ANTICORROSION PROPERTIES OF EPOXY ZINC OIL PALM FROND CELLULOSE NANOCRYSTAL COMPOSITE COATING FOR
MILD STEEL CORROSION PROTECTION
NUR FATIN SILMI MOHD AZANI
UNIVERSITI SAINS MALAYSIA
2021
ANTICORROSION PROPERTIES OF EPOXY ZINC OIL PALM FROND CELLULOSE NANOCRYSTAL COMPOSITE COATING FOR
MILD STEEL CORROSION PROTECTION
by
NUR FATIN SILMI MOHD AZANI
Thesis submitted in fulfilment of the requirements for the degree of
Master of Science
March 2021
ACKNOWLEDGEMENT
First and foremost, I would like to extend my uttermost gratitude to my main supervisor, Dr. Mohd Hazwan Hussin for his continuous guidance, support and advice throughout this research. A greatest appreciation to the Institute of Graduate Studies, USM for the financial assistance provided to me via Fellowship Scheme throughout 2 years of my study. An utmost thanks to Universiti Sains Malaysia for the Research University Incentive, RUI grant-1001/PKIMIA/8011077 given to support this research.
I would also like to thank all the administrative, science officers and laboratory assistance at School of Chemical Sciences, Archaeology Research Centre USM, School of Physics, School of Biology, Science and Engineering Research Centre (SERC) and School of Mechanical Engineering for the assistance provided to me to complete my research’s analyses.
Last but not least, I would like to offer my sincere gratitude to all my laboratory members; Ms. Hanis, Mrs. Adilla, Ms. Zi Hui, Ms. Amirah, Mr. Majid and Mr.
Sherwyn for their help. Special thanks to my parents Mr. Mohd Azani Awang and Mrs. Salina Nawi for their highest support and encouragement to me. Thank you all.
TABLE OF CONTENTS
ACKNOWLEDGEMENT ... ii
TABLE OF CONTENTS ... iii
LIST OF TABLES ... vii
LIST OF FIGURES ... viii
LIST OF SYMBOLS ... xii
LIST OF ABBREVIATIONS ... xiv
ABSTRAK ... xvi
ABSTRACT ... xviii
CHAPTER 1 INTRODUCTION ... 1
1.1 Background of the study ... 1
1.2 Problem statement ... 3
1.3 Research objectives ... 5
1.4 Scope of studies ... 6
CHAPTER 2 LITERATURE REVIEW ... 7
2.1 Corrosion of mild steel ... 7
2.2 Corrosion measurement technique ... 12
2.2.1 Electrochemical impedance spectroscopy ... 13
2.2.2 Potentiodynamic polarization ... 19
2.2.3 Open Circuit Potential ... 22
2.3 Effect of corrosion and its cost ... 24
2.4 Corrosion control ... 27
2.5 Epoxy-Zn coating ... 29
2.6 Epoxy-Zn nanocomposite coating ... 33
2.7 Cellulose nanocrystal ... 35
2.8 The application of CNCs as a reinforcing material ... 39
2.9 The extraction of CNC ... 42
2.9.1 Biomass pre-treatment and post-treatment ... 42
2.9.2 Autohydrolysis pre-treatment ... 43
2.9.3 Bleaching and mercerization ... 47
2.9.4 Acid hydrolysis ... 48
2.10 Cellulose ... 52
2.11 Oil palm tree (Elaeis guineensis Jacq.) ... 58
CHAPTER 3 EXPERIMENTAL ... 65
3.1 Chemicals ... 65
3.2 Materials ... 65
3.3 Preparation of oil palm frond cellulose nanocrystal ... 66
3.3.1 Autohydrolysis pre-treatment ... 66
3.3.2 Pulp bleaching ... 69
3.3.3 Mercerization ... 69
3.3.4 Acid hydrolysis ... 70
3.4 Characterization of OPF-CNC ... 71
3.4.1 Fourier transform infrared analysis ... 71
3.4.2 CP/MAS 13C NMR analysis ... 71
3.4.3 X-ray diffraction analysis ... 72
3.4.4 Thermogravimetric analysis ... 72
3.4.5 Differential scanning calorimeter analysis ... 73
3.4.6 N2 Brunaeur-emmett-teller analysis ... 73
3.4.7 Scanning Electron Microscope analysis ... 73
3.4.8 Energy Dispersive X-ray Spectroscopy ... 73
3.4.9 Transmission Electron Microscopy analysis ... 74
3.5 Corrosion inhibition studies ... 74
3.5.1 Preparation of epoxy-Zn rich OPF-CNC coating ... 75
3.5.2 Preparation of mild steel substrate and corrosion testing media ... 76
3.5.3 Electrochemical cell and electrodes ... 76
3.5.4 Preliminary analyses ... 81
3.5.5 Electrochemical impedance spectroscopy ... 81
3.5.6 Potentiodynamic polarization analysis ... 82
3.5.7 Effect of temperature ... 83
3.5.8 Open circuit potential analysis ... 83
3.6 Surface studies on uncoated and coated mild steel ... 84
3.6.1 Attenuated Total Reflection analysis ... 84
3.6.2 Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy analysis ... 85
3.6.3 Salt spray analysis ... 85
3.6.4 Water contact angle ... 86
3.6.5 Atomic force microscopy ... 86
3.6.6 Hardness test ... 87
3.6.7 Adhesion Test ... 87
CHAPTER 4 RESULTS AND DISCUSSION ... 89
4.1 Chemical composition of OPF biomass and treated OPF ... 89
4.2 Characterizations of OPF-CNC ... 91
4.2.1 Fourier transform infrared spectroscopy ... 91
4.2.2 CP/MAS 13C NMR ... 94
4.2.3 XRD analysis ... 98
4.2.4 TGA analysis ... 100
4.2.5 DSC analysis ... 103
4.2.6 BET analysis ... 106
4.2.7 Scanning electron microscopy and Energy dispersive X-ray analysis ... 107
4.2.8 TEM analysis ... 112
4.3 Corrosion inhibition studies ... 114
4.3.1 Electrochemical impedance spectroscopy ... 114
4.3.2 Potentiodynamic polarization analysis ... 124
4.3.3 Effect of temperature ... 129
4.3.4 OCP analysis ... 134
4.4 Surface studies on uncoated and coated mild steel ... 138
4.4.1 Attenuated total reflection analysis ... 138
4.4.2 Scanning electron microscopy and Energy dispersive X-ray analysis ... 141
4.4.3 Salt spray ... 144
4.4.4 Water contact angle ... 148
4.4.5 Atomic force microscopy ... 151
4.4.6 Hardness test ... 154
4.4.7 Adhesion test ... 156
CHAPTER 5 CONCLUSION AND FUTURE RECOMMENDATION ... 159
5.1 Conclusion ... 159
5.2 Future recommendations ... 163
REFERENCES ... 164 APPENDICES
LIST OF PUBLICATION AND CONFERENCE
LIST OF TABLES
Page Table 2.1 Extraction of CNC from various biomass sources including
their process and CNC dimension ... 37
Table 2.2 The reaction conditions of autohydrolysis for various biomass ... 44
Table 2.3 Extraction of CNC from variety of cellulosic source by using different type of acid hydrolysis and their crystallinity index ... 49
Table 2.4 The chemical composition (wt.%) of the oil palm biomass ... 64
Table 4.1 Thermal properties of OPF-CNC and a-cellulose ... 102
Table 4.2 BET summary for OPF-CNC and a-cellulose ... 107
Table 4.3 Electrochemical impedance parameters for uncoated mild steel, mild steel coated with epoxy-Zn with and without the inclusion of OPF-CNC and commercial zinc chromate coated mild steel in 3.5 wt% NaCl at 298 K ... 121
