effect of nanoparticle addition in hybrid sol-gel silane coating on corrosion resistance of low...
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Effect of nanoparticle addition in hybrid sol-gel silane coating on corrosion resistance of low carbon steel
POOVARASI BALAN 1, a, AARON NG2,b , CHEE BENG SIANG3,c, R.K. SINGH RAMAN4,d and CHAN ENG SENG5, e
1, 2, 3, 5 Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia
4 Departments of Mechanical & Aerospace Engineering and Dept of Chemical Engineering, Monash University (Melbourne), VIC-3800, Australia
a [email protected], [email protected], [email protected], draman.singh.monash.edu, [email protected]
Keywords: silanes; silica nanoparticles; alumina nanoparticles; low carbon steel, EIS, SEM
Abstract
Chromium pre-treatments of metal have been replaced by silane pre-treatments as more
environmental friendly option. Nanoparticles can be added in the silane sol-gel network have been
reported to improve corrosion resistance. In this work, the electrochemical corrosion resistance of
low carbon steel coated with hybrid organic-inorganic sol-gel film filled with nanoparticles was
evaluated. The sol-gel films have been synthesized from 3-glycidoxy-propyl-trimethoxy-silane (3-
GPTMS) and tetra-ethyl-ortho-silicate (TEOS) precursors. These films have been impregnated with
300 ppm of silica or alumina nanoparticles. The electrochemical behavior of the coated steel was
evaluated by means of electrochemical impedance spectroscopy (EIS) and scanning electron
microscopy (SEM). Equivalent circuit modeling, used for quantifying the EIS measurements
showed that sol-gel films containing silica nanoparticles improved the barrier properties of the
silane coating. The silica nanoparticle-containing films showed highest initial pore resistance over
the five days of immersion in 0.05M NaCl.
Introduction
The organo-functional silanes are excellent environmental friendly replacements for chromate
pretreatments which are carcinogenic in nature [1]. Silane coatings primarily act as a barrier by
impeding the rate of water and electrolyte transport to the substrate. However, the silane network
decomposes after a short while as a result of release of hydroxyl ions during corrosion process.
Addition of nanoparticles such as silica, zirconia and alumina are known to improve the mechanical
properties of silane network, thus improving its corrosion protection [2-5] . In the present work,
organic-inorganic hybrid silane films were impregnated with silica and alumina nanoparticles in
order to study their influence on corrosion resistance of low carbon steel. The hybrid sol-gel silane
films have been synthesized using 3-glycidoxy-propyl-trimethoxy-silane (3-GPTMS) and tetra-
ethyl-ortho-silicate (TEOS) precursors. The silanes films impregnated with silica and alumina
nanoparticles were also inesvstigated for the first time.
Materials and Methods
GPTMS, TEOS, silica nanoparticles (10-25nm), and alumina particles (<50 nm) were purchased
from Sigma Aldrich and were used as received. 0.05M nitric acid (HNO3) was prepared by diluting
down 75% concentrated nitric acid purchased from Sigma Aldrich. GPTMS: TEOS: Water was
mixed at molar ratios of 2:1:10, with 0.05M of nitric acid and used as a catalyst [6]. The mixture
was left to hydrolyse for 72 hours. For each batch of silane coating, a total of 15 ml of the
hydrolysis mixture was prepared. 300 ppm of silica nanoparticles in the silanes solution was
Advanced Materials Research Vol. 686 (2013) pp 244-249Online available since 2013/Apr/24 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.686.244
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prepared by ultrasonic dispersion of nanoparticles in deionised water for 20 minutes at 20%
amplitude setting, followed by mixing with silanes solution. The same procedure was employed
also for incorporation of alumina nanoparticles in silanes solution. Blank silane coating on metal
was also prepared, in which no nanoparticles were added to silane solution. The metal substrate
employed was mild or low-carbon steel supplied by Q-Lab Corporation. The metal strips were
ultrasonically cleaned using soap water, deionised water, acetone and then treated with sodium
hydroxide solution (pH 10.8) at room temperature. After hydrolysis of silanes for 72 hours, coating
of the cleaned metal strips with silanes was performed through dip-coating technique, in which the
strips were dipped into silane solution and immersed for 60 seconds. This procedure was repeated
twice, i.e., each metal strip was immersed in solution for total of 3 times. After the dip-coating step,
the coated strips were cured in oven at 90˚C for 5 hours.
