ENGINEERING CHEMISTRY: Module 2-corrosion & its control - metal finishing

CORROSION & ITS CONTROL
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ENGG.CHEMISTRY [RM] Page 1
MODULE-2
Definition of corrosion:
Corrosion is defined as the destruction of metals or alloys by the surrounding environment through chemical or
electrochemical reaction.
Example:
i. Formation of rust on the surface of iron,
ii. Formation of green film on the surface of copper.
Corrosion is also called as extractive metallurgy in reverse.
Types of corrosion:
DRY CORROSION: Dry corrosion occurs due to direct chemical reaction between the metal and the gasses
present in the corrosive environment.
Example: Metals when exposed to dry gasses like O2, SO2, CO2, H2S etc.
WET CORROSION: It is a common type of corrosion of metal in aqueous corrosive environment. This type of
corrosion occurs when the metal comes in contact with a conducting liquid or when two dissimilar metals are
immersed or dipped partly in a solution.
Electrochemical theory of corrosion:
According to electrochemical theory, when a metal such as iron is exposed to corrosive environment, following
changes occur.
1. Formation of galvanic cells: A large number of tiny galvanic cells with anodic and cathodic regions are
formed.
2. Anodic reaction: Oxidation of metal takes place at the anodic region.
Fe  Fe2+
+ 2 e-
The Fe2+
ions dissolve, so corrosion takes place at the anodic region.
3.The electrons travel through the metal from the anodic region to cathodic region.
OH- OH-Fe2+
Fe2+
IRON METAL
O2
H2O
Electrons
CATHODEANODE RUST

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MODULE-2
4. Cathodic reaction : Reduction of O2 or H+
takes place at the cathodic region.
In acidic medium,
2H+
+ 2e-
H2
In neutral medium,
O2 + 2H2O + ne-
4OH-
The metal is unaffected at the cathodic region.
5. Fe2+
and OH-
ions travel through the aqueous medium and form corrosion product.
Fe2+
+ 2OH-
Fe(OH)2
6. The corrosion product may undergo further oxidation to form rust.
2Fe(OH)2 + 11/2 O2 + H2O Fe2O3.3H2O
[Yellow rust]
The cathodic and anodic reactions must occur at the same rate. If 𝑖 𝑐𝑜𝑟𝑟 is the current (corrosion current) flowing ,
then the rate of corrosion of iron is given by the equation,
𝑅𝑎 𝑖 =
𝑖 𝑐𝑜𝑟𝑟
𝐹
𝑎
𝑤ℎ 𝑖 ℎ𝑎 ℎ 𝑖 𝑎 𝐹 𝑖 𝑎 𝑎 𝑎𝑦 𝑎
The total current due to the cathodic reaction [ ∑ 𝑖 𝑐] must be equal ,but opposite in sign , to the total current
flowing out due to the anodic reaction [ − ∑ 𝑖 𝑎] .
𝑖 𝑐𝑜𝑟𝑟 = − ∑ 𝑖 𝑎 = ∑ 𝑖 𝑐
Reactions at cathodic region: At cathode, the reaction is either a) liberation of hydrogen or b) absorption of
oxygen.
Liberation of hydrogen (in the absence of oxygen) Absorption of oxygen(in the presence of oxygen)
In acidic medium
2H+
+ 2e-
H2
In acidic medium,
4H+
+ O2 + 4e-
2H2O
In neutral,
2H2O + 2e-
2OH-
+ H2
In neutral,
O2 + 2H2O + ne-
4OH-

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MODULE-2
Galvanic Series:
Galvanic series is a series in which the metals and alloys are arranged in the order of their corrosion tendencies
or corrosion resistance.
According to galvanic series,
1. The metal/alloy higher up in the series corrodes faster than the metal/alloys in the bottom of the series.
2. Metals like Ti (placed below Ag in galvanic series but above in emf series) and Al (placed below Zn in
galvanic series but above in emf series) exhibit resistance to corrosion due to phenomenon called Passivation.
3. Passivation: It is the phenomenon of protection of metals against atmospheric corrosion due to
formation of a thin layer of non-porous film of metal oxide. The film forms a barrier between the
corrosive medium and metal [protective layer].
Comparison between Galvanic Series Vs Electrochemical Series:
# Galvanic Series Electrochemical Series
1 It predicts the corrosive tendencies of metal alloys It predicts the relative displacement tendencies
2
Calomel electrode is used as a reference electrode
Standard hydrogen electrode is used as reference
Electrode
3 Positioning of metal or alloy may change Position of metal is fixed. That cannot be changed
4 The metals and alloys are immersed in the sea
water for study
concentration of salts of the same metal that was being used
5 Electrode potentials are measured for both metals
and alloys.
Electrode potentials measured only for metals and non-metals

