HIGH SPEED ELECTROPLATING OF NICKEL OVER
STAINLESS STEEL
AHMAD ABDOLAHI
UNIVERSITI TEKNOLOGI MALAYSIA
HIGH SPEED ELECTROPLATING OF NICKEL OVER STAINLES STEEL
AHMAD ABDOLAHI
A dissertation submitted in partial fulfillment of the
requirements for the award of the degree of
Master of Engineering (Materials)
Faculty of Mechanical Engineering
Universiti Technologi Malaysia
DECEMBER, 2010
To my beloved parents and wife thanks for all your affectionate caring and
supporting, and above all your sacrifices and prayers accorded to me until the successful
completion of this project.
“ My Success Is Yours Too”
ACKNOWLEDGEMENT
In the Name of God Most Gracious, Most Merciful
First of all I would like give thanks to Almighty God that has protected and guided me
throughout my academic pursuit, then my sincere gratitude to my supervisor Assoc,
Prof. Dr. M. S. Hussain, Faculty of Mechanical Engineering, (Material Department),
University Technology Malaysia. His knowledge and logical way of thinking have been
of great value to me. His guidance has provided a good basis for this thesis.
ABSTRACT
Electrodeposition of nickel has been investigated intensely during the past
decades in relation to its particular mechanical properties and numerous applications in
industry. Electroplating of nickel coatings is frequently used for corrosion protection of
stainless steel, also nickel electroplating plate is one of the protective-decorative
electrodeposited metallic coating for stainless steel. Usually the electroplating process of
nickel over stainless steel is done by common methods needed to some pretreatments
such as preparation of surface, activating of the surface, striking a thin layer of nickel on
the surface. In these methods there are some problems including: Poor level of
Adhesion. Peeling off, Sometimes even after following all the proper pre-plating
treatment the adhesion is also poor. Another problem is that a strike deposits usually
very thin and examination of the strike layer may not show any signs of pitting and
roughness. Because of these problems the nickel layer can not stick to stainless steel
properly and it can peel off from the surface. In this study high speed electroplating will
be applied to solve the problems and without any preparation the nickel will deposited
on stainless steel
ABSTRAK
Penyelidikan mengenai pemendapan nikel telah lama di lakukan dan ia
berhubung kait dengan sifat-sifat mekanikal dan banyak kegunaannya di dalam industri.
Penyaduran nikel sering digunakan untuk melindungi keluli tahan karat dan melindungi
bahan hiasan yang diperbuat daripada keluli tahan karat dari berkarat. Lazimnya, proses
penyaduran nikel pada keluli tahan karat di lakukan dengan kaedah biasa dimana
beberapa pra-rawatan perlu dilakukan seperti penyediaan awal permukaan keluli tahan
karat, pengaktifan permukaan keluli tahan karat dan
penghasilan lapisan nikel di
permukaan keluli tahan karat. Bagaimanapun, melalui kaedah ini terdapat beberapa
masalah yang di hadapi seperti tahap perlekatan yang rendah, mudah tertanggal dan
walaupun selepas melakukan semua rawatan pra-saduran, lekatan yang dihasilkan juga
tidak memuaskan. Selain daripada itu, saduran yang dihasilkan juga sangat nipis dan
ujian yang dilakukan tidak dapat menunjukkan sebarang tanda bopeng dan kesat.
Disebabkan oleh masalah ini, lapisan nikel tidak dapat melekat di permukaan keluli
tahan karat dengan baik dan mudah tertanggal.
Dalam kajian ini, high speed
electroplating akan digunakan untuk menyelesaikan masalah yang dihadapi tanpa
melakukan sebarang penyediaan untuk menyadur nikel pada keluli tahan karat.
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
INTRODUCTION
1
1.1 Background of the Project
1
1.2 Problem statements
2
1.3 Objectives
2
1.4
Scope of project
2
1.5
Thesis outline
3
LITERATURE REVIEW
4
2.1
4
Overview
2.2 Properties of Nickel Electroplating
5
2.3 Solution are used for nickel electroplating
7
2.4 Watts bath operation
9
2.5
2.4.1 pH
9
2.4.2 Agitation and Temperature
10
2.4.3
Filtration
11
2.4.4
Additives
11
Problems with Watts bathe
12
2.5.1
Roughness
12
2.5.2
Adhesion
13
2.5.3
Ductility and stress
14
2.5.4
Dull deposits
15
2.5.5
Purification of Nickel Solution
15
2.6 Problems with Ni striking
17
2.7 Process of nickel electroplating over stainless steel
2.8
2.9
3
17
2.7.1 Background of process
18
2.7.2 Summery of process
19
Factors for good electroplating
21
2.8.1 Surface preparation
21
2.8.2 Anode
21
2.8.3 Current efficiency
21
2.8.4 Anti-pitting additive
22
2.8.5 Filtration
22
2.8.6 Air agitation
22
2.8.7 Temperature
23
Problems and corrective action
23
2.9.1 Roughness
23
2.9.2 Pitting
24
2.9.3 Poor adhesion
24
2.9.4 High stress and low ductile
24
2.9.5 Brighteners
25
2.9.6 Current density
25
2.9.7 Nickel striking
26
2.9.8
26
Pre-treatment process
2.10 High speed electroplating
27
METHODOLOGY
30
3.1
Introduction
30
3.2
Experimental Setup
30
3.3
Stainless steel sample preparation
32
3.4
Solutions preparation
32
3.5
Experimental Setup
33
3.6.
Sample preparation
35
3.7. Preparation of samples (for SEM)
3.8.
Sample preparation for TEM
36
37
3.9.
Adhesion testing
38
3.9.1
Scratch test
39
3.9.2.
Nano scratch test
40
3.9.3
Tape test
40
3.10
4
5
RESULTS AND DISSCUSION
42
4.1
Introduction
42
4.2
Effect of temperature on rate of deposition
42
4.3
Effect of solution on rate of deposition
44
4.4
Effect of current density on rate of deposition
45
4.5
Nano scratch test Analysis
48
4.6
Tape test analysis
50
4.7
Type of failure
51
4.8
Ni Electro crystallization
55
CONCLUSION
5.1
REFERENCES
Expected findings
Conclusion
59
59
60
LIST OF TABLES
TABLE NO
TITLE
PAGE
2.1
Typical ranges for the components in Watts bath
8
2.2
Typical Operating Parameters for Watts Nickel
9
3.1
shows various current densities applied at three different pre-set
temperatures
3.2
36
Rate of deposition is increased by increasing temperaturer and
current density
41
LIST OF FIGURES
FIGURE NO
TITLE
PAGE
2.1
Schematic diagram of nickel electroplating set up
5
3.1
Flowchart to conduct the electroplating process
31
3.2
Project specimen dimensions of the stainless steel rod
3.3
Schematic diagram showing the setup of high speed
Electroplating of nickel
3.4
on stainless steel.
