STUDIES ON THE PARAMETERS THAT

AFFECT THE PROPERTIES OF

TRIPLE NICKEL ELECTROPLATING

by

001 CHEE HENG

Thesis submitted in fulfillment of the

requirements for the degree

of Master of Science

June 2003

I,:, "

Saya isytiharkan bahawa kandungan yang dibentangkan di dalam tesis ini adalah hasil keIja saya sendiri dan telah dijalankan di Universiti Sains Malaysia kecuali dimaklumkan sebaliknya. Tesis ini juga tidak pemah diserahkan untuk ijazah yang lain sebelum ini.

I declare that the content which is presented in this thesis is my own work which was done at Universiti Sains lvfalaysia unless informed otherwise. The thesis has not been previously submittedfor any other degree.

Tandatangan Calon Signature of Student

Nama Calon: 001 CHEE HENG Name of Candidate KiP: 661209-02-5291

.. Disaksikan oleh: Witness by:

..

andatangan Penye 1 e Signature of Supervisor/Dean

Cop Jabatan: Chop of Department

PUSAT PENGAJIAN SAINS KI~A UNIVERSITI SAINS MALAYSIA 11800 USM PUlAU PlNANG.

DEDICATION

This work is dedicated to my wife - Fong Won Ho for her full support and

understanding of the importance of my studies i.e. to acquire additional knowledge in

the field of electroplating and a springboard for my future career development.

..

11

ACKNOWLEDGEMENTS

I would like to take this opportunity to express my sincere thanks to all those who

have helped me accomplish this work.

I would like to thank the Chemistry Department of USM for offering me this Master ..

Degree Course. My special thanks to my supervisor, Prof. Zainal Abidin Ahmad who

has accepted me as his student. Many thanks to him for his invaluable guidance,

encouragement, assistance and support in my research work. More importantly his

patience and kind understanding of me being a part time student, and my necessity to

give priority to my career, has take much longer time to complete my research work. I

would like to thank my co-supervisor, Dr. Mohamad Nasir Mohamad Ibrahim for his

tremendous assistance during my thesis submission.

I also like to take this opportunity to thank Ms. Lim Shui Thing and Ms. Sharon Looi

who have helped me on the preparation for this thesis and skillfully drawing the

diagrams. Special thanks to Mr. Lean Eng Hock who has helped me to get the relevant

pretreatment and nickel plating additives from Modenas SIB; Mr. Vincent Ng who has

helped me to conduct the chemical analyses; Mr. Lee Chong Chew who has helped me

to translate the abstract to Bahasa Malaysia, and Mr. Chan Kit Whye who has helped

me to check on the language errors.

Also special thanks to Mr. O.Takeda of Ebara-Udylite, Japan for his kind assistance on

the STEP test; En. l-Iisham Abdullah Chua for helping me to perform the CASS test at

III

Modenas SIB; Mr. Goh who has helped me to check the thickness at Hightec Plating,

and Cik lamilah of School of Biological Sciences, USM who has helped me on the

SEM and EDS Analyses.

Sincere thanks to my family especially my wife, for her full support and kind

understanding by travelling back to my hometown in Alor Setar alone on almost every

Saturday to look after my two children for the past 4 Yz years because of my research

work at USM's Laboratory.

For those whose name I did not mention and have directly or indirectly help me on my

work, their contributions are always remembered. To all of them - THANK YOU

VERY MUCH.

IV

TABLE OF CONTENTS

Page

DEDICATION 11

ACKNOWLEDGEMENTS 111

TABLE OF CONTENTS v .. LIST OF TABLES ix

LIST OF FIGURES x

LIST OF APPENDICES xiii

ABSTRACT xv

ABSTRAK xvii

CHAPTER 1 : INTRODUCTION AND LITERATURE REVIEW

1.1 Principles of Electrolysis 1

1.2 Principles of Nickel Electroplating 2

1.3 The Nature of Nickel 3

1.4 The Development of Nickel Plating 4

1.5 Common Process Steps in Triple Nickel Plating 6

1.5.1 Pretreatment 7

1.5.1.1 Soak Cleaning 8

1.5.1.2 Electrolytic Cleaning 10

1.5.1.3 Acid Pickling 12

1.5.1.4 Acid Activation 13

1.5.2 Triple Nickel Plating 13

v

1.6

1.7

1.8

1.5.3

l.5.2.1

1.5.2.2

l.5.2.3

Semi-Bright Nickel

High Sulphur Nickel

Bright Nickel

Post Treatment

Function of Bath Constituents

1.6.1

1.6.2

1.6.3

1.6.4

Nickel Sulphate

Nickel Chloride

Boric Acid

Ad~ition Agents (Additives / Brighteners)

l.6.4.1 Wetting Agents (Anti-Pitting Agents)

Effects of Plating Parameters

Distance between Cathode and Anode

Current Density

Plating Time

..

