M. CONRADI, A. KOCIJAN: SURFACE AND ANTICORROSION PROPERTIES OF HYDROPHOBIC ...
967–970
SURFACE AND ANTICORROSION PROPERTIES OF
HYDROPHOBIC AND HYDROPHILIC TiO2 COATINGS ON A
STAINLESS-STEEL SUBSTRATE
POVR[INSKE IN PROTIKOROZIJSKE LASTNOSTI HIDROFOBNIH
IN HIDROFILNIH TiO2 PREVLEK NA JEKLENI PODLAGI
Marjetka Conradi, Aleksandra Kocijan
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
marjetka.conradi@imt.si
Prejem rokopisa – received: 2016-04-21; sprejem za objavo – accepted for publication: 2016-05-03
doi:10.17222/mit.2016.068
We compare the wetting, morphology and anticorrosion properties of fluorosilane-modified TiO2 (FAS-TiO2/epoxy) and
as-received TiO2/epoxy coatings. An array of double-layer TiO2 nanoparticles of two sizes (30 nm and 300 nm) were spin coated
onto a steel substrate AISI 316L. The static water contact angles were measured to evaluate the wetting properties of the
FAS-TiO2/epoxy (hydrophobic) and the as-received TiO2/epoxy (hydrophilic) coatings. The morphology of the coatings was
analyzed with average surface roughness (Sa) measurements and SEM imaging. We show that the order of the deposition in a
double layer composed of dual-size nanoparticles plays an important role in the surface roughness and hence the wettability.
SEM images reveal a typical morphology and Sa difference between the FAS-TiO2/epoxy and the as-received TiO2/epoxy
coatings, reflected in the discrepancy of the average size of the agglomerates that are coating the substrate. Potentiodynamic
measurements show an enhanced corrosion resistance for the FAS-TiO2/epoxy-coated AISI 316L stainless steel compared to the
as-received TiO2/epoxy-coated AISI 316L.
Keywords: TiO2, epoxy, coatings, wetting, corrosion
V ~lanku primerjamo omo~itvene lastnosti, morfologijo in antikorozijske lastnosti s fluorosilanom oble~enih TiO2
(FAS-TiO2/epoksi) in ~istih TiO2/epoksi prevlek. TiO2 nanodelce dveh velikosti (30 nm in 300 nm) smo na jekleno podlago tipa
AISI 316L nanesli s "spin coaterjem". Omo~itvene lastnosti prevlek smo dolo~ili z meritvami stati~nih kontaktnih kotov. Le-te
so pokazale hidrofobno naravo FAS-TiO2/epoksi prevlek in hidorfilno naravo ~istih TiO2/epoksi prevlek. Morfolo{ke lastnosti
prevlek smo analizirali z meritvami povpre~ne hrapavosti povr{ine (Sa) ter SEM-mikroskopijo. Pokazali smo pomen vrstnega
reda nalaganja nanodelcev dveh velikosti na hrapavost povr{ine in njeno omo~ljivost. SEM-posnetki prikazujejo razliko v
morfologiji in hrapavosti povr{in FAS-TiO2/epoksi in ~istih TiO2/epoksi prevlek, ki se odra`a v tvorbi aglomeratov razli~nih
velikosti na eni in drugi povr{ini. Potenciodinamske meritve ka`ejo izbolj{ano odpornost proti koroziji FAS-TiO2/epoksi prevlek
v primerjavi s ~istimi TiO2/epoksi prevlekami na jekleni podlagi tipa AISI 316L.
Klju~ne besede: TiO2, epoksi, prevleke, omo~itvene lastnosti, korozija
1 INTRODUCTION
Austenitic (AISI) stainless steel is an important engi-
neering material because of its generally high corrosion
resistance combined with favourable mechanical proper-
ties, such as its high tensile strength.1,2 Its high corrosion
resistance is attributed to the presence of a passive film,
which is stable, invisible, thin, durable and extremely
adherent and self-repairing.3 However, in many aggres-
sive environments, such as a chloride-ion-rich environ-
ment, AISI 316L stainless steel is still observed to suffer
from pitting corrosion.4 Therefore, the modification of
metallic surfaces using various coatings is an important
subject in the field of enhancing particular surface pro-
perties, mechanical as well as anticorrosion properties.