Table 4.4 Potentiodynamic polarization parameters for uncoated mild steel, epoxy-Zn coated mild steel with and without inclusion of OPF-CNC, and commercial zinc chromate in 3.5 wt% NaCl at 298 K ... 128
Table 4.5 Effect of temperature parameters on the corrosion of uncoated mild steel (blank) and coated mild steel (E-Zn- 0.5) in 3.5 wt % NaCl solution ... 130
Table 4.6 The percentage of an element for uncoated mild steel (blank) and coated mild steels ( E-Zn and E-Zn-0.5) obtained using EDX analysis ... 142
Table 4.7 The maximum depth and hardness values for the commercial zinc chromate paint and epoxy-Zn coating without addition of OPF-CNC and with addition of 0.5 wt % OPF-CNC acquired using nanoindentation analysis. ... 156
LIST OF FIGURES
Page Figure 2.1 Corrosion of mild steel in an atmospheric marine
environment ... 9
Figure 2.2 The sinusoidal wave signal of excitation potential and measured current response ... 15
Figure 2.3 A typical Nyquist plot for a simple corrosion process ... 17
Figure 2.4 a) Randle’s equivalence circuit for a typical Nyquist plot and b) Warburg’s equivalence circuit for diffusion control- type corrosion ... 18
Figure 2.5 A typical Bode plot for a simple corrosion process ... 18
Figure 2.6 A typical Tafel polarization curve ... 20
Figure 2.7 Open circuit potential profile for epoxy-Zn nano-clay nanocomposite coating showing cathodic protection and barrier protection as function of immersion time ... 23
Figure 2.8 The cost of corrosion in industry categories ... 25
Figure 2.9 Epoxide ring of epoxy resin ... 30
Figure 2.10 A proposed polymerization reaction of epoxy ... 30
Figure 2.11 A proposed zinc cathodic protection mechanism ... 32
Figure 2.12 A proposed corrosion protection mechanism of epoxy-Zn with the presence of SPANI nanofibers ... 34
Figure 2.13 TEM micrographs of a) garlic straw b) tunicin c) wood d) ramie e) tomato peel and f) maize straw ... 38
Figure 2.14 A proposed pathway mechanism of polymer matrix with and without addition of CNC (refer as crystal) ... 40
Figure 2.15 The schematic diagram of effect of pre-treatment on the cellulose structure of lignocellulosic biomass ... 45
Figure 2.16 A proposed mechanism during auto-hydrolysis process ... 45
Figure 2.17 Hydrolytic mechanism of cellulose into cellulose nanocrystalline using acid hydrolysis process ... 50
Figure 2.18 The formation of negatively charged sulfate group on the CNC surface after hydrolysis process ... 52
Figure 2.19 The structural level of cellulose from its sources to the
macromolecules of cellulose ... 54 Figure 2.20 a) chemical structure of cellulose and b) isolation of
cellulose nanocrystal from cellulose chain ... 55 Figure 2.21 The intramolecular (blue line) and intermolecular (red line)
hydrogen-bond between the same and neighbouring of
cellulose chain ... 56 Figure 2.22 The statistical trend of oil palm plantation’s expansion and
average production of fruit fresh bunches ... 60 Figure 2.23 The main biomass produced by oil pam tree ... 61 Figure 3.1 The preparation of oil palm frond cellulose nanocrystal
(OPF-CNC) ... 67 Figure 3.2 Research flow chart of preparation of OPF-CNC and its
characterization along with the corrosion studies for mild
steel’s corrosion protection ... 68 Figure 3.3 The preparation of epoxy-Zn coating with addition of OPF-
CNC ... 78 Figure 3.4 The schematic diagram of epoxy-Zn-OPF-CNC coating
formulation and the illustration of OPF-CNC incorporated
onto the coating layer as a reinforcing filler ... 79 Figure 3.5 A) The experimental set up for the electrochemical
analyses (corrosion resistance measurement) via potentiostat (GAMRY reference 600) for coated and uncoated mild steels and B) The jacketed three-electrode
cell system used for the electrochemical analyses ... 80 Figure 3.6 The adhesion test scale ... 88 Figure 4.1 The percentage yield of chemical composition of the
treated OPF and the untreated OPF ... 91 Figure 4.2 The FTIR spectra of (a) OPF, (b) OPF-CNC and (c) a-
cellulose ... 92 Figure 4.3 The CP/MAS 13C NMR spectra for (A) OPF-CNC (B) a-
cellulose ... 97 Figure 4.4 X-ray diffractograms of (a) OPF, (b) a-cellulose and (c)
OPF-CNC ... 99 Figure 4.5 TGA and DTG thermograms of OPF-CNC and a-cellulose
... 101
Figure 4.6 DSC thermograms of OPF-CNC and a-cellulose ... 105 Figure 4.7 SEM micrograph at 500x magnification of (A) OPF-CNC
and (B) a-cellulose ... 110 Figure 4.8 EDX spectra of OPF-CNC (Spectrum 1) and a-cellulose
(Spectrum 2) ... 111 Figure 4.9 TEM image of OPF-CNC at 16000 X magnification ... 113 Figure 4.10 The Randle’s CPE equivalent circuit for the uncoated mild
steel (Model A) ... 115 Figure 4.11 The proposed equivalent circuit to fit the Nyquist plot of
EIS data for the coated mild steels (neat epoxy-Zn coating, epoxy-Zn-OPF-CNC coating and commercial zinc
chromate coating) (Model B) ... 115 Figure 4.12 Nyquist plot for epoxy-Zn rich coated mild steel with and
without the inclusion of OPF -CNC and commercial zinc
chromate coated mild steel in 3.5 wt% NaCl at 298 K ... 120 Figure 4.13 Corrosion illustration of epoxy-Zn without addition of
OPF-CNC ... 122 Figure 4.14 Corrosion illustration of epoxy-Zn coating with the
addition of OPF-CNC ... 123 Figure 4.15 Tafel plots for bare mild steel, epoxy-Zn coated mild steels
with and without the inclusion of OPF-CNC , and commercial zinc chromate coated mild steel in 3.5 wt%
NaCl at 298 K ... 127 Figure 4.16 Arrhenius plot for coated mild steel (E-Zn-0.5) in 3.5 wt %
NaCl solution at different temperatures ... 131 Figure 4.17 Arrhenius plot for uncoated mild steel (blank) in 3.5 wt %
NaCl solution at different temperatures ... 132 Figure 4.18 The evolution of open circuit potential for the uncoated
mild steel (blank) and coated mild steels (Commercial zinc chromate, E-Zn and E-Zn-0.5) in 3.5 wt % NaCl solution
at different time interval for a 12-h immersion at 298 K ... 136 Figure 4.19 ATR spectra for epoxy-Zn coating with and without
addition of OPF-CNC ... 140 Figure 4.20 SEM micrograph with 500 x magnification and EDX