32
34
High speed electroplating equipment invented by Dr. Sakhawat
Hussain A)Surface of cathode B)anode and cathode positioned
close togetherC)Water pomp to supply the speed of solution 35
3.5
a) Cross section of sample b) Mounted sample
c ) polished sample
3.6
a)make a layer as thin as possible b) punching c)very small
sample for
3.7
37
ion polishing
Schematic image of scratch test
37
39
3.8
scratch test on samples
39
3.9
Tape test
41
4.1
SEM image shows the thickness of nickel layer at T=60°C,
C.D=1.3A/cm2
4.2
42
SEM image shows the thickness of nickel layer
at T=55°C,C.D=1.3A/cm2
4.3
43
SEM image shows the thickness of nickel layer at T=60°C,
C.D=0.13 A/cm2 Watts solution
4.4
44
SEM image shows the thickness of nickel layer
at T=60°C, C.D=0.13 A/cm2 Sulphate based solution
4.5
44
SEM image shows the thickness of nickel layer
at T=55°C,C.D=0.25A/cm2
4.6
45
SEM image shows the thickness of nickel layer
at T=55° C, C.D=1.14 A/cm2
4.7
46
Optical micrograph showing dendritic growth of the nickel
deposits at higher current density
47
4.8
SEM image showing the dendritic growth
47
4.9
Nano scratch tests across interface
48
4.10
Friction against distance
49
4.11
Oblique scratch across interface
4.12
nickel layer is peeled off (T=60°C, C.D=2.6A/cm2)
50
51
4.13
SEM image of exterior layer of stainless steel
at C.D <1.3 A/cm2 by peel test
4.14
SEM image of interior layer of nickel
at C.D <1.3 A/cm2 by peel test
4.15
51
52
EDAX of exterior layer of stainless steel
at C.D <1.3 A/Cm2
52
4.16
EDAX of interior layer of nickel at C.D <1.3 A/Cm2
53
4.17
SEM image of exterior layer of stainless steel
at C.D>1.3 A/cm2
4.18
SEM image of interior layer of nickel
at C.D>1.3 A/cm2
4.19
4.21
Schematic diagram of electrochemical growth
4.22
SEM image of nickel layer was peeled off
At C.D=2.6A/cm2,T=60°C
55
56
57
Optical micrograph showing dendritic growth of
the nickel deposits at higher current density
4.24
54
EDAX of interior layer of nickel
at C.D> 1.3A/cm2
4.23
54
EDAX of exterior layer of stainless steel
at C.D> 1.3A/cm2
4.20
53
57
a)TEM image of nano-crystalline nickel over stainless steel
b) particle size of nickel deposites
58
CHAPTER 1
INTRODUCTION
1.1. Background of the Project
It is common practice to nickel plate many different types of industrial parts
which are made of stainless steel in order to give the parts a bright, shiny surface. By
way of example, automotive vehicle body moldings, door handles, and other functional
or decorative parts are nickel-plated for appearance purposes. In commercial practice,
nickel plating stainless steel parts typically is accomplished by initially buffing the
stainless steel to achieve a high luster and then electroplating with chromium to retain
the high luster and to make the finished surface more durable[1].
In the past, stainless steel parts have been electroplated with nickel before the
chromium electrolytic plating step. But, the adhesion between the nickel plating and the
stainless steel part has been erratic. It has been understood that proper adhesion and
good red rust resistance could not be consistently achieved when electroplating nickel
over stainless steel. Particularly, conventional pre-plating surface treatment followed by
conventional plating has not been effective in producing sufficient chemical bond
between the stainless steel surface and the nickel coating [1]. This has been especially
true in the case of bright annealed stainless steel.Thus, there has been a need for a
process to strongly adhere nickel plating to a stainless steel parts. This study relates to
an improvement in the nickel-plating process by using high speed electroplating method
1
which causes the plating to better adhere to the stainless steel while, simultaneously, not
destroying the red rust resistance of the stainless steel surface[1,3].
1.2. Problem Statement
Common electroplating methods of nickel over stainless steel have some
problems. The adhesion between the nickel plating and stainless steel part has been
erratic. Also the preparation of the surface for electroplating is difficult, for example
wood’s nickel strikes are very sensitive to metallic impurities or the strike deposit is
usually very thin and examination of strike layer may not show any signs of pitting and
roughness. Sometimes even after following all the proper pre-plating treatment the
adhesion is also poor .Further, the red rust resistance of the nickel plated part has been
erratic because this oxide layer is not electrical conductive and so it should be removed
from the surface and it’s removing is a hard work. These problems lead to use better
methods for nickel electroplating over stainless steel such as high speed electroplating.
1.3. Objectives
Study on:
Common methods for nickel electroplating over stainless steel.
Problems with common methods.
High speed electroplating of nickel over stainless steel.
Characteristics of nickel deposited on stainless steel.
1.4. Scope of Project
The scope of this research is:
•
Using equipments for high speed electroplating of nickel over
stainless steel.
2
•
Using SEM / EDX to study the microstructure of plating interface.
•
Using nano hardness equipments to study the nature of adhesion
between nickel and stainless steel.
1.5. Thesis Outline
This thesis consists of six main chapters that are covering introduction,
literature review, research methodology, experimental working, results and
discussion and conclusion. First three chapters are covering proposal for the research
and next chapters are focusing on proposed method and validating it.
3
CHAPTER2
LITERATURE REVIEW
2.1. Overview
Electroplating is the electrodeposition of a layer of metal on a substrate. The aim of the
former is to manufacture metallic articles and the latter is to produce surface coatings [1]. To a
large extent, articles and coatings with different properties can be obtained by properly selecting
and control the conditions of electrodeposition.
Electroplating is the application of a metal coating to a metallic or other conducting
surface by an electrochemical process. The article to be plated (the work) is made the cathode
(negative electrode) of an electrolysis cell through which a direct electric current is passed. The
article is immersed in an aqueous solution (the bath) containing the required metal in an
oxidised form, either as an equated cation or as a complex ion. The anode is usually a bar of the
metal being plated. During electrolysis metal is deposited on to the work and metal from the bar
dissolves:
At cathode: Mz+(aq) + ze- → M(s)
At anode:
M(s) → Mz+(aq) + ze-
Articles are electroplated to (i) alter their appearance, (ii) to provide a protective
coating,(iii) to give the article special surface properties,(iv) to give the article engineering or
mechanical properties.[2]
4
Figure 2.1: simple schematic of nickel electroplating
In figure 2.1 first the cell is filled with a nickel chloride which is dissolved in
water and a little acid. The NiCl2 salt ionizes in water into Ni++ ions and two parts of Clions. A wire is attached to the object, and the other end of the wire is attached to the
negative pole of a battery (with the blue wire in this picture) and the object is immersed
in the cell. A rod made of nickel is connected to the positive pole of the battery with the
red wire and immersed in the cell. Because the object to be plated is negatively charged
(by being connected to the negative pole of the battery), it attracts the positively charged
Ni++ ions. These Ni++ ions reach the object, and electrons flow from the object to the
Ni++ ions. For each ion of Ni++, 2 electrons are required to neutralize its positive charge
and reduce it to a metallic atom of Ni0. Thus the amount of metal that electroplates is
directly proportional to the number of electrons that the battery provides [2,3].
Meanwhile back at the anode, electrons are being removed from the nickel metal,
oxidizing it to the Ni++ state. Thus the nickel anode metal dissolves as Ni++ into the
solution, supplying replacement nickel for that which has been plated out. As long as the
battery doesn’t go dead, nickel continues to dissolve from the anode and plate out onto
the cathode. [2]
2.2. Properties of Nickel Electroplating
Electroplated nickel is used extensively in many engineering applications,
ranging from simple thin film for decorative purposes to corrosion and wear-resistant
5
coating [2,3].Nickel is plated for many reasons. First and foremost, nickel provides a
decorative appearance because of its ability to cover imperfections in the basis metal
(leveling). This deposit can be made brilliant and, when covered by a thin layer of
decorative chrome, will maintain its brilliance even under severe conditions [3]. When
nickel is applied in “duplex” form, excellent corrosion protection can also be achieved.
This requires plating two different kinds of nickel (semi-bright and bright nickel).
Nickel deposits also offer more wear ability than softer metals such as copper or zinc
and thus can be used when wear resistance is needed [3]. Because nickel is magnetic, it
can sometimes be plated where the ability to be magnetized is needed. Finally, nickel
can be made to plate with little or no stress and is therefore used for electroforming or
aerospace applications where stress needs to be held to a minimum [5]. In many
applications, many of these requirements are specified simultaneously, so that nickel is
often plated for more than just one reason [4].
There are some properties we can get from nickel plating:
•
Decorative appearance. Lustrous bright, satin semi-bright or black nickel
coatings may be obtained by different plating methods.
•
Corrosion protection.
•
Wear resistance. Nickel deposited on a part made of a softer metal protects the
part from wear. Hardness of nickel plating may be controlled by the plating
process parameters.
•
Low coefficient of friction.
•
Ferromagnetism. Ferromagnetic parts (steel) may be plated by nickel without
changing their magnetic properties.
•
Controllable internal mechanical stresses. Low stress coatings are important in
electroforming and applications, in which Fatigue strength is critical.[3,4]
Bright nickel plating is used extensively in automotive applications such as on
plated wheels, bright trim, truck exhausts, bumpers and restorations. In other
transportation areas, nickel is used for the bright work on motorcycles and bicycles.
