Corrosion 1.ppt

7/25/2019 Corrosion 1.ppt

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Corrosion contd.Corrosion contd.

Galvanic or contact corrosionGalvanic or contact corrosion


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Contd.

Occurs when two dissimilar

metals are in electrical contact

with each other and areexposed to an electrolyte

Severity of the corrosion of the

anodic metal depend upon therelative position of the two

metals in the galvanic series


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Contd.

Zn less noble readily dissolves forms anode Cu more noble acts as cathode

Cathodic reaction may occur either by hydrogen evolutionprocess or by the oxygen absorption process depending on thecorrosive environment

Dissimilar metals in contact with each other corrosion israpid due to galvanic reaction

If direct contact between dissimilar metals can be avoided norapid corrosion of the anodic metal Hence fabrication of metals by riveting or welding should be

done with materials having the same chemical composition asthe base metal


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High temperature corrosion

!nown as oxidation different from wet

corrosion

"etals are widely used at hightemperatures therefore behaviour of

metals at high temperature is of great

interest

High temperature oxide films and

scales are formed


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Contd.Contd.

Corrosion like rusting of the ferrousCorrosion like rusting of the ferrous

alloys are formedalloys are formed

Oxide films formed or some metalsOxide films formed or some metalsact as diffusion barriersact as diffusion barriers

Alloying elements like Cr, Al, Si etc.Alloying elements like Cr, Al, Si etc.

get oxidized and concentrated in theget oxidized and concentrated in the

scale layer corrosion resistancescale layer corrosion resistance

improvesimproves


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passivity Defined as a phenomenon in which a metalor an alloy exhibits a much highercorrosion resistance than its position inthe electrochemical series would indicate

Passive state less reactive

Ti, Cr, Al and stainless steel passive

metals


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Reasons for passivityReasons for passivity

Formation of a highly protective, butFormation of a highly protective, but

very thin and quite invisible lm on thevery thin and quite invisible lm on the

surface of the metalsurface of the metal

Films must be nonporous, insolubleFilms must be nonporous, insoluble

and self-healing when mechanicallyand self-healing when mechanically

damageddamaged

Metal also becomes passive becauseMetal also becomes passive becauseof a chemically absorbed layer ofof a chemically absorbed layer of

oxygen on the metal surfaceoxygen on the metal surface


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Contd.Contd.

#his layer is only one molecule thic$ and#his layer is only one molecule thic$ and

will be self%healingwill be self%healing

&assivation applies only to certain&assivation applies only to certainenvironmental conditions which tend toenvironmental conditions which tend to

maintain a protective oxide film on themaintain a protective oxide film on the

metal surface.metal surface.


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'actors influencing corrosion rate'actors influencing corrosion rate

Depending on metalsDepending on metals

!" #lectrode potential of metal or its!" #lectrode potential of metal or its

position in the galvanic series $usuallyposition in the galvanic series $usually

anodic metals corrode faster%anodic metals corrode faster% &" 'ydrogen over voltage $i"e" ease of&" 'ydrogen over voltage $i"e" ease of

hydrogen evolution%hydrogen evolution%

(" )hemical inhomogeneities, e"g",(" )hemical inhomogeneities, e"g",

inclusions, impurities, di*erent phases,inclusions, impurities, di*erent phases,

which promote the formation of anodic andwhich promote the formation of anodic and

cathodic sitescathodic sites


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Contd.Contd.

(. &hysical inhomogeneities i.e. regions(. &hysical inhomogeneities i.e. regions

of high residual stress in the metal. #hisof high residual stress in the metal. #his

region tend to become anodic with respectregion tend to become anodic with respect

to those regions which are free from stressto those regions which are free from stress

). &assivity of the metal i.e. formation of). &assivity of the metal i.e. formation of

thin films of oxidethin films of oxide


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actors depending onactors depending onenvironmentenvironment

! ". p# of the solution$ p# % & andalkali solution less corrosive$p#'&, more corosive

! (. )ature * concn. Of other ions, andthe degree of agitation + excessiveagitation is detrimental because of

abrasion and breakage of theprotective film


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Contd.Contd.

