CORROSION
Corrosion is a natural process that coverts a refined metal into a more
chemically stable oxide. It is the gradual deterioration of materials ( usually
metal ) by the chemical or electrochemical reaction with their environment.
Corrosion engineering is the field dedicated to controlling and preventing
corrosion.
Chemical equation of corrosion of Iron:-
Fe + O2 + H2O → Fe(OH)3
In the most common use of the world, this means electrochemical oxidation of
metal in reaction with an oxidant such as oxygen, hydrogen, or hydroxide.
Rusting, the formation of red-orange iron oxides, is a well-known example of
electrochemical corrosion. This type of corrosion typically produces oxides or
salts of the original metal and result in a distinctive coloration.
Corrosion can also occur in material other than metals, such as ceramics or
polymers, although in this context, the term "degradation" is more common.
Corrosion degrades the useful properties of materials and structures including
mechanical strength, appearance, and permeability to liquids and gases.
Many structural alloys corrode merely from exposure to moisture in air, but the
process can be strongly affected by exposure to certain substances. Corrosion
can be concentrated locally to from a pit or crack or it can extend across a wide
area, more or less uniformly corroding the surface. As a result, methods to
reduce the activity of the exposed surface, such as passivation and chromate
conversion, can increase a material’s corrosion resistance. However, some
corrosion mechanisms are less visible and less predictable.
The chemistry of corrosion is complex; it can be considered an electrochemical
phenomenon. During corrosion at a particular spot on the surface of an object
made of iron, oxidation take place and that spot behaves as an anode. The
electrons released at this anodic spot move through the metal to another spot on
the object, and reduce O2 at that spot in presence of H+. This spot behaves as a
cathode.
, TYPE OF CORROSION
(I) Crevice Corrosion
It is another destructive form of localized corrosion. It is usually
occurs in the areas under deposits where free access to the surroundings
environments is restricted. It is caused on the contact of metals with
metals or metals with non- metals, for example, gaskets, couplings and
joints.
(II) Stress Corrosion Cracking
It can be abbreviated to ‘SSC’ and refers to the cracking of the
metal; as a result of the corrosive environment and the tensile
strength placed on the metal. It often occurs at high temperatures.
For example, SSC of austentic stainless steel in chloride solution.
(III) Intergranular Corrosion
It occurs due to the presence of impurities in the grain boundaries
that separates grains formed during the solidification of the metal
alloy. It can also occur via depletion or enrichment of the alloy at
these grain boundaries. For example, Aluminium base alloys are
affected by IGC.
(IV) Pitting Corrosion
It is a localized form of corrosion by which cavities or ‘holes’ are
produced in the material. Pitting is considered to be more dangerous
because it is more difficult to detect, predict and design against.
Corrosion product often cover the pits. For example, it occurs in
stainless steel etc.
(V) Galvanic Corrosion
It is an electrochemical process whereby one metal corrodes in the
preference to another metal that it is in contact with through an
electrolyte. It occurs when two dissimilar metals are immersed in a
conductive solution and are electrically connected. For example, if
zinc is in contact with mild steel, the zinc udergoes galvanic
corrosion.
(VI) Uniform Corrosion
It is most common type of corrosion. It can be defined as the attack of
the entire metal surface exposed to the corrosive environment resulting
in uniform loss of metals from the exposed surface. The metal
becomes thinner and eventually fails.
For example, corrosion of anchor chain used in the heavy ships to
stable them.
(VII) Metal Dusting
It is a catastrophic form of corrosion that occurs when susceptible
materials are exposed to environment with high carbon activities. For
example, metal dusting in Ammonia.
Corrosion is a natural process that coverts a refined metal into a more
chemically stable oxide. It is the gradual deterioration of materials ( usually
metal ) by the chemical or electrochemical reaction with their environment.
Corrosion engineering is the field dedicated to controlling and preventing
corrosion.
Chemical equation of corrosion of Iron:-
Fe + O2 + H2O → Fe(OH)3
In the most common use of the world, this means electrochemical oxidation of
metal in reaction with an oxidant such as oxygen, hydrogen, or hydroxide.
Rusting, the formation of red-orange iron oxides, is a well-known example of
electrochemical corrosion. This type of corrosion typically produces oxides or
salts of the original metal and result in a distinctive coloration.
Corrosion can also occur in material other than metals, such as ceramics or
polymers, although in this context, the term "degradation" is more common.
Corrosion degrades the useful properties of materials and structures including
mechanical strength, appearance, and permeability to liquids and gases.
Many structural alloys corrode merely from exposure to moisture in air, but the
process can be strongly affected by exposure to certain substances. Corrosion
can be concentrated locally to from a pit or crack or it can extend across a wide
area, more or less uniformly corroding the surface. As a result, methods to
reduce the activity of the exposed surface, such as passivation and chromate
conversion, can increase a material’s corrosion resistance. However, some
corrosion mechanisms are less visible and less predictable.
The chemistry of corrosion is complex; it can be considered an electrochemical
phenomenon. During corrosion at a particular spot on the surface of an object
made of iron, oxidation take place and that spot behaves as an anode. The
electrons released at this anodic spot move through the metal to another spot on
the object, and reduce O2 at that spot in presence of H+. This spot behaves as a
cathode.
, TYPE OF CORROSION
(I) Crevice Corrosion
It is another destructive form of localized corrosion. It is usually
occurs in the areas under deposits where free access to the surroundings
environments is restricted. It is caused on the contact of metals with
metals or metals with non- metals, for example, gaskets, couplings and
joints.
(II) Stress Corrosion Cracking
It can be abbreviated to ‘SSC’ and refers to the cracking of the
metal; as a result of the corrosive environment and the tensile
strength placed on the metal. It often occurs at high temperatures.
For example, SSC of austentic stainless steel in chloride solution.
(III) Intergranular Corrosion
It occurs due to the presence of impurities in the grain boundaries
that separates grains formed during the solidification of the metal
alloy. It can also occur via depletion or enrichment of the alloy at
these grain boundaries. For example, Aluminium base alloys are
affected by IGC.
(IV) Pitting Corrosion
It is a localized form of corrosion by which cavities or ‘holes’ are
produced in the material. Pitting is considered to be more dangerous
because it is more difficult to detect, predict and design against.
Corrosion product often cover the pits. For example, it occurs in
stainless steel etc.
(V) Galvanic Corrosion
It is an electrochemical process whereby one metal corrodes in the
preference to another metal that it is in contact with through an
electrolyte. It occurs when two dissimilar metals are immersed in a
conductive solution and are electrically connected. For example, if
zinc is in contact with mild steel, the zinc udergoes galvanic
corrosion.
(VI) Uniform Corrosion
It is most common type of corrosion. It can be defined as the attack of
the entire metal surface exposed to the corrosive environment resulting
in uniform loss of metals from the exposed surface. The metal
becomes thinner and eventually fails.
For example, corrosion of anchor chain used in the heavy ships to
stable them.
(VII) Metal Dusting
It is a catastrophic form of corrosion that occurs when susceptible
materials are exposed to environment with high carbon activities. For
example, metal dusting in Ammonia.