1.7.1

1.7.2

1.7.3

1.7.4

1.7.5

1.7.6

Combination of Different Layers of Nickel Deposits

pH

Temperature

1.7.7 Agitation

The Properties of Triple Nickel Deposits

1.8.1

1.8.2

1.8.3

1.8.4

1.8.5

Cathode Current Efficiency

Thickness Distribution

Brightness and Leveling

Electrode Potential and Sulphur Co-Deposition

Corrosion Resistance Performance

1.9 Application of Triple Nickel Plating

1.10 Objective

VI

14

14

15

16

17

17

17

18

18

18

19

19

20

20

23

23

23

24

24

25

25

26

27

28

34

34

CHAPTER 2: PREPARATION FOR EXPERIMENTS

2.1 Setting Up for the Experiments 36

2.2 Plating Process Flow 37

CHAPTER 3: EXPERIMENTAL SECTION

3.1 Materials and Reagents 48

3.2 Preparation and Establishment of Operating Conditions for 49 Pretreatment Chemicals

3.3 Preparation and Establishment of Operating Conditions for Triple 49 Nickel P-Iating Solutions

3.4 Maintenance of Solutions and Plating Jigs 52

3.5 Cathode Current Efficiency 52

3.5.1 Effect of Distance between Cathode and Anode

3.5.2 Effect of Current Density

3.6 Thickness Distribution

3.7 Brightness and Leveling

3.7.1 Appearance by Visual Inspection

3.7.2 Appearance by SEM

3.8 Sulphur Content in the Triple Nickel Deposits

3.9 Electrode Potential Differences between Triple Nickel Layers

3.10 Corrosion Resistance Performance

CHAPTER 4: RESULTS AND DISCUSSION

4.1 Cathode Current Efficiency

4.1.1

4.1.2

Effect of Distance between Cathode and Anode

Effect of Current Density at 10 cm and 14 cm Cathode and Anode Distances

Vll

54

55

58

60

60

60

62

62

62

65

65

66

-4.2 Thickness Distribution 70

4.2.1 Effect of Distance between Cathode and Anode 71

4.2.2 Effect of Current Density at 10 cm and 14 cm Cathode and 74 Anode Distances

4.3 Brightness and Leveling 79

4.3.1 Appearance by Visual Inspection 79 , .~ _.:-,

4.3.2 Appearance by SEM 81 ..o.:.-~-

::;~ . . ",;" ~

4.4 Sulphur Content in the Triple Nickel Deposits 86 #~~:

<-

4.5 Electrode Potential Differences between Triple Nickel Layers 86 .•. - .

4.6 Corrosion Resistance Performance 90

CHAPTER 5: CONCLUSION 103

5.1 Recommendations for Future Research 105

REFERENCES 106

APPENDICES

A Make Up Procedure for Nickel Plating Solutions 111

B Experimental Results 112

C STEP Test Results from Ebara-Udylite 124

D EDS Analysis Results 126

Vlll

LIST OF TABLES

.... Table Page

2.1 Setting up for the experiments 38

2.2 Plating process flow for the experiments 47

3.1 Concentrations and operating conditions for pretreatment chemicals 50

3.2 Concentrations and operating conditions for triple nickel solutions 51

3.3 Frequency of chemical analysis and replenishment rate for nickel 53 plating solutions

3.4 Plating conditions at different distance between cathode and anode 56

3.5 Plating conditions at different current density (at 10 cm distance) 57

3.6 Plating conditions at different current density (at 14 cm distance) 57

3.7 Plating conditions of brightness and leveling experiments 61

3.8 Plating conditions of electrode potential difference experiments 61

3.9 Plating conditions of corrosion resistance experiments 64

4.1 Summary of calculated thickness versus point thickness at difterent 73 distance between cathode and anode

4.2 Summary of calculated thickness versus point thickness at 10 cm and 77 14 cm distance between cathode and anode

4.3 Thickness ratio versus current density ratio 78

4.4 Appearance by visual inspection 80

4.5 Appearance by SEM 82

4.6 Results of EDS 87

4.7 Results of STEP test 88

4.8 Results of CASS test 91

4.9 Summary of corrosion resistance performance 102

IX

LIST OF FIGURES

Figure Page

1.1 Pre-cleaning 9

1.2 Soak cleaning (Emulsion cleaning) 9

1.3 Soak cleaning (Displacement cleaning) 9 ..

1.4 Anodic cleaning 11

1.5 Cathodic cleaning 11

1.6 Acid dipping 11

1.7 Current distribution (The dogbone effect) 21

1.8 Current distribution (Not uniform over a shaped article) 21

1.9 High sulphur nickel- Relationship between current density and 22 corrosion resistance (at 4 cycles CASS test)

1.10 High sulphur nickel- Relationship between pH and corrosion 22 resistance (at 4 cycles CASS test)

1.11 Standard STEP test pattern for triple nickel deposits 29

1.12 Corrosion mechanism of single bright nickel - chromium deposits 30

1.13 Corrosion mechanism of duplex nickel - chromium deposits 31

1.14 Corrosion mechanism of triple nickel - chromium deposits 32

2.1 Pretreatment and triple nickel plating tank 39

2.2 Water rinsing tank 39

2.3 Copper rod for cathode and anode 40

2.4 Brass rod for hanging nickel anode 40

2.5 Plating jig design 41

2.6 Stainless steel plate for electrolytic cleaner 42

x

2.7 Steel panel 42

2.8 Nickel anode 43

2.9 Polypropylene anode bag 43

2.10 Rectifier with CC/CV control 44

2.11 Heating devices 44

2.12 Air agitation devices 45

2.13 Experimental layout diagram 46

3.1 Thickness measurement points on steel panel 59

• 4.1 Cathode current efficiency versus distance between cathode and 67

anode at 3 Aldm2 .. •

4.2 Weight gain versus current density at 10 cm and 14 cm distance 68 between cathode and anode

4.3 Cathode current efficiency versus current density at 10 and 14 cm 69 distance between cathode and anode

4.4 Thickness distribution at different distance between cathode and 72 anode at 3 Aldm2

4.5 Thickness distribution at different current density at 10 cm distance 75 between cathode and anode

4.6 Thickness distribution at different current density at 14 cm distance 76 between cathode and anode

4.7 SEM for semi-bright nickel (Point 1) 83

4.8 SEM for semi-bright nickel (Point 2) 83

4.9 SEM for semi-bright nickel (Point 3) 83

4.10 SEM for high sulphur nickel (Point 1) 84

4.11 SEM for high sulphur nickel (Point 2) 84

