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CHE4180: Final Report Narender Jambunathan

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MONASH UNIVERSITY

CHE4180: CHEMICAL ENGINEERING RESEARCH PROJECT

Semester 1, 2011

IMPROVED CORROSION PROTECTION OF METALS THROUGH

NANOPARTCILE INCORPORATION IN SILANE FILMS

By: Narender Jambunathan

Student ID: 20729057

Supervisor: Ms. Poovarasi Balan


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Acknowledgements

I would like to express my sincere gratitude and appreciation to my supervisor Ms. Poovarasi Balan

for her invaluable guidance and support throughout the semester. She has immense interest in her

students and has always been easy to approach, encouraging and optimistic. My sincere thanks also

go to Ms. Lithnes Kalaivani, Ms. Baljit Kaur and Mr. Mohamed Nasrun for their patience and

assistance in the laboratory, no matter how many times I asked them to show me how to operate a

particular equipment, and the great lengths they went to, to get me spare magnetic stirrers which

were in high demand during weeks 7 and 8. And last but not least, I wish to thank my fellow lab

mates: Yong Tai; Mar Vin Tim; Kok Bing Tan; and Jack Leong for the wonderful memories. It has been

a pleasure working with you all.

Abstract

Chromates, which are bundled with environmental, safety and cost issues, are widely used in metal

finishing industries. Environmental legislations are seeking to curtail its use and thus, there is an

urgent need for environmentally benign alternatives. Amongst the proposed alternatives, those

based on silanes are emerging as outstanding replacements. However, in comparison to chromate-

based treatments, silane films are comparatively thin and only passively protect the metal surface.

Literature has reported that the former issue can be resolved with the addition of nanoparticles.

Recently, it has been demonstrated that inhibitors such as rare-earth cations can be added to the

film. These inhibitors can slowly leach out of the film, act upon actively corroding regions on the

metal surface and ultimately confer the self-healing properties that make chromate-based coatings

so potent. The present work seeks to determine whether the addition of both nanoparticles and

inhibitors can synergistically improve the corrosion protection of silane films. Electrochemical

spectroscopy is used to study the effects of these additives on corrosion inhibition. Complementary

studies including salt immersion test and contact angle measurements were also performed. The

results indicate that the effects of silica nanoparticles and lanthanum are cumulative. Extensive

inferences could have been made from the results had the polishing technique been perfected.


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Table of Contents

Introduction 1

Scope 4

Objectives 5

Research Questions 5

Literature Review 6

Problem Statement 17

Experimental Design 18

a) Surface Preparation

b) Analysis and Characterization

Safety 23

a) Silanes

b) Nanoparticles

c) Polishing Equipment

Gantt Chart 25

Results 25

a) Microscopy

b) Salt Immersion Tests

c) Thickness Measurements

d) Video Goniometer

e) FTIR Spectrum

f) EIS

g) Electrodeposition

Discussion 32

a) Microscopy

b) Salt Immersion Tests



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d) Video Goniometer

e) FTIR Spectrum

f) EIS


Errors, Improvements and Limitations 39

Conclusion 40

Suggested Future Work 41

References 43


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List of Figures

Figure 1. Schematic of bonding mechanism between metal surface hydroxide layer immediately: the

diagram on the left shows the bonding mechanism immediately after adsorption on to the surface,

and the diagram on the right shows the covalent-bond interface that forms after condensation

Figure 2. Dipodal Silane

Figure 3. Wrought aluminium alloy panels following the 336-hour SST: The panel on the left is of

untreated wrought aluminium alloy; the centre panel has been chromate; and the panel on the right

has been treated with alumina-loaded bis-amino/VTAS

Figure 4. DC polarization curves of AA2024-T3 in the different scenarios tested

Figure 5. Ecorr and Icorr of bis-sulfur treated AA2024-T3 systems as a function of silica concentration in

the bis-sulfur silane solution

Figure 6. EIS Bode plots for galvanized steel pre-treated with silane films that had been modified

with SiO2 nanoparticles


with cerium-activated SiO2 nanoparticles


with CeO2 nanoparticles


with cerium-activated CeO2 nanoparticles

Figure 10. SVET maps of the scratched SiO2 films while being immersed in 0.005 M sodium chloride

solution for 6 hours. The scale represents the ionic current activities

Figure 11. SVET maps of the scratched SiO2 films after being immersed in 0.005 M sodium chloride

solution for 6 and 72 hours. The scale represents the ionic current activities

Figure 12. Phase angle curves for the electrodeposited and immersed BTSE films on Al-6111

Figure 13. Pore resistance of electrodeposited and immersed BTSE films on Al-6111 plotted against

electrolyte immersion time

Figure 14. Thickness plotted against time, comparing electrodeposited and immersed films on ferro-

plate panels. The silane used was BTSE


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Figure 15. Micrographs of: a) Unpolished Low-carbon steel surface b) Polished Low-carbon steel

surface; c) Low-carbon steel surface coated with BTSE silane

Figure 16. Micrographs of: a) BTSE + lanthanum solution; b) BTSE + silica nanoparticles and

lanthanum solution; c) BTSE + silica nanoparticles

Figure 17. Samples at day 0 of the salt immersion test

Figure 18. Samples after 24 hours of salt immersion

Figure 19. Samples after 48 hours of salt immersion

Figure 20. Comparison of all samples at day 0 against day 5

Figure 21. Photographs after day 5 of: a) control; b) low-carbon steel with coat of BTSE with Si and La

Figure 22. Thickness measurement results of all five samples

Figure 23. Contact angle measurements and photographs of the sessile drop test on all five surfaces

Figure 24. Merged FTIR spectrum of all five samples

Figure 25. EIS Bode plot of control against low-carbon steel with BTSE, Si nanoparticles and

lanthanum

Figure 26. EIS Bode plot of low-carbon steel with BTSE, Si nanoparticles and lanthanum after 24

hours

Figure 27. EIS Bode plot of low-carbon steel with BTSE, Si nanoparticles and lanthanum after 48

hours

Figure 28. Micrographs of a low-carbon steel surface following electrodeposition: a) performed at 5V

for 1800s; b) performed at 2V for 7200s

Figure 29. Peaks observed in the FTIR spectra and the molecules they correspond to


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Introduction

Underpinned by over half a century of research and development, silicon compounds, particularly

organofunctional silanes, are now rapidly emerging as an excellent, eco-friendly alternative to treat

metal surfaces and thereby impede corrosion [1]. Silanes are simply silicon analogues of alkane

hydrocarbons. Silanes that comprise at least one silicon-carbon bond are classed as organosilanes;

the carbon-silicon bond is highly stable, non-polar, and confers hydrophobicity to the molecule when

an alkyl group is present. Silicon molecules that contain two different functional groups are termed

organofunctional silanes. These functional groups can facilitate reaction and coupling between a

vast range of, normally incompatible, organic resins and inorganic surfaces [4].

Silicon possesses a normal oxidation state akin to that of its chemical analog, carbon. Silicon is

however, less electronegative, and therefore forms bonds with different strengths, angles and

lengths compared to organic compounds. These inorganic analogues often exhibit more desirable

properties such as: greater thermal endurance; resistance to moisture and chemical stressors; and

improved mechanical strength and electrical properties.

At present, chromate or chromate pigments are widely used to passivate metals; however, their use

is discouraged as a growing body of research has revealed that hexavalent chromium compounds

can be toxic, environmentally hazardous, and can traverse cell membranes, eliciting mutagenic DNA

lesions which can culminate in cancer. This has led to most environmental legislations heavily

restricting and monitoring use and disposal of chromium [2]. Furthermore, many conventional

coatings also incorporate volatile organic compounds (VOC) and hazardous air pollutants (HAPs),

both of which contribute to the formation of tropospheric smog, thus exacerbating respiratory

ailments and reducing immunity of those who breathe it [3].

Notwithstanding its toxicity and environmental drawbacks, chromate is a highly effective corrosion

inhibitor. Chromate sealers and chromate conversion coatings are able to create a self-healing

conversion coating on alloys of iron, aluminium, magnesium and zinc, protecting the metal surface if

scratched. In addition, they can act as inhibitive pigments in several general coating and primer

formulations. Chromic acid is traditionally used to rinse the exposed metal; during this process, it

reacts with the surface, dissolving it, along with the layer of metal immediately beneath the surface.

The corresponding reduction reaction leads to the precipitation of chromium(III) hydroxide on the

surface while unreacted hexavalent chromium ions are retained within the film. These treatments

are conducted prior to painting and finishing as they often improve adhesion and stability of


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overcoats [5]. The soluble hexavalent chromium ions interspersed in the passivation film will react

with actively corroding regions, repassivating exposed areas, and are responsible for the

abovementioned self-healing properties of chromate-based coats [6].

With escalating demand for environmental technology and pressure to solve the VOC, HAP and

chromate problems, trialkoxysilanes have become the primary focus of research in the adhesive and

metal-finishing industries. The present challenge centres on duplicating the potent corrosion

inhibiting effects of chromium, in the long-term, using environmentally benign silane coatings and

their associated hydrophobic properties.

Monosilanes, which have the structure X3Si(CH2)nY, where a hydrolyzable group (e.g. alkoxy, such as

methoxy OCH3 or ethoxy OC2H5) represents X and an organofunctional (non-hydrolyzable, e.g.

amine, epoxy or isocyanate) group represents Y, have received industrial acceptance as compounds

that promote adhesion during surface treatment of metals. A reactive silanol group, capable of

condensing with other silanol groups, is formed upon hydrolysis. The organofunctional group and n

are determinative of water solubility. Monosilanes have been traditionally applied by sol-gel

processes, with residual liquid being removed by drying; concomitantly, a condensation reaction

takes place, forcing any hydroxyl groups on the metal substrate to hydrolyse and form stable

siloxane [Si-O-Si] bonds [1].

After spontaneous adsorption of the silane solution onto the surface of the metal via hydrogen

bonds, the reactions that take place in the sol-gel process are as follows:

(1) SiOH (silanols in silane solution) + MeOH (metal surface) -> SiOMe (silane-metal interface) + H2O

Here, covalent metallo-siloxane bonds are formed by interaction between silanol groups in the

silane solution with metal hydroxyls. The metal hydroxyl groups would be generated during the

surface preparation step, which precedes silane application. This surface preparation step involves

alkaline treatment, whereby a solution of NaOH is used to clean the metal surface, during which OH

groups bind to the substrate [22].