Epoxy coatings have been widely used for metallic-
surface protection because of their good mechanical and
electrical-insulating properties, chemical resistance and
strong adhesion to heterogeneous substrates. However,
the highly cross-linked structure of an epoxy resin often
makes epoxy coatings susceptible to the propagation of
cracks and damage due to surface abrasion and wear.5
Therefore, a lot of research has been done to improve the
performance of epoxy coatings by adding various nano-
particles, like SiO2, TiO2, ZnO, CuO etc.6 In addition,
nanoparticles also enhance the corrosion-protection pro-
perties of the epoxy coatings by decreasing the porosities
due to the small size and high specific area. TiO2 nano-
particles are well-known anticorrosion additives used in
several applications, such as aerospace, marine, bio-
medicine, etc. because of their unique physiochemical
properties and good chemical stability.7–9
Here we report on a comparison of the surface and
anticorrosion properties of double-layer, dual-size
(30 nm and 300 nm) FAS-TiO2/epoxy and as-received
TiO2/epoxy coatings. We show that the order of the
nanoparticle deposition plays an important role in the
wetting and the morphological properties of the coatings.
Potentiodynamic measurements reveal that the hydro-
phobic coating has better anticorrosion properties than
the hydrophilic coating.
Materiali in tehnologije / Materials and technology 50 (2016) 6, 967–970 967
UDK 67.017:620.193:669.148:669.295 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)967(2016)
2 EXPERIMENTAL PART
Materials – Epoxy resin (Epikote 816, Momentive
Specialty Chemicals B.V.) was mixed with a hardener
Epikure F205 (Momentive Specialty Chemicals B.V.) in
the ratio of mass fractions of 100:53 %. TiO2 nano-
particles with mean diameters of 30 nm were provided
by Cinkarna Celje, whereas the 300-nm particles were
provided by US Research Nanomaterials, Inc.
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 % S in mass fraction) was used as a substrate.
Surface functionalization – For the hydrophobic
effect, 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 TiO2 nano-
particles’ adhesion, the diamond-polished AISI 316L
substrate was spin-coated with a 300-nm layer of epoxy
(as determined by ellipsometry)10 and then cured for 1 h
at 70 °C and post-cured at 150 °C for another hour. The
nanoparticles were then coated onto the AISI 316L +
epoxy (AISI + E) surface by spin-coating 20 μL of 3 %
of mass fractions of TiO2 nanoparticle ethanolic solution.
We prepared dual-size, double-layer coatings consisting
of 30 nm and 300 nm FAS-TiO2 nanoparticles. Both
possibilities of the order of TiO2 nanoparticles were
analyzed for the FAS-TiO2/epoxy coatings’ preparation:
AISI+E+30+300 and AISI+E+300+30. Finally, the
coatings were dried in an oven for approximately 20 min
at 100 °C.
The same procedure was repeated with the as-
received, non-functionalized, TiO2 nanoparticles to pre-
pare the TiO2/epoxy coatings.
Scanning electron microscopy (SEM) – SEM analysis
using a FE-SEM Zeiss SUPRA 35VP was employed to
investigate the morphology of the TiO2 coatings’ surfa-
ces, which were sputtered with gold prior to imaging.
Contact-angle measurements – The static contact-
angle measurements of water (W) on the TiO2/epoxy-
coated AISI 316L substrates and on the FAS-TiO2/
epoxy-coated AISI 316L substrates were performed
using a surface-energy evaluation system (Advex Instru-
ments 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 an 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 measurements were carried out at
20 °C and ambient humidity.
Surface roughness – Optical 3D metrology system,
model Alicona Infinite Focus (Alicona Imaging GmbH)
was employed 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 used for the roughness
analysis. The software offers the possibility 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 yx
= ∫∫
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 analyzed area was (714×542) μm. To
level the profile, corrections were made to exclude the
general geometrical shape and possible measurement-
induced misfits.
Electrochemical measurements – Electrochemical
measurements were performed on the TiO2/epoxy-coated
AISI 316L stainless steel and on the FAS-TiO2/epoxy-
coated AISI 316L stainless steel. The experiments were
carried out 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/Galva-
nostat/FRA Model SP-300 with EC-Lab Software and a
three-electrode flat corrosion cell, where the working
electrode (WE) was the investigated specimen, the refe-
rence electrode (RE) was a saturated calomel electrode
(SCE, 0.242 V vs. SHE) and the counter electrode (CE)
was a platinum net. The specimens were immersed in the
solution 1 h prior to the measurement in order to stabi-
lize the surface at the open-circuit potential (OCP). The
potentiodynamic curves were recorded, starting the
measurement at 250 mV vs. SCE more negative than the
open-circuit potential (OCP). The potential was then
increased, using a scan rate of 1 mV s–1, until the trans-
passive region was reached.
