FABRICATION OF NIOBIUM COATED MAGNESIUM ALLOY FOR AUTOMOTIVE &
AEROSPACE APPLICATIONS
A Thesis by
Jehanzeb Hakim
Bachelor of Industrial & Manufacturing Engineering, NED University, 2008
Submitted to the Department of Industrial and Manufacturing Engineering
and the faculty of the Graduate School of
Wichita State University
in partial fulfillment of
the requirements for the degree of
Master of Science
December 2014
© Copyright 2014 by Jehanzeb Hakim
All Rights Reserved
FABRICATION OF NIOBIUM COATED MAGNESIUM ALLOY FOR AUTOMOTIVE &
AEROSPACE APPLICATIONS
The following faculty members have examined the final copy of this thesis for form and content
and recommend that it be accepted in partial fulfillment of the requirement for the degree of
Master of Science with a major in Industrial Engineering.
Anil Mahapatro, Committee Chair
Deepak Gupta, Committee Member
Ramazan Asmatulu, Committee Member
iii
DEDICATION
Dedicated to my parents, brothers, relatives and my dear
friends
iv
ACKNOWLEDGEMENTS
There are many people without whom this thesis would not have been possible. First and
foremost, I am grateful to my parents Mr. and Mrs. Hakim for establishing me to complete this
thesis and my brothers for their prayers and continual moral support throughout my life.
I am greatly thankful to all the people who have helped and supported me in completing
this thesis. I would like to express a sincere appreciation to my advisor, Dr. Anil Mahapatro for
his continued support and advice throughout my thesis and master’s program. His guidance
helped me to complete this work. I am so grateful and indebted to him for his expert, sincere and
valuable guidance. I also take this opportunity to record my sincere thanks to Dr. Ramazan
Asmatulu for his continuous encouragement and Dr. Deepak Gupta for giving their valuable time
to review this thesis and be a part of my thesis committee.
I would especially like to thank my friend Shahzeb Khan, relatives and all office
colleagues Sai Abhishek Arshanapalli, Santosh Suggu, Syef Seraz, Zeinab, Steffen, Vamsi and
Haymer for their valuable suggestions and support during my thesis.
v
ABSTRACT
Magnesium (Mg) and its alloys are being considered as potential materials to enhance fuel
efficiency in the aircraft and automotive industries because of the light weight. However, the major
limitation of Mg is its high corrosion rate, which needs to be controlled to avoid the failure of the
components. Therefore, the present, key challenge lies in controlling Mg degradation rate.
The main objective of this thesis was to electrodeposit Niobium (Nb) layer on Mg alloy for
corrosion protection. An environmentally friendly solution is demonstrated by forming Nb coating
on Mg alloy by the electrodeposition process. The characteristic of Nb electrodeposited layer at
room temperature using Nb salt and ionic liquid were discussed in this study.
The optimal electrolyte solvent was found to be water as compared to other solvents such as
acetone and dimethyl sulphoxide. The morphology of the coating was studied using SEM-EDS and
the maximum deposition of 36 weight % Nb in aqueous solution was obtained through
electrodeposition at pH 2.5 with ionic liquid 1 butyl-2-3-dimethylimidazolium tetrafluoroborate at
room temperature. When considering electrolytes, other than water, such as acetone, the maximum
deposition was found to be 22%; whereas in dimethyl sulfoxide, no deposition was found. The
environmental effect on deposition rate using glove box in aqueous solution showed no significant
difference. Detailed surface characterization using X-Ray diffraction (XRD) and X-Ray
photoelectronscopy (XPS) showed Nb peaks that futher confirmed Nb deposition. Electrochemical
testing showed an increase in the corrosion resistant property to the Nb coated Mg sample. Salt
Spray and Humidity tests showed enhanced corrosion resistant properties and, thus, could be a
potential use in automotive and aerospace applications.
vi
TABLE OF CONTENTS
Chapter
Page
1. INTRODUCTION…………………………...……………………….……………………..….……….....1
1.1 Project Objective and Motivation………....……………………..………...………………………......1
1.2 Current Strategies of Weight Reduction.......................................................................................….....3
1.3 Magnesium and Its Alloys for Aerospace and Automotive Applications…….........……………….....4
1.4 Potential Cost, Fuel and Environmental Benefits…...............................................................................9
1.5 Limitations of Magnesium…….……….................................................................................................9
1.6 Efforts to Overcome Metal Corrosion..................................................................................................11
1.6.1 Alloy Development…………………………………...………………………………...................11
1.6.2 Rapid Solidification Processing………………....………………………………………….….…11
1.6.3 Ion Implantation…..........................................................................................................................12
1.6.4 Laser Annealing..............................................................................................................................12
1.6.5 Protective Coating Strategy……………………………………………….………………...….....12
1.7 General Applications of Niobium Coated Material…..........................................................................13
1.8 Thesis Outline..............................................................................................................…….……........16
2. LITERATURE REVIEW…………………………………………………………………………...........17
2.1 Magnesium as a Light Weight Material…...……………………………………..……………….......17
2.2 Magnesium Alloys and their Properties.....................................................................................….......18
2.3 Corrosive Nature of Magnesium………...............................................................................................22
2.4 Review of Surface Coatings of Magnesium Alloys………………….…………………………..…...23
vii
TABLE OF CONTENTS (continued)
Chapter
Page
2.4.1 Conversion Coatings……...…………………………………….…………………….………...…24
2.4.2 Anodizing……….............................................................................................................................25
2.4.3 Organic Coatings….........................................................................................................................25
2.4.4 Gas Phase Deposition Process.............................................................................................……....26
2.4.5 Electrodeposition Process Using Magnesium………………….……………………….…….…..27
2.5 Electrodeposition of Niobium Using Ionic Liquid.......................................................................…....28
3. EXPERIMENTAL PROCEDURE AND TECHNIQUES.........................................................................30
3.1 Introduction….................................................................................................................................…..30
3.2 Experimental Procedure…………........................................................................................................30
3.2.1 Selection of Substrate…….............................................................................................................30
3.2.2 Surface Treatment Techniques..................................................................………………..……...31
3.2.3 Electrodeposition Experiments…...………...……………..…………………………..........…….31
3.2.4 Ultrasonication Cleaning………...………….………….………………………………………...32
3.2.5 Parameters of Electrolytic Solution………….…....................................................................…...33
3.2.6 Effect of Deposition Environment…........................................................................................…..34
3.3 Characterization Techniques…..............................................................................................................35
3.3.1 Optical Microscope..........................................................................................................................35
3.3.2 SEM & EDX................................................................................................................………..…..36
3.3.3 X-Ray Photoelectronscopy…………....……………….……………………………………...…..38
3.3.4 X-Ray Diffraction……………........................................................................................................38
3.4 Electrochemical Tests….………....………………………………….………….………..………........40
3.5 Salt Spray Test………………...............................................................................................................40
viii
TABLE OF CONTENTS (continued)
Chapter
Page
3.6 Humidity Chamber...............................................................................................………………....….41
4. RESULTS AND DISCUSSIONS...............................................................................................................43
4.1 Optimization of Process Parameters……………………………………………………….….……....43
4.1.1 Selection of Ionic Liquid…………......……………………………………………………….......44
4.1.2 Depositing Nb on Mg Alloy……………………………...…………………………………….....45
4.1.3 Effect of Electrolytic Solution .......................................................................................................45
4.1.4 SEM and EDS..................................................................................................................................47
4.1.4.1 SEM Image of Control Mg AZ31 Substrate.............................................................................47
4.1.4.2 Effect of Dimethyl Sulphoxide (DMSO)…...…………...……………...………………….....48
4.1.4.3 Effect of Acetone Solution……………………..………………………………………...…...49
4.1.4.4 Effect of Aqueous Solution……..……...…………………………………………………......50
4.1.5 Effect of pH on Deposition…………….……………………...………………………………......51
4.1.5.1 pH Study Chart with Nb % Deposition……......………...……………………………….........58
4.1.6 Effect of Environment on Deposition…………..………………………………………………....58
4.1.6.1 Deposition Using Glove Box…………………….…………………………………………....59
4.2 Surface Characterization of Nb Coated Mg……….………………………………………………......61
4.2.1 X-Ray Photoelectron Scopy(XPS)………….………………………………………………......... 62
4.2.2 X-Ray Diffraction (XRD)…………….…………………….……………………………….…......64
4.3 Electrochemical Corrosion Evaluation.........…......................................................................................65
4.4 Effect of Salt Spray Test….....................................................................................................................67
4.5 Effect of Humidity…..………...............................................................................................................68
ix
TABLE OF CONTENTS (continued)
Chapter
Page
5. CONCLUSION AND FUTURE RESEARCH...........................................................................................69
5.1 Conclusion…………................................………………………………………………………….....69
5.2 Future Research.....................................................................................................................................70
6. REFERENCES ……………………………………………………………………………………….......71
x
CHAPTER 1
INTRODUCTION
1.1 Project Objective and Motivation
The rise in petroleum production in the recent couple of decades and concerns about global climate
change requires efforts to increase fuel efficiency. Therefore, at the Kyoto conference on climate
change, in December 1997, 159 countries participated and agreed on the critical issue of reducing
the emission of green house gases [1]. The target was to reduce emissions by 8 % between 2008
and 2012. As shown in Fig 1.1, according to United States Environmental Protective Agency,
transportation accounts for about 32% of total energy use in USA and is a significant source of the
CO2 in the environment [2].
Fig.1.1 All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2012 [2] .
1
The United States transportation system is 95% dependent on petroleum based fuel [3, 4]. As
mentioned in Table 1.1, the weight of the vehicle is the primary source of fuel consumption and
creates the scope in improving fuel efficiency for light-weight vehicles and aircraft.
Table 1.1 Fuels Economy Improvement % per 100 lb. Weight Reduction – Gasoline Engines [5]
2
Fig 1.2 Fuel Economy Improvement % - Gasoline Engines [5]
Fig 1.2 shows research conducted by Ricardo Inc. that weight reduction from 5-20 % in vehicles
can increase fuel efficiency up to 18 %. Manufacturing industries must find ways to reduce weight
reduction by introducing lighter weight and stronger materials. At the top of the list is magnesium,
which offers a set of highly attractive properties [6].
