Transcript Chemistry-Unit-4-Metal-Finishing

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Process of electro deposition of an adherent
metallic film of uniform thickness on the
surface of the substrate for the purpose of
modifying its surface properties.
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In general a metal or alloy used for a purpose
does not meet all the requirements.
Metal finishing is one of the methods
employed to improve their lacking properties.
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Improving resistance to corrosion, scratch or
abrasion and wear.
Enhancing decorative appearance.
Improving solder ability and electrical
properties
Reducing frictional loss etc.
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Metal finishing was introduced as a
decorative finish, but the increasing demand
for parts with prescribed specifications has
led to vast technological developments in the
field.
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Techniques of metal finishing:
Electroplating of metals & alloys.
Electro less plating of metals & alloys.
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The process by which coating metal is
deposited on the base metal by passing direct
current through an electrolytic solution,
containing the soluble salt of the coating
metal.
Common metals used are Cu, Ni, Zn, Au, Pt
etc.
Used in industries for producing metal
coating.
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 Decomposition Potential
 Polarization
 Over voltage
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The minimum voltage required for the
continuous electrolysis of an electrolyte in an
electrolytic cell.
In the electrolysis of water, a pair of Pt
electrodes is immersed in a solution of an
acid. It is found experimentally that a
potential of about 1.7 V must be applied to
the cell before there sets in a continuous
evolution of H2 and O2.
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During electrolysis accumulation of products
of electrolysis around the electrodes causes a
back emf which opposes the applied emf.
To overcome the back emf, the applied emf
will have to be increased ;then only
electrolytic deposition starts again.
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Gradually increasing voltage is applied
between the electrodes.
Milliammeter reading is noted down for every
voltage applied.
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Decomposition potentials of different
electrolytes are different.
Strength of the current flowing through the
cell
Chemical nature of the electrodes.
Physical nature of the electrodes
Activity of the electrolyte
Absolute temperature.
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Used to carry out all electrolytic processes.
(electroplating, electrorefining etc.)
Used in the separation of ions from a solution
by electrolysis.
Two types of ions can be discharged
simultaneously by equalising their
decomposition potentials.
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The decomposition of an electrolyte is
expected to start as soon as applied
potential reaches the value of reversible
emf of the cell.
But when products discharged at the
electrodes are gases, then the actual
decomposition potential is invariably much
higher than its theoretical reversible
electrode potential.
The excess voltage is referred to as over
voltage of the gas.
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Over voltage is the difference between the
actual applied emf to bring about
continuous electrolysis and the theoretical
emf needed for such electrolysis.
For instance the reversible potential of
oxygen gas with smooth Pt electrode is
1.23 V. But actual gas evolution takes place
at a potential of 1.68 V. The excess 0.45 V
is the over voltage of oxygen on smooth Pt
surface.
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Electrode surface.
Nature of the electrolyte.
Temperature.
Current density.
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Of the many steps involved in gas evolution
at electrodes, at least one is rather a slow
process and requires energy.
Electrolysis of water:
(i) H+ + e -----> H (fast)
(ii) H + H -----> H2(slow)
The step (ii) being slow, consumes energy
and accounts for overvoltage.
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Development of over potential in an
electrolytic cell is called polarization.
Electrolytic processes occur at the electrodes
of the cell.
Polarization sets in the cell when the
electrode processes become slow and
irreversible.
Magnitude of over potential is directly
proportional to the extent of polarization.
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The observed polarization can be of two
types.
*Concentration polarization.
*kinetic or activation polarization.
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An electrochemical reaction occurs only at
the surface of the two electrodes.
Adjacent to the electrode surface reactant
concentration decreases.
The reactant species are transported to the
electrode surface by processes like diffusion,
migration and convection from the bulk of
the solution.
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Current flowing through the cell drops when
the rate of transport is insufficient to match
the electrode reaction.
A potential than the theoretical value is
required to maintain the current at the
necessary level.
This phenomenon is called concentration
polarization.
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Steps:
*Diffusion of reactant species towards the
electrode.