4.12 SEM for high sulphur nickel (Point 3) 84

4.13 SEM for bright nickel (Point 1) 85

4.14 SEM for bright nickel (Point 2) 85

4.15 SEM for bright nickel (Point 3) 85

XI

4.16 STEP test for triple nickel at 14 cm distance (Point 1) 89

4.17 STEP test for triple nickel at 14 cm distance (Point 2) 89

4.18 STEP test for triple nickel at 10 cm distance (Point 1) 89

4.19 STEP test for triple nickel at 10 cm distance (Point 2) 89

4.20 CASS test for single bright r.ickel at 20 min plating time 92

4.21 CASS test for single bright nickel at 30 min plating time 93

4.22 CASS test for duplex nickel at 20 min plating time 94 (Semi-bright/Bright = 10/10 min)

4.23 CASS test for duplex nickel at 30 min plating time .. 95 (Semi-bright/Bright ~ 20110 min)

4.24 CASS test for duplex nickel at 30 min plating time 96 (Semi-bright/Bright = 10/20 min)

4.25 CASS test for triple nickel at 20 min plating time 97 (Semi-brightlHigh sulphurlBright = 8/2110 min)

4.26 CASS test for triple nickel at 30 min plating time 98 (Semi-brightlHigh sulphurlBright = 181211 0 min)

4.27 CASS test for triple nickel at 20 min plating time 99 (Semi-brightlHigh sulphurlBright = 10/2/8 min)

4.28 CASS test for triple nickel at 30 min plating time 100 (Seini-brightlHigh sulphurlBright = 10/2118 min)

Xli

LIST OF APPENDICES

Appendix Page

A Make up procedure for nickel plating solutions 111

B1 Effect of distance between cathode and anode at 3 Aldm2 112

B2 Effect of current density at 10 cm distance 113 .. B3 Effect of current density at 14 cm distance 114

B4 Thickness distribution at 7 cm distance 115

B5 Thickness distribution at 10 cm distance 116

B6 Thickness distribution at 14 cm distance 117 ii!'

B7 Thickness distribution at 2 Aldm2 and 10 cm distance 118

B8 Thickness distribution at 3 Aldm2 and 10 cm distance 119

B9 Thickness distribution at 4 Aldm2 and 10 cm distance 120

BI0 Thickness distribution at 2 Aldm2 and 14 cm distance 121

Bll Thickness distribution at 3 Aldm2 and 14 cm distance 122

B12 Thickness distribution at 4 Aldm2 and 14 cm distance 123

C STEP test results from Ebara-Udylite, Japan 124

01 EDS analysis results for semi-bright nickel (Point 1) 126

02 EOS analysis results for semi-bright nickel (Point 2) 127

03 EOS analysis results for semi-bright nickel (Point 3) 128

04 EOS analysis results for high sulphur nickel (Point 1) 129

05 EOS analysis results for high sulphur nickel (Point 2) 130

06 EOS analysis results for high sulphur nickel (Point 3) 131

07 EOS analysis results for bright nickel (Point 1) 132

Xlll

--D8 EDS analysis results for bright nickel (Point 2) 133

D9 EDS analysis results for bright nickel (Point 3) 134

..

XIV

ABSTRACT

The triple nickel electroplating consists of three layers of nickel deposits with different

sulphur content, i.e. the first layer plated on the substrate is semi-bright, followed by

high sulphur and bright nickel. This research is aimed at studying the parameters that

affect the various properties of triple nickel deposits such as cathode current ..

efficiency, thickness distribution, brightness and leveling, co-deposited sulphur

content, electrode potential differences and the corrosion resistance performance. The

parameters being studied are the distance between cathode and anode, current density,

plating time and the combination of different layers of nickel deposits.

All the test panels plated with bright nickel at the outer layer exhibited good brightness

and leveling, and these effects are dependent on the plating time and current density.

In addition, brighter appearance has shoViTI finer gain structure. The cathode current

efficiency is dependent on the cathode and anode distances, i.e. tends to be higher at

shorter distances (7cm > 10cm > 14cm), but is not influenced much by the current

density. The thickness values tend to be higher at shorter distances, and when the

current density is increasing (4 Aldm2 > 3 A/dm2 > 2 A/dm2). The thickness

distribution has shown a consistent pattern, i.e. always thicker at the edges (high

current areas) as compared to the areas in the middle of the panel (low current areas).

The electrode potential differences measurement between triple nickel layers have

shown that the electrode potential for all the different nickel layers is not similar, i.c.

the semi-bright is always more positive (noble) than high sulphur and bright nickel

xv

layers, and high sulphur is more negative (active) as compared to bright nickel layer.

Although sulphur was not detected in all the different nickel deposits, however, the

differences in the electrode potential results have indicated on the possibility of

sulphur co-cleposition in different percentage onto each layer of triple nickel deposits.

The CASS test results have shown that triple nickel deposits have better corrosion

resistance performance as compared to duplex and single nickel deposits with similar

total plating time (or thickness). In addition, both duplex and triple nickel deposits

with thicker semi-bright layer out-performed those with thicker bright nickel layer.

By optimizing the plating parameters such as cathode and anode distance, current

density, plating time and the combination of different layers of nickel deposits, we are

able to produce plated components that can be accepted by commercial application in

terms of cost, appearance and corrosion resistance performance.

xvi

--

PENYELIDlKAN TERHADAP PARAMETER-PARAMETER YANG

MEMPENGARUHI SIFAT-SIFAT SADURAN TRIPLE NlKEL

ABSTRAK

Proses pengelektrosaduran triple nikel melibatkan tiga lapisan nikel dengan kandungan

sulfur yang berbeza, iaitu lapisan pertama disadurkan dengan nikel separa cerah,

kemudian dengan nikel sulfur tinggi dan seterusnya dengan lapisan nikel cerah. -.

