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STUDIES ON THE ELECTRODEPOSITION OF CHROMIUM AND ITS ALLOY FROM ECOFRIENDLY Cr(III) ELECTROLYTES AND ROOM TEMPERATURE IONIC LIQUIDS-RTIL THESIS SUBMITTED TO BHARATHIDASAN UNIVERSITY



FOR THE AWARD OF THE DEGREE OF DOCTOR OF PHILOSOPHY IN CHEMISTRY BY



G. SARAVANAN 18111/ Ph. D.1/Chemistry/Full time/Oct. 2009



Dr. S. MOHAN (Research Supervisor)



EMFT Division CSIR - Central Electrochemical Research Institute Karaikudi – 630 006 December 2011



G. Saravanan Senior Research Fellow (CSIR)



Karaikudi 630 006 Tamil Nadu, INDIA Date:



DECLARATION



I hereby declare that the thesis entitled “Studies on the electrodeposition of chromium and its alloy from Eco-friendly Cr(III) electrolytes and Room Temperature Ionic Liquids-RTIL” has been originally carried out by me at Central Electrochemical Research Institute-CSIR, Karaikudi 630 006, India under the guidance of Dr. S. Mohan, Senior principal scientist, Central Electrochemical Research Institute-CSIR, India. This work has not previously formed the basis for the award of any Degree, Diploma, Associateship, Fellowship or other similar title of Bharathidasan University or any other University.



(G. SARAVANAN)



Acknowledgement First and Foremost, I praise and thank the Almighty God for his countless blessings that he showered upon me to complete my research successfully. I would like to record my gratitude to Dr. S. Mohan, M. Sc., Ph. D., Senior Principle Scientist, CSIR─Central Electrochemical Research Institute, for his supervision, advice and guidance from the beginning of this research as well as making me to gave extraordinary experience throughout the research work. Above all and the most needed, he provided me constant encouragement and support in various ways. His truly scientist instinct has made him as a constant oasis of ideas and passions in science, which exceptionally inspire and enrich my growth as a student, a researcher and as a scientist. I am indebted to him more than he knows. I gratefully acknowledge Dr. Vijayamohanan K. Pillai, Acting Director, CECRI, Dr. V. Yegnaraman, former Director, CECRI, Dr. N. Palaniswamy, Head Corrosion Preotection, Dr. N. G. Renganathan, Head, Lithium Ion Battery, Dr. Shoba Jayakrishnan, Head EMFT Division, Dr. GNK. Ramesh Bapu, Dr. K.L.N. Phani, Chairman (SAC) and Dr. R. H. Suresh Bapu for their involvement with their originality has triggered and nourished my intellectual maturity that I will benefit from, for a long time to come. I am grateful to them in all possible way. I am much indebted to Mr. R. Ravishankar, Mr. A. Rathishkumar, Ms. S. Krithika, Dr. N. Radhakrishnan, Mrs. Nalini Thondiraj, Mr. P. Kamaraj, Mrs. S. Bagyalakshmi and Dr. G. V. M. Kiruthika, for their help in using instrumentation facilities. They were generous with their helpful comments and shared their valuable time for discussions.



I wish to express my sincere thanks to Dr. V. Raj, Associate Professor, Department of Chemistry, Periyar University and Dr. G. Paruthimal Kalaignan, Professor, Department of Industrial Chemistry, Alagappa University for having assessed my research work during the doctoral committee meeting and making valuable suggestions towards improving the scientific contents of the work. I take great privilege in thanking those who have provided me personal foundation, which has greatly influenced my professional life. I whole heartedly thank my father Mr. Chinnaswamy Gengan, my mother Mrs. Kasthuri, my beloved brothers and sisters Mr. G. Pughazanthi, Mr. G. Muthukumaran, Mrs. S. Vasugi and Mrs. P. Suganthi for providing me the comfort and support on which my career is built. It is because of their sacrifice, efforts and encouragement. I stand at this stage of my professional life and without which I would have never achieved anything that I own today as something great. Collective and individual acknowledgments are also owed to my colleagues at CECRI, Karaikudi whose encouragements are helpful and memorable. Many thanks go in particular to Mr. RM. Gnanamuthu, Mr. J. Vijayakumar, Mr. N. Rajasekaran, Mr. R. Pavulraj, Mr. J. Maharaja, Mr. C. Mohan, Mr. J. Vijay, Mr. K. Soundara Pandian, Mr. P. Thangapandian and Mr. M. Vinoth Kumar, Mr. B. Venkata Rami Reddy, Mr. M. Jeevan Kumar Reddy and Mr. V. Selvamani for giving me such a pleasant time when working together with them at CECRI. It is a pleasure to acknowledge Ms. C. Nithya, SRF, CECRI for constant engagement and helpful during the preparation of my thesis.



I would like to thank my CECRI scientist and friends, Dr. M. Selvam, Dr. V.V. Giridhar, Dr. G. Radhakrishnan, Mr. N. Rajasekar, (S&T) Mr. T. Arun Kumar, Mr.



MV.



Geathesh



Menon,



Mr.



S.



Varatharajan,



Mr.



A.



Kumaravel,



Mr. M. Sankararao and Mr. P. Arunkumar. Finally, I am thankful to all staff members of CECRI library, the administrative staff of CECRI and Bharathidasan University for their support and the Department of Science and Technology (India), and the Council of Scientific and Industrial Research (India, CSIR–SRF) for the financial assistance, which has enlightened the task of this tenure.



(G. SARAVANAN)



TABLE OF CONTENTS Abstract Chapter 1.0 Introduction 1.1 Definition of electrodeposition



1



1.2 Principle



1



1.3 Factor affecting the electrodeposition process



4



1.4 Types of electrodeposits



6



1.5 Application of electrodeposition



6



1.6 Definition of alloys



7



1.7 Electro deposited alloy



8



1.8 Factor affecting the alloy deposition



8



1.9 Health and environmental effects of using hexavalent chromium electrodeposition



11



1.10 Hexavalent chromium electroplating alternatives



13



1.11 Trivalent chromium electroplating



15



1.12 Electroplating in non-aqueous solution



18



1.13 Ionic liquids



20



1.14 Deep eutectic solvents



23



1.15 Applications of deep eutectic solvents



26



1.16 References



29



Chapter 2.0 Literature review 2.1 Chromium electrodeposition from Cr(VI) electrolyte



36



2.2 Decorative and hard chromium electrodeposition using hexavalent chromium



37



2.3 Problems and developments in functional electrodeposition using trivalent chromium 2.4 References



43 59



Chapter 3.0 Experimental procedure 3.1 Electrolyte preparation



65



3.2 DES preparation



65



3.3 Preparation of substrate



66



3.4 Electrodeposition methods



66



3.5 Characterization of electrodeposited metals and alloys



71



3.6 References



81



Chapter 4.0 Aim and scope of the research 4.1 Scope of research work



90



4.2 Methodology



91



4.3 References



93



Chapter 5.0 Electrodeposition of Fe-Ni-Cr alloy from deep eutectic system containing choline chloride and ethylene glycol 5.1 Introduction



94



5.2 Experimental



95



5.3 Results and discussion



97



5.4 Conclusions



101



5.5 References



103



Chapter 6.0 Pulsed electrodeposition of microcrystalline chromium from trivalent Cr−DMF bath 6. 1 Introduction



112



6. 2 Experimental



113



6.3 Results and discussion



114



6.4 Conclusions



118



6.5 References



120



Chapter 7.0 Structure, current efficiency and corrosion properties of brush electrodeposited (BED) Cr from Cr(III)− dimethylformamide (DMF) bath 7.1 Introduction



128



7.2 Experimental



129



7.3 Results and discussion



130



7.4 Conclusions



134



7.5 References



135



Chapter.8.0



Electrodeposition of Fe-Ni-Cr alloy from deep eutectic system containing choline chloride and ethylene glycol



8.1 Introduction



145



8.2 Experimental section



146



8.3 Results and discussion



147



8.4 Conclusions



152



8.5 References



154



Chapter 9.0 Conclusions



159



List of publications



162



ABSTRACT



Electrodeposition of chromium and its alloy from eco-friendly Cr(III) electrolyte is an environmentally benign process for both decorative and functional applications. It has been amply demonstrated that electrodeposited Cr by direct current (DCD) and pulse current (PCD) on metal such as mild steel, can provide aesthetic appearance and protection against corrosion. Glycine and dimethylformamide (DMF) used as Crcomplexing agent, pulse and brush electrodeposition techniques have been used to enhance current efficiency and deposit morphology. We obtained current efficiency maximum of 36% using pulsed current which is two times higher than that of reported in literature. Eco-friendly Cr(III) electrolyte with suitable complexing agent is a good candidate to replace toxic Cr(VI) electrolyte. Ionic liquid/deep eutectic solvents have been prepared using choline chloride and ethylene glycol (hydrogen bond donor) for electrodeposition of Fe-Ni-Cr alloy. The influence of electrodeposition potential upon the Fe, Ni, and Cr composition has been investigated. Ionic liquid/deep eutectic solvent provides nanosized metal and alloy with negligible amount of H 2 evolution, therefore current efficiency increased. Electrodeposit has been characterized mainly by using XRD, SEM, AFM and EDS techniques. Anticorrosive property has been studied using potentiodynamic polarization and electrochemical impedance spectroscopy.



gF¡ß ·òÂïD ·u® ówK¦[ åâAç¦B Cr(III) t[Ãz¹l_ ÖòÍm t[ªVuýBçkÝ>_ xçÅl_ ØÃÅ©Ã⦠z¼«VtBD \u®D ¶>[ c¼éVï ïéçk¥D, ·u®ówKÂz å[ç\ ÃBÂzD kçïlKD, ΩÃçª \u®D ØÄB_ÃV| ïòsïÓÂzD ÃB[Ã|þÅm. ¼åì]çÄ \u®D m½©A t[ûâ¦Ýç> ÃB[Ã|Ý] t[ªVuýBçkÝ>_ xçÅl_ ¨¹B ¨àþ[ *m ØÃÅ©Ã⦠z¼«VtB\Vªm sökVï sáÂï©Ã⦠Î[® ¼\KD ΩÃçªÂz, ¶ö\Vª >|©A ]Å窥D ¶¹ÂþÅm. þçáE窥D \u®D ç¦Ø\Ý]_ àÃVì\ç\禥D, z¼«VtBÝ][ ¶çª¡ ¼Äì\ºïáVï ÃB[Ã|Ý] t[û⦠ØÄB_]Å[ \u®D ýkÝ][ ¶ç\©¸Bçé ¶]ï©Ã|Ýmk>uz m½©A x[ûâ¦D \u®D #öçï t[ªVu ýBçkÝ>_ Oâúï^ ÃB[Ã|Ý>©Ã⦪. m½©A t[ûâ¦Ýç> ÃB[Ã|Ý] ¶]ï\Vï 36% t[û⦠ØÄB_]Åçª åVºï^ ïõ¦¤Í¼>VD, Öm \uÅ gF¡ ¶¤Âçïï¹_ íÅ©Ãâ¦ç>s¦ Ö«õ| \¦ºz ¶]ï\Vï ÖòÂþÅm. ØÃVòÝ>\Vª ¶çª¡ ¼Äì\ºïÓ¦[ ·w_ ‡ åâAØïVõ¦ Cr(III) t[Ãz¹BVªm åâ·Ý>[ç\ ØïVõ¦ Cr(VI) Âz Ã]éVï Îò å_é ¼>ìkVï c^ám. t[ªVuýBçkÝ>_ xçÅl_ Fe-Ni-Cr c¼éVï ïéçkçB ØÃÅ ØïVçé[ z¼áVç«| \u®D ¨Ý]o[ þçáÂïV_ (çÇâ«Û[ ¸çð©A) ~MçB ÃB[Ã|Ý] Àì\ ¶BM ¶_ém tï¡D ¨¹mòïç«©ÃV[ï^ >BVöÂï©Ãâ¦m. Fe, Ni \u®D Cr ïéçkl[ *m t[ªVu ýBçkÂzD t[ûâ¦Ý][ >VÂïÝç> g«VB©Ãâ¦m. çÇâ«Û[ Øk¹¼Bò>_ ¶á¡ A«ÂïMÂïÝ>Âï>Vï¡D tï OõèB c¼éVï \u®D c¼éVï ïéçk ]«k ¶BM ¶_ém tï¡D ¨¹mò ïç«©ÃV[ïáVï sáºzþÅm. gï¼k t[û⦠ØÄB_]ÅÐD ¶]ïöÂþÅm. xÂþB\Vï XRD, SEM, AFM \u®D EDS Oâúïçá ÃB[Ã|Ý] t[ªVuýkÝ][ Ã[äB©Ã|þÅm. t[ª¿Ý> \V®ÃV| xçªkVÂïD \u®D t[¼k]lB_ t[\®©A WÅ\Vçé ïVâ½lBçé¥D ÃB[Ã|Ý] ¶ö\Vª >|©A© Ãõçà ïõ¦¤B©Ãâ¦m.



Dedicated To My Beloved Parents



CHAPTER I



INTRODUCTION 1. 1 Definition of electrodeposition Electrodeposition is process of coating usually metal onto a surface by passing the direct current. This process is done by passing the direct current through the electrolyte and it is associated with the movement through it of charged particles called ions. The terminals leading the current are electrode. 1. 2 Principle The electrodeposition was limited primarily to fulfilling decorative needs. But today it is an important industrial process and it has the following functional properties such as 



Corrosion resistance







Wear and abrasion resistance







Tarnish resistance







Heat resistance







Electrical resistance







Oil retention surface



The metallic ions in salt carry positive charge and get attracted to the substrate during passing negative current. When they reach surface of the substrate, it provide electron to reduce the positive charged ions into metallic form. Figure 1. 1 shows schematic diagram of electrodeposition cell.



1



General reaction involves At cathode Mn+ + ne−



M



At anode In case of soluble anode Mn+ + ne−



M



In case of inert anode H2O 2Cl−



2H+ + O2 + e− Cl2 + 2e−



The deposition reaction involves four types. They are 



Metal – solution interface as the focus of the deposition process.







Kinetics and mechanism of the deposition process.







Nucleation and growth process of the metal lattice.







Structure and properties of the deposits.



