Corrosion is the deterioration and loss of a material and its critical
properties due to chemical, electrochemical and other reactions of the
exposed material surface with the surrounding environment.
Corrosion of metals and nonmetals takes place due to the gradual
environmental interaction on the material surface. The structures and
facilities of different materials are affected by this interaction. Even the
ambient air, laden with moisture and oxygen, can start this process,
known as rusting, on steel surfaces.
3. CAUSES OF CORROSION
• Too much humidity or condensation of water vapour on metal
surfaces are the primary causes of corrosion.
• Corrosive gases such as chlorine, hydrogen oxides, ammonia, sulfur
oxides, amongst others can result in corrosion of parts of electronic
equipment, etc. Corrosion can also occur due to hydrogen and
• Corrosion in steel can occur when it is placed under too much stress
and the material develops a crack in it.
• Metals exposed to electrical currents for a long time can experience
• Exposure to dirt and bacteria can cause corrosion in metals.
4. Theories of corrosion (dry/chemical and
Theories of corrosion (or) mechanism of corrosion
1. Dry or Chemical corrosion.
2. Wet or Electrochemical corrosion.
5. Dry or Chemical corrosion
Dry or Chemical corrosion:
It is due to direct chemical action of environment/ atmospheric
gases[Electronegative] (O2, Cl2, H2S , N2) and anhydrous inorganic
liquids on the metal surface[Electropositive]. Chemical corrosion is best
explained by oxidation corrosion Oxidation corrosion It occurs due to
direct action of oxygen at low or high temperatures on metals in the
absence of moisture.
6. Dry or Chemical corrosion.
Oxidation occurs at the surface of metal and the resulting metal oxide
scale that acts as a barrier for further oxidation. For further oxidation
a) the metal ion must diffuse outwards
b) oxide ion must diffuse inwards. Former occurs more readily as
because of the smaller size of the metal ion. In oxidation corrosion,
the nature of the oxide film plays an important role. It may bea)
Stable – A stable layer of metal oxide is formed which is impervious
and this protects the metal from further corrosion. Eg :- Al, Sn. b)
Unstable- The oxide layer decomposes back to metal and oxygen.
7. Dry or Chemical corrosion.
c) Volatile- Oxide layer evaporates as soon as it is formed. Then by the metal
surface is exposed for further attack. Eg :- MoO3.
d) Porous- Oxide layer has porous or cracks, so oxygen has access to the
underlying metal. Hence, corrosion continues.
Pilling – Bed worth rule
It tells about the nature of the oxide layer- protective or non – protective. "If
the volume of the oxide film is greater than the volume of the meta surfacel,
then the oxide layer is protective and non-porou"s. On the other hand," if
the volume of the metal oxide is less than the volume of the metal, then the
layer is non-protective and porous". Eg :- Alkali and alkaline earth metals
form non-protective oxide film. Al forms protective oxide film.
8. Wet or Electrochemical corrosion
This Corrosion occurs when the environment favors the formation of
anodic and cathodic areas on the metal, i) A metal is exposed to two
different concentrations of environment. ii) Two different metals are in
contact in a medium. At anode:- Oxidation (Corrosion) takes place
ie M 𝑀𝑛+
At cathode:- Reduction takes place, Where electrons are consumed.
Corrosion of an anode is based on how electrons are consumed at
cathode At cathode consumption of electron takes place either by
9. Wet or Electrochemical corrosion
a) Liberation / evolution of hydrogen –occurs in acidic medium
Eg: - Fe Fe2+ + 2e- (oxidation at anode)
2𝐻+ + 2e- H2(reduction at cathode)
Fe + 2 𝐻+ 𝐹𝑒2+ + H2 ----b) Absorption of oxygen- Occurs in
neutral aqueous solution (NaCl solution) in the presence of
Eg:- Rusting of iron in presence of NaCl solution.
10. Factors effecting corrosion
Factors affecting the corrosion : The rate of corrosion mainly depends
on two factors.
A. Nature of the metal B. Nature of corroding environment.
Nature of metal Galvanic series
1. Position in Galvanic series:- When two different metals are in
contact in presence of Medium, the more active metal (top in galvanic
series) undergoes Corrosion. The rate of corrosion depends on
difference in their positions
11. 2.Over voltage:- When Zn is placed in 1N-H2SO4, it undergoes
corrosion by evolution of H2.