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MODULE-2
Different types of corrosion:
Differential metal corrosion:
This type of corrosion occurs when two dissimilar metals are in contact with each other and are exposed to a
corrosive environment. The two metals differ in their electrode potentials. The metal with lower electrode
potential acts as anode and the other metal with higher electrode potential acts as cathode. A galvanic cell
develops between the two metals.
The anodic metal undergoes oxidation and gets corroded. A reduction reaction occurs at the cathodic metal. The
cathodic metal does not undergo corrosion.
The reactions may be represented as follows:
Cell reactions:
At anode : M
At cathode : O2 + 2H2O + 4e-
4OH-
(Reduction of oxygen)
Mn+
+ ne-
(Oxidation of metal M)
Higher the potential difference between the anodic and cathodic metals, higher is the rate of corrosion.
Other examples:
1.Steel screws in copper sheet.
2.Steel screws with copper washer.
3.Bolt & nut are made of different metals.
Differential aeration corrosion:
This type of corrosion occurs when two different parts of the same metal are exposed to different oxygen
concentrations. (e.g. An iron rod partially dipped in water.) The part of the metal which is exposed to less
oxygen concentration acts as anode. The part which is exposed to more oxygen concentration acts as cathode.
The anodic region undergoes corrosion and the cathodic region is unaffected.
Zn metal
[Anode]
Fe metal
[Cathode]
Fe metal
[Anode]
Cu metal
[Cathode]
Fe metal
[Anode]
Sn metal
[Cathode]
Less O2, (Anode)
Water
More O2,
(Cathode)
Iron rod

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Cell reactions:
At anode : M
4OH-
Mn+
+ ne-
Other examples:
1. Part of the nail inside the wall undergoes corrosion.
2. When a dirt particle sits on a metal bar, the part under the dirt undergoes corrosion.
3. Partially filled iron tank undergoes corrosion inside water.
Water line corrosion: This is an example of differential aeration corrosion.
When a steel tank is partially filled with water for a long time, the inner portion of the tank below the water line
is exposed only to dissolve oxygen, whereas, the portion above the water line is exposed to more oxygen. Thus
the portion below the water line acts as anode and undergoes corrosion. The upper portion acts as cathode and is
unaffected.
A distinct brown line is formed just below the water line due to the deposition of rust.
Cell reactions:
At anode : M
4OH-
Mn+
+ ne-
Other example: Ships which remain partially immersed in sea water for a long time undergo water line
corrosion.
Pitting corrosion: This is an example of differential aeration corrosion.
Rust
Water
More
oxygen,
(Cathode)
Less
Oxygen
(Anode)

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MODULE-2
When a small dust particle gets deposited on a steel surface, the region below the dust particle is exposed to less
oxygen compared to the remaining part. As a result, the region below the dust particle acts as anode undergoes
corrosion and forms a pit. The remaining region of the metal acts as cathode and is unaffected.
Cell reactions:
At anode : M
4OH-
Mn+
+ ne-
Formation of a small anodic area and a large cathodic area results in intense corrosion below the dust
particle.
Stress corrosion: In a metallic structure, if there is a portion under stress, it will act as anode and
rest part of the structure will act as cathode. It is now a galvanic system and hence anodic part which is small
in area will corrode more.
Example: Caustic embrittlement in boilers -
1. It is a type of boiler corrosion which makes boiler material brittle. This is caused by using highly
alkaline water in the boiler, most commonly in high pressure boiler. During lime soda process, free
Na2CO3 is usually present in small proportion in the softened water.
2. Na2CO3 in high pressure boilers decomposes to give sodium hydroxide and carbon dioxide. This
makes boiler water caustic.
Na2CO3+H2O2NaOH+CO2
3. This causes embrittlement of boiler parts, particularly stressed parts like bends, joints etc.
4. The water containing NaOH flows into the minute hair-cracks, in the inner wall of boiler, by capillary
action. Here, water evaporates and the concentration of NaOH increases progressively. When the
concentration of NaOH increases to 10%, caustic soda attacks the surrounding areas, thereby dissolving
iron of boiler wall as sodium-ferroate. This causes embrittlement of boiler wall at stressed parts like bends,
joints, etc.
3Na2FeO2 + 3H2O Fe3O4 + H2 + 6NaOH
Addition of Na2SO4 and phosphates to boiler water prevents caustic cracking.
Cathode
Boiler
Soft water =
Very dilute NaOH
Anode

