CHAPTER III EXPERIMENTAL TECHNIQUES

Transcription

1 76 CHAPTER III EXPERIMENTAL TECHNIQUES 3.1 Electrodeposition Since electrodeposition is highly diverse in nature, no single universal experimental procedure has been found satisfactory to cover all aspects of electrodepositon of metals and alloys. However, it is very essential to select relevant methods to correlate laboratory experiments with actual industrial operations [1-5]. Though absolute reproducibility is rather difficult to achieve due to several factors, reproducibility with minimum error is of considerable importance Chemicals and materials used Chemicals Make Tri chloro ethylene Fischer, India Agar-Agar CDH, India Potassium chloride Ranbaxy, India Sulfuric acid Fischer, India Nitric acid Fischer, India Hydrochloric acid Fischer, India Sodium hydroxide ` Fischer, India Sodium carbonate Fischer, India Copper sulfate BDH, India Copper carbonate (basic) S.d.fine chemicals, India Methane sulphonic acid (100 %) Merck India EDTA BDH, India Lead carbonate Merck, India Lead fluoborate Madras fluorine chemicals, Chennai Hydrofluoric acid S.d.fine chemicals, India

2 77 Boric acid Sodium chloride Potassium ferry cyanide Peptone Gelatin Poly ethylene glycol (4000) Triton-X-100 Stannous oxide Tin fluoborate Iodine Potassium Iodide Sarabhai chemicals, India Ranbaxy, India Ranbaxy, India CDH, India CDH, India Ranbaxy, India Merck, India CDH, India Madras fluorine chemicals, Chennai. Merck, India S.d.fine chemicals, India Double distilled water was used for the preparation of experimental solutions. All glassware and electrochemical apparatus were fabricated at Central electrochemical research institute, Karaikudi, India. A single pan digital balance (Afcoset Model ER-180A) was used for mass measurements to determine current efficiency, throwing power, thickness etc. The current and potential were measured using digital multimeters. The digital power source (Aplab 0-2 A, 0-32 V) was employed as the DC power supply unit for electrodeposition. Polarisation studies were carried out using a constant current regulator (fabricated at Central electrochemical research institute, Karaikudi, India.) 3.2 Preparation and purification of electrolytes Copper methane sulphonate bath Required amount of basic copper carbonate was weighed and transferred into a beaker. Required amount of methane sulphonic acid was added to this and the dissolved in double distilled water. The suspended impurities present in the solution were removed

3 78 by filtration. The organic and inorganic impurities were removed by treatment with activated charcoal and dummy electrolysis respectively. The amount of copper present in the electrolyte was estimated by complexometric titration using EDTA. The basic bath compositions of copper methane sulphonate bath used for the studies such as Hull-cell, current efficiency, throwing power measurements; polarization, and samples preparation for testing mechanical properties and corrosion studies is given in the table Copper sulphate bath The sulphate based copper bath was prepared as and when required. A calculated quantity of copper sulphate was exactly weighed and transferred into the beaker, with this required volume of sulphuric acid was added slowly and made up with distilled water. The suspended impurities present in the solution were removed by filtration. The organic and inorganic impurities were removed by treatment with activated charcoal and dummy electrolysis respectively. The basic bath composition of copper sulphate bath used for the studies such as Hull-cell, current efficiency, throwing power measurements; polarization, and samples preparation for testing mechanical properties and corrosion studies is given in the table Lead methane sulphoante bath Required amount of lead carbonate was weighed and transferred into a beaker. To this the required amount of methane sulphonic acid was added and heated to o C, After complete dissolution of the salt dissolved, the solution was made up to the required volume with double distilled water. The suspended impurities present in the solution

4 79 Table 3.1 Composition of baths studied Bath Constituents A Copper (as methane sulphonate) M MSA M Temperature RT (30 o C) B Copper (as copper sulphate) M Sulfuric acid M Temperature RT (30 o C) C Lead (as methane sulphonate) M MSA M Temperature RT (30 o C) D Lead (as fluoborate) M Fluoboric acid M Boric acid 0.48 M Temperature RT (30 o C) E Tin (as methane sulphonate) M MSA M Temperature RT (30 o C) F Tin (as fluoborate) M Fluoboric acid M Boric acid 0.48 M Temperature RT (30 o C)

