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Definition:-The Process of destruction or decoration of metal or its
alloys by chemical or electrochemical attack through its environment
starting at its surface is called corrosion.
Ex. Rusting of iron (Fe2O3.2H2O)
Types of Corrosion:Corrosion of the metals occurs by the attack of surrounding
environment on the surface of the metals. The attack occurs in two
1. Dry Corrosion or chemical corrosion
2. Wet or electrochemical Corrosion
Dry or Chemical theory of Corrosion
This type of corrosion occurs mainly through the direct chemical
action of atmospheric gases such as oxygen, halogens, hydrogen
sulphide, SO2 or anhydrous inorganic liquids with metal surfaces in
immediate proximity. The extent of corrosion depends on
1. The chemical affinity between the corrosive environment and
solid metals and
2. Ability of reaction product on metal surface to form a protective
There are three main types of corrosion.
Oxidation Corrosion: It Occurs by the direct action of
oxygen at low temperature on alkali and alkaline earth metals. At
high temperature almost all metals are oxidized to form metal oxides.
Metals like Ag, Au, and Pt are not oxidized as they are noble metals.
2Mn+ + ne1/2O2 + 2nenO2---------------------------------2M + 1/2O2
During oxidation of metal, metal oxide is formed as a thin film on the
metallic surface which protects the metal from further corrosion. If
diffusion of either oxygen or metal is across these layer, further
corrosion is possible. Thus the layer of metal oxide plays an
important role in the process of corrosion. Oxides of Pb, Al and Sn
are stable and hence inhibit further corrosion. Depending the layer
formed it can be classified as Unstable oxide layer, Stable non-porous
layer, Stable porous layer and volatile layer.
In case of porous and volatile oxides, the rate of further
corrosion is faster. MoO3 is a volatile oxide and accelerates corrosion.
2. Reaction with other gases:- In dry corrosion, Cl2, SO2, H2S and
NOx gases react with metal and form corrosive products. Corrosion
product AgCl is a protective layer while SnCl4 is volatile.
3. Liquid metal corrosion :- In several industries, molten metal
passes through metallic pipes and causes corrosion due to dissolution
or due to internal penetration. For example, liquid metal mercury
dissolves gold form corresponding amalgam, thereby corroding gold.
Wet or Electrochemical Theory of Corrosion:Oxidation of metal (ex. Fe) takes place at anode while the reduction
process takes place at cathode. It may occur in two ways
(i) Evolution of H2 (ii) Absorption of O2
At anode :
Fe2+ + 2 e-
While at cathode reduction reaction is carried by evolution of H2 or
absorption of O2.
Case-I : Evolution of H2
The hydrogen ions (H+) are formed due to the acidic environment and
the following reaction occurs in the absence of oxygen.
Corrosion Chemistry
2H+ + 2eH2
In this case metal react in the acidic environment and are dissolved
(Undergo corrosion) to release H2 gas. All metal above hydrogen in
electrochemical series can show this type of corrosion. In hydrogen
evolution type of corrosion, anodic area is large as compared to its
cathodic area.
Case –II : Absorption of O2
This type of corrosion takes place in neutral or basic medium in the
presence of oxygen. The oxide of iron covers the surface of the iron.
The small scratch on the surface creates small anodic area and the rest
of the surface acts as cathodic area. The following reaction occurs at
anode and cathode
2Fe2+ + 4eO2 +2H2O + 4e4OH------------------------------------------------------------2Fe + O2 +2 H2O
Fe2+ + 4 OH- or Fe(OH)2
------------------------------------------------------------------Ferrous oxide is easily oxidized to form Fe(OH)3 in the presence of
4Fe(OH)2 + O2+ H2O
4 Fe(OH)3
Ferric hydroxide is actually hydrated ferric oxide, Fe2O3.H2O, which
is yellowish rust. Anhydrous magnetite, Fe3O4 [a mixture of (FeO +
Fe2O3)], is also formed, which is brown-black in colour. It is mark
able that the corrosion occurs at anode but the corrosion product is
formed near cathode. It is because of the rapid diffusion of Fe2+ as
compared to anion.
At anode:
At cathode:
Factors Influencing Corrosion:Since corrosion is a process of destruction of metal surface by its
environment. It is influenced by two factors (i) Nature of the metal
(ii) Environmental
Nature of Metal:(a) Position of metal in galvanic series:(b) Hydrogen over voltage:(c) Purity of metal
(d) Relative areas of anode and cathode
(e) Physical state of the metal
(f) Nature of metal oxide film
(g) Volatility and solubility of corrosion product.