spectra of A) uncoated mild steel B) mild steel coated with epoxy-Zn coating (E-Zn) and C) mild steel coated with epoxy-Zn with addition of (E-Zn-0.5) after a 12-h OCP
analysis ... 143
Figure 4.21 The images before and after a 24 h salt spray analysis for
the coated mild steels ... 145 Figure 4.22 The image of water drop on the surface of A) uncoated
mild steel B) mild steel coated with epoxy-Zn (E-Zn) C) mild steel coated with epoxy-Zn with addition of 0.5 wt % OPF-CNC (E-Zn-0.5) and D) mild steel coated with
commercial zinc chromate paint at 298 K ... 150 Figure 4.23 AFM micrograph of the uncoated mild steel and the coated
mild steel (E-Zn-0.5) after a 7-day immersion in 3.5 wt %
NaCl solution at 298 K ... 152 Figure 4.24 The before and after image of a pull- off adhesion test for
the coated mild steels ... 158
LIST OF SYMBOLS
Cdl Double layer capacitance
CPE Constant phase element
CPEcoat Coating constant phase element CPEdl Double layer constant phase element
CR Corrosion rate
Ea Activation energy
Ecorr Corrosion potential
F Faraday constant
g Gram
h Hour
icorr Corrosion current density
IE Inhibition efficiency
Mew Equivalent weight
mg Milligram
min Minute
mL Millilitre
mmy-1 Millimetres per year
mpy Mils of penetration per year
mV Milivolt
R Universal gas constant
Rcoat Coating resistance
Rct Charge transfer resistance
Rp Polarization resistance
Rs Solution resistance
v/v Volume per volume
wt Weight
Z Magnitude of the impedance
ba Anodic Tafel constant
bc Cathodic Tafel constant
DH Enthalpy
h Overpotential
q Phase angle
w Angular frequency
r Density
µA cm-2 Microampere per centimetre square
LIST OF ABBREVIATIONS
Cdl Double layer capacitance
FTIR Fourier Transform infrared
AC Alternating current
AFM Atomic force microscopy
ATR Attenuated total reflection
BET N2 Brunauer-Emmett-Teller
CE Counter electrode
CNCs Cellulose nanocrystals
CP/MAS 13C NMR Solid state 13C cross polarization/magnetic angle spinning nuclear magnetic resonance
DSC Differential scanning calorimeter
DTG Derivatized thermogravimetry
EDX Energy dispersive X-ray
EIS Electrochemical impedance spectroscopy
FTIR Fourier transform infrared
IE Inhibition efficiency
OCP Open circuit potential
OPF Oil palm frond
PD Potentiodynamic polarization
RE Reference electrode
SCE Saturated calomel electrode
SEM Scanning electron microscope
TEM Transmission electron microscopy
TGA Thermogravimetric
WE Working electrode
XRD X-ray diffraction
SIFAT ANTI-KAKISAN PELITUP EPOKSI ZINK SELULOSA
NANOKRISTAL KOMPOSIT BAGI PERLINDUNGAN KAKISAN KELULI LEMBUT
ABSTRAK
Pelitup epoksi zink (epoksi-Zn) telah digunakan secara meluas sebagai perlindungan kakisan. Walau bagaimanapun, keliangan tinggi epoksi-Zn menyebabkan kelemahan kepada sifat perlindungan salutan. Oleh itu, pengubahsuaian pelitup epoksi-Zn adalah sangat penting. Kajian ini tertumpu kepada kesan pelepah kelapa sawit selulosa nanokristal (OPF-CNC) sebagai pengisi penguat untuk mempertingkatkan sifat perlindungan pelitup epoksi-Zn. Dalam kajian ini, OPF biojisim telah digunakan untuk menghasilkan OPF-CNC menggunakan hidrolisis H2SO4. OPF biojisim pada permulaanya telah dirawat menggunakan rawatan awal autohidrolisis. Analisis permukaan menggunakan mikroskop elektron penghantaran (TEM) mengesahkan bahawa OPF-CNC adalah pengisi bersaiz nano dengan purata panjang dan diameter masing-masing iaitu 95.09 dan 6.81 nm. Prestasi anti-kaksian pelitup epoksi-Zn dan pelitup epoksi-Zn yang diubah suai telah dianalisis menggunakan analisis elektrokimia melalui spektroskopi impedens elektrokimia (EIS), polarisasi potensiodinamik (PD) dan potensi litar tertutup (OCP). Prestasi anti- kakisan dan sifat perlindungan pelitup epoksi-Zn yang diubah suai (E-Zn-0.5) dibandingkan dengan cat komersial zink kromat. Analisis menunjukkan E-Zn-0.5 memberikan kecekapan perencat tertinggi (99 %) berbanding cat komersial (70 – 80
%). Berdasarkan kajian keterbasahan, E-Zn-0.5 menunjukkan sifat hidrofobik tertinggi (100.5 ± 0.70°) berbanding cat komersial (91.04 ± 1.20°). Ujian kekerasan
membuktikan kekerasan salutan E-Zn-0.5 lebih tinggi berbanding cat komersial, iaitu masing-masing 0.61 dan 0.25 GPa. Maka, peningkatan prestasi perlindungan kakisan oleh epoksi-Zn dengan penambahan OPF-CNC membuktikan potensi sisa buangan OPF sebagai alternatif bahan hijau baharu.
ANTICORROSION PROPERTIES OF EPOXY ZINC OIL PALM FROND CELLULOSE NANOCRYSTAL COMPOSITE COATING FOR MILD
STEEL CORROSION PROTECTION
ABSTRACT
Epoxy zinc (epoxy-Zn) coating has been widely used as corrosion protection.
However, the high porosity of epoxy-Zn is tend to drawback the barrier properties of the coating. Thus, the surface modification of epoxy-Zn is imperative. This study focused on effect of oil palm frond cellulose nanocrystal (OPF-CNC) as a reinforcing nanofiller to improve epoxy-Zn barrier properties. In this work, OPF biomass was utilised in preparing OPF-CNC by H2SO4 hydrolysis. OPF was initially pre-treated with autohydrolysis pre-treatment. The surface analyses via transmission electron microscopy (TEM) confirmed that the OPF-CNC is a nanofiller with an average length and diameter of 95.09 and 6.81 nm, respectively. The anticorrosive performance of epoxy-Zn coating and modified epoxy-Zn coating was investigated using electrochemical analyses via electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PD) and open circuit potential (OCP). The anticorrosive performance and surface properties of modified epoxy-Zn (E-Zn-0.5) were compared with commercial zinc chromate paint. It was revealed that E-Zn-0.5 has higher inhibition efficiency (99 %) than commercial paint (70 – 80 %). Based on wettability study, E-Zn-0.5 shows the highest hydrophobicity (100.5 ± 0.70°) than commercial paint (91.04 ± 1.20°). The hardness test revealed that the coating’s hardness value for E-Zn-0.5 is higher than commercial paint of 0.61 and 0.25 GPa, respectively. Thus, the improved corrosion protection properties of epoxy-Zn with the addition of OPF-
CNC proof the potential of OPF biomass waste to be utilized as an alternative in renewable green materials.
CHAPTER 1 INTRODUCTION
1.1 Background of the study
Mild steel is known as low carbon steel composed of a high content of iron (Fe).