The surface morphology of silane films was observed using Scanning Electron Microscopy (SEM)
utilising Hitachi S-3400 system. Electrochemical Impedance Spectroscopy (EIS) test was conducted
to investigate the degradation of the coating when exposed to 0.05M NaCl solution over period of 7
days. A three-electrode arrangement was utilised, consisting of saturated calomel reference
electrode, platinum foil as counter electrode and exposed sample as working electrode. EIS
measurements were performed using a Gamry FAS1 Femtostat with a PC4 controller board in the
frequency range, 10-2
to 105
Hz. Modeling of EIS measurements using equivalent circuit model was
carried out using Zsimpwin software to obtain quantitative evaluation of electrochemical parameter
of the coatings, namely the pore resistance, Rpore, of the coating.
Results and Discussion
i) Coating Morphology
Fig. 1 below shows morphologies of hybrid sol-gel silane films deveoped on a low carbon steel
(GT), as well as the films incorporated with 300 ppm of alumina (GTA) and silica nanoparticles
(GTS) respectively.
Fig. 1: SEM images of low carbon steel coated with silane films; (a) without incorporation of
nanoparticles, (b) incorporated with Al2O3 nanoparticles, and (c) incorporated with SiO2
nanoparticles.
The images in Fig. 1 show the coatings to have developed several cracks. This observation can be
attributed to the evaporation of water and solvents during curing of the silanes which can cause a
substantial volumetric contraction of the film and large internal stress build up within the silane
network, leading to the formation of cracks as defects within the film. Such cracks were also
observed by Suegama et al [1, 7] in their study on bis-1, 2-(triethoxysilyl) ethane (BTSE) silane
coatings on aluminium alloy, AA2024-T3.
Fig. 1(b) and 1(c) showed the presence of white agglomerates within the silane coating for GTA
and GTS samples. Such agglomerates were absent in samples without nanoparticles (GT).
Palomino et al have also reported such when aluminium alloy AA2024-T3 was coated with a
GTS GTA
GT
a) b) c)
Advanced Materials Research Vol. 686 245
cerium-silane incorporated with SiO2 nanoparticles [1]. They analysed the region of the white
agglomerates through Energy Dispersive X-ray Spectroscopy (EDX), and detected them to be Si-
rich, suggesting the agglomerates to be the clusters of SiO2 nanoparticles. Montemor et al,
characterized bis-1,2-[triethoxysilylpropyl]tetrasulfide silane films impregnated with CeO2 (ceria)
and ZrO2 (zirconia) nanoparticles deposited on galvanised steel substrates, and found formation of
similar agglomerates [8]. Based on the findings by Palomino et al and Montemor et al [1,8], the
white agglomerates seen in SEM images of GTA and GTS samples are attributed to Al2O3 and SiO2
nanoparticles.
ii) Electrochemical Impedance Spectroscopy (EIS) measurements
EIS is essential in assessing the role of nanoparticles on the barrier properties of the hybrid silane
films on low carbon steel. Fig. 2 shows impedance spectra for GT, GTS and GTA after 2 hours of
immersion period. The low frequency resistance was found to be around 1E6 Ωcm2 for GTA and
1E5 Ωcm2 for both GTS and GT. In order to observe the evolution of barrier properties with time,
similar data were collected over the next five days.
Fig. 2: EIS bode plots obtained for silane coatings with alumina nanoparticles (GTA), with silica
nanoparticles (GTS) and without any nanoparticles (GT). Spectra were obtained during immersion
in 0.05M NaCl, after 2 hours.