Dr. Benarji Patrudu | [GITAM University,HYD]
Corrosion Chemistry
Effect of Environment:
(a) Temperature
(b) Humidity
(c) Impurity of atmosphere
(d) pH value
(e) Conducting effect
(f) Oxygen concentration
1. Protection from soil corrosion of underground cables and
2. Protection from marine corrosion of cables, ship hulls, piers,
water tanks etc.,…
Cathodic Protection:The principle involved in cathodic protection is to force the metal to
be protected to behave like a cathode. Cathodic protections are two
1. Galvanized or Sacrificial Anodic method and
2. Impressed Current methods.
In galvanic protection, a more active metal is connected to the
metal to be protected, so that all corrosion is concentrated on the
more active metal and thus saving the metal structure from the
corrosion. The more active metal used is called auxiliary anode or
Sacrificial anode.
Three metals are commonly utilized for cathodic protection of steel.
The selection of the anodic metal is dependent upon resistivity and
electrolyte. General application guides for these metals are
 Magnesium – non-sea soil and freshwater
 Zinc – sea soils and saltwater
 Aluminum - saltwater and limited freshwater applications
In this method, an impressed current is applied in opposite direction
to nullify the corrosion current and convert the corroding metal from
anode to cathode. Usually the impressed current is derived from
direct current source with an insoluble anode like graphite or
platinum. Usually a sufficient D.C. current is applied to an insoluble
anode buried in the soil and connected to the metallic structure to be
protected. The anode is usually in a black fill(coke), so as to increase
the electrical contact with the surrounding soil.
1. Protection of underground pipe lines carrying oil or water
2. Applied to open water – box coolers, water tank transmission
towers, marine piers etc.,
Even though impressed current method is useful for cathodic
protection, in practice some tray currents are developed and the
Dr. Benarji Patrudu | [GITAM University,HYD]
Corrosion Chemistry
purpose is spoiled, hence sacrificial anodic method is superior to
this method.
Corrosion Inhibitors:
A corrosion inhibitor is a substance, which when added in small
quantities to the aqueous corrosive environment effectively decrease
the corrosion of a metal.
They are two types
(1) Anodic Inhibitors:-These are oxygen carriers or oxidizing agents.
They combine with anodic metal forming an oxide film or layer
which snaps the galvanic circuit. Although this type of control is
affective, it may be dangerous since severe local attack can occur, if
certain areas are left unprotected by the depletion of the inhibitor
Ex:- Transition metal chromates, Phosphates and Tungstates.
(2) Cathodic Inhibitors:a) In Acidic solution, the main cathodic reaction is evolution of
2H+ (aq) + 2eH2(g)
Consequently corrosion may be reduced by either slowing
down the diffusion of H+ ions to the cathode or by increasing the
over voltage of hydrogen evolution. The diffusion of H+ ions is
decreased by adding organic inhibitors like amines, mercaptans,
heterocyclic compounds, substituted urea and thio-urea, metal soaps
Antimony and arsenic oxides are used as inhibitors, because
they deposit adherent film of arsenic or antimony at the cathodic
areas, thereby increasing hydrogen voltage.
In neutral solution, the cathodic reaction is
H2O + 1/2O2 + 2e- ---- 2OH- (aq)
The corrosion can be controlled by elimination of oxygen from the
corroding medium, this carried by adding reducing agen6ts like Na2S,
Na2SO3 by deareation or by retarding its diffusion to the cathodic
areas by adding inhibitors like Mg, Zn and Ni salts.
Metallic coatings:a.Anodic Metal Coating:The metal is coatings which are anodic to the bae metal are known as
anodic metal coatings. In order to protect a metal from corrosion, any
metal which is higher up in the galvanic series such as Zn, Mg, Al etc., are
coated on the surface of base metal.These active metals act as anodic to
base metal(cathode). If any pores , breaks occur in such anodic coating, a
galvanic cell is formed between coating metal and base metal. Thus no
attack of base metal occurs until all the anodic metal corrodes. Ex- Zn
coating on iron sheets.Used in roof sheets, barede wires, nuts and bolts
Dr. Benarji Patrudu | [GITAM University,HYD]
Corrosion Chemistry
Cathodic Metal Coating:The surface of a metal can be protected from corrosion by coating a base
metal with a more noble metal whose tendency for corrosion is lower than
the base metal in the galvanic series. A thin film coating of metal like tin ,
copper, nickel, etc are deposited on the base iron metal by cathodic
coating techniques to prevent corrosion. Cathodic coating provide effective
protection to the bsae metal only when they are completely continuous,
non-porous and free from crack or breaks. If such coating are puncture
more corrosion and severse damage can be done to the base metal than to
the metal without it.