Nickel is used to achieve brightness on hardware, such as hand tools [6]. In the home,
6
bright nickel is used on plumbing fixtures, light fixtures, appliances and wire goods
(racks). Bright nickel is also used for tubing applications such as on furniture and wheel
chairs. Most of these applications for bright nickel rely on the nickel for a decorative
appearance with corrosion protection and wear ability [3,5].
Bright nickel coating has been widely used in the area of decoration because of
its sound appearance and protective performance. Also, bright deposit is useful for the
electroforming of articles because it is always attended by the improvement of other
properties, such as leveling, grain refined [5]. Nickel is also used for engineering
purposes where brightness is not required. Thus, nickel is used on molds to provide wear
ability. As a barrier layer, nickel is plated on coins, jewelry and circuit boards. On strip
steel and in aerospace applications, it is used for low stress or for resizing. And nickel is
used in composites where a dispersed inorganic is codeposited (such as silicon carbide).
Most engineering applications use sulphate nickel, although nickel-plated strip steel uses
a nickel chloride/nickel sulfate bath. [3,28].
2.3. Solution are used for nickel electroplating
Different electrolyte solutions can be used for nickel electroplating
•
Watts nickel plating solutions
•
Nickel sulphate solutions
•
All-Chloride solutions
•
Sulfate-Chloride solutions
•
All-Sulfate solutions
•
Hard nickel solutions [3]
The most commonly used nickel baths are Watts baths, which use a combination
of nickel sulphate and nickel chloride. This combination of nickel salts allows for a
variety of characteristics. [4]
7
Table 2.1: Typical ranges for the components in watts bath
A typical Watts bath contains nickel sulphate, nickel chloride and boric acid.
Table 1 presents typical ranges for the components. Each component of the Watts
formulation performs a very important and necessary role in the production of
satisfactory deposits. [4].The mechanism of nickel electrodeposition from Watts
electrolytes was extensively studied. It was suggested that there are two successive
faradic reactions, the first involving the formation of nickel ads, followed by subsequent
reduction of nickel. However, in acidic electrolytes in the presence of freshly deposited
nickel, H+ is reduced to H ads, which strongly adheres to the electrode surface and
inhibits further reduction. The Watts electrolyte that contains nickel sulphate, nickel
chloride, and boric acid is widely applied for nickel electrodeposition, and its impact on
the development of modern nickel electroplating technology cannot be overestimated.
The dominant position of Watts solutions in industrial processes has been challenged
from time to time, but the only alternative adopted on a substantial scale are nickel
sulphamate solutions.[13]
8
2.4. Watts operation
The operating conditions for almost all Watts-type nickel baths are similar.
These typical parameters are given in Table 2.
Table 2.2: Typical Operating Parameters for Watts Nickel
2.4.1. pH
Bright or semi-bright baths are generally operated between pH 3.5–4.2.
Most organic addition agents give optimum brightness and leveling in this range. Higher
pH values always present the danger of adverse effects from the precipitation of metallic
contaminants and increased consumption of brightener components [4].
The pH should rise slowly during operation, since cathode efficiency is
slightly lower than anode efficiency. Sulfuric acid should be used for pH adjustment,
although hydrochloric acid may also be used with the added advantage of maintaining
the chloride ion concentration. However, the disadvantages of using hydrochloric acid
include not only the higher amounts required but the escaping hydrogen chloride gas,
9
especially from a hot, air-agitated solution. Nickel carbonate is preferred for increasing
the pH. It dissolves quite readily below a pH of 4.0. Very small adjustments to air
agitated solutions can be made below this value by adding water-carbonate slurry while
the tank is not in operation. Larger adjustments are best made in a treatment tank,
followed by filtration [4].
If the pH requires no adjustment or if it decreases, look for anode problems.
Insufficient anode areas, the overuse of inert auxiliary anodes, plugged anode bags or
poor anode contact might be the cause. If not eliminated, these problems can quickly
lead to salt depletion, poor plate distribution and off-color deposits from brightener
decomposition. If the pH rises abnormally, it is rarely a cathode efficiency problem,
provided the solution is in chemical balance. It is more likely that the acid is reacting
with dropped parts, a portion of the tank wall or alkaline cleaner solution carried in on
poorly maintained racks.[4]
2.4.2.Agitation and Temperature.
Agitation and temperature increase the diffusion rate of ions into the
cathode film. This is required in order to prevent burning and also to allow the
brightener additives to reach the cathode film. Air agitation from a low-pressure blower
has been universally accepted and is a contributing factor in many improvements in
nickel plating, especially in the decorative area. Air agitation has broadened operating
ranges of bath ingredients, reduced the required concentrations of addition agents and
minimized the use of wetting agents and hydrogen pitting problems. Note that the use of
air agitation will cause particulate matter to become suspended in the solution, resulting
in rough deposits unless good filtration practices are used.[4]
The temperature range is important in terms of physical properties and, along
with agitation, aids in keeping the bath components mixed and solubilized.
The temperature range is also an important factor in addition to agent response. If the
temperature is too high, the addition agent consumption is increased, adding to the
10
expense of operation and possible plating problems. If the temperature is too low, the
boric acid will begin to precipitate and the brighteners will not respond efficiently. [4]
2.4.3.Filtration.
The value of adequate continuous filtration for prevention of roughness and
pitting cannot be over-emphasized. Most bright nickel addition agents are not removed
to any great degree by activated carbon. Therefore, good filtration over an activated
carbon pack tends to keep concentrations of foreign organics, brightener decomposition
products and particulate matter at a minimum. A well-maintained, carbon-packed filter
of adequate capacity tends to keep the physical properties of the deposit near optimum
and minimizes the need for frequent batch treatments. It is better to apply smaller
amounts of carbon at regular intervals over the normal repacking cycle than to add the
total amount in one charge. This maintains the efficiency of the carbon pack by keeping
fresh carbon on the surface and minimizing the tendencies of channeling of the solution
through less restricted areas. A suggested rate of use for carbon packing filters is 1–2 lb
of carbon per 1,000 gal of nickel solution per 40–80 hr of operation. The rule of thumb
is that the minimum hourly discharge rate of the filter should equal the volume of the
solution. To achieve this with a carbon pack and as insoluble are collected, the filter
should have two to three times the capacity in order to avoid frequent repacking. [4]
2.4.4. Additives
It is known that organic additives are introduced in trace amounts to the plating
solutions to modify the structure, morphology and properties of the deposits. Thus
11
search and studies of the news additives are of large interest. For nickel plating from the
Watts bath, two types of additives, such as aromatic sulphones or suphonates and
compounds containing unsaturated groups such as >CfO, >CfS, −CfN, etc., are
recognized as brighteners. The influence of the additives on mechanism of
electrodeposition is not yet clearly understood. However, it is known that the additives
can act as wetting, leveling, brightening or buffering agents. [14]
2.5. Problems with Watts bathes
2.5.1. Roughness.
Roughness is generally the result of particulate matter suspended in the solution
and adhering to the work, especially on shelf areas. Gross roughness may be traced to
improper cleaning, torn anode bags, airborne dirt, dropped parts, precipitated calcium
sulfate, inadequate filtration or carbon and filter aid from an improperly packed filter. A
very fine type of roughness may be caused by precipitation of metallic contaminants in
the cathode film where the roughness may be confined to a particular current density
region. Chromium, iron and aluminum can precipitate as hydrates in the higher-currentdensity areas, where the film pH is normally higher than that of the body of the solution.
A lower operating pH may be helpful in such cases. On occasion, high-current-density
roughness has also been traced to a magnetic condition of the work. Another source of
roughness can be the air blower used for air agitation. Inspection of the filter on the air
blower may reveal that it could be defective or missing.
