SYNTHESIS, CHARACTERISATION AND ANTI-CORROSION BEHAVIOUR OF NiO-CNTs-SnO2 NANOCOMPOSITES COATINGS ON AISI 1020 STEEL

ABSTRACT

In this study, synthesised NiO-CNTs-SnO2 nanocomposites based coatings were applied on AISI 1020 steel samples to prevent corrosion. NiO-SnO2 based nanocomposites were obtained via combination of green-impregnation and sonochemical methods followed by incorporation of CNTs at different ratios. The influence of process parameters on the synthesis of NiO and SnO2 nanoparticles such as; solution pH, precursor concentration, and synthesis temperature on particle size were investigated via surface response methodology (RSM) using the Box-Behnken method. The synthesised nanocomposites were characterised with the aid of High resolution scanning electron microscopy (HRSEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), High Resolution   Transmission   Electron   Microscope   (HRTEM)   and   Selective   Area Diffraction  Spectroscopy  (SAED)  and  X-ray  Photoelectron  Spectroscopy. Subsequently, NiO-SnO2, NiO-CNTs, SnO2-CNTs and NiO-CNTs-SnO2, were dip- coated on the surface of AISI 1020 steel coupons. The corrosion performance of the (NiO-SnO2,   NiO-CNTs,   SnO2-CNTs)   and   NiO-CNTs-SnO2,     coatings   in   soil environment as  corrosive media were investigated by Gravimetry,  Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy methods (EIS). HRSEM analysis showed that the obtained NiO-CNTS-SnO2 nanocomposites were aggregated, with combined spherical, tubular and cubic structures with an average crystalline size of 25 nm.  Energy Dispersive Spectroscopy (EDS) analysis revealed the presence of Sn, Ni, C and O as major elements in the nanocomposites. The XRD analysis of as-synthesised nanocomposites revealed the formation of crystallinity bunsenite, graphite and cassiterite phases belonging to NiO, CNTs and SnO2 respectively. The X-ray Photoelectron Spectroscopy (XPS) analysis of the nanocomposites revealed the formation of Nickel Oxycarbide-Tin alloy responsible for the improved anti-corrosion properties of the developed nanomaterial. Gravimetry analysis showed its lowest corrosion rate at 8.76E-08 mpy for a coupon coated and buried with NiO-CNTs-SnO2 composite in the ratio (1:1:2) and a Protection Efficiency of 92.61% after 12 months of reweighing. The PDP showed that NiO-CNTs-SO2, in the ratio (1:1:2), had excellent corrosion resistance behavior with lowest corrosion current of 0.0064 µA/cm2 and Protection Efficiency of 93.30% while the charge transfer resistance, Rct of 6.244 KΩ·cm2 and Protection Efficiency of 93.90% were established with the same NiO-CNTs-SnO2  composite coating of the same ratio (1:1:2) for EIS measurement against the corrosion of AISI 1020 steel substrate. The corrosion rate was also found decreased from 0.0335 to 0.0022 mpy showing evidence of a slow rate of uniform   corrosion   and   better   Protection   Efficiency   of   the   steel   substrate. HRSEM/Cross-Sectional analysis of steel coated with NiO-CNTs-SnO2 (1:1:2) buried in the soil revealed successful protection of the steel coupon. XRD analysis also showed non-destruction of the diffraction peaks of NiO, CNTs and SnO2, respectively. This study demonstrated NiO-CNTs-SnO2  based coatings in the mixing ratio 1:1:2, as the most effective material to protect the steel coupon.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background to the Study

Steel is an engineering material widely used in the area of manufacturing, construction, defense, medical and transportation, to mention but a few. However, the corrosion of steel via chemical and electrochemical reaction with its service environment compromises  the  material  integrity  and  aesthetic  value  (Herrera  et  al.,  2020).  In addition, corrosion of steel may take varying forms such as uniform corrosion, and galvanic corrosion, pitting corrosion, crevice corrosion, under-deposit corrosion, dealloying.  Others  include  stress  corrosion  cracking,  corrosion  fatigue,   erosion corrosion and microbially influenced corrosion (Kokilaramani et al., 2021).

The harmful impacts of metallic corrosion are in multifolds on the economy, including the safety hazard and conservation of resources. Economically, the losses due to corrosion are enormous and most visible in iron pipes, ships, tanks, bridges, among others. The failure of equipment might be proven fatal for human life and replacing corroded metals is not only expensive, but also hazardous and put a burden on landfills (Abdeen et al., 2019). Moreso, the act of constantly replacing metals depletes natural resources. Hence, corrosion has negative impact on economy of any nation and needs to be addressed for sustainability (Kausar, 2019).