(2) SiOH (silanols in silane solution) + SiOH -> SiOSi (silane film) + H2O

The above equation shows how surfeit silanol groups adsorbed onto the metal surface condense to

form a siloxane film. The outstanding bonding of silane film to metal substrate can be attributed

largely to these Si-O-Me and Si-O-Si covalent bonds [10]. It is important to bear in mind that Si-O-Me

bonds are not resistant to water; this is why it is imperative that silane films need to be hydrophobic.


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If water were to reach the interface, Me-OH and Si-OH groups will be formed through hydrolysis.

These hydrophilic groups will attract more water to the interface, expediting the rate of corrosion.

Fig. 1. Schematic of bonding mechanism between metal surface hydroxide layer immediately: the diagram on the left shows the bonding mechanism immediately after adsorption on to the surface, and the diagram on the right shows the covalent-bond interface that forms after condensation [10].

Dipodal silanes (sometimes referred to as bis-silanes) are a series of newfound adhesion promoters

that exhibit remarkable protection on metal surfaces when coupled with organofunctional mono-

silane treatments. Achieving intrinsic hydrolytic stabilities orders of magnitude greater than

conventional silanes, dipodal silanes have superior resistance to deterioration caused by the ingress

of water between the metal surface and the substrate, and have already gained industrial approval

for use in multi-layered printed circuit boards [7]. Dipodal silanes have the following structure:

X3Si(CH2)nY(CH2)nSiX3, or X3Si(CH2)mSiX3 in the absence of functional groups; as can be seen in figure 2

below. Their primary advantage stems from the fact that they are able to form six bonds to a

substrate while conventional monosilanes are only able to form three.

Dipodal silanes are able to condense to form a larger, three dimensionally cross-linked hydrophobic

polysiloxane layer while permitting coupling through either their intrinsic functional groups or

functional groups present in the monosilane solution. Hence, monosilane-dipodal silane mixtures

generally outperform films composed by single series of silanes [8].

The primary drawback of dipodal silanes is that they are often insoluble in

water and therefore are currently only of industrial use when coupled with

functional groups that stabilize water, e.g. amines, (majority of the

hydrophobic silane films that show outstanding corrosion resistance are only

soluble in organic solvents, thus are incompatible with the environmental


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low-VOC requirement). Nonetheless, some bis-silane mixtures have been optimized to provide

short-term corrosion protection to metals, mirroring the performance of chromate, with and

without a paint overcoat above the silane layer, i.e. they can be used as pre-paint treatment or

passivation treatment [9]. However, long-term use presents problems as silane films are extremely

thin (ranging from 200-300nm) and cannot heal defects like chromate. Over time, water contact

angle decreases from 90 degrees, and siloxane groups gradually hydrolyse back into hydrophilic

silanol groups. Thus, water can traverse the silane layer, reach the metal-silane interface, disrupt the

metallo-siloxane bonds present and ultimately subvert coating performance. Thicker films were

investigated but did not produce desirable outcomes; they were brittle and the high-concentration

silane solutions used to deposit thicker layers were unstable. Despite this, researchers are making

advancements. It has been demonstrated that silane films can be modified by inhibitors,

nanoparticles and colorants, and with the former two additives producing further improvements in

corrosion protection and mechanical properties [10]. If these shortcomings are overcome, silanes

have the potential to not only offer greater end-user benefits, but can be used in applications that

chromates normally cannot address. Prime examples include: application of silanes modified with

nanoparticles in anti-fingerprint coatings; and, coatings that provide protection against erosion

corrosion and microbiologically induced corrosion (MIC) [9].

Silanes primarily act as barrier coatings, i.e. they impede the rate of water and electrolyte intrusion

to the interface, where they would initiate corrosion reactions. Therefore, hydrophobicity can be

labelled as their most important property. Silane film performance is also dependent on crosslink

density, film thickness, and the wettability of the metal substrate by the silane solution. Although

silanes may not gain widespread use in the immediate future, substantial progress is being made,

and with the recent discovery of the synergistic benefits of electrodeposition and nanoparticle

incorporation, there is good reason for optimism. Furthermore, the addition of rare-earth cations as

inhibitors has shown some promising results, including ‘self-healing’ properties [32][40]. Therefore,

nanoparticle incorporation and the addition of inhibitors will be the primary focus of the literature

review that follows.

Scope

The corrosion behaviour of low-carbon steel pre-treated with bis-1,2-(triethoxysilyl)ethane (BTSE)

silane solutions modified silica nanoparticles and lanthanum nitrate have not been reported in

literature. Cerium, a rare earth metal that has similar chemical properties to lanthanum, has been

found to form insoluble hydroxides which work as cathodic inhibitors. A study that incorporated

both cerium and silicon nanoparticles in a silane solution of tetraethoxysilane (TEOS) reported an


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increase in low-frequency electrochemical impedance with time during electrochemical tests; low-

frequency impedance values are associated with corrosion resistance at the metal surface. This self-

healing effect was not observed in the TEOS films that only had silica nanoparticles, whereby

impedance decreased with time. Zheludkevich et al. reported similar findings, using GPTMS and

TEOS on AA2024-T3 substrates, where EIS results showed increase in pore resistance with time in

silane solutions that contained cerium-activated zirconia nanoparticles. It was concluded that the

addition of cerium stabilized the oxide intermediate layer, leading to far superior long-term

corrosion protection [37]. Perhaps lanthanum too can perform inhibitory actions in silane-based films

[29]. The present research seeks to fill this void in knowledge.

Objectives

The environmental, safety and cost issues associated with chromate have called for alternative

chemicals to be used in the metal finishing industries. Silanes have emerged as promising

alternatives; however, there is still a considerable amount of research left to be done until they can

universally replace chromate-based treatments. This study will seek to:

1) Dip-coat non-organofunctional bis-silanes, in a manner that can be reproduced, to create a

compact silane film on a low-carbon steel surface.

2) Demonstrate improved corrosion protection offered by a mixture of silica nanoparticles and

lanthanum ions; these will be added into a bis-silane solution, and will be applied as a film onto low-

carbon steel.

3) Attempt electrodeposition to produce films that more consistent and uniform that those that

have been applied by dip-coating

4) The long-term objective is to help uncover novel information about silanes and help expedite the

incorporation of these environmentally friendly compounds into industries that rely heavily on

chromate.

Research Questions

One of the largest chromite mines in the world operate in Sukhinda Valley, India. Most of the mining

here is performed opencast, i.e. pit dug to extract the ore, with the chromite extracts being left on

the open ground, which may subsequently contaminate groundwater or seep from the bottom of

the pit into nearby aquifier systems. Over 2 million people inhabit Sukhinda Valley and 60% of the

drinking water contains hexavalent chromium at levels double that of international standards. More

alarmingly, it has been estimated that more than 80% of the deaths in the mining areas are due


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chromite-related illnesses such as lung cancer [30]. Regulations at Sukhinda Valley are virtually non-

existent, therefore, as long as there is demand for chromite, the opencast mining will continue.

Having realized the impact that silane films, as chromate replacements, would have on the world

around us, we proceeded to use the following questions as driving force behind our study:

Can silane films effectively replace chromate coatings?

Can the addition of nanoparticles overcome some of the key weaknesses of silane films?

Can lanthanum ions confer self-healing properties to silane-based coats?

Literature Review

Despite possessing outstanding paint adhesion and anticorrosion properties, the widespread

adoption of silanes and subsequent replacement of conventional chromate treatments has been

stalled by three major drawbacks: i) with early silane technologies, thick films could not be applied

effectively and therefore restricted silane application to short-term corrosion protection; ii) unlike

silanes and potential chromate replacements, chromates have the ability to seal small defects by

passivation through the formation of trivalent chromium oxide from free hexavalent chromium ions

in the coating; and iii) chromate films are coloured yellow, whereas silane films are colourless (see

figure 3), making it difficult for observers to see the film upon coating. These three issues are

currently being addressed with the incorporation of nanoparticles, inhibitors and pigments

respectively.

Thicker silane films facilitate long-term corrosion resistance and improvement mechanical properties

of the film. Initially, solutions used to apply thicker films were unstable and the resulting films were

brittle. Researchers have discovered that loading a small amount of nanoparticles such as silica or

alumina into the silane films overcomes the aforementioned problems. Nanoparticles have also

found to reduce porosity and increase reinforcement of the inner layers of the silane film [31].A

preliminary study was performed on wrought aluminium alloy (AA5005) treated with a mixture of

bis-[trimethoxysilylpropyl]amino silane and vinyltriacetoxy silane (bis-amino/VTAS). The results of

the 336-hour salt spray test (SST) are shown in the figure below [10].


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Fig. 3. Wrought aluminium alloy panels following the 336-hour SST: The panel on the left is of untreated wrought aluminium alloy; the centre panel has been chromate; and the panel on the right has been treated with alumina-loaded bis-amino/VTAS [10].

The results illustrate that silane films impregnated with nanoparticles offer similar protection to that

of the chromate used in the control; there was no corrosion in the treated surfaces while the

untreated AA5005 panel had corroded severely.

The same study also explored the anticorrosion properties of colloidal silica nanoparticle-loaded bis-

[triethoxysilylpropyl]tetrasulfide (bis-sulfur) silane films on substrates of the aluminium alloy,

AA2024-T3. Before application of the silane film, the hydrolyzable groups of the silane needed to be

converted to silanol for the condensation reactions to ensue. By hydrolysing the silane in its aqueous

solution, the conversion is achieved. The researchers involved in the study used a water-ethanol

mixture to prepare a 5 vol.% bis-sulfur solution; this solution was then aged in ambient conditions

for two days such that a ‘workable’ silane solution could be created, i.e. to allow time for a sufficient

amount of silanols to be generated from hydrolysis and subsequent condensation between metal

hydroxide and silanols, and between silanols themselves [10][13]. If insufficient time is given, an oily

film is the result. Silica nanoparticles were incorporated into the silane solution in a procedure that

can be broken down into two steps: (1) a range of uniform silica colloidal solutions from 0.01 – 0.1

wt.% of silica nanoparticles in water were prepared; (2) five parts of the colloidal solution was then

blended into the bis-sulfur solution at the mixing ratio of 5/95 (v/v). Consequently, the

concentration of silica nanoparticles in the bis-sulfur solution ranged from 5 – 50ppm. After

degreasing the AA2024-T3 panels, they were dipped into the bis-sulfur silane solution for 30s, and

then cured, allowing the formation of a highly crosslinked film. Electrochemical tests, DC

polarization and electrochemical impedance spectroscopy (EIS), were run to gauge the corrosion

performance of the silane incorporated with silica nanoparticles. Following this, mechanical tests

were carried out and thickness was measured.