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Factors affecting the rate of corrosion:
1. Nature of the metal: Metals with lower electrode potentials are more reactive and are more susceptible
to corrosion. For example, elements such as Mg and Zn, which have low electrode potentials, are highly
susceptible to corrosion. Noble metal such as gold and platinum, which have higher electrode potentials, are less
susceptible to corrosion.
Exceptions: Metals and alloys which show passivity are exceptions for this general trend. Such metals form a
protective coating on the surface which prevents corrosion.
2. Nature of corrosion product:
If the corrosion product [OXIDE LAYER] is insoluble, stable and non-porous, then it acts as a protective
film which prevents further corrosion. The film acts as a barrier between the fresh metal surface and the
corrosive environment.
On the other hand, if the corrosion product is soluble, unstable and porous, then the corrosion process continues
even after the formation of corrosion product.
Example: Aluminium, titanium and chromium form a protective film of metal oxide on the surface. Stainless
steel forms a protective film of Cr2O3 on the surface. But in the case of Zn and Fe, the corrosion products
formed do not have protective value.
3. Difference in potential between anodic and cathodic regions: Larger the potential
difference between the anodic and cathodic regions, higher is the rate of corrosion. For example, the potential
difference between iron and copper is 0.78 V, and between iron and tin is 0.3 V. Therefore, corrosion is faster
when iron is in contact with copper.
The use of dissimilar metals should be avoided wherever possible. Otherwise, the anodic metal gets corroded.
4. Anodic and cathodic areas:
Smaller the anodic area and larger the cathodic area exposed to corrosive atmosphere, more intense and faster is
the corrosion occurring at anode.
When anode is smaller and cathode region is larger the liberated electrons at anode are rapidly consumed. If the
cathode is smaller and reverse process takes place, decrease rate of corrosion. .
𝑪 𝑹𝒂 =
𝑪𝒂 𝒂 𝒂
𝑨 𝒂 𝒂
Larger the anodic area and smaller the cathodic area, decreases the rate of corrosion.
Ex: A small steel pipe fitted to copper tank, increases the rate of corrosion.

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MODULE-2
5. pH
of the medium: Rate of corrosion increases with decrease in pH.
a) Metals do not undergo corrosion at pH
greater than 10. This is due to the formation of protective
coating of hydrous oxides of iron.
b) Between pH
10 and 3, the presence of oxygen is essential for corrosion.
c) If the pH
is less than 3, corrosion occurs even in the absence of oxygen.
6. Temperature: Higher the temperature, higher is the rate of corrosion.
Increase in temperature increases the ionic conductivity of the corrosive medium. This also contributes to
the increase in corrosion rate.
7. Conductance: As the conductivity of the corrosion medium increases, the corrosion rate also increases.
Higher the conductivity of the medium, faster the ions can migrate between the anodic and cathodic regions
of the corrosion cell, in turn, faster will be the exchange of electrons at the electrode surfaces. This
facilitates higher corrosion rate.
Corrosion control
1. Anodizing (Anodizing of aluminum):
When aluminum metal is made anodic in an electrolytic bath with sulphuric acid or
chromic acid as the electrolyte, a thin layer of aluminium oxide (Al2O3) is formed on the
surface. This process is called anodizing of aluminium or anodic oxidation of aluminum.
Anodizing is carried out as follows:
The article is made as anode and steel or copper is made as cathode. The electrodes are dipped in a
solution of 5 – 10% chromic acid, the temperature of the bath is maintained at 350
c. A potential is applied and
gradually increased from 0 to 40V during the first 10 min. Anodizing is carried out for 20 min at 40V. After 20
min, the potential is increased to 50V and held at this potential for 5min. An opaque layer of 2-8 µm thickness is
obtained.
Anodized aluminium is exposed to a corrosive
environment, the Al2O3 layer on the surface acts as a
protective coating. Hence corrosion is prevented.
Other metals such as Mg, Ti etc. can also be
anodized.
Electrolyte 5-10% of chromic acid
Temperature 350
c
Thickness of oxide layer 2-8µm
Dc Power
Al2O3 H2CrO4
Cathode
Anode Al