Penyelidikan ini bertujuan untuk mengkaji parameter-parameter yang memp.engaruhi

pelbagai sifat saduran triple nikel seperti kecekapan arus pada katod, pengagihan

ketebalan, kesan kecerahan dan kerataan, kandungan sulfur, perbezaan potensi

elektrod dan pre stasi ketahanan terhadap kakisannya. Parameter-parameter

pengelektrosaduran yang diselidik ialah jarak di antara katod dan anod, ketumpatan

arus, masa saduran dan dengan kombinasi pelbagai gabungan lapisan nikel yang

berbeza.

Kesemua plat-plat kajian yang disadurkan dengan nickel cerah sebagai lapisan luar

mempamerkan kesan kecerahan dan kerataan yang bagus, dan kesan-kesan ini adalah

bergantung kepada masa dan ketumpatan arus saduran. Tambahan pula, saduran yang

cerah akan memberikan struktur yang lebih halus. Keputusan-keputusan ujikaji telah

menunjukkan bahawa kecekapan arus pada katod adalah bergantung kepada jarak di

antara katod dan anod, iaitu lebih tinggi pada jarak yang lebih dekat (7cm > lOcm >

14cm), tetapi tidak banyak dipengaruhi oleh ketumpatan arus. Nilai-nilai ketebalan

akan bertambah apabila jaraknya semakin dekat, dan apabila ketumpatan arus

meningkat (4 A/dm2 > 3 A/dm2 > 2 Aldm\ Pangagihan ketebalan telah menunjukkan

sesuatu paten yang tertentu, iaitu scntiasa lcbih tinggi pada sekitar penjuru plat

XVIl

(kawasan berarus tinggi) jika dibandingkan dengan di tengah plat (kawasan berarus

rendah).

Pengukuran potensi elektrod telah menunjukkan nilai-nilai potensi yang' berbeza di

antara ketiga-tiga lapisan nikel, iaitu nikel separa cerah adalah lebih positif (lengai)

berbanding dengan nikel sulfur tinggi dan nikel cerah, dan nikel sulfur tinggi adalah

lebih negatif (aktif) berbanding dengan nikel cerah. Walaupun kandingan sulfur tidak

dapat dikesan dalam ketiga-tiga Iapisan nikel, akan tetapi, pemerhatian dari perbezaan

• ,potensi elektrod telah menunjukkan kemungkinan kehadiran sulfur yang tersadur

bersamaan dengan ketiga-tiga lapisan nikel dalam peratusan yang berbeza.

Kesan ketahanan terhadap kakisan telah menunjukkan bahawa saduran triple nikel

mempunyai prestasi ketahanan yang lebih baik jika dibandingkan dengan duplex nikel

dan lapisan tunggal nikel cerah dengan mas a saduran (atau ketebalan) yang sama.

Tambahan pula, duplex nikel dan triple nikel yang mengandungi Iapisan nikel separa

cerah yang lebih tebal akan menunjukkan tahap ketahanan yang lebih baik berbanding

dengan yang mengandungi lapisan nikel cerah yang lebih tebal.

Dengan mengoptimakan parameter-parameter seperti diatas, kita boleh menghasilkan

komponen-komponen tersadur yang dapat diterima oleh pihak komersial dari segi kos,

rupabentuk dan pretasi ketahanan terhadap kakisan yang amat baik.

XVlll

CHAPTER 1

INTRODUCTION AND LITERATURE REVIEW

1.1 Principles of Electrolysis

Michael Faraday (1791 - 1867) [1], is the first scientist who studied electrolysis .. quantitatively. From his work on electrolysis (1832 - 33), he believed· that the

electrolysis process is based on charged carriers called as ions. (Ion is atom or

collection of atoms and has a charge). Those carrying positive charges (metal atoms)

being cations and traveling towards the cathode, and those carrying negative charges

(non metallic atoms) being anions and traveling towards the anode [2].

The main results of his work on electrolysis are summed up in the following two

important.Laws:

(I) The amount (or weight, W) of metal deposited by a current is proportional to

the quantity of electricity (Q) passed, i.e.

W ex::Q

W ex:: It [Q = Current (I) x Time (t)] ---------- ill

(II) The masses of different metals deposited or dissolved by the same quantity of

electricity are proportional to its chemical equivalent weights (AlZ) at 100% current

efficiency, i.e.

WocA Z

(A = Atomic weight, Z = Valency no)

From the combination of (1) and (2) as above,

W oc ItA Z

W = ItA ZF

---------- @

---------- @

.. Where F Faraday constant (Quantity of Electricity)

96490 coulombs (Ampere - seconds)

In accordance with Faraday's Laws, the same quantity of electricity will liberate

58.7/2 g of Nickel (from a Nickel salt solution), or 63.5/2 g of Copper (from a Copper

salt solution) at the cathode.

The principles that govern the electrolysis process will firstly be initiated by

conduction of electricity through a solution and, secondly, the discharge of an ion at an

electrode.

1.2 Principles of Nickel Electroplating

Electroplating is a process of depositing a coating having a desirable form by means of

electrolysis. Its purpose is generally to alter the characteristics of a surface so as to

provide improved appearance, ability to withstand corrosive agents,. resistance to

abrasion, or other desired properties or a combination of them. It is carried out in a

2

solution of various kinds and in commercial practice it is almost invariably a water

solution [3,4].

Nickel plating is the electrolytic deposition of a layer of nickel on a s~bstance. The

process involves the dissolution of one electrode (the anode) and the deposition of

metallic nickel on the other electrode (the cathode). Direct current is applied between

the anode (positive) and the cathode (negative). Conductivity between the electrodes is ~-

provided by an aqueous solution of nickel salts. When nickel salts are dissolved in ,

water, the nickel is present in solution as d~valent, positively charged ions (Ni 2+).

When current flows, divalent nickel ions react with two electrons and are converted to

metallic nickel (Ni 0) at the cathode. The reverse occurs at the anode where metallic

nickel dissolves to form divalent ions [5].

