CORROSION AND METAL FINISHING

Corrosion is a destructive attack of a metal by its environment either directly or

by electrochemical phenomenon. It can also be defined as “The spontaneous
destruction and consequent loss of a metal or alloy due to chemical or
electrochemical attack by the environment”
The most familiar examples are
1. Rusting, the formation of iron oxides is a well-known example of electrochemical
corrosion. When iron metal is exposed to the corrosive environment, the metal
forms a loosely adherent product of hydrated ferric oxide called rust.
2. When Cu is exposed to the corrosive environment it forms an adherent green
coloured product of basic cupric carbonate.
Corrosion of metals or alloys occurs in two ways- either by direct chemical attack

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or electrochemical attack by the corrosive environment.

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Direct chemical attack is reaction of metals directly with corrosive medium to form

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corresponding product in the absence of moisture. This type of corrosion is known as
dry corrosion.

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Most of the metals and alloys undergo corrosion by following electrochemical attack in
the presence of moisture or a conducting medium. This type of corrosion is known as

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wet corrosion.
Electrochemical theory of corrosion
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Electrochemical theory explains the wet corrosion, i.e. corrosion in presence of
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moisture. Electrochemical theory tells that corrosion of metals takes place due to the
formation of anodic and cathodic regions i.e. small galvanic cells on the same metal
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surface or when two different metals are in contact with each other in the presence of a
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conducting medium. At anodic region oxidation takes place and metal dissolves and get
converted it to ions, liberating electrons. At the cathode region, reduction takes place.
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We know that metals cannot be reduced further, metal atoms at the cathode region are
unaffected by the cathodic reaction. Only the surrounding atmosphere undergoes
reduction by accepting the electrons.
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This specify that only anodic region or a metal/alloy acting as anode undergoes
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corrosion where as cathodic region or metal/alloy acting as cathode remains

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unaffected.
Corrosion reactions
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We know that a metal or region which is more active will undergo corrosion i.e. it will
undergo oxidation. Reactions at anode are simple in which metal is converted in to its
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corresponding ion.
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n+ -
At anode (oxidation reaction): M →M + ne
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+2 -
At anode: Fe → Fe + 2e
The reactions at cathode are more complicated, (reduction reaction) depends on
the nature of the corrosive environment. Common reactions are of two types

1. Hydrogen liberation type 2. Oxygen absorption type 1

1. Hydrogen liberation type of reactions takes place only in the absence of dissolved
oxygen
a. If the medium is acidic, the reaction is
+ -
2H + 2e → H2 ↑
b. If the medium is alkaline / neutral, the reaction is
- -
2H2O+2e → 2OH + H2↑
2. Oxygen absorption type of reactions takes place in the presence of dissolved oxygen
a. If the medium is acidic, the reaction is
+ -
2H + ½O2 + 2e →H2O
b. If the medium is alkaline / neutral, the reaction is
- -
H2O+½ O2 + 2e →2OH
The products formed at anode and cathode diffuses towards each other through
conducting medium and form a corrosion product between the anode and cathode.
+2 -
Net reaction: 2Fe +4OH → 2Fe(OH)2
anodic area Cathodic area

+2 - - -
Fe → Fe + 2e -----> Fe(OH)2 <------ H2O+½ O2 + 2e →2OH

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In the presence of excess of oxygen Fe(OH)2 is oxidized to rust.

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2Fe(OH)2+ ½ O2 → Fe2O3.2H2O

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(Rust)

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In the limited supply of oxygen Fe(OH)2 is oxidized to magnetic oxide which is
commonly known as black rust.

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3Fe(OH)2 + ½ O2→ Fe3O4.3H2O
(black rust)
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Types of corrosion
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CORROSION
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Differential metal Differential aeration Stress

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Waterline Pitting corrosion

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Differential metal corrosion
When two different metals are in direct contact with one another and are
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exposed to a corrosive conducting medium, the more active metal behaves as anode
and suffers from corrosion, whereas the less active metal becomes cathode and
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protected from corrosion. The potential difference between the two metals facilitates
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the corrosion reaction. This type of corrosion is also known as galvanic corrosion.
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Following reaction stakes place during galvanic corrosion

At anode oxidation takes place:
n+ -
M→M + ne
The reduction at cathode depends on the nature of the corrosive environment mainly 2
it follows hydrogen evolution or oxygen absorption reaction.
Reduction Reaction:
+ -
2H + 2e → H2 ↑
- -
H2O+½ O2 + 2e → 2OH
4Fe(OH)2+ O2 + 2H2O→ 2(Fe2O3.3H2O)
Examples:
1. When Zn and Cu metals are electrically connected and exposed to an electrolyte, Zn
(more active) forms anode and suffers from corrosion whereas Cu (less reactive) forms
cathode and protected from corrosion.
2. Steel screws in a brass marine hardware
3. Steel pipe connected to copper plumbing
4. Lead antimony solder around copper wire
Differential aeration corrosion
This type of corrosion is due to the formation of differential aeration cell or
oxygen concentration cell. When a metal surface is exposed to differential air or oxygen
concentrations it forms differential aeration cell (oxygen concentration cell). The more
oxygenated region of the metal behaves as cathode and less oxygenated region
becomes anode. That means less oxygenated or less aerated region undergoes
corrosion.
Reactions are as follows