Commonly, mild steel is the most used as industrial metallic material [1]. It has been used as a raw material in the fabrication of equipment such as industrial processing tanks, pipes, wires, military equipment and automobile [2]. In the construction sector, mild steel is highly used as a structural steel. Mild steel possesses distinctive properties such as good mechanical strength, low cost and abundance [3, 4]. However, manufactured mild steel is highly exposed to an aggressive environment during its storage and transportation period [2]. In an atmospheric environment, mild steel is exposed to high polluted and an aggressive environment. An aggressive environment is referring to the high-temperature corrosive environment. Consequently, it will lead to severe destruction to its metallic properties due to the corrosion activity [3]. This will eventually decrease its mechanical strength as well as the quality of metal. Thus, metallic corrosion is a global challenge and concern to many industries. The corrosion prevention should be the main priority in mitigating the economic loss due to the corrosion effect.
The application of protective coating is the most fundamental and reliable approach to prevent the corrosion of mild steel. High durability and high corrosion inhibition efficiency should be the key properties for the designed coating. Epoxy zinc (epoxy-Zn) coating has monumental properties to protect the mild steel against
corrosive environment at maximum potential. It possesses high cross-linking density, high chemical stability, good thermal stability, and good mechanical properties [5-7].
Epoxy resin acts as a primary barrier to protect the mild steel surface. The addition of zinc into the epoxy resin is aimed to provide secondary protection to the mild steel by scarifying itself as a sacrificial anode [8]. Theoretically, Zn is more reactive than Fe. Hence, Zn is more susceptible to corrode in comparison to the Fe in the corrosive electrolyte environment. However, past studies demonstrated that the epoxy-Zn coating has high possibility to become fragile when exposed to an aggressive environment for an extended period [9, 10]. Its fragility is due to its high porosity, which might cause a considerable damage to the coating’s protective barrier [11-13]. This limitation can be improved by incorporating a nanofiller as a reinforcing material into the epoxy-Zn matrices. It is reported to be an effective approach to improve the properties of epoxy-Zn coating. A nanofiller derived from the organic composite is able to reconstruct the epoxy-Zn network via a strong hydrogen bond to reinforce the epoxy-Zn coating system [14].
The utilisation of CNC as a reinforcing filler in polymer matrices has positively received scholarly attention [15-18]. This is due to the unique characteristics of CNC, which are biocompatible, large specific surface area, high aspect ratio, high crystallinity, high thermal stability, and good mechanical properties [19, 20]. These unique characteristics, possessed by CNC, classify it as the best choice as a reinforcing filler to improve epoxy-Zn matrices. In terms of chemical bonding, CNC network was covered by an abundant of hydroxyl groups [21]. The hydroxyl groups provide the high possibility of active sites for chemical bonding between OPF-CNC and epoxy- Zn matrices. A study by Ma et al.[22] reported an improvement in thermal stability,
mechanical, and barrier properties for the modified epoxy which was reinforced with CNC.
OPF is abundantly available from palm plantations. OPF is a carbohydrate-rich material composed of cellulose, hemicellulose and lignin [23]. Interestingly, as a lignocellulosic biomass which is highly fibrous and spongy, OPF contains 40-43 % of cellulose [24, 25]. Cellulose is a semi-crystalline polysaccharide which can be found in plant’s cell wall [21]. The cellulose fibrils act as reinforcing phase to strengthen the plant structure [26]. Cellulose has good mechanical strength and high accessibility to -OH groups, which are essential for further surface modification and chemical bonding [27]. Despites the high content of cellulose, OPF is a promising fiber source to extract cellulose nanocrystal (CNC). The past work by Dungani et al.[20] and Nordin et al.[25] reported the extraction of CNC from OPF biomass via acid hydrolysis. The utilisation of chemical pre-treatment is also reported [20, 25]. The pre-treatment is indeed crucial to obtain a high crystalline CNC. The removal of non-crystalline fragments of cellulose fiber (hemicellulose and lignin) can be achieved via pre- treatment. Moreover, autohydrolysis is an environmental friendly pre-treatment, where the biomass is pre-treated with water at an elevated temperature.
1.2 Problem statement
Corrosion is defined as the destruction of metallic properties of metal [28]. The metallic corrosion has become a worldwide challenge and concern to many industries.
Without any prevention initiatives, corrosion will affects the country’s economy as it will require very high cost to overcome this problem. National Association of Corrosion Engineers (NACE) International has released a report on the cost of corrosion, which is estimated to be US$2.5 trillion globally [29]. At the nation level,
Malaysia was reported spending an average of RM 6.7 billion annually for the cost of corrosion effect covering most of the economies sector [30]. Additionally, corrosion also might endangers human life. In 2006, a pedestrian bridge in Raleigh, North Carolina collapsed, which injured more than 100 people and some were in critical condition [31]. It was reported that 11 steels cables which supported the bridge were highly corroded due to the excessive amounts of calcium chloride, which is highly corrosive to the steel structure. Thus, it is imperative to minimise the corrosion effect via efficient corrosion control such as application of coating on the steel-substrate.
The conventional coating such as chromate-based coating is usually used as protective coating for the steel substrate. It is widely available in the market and possesses good anticorrosive properties. However, the use of chromate-based coating has been limited due to its high toxicity. Hence, epoxy-Zn coating that is less toxic and has good anticorrosive performance can be employed to protect the metal from corroding. While, the problem lies in the fragility of epoxy-Zn coating for the long- immersion periods in corrosive environment. This has limit the superior mechanical strength of epoxy-Zn to prevent the penetration of water and corrosive ions into the coating network. Thus, the coating modification is indeed crucial to improve epoxy- Zn properties. The introduction of CNC as a reinforcing filler has shown satisfactory result to improve the epoxy coating properties [22]. The production of nanofiller as a reinforcing material derived from biowaste is highly encouraged. This indicates a greener approach to prepare a less toxic composite coating.
The oil palm tree (Elaeis guineensis Jacq.) planted in Malaysia accounts for 11
% of the world’s oils and fats production [32]. Thus, palm oil industry is imperative for the growth of Malaysia’s economy. In the past years, the government has taken a big step to rapidly expand and increase the oil palm‘s plantation [33]. Consequently,
the rapid growth of plantation has generated a vast amount of biomass. Loh and Choo [34] reported that the oil palm tree only produced 10 % of oil whereas the remaining of 90 % is accounted as biomass. From the plantations, oil palm frond (OPF) constitutes approximately 75 % of total biomass. The OPF commonly will be left- overs to naturally decompose on the ground, which is useful for soil fertilization. It is important to fully utilise the waste to produce a beneficial product. Therefore, the utilisation of OPF biomass to produce CNC is the best approach.
1.3 Research objectives
The present study modifies the epoxy-Zn coating properties for the mild steel corrosion protection, by incorporating OPF-CNC as a reinforcing filler derived from OPF biomass. Thus, the objectives of this research are:
1) Preparation of OPF-CNC from OPF biomass and characterization of the obtained OPF-CNC using complementary analyses to study the effect of autohydrolysis pre-treatment on the physicochemical properties of OPF-CNC.
2) Analyze the corrosion protection performance of the unmodified epoxy-Zn coating and modified epoxy-Zn coating at varying loading ratio of OPF-CNC via electrochemical measurement and surface analyses.
3) Comparison of anticorrosive performance of the commercial zinc chromate paint with the modified epoxy-Zn OPF-CNC coating.
1.4 Scope of studies
The present study highlights the modification of epoxy-Zn coating by incorporation of OPF-CNC to enhance the corrosion inhibition efficiency of the coating. The low cost and environmental friendliness of OPF-CNC was used as a reinforcing filler to improve the epoxy-Zn coating properties. The study reported the preparation of OPF-CNC derived from OPF biomass via acid hydrolysis. The effectiveness of autohydrolysis pre-treatment to improve the OPF-CNC properties was reported. The characterizations of OPF-CNC was conducted to identify the key properties of OPF-CNC as a good reinforcing filler.