*Diffusion of product away from the
electrode cause concentration polarization.
This can be minimized by:
* Increasing the convection.
* By raising the temperature.
* Mechanical agitation of the solution.
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This is caused when any of the following
steps become slow and need activation
energy.
*Adsorption of the reactant on the
electrode.
*Charge transfer across the metalsolution
interface.
*Desorption of the product from the
electrode
become slow and need
activation energy.
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A higher potential than the theoretical value
is to be applied to maintain current at the
necessary level.
It is not possible to eliminate or minimize
kinetic polarization by any means.
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Activation polarization is
*Low: When metal deposition or metal
dissolution is the electrode
process.
*High: When the electrode process is a
gas (H2 ,O2, etc) evolution,
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The electroplating device is essentially an
electrolytic cell, in which two electrodes,
anode and cathode are dipped in an
electrolyte solution.
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A d.c. electrical power source.
Anode : Inert material / coating metal itself.
Cathode : Article to be plated.
Electrolytic bath: Solution of conducting
salt, metal to be
plated
in a soluble form,
buffer
and additives.
Container: Made up of rubber lined steel,
wood or concrete.
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Anode : Made of coating metal.
Cathode: Article to be plated.
Electrolyte: CuSO4 solution.
CuSO4
Cu2+ + SO42*On passing current,
Cu2+ + 2e
Cu (Cathode)
Cu + SO42CuSO4 + 2e.(Anode)
Thus there is a continuous replenishment of
electrolytic salt during electrolysis.
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If the anode is made of some inert material
like graphite, then electrolytic salt is added
continuously in order to maintain proper
coating metal ion concentration in the bath.
The process of electroplating goes on
nonstop, since the anode is not consumed.
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The deposit should be bright and lustrous.
The deposit should be continuous, uniform,
non porous and adhesive.
It should be hard and ductile.
It should be of fine grained nature.
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Current Density:
An optimum current density should be applied.
If low- *leads to slow process
*results in coarse grained deposit.
If high- *leads to rough and treed deposit.
*results in burnt and spongy deposit.
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Metal salt and electrolyte concentration:
Optimum electrolyte concentration should be
maintained in bath to get a better deposit.
The advantages of using a strong solution:
*higher conductivity.
* cathode efficiency.
* Firm and adherent deposits.
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Temperature:
It should be between35oc and 65oc.
Lower temperature:
*reduces the solubility of the metal salt.
*reduces the mobility of the ions.
Higher temperature
*affects the lining of the equipment,
electrode surface and the substrate.
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Agitation:
Agitation of the solution brings up a fresh
supply of metal salts to the cathode.
It also sweeps away gas bubbles which may
otherwise cause pits.
Rapid agitation may
*detach the particles.
* stir up the sludge.
*results in rough and porous deposits.
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pH of the electrolytic bath:
*Suitable pH is maintained using
appropriate buffers.
In Ni plating borate buffer is used to maintain
pH 4.5.
At low pH more H2 evolution takes place on
the cathode.
At higher pH precipitation of hydroxides of
metal takes place.
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Addition agents:
The wide range of additives added in low
concentrations serve to electroplating bath
modify the structure and properties of the
electrodeposit.
Function:
Makes the deposit smooth, fine grained, hard and
adherent.
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Types of additives:
Complexing agent:
Eg: Cyanide, sulphamate ions.
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Brightners:
Eg: Aromatic sulphonates, thiourea.
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Wetting Agents:
Eg: Sodium lauryl sulphate.
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Levellers:
Produce a level deposit by getting adsorbed
at places where rapid deposition of metal
takes place.
Structure Modifiers:
These additives change the structure of the
deposit.
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Throwing power:
It is used as a measure of the distribution
of the deposit .
This property is particularly important when
the article is of an irregular shape.
This can be improved by:
* separating the distances between the
electrodes.
* agitating the solution to minimize high
local electrolytic resistance.
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Metal ion concentration:
Low metal ion concentrations give adherent
coating films.
This can be achieved by:
* addition of a compound with a common
ion.