1.2 Current Strategies of Weight Reduction
The use of polymers and fiber reinforced composites has been made for weight reduction in
low density aerospace and automotive structural applications; however, they have many
limitations. Applications of these materials in impellers, turbo charges and jet engines parts are
not possible due to limited properties under low or elevated temperatures, missing electric
conductivity or low damage tolerance [7, 8]. Fiber reinforced plastic is costly material that can
3
only be used for primary structured applications such as wings, load bearing portions of the
fuselage, crew and passengers seats- items with the highest requirements. Therefore, different
research has been conducted to find other metallic weight reduction techniques. Current
technology for metallic weight reduction strategies includes aluminum and steel alloys. These
have already been optimized for fatigue and damage tolerance properties [9, 10]. Further
progress in these areas is becoming very difficult due to the slow development of these
materials.
1.3 Magnesium and its Alloys for Aerospace and Automotive Applications
Magnesium is the eighth most abundant element on Earth, making up approximately 1.93 % by mass
of the Earth’s crust and 0.13 % by mass of the oceans [10]. Magnesium is the second most abundant
metal in salt water, following sodium [10]. Therefore, magnesium is considered as an inexhaustible
resource. Since magnesium has a lower specific heat, it gives magnesium the advantage of using
lesser energy in recycling [11]. Across the industrial landscape, the world has witnessed the rapid
growth in the application of magnesium and its alloys. Magnesium gives the characteristics of both
structural and chemical elements to other alloys. Magnesium alloy demands have traditionally been
driven by the need for light weight materials to operate the components significantly under very
challenging conditions. Magnesium is in demand not only in the aerospace industry, but also in
electronics and automotive sector. Fig 1.3 shows a magnesium alloy auto casting part is currently
being used, and with significant results. Other applications include wrought products, powder
metallurgy components, office equipments, nuclear applications and flares [12].
4
Fig 1.3 Magnesium Alloy Auto Casting [13]
Other than these areas, the most significant usage or market for magnesium alloy is in the automotive
and aerospace industries. Fig.1.4 shows applications of Mg alloys in motor vehicles, which include
transmission cases, seat frames, intake manifolds, steering wheels etc. Magnesium alloys have been
used widely in aerospace for lighter weight components [14].
Figure 1.4 Applications of Mg Alloys in Motor Vehicles [14]
5
Fig.1.5 Convair B-36 [15]
Fig.1.5 shows a Convair B-36 plane that was manufactured using a total of 8600 kg of magnesium,
including 5555 kg of magnesium sheet and 700 kg of forged. This shows that magnesium alloys are
being used successfully in the aircraft industry as well.
6
Table 1.2 Magnesium Alloys Application [16]
Alloy
AZ91
AZ31
AM60/AM50/AM20
Application
Notes
Cast Helicopter Transmission
Less costly, good tensile
Housings, space station, artificial
strength, very susceptible to
satellite, space shuttle, Drive brackets,
creep above 120 degree
manual transmission case
Celsius
Intricate components of Automotive
Medium strength alloy,
parts, concrete tools, aircraft fuselages,
ductile and weld able, good
cell phones and laptop cases
formability
Automotive (wheels , seat frames,
Greater ductility and fracture
steering wheels) and other parts
toughness
requiring elongation
AS41
QE22
Automotive (crank case, transmission
Easier to cast with better
housing)
fluidity than AS21
Aerospace ( landing wheels, gearbox
Superior tensile properties,
housings, helicopter rotor fittings)
Expensive due to silver
The automotive and aerospace industries have explored the use of magnesium in several applications
(Table 1.2) with considerable benefits using magnesium [17]. The use of magnesium significantly
decreases the load on the engine, suspension and front-end of the car, which is helpful in reducing
7
the fuel consumption and emissions. Magnesium helps in improving the vibration dampening with
the suspension system and contributes to braking safety. Shock absorber, made of aluminum can be
improved 100 times by using magnesium [18]. Magnesium is used in civil aircraft, helicopters, and
satellites. Aside from these applications, magnesium is commonly used for building pumps, fuel
filters, valve covers, fuel injectors and oil pan. Forged magnesium wheels (cast magnesium) are
usually used for motorcycles. Magnesium alloys are now used in aircraft components fabrication
because of their low density and good mechanical property [19].
Table 1.3 Comparative Properties of Metals [20]
Property
Magnesium
Aluminum
Steel
CFRP
Titanium
Density (g/cm3)
1.8
2.7
7.9
1.1-1.6
4.4
Density(oz/cubic inch)
1.04
1.56
4.57
0.64-0.92
2.56
YieldStrength (0.2% PS MPa)
300
500
1240
200
910
Yield Strength (0.2%si)
43.5
72.5
179.8
29
132
strength(kNm/kg)
167
185
157
182
205
Youngs Modulus (GPa)
44
70
220
26
120
Specific stiffness(MNm/kg)
24.4
25.9
27.8
23.6
27.1
Poissons Ratio
0.35
0.33
0.27-030
0.06-0.32
0.34
Machineability (kWh/kg)
0.085
0.15
0.54
n.a.
0.65
(W/m°C)
51 – 156
130-230
75
0.23
22
Thermal Expansion (x10-6/K)
24-27
24
12
12
8
Melting Point (°C)
650
658
1540
250
1660
2.2
3.7
0.9
n.a.
0
Specific yield
Thermal conductivity
Electrical Conductivity(x
107S/m)
From Table 1.3, we find that magnesium is a lightweight and structurally versatile metal as
8
compared to others and contributes significantly to fuel saving. Magnesium alloy can be recycled at
5 % of initial energy requirements and therefore, is ideal for use in manufacturing applications.
Magnesium alloys are frequently used in the fabrication of aerospace parts due to their excellent
mechanical properties and low density. Magnesium sheets could be used in body structural and semi
structural applications, while extruded products could be used in structural applications such as
space frames.
1.4 Potential Cost , Fuel and Environmental Benefits
Magnesium’s higher strength to weight ratio makes it favorable for automotive and aerospace
applications [21]. According to Reuters, by 2020, the usage of Magnesium in the automotive
industry will reduce the weight of the vehicle up to 15%, which will lower the fuel consumption to
9-12 % [22]. In the aerospace industry, it is estimated that the fraction of fuel in total operation cost
is 35-40 % and reduction of 20% weight in aircraft saves about 10 % of the fuel and reduction of 30
% weight in aircraft will save about 10 % of operation cost [23]. Reducing in vehicle weight saves
energy, minimizes the brake and tire wear and also lowers emissions [24]. Vehicles with less weight
are having a direct impact on cutting the carbon dioxide emissions and improving fuel economy
[24]. When taken into a broader perspective, reducing 11 kg from every 70 million light vehicle
engines produced during 2011 could save up to 908 million liters of refined fuel. Cutting down 5 kg
from the 40 million automotive transmissions manufactured in 2011could save 350 million liters of
refined gasoline [25].
1.5 Limitations of Magnesium
Several drawbacks have limited the growth of magnesium in automobiles. One of the most important
is a relatively poor corrosion resistance and flammable nature when powdered or in molten form
9
[26]. Magnesium has a high chemical affinity and high reactivity that makes it more difficult to
handle. Magnesium hydroxide tends to react with the environment and transform to carbonates of
magnesium. These salts formed on the surface destroy the passive oxide layer, leading to an
acceleration of general corrosion. Magnesium corrodes even in moist air and in distilled water.
Figure 1.6 gives a comparison of corrosion rates of magnesium and magnesium alloys in sodium
borate determined from electrochemical and gravimetric techniques.
Figure 1.6 Comparison of Corrosion Rates of Pure Magnesium (99.98% pure) and Magnesium
Alloys Determined by Electrochemical and Gravimetric Techniques [27].
The effect of aqueous solution on magnesium varies considerably depending on the nature of the
solute. Magnesium is resistant to attack by alkalis but is attacked by most acids [28]. The use of
magnesium has been hindered by its prominent tendency to staining and corrosion in wet and saltladen atmospheres [29]. The poor wear resistance also limits the contact loads. Such drawbacks of
magnesium alloys didn’t persuade the manufacturers to initially use magnesium alloys in the
industrial sector. Magnesium alloys are attractive for the manufacturers of complex shapes, such as
10
transmission gearboxes, in that they are readily cast and machined [30]. However, the corrosion
resistance of Mg alloys is very poor in wet salt-laden environments [26, 29]. Furthermore, it is
highly susceptible to galvanic corrosion when coupled to another metal [31, 32]. The undesirable
corrosion properties of Mg alloys are due to magnesium being the most electrochemically active
structural metal and the detrimental effect of many alloying elements on the corrosion resistance of
Mg [31, 33]. To overcome the corrosion resistance of magnesium, generally the aircraft industry
requires high maintenance costs that restrict them to use magnesium for potential applications
frequently, such as gearbox casings, aircraft engines, generator housings and canopies.
1.6 Methods to Overcome Metal Corrosion
Efforts have been taken to slow and control the corrosion issue of metals. A few surface modification,
techniques to enhance the corrosion resistant properties of metals, are as follows;
1.6.1
Alloy Development
It is the development in new alloys which shows promising improvement in corrosion resistance by
removing the presence of iron, nickel, and copper which can increase the corrosion of magnesium
and its alloys [34]. A combination of these elements below the tolerance limits can increase the
resistance of magnesium alloys [34].
1.6.2 Rapid solidification processing
Rapid solidification processes allow the creation of new alloys. This technique is used for refining
the primary alloy element such as refinement of silicon in production of high Si-Al-Si alloys. It is
the transformation of liquid to solid by removing superheat and latent heat at a cooling rate of 102106 k/s [35].
11
Rapid solidification (RS) reduces the effects of impurities. It increases the limits of solid solubility,
helping in formation of compositional and new phases and makes impure elements ineffective in
other phases. Rapid solidification process also helps in homogenization of material therefore not
allowing the local cell corrosive action [36, 37].