The effect of loading ratios of OPF-CNC into epoxy-Zn formulation was investigated by calculating the corrosion inhibition efficiency of the coatings via electrochemical analyses. The corrosion study also covers the effect of temperature on corrosion, potential stability of the coated substrates and surface analyses of the coatings. The obtained results are utilize to propose the corrosion protection mechanism of the unmodified and modified coatings. The present study also includes a comparison between the commercial zinc chromate paint and the modified coating to evaluate the corrosion inhibition efficiency via electrochemical and surface analyses.
CHAPTER 2
LITERATURE REVIEW
2.1 Corrosion of mild steel
The definition of corrosion is originally derived from the Latin word “corrodere”
that means “to gnaw to pieces” [28]. Generally, corrosion is a destruction of metal by chemical or electrochemical reaction with its surrounding. The destruction of metal refers to the deterioration of metal and its metallic properties. Meanwhile, the surrounding refers to the condition that is in contact with a metal. For examples, in the marine environment, metallic corrosion is caused by its reaction with chemical composition, temperature, pH and concentration of seawater. The severity of metallic corrosion is controlled by several condition factors such as physical state (solid, liquid or gas), chemical composition (constituent and concentration) and temperature [3].
In nature, metallic corrosion occurred by chemical reaction of water and oxygen with metal. In the industrial sector, metallic corrosion is commonly caused by electrochemical reaction in the presence of corrosive electrolyte at elevated temperature condition. On the other hand, rusting also one of descriptive for corrosion.
Rusting is more specific definition for the corrosion of iron or iron-base alloys by formation of hydrous ferric oxides as the main corrosion products [35]. The common types of corrosion are pitting corrosion, crevice corrosion, intergranular corrosion, galvanic corrosion and stress corrosion.
Mild steel is a versatile engineering material that has been used widely especially in the construction sector. Owning to its immense mechanical strength, malleability and low cost, it has become a favourable raw material in building structure, piers and pipeline [36, 37]. Moreover, mild steel also possesses good machinability to be used as a hull structure in the fabrication of boat and ship. The concern on the corrosion issue of steel in the marine environment has escalated to a greater extent. Indeed, metallic structure used in offshore oil platforms, ships, pipeline planted in the seawater and building structure near the coast were exposed to high vulnerability of uncontrolled atmospheric corrosion process [36]. Hence, the study on the corrosion process of mild steel is vital to reduce the further loss on corrosion effect to the industries. A various study has been done to investigate the corrosion reaction of mild steel in the marine environment by using 3.5 wt % of NaCl solution as an artificial seawater [35, 38-41].
In this study, the corrosion process of mild steel is governed by an electrochemical reaction. In electrochemical reaction, electrolyte act as a medium for electron transfer from anode to cathode. The oxidation and reduction reaction occurred simultaneously at anode and cathode, respectively (Figure 2.1). Figure 2.1 shows the illustration of the corrosion process of mild steel in an atmospheric marine environment.
Figure 2.1: Corrosion of mild steel in an atmospheric marine environment [42]
Generally, the corrosion process of mild steel is a complex process that involves three main stages of rust formation. Firstly, the formation of stable thin oxide/hydroxide layer on the mild steel surface. This is due to the reduction reaction at the cathode. Upon exposure to the open air space, oxygen is dissolved in water and subsequently is reduced rapidly to form hydroxide ion (OH-) as shown in Equation 2.1 [12]. Simultaneously, an oxidation process occurred at the anode as shown in Equation 2.2 and 2.3. Initially, Fe (iron) is oxidised to Fe2+ and then further oxidized into Fe3+
ions [43].
Cathodic reaction:
H2O + !" O2 + 2e ® 2OH- (2.1)
Anodic reaction:
Fe ® Fe2+ + 2e (2.2)
Fe2+ ® Fe3+ + e (2.3)
The available Fe2+/ Fe3+ ions are free to migrate through an aqueous solution to react with OH- cations to form oxide/hydroxide intermediate rust. Apparently, the green precipitate (intermediate rust) will form on the mild steel surface due to the reaction of Fe2+ ions with OH- as shown in Equation 2.4 [44]. Green rust is an indicative of the formation of an intermediate corrosion product, which is iron (II) hydroxide (Equation 2.4). The formation of ferrous hydroxide is known as the second stage of corrosion process. As an intermediate product, ferrous hydroxide can further reacts with oxygen to form brown-colour rust (iron oxide) on the mild steel surface (Equation 2.5) [44]. Besides, Zou et al.[45] suggested that Fe3+ ions can chemically reacts with OH- to form iron (III) hydroxide and then subsequent oxide/hydroxide product such as FeOOH and Fe2O3 are also generated (Equation 2.6 - 2.8).
Fe2+ + 2OH- ® Fe(OH)2 (2.4)
4Fe(OH)2 + O2 ® 4H2O + 2Fe2O3.H2O (2.5)
Fe3+ + 3OH- ® Fe(OH)3 (2.6)
Fe(OH)3 ® FeOOH + H2O (2.7)
4Fe + 3O2 ® 2Fe2O3 (2.8)
In an alkaline solution of 3.5 wt % NaCl, the Na+ and Cl- ions are highly available. The present cation and anions are responsible to enhance the transportations
of electrons in electrolytes. The increment of electrons transfer results in accelerating corrosion process. Hence, Fe2+ ions are available to react with Cl- or OH- ions to form FeCl2 or ferrous hydroxide (Equation 2.9 and 2.10) [46]. In addition, the reaction of Fe3+ ions with Cl- and OH- ions lead to the formation of mixed complexes precipitate of hydroxychloride salt such as Fe2Cl(OH)2 and FeClO.
Fe2+ + 2Cl- ® FeCl2 (2.9)
FeCl2 + 2OH- ® Fe(OH)2 + 2Cl- (2.10)
Furthermore, in an observation study of corrosion of mild steel in seawater by Zou et al.[45], the findings have shown that the rust layer consisted of mixed compound after 6 weeks and 18 weeks immersion time. It was revealed that after 6 weeks of immersion period, the rust layer consisted of mixed compound which are a- FeOOH and b-FeOOH as the major constituent in rust formation. After 18 weeks, the outer layer of rust was found to contain only g-FeOOH. However, the inner layer of rust consist of mixed of b-FeOOH , g-FeOOH and Fe3O4. For example, the formation of magnetite (Fe3O4) is shown in Equation 2.11. Leygraf et al.[46] proposed that the beginning of the third stage of rust development is indicated by the formation of mineral lepidocrocite (g-FeOOH).
Fe2+ + 8FeOOH + 2e ® 3Fe3O4 + 4H2O (2.11)
Hence, it can be concluded that the capability of Fe (iron) to be readily oxidized to Fe2+/ Fe3+ ions play a significant role for the formation of mixed Fe2+/ Fe3+ complex.