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(Note: On anodizing, Al2O3 is formed on the surface as a porous layer. The layer may be made compact
by sealing, which involves heating with boiling water or steam. During sealing, Al2O3 is converted into
Al2O3.H2O which occupies higher volume. Therefore, the pores are sealed.)
Applications: Anodized aluminium is used in computer hardware, roofs, floor, ceilings, curtains, escalators and
commercial buildings.
2. Phosphating: Phosphating is a process of Conversion of surface metal atoms into their phosphates by
chemical or electrochemical reactions is called phosphating.
The phosphating bath contains three essential components:
(i) free phosphoric acid,
(ii) a metal phosphate such as Fe, Mn phosphate and
(iii) An accelerator such as H2O2, nitrites, nitrates.
(iv) Temperature – 35o
C
(v) pH – 1.8-3.2
Phosphating not only improves the corrosion resistance but also imparts good paint adhesion quality to the
surface.
Applications: Phosphate coating is given as an under layer [primer coat] before painting the car bodies,
refrigerators and washing machines.
Metal coatings:
Anodic metal coating: It is a process of coating of base metals with anodic metals such as Zn, Al, Mg, and
Cd etc.
Example: Galvanizing
Galvanizing of iron : Galvanizing is the process of coating a metal surface such as iron with zinc metal.
Galvanization is carried out by hot dipping method. It involves the following steps.
Iron
sheet
Dil .H2SO4
Organic
Solvent
Water Molten Zinc +
NH4Cl (flux)
At 420 -5000C
Air drier
Pair of hot
rollers
Excess of Zn
Galvanized
sheet

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i) The metal surface is washed with dilute sulphuric acid.(Pickling process) to remove ant dirt ,rust on the
surface. [descaling].
ii) Oil, grease is removed by washing organic solvents (CCl4, toluene) [degreasing].
iii) Finally, the article is washed with water and air-dried.
iv) The article is then dipped in a bath of molten zinc. (Molten zinc is covered with a flux of ammonium
chloride to prevent the oxidation of molten zinc.).
v) The excess zinc on the surface is removed by passing through a pair of hot rollers.
Application: Galvanization of iron is carried out to produce roofing sheets, fencing wire, buckets, bolts, nuts,
pipes etc.
Cathodic metal coating : It is a process of coating of base metals with cathodic metals such as Sn , Ni
, Cr and Cu etc.
Example: Tinning
Tinning: Tinning is the process of coating the surface of a base metal (such as iron) with tin. Tinning of iron
metal is an example of cathodic metal coating on an anodic base metal.
Tinning of iron is carried out by hot dipping method. It involves the following steps.
i) The metal surface is washed with dilute sulphuric acid.(Pickling process) to remove any dirt ,rust on the
surface. [descaling].
ii) Oil, grease is removed by washing organic solvents (CCl4, toluene) [degreasing].
iii) Finally, the article is washed with water and air-dried.
iv)It is then passed through molten zinc chloride flux. The flux helps the molten tin to adhere strongly on the
surface.
v) It is then dipped in a bath of molten tin.
vi)The coated tin is immersed in palm oil. The oil prevents the oxidation of tin coating.
vii) The excess zinc on the surface is removed by passing through a pair of hot rollers.
Applications: Tin-coated steel is used for manufacturing containers.
(Note: Copper utensils are coated with tin to prevent contamination of food with poisonous copper salts.)
Iron
sheet
Dil .H2SO4 Organic
Solvent
Water
Air drier
Pair of hot
Rollers
Excess of Sn
ZnCl2
flux
Palm oil
Molten tin