Figure 4 shows DC polarization curves of the AA2024-T3 panels and the scenarios tested. Looking at

scenario 2, we see a notable reduction in anodic and cathodic current densities. With 5 ppm of silica

incorporated into the silane mixture, a significant shift, from -0.6 to -1.0 in the cathodic direction can


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be seen. Such a shift suggests that the addition of nanoparticles changes the cathodic kinetics on the

surface of the alloy. The cathodic shift is not seen in scenario 4, when considerably higher

concentrations of silica are present in the silane solution. The cathodic shift indicates the silane film

formed with 5 ppm of silica nanoparticles in the silane solution behaves as a cathodic barrier,

inhibiting cathodic reactions and consequently suppressing corrosion.

Fig. 4. DC polarization curves of AA2024-T3 in the different scenarios tested [10]

Figure 5 compares the values of corrosion potential (Ecorr) and corrosion current (Icorr) against

different concentrations of nanoparticles in the bis-sulfur silane solution. Icorr, which is directly

proportional to corrosion rate, reaches its nadir when the nanoparticle content is 15 ppm. At

concentrations over 40ppm, the presence of silica particles degrades corrosion performance; this is

potentially attributable to increased film porosity with larger concentrations of nanoparticles,

facilitating heavy water ingress.

Fig. 5. Ecorr and Icorr of bis-sulfur treated AA2024-T3 systems as a function of silica concentration in the bis-sulfur silane solution

[10]


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EIS tests corroborated the above findings with low-frequency impedance values (Zlf) of the system

increasing proportionally with silica content until 15 ppm, and dropping sharply thereafter. A large

amount of silica particles (i.e. 50ppm) resulted in two time-constants, indicative of the formation of

a double-layer, which adversely impacts film adhesion. Hardness tests indicated that the addition of

nanoparticles preferentially hardened the interfacial layer up until 15 ppm; higher concentrations of

nanoparticles only produced a stronger silane surface and had no impact on the interfacial layer. The

study successfully demonstrated that nanoparticle loading did improve both mechanical properties

and anticorrosion performance of the bis-sulfur silane solution, provided their concentrations were

kept between 5-15ppm [10].

Constituting the majority of the silane film, the siloxane network resists water uptake and chemical

attack, thus providing excellent barrier properties that impede corrosion. However, because they are

not electrochemically active, siloxane networks only form a passive role in the prevention of

corrosion. Small defects in the film such as cracks can form preferential pathways for electrolyte

uptake, and therefore, for the onset of corrosion. An active means of inhibiting corrosion is required

to further improve silane film performance. It has been shown that active inhibition of corrosion,

without negative impacts on the integrity of the silane film, can be achieved by adding controlled

amounts of rare-earth nanoparticles or ions such as cerium or lanthanum [32]. Studies performed

galvanized steel substrates incorporating lanthanum ions have revealed the formation of an

additional film composed of La2O3 and insoluble La(OH)3 which function as cathodic inhibitors [33].

Furthermore, in a study performed by Montemor’s group, EIS has revealed that the addition of

lanthanum nitrate into BTSE silane films increases low-frequency resistance by one order of

magnitude relative to a control, which was coated with just the BTSE silane, while the addition of

cerium nitrate into BTSE films increase low-frequency resistance by two orders of magnitude. Upon

conducting atomic force microscopy (AFM), the researchers hypothesized that the discrepancy in the

increased resistance conferred by the two rare-earth nanoparticles arose from their distribution

within the siloxane network during polymerization. Most lanthanum particles were found in the

outermost layers, causing them to quickly leach out of the silane film. Ceria particles were found to

have stabilized in the inner layers of the silane film, therefore, a greater proportion of ceria would be

retained within the film after ageing. The increase in impedance observed was attributed to an

increase in film thickness, a reduction in porosity and a decrease in film conductivity, as oxides of

lanthanum and ceria are highly insulating [32].

Montemor and Ferreira pre-treated galvanized steel substrates with bis-[triethoxysilylpropyl]-

tetrasulfide silane (BTESPT) solutions that had been modified with SiO2 or CeO2 nanoparticles.


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Cerium salts were used to activate the nanoparticles. EIS was used to assess the influence of

nanoparticles on silane film capacitance and resistance; by artificially inducing defects, scanning

vibrating electrode technique (SVET) was used to study the ability of Ce-activated nanoparticles to

heal these defects and impede corrosion at the microscopic level. Nanoparticle addition aims at

enhancing the barrier properties of the silane film, while the presence of cerium ions is believed to

introduce corrosion inhibition properties in the bulk of the silane film. Silica nanoparticles between

30-40nm in diameter and ceria nanoparticles between 10-20nm were ultrasonically dispersed in a

cerium nitrate solution, such that a concentration of 250ppm of nanoparticles and 250ppm of

cerium nanoparticles was achieved. This solution would then be used for the preparation of the

silane solution. The BTESPT silane solution was prepared by mixing 5% (v/v) of the silane with a

mixture of methanol 95% (v/v) and the aforementioned nanoparticle-cerium nitrate solution. To

acquire detailed information about the corrosion inhibition performance of cerium-ion-activated-

nanoparticles in a silane film, a scratch was made with a knife edge on the surface of one of the steel

plates.

EIS immediately revealed the effects of SiO2 nanoparticles activated by cerium ions on the intact

steel substrate, which was immersed in 0.005M NaCl. The Bode plots (figures 6 and 7) show that in

that presence of cerium ions, the impedance values remained stable throughout the week-long

experiment. The phase angle plots in figures 6-9 reveal the presence of two overlapped time-

constants, one distinguishable at high frequencies and the other at low frequencies. The low

frequency time constant indicates the presence of a silane-metal layer, while the high frequency

time constant represents the silane film.

Fig. 6. EIS Bode plots for galvanized steel pre-treated with silane films that had been modified with SiO2 nanoparticles [19].


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Figures 8 and 9 show EIS Bode plots for silane coatings that had been modified by CeO2

nanoparticles. Similar to figure 6, figure 8 shows that impedance values are somewhat labile when

nanoparticles, which have not been activated by cerium ions, are added. However, figure 9 shows

that the addition of cerium ions led to a notable increase in impedance. In day 1, this increase was

an order of magnitude higher that what was obtained with the addition of CeO2 nanoparticle that

had not been modified with cerium ions.

Fig. 7. EIS Bode plots for galvanized steel pre-treated with silane films that had been modified with cerium-activated SiO2 nanoparticles [19].

Fig. 8. EIS Bode plots for galvanized steel pre-treated with silane films that had been modified with CeO2 nanoparticles [19].

Based on these EIS results, it is clear that the greatest effect on barrier properties is imparted by

silane films that contain cerium nanoparticles that have been activated by cerium ions. It is also

evident that the addition of nanoparticles greatly improves barrier protection. Literature has

reported that SiO2 nanoparticle addition reduced porosity and conductivity of the silane film, leading

to improved barrier properties. The addition of cerium ions has further improved these effects. It is


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believed that cerium ions induce changes in the viscosity of the solutions and enhance cross-linking

within the silane film, and consequently lead to the formation of thicker films.

Fig. 9. EIS Bode plots for galvanized steel pre-treated with silane films that had been modified with cerium-activated CeO2 nanoparticles [19].

In substrates that have been pre-treated with silanes, corrosion is initiated at small defects, pores or

scratches on the surface of the film. Unlike chomate treatments, passivation does not occur to

inhibit the spread of corrosion in base silane films; therefore it is crucial that inhibiting species that

impede corrosion are present in the silane film. SVET was used to gather detailed information about

the mechanisms of corrosion and to determine whether nanoparticle incorporation would reduce

corrosion activity at scratches.

Fig. 10. SVET maps of the scratched SiO2 films while being immersed in 0.005 M sodium chloride solution for 6 hours. The scale represents the ionic current activities [19].

Figure 10 shows the development of anodic activity over the scratch, whereas the remainder of the

surface behaves cathodic. Greater anodic currents were seen in the system in which SiO2 was

incorporated. The image on the right shows the significant reduction in anodic current density in the

system where SiO2 was activated by cerium ions. After being immersed in the 0.005 M NaCl solution


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for 24 hours, the silane film created with the addition of SiO2 nanoparticles was damaged

completely. Anodic activity increased in the film that contained cerium activated SiO2 nanoparticles,

and delamination became apparent; however, current densities remained lower than those

measured in SiO2 films that did not contain cerium.

Fig. 11. SVET maps of the scratched SiO2 films after being immersed in 0.005 M sodium chloride solution for 6 and 72 hours. The scale represents the ionic current activities [19].

As a result of more pronounced anodic inhibition effects, cerium nanoparticles again proved to

provide superior protection to substrates than silicon nanoparticles. Initial current densities were

lower for the system that had been filled with CeO2 only. However after 72-hours, delamination

could be seen in the area immediately surround the initial scratch. Current densities increased

beyond those of films created by cerium-activated CeO2. The cerium-activated CeO2 films displayed

negligible anodic activity and minimal delamination around the damaged area. It was concluded that

when compared to films filled with CeO2, anodic activity was significantly higher in films that had

incorporated SiO2 nanoparticles. In turn, this suggests a more protective and stable silane film is

formed under the presence of CeO2 nanoparticles. Moreover, the addition of cerium ions evidently

improved the protective properties of nanoparticles.

Anodic dissolution of zinc generates zinc cations, which in turn react with hydroxyl ions, leading to

the formation zinc hydroxide or zinc oxides, forming a passive layer.