3 RESULTS AND DISCUSSION
3.1 Wetting properties
To analyze the surface wettability, we performed five
static contact-angle measurements with water (W) on
different spots all over the sample and used them to de-
termine the average contact-angle values of the coating
with an estimated error in the reading of ±1.0°.
To fabricate a surface that is as hydrophobic as possi-
ble we followed the trend of increasing hydrophobicity
based on dual-scale roughness.11,12 For this purpose, the
surface roughness was adjusted via spin-coating 30-nm
M. CONRADI, A. KOCIJAN: SURFACE AND ANTICORROSION PROPERTIES OF HYDROPHOBIC ...
968 Materiali in tehnologije / Materials and technology 50 (2016) 6, 967–970
and 300-nm FAS-TiO2 nanoparticles onto the flat
AISI+E surface. The substrate was consequently modi-
fied by self-assembled FAS-TiO2 nanoparticles resulting
in micro- to nanoparticle-textured surfaces with a refined
roughness structure. The static water contact angles, W,
and average surface roughness, Sa, for both possibilities
of the dual-size, double-layer, FAS-TiO2/epoxy coatings,
(30 + 300) nm and reversed, (300 + 30) nm, are listed in
Table 1. The measured contact angles indicate that the
surface is more hydrophobic when the bottom layer is
composed of 30-nm and the top layer of 300-nm FAS-
TiO2. The difference in W between the two coatings is
approximately 11° and this behavior can be attributed to
the increased roughness implemented by the larger nano-
particles on the top, which is reflected in the average
surface-roughness measurements, Sa (Table 1).
Table 1: Comparison of static water contact angles (W) and average
surface roughness (Sa) of dual-size, double-layer FAS-TiO2/epoxy and
as-received TiO2/epoxy coatings
Tabela 1: Primerjava stati~nih kontaktnih kotov (W) in povpre~ne
povr{inske hrapavosti (Sa) dvoplastnih FAS-TiO2/epoksi in ~istih
TiO2/epoksi prevlek
Contact
angle
Rough-
ness
Contact
angle
Rough-
ness
FAS-TiO2 TiO2
Substrate W/° Sa/nm W/° Sa/nm
AISI+E+30+300 126.0 250.9 80.3 89.8
AISI+E+300+30 115.2 160.2 79.2 95.1
We prepared, in the same manner, a double-layer of
(30 + 300) nm and (300 + 30) nm with as-received TiO2,
nanoparticles. These coatings are hydrophilic due to the
hydroxyl groups on the surface of the as-received TiO2
nanoparticles. The static water contact angles of both
possibilities were comparable, around 80°. In addition,
the average surface roughness, Sa, was much lower com-
pared to the FAS-TiO2/epoxy coatings (Table 1). This
result indicates that FAS functionalization significantly
changes not only the wetting properties of the coating
but also its morphology, as will be shown in the follow-
ing section.
3.2 Surface morphology
Figure 1 compares the morphology of the double-
layer FAS-TiO2/epoxy (a, b) and the as-received
TiO2/epoxy (c, d) coatings. SEM images reveal an obvi-
ous difference in the morphology between layers of
FAS-TiO2 and as-received TiO2 nanoparticles, which is
reflected mostly in the different length scale of the
average size of the nanoparticle agglomerates and conse-
quently in a discrepancy of the average surface rough-
ness, Sa, as reported in Table 1. FAS functionalization
apparently does not homogenize the particle distribution
as the formation of large agglomerates up to a few tenths
of microns is observed (Figure 1a and 1b). In contrast,
for the as-received TiO2 nanoparticle coatings, the nano-
particles are more finely dispersed and agglomerates of
the order of few microns are observed (Figure 1c and
1d).
SEM images also reveal that TiO2 nanoparticles were
not able to cover completely the underlying substrate
This might additionally influence the contact-angle va-
lues and the wetting properties of the FAS-TiO2 and the
as-received TiO2 layers as the epoxy substrate is hydro-
philic with a static water contact angle of 74.3°. This
effect is, however, probably more pronounced in coatings
prepared with hydrophobic FAS-TiO2 nanoparticles, as
the uncovered fractions allow the water to impregnate
between the nanoparticles and the agglomerates to come
into contact with the exposed hydrophilic epoxy and,
consequently, reduce the static water contact angle. On
the other hand, this effect does not play an important role
in the as-received TiO2/epoxy coatings as both the as-
received TiO2 nanoparticles and the epoxy are hydro-
philic.
