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EFFECT OF CORROSION IN
STRUCTURES
By
A.SRIKANTH VIHARI11131D8701
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INTRODUCTION
When a metal is attacked by substances around it, it is said to corrode
and this process is called corrosion. Corrosion causes deterioration ofessential properties in a material.
Billions of rupees are lost each year because corrosion and a huge
amount of money is spent in prevention of corrosion and tarnishing
of metals.
Corrosion involves the reaction of a metallic material with its
environment and is a natural process in the sense that the metal is
attempting to revert to the chemically combined state in which it is
almost invariably found in the earths crust.
corrosion may be regarded as resulting in a variety of changes in thegeometry of structures or components that invariably lead,
eventually, to a loss of engineering function e.g. general wastage
leading to decrease in section, pitting leading to perforation, cracking
leading to fracture.
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CORROSION IN STEEL REINFORCED
CONCRETES
Corrosion-induced deterioration of reinforced concrete can be
modelled in terms of three component steps:
Time for corrosion initiation, Ti;
Time, subsequent to corrosion initiation, for appearance of a crack on
the external concrete surface (crack propagation), Tp; and
Time for surface cracks to progress into further damage and develop
into spalls, Td, to the point where the functional service life, Tf, is
reached. Figure illustrates these schematically as a plot of cumulative
damage versus time.
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CORROSION REACTIONS:
Some of the anodic and cathode reactions that occursimultaneously on a metal surface in a "corrosion cell" are as
follows.
A typical anodic oxidation that produces dissolved ionic
product, for example for iron metal is:
[1] Fe ==> Fe2++ 2e-
Examples of cathodic reduces involved in corrosion process
are:
[2]O2+ 2H2O + 4e-==> 4OH-
[3] O2+ 4H++ 4e-==> 2H2O
[4] 2H++ 2e-==> H2
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The cathodic reaction represented by Equation [2] exemplifies
corrosion in natural environments where corrosion occurs at
nearly neutral pH values. Equations [3] and [4] represent
corrosion processes taking place in the acidic environmentsencountered in industrial processes or for the confined
volumes (pits, crevices) where the pH can reach acidic values
because of hydrolysis reactions such as:
[5] Fe2+ + 2H2O ==> Fe (OH)2 + 2H+
This reaction produces H+ ions, the concentration of which
can, under certain conditions, become large if the H+ ions
cannot readily move out from a confined volume. The overall
corrosion reaction is, of course, the sum of the cathodic and
anodic partial reactions. For example, for a reaction producingdissolved ions (sum of reactions [1] and [4]):
[6] Fe + 2H+ ==> Fe2+ + H2
[7] 2Fe + O2 + 2H2O ==> 2Fe(OH)2
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FACTORSASSOCIATEDMAINLYWITHTHEMETAL
Effective electrode potential of a metal in a solution Over voltage of hydrogen on the metal
Chemical and physical homogeneity of the metal surface
Inherent ability to form an insoluble protective film
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FACTORSWHICHVARYMAINLYWITH
THEENVIRONMENT Hydrogen-ion concentration (pH) in the solution
Influence of oxygen in solution adjacent to the metal
Specific nature and concentration of other ions in solution
Rate of flow of the solution in contact with the metal
Ability of environment to form a protective deposit on the metal
Temperature
Cyclic stress (corrosion fatigue)
Contact between dissimilar metals or other materials as affecting
localized corrosion.
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TYPES OF CORROSION Uniform Corrosion
Pitting Corrosion
Galvanic Corrosion
Crevice Corrosion
Concentration Cell Corrosion
Graphitic Corrosion
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1) Uniform Corrosion:
The metal loss is uniform from the surface. Often combined with
high-velocity fluid erosion, with or without abrasives. Generally
noticed with industrial and hydraulic structures.
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2) Pitting Corrosion:
The metal loss is randomly located on the metal surface. Oftencombined with stagnant fluid or in areas with low fluid
velocity, such as water tanks.
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3) Galvanic Corrosion: Occurs when two metals with different electrode potential is
connected in a corrosive electrolytic environment. The anodic metaldevelops deep pits and groves in the surface.