The rate of corrosion is slow because of high overvoltage (0.77V) of Zn
metal. If few drops of CuSO4 is added, the corrosion of Zn is
accelerated, because Cu deposited on Zn acts as minute cathode,
where the evolution of H2 is 0.33V. Reduction in overvoltage
accelerates the corrosion rate.
12. 3.Relative areas of anodic and cathodic parts:- Smaller the anodic area
and larger the cathodic area severe will be the corrosion.
4. Purity of the metal:- Impurities in a metal cause heterogeneity and
forms minute electrochemical cells and anodic area gets corroded.
5. Physical state of metal:- Rate of corrosion depends on grain size,
orientation of crystals, stress etc. Smaller the grain size and area under
stress, greater will be the corrosion.
13. . Nature of surface film:- The ratio of volume of metal oxide to metal is
called “Specific volume ratio”. Greater the specific volume ratio, lesser
will be the oxidation corrosion. Eg: Specific volume ratio of Ni, Cr, W are
1.6, 2.0. 3.6. Oxidation corrosion is least is W. 7.
Passivity:- Metals like Ti, Al, Ca, and Mg are passive and they have high
corrosion resistance. This is due to formation of protective oxide film
having salt-healing nature.
14. CORROSION PREVENTION METHODS
One of the easiest and cheapest ways to prevent corrosion is to
use barrier coatings like paint, plastic, or powder. Paint acts as a
coating to protect the metal surface from the electrochemical
charge that comes from corrosive compounds. Today’s paint
systems are a combination of different paint layers that serve
different functions. The primer coat acts as an inhibitor, the
intermediate coat adds to the paint’s overall thickness, and the
finish coat provides resistance to environmental factors.
16. HOT-DIP GALVANIZATION
• This corrosion prevention method involves dipping steel
into molten zinc. The iron in the steel reacts with the
zinc to create a tightly bonded alloy coating which
serves as protection. The process has been around for
more than 250 years and has been used for corrosion
protection of things like artistic sculptures and
• Unfortunately, galvanization can’t be done on-site,
meaning companies must pull equipment out of work to be
treated. Some equipment may simply be too large for the
process, forcing companies to abandon the idea
altogether. In addition, zinc can chip or peel. And
high exposure to environmental elements can speed up
the process of zinc wear, leading to increased
maintenance. Lastly, the zinc fumes that release from
the galvanizing process are highly toxic.
17. ALLOYED STEEL (STAINLESS)
• Alloyed steel is one of the most effective
corrosion prevention methods around, combining
the properties of various metals to provide
added strength and resistance to the resulting
product. Corrosion-resistant nickel, for
example, combined with oxidation-resistant
chromium results in an alloy that can be used
in oxidized and reduced chemical environments.
Different alloys provide resistance to
different conditions, giving companies greater
• Despite its effectiveness, alloyed steel is
18. CATHODIC PROTECTION
Cathodic protection protects by electrochemical
• To prevent corrosion, the active sites on the
metal surface are converted to passive sites by
providing electrons from another source,
typically with galvanic anodes attached on or
near the surface. Metals used for anodes
include aluminum, magnesium, or zinc.
• While cathodic protection is highly effective,
anodes get used up and need to be checked
and/or replaced often which can drive up costs
of maintenance. They also increase the weight
of the attached structure and aren’t always
effective in high-resistivity environments.
20. EONCOAT – A NEW WAY TO PROTECT
ASSETS FROM CORROSION
EonCoat is a cost-effective, maintenance-free,
and easily applied solution which protects the
life of the asset. It works by a combination of
the above methods. First, it alloys the metal,
then provides a thick layer of inhibitors that
will repair any damage to the alloy layer.
EonCoat uses no toxic chemicals and has no VOC’s
so it is the most environmentally friendly
solution. Independent testing shows this
solution to be the most effective and long-
lasting of all the alternatives
22. Cathodic protection
Principle- The metal to be protected is forced to behave like cathode,
there by corrosion does not occur.
There are two types of cathodic protection
i) Sacrificial anodic protection- In this method, the metallic structure
(to be protected) is connected to more anodic material. So, that the
corrosion is concentrated at more active metal. The more active metal
sacrifies itself by corrosion and protects the structure (cathode). The
sacrificial anode are Zn, Mg, Al. This method is mainly used for
protection of underground pipeline, cables, and marine structures.
23. Impressed current cathodic protection
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.