The role of the order of nanoparticle deposition
seems to be more pronounced in the FAS-TiO2/epoxy
coatings (Figure 1a and 1b), which is also reflected in
the discrepancy in static water contact angles and the
average surface roughness between AISI+E+30+300 and
AISI+E+300+30, as reported in Table 1. Larger particles
on the top seem to create larger agglomerates and con-
sequently a rougher surface.
The morphology of the as-received TiO2/epoxy coat-
ings, AISI+E+30+300 and AISI+E+300+30 (Figure 1c
and 1d) is, however, comparable, as are the static water
contact angles and the average surface roughness (Table
1).
3.3 Potentiodynamic measurements
For an analysis of the anticorrosion properties we
chose the more hydrophobic coating, FAS-TiO2/epoxy
coating, AISI+E+30+300. The comparison was made
M. CONRADI, A. KOCIJAN: SURFACE AND ANTICORROSION PROPERTIES OF HYDROPHOBIC ...
Materiali in tehnologije / Materials and technology 50 (2016) 6, 967–970 969
Figure 1: Comparison of surface morphology of double-layer,
FAS-TiO2/epoxy (a, b) and as-received, TiO2/epoxy (c, d) coatings
Slika 1: Primerjava morfologije dvoplastnih FAS-TiO2/epoksi (a, b) in
~istih TiO2/epoksi (c, d) prevlek
with the as-received TiO2/epoxy coating using the same
order of particle deposition (30+300). Figure 2 shows
the potentiodynamic behaviour of the as-received TiO2/
epoxy-coated AISI 316L and FAS-TiO2/epoxy-coated
AISI 316L stainless steel in a simulated physiological
Hank’s solution. We studied the polarization and the
passivation behaviour of the tested material after the
surface modification. After 1 h of stabilization at the
OCP, the corrosion potential (Ecorr) for the as-received
TiO2/epoxy-coated AISI 316L in Hank’s solution was
approximately –0.13 V vs. SCE. Following the Tafel
region, the alloy exhibited a broad range of passivity.
The breakdown potential (Eb) was approximately 0.25 V
vs. SCE. In the case of the FAS-TiO2/epoxy-coated AISI
316L stainless steel, the Ecorr in Hank’s solution was
approximately –0.27 V vs. SCE. The passivation range
was significantly broader, i.e., 0.4 V vs. SCE, and at
lower corrosion-current densities compared to TiO2/
epoxy-coated AISI 316L specimen. The results show an
enhanced corrosion resistance for the FAS-TiO2/epoxy-
coated AISI 316L stainless steel compared to as-received
TiO2/epoxy coated AISI 316L.
4 CONCLUSIONS
We analyzed the wettability behavior of double-sized,
double-layer, FAS-functionalized TiO2 and as-received
TiO2 nanostructured surfaces. We showed that the order
of the TiO2 nanoparticle deposition determines the sur-
face roughness and hence the wettability, as confirmed
by the average surface-roughness measurements and the
SEM imaging. This effect was more pronounced in coat-
ings with FAS-TiO2 nanoparticles. The morphology
analysis also revealed a typical morphology and Sa diffe-
rence between the FAS-TiO2/epoxy and the as-received
TiO2/epoxy coatings reflected in a discrepancy in the
average size of the agglomerates that are coating the sub-
strate. The corrosion stability of double-sized, double-
layer, FAS-functionalized TiO2 and the as-received TiO2
nanostructured coatings on the surface of the AISI 316L
stainless steel was studied in a simulated physiological
Hank’s solution. The results showed the superior corro-
sion stability of the FAS-TiO2/epoxy-coated AISI 316L
stainless steel compared to the as-received TiO2/epoxy-
coated AISI 316L.
Acknowledgement
This work was carried out within the research project
J2-7196: "Antibakterijske nanostrukturirane za{~itne pla-
sti za biolo{ke aplikacije" of the Slovenian Research
Agency (ARRS).
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M. CONRADI, A. KOCIJAN: SURFACE AND ANTICORROSION PROPERTIES OF HYDROPHOBIC ...
970 Materiali in tehnologije / Materials and technology 50 (2016) 6, 967–970
Figure 2: Potentiodynamic curves for as-received TiO2/epoxy- and
FAS-TiO2/epoxy-coated AISI 316L substrate in a simulated
physiological Hank’s solution
Slika 2: Potenciodinamske krivulje ~istih TiO2/epoksi in FAS-TiO2/
epoksi prevlek na AISI 316L podlagi, izmerjene v simulitani fiziolo{ki
Hankovi raztopini