14

Zn -> Zn2+ + 2e (1)

O2 + 2H2O + 4e -> 4OH- (2)

In the presence of sodium chloride solution, chloride ions dissociate and react with zinc ions,

forming ZnCl2- or ZnCl2 and ZnOHCl, which then form zinc hydroxycloride, Zn5(OH)8Cl2 – a corrosion

product. Normally species such as Zn2+, ZnO, ZnCl+ and Zn5(OH)8Cl2 contribute to the stability of the

protective layer. However, under the presence of excess chloride ions, localized film breakdown

takes place, allowing corrosion activity to proceed. With CeO2 nanoparticles that had been activated

by cerium ions, there was a subsequent increase in film stability and fewer free chlorides available

for adsorption. Ce4+ ions are responsible for the high stability of CeO2 nanoparticles. In addition,

CeO2-based films are stable in a wide range of pH, and therefore are not readily affected by the

increase in pH produced by the generation of hydroxyl ions (see equation 2).

The following reaction has been proposed to explain the behaviour of films consisting of SiO2

nanoparticles:

SiO2 + 2OH- -> SiO32- + H2O (3)

In alkaline environments, the silicates formed by the reaction are believed to react with zinc ions,

precipitating to form a stable layer of zinc silicate on the surface. Zinc silicate is believed to enhance

the stability of the protective film and impede corrosion. However, increases in pH will lead to

alkaline decomposition and the protective effects of silica will be attenuated. The activation by

cerium ions leads to the formation of a passivation film with greater stability, explaining why

degradation of the protective effects of silica was not observed when the nanoparticles had been

activated by cerium (see figure 10) [19].

Various techniques have been developed to deposit silane coupling agents onto metal surfaces. Such

techniques include deposition by simply dipping metal substrates into pre-hydrolysed aqueous

alcohol (solution technique), vapour deposition, spin-on application and electrodeposition [11]. Due

to its simplicity and low cost, deposition from solution is presently the most popular technique;

however it is not the most effective. There is mounting evidence suggesting that the

electrodeposition could be used to form superior coats with greater density, uniformity and a

stronger interfacial layer between the metal substrate and silane [12] [14][15].

Electrodeposition is a technique that utilizes electrical current to apply a thin film of a material onto

a metal substrate; the technique can be divided into two types, anodic and cathodic. In anodic

processes, anions are deposited into the anode, and in cathodic processes, cations are deposited


15

into the cathode. Because hydrolysed silanes are ionized molecules (they dissociate into anions and

cations) that are soluble in water, electrodeposition can be used to deposit them on metals. As

mentioned previously, electrodeposition presents various advantages over the traditional immersion

process, providing more uniform coats with controlled thickness, better adherence, and achieves

greater cross-linking of the silane coatings, resulting in reduced porosity and improved corrosion,

wear and oxidation resistance. The reaction cell comprises the silane solution as the electrolyte, the

metal substrate forming the cathode, and graphite is usually used as the anode.

van Ooij’s group used a series of analytical methods, including ellipsometry, DC polarization

technique and EIS to observe and document the surface structure of aluminium and iron substrates

that had been coated with silanes using the electrodeposition technique. To quantify the advantages

of electrodeposition, they then compared the results with those of films that had been prepared by

immersion. BTSE was used to coat polished ferroplate and Al-5052 and unpolished Al-6111. Using

graphite as the anode and the metal substrate as cathode, a constant 5-V was applied to the

reaction cell; this was done for 60 minutes on the Al-6111 substrate, 30 minutes on the Al-5052

substrate, and to observe the variation of film thickness as a function of time, a range of deposition

times were used on the polished ferroplate. The advantages of electrodeposition emerged

immediately; DC polarization tests revealed a reduction in current densities for the electrodeposited

sample when compared with the immersed sample, and thus, it can be inferred that that the films

created with electrodeposition were more effective at resisting corrosion. Likewise, EIS tests showed

higher impedance values for films that were electrodeposited. Figure 12 portrays the phase angle

curves obtained for electrodeposited and immersed BTSE films on the unpolished Al-6111. A second

time constant can only be seen in the electrodeposited layer, indicating the presence of an

interfacial layer between the metal substrate and silane film. The immersion technique only

produced a siloxane layer[1]. Subsequently, ellipsometry verified these findings.

Fig. 12. Phase angle curves for the electrodeposited and immersed BTSE films on Al-6111 [1].

Pore resistances were also analysed and the results are shown in figure 7.


16

Fig. 13. Pore resistance of electrodeposited and immersed BTSE films on Al-6111 plotted against electrolyte immersion time [1].

The above diagram shows that the interfacial layer formed by electrodeposition conferred an

extremely high pore resistance to the film; however, the advantages were short-lived as electrolyte

eventually penetrated the coating. Ellipsometry revealed that when the film was applied by

immersion, thickness was independent of immersion time; however, when applied by

electrodeposition, thickness was found to increase almost linearly with time up to approximately 30

minutes, after which it ceased to increase [1]. A study performed by Gandhi and van Ooij observed

almost identical results, see figure 14 [17].

Fig. 14. Thickness plotted against time, comparing electrodeposited and immersed films on ferro-plate panels. The silane used was BTSE [17].

Electrodeposited films were found to have a highly cross-linked interfacial layer 5 nm thick, and were

so tightly bound to the substrate that they could not be washed away by ethanol, even immediately

after deposition. On the other hand, an ethanol rinse immediate after deposition completely washed

away the film deposited by immersion. Scanning electron microscopy (SEM) was used to acquire

images of the deposited BTSE films. Films deposited by the immersion method had silicon-rich

patches and regions where silicon was barely detectable, a hallmark of non-uniform distribution. On


17

electrodeposited films, panels that underwent 10 minutes of electrodeposition revealed tears at

certain spots, devoid of silicon, but the application of the film was uniform otherwise. These tears

were no longer observed when electrodeposition time was increased to 30 and 60 minutes. It was

concluded that electrodeposition produced an organized, void-free film with high uniformity and low

porosity. Many of these properties were attributable to the interfacial layer between the silane and

the metal oxide, which had high ohmic resistance and low electrolyte permeability [1]. van Ooij’s

group could have acquired more comprehensive results had they not restricted their tests to one

fixed potential.

Problem Statement

Environmental, safety and cost issues stemming from the use of chromates have been the driving

force behind research and development to find alternative materials. Silanes have emerged as

promising alternatives to corrosion-resistant coatings; however there is still more research to be

done until they can convincingly replace chromate-based treatments.

The vast majority of studies that investigated the anti-corrosion properties of silane films have used

aluminium alloy as their substrate. Low-carbon steel is a widely used, versatile, inexpensive material

that contains roughly 0.2% carbon; due to its low carbon content, it is malleable while remaining

strong, and therefore is of great importance in the automotive industry and is also frequently used

to manufacture oil and gas pipelines. There has been relatively little research performed on low-

carbon steel, hence this will become the metal which we conduct our study on.

Plenty of studies have used silane films based on the non-organofunctional silane, BTSE, which has

the chemical formula, (C2H5O)3Si(CH2)2Si(OC2H5)3. BTSE’s dipodal structure, acidic nature, ability to

improve hydrophobicity of the coating and hydrolyze in water to form silanols, has made it a popular

choice among researchers. Its six hydrolysable groups mean it can form six bonds to a substrate

while conventional monosilanes can only form three. And because of this, silane films comprising

BTSE can condense to form a larger, three-dimensionally cross-linked, siloxane layer. Furthermore,

the presence of a carbon substituent in BTSE’s silicon atoms, mean that its siloxane films are highly

hydrophobic [36]. For these reasons, we too will make use of BTSE.

Dip coating will be used to deposit the BTSE film onto the metal surface. This technique is quick and

easy to perform. The electrodeposition technique will also be trialled. Electrodeposition is capable of

providing more uniform coats and thickness can be controlled almost linearly with deposition time.

In addition, electrodeposited coats often adhere better and have improved cross-linking, resulting in


18

reduced porosity and improved resistance to corrosion. EIS and DC polarization tests will be

conducted to gauge the effectiveness of the film.

Silica nanoparticles, lanthanum ions and a mixture of silica nanoparticles and lanthanum ions will be

incorporated into the silane film. There has been numerous research performed on silica

nanoparticles and a few on lanthanum ions, but none of the papers have tested a combination of

both..

Light microscopy will be used to study the surface before and after film application. Fourier

transform infrared spectroscopy (FTIR) will be used to verify the presence and quantify the

concentrations of chemical moieties in the films. A video goniometer will be used to measure

contact angles, which in turn, will indicate the hydrophobicity of the films we deposit. Thickness

measurements will then be performed to confirm that nanoparticles increase film thickness and

viscosity will be measured to determine whether the addition of nanoparticles does indeed improve

viscosity of the silane solutions.

Experimental Design The following samples will be used in this study:

1. Low-carbon Steel (Control)

2. Low-carbon Steel with BTSE

3. Low-carbon Steel with BTSE and Si nanoparticles

4. Low-carbon Steel with BTSE and La nanoparticles

5. Low-carbon Steel with BTSE, Si and La nanoparticles

The substrates are rectangular pieces, measuring 2cm by 6cm. Tests to analyze coating performance,

outlined in part (b) of this section, will be run on the abovementioned samples.

a) Surface Preparation

Surface preparation is the first critical step in preparing the substrates for microscopic examination.

