As re-
ported in Table 1, the FAS-SiO2/epoxy coating is rougher
and more hydrophobic compared to the FAS-TiO2/epoxy
coating. The difference in the average surface roughness
was already expected from the surface morphology
analysis (Figure 1) due to the severe SiO2 nanoparticle
agglomeration compared to the TiO2 coated surface. The
increased hydrophobicity of the FAS-SiO2/epoxy coating
can, on the other hand, be attributed to the more
pronounced micro- to nanoparticle-textured surface with
a refined roughness structure.9
Table 1: Comparison of the static water contact angles (W) and the
average surface roughness (Sa) of the FAS-TiO2/epoxy and FAS-SiO2/
epoxy coatings
Substrate Contact angle 
w
(°)
Roughness Sa
(μm)
epoxy 71.8 0.02
AISI+epoxy+TiO2 150.1 0.41
AISI+epoxy+SiO2 155.4 3.69
AISI+epoxy+TiO2+epoxy 121.1 0.23
AISI+epoxy+SiO2+epoxy 130.6 2.34
3.3 Potentiodynamic measurements
Figure 2 shows the potentiodynamic behaviour of the
FAS-TiO2/epoxy-coated, FAS-SiO2/epoxy-coated and
epoxy-coated AISI 316L stainless steel in a simulated
physiological Hank’s solution. The polarization and the
passivation behaviour of the tested material after the
surface modification was studied. The corrosion poten-
tial (Ecorr) for the epoxy coating in the Hank’s solution
was approximately –256 mV vs. SCE, for the FAS-
TiO2/epoxy-coated AISI 316L it was –335 mV vs. SCE
and for FAS-SiO2/epoxy-coated AISI 316L it was –407
mV vs. SCE. After the Tafel region, the investigated
sample exhibited a broad passive range followed by the
breakdown potential (Eb). The passivation range of the
FAS-TiO2/epoxy-coated and FAS-SiO2/epoxy-coated
AISI 316L specimen was moved to the significantly
lower corrosion-current densities compared to the pure
epoxy-coated AISI 316L. We established that the addi-
tion of nanoparticles to the epoxy coating significantly
M. CONRADI, A. KOCIJAN: COMPARISON OF THE SURFACE AND ANTICORROSION PROPERTIES ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1043–1046 1045
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Potentiodynamic curves for FAS-TiO2/epoxy-coated,
FAS-SiO2/epoxy-coated and epoxy-coated AISI 316L substrate in a
simulated physiological Hank’s solution
enhanced the corrosion resistance of the AISI 316L
stainless steel compared to the pure epoxy coating,
especially in the case of the FAS-TiO2/epoxy coating.
The corrosion parameters calculated from the potentio-
dynamic measurements showed decreased corrosion-
current densities and increased the polarisation resis-
tances of the specimens coated with FAS-TiO2/epoxy
and FAS-SiO2/epoxy coating compared to the pure
epoxy coating (Table 2). The superior protective pro-
perties of the FAS-TiO2/epoxy coating were attributed to
the more uniform distribution of the FAS-TiO2 nano-
particles within the coating.
Table 2: Corrosion parameters calculated from the potentiodynamic
measurements
Material E(I=0)(mV)
Icorr
(μA)
Rp
(k)
vcorr
(nm/year)
epoxy coated AISI
316L –256 0.33 78 1.1
SiO2/epoxy coated
AISI 316L –407 0.06 557 0.7
TiO2/epoxy coated
AISI 316L –335 0.02 1500 0.2
4 CONCLUSIONS
We analysed the morphology of FAS-TiO2/epoxy and
FAS-SiO2/epoxy coatings showing that the FAS func-
tionalization works well with the homogenization of the
TiO2 nanoparticle distribution, but has no effect on the
homogenization of the SiO2 nanoparticle distribution, as
these tend to agglomerate. This is directly reflected in
the SEM image analysis and the average surface-rough-
ness measurements. The wetting properties evaluation
reveals the superhydrophobic nature of the FAS-TiO2 and
FAS-SiO2 nanoparticle coatings prior to the epoxy
layer’s deposition for the nanoparticle fixation. This
suggests that hydrophilic epoxy eliminates the super-
hydrophobic effect, retaining, however, a high degree of
hydrophobicity that is a consequence of the combination
of surface roughness and the originally superhydro-
phobic nature of the FAS-TiO2 and FAS-SiO2 nano-
particles. There is also a noticeable difference in the
static water contact angles between the FAS-TiO2/epoxy
and the FAS-SiO2/epoxy coatings, the FAS-SiO2/epoxy
coatings being more hydrophobic due to the more
pronounced micro- to nanoparticle-textured surface with
a refined roughness structure. The corrosion evaluation
showed the significantly enhanced corrosion resistance
of the AISI 316L stainless steel with the addition of
nanoparticles to the epoxy coating compared to the pure
epoxy coating, especially in the case of the FAS-TiO2/
epoxy coating. The uniform distribution of the FAS-TiO2
nanoparticles within the coating plays a crucial role in
the superior protective properties of the coating.
Acknowledgements
This work was carried out within the framework of
the Slovenian research project J2-7196: Antibakterijske
nanostrukturirane za{~itne plasti za biolo{ke aplikacije,
financed by the Slovenian Research Agency (ARRS).
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1046 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1043–1046
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
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Letnik / Volume 51 2017
ISSN 1580-2949
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