If an external cause of roughness is not apparent, the quickest remedy is to
pump the solution to a spare tank and inspect the plating tank. The cause may be
apparent; dropped parts and torn anode bags are the most common sources. [4]
12
2.5.2. Adhesion.
Poor adhesion appears in many forms: nickel from basis metal; nickel from
nickel; bright nickel from semi-bright nickel; or subsequent chrome plate from nickel
plate. Separation from the basis metal generally indicates that undesirable surface films
are present and thus surface preparation has been inadequate. Poor cleaning may be
caused by improper chemical maintenance and control of cleaners and acid dips;
contamination and deterioration from prolonged use; poor rinsing; acid dips
contaminated with copper, chromium or oil; or an inadequate process cycle for a
particular soil or basis metal. Surface contamination will often be clearly visible or may
be indicated by water breaks after rinsing. Cleaning problems generally involve much
trial and error to identify their source. Try hand scrubbing between and skipping certain
operations, hand precleaning or hand dipping parts in buckets of fresh acid solutions.
If poor adhesion to the basis metal is traced to the nickel solution, severe contamination
is indicated. Chances are that other problems such as poor ductility and stress will have
given prior warnings. Of course, this does not rule out accidental spills and additions of
wrong chemicals.
Nickel peeling from nickel is generally caused by complete or partial loss of
contact during nickel plating. Total loss may result in an overall peeling condition.
Momentary or partial loss creates a bipolar condition in which current flow is from the
lesser negative (poor or no contact) rack to the more negative (good contact) rack
adjacent to it, resulting in an anodic oxide film. This will normally be confined to one
area, such as the trailing edges of parts plated in an automatic machine. Bipolarity
toward the end of the nickel cycle may appear as though the chromium is coming off as
a powder. A thickness check of the peeled versus the adherent portion will help locate
the general area of the problem. If there is no clear pattern and the condition is
intermittent, a faulty rack is indicated. Knowledge of bipolarity and other electrically
related problems is essential in nickel and chromium plating.
Poor adhesion of bright nickel from semi-bright nickel or chrome plate from
bright nickel, if not the result of electrical problems, can be caused by the nickel
13
passivating during transfer. Long transfer times or warm rinses will increase the chances
for nickel passivation. In these situations, the most common remedy is to activate the
nickel prior to plating using an acid or acid salt [4].
2.5.3. Ductility and stress.
Poor ductility and high stress are primarily an indication of a poor condition of
the plating solution. These properties are influenced by metallic and organic
contaminants, improper chemical or brightener balance and, in some cases, brightener
decomposition products. In all bright nickel processes, a balance of primary and
secondary addition agents is required, as they function synergistically to maintain
minimum stress and maximum ductility at the optimum degree of leveling and
brightness. Many ductility, stress and chromium plating problems have been traced to
out of balance secondary brightener levels.
Abnormally high voltages resulting from a lack of anode area may result in
oxidation or chlorination of some organic additives, which may not be removed by
carbon. Check all materials that are to come in contact with the solution, such as filter
aids and anode bags, for soluble organics that may be harmful. Good housekeeping,
solution control, continuous carbon filtration and periodic batch carbon treatments are
essential to control ductility problems [4].
14
2.5.4. Dull Deposits.
Lack of brightness can be the result of poor cleaning, solution contamination,
non-uniform agitation, improper chemical or brightener balance or failure to exercise
proper control of operating conditions. A low pH or low temperature may cause an
overall loss of brightness and poor leveling. Loss of brightness in a particular current
density may be the first clue to organic or metallic contamination. Dullness from poor
cleaning or organic contamination may appear in any current density area. Metallic’s
generally exhibit their effects by either co-deposition in the low-current-density area or
as hydrates in the high-current-density areas. Chemical analyses and plating tests will, in
most cases, reveal the course of corrective action that should be taken if the problem is
in the plating solution [4].
2.5.5. Purification of Nickel Solution.
There has been so much progress in nickel plating, and especially bright nickel,
that prolonged and frequent purification treatments are rare. A simple carbon treatment,
which may include peroxide, is generally sufficient and can be performed at some
convenient production interval. When the need for purification is indicated and the
cause of the problem is not readily apparent, chemical analyses and plating tests should
always be performed to determine the best course of action. If the tests duplicate the
plating results, the task is somewhat easier, but, if they do not, further investigation in
other areas would be in order. [4,11]
Too often, oxidation with peroxide or permanganate is tried without sufficient
investigation. Commonly, one hears that these oxidizers “burn out” organics and oxidize
15
them to carbon dioxide and water. In fact, sometimes the organic material is altered
structurally, making carbon adsorption more efficient, or it may be oxidized to a more
soluble form that has less deleterious effects on the deposit. But the oxidation could also
result in a more soluble product that has a greater detrimental effect. Carbon treatment is
usually better as the first step. First carbon treat, then filter, then determine if an
oxidization treatment is required.
Permanganate is a more powerful oxidizer than peroxide, but its use as a
treatment must include increasing the solution pH to precipitate and remove the
manganese dioxide. This, coupled with unreacted carbonate and carbon, may result in
filtration difficulties and abnormal solution losses. To avoid using excess permanganate,
which can result in serious loss of ductility and other deposit properties, dilute a 25–50
ml bath sample to 100–150 ml, adjust to a pH of 3.0–3.5, heat to 150°F and titrate with a
standard permanganate solution to a pink endpoint. Calculate the amount of
permanganate reacted; then try about one-half of this amounts in the plating bath in the
lab. This technique is also useful in checking the effectiveness of other organic removal
treatments.
.
Several suppliers offer equipment that purifies nickel and dye-free acid copper
plating solutions that operate like an ion-exchange unit. Like ion-exchange, this
purification system can be regenerated giving the purifying material many years of
useful life. These units can replace batch carbon treatments by keeping the plating
solutions at the purity level of almost new solutions for optimum plating performance.
Some of these purification units remove more organic contaminates than carbon (even
with peroxide/permanganate) and some have additional columns to remove metallic
impurities. Copper, lead, zinc, cadmium and some organics can be removed by lowcurrent-density electrolysis.[6,28] The most efficient current density may vary to some
degree for each metal, but 2–5 asf of cathode surface will be effective. Corrugated iron
is ideal for cathodes since it will provide a favorable distribution of current density. The
16
pieces of corrugated iron should be as long as the plating rack and should be cleaned,
pickled and nickel plated first at normal current density in order to avoid additional
contamination of the solution being dummied. The cathode area should be as large as
possible and good circulation or agitation of the solution should be employed. Inspect
the cathodes for flaky or powdery deposits and occasionally raise the current density a
few minutes as a seal. When finished, be sure to raise the current density again to seal in
the contaminants. [4,16]
2.6. Problems with woods nickel striking
1. Wood’s nickel strikes are very sensitive to metallic impurities. When the strike bath
is contaminated with heavy metals, the usual results are brittleness and a darking of the
deposit.
2. A strike deposit is usually very thin and examination of the strike layer may not show
any signs of pitting and roughness. [5]
2.7. Process of nickel electroplating over stainless steel
The stainless steel is very difficult to be coated with nickel because the oxide
layer which form on it’s surface is non conductive and it’s insulator so it can not transfer
electrons. It requires a different preplate sequence including a nickel strike. . A nickel
strike is a very thin coat of a nickel that will stick to stainless that has been properly
cleaned and activated. If a plating process is done wrong, there is no upper limit on the
percentage of rejects. Wood's nickel is the most popular. Typically a Woods Strike is
used. It is a very high chloride bath at a very low pH that plates very slowly and is
extremely highly compressive stressed. [6].
17
In a process for plating nickel upon a surface of a workpiece made of stainless
steel, the workpiece is immersed in an electrolytic sulfuric acid bath, with the workpiece
anodically connected. Thus, DC current flows from the workpiece, through the bath, to a
separate cathode in the bath. Thereafter, the sulfuric acid is rinsed from the workpiece
and the workpiece surface is electroplated with nickel which will strongly adhere to the
surface. Finally, the nickel-plated part may be conventionally electroplated with chrome
.A process for electroplating a stainless steel part comprising essentially the steps of:
(a) Cleaning the surface of the part;
(b) Treating the surface of the part by immersing it in an electrolytic bath, sid
electrolytic bath consisting of sulfuric acid, the concentration of said sulfuric acid being
about 10% by volume, with the part connected to be anodic, such that DC current flows
from the part through the bath to a separate cathode in the electrolytic bath;
(c) Rinsing the sulfuric acid off the part;
(d) Electroplating nickel upon the surface of the part by immersing it in an electrolytic,
nickel solution; whereby the nickel plating will strongly adhere to the surface of the
part without destroying the red rust resistance of the stainless steel part.[7]
2.7.1. Background of Process
It is common practice to nickel plate many different types of industrial parts
which are made of stainless steel in order to give the parts a bright, shiny surface. By
way of example, automotive vehicle body moldings, door handles, and other functional
or decorative parts are chrome-plated for appearance purposes. Frequently, such parts
are made of stainless steel to prevent rusting [4].