It begins with polishing, a process that removes deformed or damaged surface material, while

introducing minimal amounts of new deformation. The carbon steel plates were initially polished

using 240, followed by 300, 400 and 600 grit emergy paper at 180 RPM; water was applied steadily

to the polishing surface to prevent any localized heating which could cause metallurgical changes in

our sample. 1 and 0.5 micron polycrystalline diamond plates were then used at the same RPM to


19

carry out fine polishing which mitigates any deformations introduced by the coarser polishing

processes and results in a mirror-finish. Remnant contaminants on the surface were removed by

ultrasonic degreasing. Transducers, consisting of elements that vibrate to specific frequencies,

within the ultrasonic cleaning machinery generate acoustic energy through electrical energy. The

frequencies are engineered to create alternating high and low pressure waves, inducing cavitation,

where millions of small bubbles form during the low pressure phase which then implode during the

high pressure phase. The end result is highly effective agitation which cleans both exposed and

hidden surfaces of the metal immersed, in our case, acetone. Acetone is an organic solvent that is

miscible in water and an effective degreaser; further degreasing is achieved through the immersion

of the substrate into commercial alkaline solution. At ultrasonic frequencies, the number of cavities

produced increases while we see a decrease in the energy released by each implosion, making this

an ideal means of removing small contaminants without damaging the metal surface [20]. The

cleaning was carried out for 4 minutes in each solution and at 65 °C. The intensity of cavitation is

strongly influenced by temperature; as we increase temperature, the intensity of the cavitation is

reduced, however, increasing the temperature can also result in fiercer chemical reaction between

the alkaline solution and impurities [21]. By setting temperature to 65 °C, we achieve a trade-off

between cavitation intensity and intensity of chemical reactions between the impurities and alkaline

solution. An alkaline solution was chosen to generate free metal surface hydroxides (MeOH) which is

needed to initiate the series of reactions that culminates in the formation of covalent bonds that

adhere the metal surface to the silane film. Upon degreasing, the panels were blow dried, resulting

in surfaces that are thoroughly wettable by water (water-break-free) which adsorb and readily react

with silanols in the silane solution.

The BTSE solution was prepared by mixing 5%(v/v) BTSE silane (Sigma/Aldrich product) with 5%(v/v)

de-ionized water and 90%(v/v) of ethanol. BTSE was chosen due to its ability to form films that

comprise covalent, hydrolytically stable bonds. When completely hydrolysed with six silanols groups,

BTSE will be acidic enough to react readily with even slightly basic OH groups on metal oxide

surfaces, forming covalent, hydrolysis-resistant bonds and a highly cross-linked film. Puomi et al.

discovered that when applying BTSE films to hot-dipped galvanized (HDG) steel, bonds of greater

hydrolytic stability were formed when the films were deposited from ethanol-water solutions than

from water-based solution [23]. Additionally, Bafna performed contact angle measurements of water

on coatings that were formed that solutions that contained 0%, 4% and 8% BTSE silane. When there

was no BTSE in the solution, the contact angle was found to be 46° indicating that the film was

hydrophilic, while the 4% BTSE solution produced films that resulted in a contact angle of 88° while

the film coated with the 8% solution displayed a contact angle of 92°. When adhesion tests were


20

performed, the coating that had been generated by a 4% silane solution showed the best results,

with this concentration of silane striking an effective balance between adhesion and hydrophobicity.

The 8% BTSE silane solution resulted in coats that did not adhere as well; the inferior adhesion was

likely the result of the excessive presence of low-molecular weight, unreacted BTSE silane [24].

The abovementioned ethanol/water/silane ratio was chosen primarily due to the presence of ethoxy

esters in BTSE and the hydrophobic nature of bis-silane. The resulting solution would have a natural

pH of 6; nitric acid was then be used to reduce pH to 4. Following hydrolysis, the silane undergoes

condensation reactions, leading to polymerization and precipitation. These condensation reactions

degrade the quality of the film when formed on the metal chiefly because we get a reduction in the

number of water-resistant bonds. At a pH of 4, BTSE will hydrolyse quickly (1000 times faster than it

does at a pH of 7 [43]) and condense slowly; however, at a pH of 6, both reactions will be relatively

quick, producing silane films that are not of good quality. At higher pH, the solution will be

comprised of cyclic oligomers which cannot react completely and form a dense film devoid of

defects [22]. Subramanian and van Ooij investigated effects of silane solution pH on the corrosion rate

of iron substrates treated with BTSE; it was reported that the lowest corrosion rate was found at pH

below 5, however the mechanism for this was not proposed [44]. The presence of more SiOH groups

in BTSE solutions prepared at low-pH is likely the reason behind the better corrosion performance.

Silanes such as BTSE, which are highly hydrophobic and therefore require a considerable amount of

ethanol in the mixture, will take a roughly 24 hours to hydrolyse completely. BTSE’s hydrolysis

kinetics dictate that the first ester group is hydrolysed rapidly while the rate of subsequent ester

groups is so low that it takes an excessive amount of time to hydrolyse all six ester groups. Because

of potential time constraints, hydrolysis time will be limited to 4 hours. After 4 hours, there will likely

be unhydrolysed molecules in the film; however, these molecules will not have a very detrimental

effect on the film as they are hydrophilic and will eventually hydrolyse when exposed to the

atmosphere [22]. Continuous stirring will take place throughout this process.

Silica nanoparticles, lanthanum ions and a mixture of silica nanoparticles and lanthanum ions will

constitute the three modified BTSE silane solutions. The nanoparticles will be incorporated into the

silane solution in a two-step procedure: (1) by mixing 30 milligrams of nanoparticles in 100 mL of de-

ionized water, a uniform colloidal solution of 0.03 wt.% (i.e. 300 ppm) of silica nanoparticles will be

prepared; (2) five parts of the colloidal solution will then be blended into the BTSE solution at the

mixing ratio of 5/95 (v/v). As a result, the concentration of silica nanoparticles in the BTSE solution

will be 15 ppm. In the same manner, 30 milligrams of lanthanum(III) triflueoromethanesulfonate

(also known as lanthanum(III) triflate) will be added to 100mL of de-ionized water, and five parts of


21

this lanthanum solution will be blended into the BTSE solution. As extensive research has not been

performed on hybrids of nanoparticles and inhibitors, we will attempt a 50:50 ratio of silica and

lanthanum in a solution BTSE silane solution containing 15 ppm nanoparticles and 15ppm of

lanthanum ions.

Dip coating was performed thrice with an immersion and withdrawal speed of 50mm/min. The

metal piece would remain in the hydrolyzed silane solution for 60 seconds, which is sufficient time

for the formation of hydrogen bonds between free hydroxide groups in the siloxane network and

hydroxide groups on the metal surface. Curing at 100°C for 30 minutes in a Memmert oven was then

done, allowing the condensation reaction to occur, resulting in a denser siloxane network and the

formation of covalent bonds between the siloxane network and the metal surface, and ultimately,

significantly improved silane film adherence. The denser siloxane network will also reduce the

number of conducting pathways for electrolytes, thus increasing corrosion resistance and shelf-life

of the coating [35].

Electrodeposition of the silane solutions was performed using the low-carbon steel substrate as the

cathode and graphite as the anode. A constant voltage of 5V will be applied for 30 minutes. The

electrolyte will be the silane solution.

b) Analysis and Characterization

Light microscopy will be performed using an Olympus BX41M microscope, to analyze the uniformity

of the surface and observe silane film deposition. A coating thickness gauge, which measures

changes in magnetic field relative to an uncoated reference point on the substrate, will be used to

quantify film thickness.

A qualitative salt immersion test will be performed over a period of five days, whereby all five

samples are placed in a beaker containing 0.1M NaCl – an electrolyte that closely resembles

seawater – and removed daily and photographed. The results from this test should give an insight

into coating performance by BTSE films and the benefits of nanoparticles, and provide us with a

point of reference when interpreting our results from EIS and DC polarization.

EIS is performed using a Gamry 600 potentionstat which subjects the working electrode to 10mV

RMS alternating potential signals of various sinusodial frequencies. The current response will be

measured and impedance (Z) will then be calculated by dividing alternating potential (input) by

alternating current (output). A range of frequencies between 10-2 Hz to 105 Hz will be sweeped

within the system, and will constitute the X-axis of the Bode plots that will subsequently be

generated, whilst impedance and phase angle will be plotted on the Y-axes. Phase angles can be


22

defined by the time lag between an input potential and output current. The Bode plots can give

insight into the kinetics of the reactions at the metal surface. A 3.5% vol. NaCl solution will be used

as the electrolyte, while our metal substrate will act as the working electrode, a graphite rod as the

counter electrode and a saturated calomel electrode (SCE) will be used as our reference electrode.

The Open Circuit Potential (OCP) will first be taken, before electrical potential is applied to the cell.

This ensures that the system has reached steady-state, meaning the various corrosion reactions

occurring within the cell have reached a constant rate which can then be compensated for when the

subsequent potentiodynamic EIS scan is carried out.

To bolster the results of EIS, DC polarization tests will be performed with samples aerated in 3.5%

vol. NaCl solution (electrolyte) at a pH of 6. The samples will be pre-immersed in 0.6M salt sample

for 2 hours in order to reach steady-state. We will use SCE as reference and graphite as the counter

electrode. The exposed area will be fixed at 0.78cm2 (circle with a diameter of 1 cm). By conducting

this test, we will be able to evaluate the polarization resistance, and therefore resistance to anodic

dissolution, of the silane-coated low-carbon steel substrates. Like EIS, this test will be performed at

room temperature and in a Faraday Cage to reduce noise. The downside to this test is that, because

it encourages anodic dissolution through polarization, it is destructive and our sample cannot be

used for further tests.

Fourier transform infrared spectroscopy (FTIR) will be used to verify the presence and quantify the

concentrations of moieties such as siloxane, hydroxyl and metallo-siloxane bonds in the film. A scan

of the background will be performed before each run to account for peaks at 1600, 2400 and 3800

cm-1 which correspond to H-O-H bending, asymmetric CO2 stretching and O-H stretching

respectively, and are associated with the presence of moisture and carbon dioxide in the air which

may interfere with the final results. The following settings will be used in the spectrometer:

Range: 4000 – 400 cm-1; Gain = 2; Optical velocity = 0.6329; Apeture = Medium resolution; Number

of scans = 64; and Resolution = 4

To measure hydrophobicity of the film, contact angle measurements was determined by the sessile

drop method using a Rame-Hart Model 200 standard contact angle goniometer. Several

measurements were taken on each sample with a distilled water drop of 4 mm3 at room

temperature. The values reported constitute an average of at least five measurements. Larger

contact angles correspond to poorer wetting and therefore a more hydrophobic film.