. -nfluence of oxygen in the solution a. -nfluence of oxygen in the solution aconstant supply of oxygen is necessary forconstant supply of oxygen is necessary forcorrosion to continuecorrosion to continue

. -ncrease in temperature increases. -ncrease in temperature increasesionization and fluidity of the solution andionization and fluidity of the solution andthe rate of reactionthe rate of reaction

/. 0resence of stress due to mechanical/. 0resence of stress due to mechanical

1orking increases corrosion of the1orking increases corrosion of thestressed zones due to potential differencestressed zones due to potential differenceon the metal surfaceon the metal surface


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*pecific types of corrosion*pecific types of corrosion

+. ,tmospheric corrosion+. ,tmospheric corrosion

-. nderground corrosion-. nderground corrosion

/. "icrobial corrosion/. "icrobial corrosion

(. 0i1uid metal corrosion(. 0i1uid metal corrosion

). uniform corrosion). uniform corrosion

2. &itting corrosion2. &itting corrosion

3. Crevice corrosion3. Crevice corrosion

4. Intergranular corrosion4. Intergranular corrosion

5. 'retting and selective corrosion5. 'retting and selective corrosion+6. *tress corrosion+6. *tress corrosion

++. Corrosion fatigue++. Corrosion fatigue

+-. 7rosion corrosion+-. 7rosion corrosion


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1. Atmospheric corrosion

&rimarily due to the combined effects of film formationand film brea$down

8ate of atmospheric corrosion depends up on 9i:nature of metal 9ii: degree of protection offered by thefilm present or produced 9iii: nature of the electrolytepresent

Crac$s and discontinuities in the film producedexpose fresh areas of the metal to humid atmosphere

locali;ed corrosion cells are formed


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Contd.Contd.

2lectrolyte usually rain 1ater and humid2lectrolyte usually rain 1ater and humidatmosphere$ dissolved gases such as COatmosphere$ dissolved gases such as CO((or SOor SO((increases the acidity of theincreases the acidity of theatmospheric moistureatmospheric moisture

3ain supplies the moisture necessary for3ain supplies the moisture necessary forelectrochemcial attack$ 1ashes a largeelectrochemcial attack$ 1ashes a largepart of the film from a metal surface$ thepart of the film from a metal surface$ thesurface is exposed for the attacksurface is exposed for the attack

Atmospheric corrosion is severe on ferrousAtmospheric corrosion is severe on ferrousmetals, 1hile non+ferrous metals form dullmetals, 1hile non+ferrous metals form dullsurfaces in the presence of naturalsurfaces in the presence of naturalenvironmentenvironment


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&" +nderground corrosion&" +nderground corrosion

?lso nown as soil corrosionlso nown as soil corrosion

?Depends up on the corrosive nature of theDepends up on the corrosive nature of the

soil. causes serious damage to materialssoil. causes serious damage to materials

?)orrosive properties of soil depend on their)orrosive properties of soil depend on theiracidity, degree of soil aeration, electricalacidity, degree of soil aeration, electrical

conductivity, salt and moisture contentconductivity, salt and moisture content

?

/acteria and micro-organisms in the soil can/acteria and micro-organisms in the soil canalso greatly enhance the rate of corrosionalso greatly enhance the rate of corrosion


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Contd.Contd.

In underground soil corrosion galvanicIn underground soil corrosion galvaniccorrosion arising from contact betweencorrosion arising from contact betweendifferent metals happens fre1uentlydifferent metals happens fre1uently

7x @ Cu pipe service line connected to a cast7x @ Cu pipe service line connected to a castiron main galvanic cell is formediron main galvanic cell is formed

Ground rod of bare Cu connected to a steelGround rod of bare Cu connected to a steel

piping galvanic corrosion cell is formedpiping galvanic corrosion cell is formed Concentration cells are responsible for muchConcentration cells are responsible for much

of the corrosion ta$ing place in the soilof the corrosion ta$ing place in the soil


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Contd.

Pipelines pass through differentsoils; when two different soils are incontact with a single piece of metal,

concentration cell is formed That portion of the pipe lying in more

concentrated soil is the anode andthat in less concentrated cell is the

cathode oisture acts as the electrolyte,

pipe itself furnishes the connectingcircuit


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Contd.