18
In commercial practice, nickel plating stainless steel parts typically is
accomplished by initially buffing the stainless steel to achieve a high luster and then
electroplating with chromium to retain the high luster and to make the finished surface
more durable. In the past, stainless steel parts have been electroplated with nickel before
the chromium electrolytic plating step. But, the adhesion between the nickel plating and
the steel part has been erratic. Further, the red rust resistance of the nickel plated part
has been erratic [14].
It has been understood by those skilled in the art that proper adhesion and good
red rust resistance could not be consistently achieved when electroplating nickel over
stainless steel. Particularly, conventional pre-plating surface treatment followed by
conventional plating has not been effective in producing sufficient chemical bond
between the stainless steel surface and the nickel coating. This has been especially true
in the case of bright annealed stainless steel. Thus, there has been a need for a process to
strongly adhere nickel plating to an annealed stainless steel workpiece and to maintain
red rust resistance of the plated part [15,20, 23].
This study relates to an improvement in the nickel-plating process by high speed
electroplating of nickel over stainless steel which causes the plating to better adhere to
the stainless steel while, simultaneously, not destroying the red rust resistance of the
stainless steel surface. [19,22]
2.7.2. Summary of Process
The study herein contemplates an improvement in the conventional, nickel
plating process for plating stainless steel workpieces wherein the pre-plating,
electrolytic sulfuric acid surface treatment step is performed with the workpiece
19
connected as an anode in the electrolytic bath circuit. That is, current flows from the
work pieces, through the bath to a separate cathode, during the time that the surfaces are
subjected to the electrolytic acid bath. Thereafter, the sulfuric acid is rinsed away, and
the nickel plating and chrome plating are applied in the conventional manner.[7]
By connecting the workpieces anodically in the electrolytic acid bath, there is a
marked improvement in the adherence between the nickel plating to the surface of a
workpiece or part made of bright, stainless steel. This adherence is unexpected and
contrary to the normal understanding of the art that the part, under electrolytic acidic
exposure, should be cathodic or neutral in the electrical system. That is, by reversing the
flow of electrons, namely by flowing them away from the part rather than to the part, the
surface is remarkably activated to strongly adhere to the subsequently applied nickel
plating.[7,21]
An object of this study is to produce good, commercial nickel plating of
stainless steel, and especially bright, annealed stainless steel, which previously could not
be satisfactorily plated because the nickel did not consistently adhere to the surface of
such metal. A further object of this study is to enable the application of a conventional
nickel electroplating procedure to be used for nickel plating bright, annealed stainless
steel without materially changing the conventional procedure or increasing the expense
of operating it. That is, by reversing the flow of current in the electrolytic circuit so that
the current flows to the anodically-connected workpieces, without otherwise changing
the procedural steps or the equipment, it becomes commercially feasible to produce
strongly adhering nickel plating upon stainless steel parts. A further object of this study
is to improve the process for electroplating nickel upon the surface of a part made of
stainless steel while not adversely affecting or destroying the red rust resistance of the
stainless steel. By this process, the part retains, and may even have improved, resistance
to red rust, i.e., iron oxide formation.
20
2.8. Factors for good electroplating
2.8.1. Surface Preparation
Prior to plating operation the cathode (work piece) surface should be cleaned
from mineral oils, Rust protection oils, Cutting fluids (coolants), greases, paints, animal
lubricants and vegetable lubricants, fingerprints, miscellaneous solid particles, oxides,
scale, smut, rust [7].
2.8.2. Anodes
Small parts of high purity primary nickel (nickel rounds or nickel squares)
loaded into titanium baskets are used as anodes for nickel electroplating. Dimensions of
nickel rounds: 1” (25 mm) diameter and up to 0.5” (12 mm) thick. Dimensions of nickel
squares: 1”x1” (25×25 mm) and up to 0.5” (12 mm) thick. Sometimes nickel bars and
rods are used as anodes [7,24].
2.8.3. Current Efficiency
Current efficiency is a ratio of the current producing nickel deposit to the
total passing current.Anode current efficiency in nickel electroplating is about 100%. It
may decrease at high PH when nickel dissolution is accompanied by discharging
hydroxyl ions (OH-).
21
Cathode efficiency of nickel electroplating is 90-97%. 3-10% of the electric
current is consumed by discharging hydrogen ions (H+), which form bubbles of gaseous
Hydrogen (H2) on the cathode surface[7].
2.8.4. Anti-pitting additives
Hydrogen bubbles formed on the cathode surface and adhered to it may cause
pitting of the deposit. In order to enhance removal of the bubbles wetting agents are
added to the electrolyte. Wetting (anti-pitting) agents (e.g. sodium lauryl sulphate)
decrease the surface tension of the cathode and force the hydrogen bubbles out of the
surface [7].
2.8.5. Filtration
Continuous filtration of nickel plating baths with active carbon filters permits
to control both presence of foreign particles and organic contaminations (products of
brightener decomposition etc). The filtration pumps should turn over the solution a
minimum 1-2 times tank volume per hour [7].
2.8.6. Air agitation
Air agitation by low pressure blowers is used in nickel electroplating to
enhance removal of the hydrogen bubbles discharged at the cathode[7].
22
2.8.7. Temperature
Nickel electroplating processes are conducted at increased temperature, which
results in lower electrolyte resistance and therefore permits to decrease the voltage.
Additionally higher temperatures aid dissolution and prevent precipitation of boric acid
and other components[7].
2.9. Problems and Corrective Actions
2.9.1. Hydrogen evaluation
While the electrodeposition of these materials has been widely attempted in
aqueous solutions, hydrogen evolution reaction often occurs in the course of
electrodeposition resulting in profound effect on current efficiency and quality of the
nickel deposits, so that different additives may be needed to suppress such difficulties
[17].
2.9.1. Roughness
Roughness of nickel coating is generally caused by foreign particles suspended
in the electrolyte solution: air dust, torn anode bags, dropped parts, precipitates of boric
acid, metallic impurities or drag-in of incompatible solutions, particles of filter carbon
powder, parts of filter paper. Roughness may be also a result of deposition in low
brightener solutions at high current density.
Corrective actions: proper filtering, preventing drug-in, temperature control[8].
23
2.9.2. Pitting
Pitting is a result of hydrogen bubbles adhered to the cathode surface. It
usually occurs at low concentrations of wetting agent, low air agitation, high current
densities, and low boric acid concentrations.
Corrective actions: check the concentrations of ant-pitting (wetting) agent and
boric acid, increase air agitation, decrease the current density[8].
2.9.3. Poor Adhesion
Poor adhesion (peeling, blisters, low adhesion strength) of nickel coatings
may be generally caused either by poor pretreatment cleaning or poor acid activation of
the part surface. Activation acid contaminated with copper or chromium or improper
activation acid cause adhesion problems. For example: lead containing alloys are
activated by methane sulphonic acid or fluorides.
Corrective actions: check cleaning operations, check the activation acid[8,10].
2.9.4. High stress and low ductility
Different nickel electroplating solutions produce coatings with different levels
of internal mechanical stress and ductility. The lowest stress and maximum ductility are
provided by nickel sulphamate solutions. Brittle coatings are caused by excessive
24
concentrations of organic agents (levelers, brighteners), decomposition products of
brighteners, nickel chloride and metallic contaminants.
Corrective actions: active carbon treatment, control of nickel chloride[8].