23

Safety

Personal protective equipment such as lab coat, safety goggles and gloves should be worn when

handling silane solutions. Gloves should completely cover the hands and the wrists and should

overlap with the sleeves of the lab coat. It is essential that a lab-coat and closed-toe shoes are worn,

particularly when handling nanoparticles. It is the responsibility of the occupants of the laboratory to

inform themselves about the location of the safety shower, fire extinguisher, eye wash and

emergency evacuation procedures.

a) Silane

Relative to conventionally used chromate, silanes are considered very safe, nonetheless, it is

important to be cognizant of the hazards they pose in the laboratory. Silane vapours can form at

ambient temperature and inhalation of these vapours can damage the delicate linings of the nose,

throat and chest [25]. Contact dermatitis could result if the silane comes into contact with the skin,

though effects will not manifest until a few hours after contact has been made. Direct contact with

the eye will result in immediate eye irritation. If swallowed, the results could be fatal as

alkoxysilanes will hydrolyse to silanols and alcohols in the stomach.

The following safety measures will be taken if the abovementioned scenarios do eventuate:

If inhaled, the individual will be moved immediately to fresh air. If the individual is unable to

breathe, artificial respiration will be given until a physician arrives.

In the case of skin contact, contaminated clothing and shoes will be removed and exposed areas will

be washed with soap and water. Medical advice will be sought if discomfort continues. If the silane

comes into contact with the eye, the victim’s eyes will be flushed with water for at least 10 minutes

while the eyelids will be held away from the eyeball, ensuring that all surfaces are flushed

thoroughly. An appointment with an ophthalmologist should be arranged immediately to check for

any chemical burns to the cornea.

Had the silane solution been swallowed, immediate medical attention will be sought; the victim will

be kept calm and will not be given any liquids to induce vomiting unless directed to do so by a

medical practitioner.

Ethanol constitutes 90% of the silane solution. Because of such a high concentration of ethanol, the

silane solution poses a small but significant risk of fire. Highly volatile, ethanol will react with oxygen

in the air and reduce to carbon dioxide and water while releasing heat, in the form of fire, during the


24

reaction. A source of heat such as a fire from a match will provide sufficient activation energy for the

aforementioned reaction to ensue. Therefore, a fume cupboard must be used when preparing the

silane solution, not only to reduce risk of fire, but to mitigate exposure time to vapours.

In the rare event that the silane solution is set alight, it is important to remember that there will be

the release of toxic fumes. Unless the fire can be extinguished immediately with a CO2 or foam-

based extinguisher, all personnel in the laboratory must be evacuated swiftly.

b) Nanoparticles

At present, there is very little knowledge about the safety of nanoparticles; we do not know where

nanoparticles will move to once they enter the body. Nanoparticles can be inhaled, dermally

absorbed and ingested. Because of its extremely small size, nanoparticles can easily traverse cell

membranes, where they can interfere with cellular processes and resist cell defence mechanisms.

Their high surface to volume ratio can make them highly reactive or catalytic.

The American Association for Cancer Research has suggested that nanoparticles could be

carcinogenic, noting that researchers had observed time and dose-dependent DNA damage in cells

that had been exposed to silica nanoparticles. In addition, there is also the risk of developing

silicosis, an irreversible lung disease with symptoms that include dyspnea, chest pain and cyanosis.

With research performed on in vitro lymphoblastoid cells showing that ultrafine crystalline SiO2

particulates can be both genotixic and cytotoxic, it is essential that proper safety precautions be

taken to reduce exposure to nanoparticles. Nanoparticles must be used sparingly; laboratory masks

and gloves must be worn when handling nanoparticles and the preparation of nanoparticle-

incorporated silane solutions should take place in either a certified fume cupboard or a local exhaust

ventilation (LEV) system with high efficiency particulate air (HEPA) filtration.

c) Polishing Equipment

Immense care must be taken when operating the polishing equipment. At speeds exceeding 300

rpm, the metal piece being polished will have sufficient centrifugal force to depart from the polisher

unless held down. At 600 rpm, a metal piece will have sufficient force to inflict a deep would on the

fingers if it is not held properly. It is recommended that a magnet be used to hold on to the metal

piece and speeds kept under 300 rpm.


25

Gantt Chart

ID Task Name Start Finish DurationMar 2011 Apr 2011 May 2011

6/3 13/3 20/3 27/3 3/4 10/4 17/4 24/4 1/5 8/5 15/5

1 2.2w16/3/20112/3/2011Designing methodology

2 2.2w16/3/20112/3/2011Literature Review Draft

.2w24/3/201124/3/2011Interview

3 .4w18/3/201117/3/2011Preparing Metal Sheet via grinding, polishing and etching

5 1w25/3/201121/3/2011Producing the silane solution including those with nanoparticles

2w8/4/201128/3/2011Electrodeposition

1w15/4/201111/4/2011Characterization (DC Polarization)

1w22/4/201118/4/2011Chartacterization (EIS)

6

8

7

1w25/3/201121/3/2011Measure viscosity of solution

10

9

5.5w18/5/201111/4/2011Final Write Up

4

Results

a) Microscopy

a b c

Fig. 15. Micrographs of: a) Unpolished low-carbon steel surface b) Polished low-carbon steel surface;

c) Low-carbon steel surface coated with BTSE silane

a b c

Fig. 16. Micrographs of: a) BTSE + lanthanum solution; b) BTSE + silica nanoparticles and lanthanum

solution; c) BTSE + silica nanoparticles


26

b) Salt Immersion Test

Fig. 17. Samples at day 0 of the salt immersion test

Fig. 18. Samples after 24 hours of salt immersion

Day 2

Fig. 19. Samples after 48 hours of salt immersion


27

Sample Day 0 Day 5

Control

BTSE

BTSE + Si

BTSE + La

BTSE + Si + La

Fig. 20. Comparison of all samples at day 0 against day 5

a

b

Fig. 21. Photographs after day 5 of: a) control; b) low-carbon steel with coat of BTSE with Si and La


28


Sample Film Thickness at Centre

(microns)

Film Thickness at Base (microns)

BTSE Not Measurable 1.0

BTSE + Si Not Measurable 5.9

BTSE + La Not Measurable 2.2

BTSE + Si + La Not Measurable 6.8

Fig. 22. Thickness measurement results of all five samples

d) Video Goniometer

Sample Control BTSE BTSE + Si BTSES + La BTSE + Si + La

Goniometer photo

Contact angle

54.8° 69.4° 68.8° 60.4° 61.2°

Fig.23. Contact angle measurements and photographs of the sessile drop test on all five surfaces


29

e) FTIR Spectrum

Fig. 24. Merged FTIR spectrum of all five samples


30

f) EIS

Day 0

Fig. 25. EIS Bode plot of control against low-carbon steel with BTSE, Si nanoparticles and lanthanum

Day 1

Fig. 26. EIS Bode plot of low-carbon steel with BTSE, Si nanoparticles and lanthanum after 24 hours


31

Day 2

Fig. 27. EIS Bode plot of low-carbon steel with BTSE, Si nanoparticles and lanthanum after 48 hours


a

b

Fig. 28. Micrographs of a low-carbon steel surface following electrodeposition: a) performed at 5V

for 1800s; b) performed at 2V for 7200s


32

Discussion

a) Microscopy

The metal surface of the control upon polishing is shown in Figure 15b. The scratch that runs from

the top left to the bottom right of the micrograph is indicative of improper polishing. Unfortunately,

polishing is an art rather than a science, which takes a considerable amount of time to perfect, and

involves much trial and error. Aside from this scratch, the surface appears smooth and uniform

relative to the unpolished surface (figure 15a). The BTSE-coated metal surface can be seen in figure

15c; the deposition of the dip-coated film does not appear to be uniform, as there are regions in the

film where the silane film appears relatively thick and some regions where the bare metal surface is

exposed. Iridescence in seen in thicker regions of the coat; this phenomenon occurs as light first

traverses the uppermost layer, where some of it is reflected and the remainder reaches the bottom

layer, where once again, some of it is reflected. A phase shift is created, and if the two reflected

waves differ by a multiple of a whole wavelength, the waves experience constructive interference,

producing colours of high intensity.

A small cluster of silica nanoparticles, surrounded by concentric circles of silane solution can be seen

on the top-left of figure 16b. Scratches on the metal surface are visible through the coat, indicating

that the silane film is extremely thin. Once again, it is evident through figures 16a and 16c that

despite being dipped thrice, the dip-coating technique does not produce highly uniform coats on the

metal surface.

b) Salt Immersion Test

The salt immersion test (see figure 17) is a qualitative test that allows us to gather information about

how our five samples fare when exposed to an environment that closely resembles sea water. The

metal pieces are removed from their respective beakers containing 0.1M NaCl and the surface is

visually inspected. At day 0, all five samples are lustrous and have a highly reflective mirror finish.

After 24 hours (see figure 18), a considerable amount of corrosion has taken place on the bare low-

carbon steel surface and BTSE-coated sample, whilst samples incorporating silica nanoparticles and

lanthanum solution exhibit relatively little corrosion, and the film containing lanthanum–activated

silica nanoparticles does not display any conspicuous corrosion. This suggests that the incorporation

of silica nanoparticles has resulted in a thicker film that is less porous, and therefore impedes

electrolyte intrusion and subsequent corrosion. Such results are consistent with findings in

literature, for example, Montemor’s group tested the effectives of microsilica in HDG steel


33

substrates and reported that BTSE films incorporating SiO2 achieved an impedance one order of

magnitude higher in EIS tests than substrates with BTSE films alone [31].

As for the substrate with BTSE and lanthanum, because the solubility product (Ksp) of lanthanum

hydroxides are less than that of iron(II) hydroxide, the lanthanum solution has likely precipitated on

the surface to form a non-conductive oxide inner layer, composed of La2O3 and La(OH)3 [33]. This

oxide layer will substantially improve the corrosion resistance offered by the originally porous oxide

layer that naturally forms on the surface of low-carbon steel. Looking at the sample incorporating

both silicon nanoparticles and lanthanum, it is ostensible that the effects of silica and lanthanum on

the silane film are cumulative, suggesting the formation of a film with both reduced porosity and a

superior oxide layer at the metal-silane interface.