1.6.3
Ion Implantation
It is the exposure of surface to a beam of ionized particles which implants the ions in the material.
Not much progress has been made on Magnesium using this technique but still corrosion protection
on AZ91 embedded with N2+ have been successfully done [36, 37].
1.6.4
Laser Annealing
It is the surface heat tempering similar to the rapid solidification process and the modification here
is only done onto the surface area through the laser beam, thus creating a meta-stable solid solution
on top of the surface. This process is still in initial phases but corrosion resistance has been
achieved on Mg-Li and Mg-Zr alloys [37].
1.6.5 Protective Coating Strategy
The protective coating strategy is useful in resisting the corrosion on magnesium. It is an
independent layer applied to the surface of magnesium alloy. The technique differs from the surface
modification or treatment as there is not any significant difference to the magnesium alloy surface
underneath the layer of the coated material. The coating helps to prevent the corrosive magnesium
alloy substrate material from exposing into the corrosive environment. There are number of
methods for surface coating on magnesium and its alloys to improve its corrosion resistance
12
property for engineering applications such as conversion coatings, anodizing, hydride coatings,
sputter coating process, spray coatings and electrodeposition [38].
Magnesium alloys are coated with organic coatings to protect the surface from chemical or
atmospheric attack. Solvents in organic coatings evaporate into the atmosphere and contribute to
volatile organic compound and airborne toxic emissions. Organic paints provide a protective skin
when applied as coating on a pretreated surface, resulting not only in a corrosion resistance surface
but also in beautification of the vehicle appearance and increasing customer satisfaction.
Conversion coatings are helpful in metals to be painted through acidic chemical products by
converting the substrate to iron or zinc phosphate surface. Thin films of overlapping crystals that
form during the pretreatment reaction make the conversion coating smooth. Electrodeposition
technique is a strategy to coat irregular surfaces. It has many advantages from low cost of
instruments, smooth deposition, and deposition at room temperatures with environment friendly
coating process. Therefore, electrodeposition method was selected as a coating strategy for
magnesium alloy to reduce the rate of corrosion.
1.7 General Applications of Niobium Coated Material
Niobium (Fig.1.7) (also called as Columbium) is the 41st element in the periodic table. Niobium is a
rare element and the percentage content of Niobium in earth’s crust is 2 wt%. Niobium is used
widely, in the form of metals and alloys, in the process industry, vacuum techniques, surgical
devices, and the automotive industry [39]. It exhibits excellent strength at high temperature and is
widely used in aerospace components and missiles [40].
13
Fig 1.7 Niobium in its Natural State [41]
Niobium additives are used to stabilize and improve modern stainless steels [42]. Super alloys
containing tantalum and niobium are successfully applied in gas turbine engines and in the
aerospace industry as well [43-45]. Other than this, niobium demand is increasing significantly in
automotive industry in production of high strength lighter weight steel, which lowers fuel
consumption and carbon dioxide [41]. According to the World Steel Association, US$9 of Niobium
per car leads to a 100 kg weight reduction and ongoing fuel savings of 1 liter per 200 km yielding a
reduction of 2.2 tons of CO2 per vehicle which is greater than the total amount of CO2 created
during the production of all the steel required for the vehicle [41]. Niobium nitride is used as a
superconductor helpful in detecting infrared light as well as in absorbing anti-reflective coatings.
The compounds niobium-germanium (Nb3Ge) and niobium-tin (Nb3Sn), as well as the niobiumtitanium alloy, which display superconductivity at relatively high temperatures, are used as a type II
superconductor wire for superconducting magnets in special applications [46, 47].
Niobium alloys are widely used in biomedical field, as in case of superconducting magnets in
magnetic resonance imaging equipment, to examine the strength and abnormalities in soft tissues
[48]. Niobium is added, in the form of carbides, to cemented carbide compositions used in
14
production of cutting tools. Pure oxides are widely used in the optical industry as additives and
deposits, and in organic synthesis processes as catalysts and promoters [39].
Niobium alloys, as mentioned in Table 1.4 have their applications in different markets including
nuclear engineering, where high temperature are needed because it is highly resistant to corrosion
by liquid sodium potassium alloys and highly suited for nuclear fuels. Niobium-1% zirconium is
used in rocketry and in the nuclear industry; the space nuclear reactor presented in is predominantly
made of this alloy. It is regarded as a low-strength alloy [49, 50]. Further applications of Niobium
in aerospace industry is its usage in springs and superconducting magnets for aerospace, nuclear
reactor heat pipes, high-energy physics, thermal couplings in aerospace, RF cavities, fasteners and
mining tools for aerospace, plasma spray coatings [51].
Table 1.4 Applications of Different Niobium Products [52]
15
1.8 Thesis Outline
The aim of this thesis is to coat niobium on to magnesium alloy to protect magnesium alloy
from corrosion for its application in aerospace and automotive industry for lighter weight aircraft
and cars in order to reduce the cost on fuel. Electrodeposition of niobium on magnesium alloy is
done for its potential application as a corrosion resistant material to enhance the corrosion
resistance of magnesium. This thesis is structured into five chapters. Chapter 2 focuses on the
literature review of magnesium and niobium used as materials for aerospace and automotive
industry. It focuses on significance of reducing the weight and its impact and necessity in current
global scenario. It also discusses the corrosion behavior of magnesium, surface coating strategies,
niobium and its significant advantage of coating on magnesium. In Chapter 3, the experiment
protocol and techniques are discussed, the pre-treatment process, and electrodeposition process
which provides a background and the related coating techniques, the data, which is collected to
conduct various experiments by using the literature review from Chapter 2. In Chapter 4, the
results of the experiment are discussed using Optical microscope, Scanning Electron Microscopy
with Electron Dispersive Spectroscopy (SEM-EDS), X-Ray Diffraction, X-Ray Spectroscopy,
Tafel plots by using GAMRY potentiostat, Salt Spray test, and Humidity test which confirmed the
deposition of niobium on magnesium substrate and also corrosion tests are discussed. Chapter 5
focuses on the conclusions and future research directions.
16
CHAPTER 2
LITERATURE REVIEW
2.1 Magnesium as a Light Weight Material
Considering light weight, high specific strength, and good castability, the material which comes into
the mind for engineering applications is magnesium, which convinces the manufacturers in making
use of this material in automotive and aerospace industries.
The weight reduction is one of the important things; the automotive industry is trying to achieve in
order to reduce the fuel efficiency. Currently the industry uses high strength steels, aluminum, and
polymers in order to decrease the weight of the vehicle but a huge potential of weight reduction
remains. Increase in use of magnesium and its alloys depend on how the corrosion on magnesium is
prevented. When compared, magnesium is 36 % and 78 % lighter than aluminum and iron
respectively. Magnesium has the highest strength to weight ratio of all the structural metals. Weight
of the vehicle is of prime importance as it contributes significantly to the fuel consumption of
automobiles. A general study suggests that about 86 % of the energy consumption of a vehicle is due
to the vehicle’s own weight and the people it carries.
Magnesium use in the automotive industry has increased in recent years. The average 1500 kg North
America vehicle has approximately 120 kg each of aluminum and plastic but only 3.8 % of the
magnesium [53] but usage of magnesium has increased by 20 % annually over the past 10 years. In
North America, 58,000 metric tons (MT) of casted magnesium in automotive industries is used
annually only by 3 companies with General Motors using 28,000 MT, Ford Motor company uses
20,000 MT, and DCX uses 10,000 MT [53].
17
Since 1990, magnesium parts have been used successfully in the area of power trains, car interiors,
chassis, and body parts, using the techniques of advanced engineering design, alloy development,
and die casting technology [54].
Automotive industries find great opportunity of fuel savings in magnesium due to its high strength
and low density. Therefore, magnesium alloys are very useful as structural materials in all
applications where weight savings are of great concern. In automotive applications weight reduction
will improve the performance of a vehicle by reducing the rolling resistance and energy of
acceleration, thus, reducing the fuel consumption and moreover a reduction of the greenhouse gas
CO2 can be achieved [55].
Parts made of magnesium which are widely used in the automotive industry are steering wheel core
and instrument panel (or cowl cross beam) in the interior part, and engine head cover in the power
train parts. Additional parts can be steering column housing, seat frame, intake manifold system and
manual transmission case in the automotive parts [54]. The automotive parts with high temperature
magnesium alloys, such as engine block, engine cradle, etc., have been recently explored, and are
expected to be widely used in the near future [56, 57].
2.2 Magnesium Alloys and their Properties
Magnesium alloys are useful for automotive, aerospace and other engineering applications due to
their good strength, light weight, ductility. In recent years, plastics for engineering usage have lost
interest from manufacturers mind and are overshadowed by magnesium alloys due to their density,
stiffness and recyclable nature. Magnesium alloys are widely used in products like chain saws,
power tools, steering wheels, crank case, camshaft, gear box housing. Magnesium alloys show
promising opportunity to the manufacturers during casting process. Compared to cast aluminum,
18
casted magnesium (Fig. 2.1) can easily be used in manufacturing of thinner walls, also the use of
magnesium alloys in casting process takes lesser cooling time as compared to aluminum [58].
Conventional casting methods are still useful in making components of magnesium alloys.
Permanent mold casting is initially expensive compared to sand casting, therefore, only useful for
high volume production such as Mg-Al-Zn alloys
Figure 2.1 Magnesium Die Casting
The use of pure magnesium can be easily affected by the corrosion; with alloys it can be improved.
Magnesium resistance can be enhanced my adding 0.2 % manganese. Metallurgical factors such as
composition microstructure and heat treatment also increase the resistance rate. Alloys which are
common in automotive applications are AS41 and AS21 due to its ability to use effectively in
casting process in the manufacturing of rear engine, crank case and crankshafts of Volkswagen
Beetle [59]. Other than these, alloys such as AM60, AM50 (Fig.2.2) and AM20 provide good
ductility and fracture toughness and are used in the manufacturing of wheels, seat frames and
steering wheels. AZ91, WE43B and Elektron 21 are suitable for high corrosion resistance
19
components. Military aircrafts and helicopters (Fig.2.3) built during World War II included heavy
magnesium alloy components.