This type is noticed on other than reinforcement in structures where
different metal fixtures / fittings are used.
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4) Crevice Corrosion:
Occurs at places with gaskets, bolts and lap joints where
crevice exists. Crevice corrosion creates pits similar to pitting
corrosion. It is noticed in industrial structures steel structuresand hybrid structures.
Crevice corrosion is a localized form of corrosion usually
associated with a stagnant solution on the micro-
environmental level. Such stagnant microenvironments tend tooccur in crevices (shielded areas).
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5) Concentration Cell Corrosion:
Occurs where the surface is exposed to an electrolytic environment
where the concentration of the corrosive fluid or the dissolvedoxygen varies. Often combined with stagnant fluid or in areas with
low fluid velocity. Dampness periodic water retention with Rcc and
steel structures are prone to this type of corrosion.
6) Graphitic Corrosion:
Cast iron loosing iron in salt water or acids. Leaves the graphite in
place, resulting in a soft weak metal. As waste water pipes and
fixtures are liable for this type of corrosion.
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REASONS OF CORROSION The two most common causes of reinforcement corrosion are
(i) localized breakdown of the passive film on the steel by chlorideions and
(ii) general breakdown of passivity by neutralization of the concrete,
predominantly by reaction with atmospheric carbon dioxide.
Sound concrete is an ideal environment for steel but the increaseduse of deicing salts and the increased concentration of carbon
dioxide in modern environments principally due to industrial
pollution, has resulted in corrosion of the rebar becoming the
primary cause of failure of this material.
The scale of this problem has reached alarming proportions invarious parts of the world.
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FOLLOWING ARE THE CONTRIBUTING
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FOLLOWINGARETHECONTRIBUTING
FACTORSLEADINGTOCORROSION:
Cracks due to Mechanical Loadingo Cracks in concrete formed as a result of tensile loading,
shrinkage or other factors can also allow the ingress of the
atmosphere and provide a zone from which the carbonation
front can develop. If the crack penetrates to the steel, protectioncan be lost. This is especially so under tensile loading, for
deboning of steel and concrete occurs to
some extent on each side of the crack, thus removing the
alkaline environment and so destroying the protection in thevicinity of the deboning.
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Stray Currentso Stray currents, arising for instance from railways, cathodic protection
systems, or high voltage power lines, are known to induce corrosion
on buried metal structures, leading to severe localized attack.
o They may find a low resistance path by flowing through metallic
structures buried in the soil (pipelines, tanks, industrial and marine
structures). a cathodic reaction (e.g., oxygen reduction or hydrogen
evolution) takes place where the current enters the buried structure,
while an anodic reaction (e.g., metal dissolution) occurs where the
current returns to the original path, through the soil.
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Metal loss results at the anodic points, where the current leaves thestructure; usually, the attack is extremely localised and can havedramatic consequences especially on pipelines.
Example of stray current from a DC railway line picked up by steelreinforcement in concrete
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Corrosion of steel reinforcement due to
atmospheric pollution
Most of the times steel reinforcement is exposed to the atmosphere
during transportation and storage in the building sites for a longperiod before their installation in the concrete structures. At any of
those stages, steel rebars can be contaminated by chloride ions from
sea spray or windblown salt. This fact leads to the formation of
corrosion products on their surface.
Fiber optical microscope images after three months at open
atmosphere conditions.
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Moisture Pathwa
If the surface of the concrete is subject to long-term wetting, the
water will eventually reach the level of the reinforcement, either
through diffusion through the porous structure of the concrete, or bytraveling along cracks in the concrete. Concrete roof decks, by their
nature, are meant to be protected from moisture.
However, the presence of moisture on roofing systems may result
from failure of the roofing membrane, poor detailing of drainage
facilities, or lack of maintenance of drainage facilities.
Overwatered leading to shrinkage cracking
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Low Concrete Tensile Strength
Concrete with low tensile strength facilitates corrosion damage in
two ways. First, the concrete develops tension or shrinkage cracks
more easily, admitting moisture and oxygen, and in some caseschlorides, to the level of the reinforcement. Second, the concrete is
more susceptible to developing cracks at the point that the
reinforcement begins to corrode.