The impressed current is derived from a direct current source like a
battery. Sufficient D.C current is applied to an insoluble anode, buried
in the soil and connected to the metallic structure to be protected.
This type of cathodic protection is applied to open water-box coolers,
water-tanks, buried oil or water pipes etc.
24. METHODS OF APPLICATIONS OF METALS
Hot dipping is used for producing a coating for low melting metals such
as Zn, Pb,Sn and Al on Iron, Steel or Copper.
The process involves in immersion of a metal in a bath of its molten
coating by a molten layer.
Hot dipping is widely applied either by
25. Hot-dip galvanization is a form of galvanization. It is the process
of coating iron and steel with zinc, which alloys with the surface
of the base metal when immersing the metal in a bath of molten
zinc at a temperature of around 450 °C (842 °F). When exposed
to the atmosphere, the pure zinc (Zn) reacts with oxygen to form
zinc oxide , which further reacts with carbon dioxide to form zinc
carbonate, a usually dull grey, fairly strong material that protects
the steel underneath from further corrosion in many
26. Uses of Galvanizing :
It is used to protect Iron used for roofing sheets, wires,pipes,
nails,bolts,screws and tubes.
It is not used in cooking untensils because the solubility of Zn
Tinning is the process of thinly coating sheets of wrought
iron or steel with tin, and the resulting product is known
A cleaned iron sheet is passed through a bath of nolten flux, then
passes through a tank of molten tin.
Finally through a layer of palm oil which protect hot tin coated
28. Uses: It is used for coating steel, copper, brass and bronze etc.
It is used for store food stuffs and refrigerator equipment.
Cladding is the bonding together of dissimilar metals. It is
different from fusion welding or gluing as a method to fasten the
metals together. Cladding is often achieved by extruding two
metals through a die as well as pressing or rolling sheets
together under high pressure.
• The advantage of cladding is that the process as well as the
material can be chosen as per the needs of the application and
the bond strength required.
• The laser beam method of cladding can be used for ceramic
coating as well as metallic coating.
• Effective cooling in such cases ensures better micro-structure,
and hence only a smaller area is thermally affected.
• With metal cladding, the surfaces can develop adequate wear
resistance. The method is suitable for complex shapes as well.
31. Metal Cladding Processes
Roll bonding –
• In roll bonding, two or more layers of different metals are
thoroughly cleaned and passed through a pair of rollers under
sufficient pressure to bond the layers.
• The pressure is high enough to deform the metals and reduce
the combined thickness of the clad material.
• Heat may be applied, especially when metals are not ductile
enough. As an example of application, bonding of the sheets
can be controlled by painting a pattern on one sheet; only the
bare metal surfaces bond, and the un-bonded portion can be
inflated if the sheet is heated and the coating vaporizes.
32. Metal Cladding Processes
Explosive Welding :
• In explosive welding, the pressure to bond the two layers is provided
by detonation of a sheet of chemical explosive.
• No heat-affected zone is produced in the bond between metals. The
explosion propagates across the sheet, which tends to expel
impurities and oxides from between the sheets.
• Pieces up to 4 x 16 metres can be manufactured. The process is useful
for cladding metal sheets with a corrosion-resistant layer.
33. Metal Cladding Processes
• Laser cladding is a method of depositing material by which a
powdered or wire feedstock material is melted and
consolidated by use of a laser in order to coat part of a
substrate or fabricate a near-net shape part (additive
• It is often used to improve mechanical properties or increase
corrosion resistance, repair worn out parts, and fabricate
metal matrix composites.
34. Metal Cladding Processes
Advantages of Cladding :
• Best technique for coating any shape => increase life-time of wearing parts.
• Particular dispositions for repairing parts (ideal if the mould of the part no longer exist or too
long time needed for a new fabrication).
• Most suited technique for graded material application.
• Well adapted for near-net-shape manufacturing.
• Low dilution between track and substrate (unlike other welding processes and strong
• Low deformation of the substrate and small heat affected zone (HAZ).
• High cooling rate => fine microstructure.
• A lot of material flexibility (metal, ceramic, even polymer).
• Built part is free of crack and porosity.
• Compact technology.
Electroplating is the process of using electrodeposition to coat
an object in a layer of metal. Engineers use controlled
electrolysis to transfer the desired metal coating from an
anode (a part containing the metal that will be used as the
plating) to a cathode.