2.9.5. Brighteners
In order to achieve bright and lustrous appearance of nickel plating organic and
inorganic agents (brighteners) are added to the electrolyte. It is known that organic
additives are introduced in trace amounts to the plating solutions to modify the structure,
morphology and properties of the deposits. Thus search and studies of the news
additives are of large interest. For nickel plating from the Watts bath, two types of
additives, such as aromatic sulphones or suphonates and compounds containing
unsaturated groups such as >CfO, >CfS, −CfN, etc., are recognized as brighteners.
The influence of the additives on mechanism of electrodeposition is not yet
clearly understood. However, it is known that the additives can act as wetting, leveling,
brightening or buffering agents.[14]
2.9.6. Current Density
Nickel electroplating involves a wide range of current density levels. Current
density directly determines the deposition rate of nickel to the base material—
specifically, the higher the current density, the quicker the deposition rate. Current
density, however, also affects plating adherence and plating quality, with higher current
density levels delivering poorer results. Therefore, the optimal level of current density
depends on the type of base material and specific type of results the final product
requires[8,25].
25
One way to avoid working at lower current densities is by employing a
discontinuous direct current to the electroplating solution. By allowing between one and
three seconds of break time between every eight to fifteen seconds of electrical current,
high current densities can produce a higher level of quality. A discontinuous current is
also beneficial for avoiding over-plating of specific sections on the base material [8,12].
2.9.7. Nickel Striking
Another solution to the current density issue involves incorporating a strike
layer to the initial electro nickel plating process. A strike layer, also known as a flash
layer, adheres a thin layer of high-quality nickel plating to the base material. Once up to
0.1 micrometers of nickel coats the product, a lower quality current density is used to
improve the speed of product completion. When different metals require plating to the
product’s base material, striking can be used. In cases where nickel serves as a poor
adherent to the base material, for example, copper can be a buffer prior to the electro
nickel-plating process [8].
2.9.8. Pre-treatment Process
Proper pre- and post-treatment of the base product has a direct correlation to
the quality and deposition rate of electro nickel plating. To help ensure uniform and
quality adhesion, chemical or manual preparation includes the following three steps:
•
Pre-treatment surface cleaning: Surface cleaning entails
eliminating
contaminants through the use of solvents, abrasive materials, alkaline cleaners,
acid etch, water, or a combination thereof.
•
Surface modification: Modifying the exterior of the base product improves
adhesion through processes such as striking or metal hardening.
26
•
Post-treatment surface cleaning: Performing finishing operations, such as
rinsing, end the electroplating process.
Once pre-treatment cleaning is complete, testing the level of cleanliness in the
base material prior to beginning the electro nickel plating process is a good idea. To do
this, the waterbreak test is recommended. In this test, the treated substrate is rinsed and
held vertical. If contaminants such as oils are absent, then a thin sheet of water remains
unbroken across the entire surface of the base material. [8]
2.10. High Speed Electroplating
The rate of electrodeposition , being governed as it is by Faraday’s law,
depends directly upon the current density applied on the cathode, but this current density
has a limiting value above which acceptable plate is not obtained. The anion present in
the nickel plating bath can affect this limiting current density, chloride and chloride
solutions has been investigated by Wesley et al amongst others, claims being made that
with a solution velocity of 23m/min, sound deposits could be obtained at
450A/dm2.Sulphate baths have been described in detail in many papers which
Hammond has recently reviewed [9].
The rate at which the electrolyte solution passes over the surface of cathode
has a considerable effect on the maximum current density at which satisfactory
electrodeposits are obtained. This has been recognized for many years and was the
reason for the introduction first of cathode movement, usually at the rate of about 0.1
m/s, and then air agitation , as means of providing solution movement over the cathode
and thus reducing the thickness of the diffusion layer. The most recent paper on this
27
effect is that by Gabe. The application of ultrasonic energy as a means of agitating
electroplating baths has been tried in the laboratory and its effect described, but this
technique does not seem to have been adopted for commercial nickel plating, possibly
because it appears that its benefits are little different from those obtained when using
violent air agitation [9,10,26].
Other means of speeding up electrolyte solution movement have been tried,
such as pumping, paddle rotation and impingement of jets, and some have been used on
a production scale. In particular, claims have been made that current densities up to
10A/dm2 can be achieved by the use of paddles or impellers which are said to give
solution speeds of about 0.5m/s, even in platting baths of large volume. However, as yet
no methods has been so revolutionary as to make electroplating fit readily into the
modern concept of high speed, continuous and automated production, as carried out in
many et al forming shops. However, General Motors did apply the principle of rapid
transfer between consecutive operations in what they termed their contour High Speed
Plating Machine. In this machine, the total time required for depositing 30 µm of multilayer nickel onto car bumper was less than 2min. The bumpers were mounted on
fixtures and passed one at a time through an automatic plant containing 25 successive
closed cells, one for each different cleaning, plating and rinsing treatment. In each
electrolytic cell, the bumper was placed only 10mm away from a conforming electrode.
Through this gap the electrolyte solution was pumped very rapidly to give solution
movement which was said to be 8m/s when the equipment was first installed, although
later it was lowered to5m/s. Nickel deposition was at first conducted at about
150A/dm2, but at the slower rate of solution movement this was reduced to about
100A/dm2.Not only did this technique result in a very high deposition rate but the use of
a conforming, insoluble anode meant that the metal distribution was much better than
when plating the same bumpers in the conventional manner. A ratio of 2:1 between
maximum and minimum thicknesses of nickel plate was achieved, compared with the
normal 8:1, thus saving 250 g nickel on each bumper. The nickel plating solutions were
conventional Watts’s type, but since lead anodes were used the bath did not contain
chlorides; even he proprietary brighteners were standard ones. The pH of the nickel
28
solutions fell as their nickel ion concentrations were depleted and so this was restored
by additions of nickel carbonate, which dissolved readily in the acidic solution (pH 3 or
less).Although the chemical a electrochemical operation f the plant was not without
difficulties, it was mainly he inability to solve the man engineering problems associate
with this machine and its ancillary equipment that led to this laudable pioneer effort
being brought to an end after two year. However, this topic is still interesting
electrochemists, as indicated by recent conference in Moscow [9,23].
29
CHAPTER 3
RESEARCH METHODOLOGY
3.1. Introduction
The main purpose of this study is learning high speed electroplating of
nickel over stainless steel, investigating of the level of adhesion between nickel layer
and stainless steel and deposition rate of nickel layer on stainless steel. To asses and
study on these purposes the need to apply SEM/TEM for microstructure of interface is
necessary. For studying the nature of adhesion nano hardness tools are needed to know
ductile or brittle nature of adhesion. To study the level of adhesion the need to some
scratch and adhesion tests including tape test are necessary.
3.2 Experimental Design
The methodology of this research is to directly electroplating of nickel over
stainless steel by using high speed electroplating technique. High speed electroplating is
a technique where the high speed movement of the plating solutions is 2.7 m/s and the
high rate of plating is more than 600 µm/h (Hussain, 2009)(20). In this research, three
parameters are involved during the electroplating processes which are different values of
current density, different values of temperature and different type of solutions used. The
type of nickel plating solution which will be used to plate nickel on the aluminium is
Watt’s bath solution and sulphate based nickel solution. Finally the nickel deposits will
30
be analyzed by using Scanning Electron Microscope (SEM) and adhesion testing. Figure
3.1 shows the flowchart used to conduct the electroplating process in order to achieve
the
study
Figure 3.1: Flowchart to conduct the electroplating process
31
3.3. Stainless steel sample preparation
Specimen coupon size for the project study will be prepared where, stainless
steelrod will be of diameter 10 mm and length 50 mm. Figure 3.2 illustrates the
dimension of the specimen coupon.
Figure 3.2: Project specimen dimensions of the stainless steel rod
3.4. Solutions Preparation
In this experiment, two different solutions were prepared for plating nickel on
aluminium which are Watt’s solution and sulphate based nickel solution. Table 3.1 shows
the summary of solution preparation according to their composition.
32
Solution I ( Watts)
Solution II
q 300g ⁄ l Nickel Sulphate
300g ⁄ l Nickel Sulphate
100 g ⁄ l Boric Acid (H3BO3)
q 100 g ⁄ l Boric Acid (H3BO3 )
q 40 g/l Nickel chloride (NiCl2.6H2O)
3.5. Experimental Setup
Stainless steel samples were electroplated as follows:
•
10 mm rod with a diameter of 1 cm was used.