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EXAMPLE OF CORROSION:Bhopal Accident
Bhopal is probably the site of the greatest industrial disaster inhistory. Between 1977 and 1984, Union Carbide India Limited
(UCIL), located within a crowded working class neighbourhood in
Bhopal, was licensed by the Madhya Pradesh Government to
manufacture phosgene, mono methylamine (MMA), Methyl Iso
Cyanate (MIC) and the pesticide Carbaryl, also known as Sevin.
The long term effects were made worse by the absence of systems to
care for and compensate the victims. Furthermore, safety standards
and maintenance procedures at the plant had been deteriorating and
ignored for months. A listing of the defects of the MIC unit runs asfollows:
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Gauges measuring temperature and pressure in the various parts of
the unit, including the crucial MIC storage tanks, were so notoriously
unreliable that workers ignored early signs of trouble.
The refrigeration unit for keeping MIC at low temperatures (and
therefore less likely to undergo overheating and expansion should a
contaminant enter the tank) had been shut off for some time.
The gas scrubber, designed to neutralize any escaping MIC, had been
shut off for maintenance. Even had it been operative, post-disasterinquiries revealed, the maximum pressure it could handle was only
one-quarter that which was actually reached in the accident.
The flare tower, designed to burn off MIC escaping from the
scrubber, was also turned off, waiting for replacement of a corroded
piece of pipe. The tower, however, was inadequately designed for its
task, as it was capable of handling only a quarter of the volume of
gas released
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PREVENTION METHODS Keep concrete always dry, so that there is no H2O to form rust. Also
aggressive agents cannot easily diffuse into dry concrete. If concrete
is always wet, then there is no oxygen to form rust.
A polymeric coating is applied to the concrete member to keep out
aggressive agents. A polymeric coating is applied to the reinforcing
bars to protect them from moisture and aggressive agents. The
embedded epoxy-coating on steel bars provide a certain degree ofprotection to the steel bars and, thereby, delay the initiation of
corrosion. These coatings permit movement of moisture to the steel
surface but restrict oxygen penetration such that a necessary reactant
at cathodic sites is excluded.
Stainless steel or cladded stainless steel is used in lieu of
conventional black bars.
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FLY ASH : Using a Fly Ash concrete with very low permeability,
which will delay the arrival of carbonation and chlorides at the level
of the steel reinforcement. Fly Ash is a finely divided silica rich
powder that, in itself, gives no benefit when added to a concretemixture, unless it can react with the calcium hydroxide formed in the
first few days of hydration
Concrete mix design modifications involve such factors as reduced
w/c, including use of water reducing admixtures or super plastizers;
type of cement; permeability reducing admixtures such as fly ash,silica fume, and blast furnace slag; and corrosion inhibiting
admixtures.
Structural design aspects of corrosion control involve factors such as
configurational (geometrical) considerations that minimize or, ifpossible, eliminate exposure to corrosives.
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Here are following pictures of building showing
corroded regions
In this image is clearly shown that the beam and slab of the
building are clearly damaged due to the effect of corrosion.
The beam was failed due to the non provision of sufficient
clear cover.
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This image is the perfect example for column failure due to
the effect of corrosion.
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CONCLUSION There are a number of ways of assessing on-line corrosion, involving
electro chemical measurements or more direct assessments of
effective section, but ins pection visual or otherwise, for all systemsthat may corrode has ramifications for the designer in ensuring that it
is possible.
In some installations this may involve the incorporation of probes,
coupons or test specimens exposed to the same environment as theplant and therefore simulating the corrosion of the latter, but in a
form which allows easier assessment of the extent of corrosion.
Finally, but as an integral part of the total design and not as an afterthought,
the means of corrosion control, by material modification or by chemical or
electrochemical treatment, should be considered with as much care as is putinto any other aspect of the design process.
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The effects of fabrication methods also should be considered in
this context, welded, brazed or soldered joints, where
applicable and providing any dissimilar metal contact problemsare taken into account, usually providing less risk of crevices
than mechanical fastening methods, although whatever method
of joining is employed only careful attention to detail can
ensure satisfactory performance.
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Thank you