5 80 were removed by filtration. The organic and inorganic impurities were removed by treatment with activated charcoal and dummy electrolysis respectively. The amount of lead present in the electrolyte was estimated by complexometric titration using EDTA. The basic bath composition of lead methane sulphonate used for the studies such as Hull-cell, current efficiency, throwing power measurements; polarization, and samples preparation for testing mechanical properties and corrosion studies is given in the table Preparation of fluoboric acid Fluoboric acid was prepared by adding analar grade boric acid in small increments with constant stirring to a chemically pure hydrofluoric acid (40 wt %) in the stochiometric ratio as per the following equation 4 HF + H 3 BO 3 HBF H 2 O The presence of unreacted hydrofluoric acid was tested by the formation of white precipitate with a sample of lead nitrate solution. The strength of fluoboric acid was estimated by acid-base titration. The concentration of fluoboric acid was varied by diluting this acid Lead fluoborate bath Required amount of commercially available lead fluoborate solution (approx.50%) was taken in a polyethylene beaker. To this, required quantities of fluoboric acid and boric acid were added and then the solution made up to the required volume with double distilled water. The suspended impurities present in the solution were removed by filtration. The organic and inorganic impurities were removed by treatment with activated charcoal and dummy electrolysis respectively.

6 81 The amount of lead present in the electrolyte was estimated by complexometric titration using EDTA. The basic bath composition of lead fluoborate used for the studies such as Hull-cell, current efficiency, throwing power measurements; polarization and samples preparation for testing mechanical properties and corrosion studies is given in the table 3.1.Boric acid was added to avoid the hydrolysis of fluoboric acid Tin methane sulphonate bath Required amount of stannous oxide was weighed and taken in a beaker. Required amount of methane sulphonic acid was added and heated to 80 o C. After complete dissolution of the salt, the solution was filtered and made up with double distilled water. The suspended impurities present in the solution were removed by filtration. The organic and inorganic impurities were removed by treatment with activated charcoal and dummy electrolysis The amount of tin present in the electrolyte was estimated by iodometric titration using starch as indicator. The basic bath composition of tin methane sulphonate bath used for the studies such as Hull-cell, current efficiency, throwing power measurements; polarization, and samples preparation for testing mechanical properties and corrosion studies is given in the table Tin fluoborate electrolyte Required amount of commercially available tin fluoborate solution (approx.50%) was taken in a polyethylene container. Required quantity of fluoboric acid and boric acid was added and then made up with double distilled water. The suspended impurities present in the solution were removed by filtration. The organic and inorganic impurities were removed by treatment with activated charcoal and dummy electrolysis respectively.

7 82 The amount of tin present in the electrolyte was estimated by iodometric titration using starch as indicator. The basic bath composition for tin fluoborate bath used for the studies such as Hull-cell, current efficiency, throwing power measurements; polarization, cyclic voltammetry, and samples preparation for testing mechanical properties and corrosion studies is given in the table Electrodes and cells Electrodes Anodes Electrolytic copper (99.5 % pure) Electrolytic lead (99.5 % pure) Electrolytic tin (99.5 % pure) Cathodes Mild steel (for tin, lead, deposition) Brass (for copper deposition) Hull Cell [6] It is a small trapezoidal cell of accurate dimensions. Figure.3.1 shows the cross sectional and over view of a Hull-cell. It is of different capacities, 1000 ml, 320 ml and 267 ml. Hull cell capacity of 267 ml was used in the present study. In this cell, the cathode is positioned at an inclined angle to the anode so that the current density at each point on the cathode is different. The optimum current density range for obtaining quality deposits from the selected plating bath was determined using Hull cell [7]. The experiments help in determining the nature of deposits that could be obtained at different current densities in a single experiment. Mechanically polished and degreased Brass/MS cathodes of 10.2 x 7.0 x 0.2 cm area and copper, lead, and tin anodes of 6.5 x 6.5 x 0.5 cm size were used