Uses:It is used for protection of iron in the form of roofing sheets, wires,
pipes, nails, bolts, screws, tubes etcs.,
Galvanizing utensils cannot be used for preparing and storing food
Tinning:*Tinning is the coating of tin over iron or steel articles. The process
consists of treating steel sheet in dil.H2SO4 to remove any oxide.
*After this it is passed through a bath of zinc chloride flux. The flux
helps molten metal to adhere to the metal sheet.
*Then the sheet is passed through a molten tin tank and then through
a series of rollers.
*Finally through a layer of plam oil. The plam oil protects the hot tin
coating surface againt oxidation.
Galvanizing is the process of coating of zinc on iron or steel to
prevent rusting. The process is carried as follows
The iron article is first cleaned by acid pickling with dil.H2SO4
solution for 15 -20 minutes at 60-70ᴼC, removes any scale, rust or
other impurities.
The article is then washed well and dried.
It is then dipped in molten zinc bath at 425-450 ᴼC. The surface of
the bath is covered with NH4Cl flux to prevent oxide formation, then
the oxide is passed through a pair of hot rollers. The process removes
any excess of zinc and produces a thin film of uniform thickness.
Then it is annealed at 650ᴼC and finally cooled slowly by annealing
Dr. Benarji Patrudu | [GITAM University,HYD]
Corrosion Chemistry
to form the oxide, thus thickening the natural oxide film. The outer
part of the oxide film formed is very porous and considerably softer
than the layer directly adjoining the metal surface. The anodized
surface is made impermeable by treatment with hot water. This
monohydrate(Al2O3.H2O) which occupies more volume, thereby the
pores are sealed.
Anodized coating on Zn can be obtained by making Zn as
anode in an electrolytic bath containing chromic acid solution or
Uses:Tinning is widely used for food storage cans, tin coated copper sheets
for cooking utensils
Anodizing or Anodised oxide coating:These coating are made by anodic oxidation process this leads
to the formation of protective, corrosion resistant, electrically
insulating oxide film on the surface of non-ferrous metals like
aluminum, zinc, magnesium and their alloys.
Anodic oxidation or anodizing is an electrolytic process of
oxidation for thickening the natural oxide film on the surface of metal
to achieve more protection against corrosion and mechanical injury.
Anodizing on aluminum is done by electrolysis in suitable
acid electrolyte (sulphuric, chromic, oxalic or phosphoric acid) at 35
to 40ᴼC and moderate current densities. Aluminum is made the anode
and a plate of lead or stainless steel is made the cathode. When the
current is passed the oxygen liberated at the anode combines with it
Organic Coatings:Organic coatings are inert organic barriers applied on metallic surface
and other construction materials for corrosion protection and
decoration. A good organic coating must have
Chemically insert to corrosive environment
Good surface adhesion
Good spreading power
Impermeability to water salts and gases
Proper application method
Important organic coating is 1. Paints 2.Varnishies 3.Lacqers and
Dr. Benarji Patrudu | [GITAM University,HYD]
Corrosion Chemistry
Paint is a mechanical dispersion mixture of one or more pigments in a
vehicle. Vehicle is a liquid consisting of non-volatile film forming
material with drying oil and a highly volatile solvent thinner.