Essential of plating should have the followings 



Cathode







Anode







Electrolytic solution







An external circuit



Faraday’s Laws of electrolysis They relate the current flow, time, and the equivalent weight of the metal with the weight of deposit and may be stated as follows:



2



1. The amount of chemical change at an electrode is directly proportional to the quantity of electricity passing through the solution. 2. The amounts of different substances liberated at an electrode by a given quantity of electricity are proportional to their chemical equivalent weights. Faraday’s Laws may be expressed quantitatively I · t Eq — (1)



W = F where W = weight of deposit in grams I = current flow in amperes t = time in seconds Eq = Equivalent weight of deposited element F = Faraday, a constant, = ~96,500 coulombs



I · t is the quantity of electricity used (coulombs = ampere–seconds) and Eq (1) the equivalent weight of the element, is the atomic weight divided by the valence change, i.e., the number of electrons involved. If the current is not constant, then I · t must be integrated — (2) From a practical standpoint, the weight of the deposit is converted to the more meaningful thickness of the deposit using the relationship, W (gms) = volume (cm3)/deposit density, with the volume of the deposit equal to the thickness (in µm) area (in m2). The Faraday, F, can be experimentally determined by rearranging Eq. (1):



3



I · t Eq — (3)



F = W



WF — (4)



I·t = Eq



permits the determination of the charge passing through a circuit by the known deposition or dissolution of an element. 1. 3 Factors affecting the electrodeposition process 1. 3. 1 Effect of impurities Impurities in the electrolyte can lead to unsatisfactory deposits. Insoluble impurities such as dust and metal particles can attach physically to the cathode and cause roughness of the plate. This is more likely if the electrolyte is agitated and the insoluble particles are not allowed to settle as sediments. Organic impurities may modify the deposit by the same mechanism as the addition agents, with an unwanted effect. 1. 3. 2 Effect of Temperature Increase in temperature of the electrolyte usually increases the solubility of the metal and the conductivity of the solution. The temperature affects the nature of the deposit and increase in temperature may lead to unsatisfactory plating. High temperature also increases the problem of water evaporation and fumes. As with other parameters, the



4



most suitable temperature is usually determined empirically to give the best compromise between plating rate and deposit quality. 1. 3. 3 Effect of agitation Thermal convection current provides some stirring in plating electrolytes. Any form of agitation increase the rate at which the metal ion reaches the cathode and thereby reducing the concentration polarization. If hydrogen is co−deposited at the cathode the bubbles leaving surface provide a very effective form of stirring. In other systems introducing air bubbles from the bottom of the tank stimulates this effect. If the electrolyte is not filtered, agitation can provide by keeping the solids in suspension. 1.3.4 Effect of leveling agent Leveling is defined as the progressive reduction of the surface roughness during deposition. Thus there are two types of leveling processes: (1) geometric leveling corresponding to leveling in the absence of specific agents and (2) true or electrochemical leveling corresponding to leveling in the presence of leveling agents. Theories of leveling by additives are based on (1) the correlation between an increase in the polarization produced by the leveling agents and (2) preferential adsorption of a leveling agent on the high point (peaks of the substrate or flat surfaces) and by this inhibition (slowing down) of deposition [1, 2]. 1. 3. 5 Effect of brightening agent The brightness of a surface is defined as the optical reflecting power of the surface. It is measured by the amount of light peculiarly reflected Weil and Paquin [3] showed a linear relationship between the logarithm of the amount of light peculiarly reflected by a surface and the fraction of the surface having roughness of less than



5



1500A. Kardos and Foulke [2] distinguish three possible mechanisms for bright deposition: (1) diffusion–controlled leveling, (2) grain refining, and (3) randomization of crystal growth. Some fundamental aspects of brightening and leveling are reviewed by Oniciu and Muresan [4]. 1. 4 Types of electrodeposits Electrodeposits are depending on the electrolyte composition, additional agents, operating conditions such as pH, current density, temperature and agitation of the plating electrolyte. Different types of electrodeposits are given below. 



Coarse or fine−grained deposit







Microcrystalline







Amorphous







Compositionally modulated







Multilayer







Composite







Nanocrystalline







Powdery







Bright/semi bright







Dull/satin







Black







Soft/hard and Smooth/rough



1. 5 Application of electrodeposition Electrodeposition is playing an important role in all walks of life. Some of the important applications of electrodeposition are



6







Household articles – Decorative items, wall clock, Fan Air conditioners, jewelers etc.,







Electronics – Control systems, radar instruments







Computers







Defense – missiles/ALH/remote controlled helicopter/fighter plane







Aerospace







Astronomy







Communication – telephone wires







Power station







Solar energy conversion system







Nuclear energy application







Marine, mining industries etc.



1. 6 Definition of alloys An alloy coating may be obtained by a co–deposition of the alloy constituents from the electrolyte containing their ions. Deposited alloy may possess properties and combination of properties, which are not achievable in the coatings of pure metals. The aim of making alloys is to make them less brittle, harder, resistant to corrosion, or have desirable color and luster. Alloy may be either homogenous or heterogeneous. 1.



Homogeneous (one phase alloy). 



Solid solution.







Intermetallic compound.



7



2.



Heterogeneous (two phase alloy) 



Solid solution







Intermetallic compound







Virtually unalloyed element



1. 7 Electro deposited alloy They are similar to thermally prepared alloy in structure in that they usually contain same phase. During deposition two are more metals deposited as alloy phases at the cathode surface. These can be frequently achieved by electrolyzing a solution of mixed salt at a sufficiently high current density. 1. 8 Factors affecting the alloy deposition 1. 8. 1 Role of electrode potentials in electrodeposition alloys According to the Faraday’s law for electrodeposition binary alloy composed of two metals M and A WM = IM*t *μM/(nM*F) WA = IA*t *μA/(nA*F) Where WM, WA – weights of the deposited metals M and A μM, μA – weights of one mole of the metals M and A nM, nA – numbers of electrons transferred by the ions M and A F – Faraday’s constant (96485 Coulombs) t – time. Then the composition of the deposited alloy (molar concentration of A) CA = IA/(IMnA/nM + IAnA/nA)



8



Under equilibrium conditions ions of a metal start to deposit on the cathode surface when its potential is brought below the electrode potential calculated according to the Nernst equation. In the real process the cathode potential, at which the deposition starts is even below the equilibrium Nernst potential due to the electrode Polarization. If the electrode potentials of the alloy components are different (EM > EA) then the following conditions are possible: 



Ec > EM > EA (Ec is the cathode potential). Under these conditions no reduction reactions occur. No electric current is passing through the electrolyte.







EA < Ec < EM. Only M is deposited. The electric current is a result of the reaction Mn+ + ne– = M.







Ec < EA < EM. Both M and A are deposited but the deposition of M is preferable. The electric current is a result of the both reactions Mn+ + ne– = M and An+ + ne– = A.



1. 8. 2 Effect of complexing agent Electrodeposition of an alloy composed of metals with different electrode potentials (more noble metal and more active metal) results in preferential of the metal with higher potential (more noble metal). In order to obtain an alloy with the composition similar to the ratio of the metals contents in the electrolyte their potentials should be brought closer together. Complexing agent (chelating agent) is a substance used for complexing particular ions in the electrolyte. Ions of the complexing agent bind the simple metal ions forming complex ions. Complexing agent not only approximates the metals potentials but it also



9



retains the more noble ions in the solution preventing their immersion deposition. In order to achieve more stable effect complexing agent is commonly added to the electrolyte solution in an amount higher than it is required by the stoichiometric composition. The complexing agent in non–binded form is called free complexing agent. A complexing agent may affect on only one of the metals shifting its electrode potential to the negative side when the second metal stays in the form of simple ions. When a complexing agent form complexes with both metal ions potentials of both of them are shifted to the negative side but the difference between them decreases (they are getting closer). Sometimes two different complexing agents are added for complexing two different metals. Examples of complexing agents 



Potassium cyanide (KCN)







Potassium tartrate (K2C4H4O6)







Methanesulfonic acid (CH3SO3H)







Glycine (C2H5O2N)







Citric acid (C6H8O7)







Thiourea ((NH2)2CS)



1. 8. 3 Effect of operating parameters 



Electric current density Generally increase of current density results in an increase of the content of the less noble metal in the deposited alloy.



10







Temperature Increase in temperature causes decrease of the cathode polarization. Activation



polarization decreases due to intensification of the gaseous hydrogen formation. Concentration polarization drops as a result of increase of the metal concentration in the diffusion layer at the cathode surface. Lower cathode polarization at increased temperature results in increase of the content of the more noble metal in the alloy. 



Agitation Agitation reduces the thickness of the cathode diffusion layer, which causes



decreased concentration polarization. Lower cathode polarization in agitated cathode or electrolyte results in increase of the content of the more noble metal in the alloy. 



Additives (addition agents) Additives (brighteners, leveling agents) act similar to complexing agents causing



increase of the less noble metal in the alloy. Addition agents are most effective in the electrolytes containing simple (non–complexed) metal ions. 1.



9



Health



and



environmental



effects



of



using



hexavalent



chromium



electrodeposition Chromic acid, which is mainly used in chromium electroplating, has been recognized as highly toxic and carcinogenic. There has been an increasing concern due to environmental, health and safety considerations associated with the emission, handling, storage and disposal of hexavalent chromium compounds [5]. Generally hexavalent chromium electroplating baths generate trivalent chromium ions and hydrogen gas at the cathode, while oxygen gas is the predominant product at the anode. These gases rise to the top of the bath and then into the atmosphere. Workers can potentially inhale this mist



11



or it could potentially escape into the environment outside of the electroplating shop [6]. Hexavalent chromium ions are strongly suspected of causing lung cancer. Other effects to health include burns, ulceration of the skin and the mucous membrane, and loss of respiratory sensation (Surface Engineering Association, 1999). In addition to technical difficulties experienced in the deposition of chromium, chromic acid has now been classified as a carcinogen. Existing regulations and legislations to control hexavalent chromium are as follows Hexavalent chromium compounds (Chromates, bichromates and chromic acid) have been classified as Category I carcinogens under the Hazard Information and Packaging for supply regulations 3 (CHIP). The U.S Environmental protection agency (EPA) has classified hexavalent chromium as one of seventeen kinds of highly hazardous and toxic substances. Control of Major Accidents Hazards (COMAH) has reclassified chromic acid from harmful to very toxic. Electroplating shop having more than 4000 litres of chromium electroplating solution will have to register as a tier 1 COMAH site, similarly electroplaters having more than 16000 litres of chromium electroplating solution will be a tier 2 COMAH site. Integrated Pollution Prevention Control (IPPC) stated that the best available technology must be used to minimize and control pollution in the electroplating industry such as that of hexavalent chromium. The End of Life Vehicle Directive (ELV, Directive) has been designed to minimize the environmental impact of vehicles, which have reached the end of their useful life and either recycled or otherwise disposed of heavy metals such as lead, mercury, cadmium and hexavalent chromium will be banned for use in manufacturing vehicles. The ELV Directive regulation came into force in July 2007 and from that point onwards, no vehicle can be



12



introduced to the European market, which contains these elements. Occupational Safety and Health Administration (OSHA) issued guidelines have significantly lowered the limit on worker exposures to hexavalent chromium (VI). OSHA has amended the existing standard that limits occupational exposure of hexavalent chromium Cr (VI) in the electroplating shop from 52 µg/m3 to 1 µg/m3 as an eight hour time weighted average. The European Parliament has recommended the average exposure level (PEL) from a current PEL of 0.1 mg/m3 to 0.005 mg/m3. Electrodeposition of Cr from Cr(VI)electrolyte may be used in metal finishing, as long as it will not appear as Cr(VI) ion in the final product. Cr(VI), which is a metal in ionic state, Cr electroplate is a metal in the atomic state. In Cr plating, only the metallic form of chromium remains. Thus, articles plated with the metal Cr are acceptable. 1. 10 Hexavalent chromium electroplating alternatives It has now been established that conventional chromium electroplating using the chromic acid process is extremely hazardous because it relies on hexavalent chromium, which is highly toxic and has been classified as carcinogenic. However, there are also technical difficulties experienced in the deposition of chromium from chromic acid electrolytes. These have been summarized by Smart et al. [7] as follows • Chromium electroplating using hexavalent chromium has a low efficiency i.e. 15–22 % where 75–85 % of the applied current is used in hydrogen evolution. • The average cathodic current densities is generally 10–15 Adm–2 • The process has poor covering power across low current density areas and also around holes and slots of components. • Burning is observed as grey deposits in high current density areas.



13



• Chromium electroplating has poor throwing power. It results in thick electrodeposits on the edges and protruding parts of cathodes and thin deposits in recesses. • Interruption of the current during electrodeposition produces milky deposits i.e. ‘white washing’. • Solid chromic acid has immediate detrimental effect on human tissue, burning the skin particularly the eyes. Even dilute solutions have the insidious effect of causing ulcers, either from splashes or inhalation of spray. • Chromic acid is a strong oxidizing agent and therefore it is a potential fire hazard. • High metal concentrations of chromium metal in chromic acid solution. It has been recognized since the inception of industrial chromium electroplating that the use of chromic acid presents many hazards in terms of the chemical reactivity and toxicity of the chromium electroplating bath. Environmental, health and safety considerations as well as disposal costs have prompted the electroplating industry to consider other electroplating alternatives to hexavalent chromium. However, the problem with hexavalent chromium electroplating process is not with the electroplating technology but with the environmental, and health consequences associated with its use. The ever–increasing regulatory requirements have therefore forced metal finishing industries to look for acceptable and cost effective options [8]. The majority of industries are relying on emission control technologies rather than switching to clean technologies or waste minimization as a means of achieving compliance. In recent years there has been a growing interest in alternative materials and deposition methods that reduce hazards associated with hexavalent chromium. A novel technique can be used as a replacement for hexavalent chromium if it matches the physical and mechanical characteristics of



14



hexavalent chromium. Trivalent chromium formulations can be used to replace hexavalent bath formulations for decorative plating applications. Trivalent chromium is at least 100 times lesser toxic to humans and the environment than hexavalent. Trivalent decorative plating has been commercially available since 1975. As these formulation have improved, they prove to not only be environmentally safer but can also result in improved productivity and cost savings over hexavalent formulations. [9]. However, promising alternatives for hexavalent chromium electroplating have been suggested which include: Dry coating alternatives (physical vapour deposition, chemical vapour deposition), Thermal spray technology (high velocity oxy fuel), Nickel based alternatives, Ionic liquids and trivalent chromium electroplating [10, 11]. However, none of these alternatives can match the unique properties of chromium electroplated from hexavalent chromium. Trivalent chromium electroplating from trivalent electrolytes is considered as a very promising technology to replace conventional hexavalent chromium electroplating partly because Cr3+ ions are less toxic and therefore could serve as an environmentally benign alternative. Trivalent chromium has now become a transitional alternative for decorative applications. However, there is no commercially viable way of producing thick deposits of chromium from electrolytes based on trivalent chromium. Although more sensitive to impurities and more complex to operate, these baths are gaining more and more acceptance due to their high tolerance to current interruptions and lower toxicity. 1. 11 Trivalent chromium electroplating The trivalent chromium electroplating process has been available to the metal finishing industry since 1973. Jeffrey and Stanley [12] developed the first commercial



15



trivalent chromium bath based on chromium chloride, boric acid, ammonium chloride and bromide ions [13]. This chloride based process was commercially used in the electroplating industry as the only viable option to replace hexavalent chromium electroplating. Barclay [14] produced a trivalent electrolyte based on chromium sulphate and thiocyanate using a divided cell arrangement to prevent anodic oxidation of trivalent chromium. In 1984, W. Canning Ltd worked with IBM in a collaborative project and produced another sulphate–based bath [15]. All these trivalent chromium processes were capable of depositing ≤ 0.5µm thick chromium layer. Chloride and sulphate based trivalent chromium electroplating processes were used successfully in the electroplating industry for decorative purposes. The deposition rate of the trivalent chromium process using sulphate and chloride based electrolytes is similar to hexavalent chromium electroplating but there was no commercially viable way of producing thick deposits for hard chromium applications. The trivalent chromium finish can be similar to the hexavalent chromium finish when it is used for decorative application [16–21]. Ibrahim and Chisholm also observed that deposits from trivalent electrolytes have similar properties to that of hexavalent chromium electrolytes. The deposits were found to be micro cracked with a similar crack density and hardness of between 800–1000 HV. Deposits from trivalent chromium baths are inherently micro–discontinuous. The pore density in a micro–discontinuous deposit was found to be about 20,000 to 60,000 pores/cm2 but it became micro cracked with higher thicknesses. The normal hardness of the deposits from trivalent chromium baths is 700–1000 HV. The trivalent chromium bath also has the ability to tolerate current interruptions without passivating or producing ‘white clouds’ and hazes in the deposit.