8 83 for the experiment. Before starting experiments the backside of the cathode panels were masked using duroflex lacquer. The cathode panels were subjected to electrochemical surface treatment using the following solutions. Sodium carbonate Sodium hydroxide Anode Current density Cathodic treatment Anodic treatment 30 g/l 30 g/l Stainless steel 1.5 A/dm 2, 4.5 A/dm 2 (for brass) 2 minutes 30 seconds. Preliminary studies were carried out with Hull-cell using various electrolytes under different conditions. After the experiments, the panels were washed, dried and the results were expressed using codes to indicate the nature of deposits. By measuring the distance from high current density and the distance of the points spanning the desired deposit pattern, the value of current densities corresponding to those points and hence the current density range for production of deposits of desired quality were determined. The technique was also employed to identify a suitable addition agent that could be used to improve the quality of deposits. The following formula enabled calculation of the current density at a desired point on the Hull cell cathode. Current density in A/dm 2 at any point on the inclined cathode = C ( logL) Where C is the total current passing through the cell, L is the distance in cm of the point of interest from the nearer end of the cathode.

9 84 Figure 3.1 Cross section of Hull cell diagram All dimensions in mm

10 Throwing power (Haring-Blum cell) [9] Throwing power is regarded as one of the important characteristic properties of any electroplating bath because it is a measure of the ability of the bath to cover the surface or throw the metal uniformly over corners and in recesses. The throwing power is commonly expressed as a percentage. After Field s contribution to the plating scene, the unit of percentage came to be expressed either as a positive (+ve) or a negative (-ve) figure with 100 as the maximum. According to field the positive figures would represent good performance of the baths and negative ones, their poor performance. He further explained the poor performance in terms of non-uniform coverage and thickness of the deposit. It is a well-known fact that the alkaline baths are always associated with a high (good) throwing power, probably because of the high over-potentials for metal deposition is such solutions. In contrast, the throwing power is low or poor in the case of acid baths due to low over-potentials. The chief factors affecting throwing power are the extent to which the cathode polarizes with increase in current density, the electrical conductivity of the electrolyte and the relationship between cathode current efficiency and cathode current density. The steeper the slope of the cathode polarization curve and greater the conductivity of the electrolyte, the more uniform will be the current distribution and hence the metal distribution over the cathode surface. If the cathode current efficiency decreases with increase in current density, the uniformity of the metal distribution is improved.

11 86 Haring-Blum cell is used to determine throwing power. It is basically a rectangular (15 x 5 x 5 cm) PVC box with an open top (Figure 3.2). Typically it has two cathodes (5 x 5 x 0.2 cm) one at each end, with a single anode perforated of the same size placed between them. The studies on throwing power were carried out for different compositions of the electrolytes of copper, lead, and tin under still and stirred conditions. Deposits were produced on pre cleaned brass/steel cathodes, positioned at both ends of rectangular cell with a distance ratio of 1:5 from the anode. The plating was carried out for 30 minutes at 1-10 A/dm 2 for all the electrolytes. From the weight of the deposits obtained at the near cathode (C n ) and far cathode (C f ) the throwing power was calculated. Under such condition the electrolyte would behave in accordance with Ohm s law and the metal distribution would be proportional to the current distribution. In this hypothetical case the current distribution is referred to as primary distribution. However, resistance at the interface between the electrolyte and the cathode is high as compared with the resistance of the electrolyte due to polarization and the resulting current distribution is known as secondary distribution. Haring and Blum formula for determination of throwing power is L-M Throwing power (%) = x 100 L In this expression C (C n /C f ) is the metal distribution ratio, and K is the ratio of the distance from the far cathode and the near cathode to the anode. Thus K is the current distribution ratio and normally it is maintained at a value of 5. However, Field s formula, a modified term of Haring and Blum formula is the one greatly used, since the values it