Fig 2.2 Magnesium (AM50) Die-Cast Helicopter Components [15]
Fig.2.3 Sikorsky S-56, Westland Aircraft Ltd. (1950) 115 kg of Magnesium [15]
20
Fig.2.4 TU-95MS: 1550 kg of Magnesium [15]
The aerospace industry of the former Soviet Union widely used magnesium alloys in military air jets
(Fig.2.4) and vehicle structural platforms [15]. Magnesium alloys are currently used in helicopter
transmission housing e.g. UH60 Blackhawk transmission.
However, due to the poor corrosion the maintenance cost of all these components is really high and
required product lifetime is not achieved. QE22 alloys are used in gear housing of helicopters and
engines of racing cars but is extremely expensive [60]. AZ91 is used in automotive and aerospace
application because of its good casting qualities. Magnesium AZ31 is used as die-cast for the
military Falcon GAR-1 stabilizer fins. AZ31 alloy is the most widely used magnesium alloy in
market, as sheets made from AZ31 have been used extensively in prototyping testing of automotive
sheet panels of cars [61]. Development on these and many other alloys will be useful in future
automotive and aerospace applications. Sheets and extruded alloys with them can be used in
manufacturing components but successful application is restricted by heavy cost incurred in alloys
There are coatings strategies on magnesium alloys such as conversion coatings, anodizing, painting,
sol-gel, but presence of chromates, cyanide and other toxic carcinogens make it hazardous for the
environment [62-65].
21
2.3 Corrosive Nature of Magnesium
In 3% salt environment, the standard electrode potential of magnesium changes significantly from 2.37 V to -1.63 V and magnesium forms a weak and imperfect oxide film on its surface, therefore, it
is difficult for it to resist corrosion in an acidic environment. Moreover, galvanic corrosion affects
the magnesium rapidly due to the heavy metals or flux contamination. Metals which have low
hydrogen overvoltage like iron, nickel and copper result in galvanic corrosion where as Al, Zn, Cd
with high hydrogen over potential are less affected by corrosion [66]. The galvanic corrosion
behavior of AZ91D, AM50 and AM60 cast magnesium alloy coupled with A3 steel, 316L stainless
steel, H62 brass and LY12 aluminum alloy were investigated in different atmospheric conditions by
Song [67]. He concluded that magnesium alloys act as anode and the resistance to corrosion of these
alloys decreased when coupled with these metals. Proper Coating and insulation of material reduces
risk of corrosion. For example, fasteners made of aluminum of 6000 series reduce galvanic corrosion
of magnesium significantly in salt spray tests [68]. Magnesium is a naturally passive metal. Pitting
corrosion occurs at free corrosion potential of magnesium, when it is exposed to chlorine
environment in a non-oxidizing medium [69]. It is generally observed that corrosion pits initiate at
flaws adjacent to a fraction of the secondary phase articles such as MgI7All2. AlMn as a result of the
breakdown of passivity [69, 70].
Corrosion equation of magnesium when in contact with aqueous solution is:
Mg (s) + 2 H2O (aq) ↔ Mg (OH) 2 (s) + H2 (g)
This reaction can be separated in three partial equations
Mg (s) ↔ Mg2 + (aq) + 2 e- (anodic reaction)
2 H2O (aq) + 2 e- ↔ H2 (g) + 2 OH- (aq) (cathodic reaction)
Mg2 + (aq) + 2 OH- (aq) ↔ Mg (OH) 2 (s) (Total reaction)
22
Although the oxide layer on Magnesium surface is very weak but still it can protect against a low
level of corrosion. It becomes susceptible when the concentration of aggressive chloride ions
increases above 30 mmol/l and therefore changes into MgCl2 with high solubility [31].
Environmental condition also influences the corrosion rate; and increase in relative humidity can
increase the corrosion rate. High humidity accelerates surface corrosion cracking during atmospheric
exposure [71]. In marine environment, where there is high exposure of salt, magnesium alloys
require protection for prolonged survival [72].
The pH value also has a significant effect on the corrosion of magnesium alloys. In neutral or
alkaline salt solutions, magnesium corrosion typically takes the form of pitting corrosion [29, 72].
Magnesium can resist corrosion in alkaline solution, in particular, when the pH value is higher than
11.5 [29, 73]. With pH 4 to pH 14, the corrosion effect on to magnesium alloys in B phase is very
less. Analysis by Auger electron spectroscopy (AES) showed that the thickness of the oxide film of
the B phase formed in solution with pH=12 were several times that formed in air, increasing with
decreasing pH value [74]. Coating on magnesium alloy can protect it from corrosion, if the coated
material provides good toughness, high hardness, and mechanical strength. That can also help in
fatigue and wear resistance of the sample.
2.4 Review of Surface Coatings of Magnesium Alloys
More advanced techniques have been developed to deposit a thin layer of a different metal which
enhance the desired properties of magnesium. These include coating methods which are in use to
protect magnesium from corrosion using electrochemical plating, conversion coating, anodizing, gas
phase deposition, organic coatings, and hydride coatings [62, 63, 65]. The challenges of various
coating technologies for the corrosion protection of metal including magnesium are described,
23
however, no coating technology has successfully achieved the desired results to protect magnesium
from corrosion in critical components such as engine crank shafts under extreme conditions in
automotive applications. The main challenge which is associated with the electrodeposition of
magnesium is its extremely high chemical reactivity resulting in quick formation of oxide or
hydroxide film. This problem can be tackled by a copper immersion technique as patented by Luan
and Elizabeth [75]. In copper immersion technique, a thin layer of zinc and copper is being deposited
on magnesium to protect it from the highly corrosive acidic bath. However, the hydrofluoric acid
was still used in copper sulfate containing immersion electrolyte [76]. The process starts with the
acid pickling, after that activation of the surface and then coating zinc layer by immersing it in a zinc
solution and after that copper immersion.
Another experiment was introduced by Neubert et al [77], where the sample was degreased and
cleaned in flowing water and
immediately put in to pyrophosphate bath to be anodically
galvanostatic–etched for a few minutes, and copper was deposited in the same electrolyte as the
etching solution. A pretreatment with galvanostatic etching for copper electrodeposition on pure
magnesium and magnesium alloys in an alkaline copper sulfate solution have been done successfully
[78]. In another process, the sample was degreased and cleaned at first, then electroplating of copper
using copper sulfate as the primary salt in bath tub and potassium sodium tartrate as the complexing
agent have been used and finally plated it with nickel plating to protect it from corrosion.
2.4.1 Conversion Coatings
Conversion coatings are produced by chemical or electrochemical treatment of a metal surface to
produce a superficial layer of substrate metal oxides, chromates, phosphates or other compounds that
are chemically bonded to the surface [79]. There are a number of different types of conversion
24
coating including chromate, phosphate/permanganate and flourozircone treatments [80-82].
Conversion coating is not much costly; most widely used type of conversion coatings are chromate
based coatings. Conversion coatings do not provide adequate corrosion and wear protection from
harsh service conditions when used alone [83]. Conversion coating is only helpful in safeguarding
the material during transportation and storage on a temporary basis. The challenges associated with
this are that; it represents a serious environmental risk due to the presence of leachable hexavalent
chromium in the coatings [84, 85] and exposure to that in production environment can cause lung
cancer, nasal irritation, atrophy and upper and lower respiratory effects.
2.4.2 Anodizing
Anodizing is an electrochemical process for producing a thick stable oxide film on metals or alloy
surfaces [64, 86]. The coating produced is brittle, insulating material and is not appropriate for load
bearing applications or where corrosion resistance is of primary importance [87]. Anodizing
treatments have also been used on magnesium to force the surface to form a complex coating of
oxide film. As the dielectric force applied, impressed voltage reaches the same point and arcing
takes place inside the solution. This arcing produces heat and resulted in deposition of oxides of
other elements. The outcome is a thick and hard coating which significantly modifies the surface
properties and favors the adhesion of post-treatment coatings, for example painting [88]. H.
Ardelean et al. enhanced the corrosion resistance of AZ91 magnesium alloy in niobium and
zirconium-containing electrolytes using anodizing method [63].
2.4.3 Organic Coatings
Organic coatings are versatile and can be applied to many metals provided and appropriate
pretreatment can be developed for the substrate [89]. Magnesium surfaces must be free from surface
25
contamination , smut and loose silicates , oxides and inter-metallic compounds [90]. Environmental
concern exists with the use of solvent based organic coatings because they can be volatilized at
normal temperatures and pressures. Therefore, there are a number of health risks involved; in
addition, solvent based coatings need careful storage and handling procedures to avoid fire and
explosion hazards. Also the adhesion, corrosion and wear resistance of these coatings in harsh
applications, including automotive, have not been widely documented [83].
2.4.4 Gas Phase Deposition Process
The gas phase deposition process produces protective coatings from the gas phase [91]. Examples
include thermal spray coatings [92, 93], chemical vapor deposition [94, 95], physical vapor
deposition [96, 97], diffusion coating [98] and ion implantation [36, 37].
The thermal spray coating technique is the melting of the deposited material, which was in the form
of powder, by feeding it into a gun and heating it to its melting point. The droplets formed when
sprayed from the gun are accelerated in a gas stream onto the substrate and the droplets flow into
thin lamellar particles and adhere to the surface [99]. It has the advantage to coat material that melts
without decomposing. It can also be recoated when it’s worn. The challenges associated with this
include health issues, due to dust and fumes and light radiation, and sealing to have a smooth finish.
The investment on all these gas phase coatings is high when compared with solution phase coating
technologies. These coating processes are based on line of sight; therefore it is extremely difficult to
coat the complex shapes, inside holes and deep recesses. Also the corrosion, adhesion and wear
properties of these coatings on magnesium have not been widely documented [87].