Axygen concentration cells are very commonon buried pipe

#emperature cell is another $indB e.g. gastransmission cell laid in soil B hot compressedgas ma$es the pipe near the compressoranode and the copper pipe down in th line isthe cathode

In acid soils corrosion mechanism is similarto that of the hydrogen evolution type


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Microbial corrosionMicrobial corrosion

0his is caused by the metabolic activity of various micro0his is caused by the metabolic activity of various microorganismsorganisms

More important micro-organisms responsible for mostMore important micro-organisms responsible for mostcorrosion arecorrosion are

$i% sulphate reducing bacteria$i% sulphate reducing bacteria

$ii% sulphur bacteria$ii% sulphur bacteria $iii% 1ron and manganese bacteria$iii% 1ron and manganese bacteria $iv% micro-organisms capable of forming microbiological$iv% micro-organisms capable of forming microbiological

lmslms Metal pipes and structures buried in the damp soilMetal pipes and structures buried in the damp soil

containing sulphates and organic matter are attaced bycontaining sulphates and organic matter are attaced bythe sulphate reducing bacteriathe sulphate reducing bacteria

0he corrosion product is iron sulphide instead of rust0he corrosion product is iron sulphide instead of rust /acteria attac is fast and locali2ed/acteria attac is fast and locali2ed


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Contd.Contd.

#he corrosion product is iron sulphide#he corrosion product is iron sulphide

instead of rustinstead of rust


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(. 0i1uid metal corrosion(. 0i1uid metal corrosion

0i1uid metals produce unusual types of0i1uid metals produce unusual types ofdeterioration in the materials which they come indeterioration in the materials which they come incontact during operation li$e steel ma$ing metalcontact during operation li$e steel ma$ing metalcasting or heat transfer using li1uid metalscasting or heat transfer using li1uid metals

"echanism not electrochemicalB no transfer of"echanism not electrochemicalB no transfer ofelectronselectrons

Instead processes involved are@Instead processes involved are@

solution of solid materialsolution of solid material formation of chemical compoundsformation of chemical compounds

diffusion of li1uid metal into the solid materialdiffusion of li1uid metal into the solid material


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Contd.Contd.

Accurs in devices for nuclear powerAccurs in devices for nuclear power

Here corrosion 9essentially a process of mass transfer: is notHere corrosion 9essentially a process of mass transfer: is notdependent upon local cell local cell potentials for its driving forcedependent upon local cell local cell potentials for its driving force

Corrosion occurs by simple dissolution of the reactor components atCorrosion occurs by simple dissolution of the reactor components atthe high temperaturethe high temperature

In the heat exchanger the li1uid metal temperature drops solubilityIn the heat exchanger the li1uid metal temperature drops solubilityof the metal in the coolant li1uid metal is lowered and is depositedof the metal in the coolant li1uid metal is lowered and is depositedon the surfaceon the surface

#hus hot ;ones are continuously thinned and the heat exchanger#hus hot ;ones are continuously thinned and the heat exchangertubes are clogged resulting in fre1uent shut downtubes are clogged resulting in fre1uent shut down

sually inhibitors are added in to the li1uid metal to slow down suchsually inhibitors are added in to the li1uid metal to slow down such

a corrosiona corrosion


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). niform corrosion). niform corrosion

4niform over a given surface,4niform over a given surface,

relatively superficial and unlocalizedrelatively superficial and unlocalized

4nlocalized attack may start in its4nlocalized attack may start in its

initial phase at certain areas andinitial phase at certain areas and

spreads over the 1hole surface thespreads over the 1hole surface the

metal graduallymetal gradually

5herefore, 1hole surface is5herefore, 1hole surface is

uniformly corroded in course of timeuniformly corroded in course of time


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6. Pitting corrosion

*erious type of locali;ed corrosion through

the electrochemical reaction

Accurs when protective films generally

continues and then brea$ down in someenvironment producing cell action

&oint of film brea$age becomes anode while

the surrounding unbro$en film acts as

cathode small galvanic cells are created

producing pits


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Contd.Contd.

! Corrosion products may form caps over pitcavities

! Shapes of the pits vary 1idely usuallythese are saucer shaped, conical orhemispherical

! Only a small amount of metal is corrodedbut perforations or holes that aredeveloped can lead to costly repair of

expensive e6uipment! 0itting causes more unexpected corrosion

loss than other types of corrosion


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3. Crevice corrosion3. Crevice corrosion

3ccurs in cracs or crevices formed between3ccurs in cracs or crevices formed between

mating surface of metal assemblies andmating surface of metal assemblies and

taes the form of pitting or etched patchestaes the form of pitting or etched patches

4ocali2ed form of corrosion4ocali2ed form of corrosion /oth surfaces may be of the same metal or/oth surfaces may be of the same metal or

of dissimilar metals or one surface may be aof dissimilar metals or one surface may be a

non-metalnon-metal

)an also occur under scale and surface)an also occur under scale and surfacedeposits and under loose tting washers anddeposits and under loose tting washers and

gasetsgasets


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Contd.Contd.