12 87 gives are more realistic and range from % to 100% irrespective of the value of K. The Field s formula is [10]. formula. L-M Throwing power (%) = x 100 L+M 2 Values of throwing power for different solutions were calculated with Field s 3.4 Current efficiency The studies on current efficiency and rate of build up were carried out for different composition of the electrolytes under still and stirred condition. Experiments were carried out in a 200 ml container containing 7.5(l) x 2.5 (b) x 0.5 (t) cm anodes (copper, lead, tin,) with an exposed area of 2.5 (l) x 2.5 (b) cm and mild steel or brass with exposed area of 2.5 (l) x 2.5 (b) cm by masking the unwanted portions by lacquer. The cathodes were degreased and electrolytically cleaned in alkaline solution prior to deposition. The specimens were weighed before and after deposition and the cathodic current efficiency in each case was calculated using the relation Weight of the deposit Cathode current efficiency = x 100 Theoretical weight The theoretical amount of metal deposition was calculated using Faraday s laws, which states that gram equivalent weight of a metal deposited at the cathode requires 96,500 coulombs of electricity. The anodic dissolution efficiencies of copper, lead and tin of above-mentioned dimensions were studied in their respective MSA baths and in various concentration of pure MSA solution.

13 88 The protective value of many electrodeposited coatings is determined mostly by their thickness. It has been established that corrosion protection depends primarily on the thickness or mass of the deposit [8]. In view of this, the thickness of the deposit and the rate of deposition were calculated from the mass of the deposit obtained. Mass of the deposit (gm) Thickness (micron) = x 10 4 Density of the metal (g/cm 3 ) x Area (cm 2 ) 3.5 Conductivity measurements Solution of strong acids and bases are much better conductors than any other aqueous solution and in electrolytic processes free acid or free alkali is often used to improve the solution conductivity. It is very important that for an electroplating solution being useful; conductivity should be adequate, avoiding the necessity of using higher applied voltage and hence high energy-consumption. Conductivity measurements were carried out for different compositions of the electrolyte using Dot-tech digital conductivity meter (model Dot 466). 3.6 Polarization method The cathodic polarizations were carried out in copper, lead, and tin electrolytes of different compositions using three-electrode assembly under still and stirred condition. Platinum foil of 1 cm 2 area and saturated calomel electrode was used as the counter and reference electrode respectively. Brass/steel of 1 cm 2 area was used as a working electrode. The anodic polarization of copper, lead, and tin, were also carried out in their respective baths and in various concentration of pure MSA solution. Current steps were applied using a constant current source and corresponding potentials were measured after attaining steady state.

14 Figure 3.2 Haring-Blum Throwing power cell 89

15 Cyclic voltammetric studies Potential sweep cyclic voltammetry is one of the most powerful electrochemical techniques used as a means of obtaining a quick electrochemical spectrum of a charge transfer system and also a method for the detailed examination of reaction mechanism. It has also proved valuable in the study of surface processes, such as formation and reduction of oxide layers on metal surfaces. The electrochemical cell is made up of an all glass one-compartment cell with three-electrode cell assembly. For all the studies platinum foil of one square cm area was used as an auxiliary or counter electrode (CE). The platinum disc electrode of specified area (0.003 cm 2 ) was used as a working electrode for copper and glassy carbon of specified area (0.196 cm 2 ) used as a working electrode for lead, tin systems. All the experiments were carried out with saturated calomel electrode (Hg/HgCl 2 filled with saturated potassium chloride) as a reference electrode along with Agar-Agar-KCl salt bridge. The pretreatments of the working electrode consisted of polishing with emery paper, degreasing using tri chloro ethylene, washing and rinsing with distilled water. For cyclic voltammetry experiments analytical grade chemicals and triple distilled water were used for the preparation of solutions. The electrolytes were prepared by mixing the appropriate solutions with stochiometric amounts of the reagents concerned. The solution under study was deoxygenated for one hour using purified nitrogen. The experimental arrangement consists of potentiostat (Wenking Model VSG 72) and X-Y recorder (Rikadenki, Japan). The potential range for electrodeposition was fixed after carrying out several experiments to get reproducible results in all the copper,