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Cathodic protection:-
In cathodic protection, the metal to be protected is completely converted into a cathode. Since cathodes do not
undergo corrosion, the metal is protected against corrosion.
1. Sacrificial anode method:
In sacrificial anode method, the metal to be protected is electrically connected to a more active metal.
For example, when steel is to be protected, it may be connected to a block of Mg or Zn. In such a situation, steel
acts as cathode and is unaffected. Mg and Zn act as anode and undergo sacrificial corrosion. When the
sacrificial anode gets exhausted, it is replaced with new ones.
Other examples: Mg bars are fixed to the sides of ships to act as sacrificial anode.
Mg blocks are connected to burried pipe lines.
2. Impressed current method (impressed voltage method):
In impressed current method, the metal to be protected is connected to the negative terminal of an external d.c.
power supply. The positive terminal is connected to an inert electrode such as graphite. Under these conditions,
the metal acts as cathode and hence does not undergo corrosion. The inert electrode acts as anode; but it does
not undergo corrosion because it is inert.
Zn or Mg block

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MODULE-2
Questions:
1. Describe electrochemical theory of corrosion with iron as example.
2. Define corrosion.
3. Describe differential metal corrosion.
4. Explain differential aeration corrosion.
5. Describe pitting corrosion
6. Explain waterline corrosion.
7. Describe stress corrosion [caustic embrittlement in boilers].
8. Describe the effect of following factors on the rate of corrosion: (i) Nature of metal, (ii)
Nature of corrosion product, (iii) Difference in potential between anodic and cathodic
regions.(iv) Anodic and cathodic areas
9. Describe the effect of pH
, temperature & conductance on the rate of corrosion.
10. What is anodizing? Describe anodizing of aluminium.
11. Explain phosphating.
12. What is galvanizing? Describe galvanizing of iron.
13. Explain tinning.
14. Explain cathodic protection by sacrificial anode method

METAL FINISHING
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MODULE-2
DEFINITION OF METAL FINISHING:
Metal finishing is the process of deposition of a layer of one metal on the surface of substrate (metal,
plastic etc) or the process of conversion of a surface layer of atoms on a metal into an oxide film. (Note: Metal
finishing is the process of surface modification of a metal)
Technological importance of metal finishing:
Importance of metal finishing are,
 A decorative appearance.
 To increase the corrosion resistance
 To increase thermal resistance
 To increase optical reflectivity.
 To impart electrical and thermal properties such as semi-conduction and fire resistance.
 To impart hardness & solderability
 To provide electrical and thermal conducting surface
 Manufacturing electrical and electronic components such as contacts, PCB, capacitors & contacts etc.
 In electroforming (to manufacture metal articles entirely by electroplating)
 In electrotyping (to produce finely engraved dies or similar finely divided articles such as gramophone records)
 In electrochemical machining, polishing and etching.
 To build up material or restoration
Electroplating:
Definition: Electroplating is the process of electrolytic deposition of a metal on the surface of another metal, alloy
or conductor by the process of electrolysis.
The three important factors governing the process of electrolysis,
i. Polarization
ii.Decomposition potential
iii.Over voltage
i. Polarization:
Definition: Polarization is defined as a process where there is a variation of electrode potential due to
inadequate [slow] supply of ionic species from the bulk of the solution to the electrode surface.
Polarization is an electrode phenomenon,
The electrode potential is given by the Nernst’s equation,
Where E0
= standard electrode potential and [ Mn+
] is the metal ion concentration surrounding the electrode surface at
equilibrium.
Explanation: Consider an electrolytic cell under operation. When current is being passed, positive ions are produced
at the anode and are consumed at the cathode. If the diffusion of ions in the electrolyte is slow, there will be an
accumulation of positive ions in the vicinity of anode. Similarly, there will be a depletion of ions in the vicinity of
cathode. Under these conditions, the anode and cathode are said to be polarized. This type of polarization is known as
concentration polarization.
]n[Mlog
n
0591.00E
cell
E 