16



There are currently at least three basic types of trivalent chromium bath available. A single electrolyte bath, chloride or sulphate based, using graphite or composite anodes, and special additives to prevent oxidation of trivalent chromium at the anodes. Another type of bath uses shielded anodes where conventional lead anodes are surrounded by boxes sealed on one side by a selective ion membrane. The membrane prevents the migrating trivalent chromium ions in the solution from reaching the anode, thus preventing their oxidation to the hexavalent state. The sulphate based trivalent chromium bath utilizes additives containing the trivalent chromium ion and a secondary additive that contains grain refiners and brighteners [22]. When soluble chromium anodes are used in trivalent chromium electroplating, they would cause a rapid buildup of chromium in the electrolyte. Oxygen is produced at the insoluble anode surface and it is usually sufficient to oxidize trivalent chromium to hexavalent chromium. This results in deposition failure, because Cr(VI) contaminates trivalent electrolytes. There have been several approaches proposed to prevent oxidation of Cr(III) to Cr(VI). Jeffrey and Stanley [12] studied trivalent chromium electroplating using carbon anodes. The electrolyte had oxidizable species at lower oxidation potential than oxidized trivalent chromium. This build up of hexavalent chromium is prevented due to the difference in potential between Cr(III) and oxidizable species in the bath. Smart et al., W. Canning Ltd [23, 15] developed perfluorinated ion selective membrane in a divided cell arrangement separating anolyte and catholyte. The chromium ions were thereby prevented from coming into contact with the anode. Conductive metal oxide coated anodes also have been successfully used in trivalent chromium electroplating. These developments have solved some of the problems of anodic oxidation of trivalent chromium. A major problem



17



with the deposition from trivalent chromium electrolytes is associated with reactions taking place at the cathode. In 1994 Shahin [24] suggested that electrodeposition of chromium from trivalent baths involves two consecutive reduction steps of Cr 3+ species to Cr2+ and then from the reduction of Cr2+ species to Cr(0). According to Lide [25], the standard reduction potential for trivalent chromium ions is given according to the following reactions Cr3+ + 3e– → Cr



Eo = –0.744V



Vs SHE



— (5)



Cr3+ + e–→ Cr2+



Eo = – 0.407V Vs SHE



— (6)



Cr2+ + 2e– → Cr



Eo = – 0.913V Vs SHE



— (7)



2Cr2+ + 2H+→ 2Cr3+ + H2



— (8)



According to Lide, the net reaction process is thermodynamically expected to produce hydrogen rather than chromium metal. Reduction from trivalent chromium metal is more predominant than divalent reduction. 1. 12 Electroplating in non–aqueous solution The first experiment to electrodeposits metal was carried out in 1805 by Luigi Brugnatelli who used a Voltaic Pile to deposit gold [26]. Later improvements by John Wright using KCN led to electrolytes for Au and Ag electrodeposits which are relatively similar to those used today. While initially limited to the deposition of decorative metals, more functional metals such as Cr and Ni were deposited by Bird in 1837 [27] and Junot de Bussey in 1848 [28]. In depth study on the history of metal electroplating can be found in several reviews [29, 30]. The electroplating industry is dominated by the deposition of Cr, Ni, Cu, Au, Ag, Zn, Cd and some Cu and Zn–based alloys from aqueous solutions. Other metals are usually deposited using chemical vapour deposition techniques (CVD).



18



While these are versatile in being able to coat any substrate (plastic, metal, ceramic, glass etc.) they are difficult to apply to large and complex surfaces. Aqueous techniques have dominated due to low cost, high solubility of electrolytes, high conductivities and good throwing power, high solubility of metal salts, and high mass transfer rates. There are, however, limitations of using water based electrolytes including limited potential windows (1. 4V), hydrogen embrittlement, passivation of the anode or cathode, the use of complexants such as cyanide and the environmental issues associated with treating large volumes of water to remove metals and electrolytes before returning to the water course. There are clearly a range of other solvents that could be used, ionic and molecular; polar and non–polar. Ideally, to dissolve electrolytes polar solvents are the most desirable and small solvent molecules will generally provide a high fluidity. Unfortunately all polar molecules have electronegative elements which by their nature make them good electron donors. These will strongly coordinate to metal ions making them difficult to reduce. Some of the more electropositive metals have been plated from polar organic solvents but these offer few technological advantages. Non–polar organic solvents, mainly aromatic hydrocarbons have been used for metal deposition. These have the advantage of wide potential windows but the obvious disadvantage of poor conductivity. The Alumiplate process was developed in the late 1980s to plate aluminium from toluene [31, 32] Triethyl aluminium was used as the source of aluminium but it is pyrophoric and this in combination with a highly flammable solvent made the process difficult to run. A review of electrochemistry in non–aqueous solutions is given by Izutsu [33].



19



1. 13 Ionic liquids High temperature molten salts have long been used for extraction of metals such as Al, Ti, Na and Li [34, 35]. The main limitation with high temperature molten salts is clearly the operation temperature which makes difficult in operation and limits the range of substrates that can be used. In an endeavour to reduce the melting temperature of salts numerous studies focused on Li+/ K+/ AlCl3 eutectics which have freezing points close to 100°C [36]. The use of quaternary ammonium salts particularly pyridinium and imidazolium salts has pushed the freezing point down to ambient conditions. The term “ionic liquids” has been used to distinguish between high temperature and low temperature systems. One of the key breakthroughs in the development of ionic liquids came with the synthesis of a 1 mol. eq. N–ethylpyridinium bromide: 2 AlCl3 by Hurley and Wier in 1951 [37] which was a eutectic liquid at 20°C. This was used for the electrodeposition of aluminium [38] and therefore sparked a large amount of research for metal deposition. Using molecular orbital theory, Wilkes and co–workers [39] developed an ionic liquid, AlCl3: 1–ethyl–3–methyl–imidazolium, which had a room–temperature liquid range between 33 and 67 mol% AlCl3. In addition these “first generation” ionic liquids had a significantly lower viscosity than the corresponding pyridinium liquids and it is the imidazolium cation that still dominates the ionic liquid literature. Discrete anion or “second generation” ionic liquids are composed of simple anions, as opposed to a mixture of anions in equilibrium. Their discovery is ascribed to Wilkes and Zaworotko who produced 1–ethyl–3–methylimidazolium tetrafluoroborate and acetate for the first time [40]. It is this class of ionic liquid which has dominated the literature ever since. Anions such as BF–and PF6–were initially used because of their wide potential window



20



[41], however, they were found to slowly hydrolyze yielding HF [42] and liquids with more hydrophobic anions such as tri–fluromethanesulphonate (CF3SO3–) and bis– (trifluoromethanesulphonyl) imide [(CF3SO2)2N–] have subsequently become popular [43]. The potential window of these liquids can be extremely large making it possible to electroplate reactive metals. The properties of these liquids are covered in a series of reviews [44, 45]. Ionic liquids are salts that have a melting temperature below 100°C. Generally one of the ionic components is organic, most commonly the cation. The ions have a low degree of symmetry. By careful choice of the components it is possible to tune the properties of the liquid. In general ionic liquids 



Are good solvents for organic and inorganic materials.







Are generally considered to be polar yet non–coordinating solvents.







Immiscible with many organic solvents providing a polar alternative for two– phase systems.







Have low vapour pressures. It is generally accepted that the first ionic liquid was [EtNH3][NO3], and had a



melting point of 12°C. Ionic liquids as a low temperature alternative are currently a popular area of research. Ionic liquids are systems with an anionic and a cationic component which are liquid below 100oC [46]. The definition is used to distance these fluids with high temperature molten salts but is completely arbitrary. Ionic liquids have been of great interest, due to them having many advantageous properties from molten salts, yet negating the difficulties of high temperature. A variety of cations and anions are described in the literature but for the application of metal deposition only a relatively



21



small quantity of ionic liquids has been used. These are shown in Table 1. 1. The possibility to choose a possible ligand as the anionic component allows control over speciation and thus redox properties of metal solutes to a much greater extent than in aqueous media since extremely high activities of the ligands can be achieved. Ionic Liquids have clearly been reported to be green solvents most notably for their low vapour pressure compared to molecular alternatives. In the application of ionic liquids to metal deposition the green credentials of this methodology could also come from a significant reduction in the volume of low level aqueous streams that would need to be processed. Today, it is generally recognized that this is only part of the picture since many ionic liquids do have significant toxicity. Some ionic liquids have been designed to contain ions which are known to have lower toxicity and these include functionalized imidazoles [47], lactams [48], amino acids [49] and choline [50] although, it is only the last of these which have been extensively applied to metal deposition. Ionic liquids in electrodeposition initially the main drive to use ionic liquids was the ability to obtain high concentrations of aluminium in a highly conducting aprotic medium. The key advantages of the Ionic liquids are 



The wide potential windows







High solubility of metal salts







Avoidance of water and metal/water chemistry







High conductivity compared to non–aqueous solvents. The use of ionic liquids heralds not only the ability to electrodeposit metals that



have hitherto been impossible to reduce in aqueous solutions but also the capability to engineer the redox chemistry and control metal nucleation characteristics. It is the latter



22



area that is only now being addressed and will no doubt be the focus of research over the forthcoming decade. The main drive for using ionic liquids for metal plating has been the ability to obtain wide potential windows which is the potential range of which the electrolyte is neither oxidized nor reduced at the electrode surface. Some ionic liquids have a very large potential window of up to 4.15 V for [Bmim (PF6)] at a Pt electrode [51], and 5.5 V for [BMP(TF2N)] at a glassy carbon electrode [52]. The importance of potential windows is well known from aqueous processes where hydrogen evolution can be hazardous and lead to brittle electrodeposits (hydrogen embrittlement). Ionic liquids may even be chemically altered and hence destroyed if potential limits are not considered. The wide potential window of ionic liquids makes it possible to electrodeposit elements with low redox potentials that cannot be reduced in other media such as Al, Mg, Ge, and Si [53–55]. 1. 14 Deep eutectic solvents The requirements for ionic liquids to be used on a bulk scale for metal deposition are that they need to be low cost, non–toxic and water insensitive. The majority of discrete anions do not achieve all of these criteria and so it is more likely that eutectic based ionic liquids will be more applicable for bulk scale electrodeposition. The systems studied so far described can be expressed in terms of the general formula Cat+ X–. z Y. where Cat+ is in principle any ammonium or phosphonium cation, X is generally a halide anion (usually Cl–). They are based on equilibria set up between X– and a Lewis or Brønsted acid Y, z refers to the number of Y molecules which complex X –. The melting point of two component mixtures is dependent upon the size of the interaction between



23



the components. For ideal mixtures (non–interacting components) the freezing point will vary linearly with mole fraction whereas large negative deviations can occur when the components interact strongly with each other. This is shown schematically in Figure 1. 2. The composition at which the minimum freezing point occurs is known as eutectic point and this is also the temperature where the phases simultaneously crystallize from molten solution. One of the key advantages of these types of ionic liquids is the ease of manufacture. The liquid formation is generally mildly endothermic and requires simply mixing the two components with gentle heating. Another key advantage is that they are water insensitive which is very important for practical electroplating systems. The final key advantage of eutectic based systems is that because they are simple mixtures of known chemicals they do not have to be registered as new entities as they revert to their constituent components upon excessive dilution in water. The ionic liquids described can be subdivided into three types depending on the nature of the complexing agent used. Eutectic Type 1



Y = MClx, M = Zn, Sn, Fe, Al [56–59]



Eutectic Type 2



Y = MClx.yH2O, M = Cr, Co, Cu, Ni, Fe [60]



Eutectic Type 3



Y = RZ, Z = CONH2, COOH, OH [61, 62]



To date the only Cat + species studied have been based on pyridinium, imidazolium and quaternary ammonium moieties. In general, as with the chloroaluminate and discrete anion systems, the imidazolium based liquids have the lowest freezing points viscosities and higher conductivities. The depression of freezing point is related to the strength of interaction between the anion and complexing agent although this has not really been quantified as yet due primarily to a lack of thermodynamic data about the



24



individual components. A fourth type of eutectic has been described involving just a metal salt and a hydrogen bond donor [63]. It was shown that ZnCl2 when mixed with acetamide or urea could also form eutectics which had all the properties of ionic liquids. The novelty of Type 3 eutectics is that they use a simple hydrogen bond donor to complex the simple anion (usually chloride). The majority of the work to date has focused on simple amides, alcohols and carboxylic acids. The first reported eutectics were those formed between choline chloride and amides such as urea and acetamide. These materials have been called Deep Eutectic Solvents (DESs) to differentiate them from ionic liquids with discrete anions. A eutectic forms when there is a large interaction between the two species in the mixture. An example of this is the choline chloride: urea mixture. By themselves they have freezing points of 303˚C and 135˚C respectively. By combining these two compounds in a ratio of 1:2 (choline chloride: urea), the product formed has a freezing point of 12˚C which is a depression of freezing point is 178˚C. For comparison the depression in freezing point for the choline chloride–zinc chloride system was much larger (272˚C) [64] due to the covalent bonds formed in the metal chloride case. The main reason behind choline chloride being such a useful quaternary ammonium salt is to do with the fact that it is an asymmetric quaternary ammonium salt with a polar functional group, but also small. The asymmetric nature of this molecule reduces the freezing point of the ionic molecular liquid, as does the polar functional group. The group of



Abbott



has



published



extensively



on



the



subject



of



choline



chloride



HOC2H4N+(CH3)3Cl–(ChCl) because it is non–toxic and readily available as a bulk commodity chemical.



25



The existence of hydrogen bonding in ChCl/urea eutectic mixtures can be observed using NMR spectroscopy. HOESY spectra of HOCH2CH2N+(CH3)3F– ·2(NH2)2CO show intense cross correlation between the fluoride ion and the NH2 protons on the urea molecule [50]. Some anion complexes have been identified using FAB–MS and it is evident that the hydrogen bond donor (HBD) is sufficiently strongly coordinated to the chloride anion to be detected by this technique. In a choline chloride: urea (1: 2) mixtures the presences of Cl– with both one and two urea molecule were observed. The freezing point of the HBD– salt mixtures will be dependent upon the lattice energies of the salt and HBD and how these are counteracted by the anion–HBD interaction and the entropy changes arising from forming a liquid. For a given quaternary ammonium salt, the lattice energy of the HBD will be related to the anion–(HBD) interaction and hence to a first approximation the depression of freezing point will be a measure of the entropy change. It has been shown that the depression of freezing point correlates well with the mass fraction of HBD in the mixture. The lowest viscosities and highest conductivities are obtained with diol based HBDs. It is thought that the comparatively weak interactions between the alcohol and the chloride mean that some free glycol is able to move decreasing the viscosity of the liquid. The glycol based liquids tend also to have comparatively large potential windows [65]. 1. 15 Applications of Deep Eutectic Solvents (DESs) Deep eutectic solvents are easy to synthesize, economically viable to produce on a large scale, relatively insensitive to water, exhibit high metal solubility, hence it can be used in metal plating, electro–polishing and metal recycling. Type III eutectics have



26



the ability to dissolve high concentrations of metal oxides which allows them to be used for metallurgy. As a consequence they can be applied to large scale processes. 1. 15. 1 Electrochemical application Deep eutectic solvents have been used for electrochemical application and these have focused mainly on electrodeposition of metals. Choline chloride based liquids using ethylene glycol as the hydrogen bond donor has been the most commonly used liquid. The main advantage over aqueous media has been increased current efficiency (typically > 70 %). Despite the short development time good reproducible surface finishes were obtained. 1. 15. 2 Electrodeposition of metals The electrochemical potential window of most DESs is typically in the range of 3V. This precludes applications such as lithium batteries and the deposition of electropositive metals such as titanium and aluminium. The applications of deep eutectic ionic liquids have focused on the deposition of metals and metal alloys such as Zn, Cu, Ni, Ag, Cr, Sn, Zn-Sn, Zn-Ni and Ni-Sn alloys. While these can be deposited from aqueous solutions, the studies to date have focused on the advantages that DES present i.e. deposition on water sensitive substrates such as aluminium and the deposition of specialized alloys. Zinc is one of the most important metals in the metal plating industry due to its low cost, environmental compatibility and corrosion resistance. Abbott et al. investigated the deposition of Zn from type II deep eutectic solvents (ChCl/ZnCl2) at 60°C and reported that the deposition morphology obtained is not different from morphology reported by Sun [66]. The current efficiency was found to be 100 % and a variety of Zn alloys can be deposited.