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At less oxygenated region oxidation takes place:

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n+ -
M→M + ne
At more oxygenated region reduction takes place

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- -
2H2O+ O2 + 4e → 4OH

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Examples:
1. Partially buried pipeline in soil or water
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2. Window rods inside the frame suffer from corrosion
3. Metal under dirt, dust, scale or water undergoes corrosion
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4. Part of the nail inside the wall

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5. Paper pin inside the paper

Two common cases of differential aeration corrosion are
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a. Water line corrosion

Water line corrosion occurs in which a part of the metal is partially dipped in
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water. Consider a steel tank containing water. The maximum corrosion takes place
along a line just beneath the level of water meniscus. The part of the metal below the
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water line is exposed to dissolved oxygen (less oxygen concentration) acts as anode
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and undergoes corrosion whereas the part which is above the water line is exposed to
atmospheric oxygen (more oxygen concentration) acts as cathode, which remains
completely unaffected by corrosion.
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Ex. Partially filled metal water tank

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Other examples of waterline corrosion are corrosion of ship hull, partially immersed
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metallic pipe.
b. Pitting corrosion is a localized accelerated attack in which only small areas of the
metal surface are attacked while the remainder is largely unaffected. This localized
attack results in small pin holes (pits). The pits may initiate and propagate to a certain
depth resulting in the formation of cavities and becomes inactive. This is also a type of
differential aeration corrosion.
This type of localized corrosion is usually created by dirt deposits, corrosion
products, cracks in paint or protective coatings, etc. the metal below the deposit is less
oxygenated acts as anode and undergoes corrosion. Small anodic and large cathodic
area increases the corrosion rate. Anode area is covered by the corrosion product
making it less aerated, which also accelerate the corrosion reaction. The shape of the
pit is often responsible for its continued accelerated growth. Initially formed pits
produce conditions which stimulate the autocatalytic process.
Pitting is very destructive and frequently ruins the metallic instruments, tubes, pipes etc.

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Stress corrosion
It is a highly localized attack on the metal because most of its surface is virtually

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un-attacked while cracks progress towards stress. This corrosion occurs only in the
presence of specific corrosive environment and the presence of tensile stress on the

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metal.
Stress can be internal stress developed during fabrication of the article or during
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etching, drawing, servicing, bending, welding, etc or stress can also be external stress
acting during service conditions like heavy load on material. The metal atoms under
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stress will always at higher energy level compared to unstressed atoms. Therefore,
stressed atoms are more active and acts as anode. This corrosion involves an attack
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along the narrow paths forming local anodic areas with respect to more cathodic area
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of the metal surface. The crack grows and propagates perpendicular to the operating
stress, and failure occurs after progressing a finite distance.
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Example: Season cracking of brass in presence of ammonia, Caustic embrittlement of

boiler in presence of chloride ions and caustics.
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Corrosion control methods
Corrosion can be completely avoided only under ideal conditions. Since it is impossible
to attain such conditions, it can be minimized by using various corrosion control
methods. They are: i. Galvanising
a. Metalic coating
ii. Tinning
1. Protective
coating
i. Anodizing of Al

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b. Inorganic

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coatings ii. Phosphating

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a. Sacrificial anodic
2. Cathodic

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protection
b. Impressed current
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Protective Coating:
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Corrosion is prevented by the application of protective coating on the surface of metal,

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thereby the metal surface is isolated from the corrosive environment. The coatings
being chemically inert to the environment under specific conditions of temperature and
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pressure, forms a physical barrier between the coated surface and its environment.
Coatings are not only prevents corrosion but also decorates the surface of the metal.
 Metallic coatings
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Coating or depositing a metal layer on base metal is known as metallic coating.

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Metal coatings can be applied on the base metal by hot dipping process, electro-
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deposition, flame spraying, diffusion coating, cladding, vapor deposition etc., This
method is used for producing a coating of low melting metals such as Zn, Al, Sn etc., on
iron / steel metals which have relatively high melting point. Metal coating is classified in
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to two – anodic coating and cathodic coating.