Crevice corrosion results as a result ofCrevice corrosion results as a result of

differential aeration celldifferential aeration cell

Accurs only at film protected metalsAccurs only at film protected metals

such as ,l "g stainless steel andsuch as ,l "g stainless steel and

titaniumtitanium


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7. -ntergranular corrosion7. -ntergranular corrosion

! A form of localized surface attack in 1hicha narro1 path is corroded outpreferentially along the grain boundariesof a metal

! 8riving force is the difference in corrosionpotential that develops bet1een a thingrain boundary zone and the bulk of theimmediate ad9acent grains

! 5he constituent may be anodic, cathodicor neutral to the base metal or ad9acentzone


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Contd.Contd.

! 2xample for anodic constituents are :g/Al7,:g;n(in Al alloys and e) in iron alloys

! 2xamples for cathodic constituents areeAland CuAl(in Al alloys and eC in ironalloys

! -f the precipitate is anodic to the ad9acentzone, precipitate corrodes preferentiallyand if the precipitate is cathodic, the

ad9acent zone is corroded! Occurs most commonly in Al, Cu and "7


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5. 'retting and selective corrosion5. 'retting and selective corrosion

retting corrosion is local surfaceretting corrosion is local surface

discolorations and deep pitsdiscolorations and deep pits

Selective corrosion another type ofSelective corrosion another type of

localized corrosionlocalized corrosion

:ost common example is:ost common example is

dezincificaion of brassdezincificaion of brass

Occurs in soft 1aterOccurs in soft 1ater


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10. Stress corrosion

Combination of tensile stress and corrosiveenvironment

Ane of the causes of failures of metal structure

!nown as stress corrosion crac$ing

*tress may be residual or applied

It may be low enough and will cause no concern inthe absence of the environment

*tress corrosion crac$ing usually occurs under only

very mild corrosive conditions o metal loss or general corrosion associated with it


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Contd.

Crac$s may be intergranular trans granular or a

combination of both

Accurrence and rate of stress corrosion are

influenced by stress level corrosive agent time and

temperature of exposure structure of the metal

amount of plastic strain behavior of protective films

etc.

sually found in ,l alloys low carbon steels and

brass Accurs on mild steel exposed to al$aline solution at

high temperature and stresses


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!!")orrosion fatigue!!")orrosion fatigue

? 1nvolves the reduction in the fatigue strength1nvolves the reduction in the fatigue strength

of a metalof a metal

? Due to combined action of fatigue andDue to combined action of fatigue and

corrosioncorrosion? 5hen a metal is sub6ected to cyclic stress,5hen a metal is sub6ected to cyclic stress,

number of cycles required to cause failures atnumber of cycles required to cause failures at

a given stress will be reduced in a corrosiona given stress will be reduced in a corrosion

environmentenvironment? ttacs existing surface 7aws such as cracsttacs existing surface 7aws such as cracs

or forms new intergranular cracsor forms new intergranular cracs


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12. Erosion corrosion12. Erosion corrosion

Refers to acceleration of corrosion byRefers to acceleration of corrosion bysimultaneous abrasion by the turbulent ow ofsimultaneous abrasion by the turbulent ow ofgases or liquidsgases or liquids

Caused by the breakdown of a protective lm atCaused by the breakdown of a protective lm atthe place of impingement and its subsequentthe place of impingement and its subsequentinability to repair itself under existing conditionsinability to repair itself under existing conditions

Removes protective lms from localized areas inRemoves protective lms from localized areas inthe metal surface thereby contributes to thethe metal surface thereby contributes to theformation of di!erential cells at such areas andformation of di!erential cells at such areas andlocalized pitting takes place at the anodic pointslocalized pitting takes place at the anodic points

of the cellsof the cells "ost commonly found in condenser tubes piping"ost commonly found in condenser tubes piping

agitator and vessels in which stream of liquidagitator and vessels in which stream of liquidemerge from a pipe and strike a wall of the vesselemerge from a pipe and strike a wall of the vessel

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Corrosion 1.ppt