Outline Curriculum (5 lectures) Each lecture ? 45 minutes

1
Outline Curriculum (5 lectures)Each lecture ? 45
minutes

• Lecture 1 An introduction in electrochemical
  coating
• Lecture 2 Electrodeposition of coating
• Lecture 3 Anodizing of valve metal
• Lecture 4 Electroless deposition of coating
• Lecture 5 Revision in electrochemical coating

2
Lecture 4 of 5Electroless Deposition of Coating
3
Electroless deposition

• Involves the oxidation of a soluble reducing
  agent which supports the cathodic deposition of
  metal on a catalytic surface
• Electroless deposition this process uses only
  one electrode and no external source of electric
  current.
• Electroless deposition the solution needs to
  contain a reducing agent so that the reaction can
  proceed
• Metal ion Reduction solution

Catalytic surface
Metal solid oxidation solution
4
Typical thickness vs. time profiles
Deposit thickness
Electroplating
Electroless deposition
Immersion deposition (thin, porous deposits?)
0
0
Time
5
Types of Metal Deposition

• Electroless deposition
• E.g., nickel deposits. open-circuit using a
  reducing agent
• Electroplating
• E,g, nickel deposited at cathode using external
• d.c. power supply
• Immersion deposition
• E.g., steel nail in copper sulfate, open-circuit,
  displaces copper metal from solution onto nail

6
Immersion deposition

• A displacement reaction occurs on the surface of
  the anode.
• The work piece (anode) dissolves to metal ions.
  Metal ions in solution deposits at the cathode,
  in the absence of an external power source.
• This is a spontaneous reaction, driven by the
  electrode potential of the reaction.

Cu2 2e ? Cu E 0.337 V vs. SHE Fe2 2e
? Fe E ? 0.440 V vs. SHE Overall
reaction Cu2 Fe ? Fe2 Cu Ecell Ecathode ?
Eanode 0.737 V
anode
cathode
Fe
Cu
Fe2
Cu2
Cu
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Limitation of immersion deposition

• The deposits properties are difficult to control
  and the deposit may be porous and poorly
  adherent.
• The rate of deposition declines with time and
  ceases when the steel surface is completely
  covered with copper.
• Hence, electroless deposition of metal is more
  favourable. But, the surface needs to be
  catalytically activated in order for the metal
  deposits to form.

8
What is the Job of the Bath?

• Provides an electrolyte
• to conduct electricity, ionically
• Provides a source of the metal to be plated
• as dissolved metal salts leading to metal ions
• Contains a reducing agent
• To reduce metal ions to metal
• Wets the cathode work-piece
• allowing good adhesion to take place
• Helps to stabilise temperature
• acts as a heating/cooling bath

9
Typically, What is in a Bath?E.g., Electroless
Ni-P

• Ions of the metal to be plated, e.g.
• Ni2 (nickel ions) added as the chloride
• Conductive electrolyte
• NiCl2, H2PO2-, CH3COO-
• Complexant
• Acetate, succinate
• Reducing agent
• Hypophosphite ion H2PO2-
• Additives
• Wetters, stabilisers, exhaltants, levellers,
  brightners, stress modifiers

• Other examples of reducing agents
• Formaldehyde
• Hypophosphorus acid
• Alkaline borohydrides
• Alkaline diboranes

10
Typical Recipe and ConditionsAcid Ni-P

• Component Concentration/g L-1
• Nickel chloride 20
• Sodium hypophosphite 20
• Sodium acetate 10
• Sodium succinate 15
• Temperature 90 C
• pH 4.5

11
Which Common Metals are Electroless Deposited?

• Copper
• - for e- conductive printed circuit tracks
• Nickel-Phosphorus (3-15wt P)
• - for corrosion resistance on, e.g., steel or
  Al
• Ni-P PTFE particles
• - for self-lubricating/anti-stick coatings
• Ni-P SiC particles
• - for wear resistance

