CORROSION(Chapter 4).pptx

7/23/2019 CORROSION(Chapter 4).pptx

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Introduction of CorrosionFacts about Corrosion

Causes of Corrosion

Types of corrosionE ects of corrosion

Factors aecting

corrosionPrevention or control ofcorrosion

CHAPTER 4CORROIO! A!" IT PRE#E!TIO!


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Corrosion

The branch of Engineering that deals with

the study of corrosion and its preventionsafely is known as corrosion Engineering.

Corrosion:

The loss of material(metals or alloys) or its

useful properties by chemical interactionwith its environment is known as corrosion.

Examples

o!usting of iron.o"lackening of silver articles in atmosphere.

o#ogging of nickel.


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Facts about corrosion

$. Corrosion is a natural process.%. Corrosion is an electrochemical

process.

&. Corrosion leads to a tremendousloss.

'. Corrosion is the reverse of metal

extraction.


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ost metals exist in nature in combined forms that is in the

form of ore. o extraction of metal from its ores is anunnatural process in which component of a stable system isbeing separated.

*re extraction etal corrosion corrosion

producttable gain es mteta stable loss of es stable

+uring the process of extraction a number of steps likeconcentration, !oasting ,and melting are involved and

-nally the ore is reduced into metal. therefore metals in their -nally re-ned state are highlyunstable and have a tendency to revert back in theiroriginal state that is to stable state by the process ofcorrosion(oxidation).

Causes of

corrosion


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popular

example of corrosion.

sually the urface of metal is covered with a thin layer ofoxide. /hen this layer is broken, bare surface is exposedto the environment and this area act as an anode whilethe remaining area act as cathode.

since the medium is exposed to the atmosphere, it

contain dissolved oxygen. Thus at cathode oxygenreduction take place while at anode iron corrodes.

' #e '#e0% 01 e *xid. 2t anode

*% 0 '3%* 0 1 e 1*34 !ed. 2t

cathode555555555555555555555555555

'#e 0 63%* 0 &*% '#e(*3)& #e%*&. n3%*


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Types of corrosion

$%&nifor' corrosion

This is also called general corrosion. Thesurface e7ect produced by most directchemical attacks (e.g., as by an acid) is a

uniform etching of the metal.


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(% )alvanic Corrosion

8alvanic corrosion is an electrochemicalaction of two dissimilar metals in thepresence of an electrolyte and anelectron conductive path. 9t occurs whendissimilar metals are in contact.


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*% Crevice or contact corrosion

Crevice or contact corrosion is thecorrosion produced

at the region of contact of metals withmetals or

metals with nonmetals. 9t may occur atwashers,

under barnacles, under applied protective-lms.


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4% Corrosion in Concrete

Concrete is a widely4used structuralmaterial that is

freuently reinforced with carbon steelreinforcing

rods, post4tensioning cable or pre4stressingwires. The

steel is necessary to maintain the strengthof the

structure, but it is sub;ect to corrosion.


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+% Erosion corrosion

Erosion corrosion is the result of a combination of an

aggressive chemical environment and high


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Reduced Strength

Downtime of equipment

Lost surface properties

Reduced value of goods

Effects of corrosion


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Factors aecting corrosion$% Presence of i'purities in 'etals

peed of corrosion increases with the presence ofimpurities in the metals because these impurities

help

in setting up the voltaic cells.

(% Presence of electrolyte

Electrolytes present in water also increases the rate

ofcorrosion e.g. corrosion of iron in sea water takes

place in large extent than in distilled water because

sea water contains salts i.e.electrolytes.


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*% Position of 'etals in electroc-e'ical series

3ighly reactive metals undergo corrosion faster than

least reactive metals. !eactivity of metals can be

found from the electrochemical series.e.g.2u Cu >b #e ?n 2l g @a A

!eactivity increases

4% Presence of carbon dio.ide in /ater

>resence of carbon dioxide in natural water also

increases the rusting of iron because it acts as an

electrolyte and increases the


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+% Presence of protective coating

/hen the iron surface is coated with themetal,

which is more reactive than the iron, then

the rate ofcorrosion is retarded e.g. when iron is

coated with

Binc, iron is protected from rusting.


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>!EDE@T9*@ *# C*!!*9*@

Corrosion is slow but continuouslyeating away the metal andconseuence of rusting.

!usting can be avoided by notletting moist air come in contactwith the metal surface .#ollowing

methods could gain fully be adoptedto minimiBe rusting and corrosion


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ethods of preventing corrosion and rusting

$. Tarring

%. >ainting

&. Enameling

'. 8alvaniBing

. heradising6. Tin plating

F. electroplating


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metal is dipped in hot coal tar so that a -lmof it is sticks to the surface which protectthe surface from rusting and corrosion.

>ipes or ends of poles to be embedded inthe earth are usually given this protectivetreatment.

$% Tarring0


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>aints usually the lead paints areapplied on the surface to be

protected. Exposed metal surfacesas in case of roof and bridgestructure are given this treatment

which has to be repeated afterregular interval of time.

(% Painting0


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Enamels consist of high grade bases likeBinc oxide or lead oxide ground in oil orvarnish. they dry slowly leaving a hardtough and elastic -lm which is smooth

and durable.Enamle painted surfaces arewashable and are not a7ected byacids,alkali,gases or steam. Even toughthey are more costly than ordinarypaints yet because of their durability.

They are eually good for use both onexternal and internal work.

*%Ena'eling0


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+epositing a -ne -lm of Binc on the ironGsteelsurfaces is termed as galvaniBing.

The surfaces to be galvaniBing is -rst clearedof all foreign matter by giving it an acid washto be followed by a wash of clean water. thesurface is then dried and dipped in moltenBinc.The -ne -lm of Binc that get depositedprotect the surface from contact with

atmosphere and conseuence oxidation.

!emoval of the Binc -lm caused by scratches.

4%)alvani1ing0


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urface to be treated is cleaned of allforeign deposits by washing it with acidsolution and then with clean water.

9t is then dried and covered with Binc dust

and enclosed in steel boxes to be heated ina furnace under controlled temperatures.

olten Binc spreads over the whole surfaceand on cooling forms a thin protective layer.

herading gives better protection thangalvaniBing.

+%-eradising0


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2fter cleaning the surface with acidwash followed by wash with cleanwater and drying, it is dipped in a

bath of molten tin.2 protective covering of tin layer ifleft on the surface.

,%Tin plating0


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"y the process of electrolysis athin -lm ofnickel,cromium,copper or Binc is

deposited on the surface to beprotected.

The surface to be protected is

made the cathode and the metalto be deposited is made theanode.

% Electroplating0

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