* formation of complex compounds.
Eg: Double cyanides of Na or K.
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A good adherent deposit is obtained only if
the base metal surface is free from dirt and
extraneous matter.
 The following methods are employed to
clean the metal surfaces.
 Solvent cleaning: To dissolve oils and
greases. Eg: CCl4,CH2Cl2,CHCl=CCl2.
 Alkali cleaning: To remove minute organic
residues.Eg: NaOH,Na2CO3,Na3PO4.
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Mechanical Cleaning: Involves removal of
the oxide layer or rust and other inorganic
deposits. Eg: Impact tools like sandpaper,
chisels, knife scrapers, wire brushes etc.
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Pickling: Removal of oxide films by means
of an acid. Eg: 10--30% sulphuric acid is used
for this acid dip. It is again rinsed with hot
water.
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Electropolishing: The metal to be cleaned iss
made as anode in a suitable acid solution.
*A surface layer of the metal gets dissolved along
with the impurities.
*This helps to remove surface irregularities.
*The metal is then thoroughly rinsed with water,
dried and used for electroplating.
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Better appearance.
Protection against corrosion.
To achieve desired engineering effect.
Plating on non-metallics.
Electroforming.
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Two types of chromium plating:
*Decorative Chromium:- A thin deposit of Cr
(0.25 --0.75/um ) is applied over either
copper-nickel or nickel undercoat.
*Hard Chromium:- Involves deposition of a
thick coating of chromium directly over the
substrate. The usual thickness range of hard
chrome is 2.5---300/um.
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Bath composition- 250g chromic acid +
2.5g H2SO4 per liter
of the solution and 1g
trivalent chromium
Operating temperature - 45--55oC.
Current density - 145--430 A/ft2
Anode - Insoluble anodes like Pb, Pb6%Sb,Pb--7% Sn.
Cathode - Article to be plated
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Bath composition- 250g chromic acid +
2.5g H2SO4 per liter
of the solution and 1g
trivalent chromium
Operating temperature - 50--55oC.
Current density – 290--580 A/ft2
Anode - Insoluble anodes like Pb, Pb6%Sb,Pb--7% Sn.
Cathode - Article to be plated
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In the case of chromium plating inert
anodes are used
*Reason:
The metal going into the solution is five
times of the metal getting deposited,
resulting in the building up of excessive
chromic acid (CrIII) concentration.
This leads to imbalance of bath
composition.
Chromium metal becomes passive in the
strongly acidic bath and a black deposit is
formed on the cathode.
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Chromium plating produces an attractive,
wear resistant and abrasion resistant
surface.
Chromium plating is not continuous; it is
micro porous. Hence an undercoat of
copper and/or nickel is essential.
Applications:
*Decorative chromium provides a durable
finish (on automobiles, furniture, surgical &
dental instruments).
*Hard chromium is applied to many
components in almost every industry.
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It is a method of depositing a metal or alloy
over a substrate (conductor or nonconductor)
by controlled chemical reduction of the metal
ions by a suitable reducing agent without
using electrical energy.
Metal + Reducing agent ---
Metal+ Oxidized product.
*It is also termed as autocatalytic plating.
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The surface to be plated should be
catalytically active.
The catalytic metals like Ni, Co, Steel, Pd etc.,
do not require any surface preparation.
Non catalytic like such as Cu, brass, Ag etc.,
need activation.
*This can be done by dipping in palladium
chloride solution.
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Non conductors like glass, plastics, etc., are
first activated in a solution of SnCl2 and HCl.
After rinsing its immersed in a solution of
PdCl2 and HCl.
This treatment yields a thin layer of Pd on the
treated surface.
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Soluble salt of the metal to be plated.
Reducing agent.
A buffer for pH control.
A complexing agent.
Stabilizer is added to prevent. decomposition
of plating bath.
Accelerator to increase the rate of plating.
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No electrical energy is required.
Better throwing power.
Plating on articles made of insulators and
semiconductors.
Electro less plated coatings possesses unique
mechanical chemical properties.