27



1. 15. 3 Chromium deposition The deposition of chromium from aqueous electrolytes suffers from low current efficiency and the use of toxic CrO3. Choline chloride forms eutectic mixtures with CrCl3.6H2O, CoCl2.6H2O, and Zn(NO3)2.4H2O and these liquids are highly conducting. Studies have shown that chromium and cobalt can be efficiently deposited from these liquids. The high ionic strength prevents the water molecules acting as bulk water as they are strongly coordinated to the ions and therefore they are not easily reduced at the electrode surface. Accordingly the deposition of metals such as chromium can be carried out with high current efficiencies. It was found that the addition of up to 10 wt % LiCl, could lead to the deposition of black chromium which is amorphous in morphology hence free from surface cracks. The process has been operated at pilot plat scale although no reports about the improvement have been given in the open literature [67].



28



References 1. H. Leidheiser Jr, Z. Elektrochemie 59 (1955) 756. 2. O. Kardos, D. G. Foulke, in Advances in Electrochemistry and Electrochemical Engineering, Vol. 2, C. W. Tobia, Ed., Wiley–Interscience, New York, 1962. 3. R. Weil, R. Paquin, J. Electrochem. Soc. 107 (1960) 87. 4. L. Oniciu, J. Muresan, J. Appl. Electrochem. 21 (1991) 565. 5. K.O. Legg, M. Graham, P. Chang, F. Rastagar, A. Gonzales, B. Sartwell, Surf. Coat Technol. 81 (1996) 99. 6. D. L. Snyder, Met. Finish. 98 (1) (2000) 215. 7. D. Smart, T.E. Such, S.J. Wake, Trans. Inst. Metal Finish. 61 (1983) 105. 8. R. Weiner, A. Walmsley, Finishing Publications Ltd., Teddington, Middlesex, England (1980). 9. D.L. Snyder, Plat. Surf. Finish. 66 (1979) 60. 10. A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, Chem. Eur. J. 10 (2004) 3769. 11. A. Baral, and R.D. Engelken, Chromium-based regulations and greening in metal finishing industries in the USA, Environmetal Science Policy, 5 (2) (2002) 121-133. 12. G. Jeffrey, R. Stanley, U. S patent 3954574 (1976). 13. J. C. Crowther, U. S. patent 4053374 (1974). 14. D. J. Barclay, W. M. Morgan, U. S patent 4062737 (1977). 15. W. Canning, U. S. patent 4473448 (1984). 16. S. Nikolova, T. Dobrev, M. Monev, Met. Finish. 93 (1995) 12. 17. M. El-Sharif, C. U. Chisholm, Trans. Inst. Metal Finish. 75 (1997) 208 18. P. Lansdell, J. P. G. Farr, Trans. Inst. Metal Finish. 75 (1997) 219.



29



19. A. Gardner, Met. Finish. 104 (11) (2006) 41. 20. S. K. Ibrahim, A. Watson, O. Gawne, Trans. Inst. Metal Finish. 75 (1997) 180. 21. M. El-sharif, A. Watson, C. U. Chisholm, Trans. Inst. Metal Finish. 66 (1988) 34. 22. I. Shuker, K. R. Newby, Trans. Inst. Metal Finish. 83 (2005) 272. 23. D. Smart, T. E. Such, S. J. Wake, Trans. Inst. Metal Finish. 61 (1983) 105. 24. G. E. Shahin, U. S patent 5294326 (1994) 25. D. R. Lide, (1998), Handbook of chemistry and physics, 7th edition, CRC press. 26. M. Schlesinger, M. Paunovic, Modern Electroplating (4th edition), (editors), Wiley, New York, 2000 27. G. Bird, Phil. Trans. 127(1837) 37 28. Junot de Bussey, Fr. Pat. 3564 (1848) 29. F. Endres, A. P. Abbott, D. MacFarlane, Eds., Electrodeposition of Metals from Ionic Liquids, Wiley VCH, Weinheim, 2007. 30. P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis Wiley-VCH Verlag, Weinheim, Germany 2003. 31. E. Peled, E. Gileadi, J. Electrochem. Soc. 123 (1976) 15. 32. L. Simanavicius, Chemija, 3 (1990) 3 33. K. Izutsu, Electrochemistry in Non-aqueous Solutions, Wiley VCH, Weinheim, 2002 34. W. H. Kruesi, D. J. Fray, Met. Trans. B. 24B (1993) 605. 35. D. J. Fray, G. Z. Chen, Mater. Sci. Technol. 20 (2004) 295 36. F. Lantelme, H. Alexopoulos, M. Chemla, O. Haas, Electrochim. Acta 33 (1988) 761. 37. F. H. Hurley, T. P. Wier, J. Electrochem. Soc. 98 (1951) 203.



30



38. F. H. Hurley, T. P. Wier, J. Electrochem. Soc., 98 (1951) 207. 39. J. S. Wilkes, J. A. Lewinsky, R. A. Wilson, C. L. Hussey, Inorg. Chem. 21(1982) 1263. 40. J. S. Wilkes, M. J. Zaworotko, Chem. Commun. (1992) 965. 41. B. M. Quinn, Z. Ding, R. Moulton, A. J. Bard, Langmuir 18 (2002) 1734. 42. R. P. Swatloski, J. D. Holbrey, R. D. Rogers, Green Chem. 5 (2003) 361. 43. P. Bonhote, A.P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Inorg. Chem. 35 (1996) 1168. 44. M. J. Earle, K. R. Seddon, Pure Appl. Chem. 72 (2000) 1391 45. T. Welton, Chem. Rev. 99 (1999) 2071. 46. K. Seddon, Nat. Mater. 2 (2003) 363. 47. N. Gathergood, P. J. Scammells, M. T. Garcia, Green Chem. 8 (2006) 156. 48. Z. Du, Z. Li, S. Guo, J. Zhang, L. Zhu, Y. Deng, J. Phys. Chem. B 109 (2005) 19542. 49. K. Fukumoto, M. Yoshizawa, H. Ohno, J. Am. Chem. Soc. 127 (2005) 2398. 50. A. P. Abbott, G. Capper, D. L. Davies, R. Rasheed, V. Tambyrajah, Chem. Commun, (2003) 70. 51. U. Schroder, J. D. Wadhawan, R. G. Compton, F. Marken, P. A. Z. Suarez, C. S. Consorti, R. F. De Souza, J. Dupont, New J. Chem. 24 (2000) 1009. 52. D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, M. Forsyth, J. Phys. Chem. B 103 (1999) 4164. 53. J. Robinson, R. A. Osteryoung, J. Electrochem. Soc. 127 (1980) 122. 54. Y. NuLi, J. Yang, J. Wang, J. Xu, P. Wang, Electrochem. Solid-State Lett. 8 (2005) C166.



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55. W. Freyland, C. A. Zell, S. Z. El Abedin, F. Endres, Electrochim. Acta 48 (2003) 3053. 56. N. Borisenko, S. Z. El Abedin, F. Endres, J. Phys. Chem. B 110 (2006) 6250. 57. F. Endres, Chem. Phys. Chem 3 (2002) 144. 58. P. Chen, I. Sun, Electrochim. Acta, 46 (2001) 1169. 59. A. P. Abbott, G. Capper, D. L. Davies, H. Munro, R. Rasheed, V. Tambyrajah, Chem. Commun, (2001) 2010. 60. J, Z. Yang, Y. Jin, W. G. Xu, Q, G. Zhang, S. L. Zang, Fluid Phase Equil. 227 (2005) 41. 61. A. P. Abbott, G. Capper, D. L. Davies, R. Rasheed Chem. Eur. J. 10 (2004) 3769 62. A. P. Abbott, G. Capper, D. L. Davies, R. Rasheed, V. Tambyrajah, Chem. Commun, (2003) 70. 63. A. P. Abbott, D. Boothby, G. Capper, D. L. Davies, R. Rasheed J. Am. Chem. Soc. 126 (2004) 9142 64. A. P. Abbott, G. Capper, D. L. Davies, H. Munro, R. Rasheed, Inorg. Chem. 43 (2004) 3447. 65. R. C. Harris, PhD Thesis, University of Leicester, 2009. 66. Y. F. Lin, I. W. Sun, Electrochim. Acta 44 (1999) 2771. 67. A. P. Abbott, K. S. Ryder, U. Koenig, Trans. Inst. Metal Finish. (2008) 196



32



Table 1. 1 A selection of cations and anions used to make ionic liquids for metal deposition. Cations



Anions F



O



_ B



R3



F



F



F



N +



R2 N



Tetrafluoroborate



R1



S



F



F



O



_ N



F



S O



F F



O



F



bis(trifluoromethylsulphonyl)imide



[BF4]–



[NTF]–



Imidazolium R1



F



N +



R2



F _ P



F F



Pyridinium



F F



Hexafluorphosphate



N



C



S



thiocyanate



[PF6]–



[SCN]–



R1 N



+ R2



R4



R3



F



C2F5 _ F _ N



P



Quaternary ammonium



C2F5



F



C



C2F5



C



N



N



tris(pentafluoroethyl)



R1 P R4



+



trifluorophosphate



dicyanamide



[FAP]–



[DCN]–



R2



R3



Phosphonium



33



N C



F



R1



R2



F



F



C



C



S+ R3



_



O



O



C



C



_



N



N



Trifluoroacetate



tricyanomethide



[ATF]–



[TCM]–



Sulphonium



N C



R1, R2, R3 and R4 = alkyl group F



_



O



B



F



C



S F



O



_



O



C C



N N



Trifluoromethanesulfonate



[OTF]–



34



trtracyanoborate



[TCB]–



N



Figure 1. 1 Schematic diagram of electrodeposition cell.



Figure 1. 2 Schematic representation of a eutectic point on a two component phase diagram.



35



CHAPTER II



LITERATURE REVIEW 2. 1 Chromium electrodeposition from Cr(VI) electrolyte Chromium electrodeposition is one of the most widely used electrodeposition processes in the electrodeposition industry due to attractive appearance, good adhesion, high degree of hardness, good wear and corrosion resistance. Chromium is normally deposited on a commercial scale from aqueous solutions of chromium trioxide (CrO 3) which are commonly called chromic acid. However, chromium cannot be deposited from an electrolyte containing only chromic acid, since another chemical, which acts as a catalyst also needed. The most commonly used catalysts are sulphate and fluoride. They are generally used in the complex form such as sulphuric acid, sodium sulphate, flurosilicate and silicofluoride. The ratio of chromic acid to catalyst is maintained within definite limits; preferably about 100: 1 in the case of sulphate [1, 2]. Generally, hexavalent chromium electrodeposition electrolytes are simple in formulation, but are more complicated to operate than most electrodeposition electrolytes, and they require rigorous control. Temperature, current density and electrolyte composition affects the film characteristics and current efficiency, therefore carefully controlled in order to obtain specific deposit properties and electrodeposition rates. A dilute conventional formulation (250 g/L chromic acid and 2.5 g/L sulphate) is extensively used in the electrodeposition industry to give good coverage, substrate activation, and consistent current efficiency. The concentrated formulation (400 g/L chromic acid and 4.0 g/L sulphate) gives better coverage and greater resistance to impurities and requires a lower operating voltage. A critical point in all electrolyte



36



formulations is the requirement for close control of the CrO3/SO4 weight ratio needed to produce consistent electrodeposition results. A low ratio of chromic acid to sulphuric acid results in relatively poor throwing, covering power and increased limiting current density. Higher ratios result in slow deposition rates which produce dull deposits, increased covering power, and decreased limiting current density. Thicker deposits tend to be dull and contain variable cracks [3]. The mechanism for chromium deposition from chromic acid is complicated. Snavely et al. [4] suggested that the final reduction to metallic chromium takes place via an intermediate hydride, of short but finite life at the cathode. Ege and Silverman [5] found that additions of SO42– to the electrolyte results in the formation of complexes, deposition ultimately takes place from a complex cation. Variation of temperature, current density or electrolyte composition results in changes in current efficiency and deposit characteristics. Chromium ions enter the cathode film in the form of CrO42– anions forming complex chromium compounds. Raub et al. [6] and Martayk et al. [7] claimed that the solution layer immediately adjacent to the cathode must have mixed activation and passivation tendencies. Thus, if the chromium deposits were too active, chromium would tend to redissolve in the strongly acidic electrolyte. Conversely, if the chromium was too passive, it would tend to electroplate dull and in the form of non–adherent layers flaking from each other, as frequently occurs when there are current interruptions. 2. 2 Decorative and hard chromium electrodeposition using hexavalent chromium Commercially, decorative and hard finish (functional) chromium electrodeposition is categorized on the basis of thickness of the chromium, their applications and the chemistry of the process. The hexavalent chromium electrolyte composition for



37



decorative applications depends primarily on whether the electrolyte is co–catalysed e.g., fluorides, fluorosilicates or fluoroborates [2, 8]. The presence of other oxides of metals such as iron, copper and nickel combined with Cr 3+ affects electrolyte performance [9]. The phenomenological model for the deposition of Cr from hexavalent electrolyte was proposed by Mandich and Hoare. According to these authors, the polychromates decompose to chromous hydroxide and a dichromate which can polymerize back to a trichromate by condensation with other chromates in the solution. The HSO 4– exerts its catalytic activity by forming a chromousoxybisulfate complex through hydrogen bonding with chromous oxide. With the successive transfer of two electrons to the specifically absorbed chromousoxybisulfate complex on the cathode surface Cr(I) and finally metallic Cr along with the regenerated HSO4–, ion are then obtained [10, 11]. Decorative chromium electrodeposition is generally electroplated over a nickel resulting in tarnish resistance, hardness and wear resistance, with an elegant blue metallic appearance [12]. Decorative chromium is not just a single chromium layer but it invariably has one or more under layers of electroplated nickel, ranging from 5–50 µm thick. The visible top layer is a thin chromium coating, typically with a thickness of 0.2–0.5 µm. For less severe conditions, a single layer of bright nickel is adequate to afford the surface improvement. This, combined with the chromium layer, provides sufficient product durability. For severe conditions, multiple or single layers of nickel and copper can precede the chromium deposit. Significant improvements can be achieved with two or more different nickel layers and a special micro–discontinuous coating or strike layer [13]. The basic formulations consist of chromic acid (H 2CrO4) and the sulphate ion (SO4)2–. Single catalyst processes have about 12% chromium electrodeposition



38



efficiency. The remaining current goes towards the formation of hydrogen and trivalent chromium Cr(III). The chromium deposition efficiency increases proportionally with chromic acid concentration, up to 250 g/L and decreases thereafter. Usually, a CrO3/SO4 ratio of approximately 100: 1 is common. The inclusion of the second catalyst fluoride can increase the average cathode efficiency to about 22%. The efficiency increases with increasing chromic acid concentration up to 300 g/L but addition of fluoride catalyst necessitates an adjustment in the CrO3/SO4 weight ratio. Co–catalyst (mixed catalyst) has an increased electrodeposition speed, better coverage, wider brightness range, and more tolerance to impurities. These processes are less sensitive to current interruption and can electroplate substrates with reduced defects [14]. Tri–catalyzed chromium processes are similar to co–catalyzed processes except for the addition of a third catalyst, a proprietary organic catalyst. It improves the cathode deposition efficiency and coverage in the low current density areas. Decorative chromium deposits are either micro–porous or micro– cracked. Deposits from decorative chromium electrodeposition have approximately 10– 17 thousand pores/cm2 or 160 micro discontinuous cracks per linear cm (standard requirement) deposits. These spread the corrosion reaction over many galvanic sites. In decorative chromium electrodeposition, temperature is closely related to current density in its effect on brightness and coverage by deposit. An optimum temperature range is always established for a given concentration of chromic acid. Hard chromium electrodeposition involves immersing the substrate to be coated into the chromic acid electrolyte and the application of a current to provide driving force for the deposition of the chromium ions from the solution. The deposits are extremely hard and for most applications corrosion resistant. According to Newby [15], there are three generic