The electrochemical reaction in its simplest fOIDl is :

Ni 2+ + 2 e- Ni 0

Because the nickel ions discharged at the cathode are replenished by the nickel ions

formed at the anode, the nickel plating process can be operated for long periods of

time without interruption.

1.3 The Nature of Nickel

Nickel is a white lustrous metal. The atomic weight is 58.71, the melting point is

14550C and the density at 20°C is 8.908 g/cm3. Nickel is present in solution as

divalent, positively charged ions. The standard electrode potential, EO (Ni 2+ / Ni) = -

0.25 V [6, 7]. It has high mechanical strength, is fairly hard and at ordinary

temperatures is ferro-magnetic. It oxidizes only with difficulty even when heated in

air. A polished surface will, however, gradually corrode on atmosphefic exposure.

Nickel is slowly soluble in hydrochloric and also in dilute sulphuric acid. Dilute nitric

acid rapidly attacks nickel, but immersion in concentrated sulphuric acid renders the

surface of nickel 'passive', in which state it is not readily attacked [8].

Nickel may be deposited as a bright, semi-bright or, dull coating. Although nickel is a

ductile metal, the ductility of nickel coatings is very dependent upon their method of

production and their purity. Many electrodeposited nickel coatings (particularly the

organic bright nickels) may be brittle and have a high degree of internal stress.

Similarly, chemically deposited nickel coatings have greater hardness, brittleness and

different corrosion characteristics because of the incorporation of phosphorus or boron

in the deposits - inherent in the method of deposition from complex solutions.

1.4 The Development of Nickel Plating

The electrodeposition of nickel was first described in 1837 by G. Bird for deposited

nickel on a platinum electrode from a solution of nickel chloride or sulphate. In 1840,

the first patent for commercial nickel plating was granted to 1. Shore of England who

specified a solution 0 f nickel nitrate [9].

The development of the electrodeposition of nickel began in 1843 when R. Bottger

described the first apparently sound nickel plate, which he obtained from a bath

4

containing nickel and ammonium sulphates. The solution developed by Bottger

remained in commercial use for seventy years, and he is acknowledged to be the

originator of nickel plating [10].

Following the lead of Roseleur in 1849, Adams in 1869 was probably the first to do

nickel plating on a truly commercial basis by using both sulphate and chloride salts

and was probably the first to recognize the importance of impurities in the plating

bath. Dr. Isaac Adam Jr., a medical doctor was one of the first to commercialize nickel

~:-- plating in the United States, and his patented process gave his company a virtual

monopoly in commercial nickel plating from 1869 to 1886. His patent covered the use

of pure nickel ammonium SUlphate [11].

Remington was the next to commercialize nickel plating. Up to 1913 many bath

compositions were proposed like Weston's introduction of boric acid into the bath, the

use of chlorides by Bancroft to promote anode corrosion and the discovery that

cadmium salts act as brightener about 1912 in the Elkington plant of Brimingham,

England.

Since 1915, the rate of development has been quite rapid, beginning with Watts with

his famous "Watts bath"; the control of quality of deposits, first emphasized by Watts

and Deverton and finally maturing into ASTM - American Electroplaters' Society

Specification in 1935; accurate pH control, introduced by Thomson; low - pH baths,

suggested by Philips; commercial introduction of Il10dern bright nickel plating by

Schlotter and the many major improvements of it, for example, the design of automatic

plating equipment for high production rates; development of bath for the ductile, low-

stress, high leveling semi-bright and bright nickel coatings; the use of combination

5

plates of sulfur-free semi-bright nickel followed by sulfur-containing bright nickel to

greatly improved outdoor corrosion protection; the use of high sulphur nickel and

micro-porous nickel in combination of semi-bright and bright nickel layers to give

~: outstanding outdoor corrosion protection with minimum thickness of total ni~kel

f deposits. , ~ ~:

f '-' if'-

f Today, nickel is one of the most important metals applied by electrodeposition which

covered a broad spectrum of end uses that include decorative, engineering and

electro forming applic.ations.

Nickel plating is used principally as a bright coating underneath a chromium

electroplate to provide a highly lustrous and corrosion protective finish for articles of

steel, brass, zinc die castings, plastic, aluminium and magnesium alloys. In addition,

thin gold or brass electroplate with a clear lacquer finish is used as a decorative

coating on bright nickel deposits. Due to its mechanical properties, nickel is widely

used for, electro forming of moulds, dies, record stampers, seamless belts and textile

printing screen.

1.5 Common Process Steps in Triple Nickel Plating

Electroplating has three major steps in sequence, which is pre-treatment, plating and

post-treatment. Pre-treatment is carried out before electroplating due to the importance

of perfect cleaning. After pre-treatment, it goes through plating process where the base

material is deposited with the metallic coatings by electrolysis. Finally, the finished

products are treated for protection purpose and drying.

6

The common base materials or substrates are steel, brass, copper alloy, copper, pewter,

aluminium, stainless steel, zinc die-cast and plastics. No matter what materials or

substrates need to be plated, they have to go through three major steps in electroplating

process as outlined above.

1.5.1 Pretreatment ..

In general, pretreatment steps involve cleaning, descaling, de greasing, activation and

other processes which prepare the basis material for plating. The purpose of the

pretreatment is to produce clean, oil-free, rust-free, scale-free and active surface for

subsequent plating processes [12].

The condition of substrates has a direct bearing on the properties and the performance

of the deposits. Consequently, basis metals are generally prepared for plating by

mechanical, chemical or electrochemical finishing. In the mechanical finishing

processes of polishing, buffing [6, 13] and grinding, the surface of the work is cut or

worked by abrasive which are moved over the surface of the work by rotating wheels

or moving belts. Metal imperfections, scales, oils and grease must be removed from

the surface if the plating process is to be successful.

