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 solutionCatalyticsurface

Metal solid + oxidation solution

Typical thickness vs. time profiles

Depositthickness

Time00

Electroplating

Electroless deposition

Immersion deposition (thin, porous deposits?)

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.

Fe CuCu2+

anode cathode

Cu

Fe2+

Cu2+ + 2e CuE = + 0.337 V vs. SHE

Fe2+ + 2e FeE = 0.440 V vs. SHE

Overall reactionCu2+ + 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

• 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

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

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

An open-circuit, redox process taking place An open-circuit, redox process taking place

spontaneously on a single autocatalytic substrate.spontaneously on a single autocatalytic substrate.

Cathodic:Cathodic: NiNi2+2+ + 2e + 2e-- = Ni = Ni

Anodic:Anodic: HH22POPO22-- + H + H22O - 2eO - 2e-- = H = H22POPO33

-- + 2H + 2H++

hypophosphite ionhypophosphite ion orthophosphite ion orthophosphite ion

Overall:Overall: NiNi2+2+ + H + H22POPO22-- + H + H22O = HO = H22POPO33

-- + +

2H2H++

The Electrochemical reactions

Spontaneous reaction:Go = 48 kJ mol-1

Gibbs free energy change, Gcell

Gcell = n F Ecell

Gcell > 0 , no spontaneous reaction

Gcell < 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 m) on mild steel

2 m 60 m

SEM image showing a branched network pore

Optical micrograph showing a pore which reveals the steel substrate

Log-log Porosity vs. thickness for electroless Ni-P deposits on steel

1

10

100

1 10

Deposit thickness/m

% P

oro

sit

y

• 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’

Properties of Electroless Deposition

• Printed circuits and resistors Printed circuits and resistors

• Temperature sensorsTemperature sensors

• Valves for fluid handlingValves for fluid handling

• Moulds for plastic and glassMoulds for plastic and glass

• Gears, crankshafts and hydraulic cylindersGears, crankshafts and hydraulic cylinders

• Magnetic tapesMagnetic tapes

• Coatings on aluminium (to enable this metal to be soldered)Coatings on aluminium (to enable this metal to be soldered)

• Corrosion-resistant coatings for components or structures exposed Corrosion-resistant coatings for components or structures exposed

to atmospheres or immersed in fresh or sea waterto atmospheres or immersed in fresh or sea water

• Plating on plastics, e.g., car door handles and marine hardwarePlating on plastics, e.g., car door handles and marine hardware

Applications of Electroless Deposition include

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.

• 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

Summary