It is a general belief that corrosion is a universal enemy that should be accepted as a process that is inevitable. As products and production processes become more complex and the penalties for corrosion failures have become more expensive and increased awareness has been generated. Moreover, an average of 10 percent of the total metal output in the world is estimated to be lost in corrosion (Fayomi et al., 2019). This affects the economy of a nation and her assets: infrastructures, transportation, utilities, nuclear and military facilities, and production and manufacturing plants. According to the  National Association  of  Corrosion  Engineers  (NACE),  corrosion  costs  globally accounted for about 3.4% of the global Gross Domestic Product as at 2013, which amounts to $2.5 trillion US dollars which is about 5 times of the GDP of Nigeria with $508  billion  dollars  (Popoola  et  al.,  2017). According  to  NACE  with  current  day technology, about a third of the costs incurred by corrosion can be saved, and there are various methods that help implement corrosion control (Akpoborie et al., 2021). The harmful effects of corrosion can be felt on society in both direct and indirect ways. Direct ways being that it affects how long we use our possession and indirect being that incurred costs obtained by manufacturers through corrosion are passed to consumers. Corrosion costs are partly related to attempts to give an attractive look to engineering equipment,  structures  and  designs.  Partly  because  of  the  direct  replacement  and maintenance costs and concurrent losses due to interruption to plant operation and additional costs associated with the use of expensive materials and other preventive measures. Generally, corrosion of steels in acidic aqueous solution is prevalent in many industries  wherein  acid  is  used  in  processes  such  as  acid  pickling,  industrial  acid cleaning, acid descaling and oil well acidizing (Farag, 2020). One of the most effective and  economical  approach  to  improve  surface  ability of  material  to  withstand  high surface stress and harsh environments is by creating surface layers that would possess a high level of corrosion and wear resistance (Abakay et al., 2017).

Several methods (such as inhibitor, cathodic protection, appropriate material selection and design) have been employed to prevent and/or control corrosion in metals (Wei et al., 2020). While operational maintenance of cathodic protection might be sometimes uneasy due to the period inspection of the control system. The appropriate material selection  on  the  other  hand  requires  economic  factor  and  application  of  inhibitors sometimes affect the quality of product the material is meant to transport (Khan et al., 2015).

One of these methods that standout and is considered acceptable in practice is isolation of the metals from the corrosive environment via coating. Metals and alloy, ceramics, polymers and their complex composites have deposited on mild steel for corrosion prevention, though having limitation such as, durability (Abdeen et al., 2019). Metal sage, weaken and deteriorate and eventually their useful life ends within a short period of time.

In contrast, the introduction of nanoparticles into organic polymers and the likes offers an effective way to improve the substrate properties such as electrical conductivity, mechanical properties, thermal stability, flame retardancy, and resistance to chemical reagents (Coetzee et al., 2020). Moreover, composite coating with nanoparticles also improved properties which simple steel and metal plating do not have. Such properties include corrosion resistance, wear resistance and stronger adhesive bond between the coating and substrate. Studies have shown the incorporation of a carbon-based coating material in  electroplating;  good  hardness improved the thermal  conductivity of the metallic substrate (Ai et al., 2020). Furthermore, coatings with carbon-based nanomaterials  improve  wear  resistance,  hardness  and  lower  frictional  coefficient (Abdeen et al., 2019).

Nanoparticles are of great interest to the material scientist due to their extremely small size and large surface to volume ratio, responsible for improved both chemical and physical properties (These properties include mechanical, biological and catalytic activity, thermal and electrical conductivity, optical absorption and melting point) compared to bulk of the same chemical composition (Khan et al., 2017). It can therefore be said that properties embedded in the materials produced at nanometer scale cannot be variously compared  with  the few properties realized  at  bulk state.  In  addition,  the incorporation of carbon nanotubes in the coating matrix improved both tribological behavior, thermal conductivity and corrosion resistance of the material. Studies have shown  that  NiO-SnO2   coatings  possess  a non-crystalline  amorphous  structure,  thus exhibit excellent protection for mining, chemical equipment and structural components when  used  in  the  highly  corrosive  environments  encountered  in  the  oil  and  gas industries (Jeevanandam, 2018). Therefore, when applied to steel surfaces, NiO-SnO2 coated CNTs will provide effective corrosion protection by creating a physical barrier that minimizes contact between the metallic substrate and aggressive environments.

This   study   focused   on   the   synthesis   of   NiO-SnO2-CNTs   nanocomposites   via combination of green-chemical vapour deposition and impregnation methods. This was followed  by  characterisation  of  the  nanocomposites  in  terms  of  microstructure, elemental composition, morphology, mineralogical phases using different analytical techniques. Subsequently the synthesised nanomaterials were applied as a coating. The anti-wear and corrosion behavior of NiO-SnO2-CNTs coating were further investigated.