Besides, the related research on environment factors such as temperature, wetness
period and dissolved oxygen done by Melchers et al.[47], Schindelholz et al.[48] and Leygraf et al.[46] , respectively divulged that the surrounding affects the development of rust. According to Melchers et al.[47], by exposing the mild steel surface to a high- temperature condition (70 – 80 °C) resulted in rapid corrosion process. Similarly, Schindelholz et al.[48] divulged that the longer the period of metal surface exposed to an aqueous solution (wetness period), the higher the corrosion rate. According to Leygraf et al.[46] , dissolved oxygen significantly accelerates the formation of rust on the mild steel surface as shown in Equation 2.5. Over the time, the formation of thick rust precipitate prevent the dissolved oxygen to approach the steel surface. It was revealed that the concentration of oxygen is diminished at the steel base as the rust precipitation thicken [38]. In addition, at severe high-temperature condition (above 80
°C) , the solubility of oxygen in water is restricted. As a result, the corrosion process is slow down due to the formation of rust layer and low solubility of oxygen. Thus, it can be proposed that the corrosion process can be restricted by protecting mild steel surface from its environment.
2.2 Corrosion measurement technique
Electrochemical technique is a well-known method to forecast corrosion process. A number of studies have reported the evaluation of coating performance during corrosion process by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PD). EIS and PD are the most common electrochemical techniques used to evaluate anticorrosion performance of inhibitor and coating. The electrochemical corrosion parameters generated by EIS and PD is useful to comprehend corrosion inhibition mechanism.
2.2.1 Electrochemical impedance spectroscopy (EIS)
EIS has been used as an important tool in massive number of corrosion control research by electrochemist. In practice, EIS has been proved to be one of useful technique in electrochemical analysis for corrosion characterization and prediction.
EIS is a non-destructive method at which the analysis can be done repeatedly by using the same sample. The capability of EIS to study the high-impedance system make it as one of the standard research tool to evaluate corrosion inhibition mechanism for coating/metal system[45, 49].
The concept of impedance measurement in EIS system correlates with the basic theory of electrical resistance introduced by Ohm’s law. Generally, Ohm’s law depicts that resistance is inversely proportional to current. Resistance (R) is represented as ratio of voltage (V) and current (I) as shown in Equation 2.12 .
R = !
" (2.12)
In the corrosion process, the electrical current flow enhance the degradation of metal surface. A high current resistance is required by a coated metal to inhibit metal dissolution. The electrical resistance of working electrode (coated metal) can be measured by using EIS method in terms of impedance measurement. EIS data is analysed by using equivalent circuit. Thus, impedance which is known as general circuit parameter is used to replace the basic concept of resistance. This is due to the consideration of a more complex behaviour of circuit elements in an electrochemical system. The complex behaviour of circuit element refers to the existence of
capacitance, inductance and mass diffusion properties of designed equivalent circuit.
Impedance element is represents as Z’ and Z” which expressing real and imaginary component of impedance respectively as shown in Equation 2.13.
Z = !
" (2.13)
where Z= Z’ + Z”
In the EIS system, impedance is measured by using three electrode-cell immersed in electrolyte that consist of working electrode (WE), counter electrode (CE) : Pt plate and reference electrode (RE) : saturated calomel electrode (SCE) connected with potentiostat [50]. During EIS analysis, a small-amplitude alternating current (AC) signal is applied to an electrochemical cell to measure the current flow through the electrode-electrolyte interfere. In other word, a small magnitude of sinusoidal potential in the range of 5 mV - 50 mV is applied to the electrochemical interfere over varying range of frequency, 10-2 – 105 Hz as an input signal [49]. The output generated is an impedance response at which current is measured at each of frequency applied. The obtained AC current response is useful to obtain resistance and capacitance values. In linear system, the impedance response is represents as the sine wave receiving the same frequency of applied sinusoidal potential (excitation signal) but shifted in phase as shown in Figure 2.2.
Figure 2.2: The sinusoidal wave signal of excitation potential and measured current response [51]
Hence, the output of impedance response acquiring the excitation frequency generates an impedance data. From the sine wave diagram shown in Figure 2.2, the relationship of the voltage with the impedance and the current can be expressed as equations below:
E (t) = I (t). Z (w) = Eo sin (wt) (2.14)
I (t) = Io sin (wt + q) (2.15)
where Eo and Io are the magnitude of potential and current, respectively. Meanwhile w and q are angular frequency and phase shift angle, respectively. The angular frequency can be expressed as w = 2pƒ. The phase shift angle (q) between excitation and current response is related with impedance and can be calculated by using Equation 2.16:
tan q = #$"
$" (2.16)
The EIS data can be interpreted by using an electrical equivalence circuit (ECC). According to Chang et al.[52], ECC can be utilized to represent the electrochemical behaviour of an electrode-electrolyte interfere. The Randles’s equivalence circuit is commonly used to represent the electrochemical interfere corresponding to a simple corrosion process. However, the interpretation of EIS data based on ECC is varied based on the several parameters. The parameters involving the electrolyte (temperature, concentration of electrolyte and inhibitor present) and type of coating used on the working electrode influence the corrosion process occurs in an electrochemical cell [53]. The EIS data also can be interpretated based on ECC with an additional element of Warburg impedance. This is due to the occurrence of complex corrosion behaviour influenced by the diffusion process. The designed circuit can be used to explain the complex electrochemical reaction due to the combination of kinetic and diffusion process [49].
The generated impedance data can be viewed in two type of plots which are Nyquist plot and Bode plot. The impedance data generated is represented as vector quantities of Z involving of real, Z’ and imaginary, Z” element of impedance. The impedance elements are frequency-dependent and corelated with the phase shift angle as shown in equation 2.16. The frequency-dependent impedance can be expressed as Z (w) = !(')
"(') .
Nyquist plot represents the impedance data as a graph of imaginary part ,-Z”
against real part Z” at the varying frequencies as shown in Figure 2.3. Based on the semicircle diagram, the solution resistance (Rs) values can be directly obtained from the abscissa at high-frequency response when angular frequency, w = 2pƒ approaches an infinite value (ƒmax or ƒ ® ¥). On the other region of low -frequency response when angular frequency approaches zero (ƒmin or ƒ ® 0), the total resistance can be readily
obtained from the abscissa. Total resistance is the sum of solution resistance (Rs) and charge transfer resistance or polarization resistance (Rct or Rp).
Figure 2.3: A typical Nyquist plot for a simple corrosion process [49]
EIS data represents by a perfect semicircle of the Nyquist plot can be well- fitted with Randle’s equivalence circuit in Figure 2.4 (a). As a result, the value of solution resistance (Rs), charge transfer resistance or polarization resistance (Rct or Rp), double layer capacitance (Cdl) and diffusion impedance (W) can be obtained. Cdl is a non-faradaic component and can be calculated by using equation 2.17 [49] :
Cdl = )
*+,#$%× .& (2.17)
Figure 2.4: a) Randle’s equivalence circuit for a typical Nyquist plot and b) Warburg’s equivalence circuit for diffusion control-type corrosion [49]
Alternatively, EIS data also can be represented as a Bode plot. In a bode plot, impedance element, Z is converted into complex number that represents the sum of imaginary,-Z” and real, Z’ parts (Z (w) = Z’ (w) + !Z”(w)). Bode plot is shown in Figure 2.5 as a plotted graph of absolute impedance, log |Z| and phase angle ,q against angular frequency, w. Bode plot is used to explain in detail of frequency-dependent impedance relationship [54].