After 48 hours, all samples appear to have experienced a considerable amount of corrosion. Upon

closer inspection, it can be seen that the most corroded samples are the control and substrate

coated with BTSE silane alone. Despite its hydrophobicity, the BTSE silane film has done little to

hinder the access of electrolytes to the metal surface; due to the porous nature of films devoid of

nanoparticles, it is likely that electrolyte contact angle slowly decreased, allowing hydrophobic Si-O-

Si bonds to be hydrolyzed to hydrophilic Si-OH and subsequently, the covalent linkages that once

anchored the film to the substrate – the Fe-O-Si bonds – were hydrolyzed into Fe-OH, leading to film

delamination [1]. The trend from the previous day continues with the sample incorporating both silica

nanoparticles and lanthanum showing the greatest resistance to corrosion.

Interestingly, the top third of the substrate incorporating both silica nanoparticles and lanthanum

has experienced significantly less corrosion than the bottom third, even after day 5 (see figure 20

and 21). Thickness measurements explained these observations.


An Elcometer 456 coating thickness gauge was used to measure the thickness of our films; the

magnetic field produced by the bare substrate was used as the reference point, therefore the

presence of a non-magnetic coating – in our case, a silane film – would produce lower magnetic field

and this would correspond to a thickness measurement. A lower the magnetic field reading,

correlates to a thicker film. Although thickness measurements through this means are not the most

accurate, some interesting trends did emerge (see figure 22). All measurements were performed at

the base of the metal piece, as the silane film appeared to be thickest here. The reason behind this is

outlined in the errors and improvements section.


34

Consistent with expectations, the BTSE silane film produced the lowest thickness reading of 1

micron. However, this value is considerably higher than what has been reported in the literature,

primarily because much of the thickness readings reported in journals have been acquired through

ellipsometry, an optical technique which can give accurate readings in the nanometer range [1]. The

BTSE silane incorporating lanthanum gave a reading of 2.2 microns, suggesting that the solution did

little to increase film thickness, but rather its presence simply encouraged the formation of an

electrolyte-resistant oxide layer. The largest readings of film thickness were acquired from samples

that had incorporated silica nanoparticles. Congruent with literature, nanoparticle addition did

indeed increase the thickness of the film [31].

The addition of silica nanoparticles to the BTSE film increased thickness by 4.9 microns. Although

nanosize, literature has reported that silica nanoparticles have a tendency to agglomerate and

aggregate, meaning only high shear stresses applied onto the silane film can break them down into

individual nanoparticles [9][38]. This tendency was observed when adding the nanoparticles to the

partially-hydrolyzed silane solution, where they would sink to the bottom of the beaker they were in

and cluster together, forming large aggregates that were visible to the naked eye. In the silane film,

the aforementioned silica agglomerates would adsorb silane molecules, allowing the formation of

thicker films.

Once again, the addition of silica nanoparticles and lanthanum to the BTSE silane were cumulative,

producing the thickest film of 6.8 microns.

d) Contact Angle Measurements

If a liquid distributes itself over a metal surface evenly, without the formation of droplets, then the

surface is said to be wetted and hydrophilic. This means that cohesive forces that hold the bulk

liquid water together are lesser than the forces associated with the interaction between water and

the surface. The opposite is true for hydrophobic surfaces, where forces associated with the

interaction between water and the surface are comparatively small. Consequently, distinct water

droplets are formed on hydrophobic surfaces. To quantify hydrophobicity and hydrophilicity, the

contact angle of a liquid droplet on a metal substrate must be measured, which in turn, gives us

insight into the extent of interaction between a liquid and a solid surface. The contact angle is a line

that is tangent to the curve of the droplet at the point where the droplet meets the metal surface.

As hydrophobicity increases, so does the contact angle between the droplets and the surface [39].

The control exhibited the lowest contact angle measurement of 54.8°, a value similar to that

reported by Bafna who used a bare aluminium surface [24]. This value indicates that the surface is


35

somewhat hydrophilic. Once a BTSE film is applied, contact angle increased considerably to 69.4°.

This increase is attributable to the formation of the siloxane network during the curing process and

the presence of hydrophobic C-H groups in BTSE. Palomino’s group, who had also used a curing time

of 30 minutes at 100°C, observed a value of 68° on BTSE-coated aluminium surfaces. However,

discrepancies arose when we compared our results with their contact angle measurements for BTSE

with silica nanoparticles, which had given a value of 86°; the paper reported that the mechanism by

which nanoparticles reduced surface hydrophobicity was still incompletely understood [40]. In our

case, the addition of silica nanoparticles did little to increase hydrophobicity. One possible

explanation is that when preparing the nanoparticle-incorporated BTSE silane solution, we would

notice silica aggregates sinking to the bottom of the beaker. They would never distribute themselves

in the solution homogenously. As a result, we may have had concentrations of silica nanoparticles

that deviated significantly from our intended concentration of 15ppm. Therefore, the increase in

hydrophobicity may not have been seen because: (1) silica nanoparticles were at too low of a

concentration to have an effect on silane film porosity, i.e. it does not reduce the rate of electrolyte

uptake; or (2) the concentration of silica nanoparticles were excessive, and therefore, silica’s ability

to reduce film porosity were mitigated [10].

Contrary to expectations, the addition of lanthanum solution to the BTSE film reduced

hydrophobicity. Palomino et al. reported that the addition of cerium to BTSE films effects just as

much of a change in hydrophobicity as silica nanoparticles, giving them a contact angle of 84°. As

observed in the tests detailed above, Palomino’s group noticed that the effects of cerium and silica

were cumulative, producing a contact angle of 89° [40]. After observing AFM images showing cerium

doped silane films, Cabral et al. stated that rare-earth ions such as cerium allow the formation of

thicker films as they are able to influence the hydrolysis and polymerization reactions, though the

mechanisms by which these ions influence the film is still incompletely understood [41]. Because our

values of 60.4° and 61.2° was very similar to our bare metal control, the only reasonable explanation

is that dip-coating was not uniform and contact angle measurements were performed on a region

where coating was thin or absent. As stated earlier, one of the major drawbacks of silane films over

chromates is that they are colourless and difficult to observe upon coating, and it is in scenarios like

these where the incorporation of colorants, which also function as fillers (e.g. TiO2), would have

been of tremendous benefit.

e) FTIR

Silane film performance is closely linked to its structure. Thus, to obtain a good understanding of

silane film structure, characterization is essential.


36

Almost all heteronuclear diatomic molecules absorb infrared light. However, these molecules only

absorb infrared light at specific frequencies which affect the dipolar moment of the molecule. The

dipolar moment is produced as a result of differences in charges in the electronic fields of the atoms

that constitute the molecule. Upon irradiating the sample with a broad spectrum of infrared light,

Fourier Transforms are performed on the data, allowing the level of absorbance at each frequency

to be reported. Because different molecules absorb infrared right at different frequencies, the

resulting spectrum represents the molecular fingerprint of the sample. The height of the peaks

correspond to the amount of chemical species present in the sample. On a fundamental level, this is

the mechanism that underlies FTIR.

The below table outlines the significant peaks observed in the FTIR spectra (figure 24).

Assignment Absorption Band (cm-1)

Atmospheric O2 3700 - 4000

O-H stretching / SiOH 3000 - 3700

C-H stretching / BTSE 2850 - 2950

C=O stretching / CH3OCH3 1700 - 1750

Si-O stretching / SiOSi 1000 - 1250

Fe-O stretching / SiOFe 1000 - 1100

O-H deformation / SiOH 880 - 900

Atmospheric CO2 600 - 700

Fig. 29. Peaks observed in the FTIR spectra and the molecules they correspond to

All of the coated substrates indicate the presence of hydroxide groups. These hydroxide groups

belong to silane molecules that have not cross-linked completely, or hydroxide groups formed on

the metal surface during alkaline treatment, which have not formed covalent bonds with the free

hydroxide groups on the silane film. There appears to be noticeably less OH groups in the silane

solution that had incorporated both lanthanum and silica nanoparticles. Perhaps both these

additives have synergistically increased the condensation of the film during curing. However, it can

also be seen in the spectra that all chemical species are present at a slightly lower quantity in the

sample containing both lanthanum and silica nanoparticles. This is likely a result of non-uniformity

created by the dip-coating technique. Nonetheless, it is worth noting that the biggest difference in

the height of the peaks between the BTSE sample incorporating silica nanoparticles and lanthanum

against the rest of the coated samples occurs in the range of wavelengths that correspond to the

presence of hydroxide groups, i.e. there is not much of a height difference between the peaks of the

coated samples when the broad hydroxide peaks are overlooked. Further tests will have to be


37

performed to add weight to the notion that both additives may work together to increase

crosslinking.

The peaks that correspond to C-H stretching are observed on all coated samples, The BTSE molecule

contains four C-H bonds, and the peaks confirm the presence of BTSE on the film.

The peak corresponding to the C=O bond, seen in the BTSE film incorporating silica nanoparticles

and lanthanum, is likely due to a contamination of acetone which was used to wipe the housing of

the infrared light source after each sample had been tested.

Si-O stretching was observed in all coated samples. Such peaks affirm the presence of a siloxane

network in the film. Interestingly, the height of the peaks produced by the BTSE sample

incorporating silica nanoparticles and lanthanum is not considerably lower when compared to the

peaks produced by the rest of the coated samples as it was for the hydroxide peaks. Once again,

further tests will have to be performed to confirm the interaction between silica nanoparticles and

lanthanum, e.g. FTIR that actively monitors that silane film before and after curing, and atomic force

microscopy (AFM) to acquire further details about the film structure.

Fe-O peaks confirm the presence of metal-hydroxide bonds which anchor the siloxane network to

the substrate. The addition of lanthanum and silica nanoparticles did not improve film adherence.

The noise seen in the far left and right of the FTIR spectra is inevitable, as it is caused by the

surrounding atmosphere.

The absence of distinct peaks at 1150 cm-1 which corresponds to C-H stretching in OC2H5 implies that

there were no detectable traces of unhydrolysed BTSE molecules in our silane solution when FTIR

was performed [42]. Such unhydrolysed molecules could have effectively reduced cross-link density.

f) EIS

Most industries do not want to wait 5 to 10 years for the completion of a comprehensive salt

immersion test. Instead, they make use of electrochemical impedance spectroscopy, whereby an

alternating potential is applied to the electrochemical cell and the alternating current response is

recorded over a range of frequencies. Impedance is calculated by dividing the input potential by the

output current and phase angle is calculated by measuring the time difference between the input

and output signals. The values are then plotted on a Bode plot, which can give insight into the

physiochemical processes that take place at the metal surface and ultimately allow us to determine

the shelf life of our coat. The larger the impedance of produced by a coating, the greater its

corrosion performance will be [45].