16 91 lead, tin electrolytes. Cyclic voltammetry studies were carried out with various electrolytes of the following compositions as given in the table Corrosion resistance studies One of the important properties of electrodeposited coatings is their corrosion resistance. The porosity of electrodeposits is closely related to its corrosion behaviour. Evaluation of corrosion resistance may be carried out by non-electrochemical and electrochemical techniques. Corrosion data like corrosion current I corr, corrosion potential E corr and weight loss were found out using various techniques like potentiodynamic polarization and weight loss method Potentiodynamic polarization method The earlier work of Wagner and Traud showed that there is a linear relationship between potential and applied current at potentials only slightly removed from the corrosion potential. They considered this relationship to be especially important because low current polarization measurements combined with corrosion rate data permit calculation of Tafel slopes. Since the measurements are carried out close to the corrosion potential, only surface changes resulting from high current polarization are eliminated. The slope of the linear portion will give the polarization resistance Rp = E/ t in ohm.cm -1. I corr can be calculated by knowing the anodic and cathodic slopes b a and b c respectively using the following equation. b a b c I corr = R p (b a + b c )

17 92 Table 3.2 Composition of baths used for cyclic voltammetry studies Bath Constituents A1 Copper (as methane sulphonate) M MSA M Temperature RT (30 o C) B1 Copper (as copper sulphate) M Sulfuric acid M Temperature RT (30 o C) C1 Lead (as methane sulphonate) M MSA M Peptone 1.0 g/l Temperature RT (30 o C) D1 Lead (as fluoborate) M Fluoboric acid M Boric acid 0.48 M Peptone 1.0 g/l Temperature RT (30 o C) E1 Tin (as methane sulphonate) M MSA M Peptone 1.0 g/l Temperature RT (30 o C) F1 Tin (as fluoborate) M Fluoboric acid M Boric acid 0.48 M Peptone 1.0 g/l Temperature RT (30 o C)

18 93 The corrosion resistance studies of electrodeposited copper, lead, and tin, of various thickness obtained from various electrolytes were carried out with polarization technique. A three-electrode cell assembly was used in these polarization studies. The electrodeposited copper, lead, and tin specimens were masked with lacquer to expose only one square centimeter area on one side of the working electrode. A platinum foil and saturated calomel electrode were employed as auxiliary and reference electrodes respectively. Polarization studies were carried out with 5 % W/V neutral sodium chloride solution for testing using Auto lab Ecochemie BSTR 10A system. The potentials were scanned at the rate of 5-mV/sec up to 200 mv from the OCP value both in the cathodic and on the anodic direction with suitable IR corrections. The intercepts of the linear portions of the two polarization curves give I corr and E corr values Electrochemical impedance measurement method Using the three electrode cell assembly, as used in potentiodynamic polarization method, impedance measurements were carried out on copper, lead and tin deposits in 5 % sodium chloride solution at OCP using Solartron Model SI 1280 B. A circuit diagram for the impedance study is shown in figure 3.3. Impedance measurements were carried out at the OCP of the working electrode using the AC impedance system. AC signal of amplitude 10 mv was impressed to the system with the frequencies ranging from 10 KHz to 1 mhz. The values of solution resistance (Rs) and charge transfer resistance (Rt) have been obtained from the Nyquist plot of real part (Z ) Vs imaginary part (Z ).

19 Figure 3.3 Circuit diagram for AC impedance measurement 94

20 Self corrosion (weight loss) method The corrosion rates can be expressed as variation in the weight per unit surface area and unit time of penetration of the corrosion process into the metallic material in unit time. Weight loss experiments were carried out for copper, lead and tin anodes in various concentrations of MSA. The anode area of (6.25cm 2 ) was exposed for 7 days and the corrosion products were removed and loss in weight was calculated [11]. 3.9 Properties of electrodeposits The characteristics of deposits mainly depend upon the nature of metals and alloys. Properties of the electrodeposits were studied by measuring adhesion, porosity, hardness and solderability Adhesion A number of qualitative and quantitative tests have been developed for adhesion. The qualitative tests are (a) bend test (b) twist test (c) heat cycling (d) burnishing (e) scratching and chisting (f) impact tests and cupping. The qualitative tests are (1) peel test (2) tensile test and (3) shear test. In the present work bend test has been used for qualitatively assessing the deposit adhesion [12]. The bend test comprises bending of the plated object to such as extent as the specimen will permit. The test piece is held firmly by vise and bent as sharply as possible. The bending is frequently reversed and repeated unit basis metal is fractured. Any evidence of peeling, flaking of the deposit is taken as cause of rejection.