26
2.4.5 Electrodeposition Process Using Magnesium
Electrodeposition is a process by which a substrate is coated with a metal that has the desired
properties necessary for the specific application [100, 101]. Typically a melt salt in solution is
reduced to its metallic form on the surface of the substrate. One of the major advantages of this
process is the low capital investment. Challenges associated with this technology are the use of toxic
chemicals such as chromium, cyanide, fluoride and corrosive acid based pretreatment, and bath
solutions [87] [102]. Magnesium reacts aggressively with most acids and dissolves in acid media,
which necessitates the avoidance of acid based electrolyte solutions. Also electrolytic contact with
other metal can cause the formation of local corrosion cells on the surface leading to pitting.
Therefore, the metal coating on magnesium must be pore free or the corrosion rate will increase
[103]. A minimum coating of 50 microns has been suggested to ensure pore free coatings of Cu-NiCr for outdoor use [87]. Noble metal plating on magnesium has been successfully deposited for
space applications. Gold plating has been achieved using the following sequence on magnesium–
lithium alloys [104]. Gold plating of magnesium alloy AZ31 has been achieved using the zinc
immersion pretreatment process followed by electroless nickel plating and subsequent gold plating
[104]. The steps involved in gold plating include degreasing, alkaline cleaning followed by chromic
acid pickling with a thin layer of nickel deposition and striking with a gold layer. Gold plating on
magnesium alloy RZ5 has been investigated using a variety of techniques, such as initial plating of
nickel by using non-aqueous solutions and then plating gold on that nickel layer [105]. However,
this technique is expensive and there is a possibility of moisture contamination in it as well.
Nickel coatings on magnesium have also been proposed for use in aerospace applications. Direct
electroless nickel plating on magnesium alloy ZM21 has been shown to produce coatings with good
mechanical, environmental, optical and soldering properties [106].
27
2.5 Electrodeposition of Niobium Using Ionic Liquid
Niobium is a soft, rare transition metal used in the production of high-grade steel. It is corrosion
resistant and exhibits superconductivity properties. Niobium finds several automotive applications in
its use as an alloying agent to develop high strength steel grades [107]. Niobium has the property to
withstand the extreme conditions of the nuclear and aerospace industries. However, the successful
electrodeposition of niobium has remained elusive [108]. Current methods of electrodepositing
niobium on other elements involve high temperature molten salts [109].
Production of niobium deposits, which are coherent and closely adherent, has been observed in
fluoride melts on low carbon steel [110]. However, the unfavorable condition that restricts this
method is the toxicity and corrosiveness of the environment and the bath tub. Therefore, the
challenges associated with this technology are the use of toxic chemicals such as chromium, cyanide,
fluoride and corrosive acid based pretreatment, and bath solutions [87] [111].
The ionic liquid is the most common in preparation of the electrolyte and carrying out the
electrodeposition process of metals is 1-ethyl-3-methylimidazolium chloride (EMIC). The deposition
of niobium was studied by Cheek and Trulove using the same ionic liquid in acidic melt and
observed corrosion resistive films on the platinum [112]. Another successful attempt at the
electrochemical deposition of niobium on surface of copper using choline chloride-based ionic liquid
has been done [113]. Electrochemical reduction of NbCl5 in an acidic melt indicates a clear one–
electron cyclic voltametric process, followed by a reduction in a more complex process,
predominantly to the trivalent state [114]. The deposition of niobium on platinum occurs with traces
of aluminum in powdered form without any thick films. This experiments with chemical reduction to
lower valence states, and then, experimenting with electrochemical reduction gives a successful
28
deposition of Nb films on platinum. The niobium deposition was also tried on low concentration
with lower current densities, but was hardly successful. In the AlCl3: n-butyl-pyridinium chloride
molten salt system, chemical reduction with aluminum powder allowed deposition of Nb-Al films
with considerable niobium content [115].
Barbato in experiments containing 0.25M NbF5 and 0.25M LiF at 100˚C for 1 hour using ionic
liquid 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]Tf2N) deposited a
thin film of niobium on nickel, but the film was not uniform [116]. Chirkov attempted Niobium
deposition on copper and stainless steel using ionic liquid 1-ethyl-3-methylimidazolium chloride
(EMIC). He observed only thin films of niobium on both metal substrates with no high peaks of
crystalline niobium. On the other hand, in case of using magnesium as a base material, the metal
aggressively reacts with most acids and dissolves in acid media and chlorine environment; therefore,
it is necessary to avoid acid based electrolytes. Investigation has been carried out using such ionic
liquid, which consist of nontoxic acid free electrolytes for electrodeposition on magnesium alloys.
29
CHAPTER 3
EXPERIMENTAL PROCEDURE AND TECHNIQUES
3.1 Introduction
The lower specific weight of magnesium is promising for applications in automotive and aerospace,
but the corrosion of magnesium and magnesium alloys limits these applications. Electrodeposition
on magnesium shows the potential to increase the resistant properties of magnesium alloys and
results in higher fuel efficiency.
3.2 Experimental Procedure
3.2.1 Selection of Substrate
The material used in the experiment is Magnesium (Mg) AZ31 alloy (Fig.3.1), which is used for
photoengraving, power general and defense applications. AZ31 is the most widely used commercial
magnesium alloy and is available in sheets. The counter electrode was a niobium rod with a length of
100 mm and a diameter of 16 mm. The surface was cleaned with acetone by putting the sample
inside the ultra-sonication machine for 5 minutes to remove the dust particles and impurities on the
surface. A magnesium AZ31 sample was also used as a control sample to compare the corrosion and
degradation rate of the coated sample with the controlled one.
Fig 3.1 Control Magnesium Alloy AZ31 Sample
30
3.2.2 Surface Treatment Techniques
The aim of the pretreatment prior to any kind of coating is to remove foreign matters from the
surface such as, grease, cutting oil and films on the substrate. Surface impurities can be composed of
scale, soil, and dust from the exposed atmosphere and, thus, affect not only the coating quality but
could possibly prevent it as well. Therefore, surface treatment is necessary to make sure that the
condition of the surface is suitable for the coating. The treatment on the substrate is uniform
throughout the substrate with all work pieces to ensure the material surface is effective for coating.
Oil and dust removal is among the most common pretreatment methods for metallic surfaces for
coating. This could be achieved by cleaning the substrate with a methanol or acetone solution.
Dipping the substrate in a beaker with acetone or methanol solution is another procedure, since over
time; it will be evaporated completely to leave the substrate oil and grease free. The cleaning
procedure which is used in experiment, is ultrasonic cleaning of samples with the help of ultrasound
(ranging from 20-400 KHz). Cleaning normally lasts between three and six minutes, but can exceed
20 minutes, depending on the object to be cleaned [117]. The base material used in the experiments
was magnesium AZ31 alloy. Before going for the electrochemical deposition, the samples were pretreated by dipping in acetone solution in a beaker and, then, placed it inside the ultrasonic cleaner for
6-8 minutes.
3.2.3 Electrodeposition Experiment
Coatings can protect magnesium and its alloys by providing a barrier between the metal and its
environment. Electrochemical plating is a process by which a substrate is coated with a metal that
has the desired properties necessary for the specific application. Here the electrodeposition of
niobium on magnesium was done using an environmental friendly green ionic liquid. Mg AZ31
samples were cleaned and used for electrodeposition.
31
The electrostatic deposition was carried out in a controlled electrochemical cell with Mg as the
cathode, niobium as anode and a proprietary developed ionic liquid as the electrolyte.
The electrolyte solution contains 0.125 M of non-corrosive Ionic Liquid (IL) and 0.005 M of
niobium bromide after examining the electrolytic bath acidity. Electrolyte is prepared by the slow
addition of niobium bromide in ionic liquid solution which is mixed in solvent at room temperature
under continuous stirring. Non-corrosive Ionic Liquid 1 butyl-2, 3-dimethylimidazolium
tetrafluoroborate was used in the electrolyte to enhance the electrodeposition process. After that a
few drops of choline hydroxide were added as well to adjust the pH level and Nb bromide coating
solution was prepared and dissolved in solvent. A niobium electrode was used as an anode and
magnesium AZ31 specimen was used as cathode. Electrodeposition was carried out at 0.7 volts for
20 minutes using a DC power supply. After 20-25 minutes of sample immersion in electrolyte, it was
taken out from the electrochemical solution and rinsed in distilled water for 2-3 minutes and finally
dried in air. The samples were put inside ultrasonication for 5 minutes to remove any dust particles
or residual from the surface of coated sample of Mg AZ31. The sample was dried for 6 hours and
subsequently characterized by techniques (See Section 3.3).
3.2.4 Ultra-sonication Cleaning Method
An ultra-sonication machine (Fig.3.2) is helpful in applying sound energy to agitate particles on to
the surface of sample to check the strength of the coated material. Ultrasonic method of cleaning Mg
sample using methanol investigates the loosening of particles adhering to material surface.
Ultrasonic frequencies (>20 kHz) are usually used, leading to the process being known
as ultrasonication or ultra-sonication [118].
32
Fig 3.2 Ultrasonic Cleaning Machine
3.2.5 Parameters of Electrolytic Solution
Table 3.1 shows the parameters of different combinations of electrolytes used in deposition. In this
experiment, aqueous solution, acetone and dimethyl sulphoxide have been used as electrolytic
solution. All the experiments with different electrolytes were conducted using the similar
electrodeposition process as mentioned in section 3.2.3.
Table 3.1 Parameters for Nb Deposition on Mg AZ31
pH Level
of Bath Tub
Amount of
Electrolytic
Solutions
used
Ionic
Liquid
(gm)
Nb Bromide
Salt (gm)
DC
Voltage
2.5
40 ml
1 gm
1 gm
0.7
15- 20
3.0
40 ml
1 gm
1 gm
0.7
15- 20
4.0
40 ml
1 gm
1 gm
0.7
15-20
6.0
40 ml
1 gm
1 gm
0.7
15-20
33
Time of Deposition
(mins.)