•
Prepare watts solution or Sulphate based solution as an electrolyte
•
Making anode and cathode close together and pass a high current density
through an electrolyte.
•
The solution flows through anode and cathode at a rate of 2.7m/s.
•
The experimental work will be done at different temperatures and current
densities but constant time.
•
Nickel layer (10-80µm) will deposit on stainless steel without peeling of later.
•
Using SEM /EDX to study the microstructure of interface and study the rate of
deposition versus changing the current density and changing the time.
•
Using nano hardness tools to know the nature of adhesion.
Figure 3.3 shows the schematic diagram of the high speed plating equipment while
Figure 3.4 show the high speed plating equipment developed by M S Hussain (Patent
pending).
33
Figure 3.3: Schematic diagram showing the setup of high speed electroplating of nickel
on stainless steel. (Patent pending)[18]
34
Figure 3.4: High speed electroplating equipment invented by Dr. Sakhawat Hussain
A)Surface of cathode B)anode and cathode positioned close together C)Water pomp to
supply the speed of solution (Patent pending)
3.6. Sample preparation
To explore the level of adhesion between nickel layer and stainless steel part
and to investigate the rate of deposition of nickel on stainless steel, experimental tests
has been done at different temperatures (55 ° C, 60 ° C, 70 ° C), different Current
densities (0.13- 1.3 A/cm2) and different electrolyte solutions (Watts ,Sulphate based
solution).Experimental working has done at these temperatures and current densities in
nickel Watts solution.
35
Table 3.1: shows various current densities applied at three different pre-set temperatures
T=55 °C
T=60 °C
T=70 °C
C.D=0.13 A/cm2
C.D=0.13 A/cm2
C.D=0.13 A/cm2
C.D=0.26 A/cm2
C.D=0.26 A/cm2
C.D=0.26 A/cm2
C.D=0.39 A/cm2
C.D=0.39 A/cm2
C.D=0.39 A/cm2
C.D=0.52 A/cm2
C.D=0.52 A/cm2
C.D=0.52 A/cm2
C.D=0.65 A/cm2
C.D=0.65 A/cm2
C.D=0.65 A/cm2
C.D=0.78 A/cm2
C.D=0.78 A/cm2
C.D=0.78 A/cm2
C.D=0.91 A/cm2
C.D=0.91 A/cm2
C.D=0.91 A/cm2
C.D=1.17 A/cm2
C.D=1.17 A/cm2
C.D=1.17 A/cm2
C.D=1.3 A/cm2
C.D=1.3 A/cm2
C.D=1.3 A/cm2
3.7. Preparation of samples (for SEM)
A selected numbers of nickel plated stainless steel samples were prepared for
SEM as follows:
I.
II.
Wire cutting the cross section of sample
Cold or hot mounting
36
III.
Grinding and polishing the surface
IV.
Gold sputtering
V.
Using SEM for characterization
Figure 3.5 a) Cross section of sample b) Mounted sample c) polished sample
3.8. Sample preparation for TEM
TEM requires a very lengthy and sensitive sample preparation. A selected numbers of
plated samples were given the following treatment:
I.
II.
III.
Using a wire cutting machine samples were cut to a thickness of 0.5 mm
These samples were ground to a thickness of 0.5 µm
Punching
Figure 3.6:a)make a layer as thin as possible b) punching c)very small sample for
ion polishing
37
IV.
Ion milling
Ion milling is the last but the most important step of the preparation process.
The sample is mounted on a specimen holder and ion beam-polished to generate an
electron transparent area. This is accomplished by turning the ion guns on and off during
the sample rotation. The left and right ion guns are tilted at 10° from the top and the
bottom, respectively. As soon as the perforation is detected, the voltage of the ion beams
is reduced to a lower level and the incident angle is changed to 4° to enlarge the
transparent area for TEM investigation. In preparing TEM, it is sometimes observed that
the films might be completely removed by ion bombardment before there was a thin
area on the metallic substrates. Because of this situation, it is not feasible to specify the
ion milling time required for the creation of a thin area in metallic substrates. For each
combination of substrate and film, the conditions and time required for the milling must
be confirmed by a specific analysis or an elaborate experiment to make sure there is a
thin area in the films suitable for TEM observation.
3.9. Adhesion testing
After the electroplating process, adhesion testing was required to quantify the
strength of the bond between the nickel layer and the stainless steel substrate. Adhesion
testing in the paint and coating industries is necessary to ensure the paint or coating will
adhere properly to the substrates to which they are applied. The adhesion between a
coating and its substrate has been measured in various ways. For example, coatings are
often evaluated by a scratch test. A calibrated scratching tool is applied to a test sample.
A high quality adhesion between the coating and the substrate prevents the penetration
of the scratching tool and the workpiece passes. If the adhesion is low, however, the tool
can penetrate and scratch the coating, in which case the sample fails.
38
3.9.1. Scratch test
Scratch Test is a method used for determining the adhesion strength of a
nickel layer on Stainless steel. Figure 3.7 shows a schematic diagram of a scratch test
equipment.During the scratch test, the stage moves in the x direction and a probe
remains stationary while applying a controlled load on the specimen and the load is
applied by a pin.
Figure 3.7: Schematic image of scratch test
Figure 3.8: scratch test on samples
39
3.9.2. Nano scratch test
Nano scratch test has been done across the interface. (Load=50.05 mn)
These experiments were carried out three times:
Trial1 = Long scratch across interface (normal to scratch)
Trial2 = 40 mm scratch across interface (normal to scratch)
Trial3 = 60 mm scratch across interface (oblique angle to scratch)
A 4.4 mm spheronical indenter used as scratch probe
3.9.3. Tape test
Another test to investigate the level adhesion is tape test. In this test, a knife
was used as a cutting device. Two cuts were made into the coating with a 30 – 45 degree
angle between legs and down to the substrate which intersects to form an “X”. Tape was
placed on the centre of the intersection of the cuts and then removed rapidly. The X-cut
area is then inspected for removal of coating from the substrate or previous coating and
rated. A standard method for the application and performance of these tests is available
in ASTM D3359.
40
Figure 3.9 shows the tape test on the sample.
Figure 3.9: Tape test
3.10. Expected findings
It is expected by increasing the temperature of the electrolyte solution the rate of
deposition will be increased and also by increasing the current density the rate of
deposition will be increase however the level of adhesion will being decrease by
increasing these two factors.
Table 3.2. shows the expected results after experiment
Parameters be change
purpose
Expection
Temperature
To look at changing of
By
thickness
→Thickness will be increase
To see different rates of
Watt’s solution→ slow rate
Solutions
electroplating
Current density
increasing
temperature
Sulphate solution→ high rate
To look at changing of
By increasing current density
thickness
→ Thickness will be increase
41
CHAPTER 4
RESULTS AND DISCUSSION
4.1 Introduction
This chapter will discuss the results of plating nickel on stainless steel samples
that are produced via high speed electroplating method. This result is divided into two
different analyses: which are adhesion analysis, and morphology analysis for adhesion
analysis, the samples were analyzed using the knife and tape test. For morphology
analysis, the samples were characterized using a Scanning Electron Microscope (SEM)
to measure the thickness of the layer and Transmission Electron Microscopy to
investigate the particle size of nickel over stainless steel. In this chapter, the effect of
current density, temperature and type of solution on thickness of nickel plated, are
discussed.
4.2. Effect of temperature on rate of deposition
By increasing the temperature deposition rate will be increased.Relatively
high temperatures from55 c° to 70c° are helpful for supporting high current densities.
Figure 4.1: SEM image shows the thickness of nickel layer at T=60°C, C.D=1.3A/cm2
42
Figure 4.2: SEM image shows the thickness of nickel layer at T=55°C,C.D=1.3A/cm2
As shown the thickness of nickel layer is increased by increasing the temperature.
Table4.1: Rate of deposition is increased by increasing temperature and current density
43
4.3. Effect of solution on rate of deposition
Watts solution showed higher rate of plating compared to the sulphate based
nickel solution.