Figure 2.5: A typical Bode plot for a simple corrosion process [49]
2.2.2 Potentiodynamic polarization (PD)
Potentiodynamic polarization is one of the constructive methods for corrosion monitoring. Comparing to the EIS method, PD method highlights corrosion current density (icorr) and corrosion potential (Ecorr) as the corrosion parameters [55]. Hence, the efficiency of the coating on corrosion protection of mild steel can be analysed by PD method. A number of studies have reported the evaluation of coating performance for corrosion protection by using PD method. Based on reported literatures, PD is a useful technique to identify corrosion phenomenon such as localized corrosion (pitting) , passivation behaviour and corrosion rate [56, 57]. Pitting is known as catastrophe corrosion phenomena. The random formation of small pits on the metal surface indicates the penetration of corrosive ions that eventually will causes severe degradation of metal. The identification of pitting corrosion on the metal surface is essential to eliminate further metal degradation. The occurrence of pitting on the metal surface can be studied by determination of critical pitting potential, Epitt that measures the rapid increment of current in polarization curves via PD analysis [57].
Fundamentally, PD is conducted by an application of wide range of potential as an input excitation signal to a test electrode at which an oxidation-reduction reaction are dominant [50]. The output signal measured is the current response of an electrode towards the applied potential. The current generated upon the application of wide range of potential is an indication of corrosion rate. At varying range of potential, the generating current is continuously monitored to obtain the polarization curve. The obtained polarization curve is presented as a graph of electrode potential (E) against current density (log i) as shown in Figure 2.6.
Figure 2.6: A typical Tafel polarization curve [49]
The current density is defined as rate of current (I) per unit corrosion surface area (A). Tafel curve depicts the linear relationship of potential and log i by considering an electrode is polarized at adequately large potential [54]. The polarization is represented by deviation of corrosion potential , Ecorr in both anodic and cathodic directions as shown in Fig. 2.6. The linear relationship of E-log i plot can be mathematically expressed based on Butler-Volmer equation as shown in equation 2.18 [49].
I = icorr [exp (*.0102
3$ ) – exp (- *.0102
3' )] (2.18)
where I is the current (A), icorr is the corrosion current (A cm-2), ba and bc is the tafel constants and h is overpotential which is can be expressed as the difference between applied potential, E and corrosion potential, Ecorr (h = E - Ecorr ).
Based on Tafel curve in Figure 2.6, an important parameter for corrosion protection efficiency can be studied by value of corrosion potential, Ecorr and corrosion current icorr. The determination of Ecorr and icorr can be determined by an extrapolation method. The intersection point between anodic and cathodic linear curve is characterised as Ecorr. From the Ecorr point value, icorr can be readily read from the log i y-axis. By considering the dissolution of mild steel in NaCl solution, Ecorr value depicts the corrosion potential at which the rate of mild steel dissolution (anodic) is equal to the rate of formation of hydroxide ion (cathodic). Thus, the determination of icorr is vital which is equivalent to the rate of mild steel dissolution when it is immersed in corrosive environment. By comparing the value of icorr for the coated metal and uncoated metal, the efficiency of coating can be well evaluated.
Additionally, Tafel curve reveals the combination of oxidation and reduction linear curves, which are represented by the Tafel slope for anodic and cathodic region , respectively. The anodic and cathodic characteristic in Tafel curve are denoted as ba
and bc, respectively and can be expressed as Equation 2.19 and 2.20 [49, 50].
β4 = *.010.5
678 (2.19)
β9 = *.010.5
()#6)78 (2.20)
where R is the universal gas constant (8.314 J mol K-1), T is the absolute temperature in Kelvin (K), F is the Faraday constant (9.65 X 104 C mol-1), a is the charge transfer coefficient and n is the number of electrons.
The other corrosion parameters such as corrosion rate (CR), polarization resistance (RP) and corrosion inhibition efficiency can be calculated based on icorr and tafel constant values as shown in Equation 2.21, 2.22 and 2.23 [50, 58].
CR (mmy-1) = 1.)0×:'())×;*+
< X 0.0254 (2.21)
Rp = 3$×3'
*.010×:'())×(3$3') (2.22)
% IE = :°'())#:'())
:°'()) X 100 (2.23)
where CR is the corrosion rate in millimeter per year (mmy-1), iocorr is the corrosion current for uncoated mild steel), icorr is the corrosion current for the coated mild steel, ba and bc is the Tafel constants, Mew is the equivalent weight of the sample and r is the density of the sample.
2.2.3 Open Circuit Potential
Open circuit potential analysis is designed to measure the potential stability of the electrode in the corrosion system. It is a useful method to measure the potential of working electrode (sample specimen) relative to the reference electrode (saturated calomel electrode) [59]. The potential is measured as a function of immersion time.
The measured potentials is known as the free corrosion potential (Ecorr). During OCP analysis, there is no potential is applied to the corrosion system , thus no current is measured. This condition indicates that the reaction rate of anodic and cathodic are in equilibrium state. Usually, OCP will be conducted as perquisite step before proceeding to the EIS and PD analysis to ensure the equilibrium condition of sample with its surrounding medium (electrolyte).
OCP profiles of electrode is one of an important thermodynamic parameter to evaluate the tendency of working electrode to corrode in the electrolyte. A OCP profile is represented as a graph of potential against immersion time. A high potential values (least negative) and a low potential values (more negative) indicates low tendency and high tendency of corrosion to occur respectively. A study of electrochemical analysis of epoxy-Zn coating via OCP analysis is beneficial to study the barrier protection and cathodic protection behaviour of the coating system.
A study by Shirehjini et al.[60] investigated the effect of nano clay in epoxy- Zn coating immersed in 3.5 wt % NaCl solution for 60 day. From the OCP profiles shown in Figure 2.7, OCP values of all coatings were compared with the free corrosion potential (Ecorr) of uncoated mild steel at -0.780 V vs SCE.
Figure 2.7: Open circuit potential profile for epoxy-Zn nano-clay nanocomposite coating showing cathodic protection and barrier protection as function of immersion time [60]
The potential below and above -0.780 V vs SCE indicates the cathodic protection zone and barrier protection zone respectively. The more negative potential values suggested a high tendency of corrosion due to the oxidation of zinc particles for all coating. This showed that mild steel is effectively protected by scarifying action of zinc. As immersion time increased, the OCP values shifted to more positive potential due to the formation of zinc corrosion products around the zinc particles. This resulted in the reduction of electrical contact among the zinc particles and between the zinc particles and mild steel. The evolution of OCP values towards more positive value also showed the role of nano clay particles enhanced the barrier protection of the coating by inhibiting the movement of corrosive ion approaching the mild steel surface.
However not all epoxy-Zn nanocomposite coating exhibit the same pattern of OCP profiles. For instance, Yang et al., utilized a sulfonated polyaniline nanofiber as a conductive filler in the epoxy-Zn coating. Based on OCP profile, Yang et al.[9], concluded that the modified epoxy-Zn coating showed barrier protection at initial immersion time and followed by cathodic protection at longer immersion period.
Hence, it can be suggested that OCP profiles is varied based on the type of nanoparticles used. Therefore, OCP analysis is vital to study the potential stability and to determine the free corrosion potential of the electrode .
2.3 Effect of corrosion and its cost
The consequent of corrosion includes two main aspect, which are economic and social. In daily life, corrosion is commonly recognized on household appliance such as charcoal grill, outdoor furniture, metal tools, water purifier and washing machines.
These appliance are commonly made up of metal and frequently exposed to the wet
environment. Rusted appliances required high maintenance cost and may require new replacement if the corrosion effect on those device is severed.
In the economic aspect, NACE International estimated that the cost of corrosion is US$2.5 trillion at a global level covering all aspect of fields and technologies [29].