3.2.6 Effect of Deposition Environment
Further experiments have been conducted using aqueous solution as an electrolyte in the controlled
Glove box environment. Studies show that deposition of certain elements requires the Glove box
environment; therefore we should look at whether it will help us in this case. The experiment was
performed under the Glove box environment using inert gas with oxygen content less than 5 %.
The Glove box (Fig.3.2), a sealed container, is designed to achieve the desired isolated environment
for experiments. The front side of the Glove box has gloves where person can place his hands for
performing the experiments inside in controlled environment. The Glove box is filled with high
purity inert atmosphere such as argon or helium. This setup will protect not only the inside
environment from outside contaminations but helps the user in handling the hazardous chemicals
easily. The transparent box allows the person to see the experiment which he is performing inside.
While creating the vacuum environment, it is necessary to ensure that no material is kept inside the
glove box. The same electrodeposition procedure as discussed in section 3.2.3 has been repeated
using water as an electrolyte, and all the materials have been placed inside the box. Before operating
inside the Glove box, set the pressure range at 2-6 mbar, reduce the moisture level to less than 0.1
ppm and oxygen level to less than 5% at room temperature. In the next step, wear the black rubber
gloves to carefully handle the materials inside the Glove box for experiment.
34
Figure 3.2 Glove Box
3.3 Characterization Techniques
Characterization techniques are useful in studying and determining the chemistry of the material at
the atomic level. It is useful in finding out the internal structure and properties of a material. The indepth study to magnify the specimen in microscopic study provides the distribution result of
elements on the surface. One of the other characterization technique used is Scanning Electron
Microscope (SEM) by Carl Zeiss, which delivers high resolution surface information and superior
materials contrast equipped with Energy Dispersive Spectroscopy (EDS). EDS confirmed the
coating of Niobium using detailed surface characterization and EDS results also showed the
elemental composition of the coated niobium material. Characterization was done on samples
using X-R Diffraction (XRD); X-Ray Photonelectroscopy, corrosion evaluation using Tafel plots
and corrosion analysis using Salt Spray and the Humidity test.
3.3.1 Optical Microscope
In our lab, we have the Optical Microscope (Fig. 3.3) from Leica Company. The microscope has a
facility of digital camera, connected to the computer, having the software installed named Leica
35
Vision through which the image of the micro structural and grain size of sample can be easily seen
and measured. The images of the samples can be taken at different magnifications depending on the
user convenience.
Fig.3.3 Optical Microscope Apparatus
3.3.2 Scanning Electron Microscopy & Electron Dispersion X-Ray
Scanning Electron Microscopy (SEM) in Fig.3.4 is a large vacuum tube that uses electrons instead of
light to form an image. With the higher level of resolution in use of SEM machine, the specimen can
be focused and magnified more as compared to optical microscope. SEM machine uses
electromagnet instead of lenses, therefore, the user controls has much more control over the degree
of magnification and can get clear strikingly clear images. A beam of electrons is produced from a
heated filament and driven by a high voltage to the specimen at the top of the microscope through
the electron gun. As the beam strikes the specimen, electrons and x-rays are ejected from the sample,
collected, converted into a signal and display to the screen. The procedure produces higher
magnification of the sample with in depth characterization.
36
The SEM uses vacuum conditions and electrons to get an image, therefore, the sample should be
completely dry. The coating on the sample should be conductive, in this case the coating is niobium
metal which is prepared and air dried and treated for analysis in scanning electron microscopy,
otherwise non metals should be covered with a thin layer of conductive material.
The electrons produce x-rays emitted from the sample that can be explained using energy dispersive
spectroscopy (EDS). An EDS software is used to analyze the energy spectrum to determine which
element is present on the surface of the sample and the percentage composition of the atoms or
elements as well. EDS is useful to find the chemical composition of materials to a size of a few
microns and shows an element composition table which gives the atomic weight and atomic number
composition of different element on to the surface.
Fig 3.4 SEM Apparatus
37
3.3.3 X-Ray Photoelectron Spectroscopy
Fig.3.5 X-Ray Photonelectron Spectroscopy (XPS) [119]
Fig.3.5 X-Ray Photonelectron Spectroscopy (XPS) referred to as Electron Spectroscopy for
Chemical Analysis (ESCA) is the technique for surface characterization. It irradiates the sample
surface with an X-ray. The electrons of the substrate atoms excite with the X-ray, and if the energy is
lower than the X-ray energy, they will be emitted from the shell of the parent atom as a
photoelectron. From the extreme outer surface the photoelectrons excites and gives the material
composition of different element.
3.3.4 X-Ray Diffraction (XRD)
X-Ray Diffraction (XRD) in Fig.3.6, measures the average spacing between layers of atoms. It helps
in determining the orientation of a single crystal or grain. XRD is useful in giving the crystalline
structure of an unknown material and can measure the size, shape and internal stress of small
crystalline regions. The atomic planes of a crystal cause an incident beam of X-rays to interfere with
one another as they leave the crystal. The phenomenon is called X-ray diffraction. It uses incident
38
rays with wavelength λ which strike the surface and diffract from different planes (h k l) with an
angle θ from the sample. Some intense diffraction peak satisfies the Bragg’s law stating that
diffraction occurs at angle of 2θ relative to incident beam and hits detector with high intensity.
Waves reveal the atomic structure of crystals according to Bragg’s Law i.e.
nλ = 2dsin θ
The peak intensity reflects the density of atoms and their types on the plane. Moreover for the cubic
structure, the lattice parameter (a) of a given plane can be found out by using miller indices (h k l)
and d represents the inter-atomic spacing.
Fig.3.6 X-Ray Diffraction Machine
39
3.4 Electrochemical Test
There are various methods for testing out the surface corrosion of sample and its ability to withstand
corrosion. One of the techniques includes electrochemical test method.
The electrochemical test is a good method analyzing the sample surface as it can depict the future
state of the sample by doing an accelerated corrosion test analysis. For this purpose, GAMRY
potentio-stat 600 instruments was used with appropriate current and voltages, all measurements were
performed in a three electrode cell. The software takes the data and displays the plots and corrosion
condition of the sample exposed to the solution. The corrosion is accelerated by varying the current.
The degradation test is evaluated by performing the test in salt solution with 3 % NaCl at room
temperature.
The potentiodynamic curves were displayed at a constant voltage scan rate of 10mV/s. The working
electrode with the deposited element niobium was exposed to the solution with a surface area of 1
cm2, sample period was set to 0.245 s and scan frequencies was 1 to -1 V and finally the Tafel plots
were obtained.
3.5 Salt Spray Test
Salt spray test was conducted in a salt spray machine as mentioned in Fig.3.7, as per ASTM B117
standard using 5% NaCl solution as corrosive medium for 96 hours. Salt spray testing is a technique
commonly used to determine the corrosion resistance in aerospace and automotive applications. It is
a standardized accelerated corrosion test, also referred as Neutral Salt Spray (NSS), Spray Fog. It is
commonly used by the industrial sector, marine, automotive, military, Aircraft. With corrosion
testing in salt spray we can create simulation that can predict the degradation rate of the substrate
coated with the desired metal film. The environmental condition can be controlled including the
40
temperature and salt concentration. The substrate of plain magnesium alloy and coated magnesium
alloy were placed in enclosed salt spray testing cabinet to a continuous fog and after the desired time
of 96 hours, the samples were taken. After that, the surface conditions and corrosion of deposited
niobium on magnesium alloy were analyzed by taking microscope images.
Fig 3.7 Salt Spray Machine
3.6 Humidity Chamber Test
Another test to check the corrosion resistance of the modified magnesium alloy sample coated with
niobium can be done by humidity test. Environmental humidity chamber test is necessary to validate
the challenging and demanding climatic conditions, a coated part will have to go through. Humidity
test was carried out in a thermostatically controlled humidity chamber (Fig.3.8). The relative
humidity in this chamber can be maintained. Here in this thesis work it was maintained at approx 95
% at temperature 50 degree Celsius for 96 hours.
41
Fig.3.8 Humidity Chamber
42
CHAPTER 4
RESULTS AND DISCUSSIONS
4.1 Optimization of Process Parameters
Ionic liquids are the salts with green and environment friendly solvents for their use in industries
chemical environment. It has been used for various applications such as organic synthesis, catalyst,
electrochemical devices and solvent extraction of a variety of compounds [120].
Ionic liquids are composed of anions and cations and are liquids at room temperature below 100 degree
celsius. Ordinary liquids like water and gasoline have electrically neutral molecules, whereas, Ionic
liquids have large number of ions and short lived in pairs which significantly increase the flow of
ions inside the electrolytic cell. The technological and scientific importance for ionic liquids have
spanned a wide range of applications, owing to their physiochemical properties such as low melting
point, thermal and chemical stability, negligible volatility, flame retardancy moderate viscosity, high
ionic conductivity high polarity and solubility with a wide variety of compounds [121-123]. The interest
in ionic liquid was initiated because of their advantageous physicochemical properties such as
negligible vapor pressure, high thermal and electrochemical stability, and high solvating power [124,
125].
Since the development of ionic liquids, different investigations have been done to search for the
ionic liquids suitable for the electrochemical processes and facilitate the metal to metal deposition.
Ionic liquids provide wide electrochemical window suitable for electrodeposition with high
conductivity. These characteristics of ionic liquid not only help the investigations into metal
electrodeposition [126-128], electro capacitor, and electro catalysis in a less demanding manner, but
also open up new possibilities for increased reactivity of processes or stability of reactants or
43
products in ionic liquids [129]. Different ionic liquids behave differently with change of material and
electrode interfaces. Therefore, investigation has been carried out to find the ionic liquid suitable for
electrodeposition.
4.1.1 Selection of Ionic Liquid
Careful study of the ionic liquids and their structures were studied and control magnesium was tested
in all these ionic liquids including Choline Chloride, Choline Hydroxide, 1-Ethyl-3methylimidazolium chloride, - Butyl - 2, 3 dimethylimidazolium tetrafluoraborate, 1-Butylvridinium chloridethe, 1-Butyl-vinidinium chloridethe. The ionic liquid 1- Butyl - 2, 3
dimethylimidazolium tetrafluoraborate (Fig.4.1) was selected based on its non corrosive nature and
less effect on the magnesium sample during the immersion test. This ionic liquid has never been
investigated in electrodeposition of niobium on magnesium, therefore, investigation on this ionic
liquid and deposition results will be significant. The structure is shown in Fig.4.1
.