1.2       Statement of the Research Problem

Premature and unpredictable failure of buried metals due to its exposure to physicochemical properties of the    soils    which    is    attributed    to    changes    of the environment   and   season   has   contributed   to   loss   of   lives,   properties   and contamination of environment and products (Quej-Ake et al., 2015; Prasannakumar et al., 2020).

Most of the synthetic coatings used as barrier between metallic substrates and their surroundings are highly toxic, hazardous and easily dispersed into the environment due to poor adhesion of these coating materials onto the surface of metallic substrate (Zhenget al., 2017).

Various methods have been employed to protect buried steel from corrosion, which includes the use of inhibitors, impressed current, sacrificial anodes and coatings, but corrosion damage in steel remains a very serious problematic due to climatic change and the time of exposure (Quej-Ake et al., 2015; Zhan et al., 2017).

Large utilization of nanoparticles in metal coating is susceptible to the formation of defects such as, cracks during micro hardness testing, which results in premature failure of the composites. Which is due to high colloidal instability and agglomeration of nanomaterials at higher concentration that affects evenly distribution and adhesion of the nanoparticles onto the surface of metallic substrate.

Corrosion protection efficiency of doping and co-doping of coaters on activated carbon or polymer is low due to poor water repellent ability of the matrics.

1.3       Aim and Objectives of the Study

The aim of this research work is to synthesis, characterise and investigate the anti- corrosion behaviour of NiO-SnO2-CNTs nanocomposites on AISI 1020 steel. The aim was achieved through the following objectives; which are to:

The aim of this research is to synthesis, characterise and investigate the anti-corrosion behaviour of NiO-SnO2-CNTs nanocomposites on AISI 1020 steel. The aim was achieved through the following objectives; which were to:

i.      synthesise  nickel  oxide,  and  tin  oxide  nanoparticles  using  response  surface methodology via Box-Behnken method;

ii.      produce  carbon  nanotubes  (CNTs)  via  catalytic  chemical  vapour  deposition method on Fe-Ni/Kaolin catalyst;

iii.      prepare NiO-CNTs, SnO2-CNTs, NiO-SnO2, and NiO-SnO2-CNTs composites via wet impregnation/sonochemical methods involving variation of the amount of NiO, CNTs and SnO2;

iv.      characterise NiO, SnO2, CNTs and their composites forms for their morphology, particle size, elemental composition and mineralogical phases using High resolution Scanning Electron Microscopy (HRSEM), Energy dispersive spectroscopy (EDS) and X-ray Diffraction (XRD) respectively.

v.      apply the developed NiO, SnO2, CNTs, NiO-CNTs, SnO2-CNTs, NiO-SnO2, and NiO-SnO2-CNTs  nanocomposites  on  the  surface  of  the AISI  1020  steel  as nanocoatings;

vi.      determine some physico-chemical parameters in the soil where the metals were buried; and

vii.    evaluate the corrosion coating efficiency of the prepared nanoparticles and nanocomposites on the AISI 1020 Steel via weight loss (WL) and potentiodynamic  polarization  (PDP)  and  electrochemical  impedance spectroscopy (EIS) methods.

1.4       Justification of the Study

Researchers  have  found  that  the  incorporation  of  nanoparticles  into  a  coating  has become an alternative way to further improve the corrosion resistance and mechanical properties of the existing surface coating to keep up with the rapidly changing needs of certain special applications (Tudela et al., 2014; Aliyu & Srivastava, 2020). Immobilisation of nanoparticles on CNTs matrices provides an excellent barricade against the ingress of corrosive ions, reduces agglomeration of the nanoparticles and release of the nanoparticles into the environment (Chhetri et al., 2019). High thermal and mechanical properties with excellent water repellant properties of CNTs favoured its choice as the matrices for anchoring the nanoparticles for coating of metallic substrate. Green synthesis of nanoparticles reduces metal precursors and also serve as capping agents for the nanoparticles produced. Fine sizes of the particle used in this nanocoating will help in filling the spaces and blocking the corrosive elements from diffusing  into  the  steel  substrate. This  will  in  turn  protect  steel  product,  eliminate hazardous effect on human health and environment.

Consequently, the synergistic integration or incorporating of NiO-CNTs-SnO2 nanocomposites on the surface of steel substrate via dip deposition technique would offer inhibition, barrier and hydrophobic action with active protection, which represents an effective and reliable potential development  strategy to obtain longtime durable coating that enhance their performance in different fields. Therefore, there is a strive to make use of environmentally friendly, non-toxic /less toxic, nanocoating material that is capable of withstanding harsh climatic change and offer superior passivation over steel in an extended period of time.

1.5       Scope of the Study

The scope of this work is limited to development of NiO-SnO2-CNTs nanocomposite as a coating material for prevention of AISI 1020 steel against corrosion in a soil environment.

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