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
Lecture 4 of 5
Electroless Deposition of
Coating
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
Typical thickness vs. time profiles
Deposit
thickness
Electroplating
Electroless deposition
Immersion deposition (thin, porous deposits?)
0
0
Time
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
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
anode
Fe2+
Fe
cathode
Cu2+
Cu
Cu
Overall reaction
Cu2+ + Fe  Fe2+ + Cu
Ecell = Ecathode  Eanode = 0.737 V
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.
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
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
Other examples of reducing agents
• Formaldehyde
• Hypophosphorus acid
• Alkaline borohydrides
• Alkaline diboranes
• Reducing agent
– Hypophosphite ion = H2PO2• Additives
– Wetters, stabilisers, exhaltants, levellers, brightners, stress
modifiers…
Typical Recipe and Conditions
Acid Ni-P
Component
Nickel chloride
Sodium hypophosphite
Sodium acetate
Sodium succinate
Temperature
pH
Concentration/g L-1
20
20
10
15
90 C
4.5
Which Common Metals are
Electroless Deposited?
Copper
- for e- conductive printed circuit tracks
Nickel-Phosphorus (3-15%wt 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
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
Overall:
orthophosphite ion
Ni2+ + H2PO2- + H2O = H2PO3- + 2H+
Spontaneous reaction: DGo =  48 kJ mol-1
Gibbs free energy change, DGcell
DGcell =  n F Ecell
DGcell > 0 , no spontaneous reaction
DGcell < 0 , spontaneous reaction
n = number of electrons
F= Faradays constant, 96485 C mol-1
Ecell = Ecathode  Eanode
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.
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.
Porosity in electroless
Ni-P deposits (<5 mm) on mild steel
2 mm
SEM image showing a
branched network pore
60 mm
Optical micrograph showing a pore
which reveals the steel substrate
Log-log Porosity vs. thickness for
electroless Ni-P deposits on steel
% P o ro s ity
100
10
1
1
D e p o s it th ic k n e s s /m m
10
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 < 5 metal turnovers
– Spontaneous decomposition can occur – ‘bombing out’
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
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.
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