Removal of scale is through mechanical means of tumbling, dry rolling, buffing,

deburring, polishing, desmutting and blasting. Such mechanical treatment eliminates

or minimizes the subsequent pickling appiications.

7

The water-break test is widely used for measuring the cleanliness of metal surfaces

after pretreatments. Other methods such as visual (optical, microscopical) and

fluorescence contact angle are used in some cases for cleanliness measurement [14,

15].

1.5.1.1 Soak Cleaning

.. Soak cleaning involves the removal of oil, grease, dirt and soil from the surface of the

substrate. This can be accomplished by solvent degreasing or alkaline cleaning

depending on the degree of cleanliness required. The detergent nature of the cleaning

solution provides the primary cleaning action [16, 17]. In general, for very heavy oil,

grease, dirt and soil, pre-cleaning by means of solvent cleaner or alkaline pre-cleaner

are recommended (Figure 1.1).

The alkaline cleaning normally will contain either emulsifiers or surfactants or both.

The function of emulsifiers is to dissolve oil and grease through emulsifying action

(Figure 1.2). The surfactants will remove oil and grease by displacement action

(Figure 1.3). The dirt and soil will be removed together as well.

Incorporation of ultrasonic, spray mechanical agitation and elevated temperature will

help to create mechanical and impact force by loosening the oil, grease, dirt and soil in

order to further enhance the cleaning efficiency [18, 19].

8

Figure 1.1 : Pre-cleaning

Surfactants Surround Oil and Grease (Wetting)

The Hydrophillc End Causes Soil to Act Like Water (Emulsion)

Figure 1.2 : Soak cleaning (Emulsion cleaning)

Surfactants Form Temporary/Weak Emulsions to Carry Soil to Surface

Figure 1.3 : Soak cleaning (Displacement Cleaning)

(Courtesy of Enthone-OMI)

Q

1.5.1.2 Electrolytic Cleaning

Electrolytic cleaning is commonly used after alkaline cleaning. Electrolytic cleaning

will remove buffing compound and some remaining oil, grease, dirt, soil and certain

metal oxide from the surface of the substrate. Moreover, certain dirt particles which

become electrically charged are repelled from the substrate surface.

Electrolytic cleaners are classified into 3 types, 1.e. anodic, cathodic and periodic ..

reverse [20].

(I) Anodic cleaning involves the substrate function as anode. Oxygen gas is

generated at the substrate surface. Like Hydrogen gas generated in cathodic cleaner it

would provide the scrubbing action that helps to remove the soil, rust, scale and

foreign inclusions (Figure 1.4).

Since the substrate will dissolved during the anodic application, there is a need to

ensure that the cleaning time is not too long and the current is not too high.

(II) Cathodic cleaning involves the substrate function as cathode during application

of current. Hydrogen gas will be generated at the substrate surface and is effective in

solubilizing certain metal oxides (Figure 1.5).

However, this benefit is countered by the increased potential for hydrogen

embrittlement [21] of the substrate and substrate contamination by foreign inclusion

which may be depusited during cathodic application. Thus, an anodic cleaner is

needed after this precess.

10 R

Figure 1.4 : Anodic cleaning

Figure 1.5 : Cathodic cleaning

Application/Operation

• Room Temperature

• Short Immersion Time (Up to 1 Min.)

Figure 1.6 : Acid dipping

(Courtesy ofEnthone-OMI)

11

(III) Periodic reverse cleaning is a combination of cathodic and anodic cleaning by

altering or change of the current. Both Hydrogen and oxygen gasses will be generated

at certain time interval.

In order to reduce the risk of hydrogen embrittlement and contamination by foreign

inclusions as highlighted in cathodic cleaner, most periodic reverse should finish when

the part is anodic.

1.5.1.3 Acid Pickling

Treatment of the surface of the work pieces with acid solutions is performed for oxide

and welding scale removal. The term pickling denotes the removal of relatively thick

oxide films and usually includes the removal of some metal substrate [22, 23]. Acid

pickling is used to remove oxide films and involves dissolution of oxide scale in an

acid. A generalized reaction may be written as:

MO + 2 HA ................. , .... " ... , ..... ,., .. , ..... ~ MA 2 + H20 ---------- ®

Where M = metal

HA = acid

Sulfuric, hydrochloric and phosphoric acid could be used in this regard. Sulfuric is

most often used because it is the least expensive. The rates of reactions are increased

by an increase in acid concentration, temperature and degree of agitation.,

1')

Hydrochloric acid is more costly and there is a fuming problem. Nevertheless, many

small establishments use it because it works well without the addition of heat. It is also

used for light acid dips before plating.

Phosphoric acid is intermediate in cost, but it forms phosphates at the surface of the

basis material. This is desirable if rust resistance is needed, but not if the electroplating

is to follow.

.. ,. - In some cases, acid electrolytic pickling cathodically is used to remove stubborn

welding scale in order to achieve better plating quality.

1.5.1.4 Acid Activation

It is a final pretreatment step before plating processes. Acid activation is used to

remove any flash rust and for the neutralization of the residual alkali films by dipping

application (Figure 1.6). This step is to ensure clean and active surface for subsequent

plating operation.

1.5.2 Triple Nickel Plating

Single and multilayer nickel coatings are used to produce decorative coatings that

resist corrosion. Single-layer bright nickel deposits are specified for mildly corrosive

service. Double- and triple-layer coatings are specified for use in severe corrosion

service [5]. In double-layer coatings (or duplex system), the first nickel layer is

deposited from a semi-bright sulphur-free bath, and the second layer is then deposited

from a conventional sulphur-containing bright bath. In triple-layer coatings, a special

thin layer of bright, high-sulphur nickel is deposited between the initial layer of semi-

bright nickel and the top layer of bright nickel.