Anodic coating which is produced by coating more active metal than the base
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metal. If any crack or pit is formed on the coated metal, it will act as anode and base
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metal as cathode and hence base metal will not corrode. Coating of zinc over iron is
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anodic coating is also known as galvanizing. Zinc is more active than iron, which
sacrifices itself to protect iron. Anodic coating of iron is done by Al and Cd also. Since
Zn is toxic, galvanized utensils are not used for cooking or food storing.

Steps involved in galvanizing are:

 Degreasing: The base metal is washed with organic solvents to remove oil/grease,
  Pickling: The base metal is washed with dilute sulphuric acid to remove scale/rust
 Washing: Then it is washed with water and dried, before coating.
 Flux Treatment: The base metal is treated with NH4Cl and ZnCl2¬ flux. This is done to
prevent the oxidation of coating metal Zn and also to get adherent coating.
 Hot Dipping process: The base metal is then dipped in a molten bath of Zn (~ 450oC)
 Stripping: The excess of coated metal is stripped off using a pair of hot rollers and
cooled gradually.
Uses: Galvanizing is applied to nails, bolts, pipes, roofing sheets etc.
If any crack is produced on the galvanized sheets, Zn undergoes severe corrosion by
protecting the base metal.

Inorganic coatings (Chemical conversion coatings)

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These coatings are produced at the surface of the metal by chemical /
electrochemical reactions. This coating is a chemical conversion process in the sense

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that, coatings are integral part of the metal itself. Inorganic coating is process of
conversion of atoms of the base metals to its compounds and which is generally

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produced by chemical dip, spraying or by electrolytic methods. These coatings are
applied on the article for decorative effect and to increase the corrosion resistance of
the base metal. This coating serves as an excellent base coating for paints, enamels

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and also acts as an excellent insulator. i
Examples:
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 Anodizing of Al
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Anodizing is a process of artificially converting protective passive oxide film on the
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surface of metal by electrochemical oxidation. Anodized coatings are generally

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produced on non-ferrous metals like Al, Zn, Mg and their alloys by anodic oxidation
process. The base metal is made as anode. It is carried out to produce a
porous/nonporous coating.
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Current Source
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Aluminium Anode
(Article to be anodized) Carbon/graphite cathode (Inert)

H2SO4/Chromic
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Acid/
Phosphoric acid
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The electrolysis is conducted in an acid bath, at moderate temperature 30-40 C, using
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moderate current densities, in which the base metal is made as anode. The commonly
used baths are H2SO4 / Chromic acid / Phosphoric acid /oxalic acid. Lead is generally
used as cathode.
Steps involved in anodizing of Al are,
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 Anodizing: Aluminum is immersed in a tank containing an electrolyte having
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around 15% sulfuric acid concentration having temperature about 40 C. Electric
current is passed through the electrolyte and the aluminum is made the anode in
this electrolytic cell; the lead is the cathode. Voltage applied across the anode
and cathode causes negatively charged anions to migrate to the anode where the
oxygen in the anions combines with the aluminum to form aluminum oxide
(Al2O3).
Reaction at anode,
+ -
2Al + 3H2O → Al2O3 + 6H + 6e
 At Cathode,
+ -
6H + 6e → 3H2
 Coloring: Anodic films are initially porous and non-continuous which is well
suited to a variety of coloring methods including dyeing, both organic and
inorganic dyestuffs, and electrolytic coloring processes.
 Sealing: These pores are sealed by exposing the metal to boiling water. In this
process the metal oxide layer changes into its metal oxide mono hydrate
(Al2O3.H2O).
Uses: Anodized articles are used as Tiffin carriers, soapboxes, household utensils,
window frames, etc.

Cathodic Protection:
Cathodic protection is a method in which the base metal to be protected from corrosion
is made to act as cathode and no part of it is allowed to act as anode.

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We all know that corrosion of the metal takes place at the anode not at the cathode.

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Therefore corrosion of the metal can be prevented by converting all of the anodic
(active) sites on the metal surface to cathodic (passive) sites. Cathodic protection

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systems protect a wide range of metallic structures in various environments. Common
applications are of cathodic protections are steel water pipelines and storage tanks,
steel piers piles, ship and boat hulls, offshore oil platforms and onshore oil well casings

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and metal reinforcement bars in concrete buildings etc.
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Cathodic protection may be achieved in either of two ways.
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1. Sacrificial anodic protection
2. Impressed current cathodic
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Sacrificial anodic protection:

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Mg
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Figure: The steel pipe is connected to a more active Mg block.

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In this method the more active metals like Zn, Mg, and Al etc are attached to base metal.
The anodic metals being more reactive undergo corrosion but base metal remains
unaffected. The more active metal so employed is called sacrificial anode and the
method is called sacrificial anode method. The sacrificial anodes have to be replaced
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from time to time after complete corrosion.

Examples: The method is used for protecting buried pipeline, ship hulls, industrial water
tank steel rods in RCC columns.