This is equivalent to an average of 3 - 4 % of gross domestic product (GDP) at each nation all over the world. Specifically in Malaysia, about RM 6.7 billion has been spent to cover the consequent of corrosion in all economies sectors [30]. Based on statistical pie chart in Figure 2.8, it can be seen that cost of corrosion are dominant in the utilities and transportation sector. The utilities sector including the industry that provide daily necessities to the household such as clean water supply, electricity and gas.
Figure 2.8: The cost of corrosion in industry categories [61]
Effect of corrosion are considered highly vulnerable in the industrial plants such as electrical power plant [62], chemical processing plant [63], desalination plant [64]
and oil and gas industrial plants [65]. These industrial plants involve a major process involving corrosive chemicals, flammable gases and petroleum stored at elevated temperature and pressure in the storage tanks. Without a proper care and a good maintenance, these tanks are highly exposed to corrosion that will affects the chemical process and the final product quality. In worst case, corrosion can lead to gas explosion or chemical leaking due to the pitting corrosion. Pitting corrosion can cause the formation of holes on the metal surface. Eventually, it will causes the machine failure and plant shutdown. Yearly, a massive amount of money has been allocated by most of industries for maintenance and corrosion control.
In social aspect, corrosion may endangers public health and safety. The corrosion pollution can causes several health issues. For example, in food and pharmaceutical industries, corrosion is highly concern due to the possibility of food and chemical contamination. The formation of iron-based product is harmful to the human’s health. The corrosion product can possibly contaminate the final product in the process of chemical or food production. This accident is fatal to the production line due to the loss of product and will causes billions dollar loss to the industries. Besides, underground corrosion is also one of major concern. According to Saupi et al.[30], underground pipeline used for the water, gas and oil transport is the most vulnerable to corrode due to the reaction with its corrosive environment. The underground corrosion in water transport can cause water contamination that is fatal to human’s health. The underground pipeline is important to deliver a clean water resources for daily uses.
In public safety concern , corrosion may endangers public due to the bridge or building collapse. The sudden failure in mechanical structure of building, bridge, electrical tower and highway’s section are possible to occur due to the corroded steel
reinforced bar used in concrete. In 2018, Morandi Bridge in Italy was collapsed during heavy rainfall [66]. The tragic incident had killed 43 people and caused 600 people loss their home. Based on the investigation and inspection report published, several factors had caused the fall of Morandi Bridge. In the report, it was highlighted that severe metal corrosion is one of the main issues found. It was proved that the bridge had been suffering from major degradation. Due to the heavy rainfall, the bridge which is suspected to be in poor condition due to the major degradation cannot withstand the heavy rainfall, causing it to collapse.
2.4 Corrosion control
Corrosion is a naturally occurring phenomena which is destructive to the metallic substances. The building and bridge structure are mainly composed of metallic substance which are fragile towards the vulnerability of corrosive environment and oxygen. Indeed, corrosion process is impossible to be diminished as it is naturally-occurring process. However, controlling the corrosion process is highly possible with the aid of invention in corrosion studies. There are a lot of potential methods that can be used to control the corrosion process such as genuine material and design selection [67], utilization of corrosion inhibitor [68], electrochemical technique [69], environmental control focusing on humidity aspect [70] and surface treatment by application of coating [71]. Those methods have been extensively used in the industry for corrosion prevention.
The proper material and design used to build the processing tank and storage tank are vital, especially for the industry that involve with the chemical processing.
The common potential corrosion-resistance materials such as stainless steel, aluminium metal and galvanised steel are widely used in the industry for myriad
applications. For instance, galvanised steel is preferred to be used in concrete as the building and construction material to prevent severe concrete damage due to the corrosion [72]. Additionally, the good design of storage tank in terms of its size, shape and wall thickness is crucial to reduce any possibility of corrosion initiation [67]. A case study by Geary et al.[73], reported that internal corrosion associate with structural fractural had caused the storage tank to severely damaged and collapsed. The site analysis revealed that the main causes of the catastrophic failure of storage tank was due to the hoop stress induced by the chemical content. Consequently, the shell and base plate of the tank cannot withstand the large tensile force, caused it to rupture and collapse. Hence, it is important to highlight the importance of proper structural design to lessen the corrosion effect on the tank structure. The storage tank are highly susceptible towards corrosion due to the chemical content. Thus, the wall thickness of shell and base plate of tank are an important compartment to lessen the possibility of uneven stress due to the weight of chemical content.
Besides, corrosion also can be prevented by an electrochemical technique. This technique is useful to mitigate corrosion of steel in concrete by performing cathodic protection. This method is usually used for bridge and building structure at the seawater region. Cathodic protection is a reliable technique to lessen the corrosion.
Fundamentally, the protected metal is subjected to the cathodic polarization resulted in more negative potential than the free corrosion value, thus lower the corrosion rate [69]. Besides, Gong et al. [70] proposed that corrosion rate can also be minimised by controlling the environment in terms of metal exposure to the humidity. In the presence of water and oxygen, metal can be easily corrode. For instance, in the automotive industry, the automotive goods usually is imported or exported to the different locations. The goods usually will be stored in the container and be shipped across the
oceans and continents. The long shipment period causes the metal substance more susceptible to corrode due to the temperature and humidity changes. Hence, the addition of supper desiccant inside the cargo container is found to be an effective method to prevent moisture damage on the metal surface. Supper desiccant is useful to limit the presence of water to prevent the corrosion during the transportation period [70].
Moreover, corrosion also can be mitigated by the utilization of inhibitor and surface treatment by application of coating. The utilization of inhibitor is a well-known method used for corrosion prevention. Recently, organic inhibitor is extensively studied by researcher due to its friendly-environmental properties [74, 75]. Inhibitor is an active chemical compound that is added into the environment. This active compound forms a protective layer on the metal surface to inhibit metal dissolution.
Similarly, the surface treatment by application of coating also can inhibit corrosion by a protective barrier formed between the metal and its surrounding environment.
Coating can be formulated in various types of forms such as paint, film, waxes and spray. The addition of active metal (e.g., zinc) , hydrophobic material (e.g., silane) and nanocomposite has benefit the efficiency of coating in better ways. Compare to all the corrosion control methods, application of coating is more reliable to be used due to its easy application, high durability and high availability.
2.5 Epoxy-Zn coating
Coating is a protective layer applied on the surface of substrate. It has been used widely for various application. Coating is a very well-known method used by most of industries to lessen the corrosion. Coating in the forms of paint and spray are widely found in the market today for daily-use applications. Epoxy resin is a thermosetting
polymer which has high chemical reactivity and high toughness properties [76]. Its chemical structure comprise of reactive and highly strained C-O-C epoxide ring as shown in Figure 2.9.
Figure 2.9: Epoxide ring of epoxy resin
The epoxide ring structure is receptive to a various of reactants and curing agent [77]. This proved that epoxy resin has a good versatility in the formulation which can be advanced for various applications. For instance, epoxy resin-based paint is formulated by being crosslinked with curing agent and widely used as surface coating for ships, steels, storage tanks and more . Epoxy resin can be cured with curing agent such as aromatic amine at room temperature. It also possess superior surface properties such as low curing shrinkage, good chemical resistance and high adhesion to various substrates [78, 79]. The opening of C-O-C ring structure can be achieved via polymerization that results in cross-linking and/or chain extension as shown in Figure 2.10. The “X” element in Figure 2.10 represents nitrogen (N).
Figure 2.10: A proposed polymerization reaction of epoxy [80]
