Download METAL FINISHING

Metal Finishing
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.
OBJECTIVES
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.
Improving resistance to corrosion, scratch
or abrasion and wear.
Enhancing decorative appearance.
Improving solder ability and electrical
properties
Reducing frictional loss etc.
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.
Techniques of metal finishing:
Electroplating of metals & alloys.
Electro less plating of metals & alloys.
ELECTROPLATING
• 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.
Factors Governing Electroplating:
 Decomposition Potential
 Polarization
 Over voltage
DECOMPOSITION POTENTIAL.
• 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.
Reason:
• 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.
Measurement of ED
• Gradually increasing voltage is applied
between the electrodes.
• Milliammeter reading is noted down for
every voltage applied.
Factors Influencing ED
• 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.
Uses of decomposition potential
• 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.
OVER VOLTAGE
• 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.
Definition
• 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.
Factors Affecting Over voltage:
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Electrode surface.
Nature of the electrolyte.
Temperature.
Current density.
• 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.
POLARIZATION
• 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.
Types Of Polarization:
The observed polarization can be of
two types.
*Concentration polarization.
*kinetic or activation polarization.
CONCENTRATION
POLARIZATION
• 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.
• 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.
• 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.
ACTIVATION POLARIZATION
• 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 metal-solution
interface.
*Desorption of the product from the electrode
become slow and need activation energy.
• 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.
• 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,
Theory Of Electroplating.
• The electroplating device is essentially an
electrolytic cell, in which two electrodes,
anode and cathode are dipped in an
electrolyte solution.
Essential Components.
•
•
•
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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.
Theory
• 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.
• 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.
Characteristics Of A Good Deposit.
• 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.
Factors Influencing The Nature Of
Deposit.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Types of additives:
• Complexing agent:
Eg: Cyanide, sulphamate ions.
• Brightners:
Eg: Aromatic sulphonates, thiourea.
• Wetting Agents:
Eg: Sodium lauryl sulphate.
• 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.
• 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.
• 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.
Methods of cleaning the metal
surface.
• 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.
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.
 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.

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.
Applications.
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Better appearance.
Protection against corrosion.
To achieve desired engineering effect.
Plating on non-metallics.
Electroforming.
Electroplating of Chromium.
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.
Decorative chromium.
• 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
Hard chromium.
• 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
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.
• 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.
Electro less Plating.
• 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.
• 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.
• 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.
Composition of Electroless bath.
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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.
Advantages of Electroless Plating.
• 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.
Electro less Plating of Copper.
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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.
• 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.
Applications.
• 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.
Electroless Plating on Plastics.
• Pre treatment and activation of the Surface:
• Etching with Chromic acid-Sulphuric acid-Water
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.
Composition of the bath.
• 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)
• 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.
Printed Circuit board (PCB)
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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.
Subtractive Preocessing
• 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.
• 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.
Semi Additive Process
• 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.
Additive Process.
• 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 2550/u m.
Applications.
• The PCBs are used in:
*Communications.
*Instrument controls.
*Consumer market items (T.V.s,
transistors, CD/DVD players etc.,).
* Military and aerospace equipment etc.
Major drawbacks.
• 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.
Electroplating
 Electrical power
is needed.
 Less throwing
power.
 Driving force is
electrolysis by
electricity.
Electrolessplating
No electrical
power is needed.
Better throwing
power.
Driving force is
autocatalytic
redox reaction
over active surface.