39



functional hexavalent chromium processes for the deposition of metallic chromium. The earliest formulation is presently known as conventional or Sergent electrolytes which are simply chromic acid (H2CrO4) and sulphate ions (SO42–) in approximately a 100:1 weight concentration ratio. This formula was developed in the late 1920s and is still in widespread use especially in small plating shops. The cathodic efficiency of this process is of the order of 10–15%. Typically with a current density of 32 Adm–2 about 25 µm/hr of deposition is obtained. In the 1950s a mixed catalyst system was commercialized in which a fluoride ion source, typically a salt of fluorosilicic acid, was added to the conventional electrolyte. This formula offered significant improvements both in cathodic efficiencies of up to 22% and deposition rates up to about 50 µm/hr. It provides superior wear and corrosion properties compared to conventional chromium deposits. Mixed catalyst chemistry has been used successfully over thirty years in the electrodeposition industry. In the mid 1980s a non–fluoride chromium (high speed) electrodeposition electrolyte was introduced into the market which replaced the halogen ion with a proprietary organic species. The process is capable of approximately 25% cathodic efficiency and deposition rate up to 75 µm/hr. Deposits from this electrolyte are thought to be the best in terms of wear and corrosion properties, surface finish available in the chromium electrodeposition. This formulation consisting of chromic acid, sulphate ions, fluorosilicic acid have now become the state of the art process and the solution is in widespread use particularly where quality and productivity are of concern [8]. According to Kirk Othmer [16] a thick layer of chromium is deposited directly on the base metal to provide wear and corrosion resistance in hard chromium electrodeposition. The thickness of the hard chromium coating varies in the range of 20–500 µm. Being very hard (~1000



40



HV) the chromium deposit will experience microcracking. This is quite normal and will not usually affect the coating in most engineering applications. Hard chromium deposits display high levels of internal stress and hardness up to 1000 HV with a micro cracked structure varying from 15–200 cracks/cm2 [8, 9]. The cracks penetrate to the interface of the base material and the chromium deposit. At high temperatures, smooth deposits with less burning and nodulation are obtained. It has been observed that adherence and roughness are the common defects in hard chromium electrodeposition. The high level of tensile stress within the chromium deposit leads to spontaneous cracking of deposits which can improve the oil retention properties of thicker deposits and this has many engineering applications in lubricated wear situations [2]. Baraldi et al. observed during cyclic potentiodynamic curves of cathodic polarization (from 0.00 to −1.25 V/SCE, 5 mV s−1) in chromic acid solution used by industries, ARMCO iron undergoes pitting corrosion, while carbon steels undergo a selective ferritic corrosion. Blisters of pure iron were also observed on ARMCO iron electrodes surface. Baraldi also suggested a direct participation of the metal substrate to the overall reaction of reduction of the chromic acid to chromium metal. Therefore, the chemical nature of the electrode metal has been changed, in order to prove that the chromium reduction goes on through an ECE mechanism involving a fast chemical reaction between unstable intermediate chromium products and the metal substrate. Cathodic potentiodynamic polarization curves were performed on copper electrodes and the scans were stopped at four different potentials (−0.90, −1.00, −1.10 and −1.25 Vs SCE). The copper surface undergoes a severe corrosion in all the experimental



41



conditions. The morphology of the attack depends on the polarization potential and in the presence of additives, like iron sulphate [17]. The electroreductive behaviour of CrO3 + H2SO4 aqueous solution is studied on poly–crystalline iron and copper cathodes. The electrochemical results have compared with XPS, AFM and SEM. In the case of iron a main role is played by the passive state of the electrodic surface caused by the presence of a protective mixed iron/chromium oxide layer and by its electrochemical dissolution. This passive to active transition of the electrodic surface and the occurrence of an irreversible reductive current peak, in a potential range where the deposition of metallic chromium does not occur and this suggests that the electrodic surface plays an active role in the electrochemical process. A major difference is found between Fe and Cu cathodes, concerning the kinetic mechanism of formation of adsorbed species [18]. Benedetto Bozzini reported the electrochemical reduction of bichromate from an aqueous solution at a Pt(1 1 1) electrode and studies were done by voltammetry and synchrotron–based photoelectron spectromicroscopy. Cathodic reduction is carried out at potentials −1.5 and −2.0 eV in an electrochemical cell attached to the UHV chamber hosting the scanning photoelectron microscope (SPEM). The Cr 2p and Pt 4f core level spectra were used as fingerprints of the Cr lateral distribution and chemical state after reduction. The Pt 4f and Cr 2p maps show inhomogeneous Cr distribution in microscopic Cr–rich and Cr–poor islands. Analysis of the Pt 4f, Cr 2p photoemission peaks recorded in these islands has disclosed that under both reduction conditions Cr(VI) is reduced principally to Cr(III), with 10 % of Cr(0) localized in the Cr–rich islands. The presence of



42



an intermediate oxidation state has also been detected, that can be assigned to a transition state from Cr(VI) to Cr(III) [19]. 2. 3. Problems and developments in functional electrodeposition using trivalent chromium A major problem with deposition from trivalent chromium electrolyte is associated with reactions taking place at the cathode as ligand formation is slow and kinetically inert which hinders deposition [20, 21]. According to Bard et al. Howarth and Pletcher [22, 23], the negative deposition potential from the trivalent chromium electrolyte and slow ligand exchange gives low cathode efficiency with much of the current going to produce hydrogen. High pH values of trivalent chromium electrolyte can cause the production of oligomeric olated species, which will reduce the reducible chromium. Benaben extensively studied about Cr deposition from trivalent electrolyte. He states that by so doing he obtains a solution containing 30 to 50 g/l of trivalent Cr together with Cl- ions. At a electrolyte temperature of 50˚C and a current density of about 100 A/dm2, he has demonstrated that deposits of at least 150 μm having a hardness of about 950 HV can be obtained at an efficiency of about 30%. These deposits are semibright in appearance with a slightly nodular appearance under the microscope. Increase of hardness in trivalent Cr by heat treatment due to chromium carbide (Cr7C3) and oxide (Cr2O3) formation during heat treatment (300-350˚C, 30 min), the microhardness was increased to 1700-1800 HV [24–27]. El–Sharif et al. [28] observed that olation reactions are catalyzed by Cr(II), however, Howarth and Pletcher [23] could not find any evidence of Cr(II). Olation of the commercial electrolytes tend to lose efficiency with increasing electrodeposition time. It was also observed that cationic species decreases but there are



43



an increase in anionic species [29]. Trivalent chromium has a marked tendency to form various types of complexes in aqueous solutions. It is almost impossible to deposit the chromium coating from a simple aqueous Cr(III) solution due to a very stable Cr(H2O6)3+ ion complex. The coordinate water molecules, OH– groups or other ligands may be replaced by anions in solution. Anions that easily enter into the coordinate sphere and displace OH–groups can effectively prevent olation [20, 12]. Some organic ligands influence the electrodeposition rate and the quality of coatings due to the formation of Cr(III) active complexes. Ibrahim et al. [30, 31] and El–Sharif et al. [28]. Handy et al. [32] also confirmed that deposition rate and the quality of chromium deposits from trivalent electrolytes are affected due to slow ligand exchange combined with unfavourable reduction potentials. Zhenmi et al. (1993) [33] and Smith [34] (1994) found that a rise in pH at the cathode surface is due to the evolution of hydrogen. As a result the deposition rate from trivalent electrolytes falls rapidly and the process usually reaches a limiting thickness of 3 to 4µm and also results in olation. Watson et al. [35] found that the pH at the cathode surface is in the range of 3.8–4.5 compared to the bulk pH of the electrolyte (2.3–3.8). At this pH value, coordinated water molecules may be converted to OH– groups, which lead to the formation of bridge species. This reaction may continue with the formation of larger molecules, where the chromium atoms are linked with OH – groups (olated compounds) as shown in Figure 2. 1 Formation of olated compound chains of trivalent chromium [36] thus, in addition to the slow ligand exchange processes, trivalent chromium has the characteristics of forming short or long polymers by bridging via a water ligand and H +. Ibrahim et al. [30] observed that the tendency to form olated species is in direct proportion with the pH of



44



the electrolyte. El–Sharif and Chisholm, [37] concluded that the presence of divalent chromium accelerates not only ligand exchange kinetics but also the rate of formation of olated species, particularly within the diffusion layer during electrodeposition. Song and Chin [38] studied the effect of electrolyte composition, mass transfer and trivalent chromium deposition from an electrolyte containing ammonium formate and sodium acetate as the complexing agent and observed that the rate of chromium deposition process is controlled by the transport of the Cr(III) complex ion to the cathode surface. Maximum chromium thickness obtained from a commercial trivalent electrolyte is generally less than 10µm, making the deposits unsuitable for functional applications. Various studies [28, 30] described the failure of sustained deposition of Cr 3+ to the formation of a stable and inert Cr 3+ complex, [(H2O)4Cr(OH)(OH)Cr(H2O)4]4+, in the aqueous solution. The deposition rate is controlled by the diffusion of the complex Cr3+ ion to the cathode surface. Mandich [12] found that hydrolysis, olation, polymerization and oxalation of trivalent chromium to metallic chromium occurred during trivalent chromium electrodeposition. Song and Chin [38] also proposed that chromium coating is inhibited by the formation of a film at the cathode composed of chromium hydroxide or chromium oxide. Some of the organic ligands influence the electrodeposition rate and the quality of the coatings due to formation of Cr(III) active complexes. Trivalent electrodeposition has several problems such as low current efficiency and solution instability. To minimize this problem an appropriate ligand capable of forming a Cr(III) complex with optimum chemical and electrochemical reactivity is used [39]. Grace and Spiccia [40] also confirmed that dimeric complexes and oligomeric species are present in Cr(III) sulphate solution at pH 3.5–4.4 and the deposit obtained from such trivalent based



45



electrolytes is less bright compared to hexavalent chromium deposits. Ibrahim and Watson [30, 35] found that such complex formation in the vicinity of the cathode is due to a sharp increase in pH due to intensive hydrogen evolution. The inclusion of oligomeric Cr(III) species into Cr deposit gives rise to a possible blackness in the chromium deposit. Many researchers [37, 41] made a lot of effort to improve the deposition of thick coatings from environmentally more acceptable Cr(III) electrolytes. However, the most successful electrolyte systems have been developed by Ibrahim et al. [30] to deposit at the rate of 1 µm/hr for prolonged electrolysis. Progress in the development of Cr(III) electrolyte has been slow, largely due to the complex nature of the chemistry and electrochemical nature of Cr(III) species in aqueous solutions. Recently, a high speed electrodeposition system (300 µm/hr) for chromium coating based on the more environmentally acceptable Cr(III) electrolyte has been developed by El–Sharif et al. [42]. The deposits obtained have similar characteristics to hard chromium coatings. Mcdougall et al. [43] found that hydrazine coating results in lower hardness, low current efficiency and solution stability of Cr(III) electrolyte. Nanocrystalline materials are of high technological and scientific interest because their internal microstructure can affect many macroscopic properties like hardness, corrosion resistance and magnetic properties. Narayanan et al. discussed the properties of electrodeposited nanocrystalline (n–Cr) and amorphous (am–Cr) samples prepared by an electrochemical method. The deposited n–Cr has crystallites with grain diameters of about 30 nm. X–ray diffraction shows completely different atomic arrangements in n– Cr and am–Cr, but the SANS results are very similar to each other. The microstructure of



46



both n–Cr and am–Cr can be described as a surface fractal with fractal dimension of 2.24(3) and 2.22(3) for n–Cr and am–Cr, respectively [44]. Fe–Cr alloy films were electrodeposited from the trivalent chromium electrolyte containing glycine as complexing agent. The effects of electrodeposition parameters such as current density, electrolyte pH and electrolyte concentration on the composition of deposited films were investigated in detail. The magnetic properties of Fe–Cr deposited films were measured by using VSM. The result indicated that the crystallographic structures of deposited films are the α–Fe solid solution for deposited films with 3.1 at.% Cr, the meta–stable phase for the deposited films with Cr content ranging from 4.5 to 22.4 at.%, and the amorphous phase for deposited films with Cr content ranging from 22.9 to 74.4 at.%. The magnetic measurement result indicated that the amorphous Fe–Cr deposited film is a paramagnetic material [45]. The electrodeposition of quaternary Fe–Cr–Ni–Mo alloy was investigated in a chloride electrolyte. The electrolyte was formulated to facilitate the deposition of different alloy over a range of current density. Electrolytes containing trisodium citrate and glycolic acid complexing agents showed possible synergistic effects, which was beneficial to quaternary alloy deposition. It was observed that the chromium content of electrodeposited alloy was increased with current density while the reduction of molybdenum appeared to be associated with the induced codeposition of Ni and Mo. The Fe–Cr–Ni–Mo alloy was deposited at a potential range from −1.2 to −1.9 V (saturated calomel electrode). A scanning electron micrograph of electrodeposited quaternary alloy showed that hydroxide particles were not present on the surface and that fine–grain, smooth and compact Fe–Cr–Ni–Mo alloy deposits were obtained. The X–ray diffraction



47



pattern of the alloy deposits revealed that the alloy were either amorphous or microcrystalline in structure [46] A novel Ni90.4Cr9.6 nanocomposite film was prepared by co–electrodeposition of Ni and Cr nanoparticles in an average size of 39 nm. The Cr nanoparticles were dispersed in the electrodeposited nanocrystalline Ni grains (31 nm in average). The oxidation at 800°C showed that the as–deposited Ni–Cr nanocomposite film has a superior oxidation resistance compared to the electrodeposited pure Ni film and the uncoated Ni matrix. X–ray diffraction characterization and cross–sectional investigation indicated that a continuous Cr2O3 scale formed on the electrodeposited nanocomposite film [47]. The effects of electrolyte composition and electrodeposition conditions on structure, morphology, and composition of amorphous Fe–Cr–P–Co deposits on AISI 1020 steel substrate, earlier plated with a thin Cu deposit, were investigated. The increase of charge density activates the inclusion of Cr in the deposit. The effect of charge density on the content of Fe and Co is not clear. However, there is a tendency of increasing of Fe content and decreasing of Co content with the raising of current density. The Co is more easily deposited than the P and its presence results in a more intense inhibition effect on the Cr deposition than the inhibition effect caused by P presence. Scanning electron microscope (SEM) analysis showed that Co increase in the Fe–Cr–P–Co alloys does not promote the susceptibility to microcracks, which led to a good quality deposit. The passive film of the Fe–Cr–P–Co alloy shows a high ability of formation, high protective capacity, and the results obtained by current density of corrosion, jcor, show that the deposit with addition of Co, Fe31Cr11P28Co30, presents a higher corrosion resistance than the deposit with addition of Ni, Fe54 Cr 21P20Ni5 [48].