METAL FINISHING
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MODULE-2
Factors affecting the electrode polarization:
1. Nature of the electrode [size, shape & composition]
2. Electrolyte concentration
3. Temperature
4. Rate of stirring of the electrolyte
5. Products formed at the electrode
Large electrode surface, low [Mn+
] concentration, continuous stirring decreases polarization
ii. Decomposition potential [Ed]
Definition: Decomposition potential is defined as the minimum voltage that must be applied in order to
carry out continuous electrolysis of an electrolyte.
The decomposition potential is determined using an electrolytic cell as shown in figure:
Example: In the electrolysis of water, a pair of platinum electrodes immersed in a solution of an acid. It is found
experimentally that a potential of about 1.7V must be applied to the cell before there sets in a continuous evolution of
H2 and O2 .The voltage at which the current increases suddenly is called Ed of the electrolyte.
iii. Over voltage (over potential) (η)
Definition: Over voltage is defined as the excess voltage that has to be applied above the theoretical
decomposition potential to start the electrolysis.
η = [Ed] experimental -[Ed] theoretical
Example: For electrolysis of water using smooth platinum electrodes,
The theoretical decomposition potential using Pt electrode is 1.23 V.
The experimental decomposition potential using smooth platinum electrode is 1.7 V.
η = 1.7-1.23 = 0.47V
A graph of variation of current w. r. to applied potential

METAL FINISHING
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MODULE-2
Factors affecting the over voltage value:
1. Nature of the electrode.
2. Nature of the product formed at the electrode.
3. Current density (i.e. current per unit area of the electrode surface.)
4. Temperature
5. Rate of stirring.
Principal components of an electroplating process: The principal components are shown in
the following figure.
The main components are:
1. 1. Electroplating bath: It contains a suitable salt
solution of the metal being plated. It also contains
other additives.
2. 2. Anode: It may be a rod or pellets of the metal
being plated. It may be an inert electrode. It
should be electrically conducting.
3. 3. Cathode: It is the article to be plated. It should
have an electrically conducting surface.
4. 4. Inert vessel: It contains above mentioned
materials. It may be a vessel made of rubber lined
steel, plastic concrete or wood.
5. 5. D.C. power supply: The positive terminal of
the power supply is connected to the anode and the
negative terminal is connected to the cathode.
Effect of plating variables on the property of electrodeposit:
1. Current density: Current per unit area of the electrode surface. [Amperes/cm2
].
 At low current density, a bright, fine grained crystalline deposit is obtained but the rate of deposition is
slow.
 At high current density, hydrogen evolution occurs at the cathode, a burnt and spongy deposit results.
In general, for a particular bath, the optimum current density is experimentally determined and applied.
Optimum current density ranges from 10 to70 mA/cm2
2. Concentration of metal ion:
 At high concentration of electrolyte, mass transfer increases leads to poor deposit.
 At low concentration, crystal size decreases and results in fine deposit. Therefore, the free metal ion
concentration is kept low.
Optimum molar concentration of an electrolyte maintained is 1-3mol/dm3
 A low metal ion concentration may be achieved by the addition of a compound with a common ion (e.g.
addition of H2SO4 to CuSO4)
3. Complexing agents:
 Complexing agents are used to maintain a low metal ion concentration, results in fine deposit.

METAL FINISHING
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MODULE-2
 Complexing agents are also used to improve the throwing power of the bath. Higher the throwing power,
more uniform is the deposit.
(e.g. addition of NaCN to CuCN to get low concentration of Cu+
)
4. Throwing power of a bath: is a “Capacity of plating bath to give a uniform deposit even on an irregularly
shaped object.”
Measurement of throwing power: Haring-Blum Cell
- +
1. It consists of two electrodes and an anode at the center. The cathodes are at different distances d1 and d2 from anode
[let d1>d2]
2. The process of electroplating is carried out and the weights [w1 & w2] of deposits at cathodes [1 &2] are noted.
When w1= w2 i.e. amount deposited is same irrespective of the placement of the electrode, then throwing power is
considered very good (100%).
When the calculated throwing power is – 100% then it is considered as very poor.
Factors affecting the throwing power of bath:
1. Concentration of electrolyte
2. Conductance of solution
3. Additives
5. pH
of an electrolytic bath:
 At low pH
values, liberation of hydrogen occurs at the cathode resulting in a burnt deposit.
2H+
+ 2e-
H2
 At high pH
values, the cathode surface gets coated with insoluble hydroxides.
2)OH(MOH2M  
Therefore, for most of the plating processes, a pH
range of 4-8 is optimum.
 The desired pH
is maintained using suitable buffers. (e.g. phosphate buffer in gold plating)
6. Temperature:
d1 d2 Cathode2
Cathode1
Electrolytic
solution
w1
w2
=y,
d2
d1
=xWhere
2)-y+(x
100×y)-(x
solutionbaththeofpowerthrowing% 