Fig.4.1. Structure of 1- Butyl -2,3 Dimethylimidazolium Tetrafluoraborate
Choline Hydroxide (Fig.4.2) is a base, a clear light yellow, stable viscous solution is used to
neutralize and control the pH value of the electrolytic cell [130].
44
Fig 4.2 Structure of Choline Hydroxide
4.1.2 Depositing Niobium on Magnesium Alloy
To the best of our knowledge no literature exists on niobium deposition on magnesium using
electrodeposition technique. Considering the light weight properties and corrosive nature of
magnesium, niobium has been selected as a possible coating material on magnesium alloy samples
to prevent the effect of corrosion with the help of surface coating technique as niobium can
withstand extreme corrosive condition. It has a high melting point of ca.2500 Celsius, excellent
resistance to wet corrosion even in a very acidic media good weld-ability and a low neutron crosssection allows it to use in nuclear reactor [131]. Moreover niobium has shown its resistant to
corrosion by fused alkalis and by acids, including aqua-regia, hydrochloric, sulfuric, nitric and
phosphoric
acids
[132].
Furthermore,
it
shows
superconductive
properties,
and
forms dielectric oxide layers helpful in deposition as well as reduces the rate of degradation.
4.1.3 Effect of Electrolytic Solutions
1- Butyl - 2, 3 dimethylimidazolium tetraflouraborate has been selected for experimental purposes
because the corrosion behavior is less as compared to other ionic liquid. As niobium deposition on
magnesium is attempted for the very first time; therefore, different electrolytic combinations have
been used for this purpose, to check the possibility of deposition capability. Table 4.1 shows
different electrolytic solutions used with ionic liquid and experimented for niobium deposition
experiment at different level of pH.
45
Table 4.1 Electrolytic Solution for Deposition Study
Selection of Different Electrolytic Solution
1) Dimethyl Sulphoxide (DMSO) + Ionic Liquid
2) Acetone + Ionic Liquid
3) Water + Ionic Liquid

DMSO
•
A protic solvent. Therefore, it cannot donate hydrogen. Hence we can restrict the formation
of acid like HCL or HBR during the process of electrodeposition.
•


Ability to dissolve both polar and non polar compounds.
ACETONE
•
High reactivity of halogen compounds present in niobium salts.
•
Acetone has excellent miscibility.
WATER
•
Water is good polar solvent and is also called Universal Solvent.
•
Its electrical conductivity is highly enhanced by adding small amount of ionic liquid.
Table 4.2 pH Range for Experiments
Solutions
Selected pH Range for
Experiments
DMSO
2.0 – 6.0
Acetone
2.0 – 6.0
Water
2.0 – 6.0
Table 4.2 shows the experiments conducted on different pH to examine the rate of deposition.
46
4.1.4 Scanning Electron Microscopy & Energy Dispersive X-Ray Spectroscopy
In this experiment, magnesium base samples prepared were characterized using Scanning Electron
Microscopy (SEM) to determine the surface topography and Energy Dispersive X-ray spectroscopy
(EDX) to determine the elemental composition of the surface.
4.1.4.1 SEM Image of Control Mg AZ31 Substrate
Fig.4.3 (a)
Fig.4.3 (b)
Fig.4.3. (a) SEM image of control magnesium sample with 2 x magnification (b) SEM image of
plain magnesium sample with 10 x magnification.
47
4.1.4.2 Effect of Dimethyl Sulphoxide (DMSO) Solution
Microstructure of Mg Alloy
Fig.4.4 a) Resolution: 10 microns
Fig.4.4 b) Resolution: 2 microns
Fig. 4.4 (a, b) above correspond to Magnesium alloy using DMSO showing no deposition. The
images on different resolution shows surface of magnesium with no niobium deposition
EDS Spectrum of Magnesium Sample using DMSO
Fig 4.5.EDS Spectrum of Mg AZ31using DMSO
48
Fig 4.5 The EDS is showing traces of Niobium with no significant deposition when conducted
experiments at pH 2. Other experiments on higher pH also confirm that electrolytic solution
containing DMSO does not support electrodeposition of niobium.
4.1.4.3 Effect of Acetone Solution
Microstructure of Mg Alloy
Fig.4.6 SEM Image of Nb Coated Mg Alloy
Fig.4.6 shows 5 Atomic % or 22 weight % of the scattered deposition of Niobium using acetone as
an electrolyte.
EDS Spectrum of Niobium using Acetone
Fig4.7 EDS spectrum of Mg AZ31 using acetone as Electrolyte
49
Fig.4.7 shows the peaks of Niobium and 21.62 weight % deposition of Niobium on Magnesium
sample at pH 3, using acetone as electrolytic solution.
4.1.4.4 Effect of Aqueous Solution
Microstructure of Mg Alloy
Fig.4.8 (a) Nb Coated Mg Alloy
Fig.4.8 (b) Nb coated Mg Alloy
Fig.4.8 (a) SEM image of niobium coated magnesium sample with 2 x magnification (b) SEM
image of niobium coated magnesium sample with 100 x magnification.
EDS Spectrum of Niobium using Water
50
Fig4.9 EDS spectrum of Mg AZ31 using Water as Electrolyte
Fig.4.9 shows the peaks of Niobium and 25.81 weight % deposition of Niobium on Magnesium
sample at pH 3, using water as electrolytic solution.
Deposition Table of Electrolytic Solution
The initial deposition experiment using aqueous solution shows approx. 26 weight % of niobium on
magnesium. Therefore, aqueous solution is selected for further studies as it shows the best deposition
percentage of niobium on magnesium when compared to all experiments in different solutions.
Table 4.3 Deposition Table of Electrolytic Solution
Solutions
pH Range for Experiments
Maximum % of Nb
Deposition (by weight)
DMSO
2.0 – 6.0
0%
Acetone
2.0 – 6.0
22 % ( @pH 2.5)
Water
2.0 – 6.0
36 % ( @pH 2.5)
Table 4.3 shows the results of deposition using different electrolytes for experiments with varying
pH levels. Detailed results will be discussed in section 4.1.5
4.1.5 Effect of pH on Deposition
Based on the electrolyte study, aqueous solution is selected for further studies as it shows the best
deposition percentage of Nb on Mg. As discussed earlier, setting up the parameters is important for
electrodeposition process therefore after careful study of the electrodeposition process, the
parameters were optimized for the niobium deposition on magnesium. The parameter includes
voltage, time of deposition, niobium salt concentration. So in this case, once the voltages, time of
electrodeposition, niobium bromide salt concentration were kept constant after playing around and
51
setting these factors, a study of pH level of the electrolytic cell have been conducted, to find out the
range on to which maximum electrodeposition takes place. The acidic environment of the bath tub
can have impact on deposition rate.
The varying of pH effects the deposition. Three different experiments were conducted on pH level 6,
4 and 2.5
Microstructure of Mg Alloy at pH 6
Fig.4.10 (a) SEM image of magnesium sample with 2 x magnification (b) SEM image of
magnesium sample with 10 x magnification.
Figure 4.10 show the SEM image of the Mg AZ31 substrate with no deposition of niobium at pH 6
for 20 minutes at room temperature prepared in electrolytic solution of ionic liquid and water.
52
Fig 4.11 EDX Analysis of Elemental Composition
Fig.4.11 shows the EDX analysis of Elemental composition spectra for the Mg sample at pH level 6
shows no signs of Nb deposition.
Microstructure of Mg Alloy at pH 4
Fig.4.11 (a) SEM image of magnesium sample with 2 x magnification (b) SEM image of
magnesium sample with 10 x magnification
53
Fig.4.11 (a, b) shows no niobium deposition on sample when pH 4 of aqueous solution have been
maintained.
Fig.4.12. EDX Analysis Elemental composition spectra for the Mg sample at pH level 4 with no
niobium deposition
Figure 4.12 show the SEM image of the Mg AZ31 substrate with no deposition of niobium at pH 4
for 20 minutes at room temperature prepared in electrolytic solution of ionic liquid and water.
Microstructure of Mg Alloy at pH 2.5
a)
b)
54
Fig.4.13. (a) SEM image of magnesium sample with niobium deposition at 2 x magnification (b)
SEM image of magnesium sample with deposition at 10 x magnification , using water with pH 2.5
before Sonication
Figure 4.13 shows the SEM image of the film of Nb on magnesium substrate prepared by
electrodeposition at room temperature for 20 min. The film is fairly homogenous. It is well known
that for either solution growth or vapor deposition. The presented electrodeposition method was
effective for making MgAZ31 substrate coated with Niobium.
Fig. 4.14 EDX Analysis of Elemental composition spectra for the Mg sample at pH level 2.5
before Ultrasonic Cleaning.
Fig. 4.14 shows 37.79 weight % of Nb deposition on Mg alloy at pH 2.5.
55
a)
b)
Fig.4.15. (a) SEM image of magnesium sample coated with niobium at 2 x magnification before
Ultransonication ; (b) SEM image of magnesium sample coated with niobium at 2 x magnification
after Ultrasonication
Fig. 4.15 (a) and Fig.4.15 (b) shows the SEM image of niobium coated magnesium sample before
and after the ultrasonication cleaning respectively.
56
=
Fig.4.16 EDX Analysis of Elemental composition spectra for the Mg sample at pH level 2.5
after Ultrasonic Cleaning
Figure 4.16 shows the SEM analysis of the magnesium surface coated with niobium. EDX analysis
confirms the presence of 35.50 weight % of Nb coated on the surface of Mg alloy sample with the
histogram depicting the elemental composition of the atoms on to the substrate. The visually
observed coating on the surface of the Mg was analyzed using SEM. The surface of the coated Mg
with Nb was fairly uniform and good. However presence of surface irregularities can be seen.