• The anode and cathode are placed in an electrolyte chemical
bath and exposed to a continuous electrical charge.
• Electricity causes negatively charged ions (anions) to move to
the anode and positively charged ions (cations) to transfer to
the cathode, covering or plating the desired part in an even
• Electroplating takes a substrate material (often a lighter and/or
lower-cost material) and encapsulates the substrate in a thin
shell of metal, such as nickel or copper.
Advantages of Electroplating:
• Engineers often use electroplating to increase the strength and
durability of various designs. You can increase the tensile strength of
various parts by coating them in metals such as copper and nickel.
• Electroplating is an effective way to add cosmetic metal finishes to
customer products, sculptures, figurines, and art pieces.
• Electroplating also offers the benefit of conductivity. Because metals
are inherently conductive, electroplating is a great way to increase
the conductivity of a part. Antennas, electrical components, and
other parts can be electroplated to increase performance.
39. Organic surface coatings
An organic coating is a type of coating formed by carbon-
based polymeric chains derived from natural (vegetable,
animal) or synthetic matter. Solid, adhesive and cohesive
organic coatings can be found in the form of paints, varnishes,
40. Organic surface coatings
These coatings are primarily used to provide additive type finishes on
the materials on which they are applied. Organic coatings can be
monolithic (consisting of only one layer) or two or more layers.
• Organic coatings act as a protective barrier against corrosion and
• These are durable coatings applied to a substrate for their decorative
or specific technical properties.
• Organic coatings depend primarily on their chemical inertness and
• Various types of organic coatings are available for industrial purposes
including primers, adhesive cements and topcoats (enamel, varnish
41. Organic surface coatings
• Organic coatings are easy to apply with the help of brushes,
sprays, rollers, dips, or by electrostatic means.
• Brush application is a slow and lengthy procedure. The coating
cures or dries by evaporation or loss of solvent, polymerization
It is a mechanical dispersion mixture of one or more pigments in a
vehicle. Vehicle is a liquid, consisting of non-volatile, film-forming
material, a drying oil and a highly volatile solvent, thinner.
Mechanism- when paint is applied, the thinner evaporates, while the
drying oil slowly oxidizes forming a dry pigmented film.
Constituents of paints
• Provides capacity, strength & color.
• Gives protection to the pain film by reflection harmful UV light.
• Provides resistance to paint film against abrasion/water.
• Pigments used whites (Titanium oxide, zinc oxide), black- carbon
black, and green- chromium oxide.
2. Vehicle or drying oil
• Film – forming constituent.
• Glycerol esters of high molecular-weight fatty acids present in animal
and vegetable oils.
• Mostly used drying oils are linseed oil, soyabean oil
. 3. Thinners
• Reduces viscosity.
• Dissolves vehicle, additives of vehicle.
• Increases the penetration power of vehicle.
• Increases the elasticity of paint film.
• Common thinners are turpentine, mineral spirits, benzene, dipentene
• Oxygen- carrier catalysts.
• Accelerates the drying of the oil-film through oxidation,
polymerization and condensation.
• Improves the drying quality of oil-film.
• Common driers are tungstates, resonates etc.
• Low refractive indices materials.
• Increases durability of paint.
• E.g:- barites, talc, gypsum, china-clay.
• Provides elasticity.
• Minimizes cracking.
• Common plasticizers are triphenyl phosphate, tributyl phthalate etc.
7. Antiskinning agent
• Prevents gelling and skinning.
• Polyhydroxy phenols are used.
It is a homogenous colloidal dispersion solution of natural or synthetic
resin in oil or thinner or both. It acts as a protective layer by
evaporation, oxidation and polymerization of the constituents.
Types of varnishes
a) Oil varnish- homogenous solution of one or more natural or
synthetic resins in a drying oil and volatile solvent.
b) b) Spirit varnish- contains resin dissolved in a volatile solvent
Constituents of varnish
1.Resin- Natural resins (shellac, dammer etc) ,Synthetic resins ( phenol-
aldehyde, urea-formaldehyde etc).
2. Drying oils- Oils like linseed oil, soyabean oils are used. They dry by
oxidation and polymerisation.
3. Solvents or thinners- Turpentine, petroleum spirits etc are used.
4. Driers- Enhance the drying rate of oil constituents. Pb, Co linoleates
5. Antiskinning agents- Tert-amyl alcohol, guaiacol etc are used.