METAL FINISHING
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 Increase in temperature increases the conductivity, increases the mobility of ions, and decreases the
polarization.
 However, too high a temperature may lead to evolution of hydrogen at the cathode, results in burnt deposit.
 Therefore, a moderate temperature range of 35 – 60o
C is used for most of the plating processes.
7. Organic additives: To improve the quality of electrodeposit, certain organic compounds are added to the electrolytic
bath.
These are a) brighteners, b) levellers, c) structure modifiers and d) wetting agents.
a. Brighteners: Brighteners are added to get bright deposits and light falling on the metal surface gets reflected.
Example: Aromatic sulphones, sulphonates, thiourea etc. in Ni plating.
(Note: When the grain size of the electrodeposit is lower than the wave length of the incident light, the light gets reflected,
but not scattered. Thus the deposit appears bright.)
b. Levellers: Levellers are added to get a level [uniform] deposit. Levellers get adsorbed at places where rapid or
excessive deposition is taking place, thus preventing the excessive growth in those places.
Example: Sodium allyl sulphonate in Ni plating.
c. Structure modifiers (Stress relievers): Structure modifiers are added to change the orientation of the crystals with
respect to surface of substrate and reduce internal stress.
Example: Saccharin.
d. Wetting agents: Wetting gents are added to remove any hydrogen sticking to the cathode surface. Thus they prevent
hydrogen embrittlement of the deposit.
Example: Sodium lauryl sulphate.
Electroplating of chromium
 The surface of the object is subjected to descaling [washing with an acid] and
 Degreasing [washing with organic solvent].
 Finally, the surface is washed with deionized water. Then, chromium plating is done under the following
conditions.
# Particulars Decorative Cr-plating Hard Cr-plating
1 Plating bath composition
Chromic acid (CrO3) + H2SO4 in the
weight ratio 100 : 1
Chromic acid (CrO3) + H2SO4 in the
weight ratio 100 : 1
2 Operating temperature 45-55 o
C 43-66 o
C
3 Current density 100 – 200 mA/cm2
215 – 430 mA/cm2
4 Current efficiency 8 – 12 % 10-15%
5 Anode
Insoluble anode: Pb-Sb or Pb-Sn alloy
coated with PbO2.
Insoluble anode: Pb-Sb or Pb-Sn alloy
coated with PbO2.
6 Cathode Object to be plated Object to be plated
7 Anodic reaction  2e
2
O
2
1
2HO
2
H
oxygen,ofl i berati on
 2e
2
O
2
1
2HO
2
H
oxygen,ofl i berati on

METAL FINISHING
---------------------------------------------------------------------------------------------------
MODULE-2
 In chromic acid, chromium is present in 6+ oxidation state. It is first reduced to 3+ state by a complex anodic
reaction in the presence of sulphate ions.

 

3SO6
CrCr
2
4
 The Cr3+
then gets reduced to Cr on the substrate surface.
Cr3eCr3
 
For a good deposit, the Cr3+
concentration must be low.
 The PbO2 oxidizes a part of Cr3+
to Cr6+
, thus reducing the concentration of Cr3+
.

  6PbO
Cr3Cr 2
During Cr coating, Cr rods are not used as anodes because:
1. In acidic solutions, chromium may undergo passivation.
2. Chromium anodes increase the Cr3+
concentration.
Electroplating of Nickel by watt’s method :
 The surface of the object is subjected to descaling [washing with an acid] and
 Degreasing [washing with organic solvent].
 Finally, the surface is washed with deionized water. Then, chromium plating is done under the following
conditions.
8 Cathodic reaction

 

3SO6
CrCr
2
4
Cr3eCr3
 

 

3SO6
CrCr
2
4
Cr3eCr3
 
9 pH 2-4 2-4
# Particulars Decorative Cr-plating Hard Cr-plating
10 Applications
1. Used in corrosion resistant coating.
2. Used to give decorative finish on
automobiles & surgical instruments
1. Extensively used in industrial &
engineering applications.
# Particulars Nickel plating (watt’s method )
1 Plating bath composition 250g of NiSO4 + 45g of NiCl2+30g of boric acid
2 Operating temperature 25-65o
C
3 Current density 10-60A/ft2
4 Current efficiency 95-100 %
5 pH 4-4.5
6 Anode Nickel pellets or nickel pieces
7 Cathode Object to be plated
8 Anodic reaction 
 2eNiNi 2