57
4.1.5.1 pH Study Chart with Nb % Deposition
Table 4.4 pH Study Chart with Nb % Deposition
pH
Water
Level
Ionic
Nb
Liquid
Bromide
Voltage
Time of Deposition
(mins.)
%
Deposition of
Nb
2.5
40 ml
1 gm
1 gm
0.7
15- 20
36 weight %
3.0
40 ml
1 gm
1 gm
0.7
15- 20
26 weight %
4.0
40 ml
1 gm
1 gm
0.7
15-20
0%
6.0
40 ml
1 gm
1 gm
0.7
15-20
0%
.
In Table 4.4, the pH study shows that at higher level of pH, the electrodeposition is not possible and
a fairly low pH gives higher deposition rate of elemental Niobium of 36 weight % which after
ultrasonication cleaning remains at 36 weight % on to the magnesium surface.
4.1.6
Effect of Environment on Deposition
Studies shows that deposition of certain elements require Glove Box environment, as in case of
aluminum deposition on magnesium[133]; therefore we would look that whether it will help us in
our case . A repeated experiment on 4-5 magnesium samples has been conducted to observe the
58
significance. This experiment was performed under glove box environment using inert gas argon
with oxygen content less than 5 %.
4.1.6.1 Deposition Using Glove Box
Experiment using aqueous solution was conducted inside the glove box using the parameter as
mentioned in Table 4.5.
Table 4.5 Percentage deposition of niobium inside the Glove Box
pH Level
of Bath
Tub
Water
Ionic
Liquid
(gm)
Nb
Bromide
Salt (gm)
DC
Voltage
Time of
Deposition
(mins.)
% deposition of
Nb
2.5
40 ml
1 gm
1 gm
0.7
15- 20
35 weight %
Microstructure of Nb Coating inside Glove Box
a)
b)
c)
Fig.4.17. (a) SEM image of magnesium sample coated with Niobium at 2 x magnification
(a)
SEM image of magnesium sample coated with niobium at 10 x magnification.
59
(b)
SEM image of magnesium sample coated with niobium at 400 x magnification.
Figure 4.17 shows the SEM image of the film of Niobium on MgAZ31 substrate prepared by
electrodeposition method under inert glove box environment at room temperature for 20 minutes.
The film looks homogeneous and comprises nano-crystallites.
EDS Spectrum of Niobium using Glove Box
Fig.4.18. EDX Analysis of Elemental composition of the Mg sample using Glove Box condition
In Fig.4.18 we can see the peaks of niobium and the 35 weight % composition of Nb deposited on
surface of Mg AZ31 alloy.
a) Without Glove Box
b) With Glove Box
60
Fig. 4.19 (a, b) Niobium deposition using without Glove box environment and with Glove box
Experiments were performed using 3 samples of magnesium alloy to deposit niobium on it, in an
inert environment. As Fig. 4.19 shows, the deposition of Nb remains at 35 weight % on an average
that does not show any significant difference.
Table 4.6 Comparison of Deposition Environments
Deposition Environment
% of Nb Deposition
With Glove Box
35 weight %
Without Glove Box
36 weight %
Table 4.6 shows that the experiments conducted without glove box and with glove box give an
average deposition of 36 weight % and 35 % weight % respectively, and thus, concluded that, glove
box has similar impact on deposition rate.
4.2 Surface Characterization of Nb Coated Mg
Detailed surface characterization has been conducted to evaluate the quality of the coating formed.
Further chemical composition has been determined using X-ray photoelectron spectroscopy (XPS)
and X-ray Diffraction (XRD)
61
4.2.1 X-Ray Photoelectron Scopy (XPS)
Fig 4.20 (a) X-Ray Photonelectroscopy (XPS)
Fig 4.20 (b) X-Ray Photoelectronscopy (XPS)
62
Table 4.7 X-Ray Photoelectronscopy (XPS)
From Fig.4.20 (a, b), we can see that the energy peaks of different elements including niobium and
oxygen. These are peaks obtained from the photoelectrons leaving the sample. These peak areas
determine the composition of material surface and detect the presence of Niobium covering 17.72 %
of area (Table 4.7). The presence of thin film of niobium can be obtained as indicated. Fig.4.20
indicated the peak position and the center of Nb3d5/2 and Nb3d 3/2 in oxidation state of Nb205.
63
4.2.2
Results from X-Ray Diffraction (XRD)
Fig 4.21 X-Ray Diffraction (XRD)
According to Bragg’s Law [134];
Angle of Incident = Angle of Diffraction
n= 2dSin ; where wavelength () = 1.54 A and 2 = 34 ; n =1
= 2dSin
Interatomic Spacing (d) = 2.56
For cubic structure:
64
With Miller indices plane given as hkl (101) and (110)
Lattice parameter “a” is calculated as 3.61 A
Crystal Structure for the deposits has been further analyzed by XRD. Fig.4.21 shows the acquired
diffraction patter. The literature study shows that value of the lattice parameter indicates the
modified hexagonal system crystallographic structure of Niobium pentaoxide (Nb205) arranged in a
body centered cubic fashion [135]. The peaks attribute to niobium pentaoxide with body centered
cubic structure can be detected clearly from the substrate. The diffraction patterns for deposits
composed of (101) and (110) faces. These peaks have high intensity, shows the orientation of the
electrodeposits.
4.3 Electrochemical Corrosion Evaluation
The corrosion resistance of niobium deposited magnesium alloy is investigated using Potentio
dynamic electrochemical test.
Table 4.8 Determination of Corrosion Rate using Tafel Plot for Magnesium AZ31 and Niobium
Coated Sample
65
1.00E+00
5.00E-01
1.00E-09
1.00E-07
1.00E-05
0.00E+00
1.00E-01
1.00E-03
-5.00E-01
-1.00E+00
Plain Mg Az31
Nb Coated Mg alloy
-1.50E+00
-2.00E+00
-2.50E+00
-3.00E+00
Fig 4.22 Overlay of Tafel plots for Magnesium AZ31 and Niobium Coated Mg Alloy
From Fig. 4.22 , it is clear that Niobium is more corrosion resistant than control Mg AZ31, from
the image, niobium coated sample is in a phase which has used less current and more voltage for
the degradation, while Mg AZ31 sample on the other side used more current and less voltage for
the degradation. The graph is further explained through experimental data, from Tables 4.8, we
can see that the corrosion rate of Nb coated Mg Alloy was 0.00111 mm per year, while the
corrosion rate for Mg AZ31 sample was 0.1458, this proves that uncoated control magnesium
sample degrade faster than niobium coated magnesium sample thus coated substrate protects the
sample from degradation.
66
4.4 Effect of Salt Spray Test
Experiment was performed in salt spray machine for 96 hours, using ASTM standard of 5 % salt
concentration.
Microscopic Image After Salt Spray Test
a)
b)
Fig.2.23 (a) Microscopic image of controlled magnesium alloy sample; (b) Microscopic image of
niobium coated magnesium alloy sample
Fig 2.23 ( a, b ) shows clearly that the surface of the controlled magnesium alloy without coating
has corroded significantly and became very rough and broken when exposed to salt environment for
96 hours , where as the niobium coated sample is protected from getting corroded .
67
4.5 Effect of Humidity Test
Experiment was performed inside Humidity Chamber for 96 hours, setting the 95 % humidity at 50
Celsius.
Microscopic Image After Humidity Test
a)
b)
Fig.2.24 (a) Microscopic image of controlled magnesium alloy sample; (b) Microscopic image of
niobium coated magnesium alloy sample.
Fig 2.24 shows clearly that the surface of the controlled magnesium alloy without coating has
corroded significantly and cracked when exposed to hot and humid environment for 96 hours,
where as the niobium coated sample is still very smooth and is protected from corrosion.
68
CHAPTER 5
CONCLUSION AND FUTURE RESEARCH
5.1
Conclusion
The main objective of this work performed in this thesis is to electrodeposit Niobium (Nb)
layer on magnesium alloy for corrosion protection and thus used the magnesium based alloys as a
potential light weight material for aerospace and automotive industry. Magnesium is selected as a
base material and niobium as a deposited material on the basis of light weight and highly corrosion
resistive nature respectively.
Niobium coatings were deposited on magnesium AZ31 sample by electrodeposition using
environmental friendly ionic liquid. The highly corrosion and heat resistant properties of niobium
was fabricated on magnesium AZ31 alloy substrate by an electrodeposition method and offered a
great opportunity to be a potential material which can be used by automotive and aerospace
industries for light weight fuel efficient material.
After experiments it is concluded that aqueous solution at pH 2.5 is found to be the best electrolytic
combination with Ionic Liquid with maximum deposition as compared to DMSO and acetone
solutions. The observation with the help of SEM-EDS, XRD and XPS gave significant information
about the morphology and structure of the coating. The XRD and XPS show the coating of niobium
pentaoxide coating on Mg AZ31. Electrochemical test indicates that the degradation rate of coated
and uncoated samples in salt solution, with the help of Tafel curves have significant difference thus
confirms that the rate of corrosion dropped in case of niobium coated sample. Experiment performed
under glove box shows no significant difference in % deposition of Nb on Mg when using water and
ionic liquid as electrolytic medium.
69
Salt Spray and Humidity test also shows that niobium coated sample is resistant to corrosion as
compared to the control magnesium alloy. Hence, the coating protects the components from early
degradation and because of its light weight in nature; it can be a potential fuel saving material in
automotive and aerospace industry.
5.2
Future Research
The present work is very first of its kind and the positive results shows that it is possible to deposit
uniform thin films of niobium on magnesium alloy from an ionic liquid. As the research and
development on different Ionic liquids continues, it can be said that the immersion and
electrodeposition should be performed on other ionic liquids as well, which could enhance the
percentage deposition of niobium on magnesium.
Further work will explore and enhance the potential use of coated material in critical components of
vehicles and flights. Also, it would be interesting to find out the coating thickness and optimize the
parameters with respect to the desired applications.
.
70
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