48



Nanocrystalline (nc) Ni films with and without dispersions of Cr nanoparticles were electrodeposited from a nickel sulfate electrolyte. The grain size of the nc–Ni films was reduced with increasing in the co–deposition content of Cr nanoparticles. Potentiodynamic polarization tests showed that increasing in the co–deposition content resulted in an enlarged passive region of the nc–Ni in 3.5% NaCl through reducing the corrosion potential and increasing the breakdown potential. Scanning electron microscopy (SEM) observation indicated that the polarized pure nc–Ni film exhibited numerous large and deep pits. However, they became smaller and shallower when 4.5 wt.% Cr nanoparticles were co–deposited, and almost disappeared when 10.9 wt.% Cr were co–deposited. X–ray photoelectron spectroscopy (XPS) analysis showed that the different electrochemical corrosion performance was associated with the ability of the Cr nanoparticles co–deposited nc–Ni film to form a continuous Cr–oxide passive film [49]. Cr–P coatings were prepared by electrodeposition from trivalent chromium plating electrolyte using malonic acid as complexing agent. The influences of electrolyte composition on the trivalent chromium electrodeposition process and deposited coating properties were studied. The effects of plating parameters such as current density, electrolyte pH and plating time on structure and morphology of deposited coatings were investigated in detail. Results show that the composition, microstructure and surface morphology of the Cr–P coatings depend on electrolyte composition and plating conditions including electrolyte pH, current density, plating time, etc. Results of EDAX and XPS indicate that the deposited coatings contain Cr(s), Cr(III), phosphorus, oxygen and carbon. The optimum plating parameters for good–quality chromium deposited coating are pH 2–3, current density 3–12 dm2, temperature 35°C and Ti/IrO2 as anode.



49



These results may be of great practical and theoretical significance for further improvement of trivalent chromium plating process [50]. Nanocrystalline Cr–C layers with excellent anti–wear performance were prepared by electrodeposition in Cr3+ electrolyte and subsequent annealing. X–ray diffraction (XRD) shows that the crystalline structure of the Cr–C layer changed from amorphous to nanocrystalline when the annealing was conducted. The hardness, Young's modulus and wear rate of the Cr–C layer were measured. The results indicate that the 400°C annealed nanocrystalline Cr–C layer exhibits a high ratio of hardness to Young's modulus and excellent wear resistance. The excellent wear resistance can be attributed due to the presence of hardness and roughness. The friction tests reveal that the friction coefficient depends on the Young's modulus and the counterpart. Comparing wear with friction, no obvious connection can be found between them [51]. Cr–Ni multilayers with thickness modulation of several tens of nanometers were prepared by pulse–current electrodeposition from a Cr(III)–Ni(II) electrolyte at 30°C. The Cr–Ni



multilayers



were



composed



of



alternate



amorphous Cr–rich



and



nanocrystalline Ni–rich layers. This amorphous Cr–rich layer is attributed to a reduction in the complex–formed Cr ion, which leads to the presence of C, a glass forming element, in the Cr–rich deposit during electrodeposition. Twins were found in the nanosized Ni– rich layer, in which stacking faults were frequently observed [52]. Chromium–phosphorus (Cr–P) coatings were electrodeposited from trivalent Cr (III) electrolytes containing hypophosphite. The electrochemical corrosion behavior of Cr–P coatings, traditional Cr coatings deposited in hexavalent Cr(VI) electrolytes, and chromium–carbon (Cr–C) coatings deposited in Cr (III) electrolytes containing formate



50



were studied by measuring potentiodynamic polarization curves in a 10 wt% HCl solution. The results of electrochemical tests showed that Cr–P coatings exhibit better corrosion resistance than traditional Cr and Cr–C coatings, which is characterized by a lower critical current density, lower passive current density, and lager passive potential range. XPS and SEM analyses confirm that the excellent corrosion resistance of Cr–P coatings is attributed to the formation of a phosphide passive film, which has high stability and self–repairing ability, and can act as a “buffer” to reject the penetration of chloride ions [53]. Fe–Cr–P amorphous alloys were electrodeposited in a trivalent chromium sulfate electrolyte containing glycine as complexing agent. The effects of electrolyte composition and electrodeposition parameters such as current density, electrolyte pH and plating time on the structure, morphology and composition of the amorphous Fe–Cr–P deposits were investigated. It showed that glycine is a suitable complexing agent to get good quality of Fe–Cr–P amorphous alloys by trivalent chromium electrodeposition. It was observed that the chromium content in the deposits increased with increasing the cathodic current density. When current density exceeds specific value, the chromium content in the deposit would decrease. Results of EDS and XPS indicate that the deposited alloys contain chromium, iron, phosphorus, nitrogen, oxygen and carbon. The XRD pattern reveals that the Fe–Cr–P alloy deposit has amorphous structure [54]. The chemical composition of the top layers of Cr–Co alloy electrodeposited from Cr (III) formate–urea electrolytes with various concentrations of CoCl2 was studied by the X–ray photoelectron spectroscopy method. The quantity of each element in at.% was calculated from the single Cr2p3/2, Co2p3/2, C1s, O1s and N1s peak areas. Analysis



51



of XPS spectra has shown that the surface of Cr–Co alloy is covered with the thick oxide film (about 20 nm) and the bulk of the deposit consists of two phases: carbide and metallic of both chromium (about 60 at.%) and cobalt (about 3 at.%). A low percentage (about 3 at%) of nitrogen in the bulk of the deposit suggests the presence of chromium nitrides, which may be formed along with carbides during electrodeposition owing to reduction of chromium–carbamide complexes and an extremely high electrocatalytic activity of chromium in the cathodic processes [55]. Singh Raman et al. investigated nanocrystalline structure and concluded that Fe– Cr alloys have oxidation resistance at much lower chromium contents. Discs of nanocrystalline Fe–10% Cr alloy were produced by ball milling of Fe and Cr powders and compact without considerable grain growth.



Corrosion resistance of discs of



nanocrystalline and microcrystalline alloys were compared by subjecting them to oxidation in air and post–oxidation characterization of the oxide scales by secondary ion mass spectroscopy (SIMS). Nanocrystalline Fe–10% Cr alloy showed greater oxidation resistance than the microcrystalline alloy of the same composition. Chromium content of the inner scale formed over the nanocrystalline alloy was detected to be nearly five times greater than that in the case of microcrystalline alloy, facilitating the formation of the passive layer and providing an explanation for the greater oxidation of the former [56] The effects of direct current and pulse current on composition and corrosion resistance of Fe–Ni–Cr alloy coatings were studied. In both direct and pulse current electrodeposition, increasing the current density has a decreasing effect on Fe and Ni and an increasing influence on Cr. In pulse current electrodeposition, duty cycle has a greater effect than frequency on composition of the alloy coating, particularly in the



52



range of 10–50%. In this range, by increasing the duty cycle, Ni decreases, Fe sharply increase and Cr shows an increasing trend. Following a study of the microhardness of coatings, it is determined that the microhardness increases about 1.5 times by pulse current electrodeposition. The corrosion resistance of the alloy coatings (electrodeposited by direct and pulse current) has been investigated in NaCl solution. The micro–cracks of alloy coating can be decreased by using pulse current, which demonstrates a better corrosion resistance than direct current coating [57]. Amorphous chromium coatings



were



electrodeposited



from Cr(III)–based



solutions containing organic (HCOONa) or phosphorus–containing (NaH2PO2) additives. Their structure was studied by a combination of X–ray diffraction (XRD), valence–to– core X–ray emission spectroscopy (XES) and X–ray absorption spectroscopy (XAS) at the Cr K–edge. Metalloid atoms (C or P) incorporated in electrodeposited structure are chemically bonded to chromium. Upon annealing at elevated temperatures in vacuum, these amorphous coatings crystallize into a mixture of phases containing metallic chromium and chromium carbides or chromium phosphides. Quantitative analysis of valence–to–core XES data demonstrates that the average local structure of chromium in the amorphous coatings does not change significantly during crystallization [58]. Ni–Cr nanocomposite coatings with different Cr particles contents were developed by electrodeposition method from a nickel sulfate solution containing different concentrations of Cr nanoparticle with an average particle size of 40 nm. The characteristics of the coatings were assessed by scanning electron microscopy and microhardness test. The friction and wear performances of Ni–Cr nanocomposite coatings and pure Ni film were investigated, with the effect of the Cr content on the friction and



53



wear behaviors to be emphasized. The results indicate the microhardness, friction and wear behaviors of Ni–Cr nanocomposite coatings are closely related with Cr particles content. The Ni–Cr nanocomposite coating with a lower Cr content of 4.0% shows somewhat increased microhardness and wear resistance than the pure Ni coating, while the Ni/Cr nanocomposite coating with a higher Cr content has much better wear resistance than the pure Ni coating. The effect of Cr nanoparticles on the microhardness and wear resistance were discussed [59]. Amorphous Cr–C alloy coatings were electrodeposited from a trivalent chromium electrolyte containing formic acid. The effects of plating parameters such as pH and current density on composition, structure, morphology and corrosion behaviour of as– deposited coatings were investigated. Results showed that the suitable current density for Cr–C alloy electrodeposition increases as the pH value decreases. Furthermore, by decreasing the pH, higher amount of carbon was introduced into the coatings. The Cr– C alloy coatings deposited at lower pH showed higher passive current density on Tafel plot. By annealing the amorphous Cr–C alloy coatings at 300°C, a nanocrystalline structure having higher hardness was obtained. However, the corrosion resistance was seen to be impaired. Annealing at 600°C led to the precipitation of Cr23C6 which resulted in a higher hardness. The formation of Cr23C6 enhanced the corrosion resistance of coatings due to an increase in corrosion potential. However, formation of micro–cells between the Cr23C6 precipitates and bulk chromium prevented the passive film formation [60]. The mechanism of chromium metal deposition from a trivalent chromium electrolyte containing formic acid and polyethylene glycol 1000 (PEG) was studied on an



54



electrochemical quartz crystal microbalance (EQCM). Reactions of PEG molecules with trivalent chromium ions



and



their



influence



on



the



plating



process



of



trivalent chromium were investigated. EQCM studies at low trivalent chromium ion concentrations show that chromium electrodeposition occurs via the formation of an adsorption layer on the electrode surface, which is called a cathodic film. Cathodic films hinder the penetration of ions from bulk solution to the cathode surface. In the inner portion of the cathodic film and at the cathode surface, intermediate complexes were formed during the deposition process. During electrodeposition, the PEG molecules decreased the reductive current of hydrogen compared with solutions without PEG; an effect that was also observed due to the pH on the electrode surface. PEG plays a decisive role in the formation of intermediate compounds during electrodeposition [61]. Roberto Giovanardi studied an alternative plating process to obtain chromium coatings through electrodeposition from electrolytes containing trivalent chromium, as aqueous solutions of Cr (III) are significantly less dangerous, in terms of human health and environmental impact, as compared to the traditional Cr (VI) electrolytes employed for this purpose. In order to overcome problems regarding the reduction of Cr (III) in aqueous solution, two approaches were followed: i) the possibility of co–depositing chromium along with a second metal, which could help the process of discharge of Cr3+ on the substrate; ii) the use of a specific ligand for the Cr3+ ion, which can generate easily reducible complexes at the metal–solution interphase. Both approaches led to interesting results: in particular, the co–deposition enabled us to obtain NiCr alloy with a high percentage of chromium, and the deposition using specific complexing agents



55



allowed optimal electrolyte compositions to be developed both for decorative and hard chromium plating [62]. Role of complexing agents were studied using the Cr(III)–formate, Cr(III)– oxalate and Cr(III)–glycine electrolytes for the Cr coatings. The deposition rate and brightness range of the electrodeposition process in these electrolytes were investigated. The geometric structures of the Cr complex ions were optimized, using density functional theory with General



Gradient



Approximation/Perdew–Wang



91



(GGA/PW91)



calculation. It was found that the deposition rate and brightness range significantly depend on the geometric structure of the Cr(III)–ligand complex ions. The expansion of the distance between Cr and H2O induced by introducing complexing ligand is a necessary condition for the electrochemical reduction of Cr(III) to metal Cr. The dehydration rate of Cr(III)–ligand complex ions determines the electrodeposition characteristics of electrolytes. The ligand accelerates the electroreduction of Cr(III) in Cr(III)-formate, due to expansion of the distance (r) between Cr and H2O as a result of introducing ligand into complex ions. Cr complex ions with larger maximum ‘r’exhibit the higher activation of electroreduction. Due to the relatively high pKa 1 of formate acid, the active complex ions, [Cr(H2O)5CHOO]2+, in Cr(III)-formate electrolytes are rare. Thus, during the plating process, as the [Cr(H2O)5CHOO]2+ ions are consumed rapidly to achieve commercial acceptable thick Cr coatings [63]. Black



chromium



coatings



were



electrodeposited



from



a



1–butyl–3–



methylimidazolium tetrafluoroborate ([BMIm][BF4]) solution containing Cr(III) under potentiostatic control on copper substrates. The electrochemical behavior of the electrolyte was studied by cyclic voltammetry. The reduction of Cr(III) species in the



56



solution is a two–step process, from Cr(III) to Cr(II) and from Cr(II) to Cr(0). The potential of each reaction shifts positively with the increase of the electrolyte temperature. The parameters for the electrodeposition of black chromium films were optimized. Homogeneous black chromium films with spectrally selective optical properties were produced by applying a potential of −1.5 V for 1800 s, at an electrolyte temperature of 85°C. The coatings consist of a mixture of chromium oxide/hydroxide and metallic chromium. They are amorphous and present a sub–micrometric granular structure [64]. Electrodeposition of chromium in a mixture of choline chloride and CrCl3·6H2O has been reported recently [65]. Green viscous liquid is obtained by mixing choline chloride with CrCl3·6H2O and the physical properties of this deep eutectic solvent are characteristic of an ionic



liquid.



The



eutectic composition was



found to



be 1:2



choline



chloride/CrCl3·6H2O. From this ionic liquid Cr could be electrodeposited efficiently to yield a crack free deposit. Addition of LiCl to the choline chloride and CrCl3·6H2O liquid was found to allow the deposition of nanocrystalline black–chromium films [66]. The use of this ionic liquid might offer an environmentally friendly process for electrodeposition of Cr instead of the current chromic acid–based electrolyte. However, some efforts are still necessary to get shining Cr deposits which can compete with the conventional Cr(VI) or Cr(III)–based aqueous galvanic process. Cr reduced via a Cr(II) state which forms an insoluble intermediate at the electrode/electrolyte interface and the deposition process is irreversible. Significant results reported by Benaben on hard Cr deposition using ionic liquids (IONMET European research program), he obtained shiny metallic hard Cr with



57



thickness up to 50



100 μm using trivalent chromium salts. Using a mixed solution of



CrCl3·6(H2O) and choline chloride, hard chromium layers were deposited on steel [67]. The addition of up to 10 wt.% LiCl leads to a matt black, amorphous Cr layer is free of surface cracks, unlike samples deposited using aqueous Cr(III) and Cr(VI) which are highly crystalline with cracked surface [66]. When the samples were electrolyzed in 1.1 M KNO3 solution for 2 min they were found to have excellent corrosion resistance. The samples withstood > 1600 hr in a salt spray test without any visible sign of corrosion. The black coloration of the film was due to the deposits being nanoparticulate and XRD analysis showed that the Cr(110) and Cr(211) were the predominant crystal faces present. The deposit thickness, adherence and morphology could be further improved using pulse–plating [68]. Solid particles of primarily hard substances, solid lubricants, ductile metals or their alloys and/or molten polymers are embedded in a network of cracks of hard chromium coatings to attain improved physical characteristics, primarily to increase wear resistance, sliding behavior, ductility and corrosion resistance. The network of microcracks in the chromium coating is widened and solid particles are embedded within the cracks. Preferred uses are as coatings on the bearing surfaces of piston rings or cylinder bearing sleeves for internal-combustion engines [69, 70].