METAL FINISHING
---------------------------------------------------------------------------------------------------
MODULE-2
ELECTROLESS PLATING:
Definition of electroless plating: Electroless plating is a method of depositing a metal over a
catalytically active surface of the substrate by using suitable reducing agent without using electrical
energy.
product.oxidized+MagentReducing+M +n

 The catalytic metals such as Fe, Ni, CO, Rh, Pd, Al etc do not require any surface preparation before electroless plating.
 But a Non-catalytic metal such as Cu, Brass, and Ag etc needs activation. This is done by dipping the base metals in
PdCl2 (Palladium chloride) in HCl.
 Non – Conductors like glass, insulators, plastics, ceramics etcare first activated in a solution of SnCl2 in HCl. After
rinsing, it is immersed in a solution of PdCl2 in HCl.
Distinction between electroplating and electroless plating:
Property Electroplating Electroless plating
1. Driving force Power supply Autocatalytic redox reaction
2. Anode Separate anode Catalytic surface of the substrate
3.Cathode Article to be plated Article to be plated ( with a catalytic surface)
4.Reducing agent Electrons Chemical reagent
5.Reactions
MneM
neMM
n
n




product.oxidized+MagentReducing+M +n

6.Applicability Only conductors Conductors, semiconductors & insulator
7.Nature of deposit Not satisfactory for intricate partsSatisfactory for all parts
1. Electroless plating of Copper
 Before electroless plating, the surface is cleaned thoroughly.
 Insulators such as plastics and printed circuit boards are activated by dipping first in stannous chloride (SnCl2) and
then in palladium chloride (PdCl2).
 Then, the electroless plating is done under the following conditions:
9 Cathodic reaction Ni2eNi2
 
10 Additives Saccharin , coumarin, aromatic sulphonaamide
11 Applications
i. As an undercoat for Cr plating, brass,gold & rhodium
plating
ii. Decorative mirror finish
iii. Black nickel plating is used for making name plate
type writer parts, camera components,optical &
electrical instruments

METAL FINISHING
---------------------------------------------------------------------------------------------------
MODULE-2
# Particulars Electroless plating of copper
1 Plating bath solution CuSO4 [12g/lt]
2 Reducing agent Formaldehyde (HCHO)
3 Complexing agent EDTA
4 Buffer Sodium hydroxide and Rochelle salt (Na-K-tartrate)
5 pH 11
6 Temperature 250
C
CuHO2H2HCOOCu4OH2HCHO:reactionOverall
Cu2eCu:Cathode
2eHO2H2HCOO4OH2HCHO:Anode
22
-2-
-2
-
22
--





Formaldehyde and copper sulphate are added to the plating bath periodically.
Applications: 1. Used for metalizing printed circuit boards, Used to produce through-hole connections.
Through-hole connection is PCB’s:
For PCB’s with double sided circuits, through-hole connection is
required. The through-hole connection is made by electroless
plating technique. Preparation of PCB by electroless plating:
1. The base material is made up of glass reinforced plastic [GRP’S]
or epoxy polymer.
2. The base material which is double sided, is electroplated with
copper
3. Selected areas are protected by photoresist.
4. The rest of copper is removed by etching to produce circuit
pattern or track
5. The connection between two sides is made by drilling hole
followed by plating-through-holes by electroless plating.

METAL FINISHING
---------------------------------------------------------------------------------------------------
MODULE-2
Questions:
1. What is metal finishing? Explain technological importance of metal finishing.
2. What is electroplating? Explain the following terms
a. Polarization b. Decomposition potential c. Overvoltage
3. Explain the following plating variables affects nature of electro deposit
a. current density b. temperature & pH c. concentration of the electrolyte
d. throwing power of an electrolytic bath d. organic additives
4. Explain electroplating of (decorative & hard) chromium & mention its application.
5. Explain electroplating of Nickel by watt’s method
6. What is electroless plating? What are the differences between electroplating & electroless
plating ?
7. Explain electroless plating of copper in the manufacturer of PCB’s

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