Effect of Corrosion in Structures

8/13/2019 Effect of Corrosion in Structures

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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

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Effect of Corrosion in Structures

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