Title page
Table of Contents
List of Abbreviations, Notations and Units

1.1       Aim and Objectives
1.2       Justification of Research
1.3       Scope of Work
1.4       Contribution to Knowledge

2.1       General Properties of Aluminium and its Alloys
2.1.1    Classification system of aluminium alloy
2.1.2    Aluminium cast alloy designation
2.2       Semi-Solid Metal (SSM) Processing
2.3       Ternary and Multi component Alloy
2.4       Heat Treatment of Cast Al-Si-Mg alloys
2.4.1    Solution treatment
2.4.2    Quenching
2.4.3    Ageing treatment
2.5       Hardness measurement
2.5.1    Rockwell hardness test
2.6       Mechanism of Electrochemical Corrosion
2.7       Corrosion Prevention and Control Method
2.8       Corrosion Inhibitors
2.8.1    Anodic inhibitors
2.8.2    Cathodic inhibitors
2.8.3    Adsorption inhibitors
2.9       Organic inhibitors
2.10     The Use of Natural Plant Extracts as Corrosion Inhibitors
2.10.1  Neem tree (Azadirachta indica) / Neem seed extracts as corrosion inhibitor
2.10.2  Guava tree (Psidium guajava) / Guava leaf extracts as corrosion inhibitor
2.11     Adsorption Consideration
2.11.1  Langmuir isotherm model
2.11.2  Temkin and Frumkin adsorption model
2.12     Application of aluminium and its alloy

3.1 Materials and equipment
3.1.1    Materials
3.1.2    Equipment
3.2       Method
3.2.1    Extraction of neem seed and guava leaf
3.2.2    Fourier transform infrared spectroscopy (FTIR) analysis of the extracts
3.2.3    Sample preparation
3.2.4    Heat treatment
3.2.5    Hardness measurement
3.3       Corrosion Test
3.3.1    Gravimetric-based mass loss method
3.3.2    Corrosion rate determination
3.3.3    Inhibition efficiency (IE)
3.3.4    Reaction kinetics
3.3.5    Potentiodynamic polarization measurement
3.4       Micro-structure and Surface morphology examination


5.1       Fourier Transform Infrared Spectroscopy (FTIR) Analysis
5.2       Hardness Measurement and Double Thermal Ageing Treatment
5.3       Gravimetric-Based Mass Loss Measurement
5.3.1    Effect of inhibitor concentration and time on corrosion rate
5.3.2    Effect of inhibitor concentration and time on inhibition efficiency
5.3.3    Effect of temperature on inhibition efficiency
5.3.4    Thermodynamic consideration
5.3.5    Kinetic consideration
5.4       Potentiodynamic Polarization Studies
5.4.1    Inhibition efficiency and adsorption behaviour
5.5       Microstructure and Surface Morphology Analyses
6.1       Summary
6.2       Conclusions
6.3       Recommendations

This study determined the corrosive effect of seawater on as-received and double thermally-aged Al-Si-Mg alloy. It also determined the functional groups present in guava leaf and neem seed extracts and their inhibitive effect on the corrosion of Al-Si-Mg alloy in seawater. This was with a view to enhancing their performance in the aggressive seawater environment. The study involved Fourier transform infrared (FTIR) analysis of the extracts, thermal ageing treatment, hardness measurement, gravimetric based-mass loss test, potentiodynamic polarization test and microstructural examination. The FTIR analysis results showed that the extracts can serve as good corrosion inhibitors due to the presence of aromatic compound and functional group with lone pair of electrons which makes them easily adsorb on surface of the alloy. The thermal ageing treatment employed was Double Thermal Ageing (DTAT) with temper condition at pre-ageing temperature of 105oC at constant time of 2 hrs. The hardness of Al-Si-Mg (SSM-HPDC) alloy was determined using the Rockwell-F scale hardness tester. The hardness value of the alloy increased from 59.1 HRF (as-received sample) to 82.8 HRF (double thermally aged sample) indicating an increase of 40%. Improvement in hardness can be attributed to fine coherent clusters of precipitates which serve as obstacles to dislocation movement. The gravimetric based-mass loss test was carried out at different inhibitor concentrations, time and temperature ranges of 0.1-0.5% v/v, 1-5 hrs and 30-70oC, respectively. From the results obtained the corrosion rate of both the as-received and age-hardened samples decrease with increase in inhibitor concentrations. The maximum inhibition efficiencies of 63.17% and 72.10% were obtained for the as-received and age-hardened samples, respectively, in the presence of guava leaf extract. While in the presence of neem seed extract maximum inhibition efficiencies of 60.49% and 64.26% were obtained for the as-received and age-hardened samples, respectively. The mechanism of inhibition was elucidated by kinetic and thermodynamic models. The polarization curves showed that the extracts act as mixed-type inhibitors. Results obtained from the potentiodynamic polarization technique indicate higher potential value for guava (Psidium guajava) leaf extract and lower potential value for neem (Azadirachta indica) seed extract. The Tafel plot showed an increase in polarization resistance (Rp) and lower corrosion current density (jcorr) for inhibited samples as compared with uninhibited samples. From the results, IE of 90.48% and 98.88% was obtained for as-received and age-hardened samples respectively in the presence of guava (Psidium guajava) leaf extract. While in the presence of neem seed extract IE of 96.55% and 80.68% were obtained for the as-received and age-hardened samples respectively. The two methods used for the corrosion evaluation were in agreement and a mixed type corrosion inhibition exist which obeys the Langmuir adsorption isotherms. From OPM result, the microstructure of the age-hardened sample showed finer grains and enhanced grain boundaries than the as-received sample. This was as a result of increase in volume of precipitated intermetallic compounds during ageing process and also refinement of precipitated constituent particles. The surface morphology of as-corroded samples was assessed with scanning electron microscope. Results showed severe damage and pits formation for as-corroded uninhibited condition than as-corroded inhibited condition.

1.0              INTRODUCTION
Aluminum is the second widely used metal due to its desirable chemical, physical and mechanical properties. It is alloyed with elements like Si, Mg, Cu, Mn, Fe etc. Addition of silicon to aluminum can improve its fluidity as well as castability and mechanical properties

(Asuke et al., 2009). Addition of Magnesium to Al-Si alloy improves its strength to weight ratio and yield stress by combining with silicon to form the age hardening phase (Mg2Si) which precipitates from a super saturated solid solution during heat treatment (Birol, 2009).

Semi-solid metal processing has gained increasing interest from automotive industry and to a lesser extent the aerospace industry because it demonstrate the capability of producing near –net shaped components with good mechanical properties (Govender et al., 2008).The application of this technique is still in its early stage and some brake cylinders and pistons have been manufactured by this process (Haizhi, 2003).

Aluminium and its alloy find application in many industries due to high strength to weight ratio, good corrosion resistance, excellent workability, high electrical and thermal conductivity. However the ability of aluminium and its alloy to resist attack in aggressive corrosive environment have been reported to be poor (Mohammed et al., 2013).The use of inhibitors for corrosion control of materials, which are in contact with aggressive environment is an accepted practice (Stephen et al., 2014). Although synthetic inhibitors when used for corrosion control, have been reported to indicate an excellent performance. But majority of these inhibitors are not eco-friendly and are expensive (Popoola et al., 2012). Therefore effort towards identifying any potential eco-friendly and less expensive corrosion inhibitors remain relevant and important. The growing interest among researchers for green inhibitors remained a top research focus. In this.....

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