58



References 1. D. J. Barclay, and W. M. Morgan, Electrodeposition of chromium, United States Patent. 4062737, International Business Machines Corporation, Armonk, New York, U.S.A, (1977). 2. J. K. Dennis, T. E. Such, Nickel and chromium plating, 3rd edition, Cambridge, England, Woodhead Publishing Limited, Chapters VIII, IX, X, and XI, (1993), 206–240, 245–257,270–302, 310–344. 3. T. Dobrev, M. Monev, S. Nikolova, S. T. Rashkov, Comparative corrosion tests of multilayer nickel chromium coatings, Surf. Coat. Technol. 31 (1987) 127. 4. C. A. Snavely, L. F. Charles, E. B. John, Electrodeposition of chromium and alloys thereof, United States Patent. 2693444, Battele Development Corporation, U.S.A, (1954). 5. J. F. Ege, L. Silverman, J. Anal. Chem. 19 (1947) 693. 6. E. Raub, K. Mueller, Fundamentals of metal deposition, Elsevier Publishing Inc, NewYork, U.S.A (1967) 46 7. N. M. Martayak, R. Weil, Mater. Charact. 28 (1992) 113. 8. A. R. Jones, D. Snyder, Decorative and hard chromium plating, Product Finishing, (2004), http:// www.pfonline.com. 9. N. Zaki, Chromium plating, Product Finishing, (2000), http:// www.pf online.com. 10. N.V. Mandich, Plat. Surf. Finish. 84(5) (1997) 108. 11. J. P. Hoare, J. Electrochem. Soc. 126 (1979) 190.



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12. N. V. Mandich, Practical considerations in bright and hard chromium electrodeposition Part I–V, Met. Finish. 97 (1999) (6–10) 42–45 79–86, 100–112. 13. R. J. Clauss, R.W .Klein, Corrosion protection studies with copper multiple nickel micro–porous chromium plate, Proceedings: 7th International Metal Finishing Conference, Interfinish, May, 1968, Hanover, CANADA 14. D. L. Snyder, Decorative chromium plating, Met. Finish.98 (2000) 215. 15. K. R. Newby, Met. Finish. 93 (1999) 223. 16. K. Othmer, Encyclopedia of chemical technology (KOECT, 1980). 17. P. Baraldi, E. Soragni, C. Fontanesi, V. Ganzerli, J. Alloys Compd 317–318 (2001) 612. 18. R. Giovanardi, E. Soragni, C. Fontanesi, V. D. Renzi, U. D. Pennino, M. L. Foresti, J. Electroanal. Chem. 576 (2005) 243. 19. B. Bozzini, M. Amati, M. Kazemian, A. L. Gregoratti, M. Kiskinova, J. Electroanal. Chem. 1 (2011) 136. 20. D. Smart, T. E. Such, S. J. Wake, Trans. Inst. Metal Finish. 61 (1983) 105. 21. M. El–sharif, A. Watson, C. U. Chisholm, Trans. Inst. Metal Finish. 66 (1988) 34. 22. A. J. Bard, R. Parsons, J. Jordan, Standard potential in aqueous solution, 1 st edition, Chapters XVI, XIV, International Union of Pure and Applied Chemistry, U.S.A, Marcel Dekker Inc, (1985), 453–460, 566–579. 23. J. N. Howarth, D. Pletcher, J. Appl. Electrochem. 18 (1988) 644. 24. P. Benaben, Plat. Surf. Finish., 76(11) (1989) 60



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25. P. Benaben, Proc. of Interfinish 88, 2, 507, Paris (1988). 26. J. Dash, J. Dehaven, U.S. Patent 5, 413, 646 (1995). 27. P. Benaben and F. Durut, Final Report, AESF Summer Research Project, December 1996, NASF Washington, DC, 1996. 28. M. El–Sharif, McDougall, C. U. Chisholm, Trans. Inst. Metal Finish. 77 (1999) 139. 29. I. Drela, J. Szynkarczuk, J. Kubicki, J. Appl. Electrochem. 19 (1989) 933. 30. S. K. Ibrahim, A. Watson, O. Gawne, Trans. Inst. Metal Finish. 75 (1997) 180. 31. S. K. Ibrahim, D. T. Gawne, A. Watson, Trans. Inst. Metal Finish. 76 (1998) 156. 32. S. L. Handy, C. F. Oduoza, T. Pearson, Trans. Inst. Metal Finish. 84 (2006) 300. 33. T. Zhenmi, Y. Zhelong, Z. Jingshuang, A. Mao–Zhong, W.L. Li, Plat. Surf. Finish. 80 (1993) 79. 34. H. Smith, D. Snyder, (1997), http:// www.pfonline.com. 35. A. Watson, C. U. Chisholm, M. El–sharif, Trans. Inst. Metal Finish. 64 (1986) 149. 36. A. Watson, A. M. H. Anderson, M. El–Sharif, C. U. Chisholm, Trans. Inst. Metal Finish. 69 (1991) 26. 37. M. El–Sharif, C. U. Chisholm, Trans. Inst. Metal Finish. 75 (1997) 208 38. Y. B. Song, D. Chin, Electrochim. Acta 48 (2002) 349. 39. M. Y. Choi, Y. Choi, J. I. Choe, M. Kim, S. C. Kwon, Mater. Sci. Forum. 475–479 (1982) 4005. 40. M. Grace, L. Spiccia, Inorg. Chim. Acta 213 (1993) 103. 41. C. Barnes, J. B. Ward, J. R. House, Trans. Inst. Metal Finish. 55



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(1977) 73. 42. M. El–Sharif, S. Ma, C. U. Chisholm, Trans. Inst. Metal Finish. 73 (1995) 19. 43. J. Mcdougall, M. EL–Sharif, S. Ma, J. Electroanal. Chem. 28 (1998) 929. 44. R. Narayanan, S. K. Seshadri, Met. Finish. 99 (2001) 84. 45. F. Wang, T. Watanabe, Mater. Sci. Engg: A 349 (2003) 183. 46. A. G. Dolati, M. Ghorbani, A. Afshar, Surf. Coat. Technol. 166 (2003) 105. 47. Y. Zhang, X. Peng, F. Wang, Mater. Lett. 58 (2004) 1134. 48. C. A. C. Souza, J. E. May, A. T. Machado, A. L. R. Tachard, E. D. Bidoia, Surf. Coat. Technol. 190 (2005) 75. 49. X. Peng,Y. Zhang, J. Zhao, F. Wang, Electrochim. Acta 51(2006) 4922. 50. B. Li, A. Lin, F. Gan, Rare Metals 25 (2006) 645. 51. Z. Zen, L. Wang, A. Liang, L. Chen, J. Zhang, Mater. Lett. 61 (2007) 4107. 52. C. A. Huang, C. Y. Chen, C. C. Hsu, C. S. Lin, Scripta Materialia 57 (2007) 61. 53. Z. Zeng, A. Liang, J. Zhang, Electrochim. Acta 53 (2008) 7344. 54. B. Li, A. Lin, X. Wu, Y. Zhang, F. Gan, J. Alloys Compd. 453 (2008) 93. 55. S. Survilienė, V. Jasulaitienė, A. Cesuniene, A. Lisowska–Oleksiak, Solid State Ionics 179 (2008) 222. 56. R. K. Singh Raman, R. K. Gupta, Corros. Sci. 51 (2009) 316. 57. H. Adelkhani, M. R. Arshadi, J. Alloys Compd. 476 (2009) 234. 58. O. V. Safonova, L. N. Vykhodtseva, N. A. Polyakov, J. Swarbrick, M. Sikora, P. Glatzel, V. A. Safonov, Electrochim. Acta 56 (2010) 145. 59. Y. Zhou, G. Zhao, H. Zhang, Nonferrous Met. Soc. China 20 (2010) 104.



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60. S. Ghaziof, M. A. Golozar, K. Raeissi, J. Alloys Compd. 496 (2010) 164. 61. N. Van Phuong, S. Kwon, J. Lee, J. Shin, B. Huy,Y. Lee, Microchem. J. 99 (2011) 7. 62. R. Giovanardi, G. Orlando, Surf. Coat. Technol. 205 (2011) 3947. 63. Z. Zeng, Y. Zhang, W. Zhao, J. Zhang, Surf. Coat Technol. 205 (2011) 4771. 64. S. Eugénio, C. M. Rangel, R. Vilar, Ana Maria Botelho do Rego , Thin Solid Films 519 (2011) 1845. 65. A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed, Chem. Eur. J. 10 (2004) 3769. 66. A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed, J. Archer. C. John. Trans. Inst. Metal Finish. 82 (2004) 14. 67. P. Benaben, Proc. AESF SUR-FIN 2007, Cleveland, Ohio, NASF Washington, DC; p. 382. 68. T. Schubert, S. Z. El Abedin, A. P. Abbott, K. J. McKenzie, K. S. Ryder, F. Endres, Electrodeposition from Ionic liquids. WILEY–VCH Verlag GmbH& Co. Weinheim 2008. 69. P. Benaben, L. Horme, U. S patent 5868917 (1999) 70. Hans-Jochem Neuhauser, Bergisch-Gladbach, Ulrich Buran, Burscheid, Rudolf Linde, Burscheid, U. S patent 4846940 (1989)



63



OH2



OH



Cr OH2



OH2



H O



OH 2



Cr



Cr



+ 2H2O



Cr O H



OH2



H O Cr



H+



+



Cr



H O



OH Cr



Cr



Cr



Cr



O H



H O



O H



O H



Figure 2. 1 Reactions of polymeric olated Cr(III) complexes.



64



CHAPTER III



EXPERIMENTAL PROCEDURE 3. 1 Electrolyte Preparation The compositions of the electrolytes used for Cr electrodeposition were shown in Table 3. 1. All components were purchased from Across Organics and were weighed using a Contech precision CB–Series balance. Chromium (III) chloride, glycine and methanol were mixed in one beaker and the other components were mixed in another. Each beaker was filled with half of the required volume of deionized (DI) water and then both solutions were mixed and heated in a water bath with reflux condenser. The chromium (III)–glycine solution and the solution containing the other components were heated at 67˚C and 35˚C, respectively, for 30 minutes. Then, both solutions were mixed together and allowed to cool to room temperature. The purpose of using complexing agents is to 1) to obtain lower potential reduction of Cr 3+ to metallic state, and 2) to prevent the formation of hexaaquo–coordinated trivalent chromium complexes. 3. 2 DES preparation Choline chloride [HOC2H4N(CH3)3Cl] (ChCl) (Acros organics 99 %) was dried under vacuum prior to use. Ethylene glycol (EG) (Fisher scientific 99 %), was used as received. The mixtures were formed by stirring the two components together (in a 1: 2 molar ratio of ChCl: EG at 60˚C until a homogeneous, colourless liquid formed. The metal halide salts are; CrCl3·6H2O (Acros organics 98 %), NiCl2·6H2O (Acros organics 98 %), FeSO4·7H2O (Acros organics). The concentrations of metal salts were 0.79 mol dm–3 of CrCl3·6H2O, 0.39 mol dm–3 NiCl2.6H2O and 0.03 mol dm–3 FeSO4·7H2O.



65



3. 3 Preparation of substrate Surface Treatment Process The Cr and its alloy electrodeposition process involved the following steps Degreasing (1min) Rinsed with distilled water (1min) Activated by immersion in electro–cleaning solution (35g/l Na2CO3 + 25g/l NaOH) for 2 min Rinsed with distilled water (30 s) Acid dipping in 5% H2SO4 Rinsed with distilled water (30 s) Electrodeposition of Cr and its alloy for various time and current densities Rinsed with distilled water (1min) Dried in air. 3. 4 Electrodeposition Methods 3. 4. 1 Comparison of DC and PC electrodeposition Pulse electrodeposition–charging of the electrical double layer (EDL) at the metal– electrolyte interface In pulse electrodeposition (PED) [1–3] the potential or current is alternated swiftly between two different values. This results in a series of pulses of equal amplitude, duration and polarity, separated by zero current. Each pulse consists of an On–time (tOn) during which potential and/current is applied, and an Off–time (tOff) during which zero current is applied as shown in Figure 3. 1. It is possible to control the deposited film composition and thickness in an atomic order by regulating the pulse amplitude and



66



width [4–7]. They favour the initiation of grain nuclei and greatly increase the number of grains per unit area resulting in finer grained deposit with better properties than conventionally plated coatings. Modern electronics and microprocessor offers the flexibility of programming of the applied current waveform. The latter is of two groups: (1) unipolar, where all the pulses are in one direction (with no polarity) cathodic pulse followed by a period without current (or an anodic pulse), (2) bipolar, where anodic and cathodic pulses are mixed. Cathodic pulses followed by anodic pulses and (3) direct current (DC) with superimposed modulations. In electroplating, a negatively charged layer is formed around the cathode as the process continues. When using DC, this double layer charges to a defined thickness and obstructs the ions from reaching the part. In PED, the output is periodically turned off to cause this layer to discharge somewhat. This allows easier passage of the ions through the layer and onto the part. High current density areas in the bath become more depleted of ions than low current density areas. During tOff, ions migrate to the depleted areas in the bath. When pulse t On occurs, more evenly distributed ions are available for deposition on the substrate. In the conventional DC plating there is only one parameter, namely current density (i), which can be varied. But in PED, we have many independent variables, viz., (i) on–time (ton), (ii) off–time (tOff) and (iii) peak current density (ip). In PC, the duty cycle (γ) corresponds to the percentage of total time of a cycle and is given by [8] γ (duty cycle) = t On / (tOn + tOff)



— (1)



where f is frequency, defined as the reciprocal of the cycle time t Total — (2)



f (frequency) = 1/ (tOn + tOff)



67



In practice, pulse plating usually involves a duty cycle of 5% or greater and tOn from µs to ms. PC will deposit metal at the same rate as DC provided the average pulse current density equals the latter. The average current density (ia), in pulse plating is defined as [9] Average current (ia) = peak current (ip) X duty cycle (γ)



— (3)



The charges are provided to electrical double layer (EDL), to raise its potential to the value required for metal deposition at the rate corresponding to (ia). The (ip) supplied consists of two parts, (1) a capacitive current, which charges the double layer and (2) a faradic current (iF), which corresponds to the rate of metal deposition. The charging time (tC) of EDL is the time required for the iF, to reach 99% of ip [10] while discharge time (tD) is the time required to decrease iF to 1% of ip. The rapid method for selection of tC and tD values in relation to the applied ip [11] is given below. tC = 17/ ip



— (4)



tD = 120/ ip



— (5)



3. 4. 2. PED vs. DC deposition The pulse parameter were selected such that in all cases the pulse on–time was less than the transition time, τ (the time required for the interfacial concentration to reach zero [12] and much longer than the charging time, t C in microseconds, of the double layer as calculated from Eq. (4) and Eq. (6), respectively [13]. τ = {πD (CbzF)2}/4 ip2



— (6)



Where ip is the peak current density (A cm–2), z is the number of electrons (3, for the case of chromium), F is Faraday’s number (96,500) coulombs), C b is the bulk concentration of Cr3+ ions (mol L–1) and D is the diffusion coefficient of Cr 3+ (cm2 sec–1). In addition, the



68



pulse off–time was selected such that the shortest pulse off–time was several order of magnitude longer than the discharge time, t D, of the double layer as calculated from Eq.(5) [13]. Ibl [10] observed that the deposit properties obtained with PC at high frequency are similar to those obtained by DC plating. They believed that deposition at high frequency during off–time is independent of the primary current distribution because no external electric field is applied during off–time. PC makes the deposition more uniform than by DC plating at the same current density. The uniformities of PC at high frequency are even better than that of DC plating at 10 mAcm–2, as can be shown by a Wagner number analysis. The Wagner number, assuming Tafel kinetics [11, 14] is defined as follows. RTK WαT =



— (7) αcFL|i*avg|



where L is a characteristic length of the system, K is the specific conductivity of the electrolyte solution, and iavg * is the average current density which is dependent on the primary current distribution. The value of Wagner number is an index of the relative influence of solution conductivity and surface kinetics over deposition distribution. The electrode response at high frequency approaches that of a DC signal due to charging and discharging the double layer as shown in Figure 3. 1. The current discharging from the double layer is independent of the primary current distribution according to Holmbom and Jacobsson [15]. The iavg * of PC at high frequency is iavg* =