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Composition of bath:
Coating solution - CuSO4 solution (12g/l)
Reducing agent - HCHO (8 g/l)
Buffer - NaOH (15 g/l) and Rochelle
salt(14g/l)
Complexing agent- EDTA di sodium salt
(20g/l)
Optimum pH 11
Optimum temperature - 250C.
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Reactions:
Cu2+ + 2e -----------> Cu. (Cathode)
2 HCHO + 4OH- ------> 2HCOO- +2H2O +
H2+ 2e
(Anode)
--------------------------------------------------------------Cu 2+ + 2HCHO + OH- -----> 2HCOO- +2H2O +
H2
--------------------------------------------------------------*Plating rate is 1-5/u mh-1.
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Used for metalising printed circuit boards.
For producing through-hole connection
necessary when double sided printed circuit
boards are fabricated.
For plating on non conductors.
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Pre treatment and activation of the Surface:
Etching with Chromic acid-Sulphuric acidWater to modify the surface by roughening
and making it hydrophilic for better
adhesion.
Neutralizing to remove all chromium from
the surface.
Catalysis-by immersing in a stabilized
concentrated solution of stannous chloride
and Palladium chloride in conc.HCl.
Usually 1-5micrograms of Pd/sq.cm is
adsorbed on the surface.
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Soluble salt of the metal to form the film.
(sulphates, chlorides)
Reducing agent such as sodium
hypophosphite or formaldehyde.
Buffer to maintain pH (8 to 10)
Complexing agent -- (succinates, citrate, or
tartarate of Na, K, disodium salt of EDTA)
Accelerator---- (NaF or CaF2)
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A plating time of 5 to 20 minutes gives a
film of 0.15 ---0.50 /um thickness, thus
giving it a metallic look.
A typical ABS plastic with a bright chrome
finish has usually three coats -Cu base
plating-15 /um.
Ni coat 15 --25/um.
Top Cr film 0.25/um.
This has an advantage of weight savings of
60% compared to an all metal part.
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Three methods of fabrication of PCBs:
Subtractive process.
Semi additive process.
Additive process.
Printed circuit boards are made of epoxy
polymers, phenolics or any other heat stable
rigid polymer.
Flexible films of polyesters and polyamides
are also used.
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Uses a flat board of 300-350/u m thickness.
.Holes are drilled whereever through
connection is required.
Cleaned and catalysed board is kept in a
flash electroless Cu bath, to obtain athin Cu
film (0.5 to 1 /u m) on the entire board.
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Board is next electrolytically coated to give
25/um thick Cu film on both sides.
Positive masking of the circute is done by
painting it with photoresist paint.
Unwanted Cu is then etched away using
etchants like chromic acid CrO3-H2SO4,
FeCl3-HCl.
Photoresist paint is then washed away to give
desired circuit.
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Thinner 25/um Cu foils are used to laminate
the epoxy board on both sides.
Drilling holes , catalysing on electroless Cu
plating is done as in the subtractive
procedure.
Circuit is painted over with photoresist paint
and rest of Cu is etched away.
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No etching away excess Cu is needed.
The epoxy board is not laminated with Cu
foils on either sides.
All the rest of the board is covered with
negative photo resist paint except where Cu
circuit is to be obtained.
The board is then plated with electroless Cu
until the thickness of the circuit is 25-50/u
m.
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The PCBs are used in:
*Communications.
*Instrument controls.
*Consumer market items (T.V.s,
transistors, CD/DVD players etc.,).
* Military and aerospace equipment etc.
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Costs more/unit weight of the deposited
metal.
It needs pure chemicals.
Chemical reductants are more expensive
than electricity.
Metal salts and reductant used in electro
less plating solutions are
thermodynamically not stable.
Impurities or dust or colloidal matter even if
present in trace amounts promote
decomposition of bath components.
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Electroplating
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Electrical power
is needed.
Less throwing
power.
Driving force is
electrolysis by
electricity.
surface.
Electrolessplating
No electrical
power is needed.
Better throwing
power.
Driving force is
autocatalytic
redox reaction
over active
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