Figure 4.3: SEM image shows the thickness of nickel layer at T=60°C, C.D=0.13
A/cm2 Watts solution
Figure 4.4: SEM image shows the thickness of nickel layer at T=60°C, C.D=0.13
A/cm2 Sulphate based solution
44
4.4. Effect of current density on rate of deposition
Nickel electroplating involves a wide range of current density levels. Current
density determines the deposition rate of nickel to the base material .The higher the
current density, the faster the deposition rate. Current density, however, also affects
plating adherence and plating quality, with higher current density levels produces poorer
results. Therefore, the optimal level of current density depends on the type of base
material
and
specific
type
of
results
the
final
product
requires.
One way to avoid working at lower current densities is by employing a discontinuous
direct current to the electroplating solution. By allowing between one and three seconds
of break time between every eight to fifteen seconds of electrical current, high current
densities can produce a higher level of quality. A discontinuous current is also beneficial
for
avoiding
over-plating
of
specific
sections
on
the
base
material.
By increasing the current densities from 0.13 A/cm2 to 1.3 A/cm2 the rate of deposition
was increased however the level of adhesion was lowering due to high current density.
By current densities above 1.3 A/cm2 nickel layer could not be electroplated on
stainless steel. On the edges of Ni layer there were dendrites growth.
Figure 4.5: SEM image shows the thickness of nickel layer at T=55°C,C.D=0.25A/cm2
45
Figure 4.6: SEM image shows the thickness of nickel layer at T=55° C, C.D=1.14
A/cm2
As shown in figure 4.5 and figure 4.6 the rate of deposition was increased by
increasing the current density because the rate of transferring of nickel atoms was
increased and more nickel atoms were deposited on nickel layer. Current density
directly determines the deposition rate of nickel to the stainless steel specifically, the
higher the current density, the faster the deposition rate. Current density, however, also
affects plating adherence and plating quality, with higher current density levels
delivering poorer results and the quality of nickel layer lowered. At currents higher than
1.3 A/cm2 the nickel layer could not deposit on the stainless steel surface and dendrite
growth on the edges of nickel could be shown. Figure 4.7 shows the dendrite growth of
nickel and it could not deposit on the stainless steel surface. So the optimum current
density for high speed electroplating is below 1.3 A/cm2, and at higher than 1.3 A/cm2
the quality of nickel layer is lowered and also the adhesion is very low.
46
Figure 4.7 and figure 4.8 show the dentritic growth of nickel at current higher than
1.3 A/cm2.
Figure 4.7: Optical micrograph showing dendritic growth of the nickel deposits at
higher current density
Figure 4.8: SEM image showing the dendritic growth
47
4.5. Nano scratch test Analysis
Figure 4.9 shows the results after first and second trials.
Scratch test was done on the samples. The results show the electroplated samples with
current density less than 1.3 A/cm2 have good adhesion and the samples with current
densities higher than 1.3 A/ cm2 did not show good adhesion.
Figure 4.9: Nano scratch tests across interface
As shown in figure 4.9 there is no debonding between nickel layer and stainless steel
part so the adhesion level is good.
48
Figure 4.10 shows the hardness of nickel layer and Stainless steel part.
Figure 4.10: Friction against distance
As shown in figure 4.10 the stainless steel part is harder than nickel layer.
49
Figure 4.11 shows the oblique scratch test across the interface of sample. This
test is useful for assessing coating adhesion.
Figure 4.11: oblique scratch across interface
As shown in figure 4.11 there is no deboning between nickel layer and stainless
steel part and this shows a good adhesion.
4.6. Tape test analysis
The results show that the adhesion of samples which electroplated in current
densities below 1.3A/cm2 have good adhesion but the samples which electroplated in
current densities greater than 1.3 A/ cm2 don’t have good adhesion and the nickel layer
peels off easily.
50
Figure 4.12: nickel layer is peeled off (T=60°C, C.D=2.6A/cm2)
4.7. Type of failure
If current densities less than 1.3 A/cm2 a ductile type of failure was observed,
because some nickel amounts are in the stainless steel and some amounts of stainless
steel elements are in nickel layer.
Figure 4.13: SEM image of exterior layer of stainless steel at C.D <1.3 A/cm2 by peel
test
51
Figure 4.14: SEM image of interior layer of nickel at C.D <1.3 A/cm2 by peel test
Figure 4.15 and figure 4.16 show the EDAX of exterior layer of stainless steel and
interior layer of nickel after peel test.
Figure 4.15: EDAX of exterior layer of stainless steel at C.D <1.3 A/Cm2
52
Figure 4.16: EDAX of interior layer of nickel at C.D <1.3 A/Cm2
As seen in figure 4.15 small amounts of nickel are present in stainless steel and
some elements from stainless steel are seen on the peeled nickel layer.
When the current density is greater than 1A/cm2 the type of failure observed after peel
test is brittle type of failure. EDAX- elemental analysis does not show any elemental
transfer from the stainless steel to the peeled nickel layer and vice versa.
Figure 4.17: SEM image of exterior layer of stainless steel at C.D>1.3 A/cm2
53
Figure 4.18: SEM image of interior layer of nickel at C.D>1.3 A/cm2
Figures 4.19 and 4.20 show the EDAX of interior layer of nickel and exterior layer of
stainless steel.
Figure 4.19: EDAX of exterior layer of stainless steel at C.D> 1.3A/cm2
54
Figure 4.20: EDAX of interior layer of nickel at C.D> 1.3A/cm2
As shown in figure and 4.20 EDAX of interior layer of nickel there is no net
elemental transfer of stainless steel to the nickel layer and vice versa.
.
4.8. Nickel Electro crystallization
Electrocrystallization is the process of absorption, nucleation and growth of
particles (12).Nickel electrodeposition takes place at electrode: electrolyte interfaces
under the influence of an electric field and include a number of phase formation
phenomena.Fundamental aspects of electrocrystallization of metals are directly related
to nucleation and crystal growth processes. There are some growing centres in the
processes of multiple nucleation and growth.
55
Figure 4.21 : Schematic diagram of electrochemical growth of a stepped crystal surface
(1) ion in the electrolyte (2) adatom on the flat terrace (3) atoms in kink site (4) vacancy
(5) atoms in two atomic cluster(1
2
3) surface diffusion mechanism (1
3) direct
attachment mechanism (a,b,c) distribution of adatoms concentration
The basic thermodynamic concepts of nucleation and crystal growth electro
crystallization includes nucleation ad growth processes.Nucleation and growth processes
in electrochemical metal deposition determine the physical, chemical, electric properties
of Ni on stainless steel.
In conventional methods such as electroplated nickel the nucleation and
growth stages take place slowly. In high speed electroplating the nucleation and growth
stages take place very rapidly. At current densities greater than 1.3 A/cm2 dendritic
growth took place very rapidly.
56
Figure 4.22: SEM image of nickel layer was peeled off at C.D=2.6A/cm2,T=60°C
Figure 4.23: Optical micrograph showing dendritic growth of the nickel deposits at
higher current density
Figure 4.24 a) shows a TEM images(top view) of nickel deposited on stainless steel by
high speed electrodeposition .
Figure 4.24(b) shows the particle size of the nickel deposits. The particle size
measurements shows that the electroplated nickel particles are very fine and are
nanocrystalline.As these are presence of particles as small as 4nm in diameters
57
Figure 4.24: a)TEM image of nano-crystalline nickel over stainless steel b) particle size
of nickel deposites
58
CHAPTER 5
5.1.
I.
Conclusion
Stainless steel was electroplated directly with nickel without the need for any
pre-treatment.
II.
The rate of electrodeposition by high speed electroplating method has been
found to be 30 times faster than conventional electroplating using Watts type of
electrolyte
III.
The rate of electrodeposition by this process increased by increasing temperature
and current density
IV.
Watt’s nickel electrolyte gave faster rate of plating than sulphate based nickel
solution.
V.
The level of adhesion was much higher at lower current densities.At current
densities
higher
than
1.3
A/cm2
the
level
of
adhesion
becomes
unacceptable/peels of easily.
VI.
The electrodeposited nickel has been found to be nano-crystalline_TEM images
show presence of particles as small as 4 nm in diameter
59
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