69



iavg x duty cycle, and equal to 20 x 0.5 = 10 mAcm–2. Hence, the Wagner number of PC at high frequency is the same as that of DC at 10 mAcm–2 as other parameters in Eq. 7 should be considered constant. With equal Wagner number, the current distribution of PC at high frequency should predictably be the same as that of DC at 10 mAcm–2. A longer on–time of 40 ms may be explained in terms of the redistribution of the partial currents for reduction of Cr and hydrogen ions. At such a long on–time, it is believed that a larger portion of the applied current is consumed by the hydrogen ion reduction reaction leading to lower efficiency for Cr deposition. The moderate on–time of 16 ms enhances the formation of very thin pulsating diffusion layer shown in Figure 3. 2, from which mass transfer and diffusion of ions occurred very effectively. It is reasonable to assume that the off–time (24 ms) is sufficient for replenishment of the Cr 3+ ions at the pulsating diffusion layer. Therefore, there is a plenty of replenished Cr 3+ ions getting deposited without significant amount of hydrogen evolution which leads to higher current efficiency [16–19]. 3. 4. 2 Brush Electro Deposition (BED) techniques BED equipment includes power packs, solutions and plating tools, anode covers and auxiliary equipment. Microprocessor–controlled selectron power pack Model 150 A– 40 V was used to transform AC current to DC current. The schematic of the brush plating system is given in Figure 3. 3. The power packs have two leads, one is connected to the plating tool and the other is connected to the work piece to be electrodeposited. The anode is covered with an absorbent material, which holds the solution. The operator dips the plating tool in the solution and then brushes it against the surface of the work piece to



70



be finished. When the anode touches the work surface a circuit is formed and deposits produced. 3. 5 Characterization of electrodeposited metals and alloys 3. 5. 1 X–Ray Diffractometry (XRD) Principle In this technique the primary X–rays are made to fall on the sample substance under study. Because of its wave nature, like light waves, it gets diffracted to a certain angle. This angle of diffraction, which differs from that of the incident beam, will give the information regarding the crystal nature of the substance. The wavelength of the X– rays can be varied for the application by using a grating plate. Phillips diffractometer with CuK (2.2 kW) Instrumentation It consists of X–ray tube for the source, monochromatic and a rotating detector are shown in Figure 3. 4. Applications The diffraction of X–rays is a good tool to study the nature of the crystalline substances. In crystals the ions or molecules are arranged in well–defined positions in planes in three dimensions. The impinging X–rays are reflected by each crystal plane. Since the spacing between the atoms and hence the planes can’t be same or identical for any two chemical substances, this technique provides vital information regarding the arrangement of atoms and the spacing in between them and also to find out the chemical compositions of crystalline substances. The sample under study can be of either a thin layer of crystal or in a powder form. Since, the power of a diffracted beam is dependent



71



on the quantity of the corresponding crystalline substance; it is also possible to carry out quantitative determinations. 3. 5. 2 Scanning Electron Microscopy (SEM) Principle In this technique, an electron beam is focused onto the sample surface kept in a vacuum by electro–magnetic lenses (since electron possesses dual nature with properties of both particle and wave an electron beam can be focused or condensed like an ordinary light) (Hitachi 3000H). The beam is then scanned over the surface of the sample. The scattered electron from the sample is then fed to the detector and then to a cathode ray tube through an amplifier, where the images are formed, which gives the information on the surface of the sample. Instrumentation It comprises of a heated filament as source of electron beam, condenser lenses, aperture, evacuated chamber for placing the sample, electron detector, amplifier, CRT with image forming electronics, etc are shown in Figure 3. 5. Applications Scanning electron microscopy has been applied to the surface studies of metals, ceramics, polymers, composites and biological materials for both topography as well as compositional analysis. An extension (or sometimes conjunction to SEM) of this technique is Electron Probe Micro Analysis (EPMA), where the emission of X–rays, from the sample surface, is studied upon exposure to a beam of high energy electrons. Depending on the type of detectors used this method is classified in to two as: Energy Dispersive Spectrometry (EDS) and Wavelength Dispersive Spectrometry (WDS). This



72



technique is used extensively in the analysis of metallic and ceramic inclusions, inclusions in polymeric materials, diffusion profiles in electronic components. Disadvantages The instrumentation is complicated and needs high vacuum for the optimum performance. 3. 5. 3 Energy–dispersive X–ray spectroscopy (EDS) Energy dispersive X–ray spectroscopy (EDS) is an analytical technique used for the elemental analysis or chemical characterization of a sample (Hitachi 3000H). It is one of the variants of XRF. It relies on the investigation of a sample through interactions between electromagnetic radiation and matter, analyzing X–rays emitted by the matter in response to being hit with electromagnetic radiation. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing X–rays that are characteristic of an element's atomic structure to be identified uniquely from one another. Schematic diagram of EDS are shown in Figure 3. 5. To stimulate the emission of characteristic X–rays from a specimen, a high energy beam of X–rays is focused into the sample being studied. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher–energy shell then fills the hole, and the difference in energy between the higher–energy shell and the lower energy shell may be released in the form of an X–ray. The number and energy of the X–rays emitted from a specimen can be



73



measured by an energy dispersive spectrometer. As the energy of the X–rays is characteristic of the difference in energy between the two shells, and of the atomic structure of the element from which they were emitted, this allows the elemental composition of the specimen to be measured. 3. 5. 4 Atomic force microscope (AFM) Principle This technique operates by measuring the forces between the sample and the tip, and the sample need not be a conducting material. Here, the tip is brought close enough to the sample surface to detect the repulsive force between the atoms of the tip material and the sample. The probe tip is mounted at the end of a cantilever of a low spring constant and the tip–to–sample spacing is held fixed by maintaining a constant and very low force on the cantilever (Molecular Imaging, Picoscan 2000). Hence, if the tip is brought close to the sample surface, the repulsive force will induce a bending of the cantilever. This bending can be detected by a laser beam, which is reflected off the back of the cantilever. Thus by monitoring the deflection of the cantilever, the surface topography of the sample can be tracked. Since the force maintained on the cantilever is in the range of inter–atomic forces (about 10–9 Newton), this technique derived the name “atomic force” microscopy. Schematic diagram of AFM are shown in Figure 3. 6. AFM operates at two modes Repulsive or contact mode ─ which detects the repulsive forces between the tip and sample; Attractive or non–contact mode ─ which detects the van der waals forces that act between the tip and sample.



74



Applications AFM find applications widely in material sciences especially for surface studies on a nano scale range. AFM finds its applications in measuring the hardness of materials. Sometimes, AFM can be used in the study of “depth profile” of the deposited oxide layer on to a material. Disadvantages These methods require special sample preparation techniques, which are tedious, like, thin sectioning, electopolishing, various mechanical cutting and polishing techniques, etc. 3. 5. 5 Hardness measurements Microhardness of the Cr deposited on suitable substrate was evaluated by using a MH6 Everyone hardness tester with Vickers intenders. A dwelling time of 5s and a load of 50g were used for the measurement. Schematic diagram of Everyone hardness tester are shown in Figure 3. 7. 3. 5. 6 Thickness and Current Efficiency Initial and final weights were taken for all the samples. By knowing the Electrodeposited weight, from which thickness and current efficiency were calculated by the following formulae. Thickness (µm) = Weight of deposit X 10000 / (Area X Density) — (8) Current Efficiency = (Actual weight of deposit X 100) /Theoretical weight of deposit— (9)



Theoretical weight of deposit = (M x I x T) / (Z x F) M  Molecular weight of metal (Zn)/alloy (Zn–Cr) I  Current Passed in Ampere



75



T  Time in (Sec) Z  No. of electrons F  Faraday's constant (96500C) 3. 5. 7 Electrochemical studies on anticorrosive properties 3. 5. 7 (A) Potentiodynamic polarization studies The principal aim of the present investigation was to study surface degradation resulting from electrochemical processes, and this necessitated in analysis of the surface deposit left after electrochemical reactions. The potentiostatic polarization experiments provided some idea of the electrochemical activity of the material. The studies were carried out using PARSTAT 2273 electrochemical impedance analyzer. However, this necessitated scanning across a wide range of electrode potentials so that the surface of the material at the end of such polarization was the result of cumulative effects at different potentials. To analyze the surface, the material was subjected to potentiostatic polarizations, one specified potential being impressed on the material at a time. The potentials were either anodic or cathodic with respect to the primary electrochemical process occurring on the surfaces as indicated by the potentiodynamic polarization curves. The schematic polarization curve shown in Figure 3. 8 current and potential of the corroding electrode is related by I=Icorr(e2.3η/ba – e2.3η/bc)



— (10)



where I corr is corrosion current; η is over potential (E–Ecorr), ba and bc are anodic and cathodic Tafel slopes. At higher overpotentials, i.e. η ≥RT/F



— (11)



76



I=Icorr e2.3η/ba



— (12)



logI = logIcorr + η/ba



— (13)



In the plot η vs. logI, extrapolation of linear line to corrosion potential gives a straight line and the slope gives both ba and bc and the intercept gives the corrosion current. The Ecorr and Icorr values have been calculated using the Tafel extrapolation method. 3. 5. 7 (B) Electrochemical impedance spectroscopy EIS Electrochemical Impedance Spectroscopy or EIS is a powerful technique for the characterization of electrochemical systems (PARSTAT 2273 electrochemical impedance analyzer). The promise of EIS is that, with a single experimental procedure encompassing a sufficiently broad range of frequencies, the influence of the governing physical and chemical phenomena may be isolated and distinguished at a given applied potential. In recent years, EIS has found widespread applications in the field of characterization of materials. It is routinely used in the characterization of coatings, batteries, fuel cells, and corrosion phenomena. It has also been used extensively as a tool for investigating mechanisms in electrodeposition, electrodissolution, passivity, and corrosion studies. It is gaining popularity in the investigation of diffusion of ions across membranes and in the study of semiconductor interfaces. 3. 5. 7 (C) Principles of EIS measurements The fundamental approach of all impedance methods is to apply a small amplitude sinusoidal excitation signal to the system under investigation and measure the response (current or voltage or another signal of interest). In the following figure, a non– linear I–V curve for a theoretical electrochemical system is shown in Figure 3. 9. A low



77



amplitude sine wave ∆Esin(ωt), of a particular frequency, is superimposed on the dc polarization voltage E0. This results in a current response of a sine wave ∆Isin(ωt+φ) superimposed on the dc current I0. The current response is shifted with respect to the applied potential. The Taylor series expansion for the current is given by ΔI = (dI/dE) E0, I0 ΔE+1/2 (d2I/dE2) E0, I0 ΔE2+ ·······



— (14)



If the magnitudes of the perturbing signal ∆E is small, then the higher order terms 1/2 (d2I/dE2) E0, I0 ΔE2+ ········



— (15)



the first equation can be assumed to be negligible. The impedance of the system can then be calculated using Ohm’s law as,



ΔE(ω) — (16)



Z(ω) = ΔI(ω)



This ratio is called impedance, Z(ω), of the system and is a complex quantity with a magnitude and a phase shift which depends on the frequency of the signal. Therefore by varying the frequency of the applied signal one can get the impedance of the system as a function of frequency. Typically in electrochemistry, a frequency range of 100 kHz – 0.1 Hz is used. The impedance, Z(ω), as mentioned above is a complex quantity and can be represented in Cartesian as well as polar co–ordinates. In polar co–ordinates the impedance of the data is represented by, Z(ω) = |Z(ω)|eφ(ω)



—(17)



78



where |Z| is magnitude of the impedance and φ is the phase shift. In Cartesian co– ordinates the impedance is given by Z(ω) = Zr(ω) + jZj (ω)



—(18)



where Zr is the real part of the impedance and Zj is the imaginary part and j = −1 The plot of the real part of impedance against the imaginary part gives a Nyquist Plot, as shown in Figure 3. 10. The advantage of Nyquist representation is that it gives a quick overview of the data and one can make some qualitative interpretations. While plotting data in the Nyquist format the real axis must be equal to the imaginary axis so as not to distort the shape of the curve. The shape of the curve is important in making qualitative interpretations of the data. The disadvantage of the Nyquist representation is that one loses the frequency dimension of the data. One way of overcoming this problem is by labeling the frequencies on the curve. The absolute value of impedance and the phase shifts are plotted as a function of frequency in two different plots giving a Bode plot, as shown in Figure 3. 11. This is the more complete way of presenting the data. The relationship between the two ways of representing the data is as follows: |Z|2 = (Zre)2 + (Zim)2



— (19)



Zre Ø = tan–1



— (20) Zim



Zre = |Z|cosø



— (21)



Zim = |Z|sinø



— (22)



For this simple process, the model circuit is that shown in Figure 3. 12. The circuit is a resistor, Rp, is parallel with a capacitor, C. The entire parallel circuit is series



79



with another resistor, Rs. The utility of AC impedance lies in the fact that Rs equals the solution resistance uncompensated by the potentiostat and Rp equals the polarization resistance.



80



References 1. M. Ghaemi, L. Binder J. Power Sources 111 (2002) 248. 2. A. Marlot, P. Kern, D. Landolt, Electrochim. Acta 48 (2002) 29. 3. K. M.Yin, S. L. Jan, C. C. Lee, Surf. Coat. Technol. 88 (1997) 219. 4. S. Kainuma, S. Ishikura, K. Hisatake, J. Magn. Soc. Jpn. 21 (1995) 889. 5. T. Houga, A. Yamada, Y. Ueda, J. Jpn. Inst. Met. 64 (2000) 739. 6. Y. Ueda, N. Hataya, H. Zaman, J. Magn. Magn. Mater. 156 (1996) 350. 7. M. Alper, K. Attenborough, R. Hart, S.J. Lane, D.S. Lash- more, C. Younes, W. Schwarzacher, Appl. Phys. Lett. 63 (1993) 2144 8. S. D. Beattie, J. R. Dahn, J. Electrochem. Soc. 150 (2003) A894. 9. J. C. Puippe, N. Ibl, J. Appl. Electrochem. 10 (1980) 775. 10. N. Ibl, Surf. Technol. 10 (1980) 81. 11. J. C. Puippe, F. Leaman, Theory and practice of pulse plating, 1st edn. American Electroplaters and Surface Finishers Society, Orland, FL, 1986, p 247–257 12. A. M. El–Sherik, U. Erb, J. Page, Surf. Coat. Technol. 88 (1996) 70. 13. S. K. Ghosh, A. K. Grover, G. K. Dey, M. K. Totlani, Surf. Coat. Technol. 126 (2000) 48. 14. A. C. West, C. C. Cheng, B. C. Baker, J. Electrochem. Soc. 145 (1998) 3070. 15. G. Holmbom, B. E. Jacobsson, Surf. Coat. Technol. 35 (1988) 333. 16. S. I. Kwak, K. M. Jeong, S. K. Kim, J. Electrochem. Soc. 143 (1996) 2770. 17. S. Roy, Ind. Eng. Chem. Res. 51 (2012) 1756 19. M. Datta, D. Landolt, Surf. Coat. Technol. 25 (1985) 97.



81



Table. 3. 1 Electrolyte composition for electrodeposition of chromium Solution composition



Glycine electrolyte



DMFelectrolyte (g/l)



(g/l) CrCl3 6H2O



212



212



NH4Cl



26



26



NaCl



36



36



B(OH)3



20



20



Glycine



75



–



DMF



–



200 ml



CH3OH



200 ml



–



Figure 3. 1 A typical time dependence of applied current in pulse wave form where t On is the On–time, tOff is the Off–time, ia is the average current density, ip is the peak current density and iF is the Faradaic current. 82



Figure 3. 2 Concentration profiles of the two diffusion layers in pulse electrolysis at the end of pulse. The broken line (– – – – –) shows the recovery of the concentration in the pulsating diffusion layer during the Off–time. (T