When paint is applied on a metal surface the thinner evaporates while
the drying oil slowly oxidises forming a light pigmented film
Characteristics of a good paint:
1. Good chemical stability, 2.Good surface adhesion 3.Good
spreading power. 4. Resistance to corrosion 5.Attractive in colour
6. Insoluble In ordinary solvents 7.Non-poisonous, 8.Good hiding
power, 9.Glossy film and 10. Reflect the uv light
Pigments: Pigments are solids. They give desire colour ,opacity and
strength . these effect the UV radiation and protect the paint,
pigments used for various colours are
 White :-zinc oxide, white lead , titanium oxide, litho phone
 Red:- red lead, ferric oxide, chrome red
 Green:- chromium oxide
 Blue:- Prussian blue
 Black :- carbon black
Vehicle or drying oil : Vehicle is a film forming constituents of
pain. These are glycerol esters of high molecular weight fatty acids,
generally present in animal and vegetable oil. drying oil contains
high percentages of unsaturated fatty acids and they dry quickly by
absorbing oxygen and polymerizes, so liquid oils is converted into
solid film. The most widely used drying oils are linseed oil, soybean
oil, and de hydrated castor oil.
Main film forming constituents
Vehicle or medium
Good toughness
Good adhesion
Good durability and
Good water-proofness
III. Thinners: thinners reduce the viscosity of paint and hence
increase the covering area. These also increase the elasticity of paint
film and increase the penetration power of the vehicle. Ex. Turpentine
oil, mineral sprits, benzene, naphtha, kerosene, methylated
naphthalene. etc.,
IV. Driers: driers are oxygen-carrying catalyst. They accelerate the
drying of oil through oxidization, polymerization and condensation
Thus their main function is to improve the drying quality of oil film.
Too much of drier tends to produce hard and brittle films. Ex.
Resinates, linoleates, tungastates and naphthanates of Cr, Mn, Pb and
V. Extenders and fillers: These are inert materials, which improve
the properties of paints and reduce the cost, increase the durability of
paint.Ex. Barytes, Talc, asbestos, silica, gypsum, mica, slate powder,
china clay, manganese silicate ,CaSO4, etc.
Dr. Benarji Patrudu | [GITAM University,HYD]
Corrosion Chemistry
VI. Plasticizers: Platizers are incorporated into the paint to provide
elasticity and to minimise cracking . Ex. Tricrysol phosphate ,
triphenyl phosphate , tributyl phosphate , dibutyl tartarate etc.
VII. Anti-skinning agents : These are added to some paints for
preventing gelling and skinning of the paint film . Important anti
skinning agents are polyhydroxy phenols.
Varnish is a homogeneous colloidal dispersion solution of natural or
synthetic resin in oil or thinner or both. It is used as a protective and
decorative coating at suitable surfaces and dries by evaporation,
oxidation and polymerization of its constituents leaving behind a hard
transparent glossy, lustrous and durable film. There are two types of
1. Oil varnish, 2. Spirit varnish.
1. Oil Varnish: - is a homogeneous solution of one or more natural or
synthetic resins in a drying and a volatile solvent. This type of varnish
dried up by the evaporation of the volatile solvent, followed by
oxidation and polymerization of drying oil. Ex. Copal varnish-used
for exterior as well as interior works
2. Spirit Varnish – contain a resin dissolved in a completely volatile
solvent. Spirirt varnishes dries quite rapidly, but leaves behind a
film, which is brittle and so has a tendency to crack or peel off. ExShellac- used for polishing wooden furniture.
Constituents of a Varnish:
a. Resins: Resin in varnish provides an element of hardening, resistant
to weathering, durability and resistance to chemical action and water
Natural resins: shellac, rosin, copal, dammer etc.,
Synthetic resins: phenol formaldehyde, urea formaldehyde etc.,
b. Drying oils: Drying oils are dried by oxidation and polymerization
and give a thin film. Ex. Linseed oil, dehydrated castoroil, soya bean
oil, fish oil, etc.,
c. Solvents or thinners: these reduce the viscosity and increase coving
area. The solvent used are kerosene, turpentine oil, acetone etc.,
d. Antiskinning agents: These are used to prevent gelling. Ex. Tertiary
amyl phenol.
1. For protection against corrosion
2. As a brightening coat to paint surface
3. For improving the appearance and intensifying the ornamental
grains of wood surfaces.
1. Define corrosion? Present the mechanism of electrochemical
2. Write at least 5 differences between dry corrosion and wet
3. Describe the factors that influence corrosion with respect to metal
and nature of environment?
4. What are varnishes and how are they used in corrosion prevention.
5. Define cathodic protection? Explain different methods of cathodic
protections with applications?
6. What is paint? Discuss the functions of Constituents of paint?
7. Describe the processes i) Galvanizing and ii) Tinning?
Dr. Benarji Patrudu | [GITAM University,HYD]