21 Porosity Porosity of the electroplated metal coated over steel is detected by the Ferroxyl test [13]. Special test papers were prepared by impregnating them in a solution containing 50 g/l sodium chloride solutions and then pressed against the electrodeposited panel and left for 10 minutes. After removal the papers were immersed in a 10g/l solution of potassium ferry cyanide. Blue marks developed in the paper in the region where steel is exposed through discontinuities in the coating were counted by viewing the surface with microscope. The porosity of the coating was expressed as the percentage of defective area Micro hardness The value of micro hardness of electrodeposits of copper, lead and tin obtained from different electrolytes at different conditions were determined by using a HMV, Shimadzu Micro hardness tester with a square base diamond pyramid, having an angle of 136 o at the vertex between two opposite faces. The deposit should be of adequate thickness so that the diamond pyramid does not penetrate it to a depth greater than 10 % of its thickness. This can be readily ascertained since the penetration depth is equal to 1/7 of the indentation diagonal. For determining micro hardness of deposit, the diamond pyramid was pressed into the deposit for 10 seconds and the indentation diagonal was measured after the load was removed. The Vicker s micro hardness of the deposit in Kg/mm 2 was determined in each case by using the formula V = 1854 x P d 2 Where P is the load applied in grams and d is the diagonal of the indent obtained in micrometers.

22 Solderability In this method [14] fixed volume of solder was placed on the test specimen, which was heated to 513 K on a hot plate. On heating the test specimen, the solder melt and spread over the surface. Then the sample was cooled and the area of spread was measured with a planimeter. Spread factor for a particular test surface is calculated by using the equation. D-H Percentage spread factor = x 100 D Where D- diameter of sphere having a volume equal to that of the solder used and H is the height of the solder spot. If there is no effect from gravity, surface tension or wetting of the solder, the solder drop will assume the shape of a sphere. As the solderability increases the height of the solder spot decreases resulting in the increase of the spread factor value Structural characterisation X-ray diffraction studies The copper, lead, and tin deposited samples obtained from various electrolytes were subjected to x-ray diffraction studies using PAnalytical model X per PRO to identify the orientation. CuKα radiation was used Scanning Electron Microscopic studies The morphology of the electrodeposits was examined under high magnification to assess the grain size, deposit nature, heterogeneities and pores present in the deposits using a scanning electron microscope. The scanning electron microscope that makes use of reflected primary electrons and secondary electrons enables one to obtain information from regions, which cannot be examined by other techniques. The plated specimens

23 98 were cut into 1 X 1 Cm 2 size and mounted suitably and examined under the microscope. The SEM photographs were taken by using Hitachi model S-3000H l with an acceleration voltage range of 20,000V and with the magnification range of 1000 and 2000.

24 99 REFERENCES 1. A.K.Graham, Electroplating Engineering Hand Book, 3 rd edition, Von Nostrand, Reinhold publishing, Princeton (1971). 2. J.B Kushner, Electroplating know-how Evansville, Indiana (1974). 3. W.Blum and B.H.George, Principles of Electroplating and Electroforming 3 rd edition Mc Graw Hill book company New York (1949). 4. A.T.Vagramyan and Sulov Evaz A, Technology of Electrodeposition, Robert Draper Ltd (1960). 5. A.G.Gray, Modern Electroplating New York (1953). 6. N.O.Ruboa, Electroplating and Finishing 63 (1992) W.Nobse, Investigation of Electroplating and Related Solution with the aid of Hull cell, Robert Draper Ltd, (1996). 8. E.Raub and K.Muller, Fundamentals of Metal Deposition Elseveir publishing co. New York (1967). 9. W.Nobse, The Hull cell Robert Drapper Ltd (1966). 10. M.Mc.Cormick and Kuhn, Trans Inst. Metal Finish.71 (1993) Peterwolfram Wild, Modern analysis of Electroplating, finishing publishing Ltd., London. 12. P.Leisner and M.E.Benzon Trans. Inst.Metal Finish. 75 (1997) C.Bocking, Trans. Inst.Metal Finish 74 (1996) J.B.Mohler, Metal Finish. 71 (1973) 86.