12
The Electrochemical reactions

• An open-circuit, redox process taking place
  spontaneously on a single autocatalytic
  substrate.
• Cathodic Ni2 2e- Ni
• Anodic H2PO2- H2O - 2e- H2PO3- 2H
• hypophosphite ion orthophosphite ion
• Overall Ni2 H2PO2- H2O H2PO3- 2H
  

Spontaneous reaction DGo ? 48 kJ mol-1
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Gibbs free energy change, ?Gcell

• ?Gcell ? n F Ecell

?Gcell gt 0 , no spontaneous reaction
?Gcell lt 0 , spontaneous reaction
n number of electrons F Faradays constant,
96485 C mol-1 Ecell Ecathode ? Eanode
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Hydrogen Embrittlement

• To describe the presence of hydrogen in metal
  deposit.
• In electroless deposition or electroplating, H
  atom or H2 molecules could be entrapped or
  absorbed into the metal deposits.
• Induces a high physical stress in the coating.
• .
• Coatings may delaminate from the substrate or
  crack.
• Reduce the mechanical properties of coating.

15
Some important characteristics for electroless
deposition

• The substrate metal and the deposited metal must
  support the electrode processes in a catalytic
  manner.
• The process must be operated so as to avoid
  spontaneous decomposition of the electrolyte or
  onto the tank surfaces.
• A pH decrease accompanies the overall process.
• The reducing agent depletes its oxidation
  product accumulates.
• The source of metal, e.g. Ni2 declines in
  concentration.
• In practice, the deposit is usually an ally, e.g.
  Ni-P, showing that the previous reactions are
  oversimplified.

16
Porosity in electroless Ni-P deposits (lt5 mm) on
mild steel
60 mm
2 mm
SEM image showing a branched network pore
Optical micrograph showing a pore which reveals
the steel substrate
17
Log-log Porosity vs. thickness for electroless
Ni-P deposits on steel
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Properties of Electroless Deposition

• Must have an autocatalytic substrate
• To allow deposition to initiate and continue
• Constant deposition rate with time
• Typically 10-15 micron per hour
• Uniform deposit thickness
• Even on complex shapes
• Baths require good analytical control
• To maintain deposit thickness and composition
• Baths have a short lifetime
• Can be lt 5 metal turnovers
• Spontaneous decomposition can occur bombing
  out

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Applications of Electroless Deposition include

• Printed circuits and resistors
• Temperature sensors
• Valves for fluid handling
• Moulds for plastic and glass
• Gears, crankshafts and hydraulic cylinders
• Magnetic tapes
• Coatings on aluminium (to enable this metal to be
  soldered)
• Corrosion-resistant coatings for components or
  structures exposed to atmospheres or
  immersed in fresh or sea water
• Plating on plastics, e.g., car door handles and
  marine hardware

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More application of electroless deposition

• Oil Gas
• Valve components, such as Balls, Gates, Plugs
  etc. And other components such as pumps, pipe
  fittings, packers, barrels etc.
• Chemical Processing
• Heat Exchangers, Filter Units, pump housing and
  impellers, mixing blades etc.
• Plastics
• Molds and dies for injecting and low and blow
  molding of plastics components, extruders,
  machine parts rollers etc.
• Textile
• Printing cylinders, machine parts, spinneret's,
  threaded guides
• Automotive
• Shock Absorbers, heat sinks, gears, cylinders,
  brake pistons etc.
• Aviation Aerospace Satellite and rocket
  components, rams pistons, valve components etc.
• Food pharmaceutical Capsule machinery dies,
  chocolates molds, food processing machinery
  components etc.

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Summary

• Electroless deposition provides important,
  speciality
• (e.g., Ni-P based) coatings on steel or aluminium
  or
• Cu printed circuit board tracks
• High degree of control over deposit thickness
• By controlling bath chemistry, temperature and
  time.
• The process requires no external current
• But is more expensive than electroplating
• The substrate must be made autocatalytic
• For deposition to start and continue
• The throwing power is very good
• uniform coatings, even on screw threads