The impedance results obtained for 5 days were fitted using proposed equivalent electrical circuit
(EEC) model shown in Fig. 3. This EEC makes use of constant phase elements, which correspond
to a capacitor when the CPE exponent (n) is one. According to Van Ooij et al [9], the pore
resistance, Rpore is inversely proportional to the defect or pores for a given coating. There will be
some development of conducting short circuit paths in the coating, facilitating transport of
electrolyte ions [10], resulting in a decrease of the Rpore. Basically, Rpore is a measure of the porosity
and degradation of the coatings where lower Rpore indicate higher porosity and presence of defects
in the film.
The Rpore for all the coated samples generally decreased over 5 days (Fig. 4), indicating an
increasing degree of degradation of the coatings escalates with time.
246 Green Technologies for Sustainable & Innovation in Materials
Fig. 3: Schematic of equivalent circuit model used for fitting of all samples. Rsol is solution
resistance; whereas Qcoat and Rpore were used for fitting of the high frequency range data,
corresponsing to coating capacitance and resistance, respectively; Qdl and Rpol represents low
frequency region.
Fig. 4: Change in: (a) Rpore (high frequency resistance) and (b) Qcoat, of low carbon steel coated with
silane coatings, without nanoparticles (GT) and those impregnated with silica (GTS) and alumina
(GTA) nanoparticles,
Rpore Rpol Warburg
Rsol
Qdl
Qcoat
a)
b)
Advanced Materials Research Vol. 686 247
The initial pore resistance of GTA sample was the highest, followed by GT and GTS, respectively.
This coating suffered cracks (Fig. 1) due to the overloading of the larger sized alumina (<50 nm)
which explains the rapid decrease in Rpore for GTA from Day 0 to Day 1 (Fig. 4(a)). Since the silica
nanoparticles were smaller (10-25 nm), the stresses in GTS were possibly less significant.
Therefore, the Rpore values of GTS did not drop as rapidly as compared to the Rpore values of the
GTA.
Fig. 4(b) shows the dependence of the coating capacitance in the high frequency region, which is a
measure of water uptake within the silane film. As can be observed, Qcoat values increases with time
for all samples, indicating increased water uptake within the silane film as time progressed. This
observation is consistent with the trend for pore resistance, Rpore (Figure 4(a)).
Generally, the silica and alumina nanoparticles incorporated in the silane coating on the low carbon
steel fill up voids, cracks, micro pores and defects in the coating which leads to an improvement in
the barrier properties. Therefore, the addition of nanoparticles is proven to improve the mechanical
properties by reducing the porosity of the silane coating. This is supported by findings from M. F.
Montemor et al in 2009 [11]. Hence, the number of areas which provides pathways for the
penetration or diffusion of the aggressive electrolyte to the coating-substrate interface will be
reduced with the incorporation of nanoparticles. However, overloading of nanoparticles beyond
optimum concentration can lead to degradation of barrier properties of the silane films [9]. This
provides possible explanation to the degradation of barrier properties of alumina incorporated silane
coatings over 5-day immersion period as seen in Fig. 4. The Rpore values of alumina incorporated
silane films (GTA) are in fact lower than films without any nanoparticles (GT). Excess of alumina
nanoparticles can also affect the interfacial adhesion adversely, leading to premature delamination
of the film from the substrate [9].
On the other hand, the silane films loaded with 300 ppm silica nanoparticles (GTS) of much smaller
size have the highest Rpore values at the end of 5th
day, indicating superior barrier properties. This
behaviour is possibly attributed to the optimum size and concentration of silica nanoparticles (300
ppm) used in hybrid sol-gel silane coatings.
Conclusion
Coatings of hybrid sol gel silane coating synthesized from 3-glycidoxy-propyl-trimethoxy-silane (3-
GPTMS) and tetra-ethyl-ortho-silicate (TEOS) precursors with 300 ppm silica nanoparticles
improved corrosion protection of low carbon steel substrate. However, the impregnation of 300
ppm alumina nanoparticles showed degradation of barrier properties due to the larger size and
overloading of nanoparticles.
Acknowledgement
The authors would like to acknowledge the financial support for this work by Monash University,
Sunway campus under Major Project Fund EM-11-08 and Seeding Fund E-CSPERS- 006 E-3-11.
248 Green Technologies for Sustainable & Innovation in Materials
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Advanced Materials Research Vol. 686 249
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