CORROSION
ENGR. REY S.A. RIMANDO
College Instructor
PARTIDO STATE UNIVERSITY
Objectives
Discuss Corrosion
Define terminologies concerning
corrosion
Identify factors affecting
corrosion
Discuss types of corrosion
Describe examples of corrosion
and how to prevent corrosion
What is Corrosion?
• Corrosion is the deterioration of materials by
chemical interaction with their environment.
• The term corrosion is sometimes also applied
to the degradation of plastics, concrete and
wood, but generally refers to metals.
• Corrosion is one of the most common
phenomena that we observe in our daily lives.
• You must have noticed that some objects
made of iron are covered with an orange or
reddish-brown colored layer at some point in
time.
• The formation of this layer is the result of a
chemical process known as rusting, which is a
form of corrosion.
Causes of Corrosion
The chemical combined state of the metal is
known as “ore”
The ore has low energy in its thermodynamical
state.
The extracted metal has high energy
,thermodynamically unstable .
Metal converts itself into stable state by reacting
with the environment and undergoes corrosion.
The corroded metal is thermodynamically stable.
Factors Affecting Corrosion
Exposure of the metals to air containing gases.
Exposure of metals to moisture, especially salt water (which
increases the rate of corrosion).
Presence of impurities like salt (For example, NaCl).
Temperature: An increase in temperature increases corrosion.
Nature of the first layer of oxide formed: Some oxides form an
insoluble protecting layer that can prevent further corrosion. Others,
like rust, easily crumble and expose the rest of the metal.
Presence of acid in the atmosphere: Acids can easily accelerate the
process of corrosion.
Effects or disadvantages of corrosion
The valuable metallic properties like
conductivity,malleability, ductility etc are lost
due to corrosion.
Life span of the metallic parts of the
machineries is reduced.
The process of corrosion is very harmful and
is responsible for the enormous wastage of
metal in the form of its compound.
The failure of the machinery takes place due
to lose of useful properties of metals.
The approximate estimate of loss of metal due
to corrosion is 2 to 2.5 billion dollars per
annum all over the world.
TYPES OF CORROSION
Galvanic corrosion
This corrosion occurs when different
metals are in contact and exposed to
corrosive atmosphere.
The metal which has higher electrode
potential value will form anode and
undergoes corrosion. Example: Zn-Cu
galvanic cell zinc behaves as anode and
corrosion occurs, copper behaves as
cathode and protected.
TYPES OF CORROSION
Pitting corrosion
When corrosion starts on a metal surface at certain spots they become less
open to oxygen with the result that corrosion proceeds faster at those very
spots making deeper and deeper cavities.
Pitting corrosion can be explained on the basis of differential aeration.
The pit created becomes deeper, its bottom becomes less open to oxygen
which makes it more anodic.
The part which has higher oxygen concentration is cathodic.
TYPES OF CORROSION
Water line corrosion
The concentration of oxygen dissolved in water is greater at the surface than
deeper down. This causes formation of a concentration cell.
Anode is the lower portion .
Cathode is at the water level.
Due to the poor conductivity of water the ions just below the water level are
available for reaction, the metal corrodes just below the water level.
TYPES OF CORROSION
Intergranular Corrosion
Intergranular corrosion can be defined as localized corrosion at and
adjacent to grain boundaries, with relatively little corrosion at the grains. As
a consequence, the metal alloy disintegrates and/or it loses its strength.
Although metal loss is minimal, IGC can cause the catastrophic failure of
equipment. IGC is a common form of attack on alloys in the presence of
corrosive media that results in the loss of strength and ductility. One should
not mistake IGC with stress corrosion cracking (SCC).
TYPES OF CORROSION
Crevice Corrosion
Whenever there is a difference in ionic
concentration between any two local areas of
a metal, a localised form of corrosion known
as crevice corrosion can occur. For instance,
this form of corrosion mostly occurs in
confined spaces (crevices).
Examples of areas where crevice corrosion
can occur are gaskets, the undersurface of
washers, and bolt heads. All grades of
aluminium alloys and stainless steels also
undergo crevice corrosion. This is mainly
because of the formation of a differential
aeration cell that leads to the formation of
corrosion inside the crevices
TYPES OF CORROSION
Hydrogen Grooving
This is a corrosion of the piping by
grooves that are formed due to the
interaction of a corrosive agent,
corroded pipe constituents, and
hydrogen gas bubbles.
The bubbles usually remove the
protective coating once it comes in
contact with the material.
TYPES OF CORROSION
Metal Dusting
Metal dusting is a damaging form
of corrosion that occurs when
vulnerable materials are exposed
to certain environments with high
carbon activities, including
synthesis gas.
The corrosion results in the break-
up of bulk metal to metal powder.
Corrosion occurs as a graphite
layer is deposited on the surface of
the metals from carbon monoxide
(CO) in the vapour phase.
Prevention of Corrosion
• Electroplating - Electroplating is basically
the process of plating a metal onto the
other by hydrolysis mostly to prevent
corrosion of metal or for decorative
purposes.
• Galvanization - It is the process in which
iron or steel is protected from corrosion by
coating with a thin layer of zinc.
• Tinning – It is the process of coating of tin
over the Iron sheet or steel articles
• Passivation - Passivation is a chemical
treatment for stainless steel and other
alloys that enhances the ability of the
treated surfaces to resist corrosion.
• Anti-Corrosion Protective Coatings
• Painting and Greasing
• Use of Corrosion Inhibitor or Drying Agents
• Periodic Cleaning of Metal Surface
FACTORS EFFECTING THE CORROSION
The factors that effect corrosion are
i) Nature of the metal
A ) Purity of a metal
B ) Position in galvanic series
C ) Over voltage
D ) Nature of oxide film
E ) Nature of corrosion product
ii) Nature of the environment
A. Temperature
B. Humidity in Air
C. Presence of impurities
D. Acidic value
E. Amount of oxygen
F. Velocity of ions