38

An EIS Bode plot of the control against the sample with BTSE, silica nanoparticles and lanthanum is

shown in figure 25. At day 0, two resistive parts appear in both impedance plots. These parts appear

at 10-2 and 5 x 103 Hz and describe the polarization resistance (Rpolar) and solution resistance (Rsol)

respectively. Cathodic and anodic reactions can only take place at finite rates; if electrical energy is

used to force these electrochemical reactions to take place at higher rates, a deficiency in electrons

liberated by the metal undergoing anodic dissolution results, and a positive potential change at the

interface occurs. This is referred to as positive polarization. As electron deficiency, and therefore

polarization becomes greater, so does the tendency for the metal to dissolve anodically. Thus, the

resistance to polarization correlates inversely to the rate of corrosion, i.e. a higher Rpolar value

represents a slower corrosion rate at the metal surface [45]. There is also one more value that can be

determined through the impedance plot for the coated sample and that is the pore resistance, Rpore

which appears at 5x103, just like Rsol did for the impedance plot of the control. Rpore represents the

resistance of electrolytes in pores and cracks in the coating which are connected in parallel to one

another [37]. The impedance value for Rpore can be calculated by subtracting the value of the plateau

produced from 5 x 103 Hz onwards by the coated sample by Rsol for the bare substrate which is

around 16Ω. There was a notable difference in low-frequency impedance values between the two

samples, likely caused by the presence of a hydrophobic BTSE siloxane network and silica

nanoparticles which reduce the active area, thus reducing anodic and cathodic currents. However,

these differences in impedance were not as significant as they were in literature, which reported

several orders of magnitude of increase coated samples [32][34]. Possible reasons behind this are

outlined in the errors and improvements section. As for the bare sample, our findings mirrored

those seen in literature. Two well-defined time constants were observed: one at frequencies around

10-2 Hz; and the other at 105 Hz. The low-frequency time constant can be ascribed to corrosion

activity and the one at high frequency to the porous oxide layer that naturally forms on low-carbon

steel surfaces [19][31][32][34].

After 24 hours of immersion, impedance spectra have revealed the presence of three distinct time

constants at 10-2, 2 x 103 and 2.5 x 104

Hz in the coated sample (figure 26). These time constants can

be ascribed to the capacitance produced by the presence of a lanthanum oxide/hydroxide layer,

Coxide, an interfacial layer produced by the Fe-O-Si bonds that anchor the metal substrate to the

silane film, Cinterface, and the external silane layer, Ccoat respectively [32]. These time constants can also

be seen more conspicuously in figure 25. The value of impedance at low-frequency has dropped

considerably, implying a slow deterioration of the silane film, however values are still higher than

those produced by the bare sample. Unlike results seen in literature, the addition of lanthanum did

not cause in increase in Rpore over time; journals had attributed the increase in Rpore to the slow


39

release of dopants, trapped in the siloxane network, into actively corroding regions [29][37]. However,

one study did report that lanthanum ions have a tendency to migrate to the outer layers of the

coating; because of this, perhaps they were leached out of the film by the electrolyte before the

potentiodynamic EIS scan even took place, thus preventing them from exerting their corrosion-

inhibiting effects [32].

After 48 hours, the low-frequency impedance of the coated sample has fallen to that of the bare

sample (figure 27). The time constants that were previously seen at day 0 and day 1 no longer exist.

This suggests that film delamination has taken place. Literature has generally reported coatings to

have lasted considerably longer than 48 hours [19][31][32][34]. Excessive polishing during the surface

preparation is to blame; this will be covered in further detail in the errors and improvements

section.


The literature review has outlined the advantages that electrodeposition has over traditional dip-

coating methods. Electrodeposition was initially attempted with parameters akin to what had been

seen in journals, however the results published in the literature had been acquired with a platinum

mesh counter-electrode, while we only had access to a graphite electrode [16][17][18]. Looking at figure

28a and b, deposits of, what appear to be, graphite is seen on the metal surface. Reducing the

voltage from 5V to 2V did not stop our counter-electode from reacting. Unfortunately, the only film

characterization technique available was FTIR, which cannot detect graphite due to its homonuclear

molecular nature. Raman spectroscopy or energy-dispersive X-ray spectroscopy (EDX) will be

required to confirm the presence of graphite. This is one of the limitations of our research.

Errors, Improvements and Limitations

Surface preparation is crucial to achieve a microscopically rough surface, devoid of contaminants,

such that the silane film adheres firmly to the substrate. Unfortunately, we polished our surface with

0.5 micron polycrystalline diamond plates which made the surface excessively smooth and nullified

the benefits of polishing. A smooth surface meant that when dip-coating was performed vertically,

the effects of gravity would easily overcome the resistances imparted by the minimal surface

roughness we had, and thus, our film would pool at the bottom of the substrate. This explains why

there were uncorroded regions at the bottom of our samples used in the salt immersion test and

why we noticed some irregular results when performing video goniometry. As stated earlier,

polishing is an art rather than a science that takes a considerable amount of time to perfect.

Although we had made steady progress in the polishing technique through the weeks, we still had


40

not mastered it. Even if we had used a coarser grit paper when polishing, the polishing we would

perform on the surface would not be uniform and we would invariably observe scratches on the

surface under the microscope.

Limitations include inability to measure viscosity of the silane solution. Our silane films were not

viscous enough to be measured by the viscometer and would stick to the surfaces sample requiring

laborious cleaning before the next sample could be used. It has been reported in literature that the

addition of silica nanoparticles can induce changes in viscosity during the cross-linking process, and

this is how nanoparticles are able to improve the mechanical properties of the film [19]. We were not

able to verify this. Also, the BTSE silane solution provided by Sigma-Aldrich was only 96% pure, and

its container did not state what the remaining 4% consisted of. The unknown compounds may have

interfered with the FTIR results.

Nanoparticle distribution was not homogenous, and they would often sink to the bottom of the

container in which they were held. Because of this, we may have had wildly fluctuating

concentrations of nanoparticles in our final silane film. Dispersing agents might be able to solve this,

by forming bubbles around individual nanoparticles and preventing large aggregations from forming

and at the same time, creating buoyant forces that would prevent them from sinking to the base of

the container.

Raman spectroscopy, a characterization technique based on the inelastic scattering of light that is

monochromatic, could have been used to identify the presence of graphite in our electrodeposited

films. Scanning vibrational electrode technique (SVET) and AFM could have been used to unveil

further information about the corrosion process at the metal surface and whether added inhibitors

function cathodically and/or anodically.

Conclusion

Polishing is a critical step in surface preparation. A surface that is too smooth will not allow the dip-

coated film sufficient time to form hydrogen and subsequently covalent bonds. This results in films

that are significantly thicker at the base of the metal coupon. Microscopy revealed highly non-

uniform coatings and scratches present on the metal surface. These flaws were also seen in the

contact angle measurements, whereby certain coats did not follow the expected trends. FTIR

revealed the entities present, and showed that complete cross-linking had not taken place due to

the presence of hydroxide peaks belonging to silanols groups. To reduce the number of unreacted

silanol molecules, curing time should have been increased.


41

Nonetheless, BTSE silane-based treatments have been used to successfully improve corrosion

resistance on low-carbon steel surfaces. Thickness measurements revealed that the incorporation of

silica nanoparticles was effective at increasing film thickness and the lanthanum ions acted

synergistically with the nanoparticles to further increase thickness. The salt immersion tests results

were consistent with the aforementioned finding.

Low-frequency impedance values in Bode plots generated by EIS showed that the best results were

achieved in substrates that had been treated with BTSE containing both silica nanoparticles and

lanthanum ions. Although self-healing effects of lanthanum were not observed, this EIS results

suggest that the incorporation of rare-earth ions did improve the overall corrosion resistance of the

film. Indeed, rare earth cations are the next step forward towards creating silane films that actively

inhibit corrosion, just like chromate-based treatment.

Suggested Future Work

1) The limitations of dip-coating were apparent in our experiment. Because of this, additional

coating methods should be trialled. Electrodeposition with a platinum mesh counter-electrode

should be attempted, as should spin-coating, whereby excess coating on a substrate is rotated at

high speed, which in turn spreads the fluid over the surface through centrifugal forces.

2) Hardness tests should be incorporated to gauge the mechanical advantages conferred by of

nanoparticle-loaded silane films.

3) Our EIS results would have been of far greater benefit if empirical circuit modelling, which models

silane coats based on electrical circuits, was performed. Circuit modelling would have given us

quantitative values for capacitances and resistances in the film and how they change with time.

However, it is a technique that takes a considerable amount of time to master, even with the use of

complex software packages.

4) Literature has reported that lanthanum ions tend to distribute themselves to the external layers

of silane films, therefore double layer coatings – using a different silane such as GPTMS as the

topcoat – could prevent premature leaching of the lanthanum ions while increasing silane film

thickness [34].

5) Due to the novelty of our research, an optimum ratio of lanthanum ions and silica nanoparticles

has not been determined. Had our research taken place over a longer time span, we could have

varied the concentration of silica nanoparticles and lanthanum ions to deduce this optimal ratio.


42

6) If our treated metal was used to manufacture conduits for drinking water, the control of

microbiologically induced corrosion (MIC) is essential. Silver nanoparticle inhibitors have been found

to have long-lasting biocidal effects when subject to controlled release, and can be useful in

preventing the deleterious effects of microbial attack [46].

7) Because dip-coated silane films were often difficult to see visually, the addition of dyes such as

phenolphthalein would have allowed us to determine any significant non-uniformity in the coats

macroscopically. Furthermore, because the colour of phenolphthalein is sensitive to pH, we could

determine whether cathodic reactions (increases local pH) or anodic reactions at taking place at the

surface without conducting exhaustive physical tests.

8) With sufficient time, a systematic study on the effects of heat-curing at various temperatures and

duration on the mechanical properties of low-carbon steel and its significance on the deposited

silane film could be performed.


43

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