1.5.2.1 Semi-Bright Nickel

.. Semi-bright nickel solutions contain nickel sulfate, nickel chloride, boric acid and a

leveling agent [24, 25]. The original process used coumarin [9, 26] as the principal

additive. Coumarin-free processes are now available. The process yields deposits that

are semi-lustrous and sulfur-free «0.005% S). The deposits are smooth and have a

columnar structure unlike the banded structure or laminar characteristics of fully bright

deposits.

The solution was developed to facilitate polishing and buffing; semi-bright nickel

deposits are easily polished to a mirror finish. Efforts to eliminate polishing led to the

combination of semi-bright and bright nickel deposits. Experience has shown that this

double-layer nickel coating has greater resistance to corrosion than a single-layer

coating of equivalent thickness [5, 9].

1.5.2.2 High Sulphur Nickel

High sulphur nickel solutions contain nickel sulfate, nickel chloride, boric acid and an

addition agent which provides the necessary contents of sulphur in the deposits [9, 11].

14

In this case, a special thin layer of bright, high sulphur nickel is deposited between the

initial layer of semi -bright nickel and the top layer of bright nickel. The very thin layer

should comprise about 10% (about 1.2 - 2.5 microns) of the total nickel coating

thickness and containing about 0.15% sulphur (as compared with 0.04% to 0.10%

nonnally found in bright nickel deposits).

1.5.2.3 Bright Nickel ..

Bright nickel solutions contain nickel sulfate, nickel chloride, boric acid and at least

two types of organic addition agents, which complement each other and yield fully

bright nickel deposits [8, 9, 10, 11].

One type produces deposits that are mirror-bright initially, but are unable to maintain

the mirror like appearance of the deposit as its thickness has increased. This class

includes compounds like benzene disulfonic acid, and benzene trisulfonic acid,

benzene sulfonamide and sulfonimides such as saccharin. The presence of the sulfon

group and an unsaturated bond adjacent to the sulfon are critical characteristics.

Adsorption of the addition agent occurs by virtue of the unsaturated bond, onto grov"ih

sites, points or edges of crystals, and at dislocations. The organic compound is reduced

electrochemically at the cathode, and this is accompanied by the reduction and

incorporation of sulfur (as the sulfide) in the deposit. Fully bright nickel deposits

typically contain 0.04% to 0.10% sulfur. These reactions control the structure and

growth of the nickel as it is deposited.

1';;

The second type may be termed leveling agents because they make the surface

smoother as the thickness of the deposit has increased. They are sulfur-free, bath-

soluble organic compounds containing unsaturated groups and generally introduce

--small amounts of carbonaceous material into the deposit. Typical examples of this

second class of brighteners are formaldehyde, coumarin, ethylene cyanohydrin, and

butynediol [27].

The combination of organic addition agents makes it possible to obtain smooth, ..

brilliant, lustrous deposits over wide ranges of current density. The deposits have a

banded structure consisting of closely spaced laminations, believed to be related to the

co-deposition of sulfur. Certain cations, for example, zinc, selenium and cadmium,

enhance the luster of electrodeposited nickel, and have been used in combination with

the organic additives.

1.5.3 Post Treatment

Normally, after triple nickel plating, the top layer will be plated with decorative

chromium, black chromium [29] and other finishing like hard chromium [30], brass,

tin-cobalt alloy plating and etc. However, this study does not cover the top layer

deposit.

Proper rinsing and drying are very important in order to make sure that any residue

remaining on the surface after the plating operation is completely removed to prevent

building up of the corrosion spot and staining [31].

1 t')

1.6 Function of Bath Constituents

1.6.1 Nickel Sulphate

Most of the nickel ion content is contributed by nickel sulfate [7, 11]. This salt is used

because it is the least expensive salt of nickel with a stable anion that is not reduced at

the cathode, oxidized at the anode, or volatilized. It is also highly soluble and readily

available commercially. The limiting cathode current density for sound nickel deposits ..

is a function of the nickel ion concentration in the cathode film, which in tum depends

on the metal ion concentration of the bath itself. The larger amount of nickel sulfate

now used in the plating bath not only raises the limiting cathode current density but

also lower the resistivity, thus improving plate distribution.

1.6.2 Nickel Chloride

A principal function of the chloride ion is to improve anode dissolution by reducing

polarization. It also increases the conductivity of the bath and has marked effects at the

cathode. It increases throwing power as a result of increasing cathode efficiency,

electrolyte conductivity and slope of the cathode potential curve. These effects are at a

maximum in the all-chloride bath studied in detail by Wesley and Carey and to a lesser

degree in the high-speed bath developed by Pinner and Kinnaman which contains

chloride and sulfate in about equal normalities [11]. The interesting effects of chloride

ion in the cathode tllm have been studied though more needs to be done with

brighteners present.

17

1.6.3 Boric Acid

Boric acid serves as a weak buffer in a nickel plating solution. Its principal effect is

that of controlling the pH in the cathode film. In the absence of a· buffer, nickel

deposits at ordinary temperatures tend to be hard, cracked and pitted as shown by

Macnaughtan.[ll]. Moreover, the presence of boric acid is able to lower the

overpotential of nickel deposition [32], and therefore act as a catalyst [33, 34] to

facilitate faster nickel deposition. Boric acid is obtained in a very pure and inexpensive

• form, is nonvolatile and stable, produces whiter deposits, is helpful in its smoothing

action on the deposit, and is unique in its cooperative effects with leveling agents.

1.6.4 Addition Agents (Additives I Brighteners)

The various types of additives and brighteners for semi-bright nickel, high sulfur

nickel and bright nickel have been highlighted in sections 1.5.2.1, 1.5.2.2 and 1.5.2.3

respectively. Therefore, this section only covers the wetting agents.

