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Electroplating is a process by which a Metal ( or Metals ) is deposited, usuallyas a Relatively thin coating , upon the surface of an article which is itself made ofdifferent Metal.
Following are the three main purpose for which Electro-deposits may be used(1):PRESERVATION
To provide a protection against corrosion.e.g. Zinc, Cadmium or Nickel deposits on Steel
(2):DECORATIVETo provide a perticular surface appearance.e.g. Brass plating on bright Nickel plated Steel
(3):MECHANICAL REASONSTo Provide Wear Resistance.e.g. Hard Chromium deposite on bearing surface.
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Choice of tank Material
The process is carried out in a tank made of Iron or Concrete or soap stone orStone ware or cement or wood.
The choice of the tank maerial is governed by the three factors. Resistance to attack by solution concerned.Mechanical strength. Cost including investment and unkeep.
The process consist in suspending the work piece to be platedIn a solution of chemical salt of the Metal to be deposited andpassing a current through the solution between the work piece(made a cathode) and solid pieces of the Metal to be deposited(Called the anodes) suspended in the solution a short distancefrom the work.
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Condition of Electrolyte
The electrolyte usually is a soluble salt of the Metal and must fulfill the following conditions.(1)SOLUBILITY: it should be highly soluble.(2)CONDUCTIVITY: it should be good conductor.(3)STABILITY: it should be stable towards hydrolysis, oxidation and reduction.(4)COST: it should be cheap(5)COVERING POWER AND THROWING POWER: it should possess good covering power
and throwing power.(6)UNIFORM ANODIC CORROSION: In certain cases anode becomes passive after sometimes
This is specially so in the case of Nickel baths, when pure Nickel is used as anode.
In such case uniform anodic corrosion is not possible.This make the process uncontrollable in case of Nickel plating, NickelChloride is added to overcome passivity.
(7)DRAGOUT: when the plated article is lifted from the solution,
Some solution stick to the cathode which is lost during washing and cleaning.The loss of solution so taking place is known as dragout losses.
The solution be such that dragout losses should be minimum particularly in case of
precious Metal solution.
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ELECTROPLATING OF SILVER
Silver can be plated from a variety of baths. The Cynide type of bath iswidely used. The basic constituents of this bath are Silver, Sodium orPotassium Carbonate. Most Silver plating is done by using two baths inSuccesion.
The first bath is called the strike solution. It has low concentrationof Silver and has high Cynide content.
Two types of strike bath may be used.
The composition of the strike bath and operating conditions are given in Table-1
Constituent present
And operating
condition
Bath 1Low Metal
Bath 2High Metal
Potassium Cynide 42 gm/litre 14 gm/litre
Potassium Cynide free 41 gm/litre -
Silver Cynide 0.9 gm/litre 05 gm/litre
Potassium Carbonate 28 gm/litre 28 gm/litre
Silver 01 gm/litre 05 gm/litre
Temperature 25 C 25 C
Current density 320 A/sq.m 43 A/sq.m
Anode Fe Fe
Voltage < 06 Volt < 06 Volt
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STRIKES
Initially, a special plating deposit called a strike may be usedto form a very thin ( typically less then 0.5 mil thick) plating with high qualityand good adherence to the substrate. This serves as a foundation forsubsequent plating processes. A strike uses a high current density anda bath with a low ion concentration. The process is slow, so more efficient
plating processes are used once the desired strike thickness is obtained.
The striking method is also used in combination with the platingof different Metals. If it is desirable to plate one type of deposit ontoa Metal to improve corrosion resistance but this metal has inherentlypooradhesion to the substrate, a strike can be first deposited that is compatible
with both. One example of this situation is the poor adhesion ofelectrolytic Nickel on Zinc alloys, in which case a Copper strike is used,
which has good adherence to both.
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The composition and operating conditions of the Silver plating baths areGiven in table-2
Constituents and
Operating conditions
Bath 1
Low Metal
Bath 2
High Metal
Bath 3
Cynide Nitrates
Sodium Cynide ( g/l ) 16 - 35 - 42
Potassium Cynide ( g/l ) 56 105 -
Potassium Cynide free
( g/l )
40 57 -
Silver Cynide ( g/l ) 34 98 21 42
Potassium Carbonate
( g/l )
42 14 - 100 -
Silver ( g/l ) 37 110 24 - 48
Carbonate Disulphide( g/l )
0.0007 - 0.0007
Temperature (C) 25 35C 44-55C 27-55C
Current Density (A/sq.m) 0.54-16.40 6.45-16.40 0.54-16.10
Anode Ag Ag Ag
Cathode efficiecy 100% 100% 100%
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PRECAUTIONS
1. Anodes used for Silver plating should be the purest not less than 997.4 fineotherwise impurities may cause black Anodes i.e. coveringof Anode by a blackish scum. This interferes with corrosion of Anode andcomposition of Anode, causing rough deposits.
2.Care should be taken not to contaminate solution with Chloride.
3.Proper attention should be paid to minimize dragout losses and enough Anodesshould be used. If these two conditions are fulfilled Silver content of the bath willbe maintained automatically.
4.If it is necessary to add Silver it is adde in the form of Silver Cynide.5.The Cynide content gradually decreases and should be replenished.
6.It is better to add small quantities of brightner often, then to add large quantitiesat one time.
7.Impurities and dirt should be ke pt out of the solution.8.The Carbonate content of the bath increases gradually. The excess should beremoved by either adding Gypsum and filtering or by lowering the Temperature to
very low values to precipitate the Sodium Carbonate.
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PROPERTIES OF DEPOSITSSilver deposits from Cynide solutions have a milky or trosted appearance.
Buffering is required to obtain bright surfaces small amount of Carbon Disulphideadded acts as a brightening agent.It has been seen that addition of 1 gm of Sodium thiosulphate and 10 cc of
Ammonia per litre is more effective in producing bright white Silver deposit.FOR COATING STEEL BEARINGS
For plating Steel bearings, by using higher Temperature greater agitation, specialracking, more concentrated solution and additions of Potassium by hydroride highercurrent densities can be permitted, thus good heavy deposits can be obtainedexpeditiously.FOLLOWING ARE STEPS FOLLOWED IN PLATING BEARINGS:
1.After decreasing it is anodically cleaned in a hot alkaline solution.2.It is then dipped in hot Hydrochloric Acid solution.
3.After dipping the base is anodically etched in 50% H2SO4.4.The prepared surface is given Nickel strike.5.The Nickel strike is then followed by two silver strikes.6.Silver is then plated using plating solution no.27.The coated Metal is then annealed at 482 to 537C for an hour.
8.Then thin coatings of Lead and Indium are applied to the plated surface and theparts are heated to 160C to obtain an alloyed surface.
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REFERENCES
1. Internet2. Electrometallurgy & Corrosion By :
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