1.6.4.1 'Vetting Agents (Anti-Pitting Agents)

Although the cathode efficiency of nickel deposition is high, nevertheless sufficient

hydrogen ions are discharged and cause pitted deposits during the electroplating

process. Certain anionic wetting agents are used in nickel baths to prevent bubbles of

hydrogen from adhering to the cathode and causing pits in the deposit [11]. They

function by reducing the surface and interfacial tension, and with the correct wetting

1 R

agent the contact angle between the liberated hydrogen bubbles and cathode is reduced

to zero. The bubbles are detached before they can grow to a size to cause pitting by

blocking the plating at the point of contact of the bubbles.

Another value derived from the use of the wetting agents is the diminished drag-out of

nickel salts because of the fast runoff from points and edges of the articles due to the

low surface tension.

1.7 Effects of Plating Parameters

Beside the bath constituents as outlined above, other plating parameters also influence

the nature of the cathode film and thereby the properties of the triple nickel deposits.

They are distance between cathode and anode, current density, plating time (or

thickness), the combination of different layers of nickel deposits, pH, temperature,

degree of agitation, inorganic impurities and orgamc impurities. These are all

interrelated [9, 10, 11].

1,7.1 Distance between Cathode and Anode

Increasing the distance between cathode and anode generally improves thickness

distribution, but this effect is limited as the distance becomes large compared with the

size of the cathode [35]. Shortening the distance will cause poorer thickness

distribution and potential burning appearance especially at the edges of the cathode

10

where the nearer surfaces will receive higher current than the further surfaces as

illustrated in Figure 1.7 and 1.8.

1.7.2 Current Density

The effect of current density is not marked over the range of 1 to 5 A / dm2, but is

usually in the direction of an increasing of burning appearance and poorer thickness ..

distribution with increasing current density for all layers of nickel deposits. Although

reduction in current density will improve appearance and thickness distribution, but

will be in the expense of longer plating time and reduced capacity.

For high sulfur nickel, lower current density facilitates co-deposition of sulfur in the

deposits and provides better corrosion resistance (Figure 1.9).

1.7.3 Plating Time

Thickness (or weight deposited) is proportional to the plating time and current density

as per Equation@. In general, longer plating time will provides better appearance and

enhances the corrosion resistance performance. However, most of the commercial

application required as shorter plating time as possible in order to achieve higher

output at lowest cost.

')()

~

The "dog-bone" effect is an uneven plating condition caused by the unequal current distribution produce in an electroplating cell

+ f----------l f--------~~+ -"'-

Anode

Figure 1.7 : Current distribution (The dogbone effect) (Courtesy ofEnthone-OMI)

Cathode Anode

..

""\ r-:. ~ ~ A

"'-': (/

A ::,.,.

~ ~ :/ :/ '==

"/

f1

Areas remote from the :mode receive a smaller share of the available current than areas near the anode

Figure 1.8 : Current distribution (Not uniform over a shaped article) (From [5])

21

10 I

Rating Number

-c r' ~(r-

High sulpl)..ur : 0.8 !-Lm

Total triple nickel:

9

8

7 Max : 14 !-Lm, Min: 8 !-LID

6

1 2 3 4 5 6 ..

Current Density (Aldm2)

Figure 1.9: High sulphur nickel- Relationship between current density and corrosion resistance (at 4 cycles CASS test)

10

9 High sulphur: 0.8 ~m

Rating Number 8 Total triple nickel:

7 Max: 14 !-Lm, Min: 8 ~m

6

2 3 4 5

pH

Figure 1.10 : High sulphur nickel - Relationship between pH and corrosion resistance (at 4 cycles CASS test)

(Courtesy of Ebara-Udylite)

...,...,

1. 7.4 Combination of Different Layers of Nickel Deposits

The combination of different layers of nickel deposits is able to enhance the corrosion

resistance of the plated substrates by selective corrosion of the specified nickel layers

because of difference in electrode potential behaviors. The selective corrosion

mechanisms between different layers of nickel deposits are illustrated in section

1.5.2.4.

..

1.7.5 pH

The effect of pH varies with the type of the plating bath. For semi-bright and bright

nickel, pH below 3.8 will cause low brightness and poor leveling; and pH above 5 will

cause poor ductility and poor chromeability due to an excess of additives / brighteners

in the deposits; high stress and burning will also occur because of the harmful effects

of some impurities, and stress may be much more pronounced at pH above 5.

For high sulphur nickel, pH below 2.0 will cause heavy attack on the substrates such

as steel and zinc die castings; pH above 3.0 will cause low co-deposition of sulfur in

the deposits and subsequently causes poor corrosion resistance (Figure 1.10).

1.7.6 Temperature

The effect of bath temperature varies with the type of plating bath and the current

density. For semi-bright and bright nickel, at temperature below 40 DC, burning occurs

at high current density and the brightness reduced because of the lesser amount of

additives / brightness being deposited. For temperature above 70 DC, some breakdown

of additives / brightness may occur and excessive electrolytes are evaporated.

1.7.7 Agitation

Agitation is accomplished by means of an air agitation, mechanical articles movement ..

and solution circulation. The agitation helps to replenish the nickel ions and additives

which are depleted during electrolysis process especially in the difficult-to-reach area

or recesses. To a certain extend, moderate agitation also helps to remove hydrogen

bubbles on the cathode surface to reduce pitted deposit. For high sulphur nickel, air

agitation will cause decomposition of the additives, and normally only mechanical

movement and solution circulation is recommended.

1.8 The Properties of Triple Nickel Deposits

In general, typical properties of triple nickel deposits such as cathode current

efficiency, thickness distribution, brightness and leveling, sulphur co-deposition,

electrode potential differences and the corrosion resistance performance are measured

in order to control the quality of electroplated articles [5, 9, 11]. The above properties

are being studied in this research. Other properties such as adhesion, ductility,

roughness and pitting, internal stress and hardness are also important, but are excluded

in these studies.

24