Factors affecting corrosion & control measures

Factors affecting corrosion on the basis of nature of the metal and Nature of Corroding medium, Protection from corrosion in reference to Sacrificial protection, Cathodic protection, Metallic coatings, Organic coatings, Proper designing.

1. Prepared by:
Prof Sarala Prasanna Pattanaik
Assistant Professor
Department of Chemistry

3. •MechanismofDry(Chemical)andWet(electrochemical)
Corrosion
•TypesofElectrochemicalCorrosion(Differentialaeration
corrosion,Galvaniccorrosion,Pittingcorrosion,Waterline
corrosion,Crevicecorrosion,Stresscorrosion)
•Factorsaffectingcorrosion:NatureofthemetalandNature
ofCorrodingmedium.
•Protectionfromcorrosion(Sacrificialprotection,Cathodic
protection,Metalliccoatings,Organiccoatings,Proper
designing.)

4. There are two factors that influence the rate of corrosion which are
involved in the process of corrosion.
1. Nature of the Metal
•Position of metal in Galvanic series
•Purity of the metal
•Physical state of the metal
•Volatility of Corrosion product
•Nature of oxide film formed
•Solubility of corrosion product formed
•Relative areas of Cathode & anode
•Passive character of the metal
Factors affecting Corrosion

5. 2. Nature of Corroding Environment
•Temperature
•Humidity in air
•Effect of pH
•Presence of impurities in atmosphere
•Conductance of the electrolytic medium
•Formation of Oxygen concentration cell
•Presence of Suspended particles in atmosphere
Factors affecting Corrosion

6. a.PositionofmetalinGalvanicseries:-Theextentofgalvanic
corrosiondependsonthepositionofthemetalsingalvanic
series.Themetalwhichisplacedathigherpositionintheseries
aremorereactiveandundergoescorrosion.Therateand
severityofcorrosion,dependuponthedifferenceintheir
positionandgreateristhedifference,thefasteristhecorrosion
oftheanodicmetal.
b.Purityofthemetal:-Impuritiesinametalcauseheterogeneity
andformstiny/minuteelectrochemicalcells(attheexposed
part)andtheanodicpartgetscorroded.Generally,puremetal
donotundergoanytypeofelectrochemicalcorrosion.Hence,
moretheimpuritygreateristherateofcorrosion.
Cont………
1. Nature of the Metal

7. c.Physicalstateofthemetal:-Rateofcorrosionisinfluencedby
thephysicalstateofthemetal(suchasgrainsize,orientationof
crystals,stressetc).Thesmallerthegrainsizeofthemetal,
higherwillbeitssolubilityandgreaterwillbeitscorrosion.
However,areaunderstresseveninapuremetal,tendtobe
anodicandcorrosiontakesplaceatthesestressedareas.
d.VolatilityofCorrosionproduct:-Higherthevolatilenatureofthe
corrosionproductincaseofdrycorrosion,greateristherateof
corrosionsuchasinthecaseofMoformvolatileoxidelayer
(MoO
3),SnformingvolatilefilmofSnCl
4whenexposedtoCl
2.
Cont ………
1. Nature of the Metal

8. e.Natureofoxidefilmformed:-Inaeratedatmosphere,practicallyall
themetalsgetcoveredwithathinsurfaceoffilmofmetaloxide.The
ratioofthevolumeofthemetaloxidetothemetalisknownas
specificvolumeratio.Greaterthespecificvolumeratio,lesseristhe
oxidationcorrosionrate.ThespecificvolumeratioofAl,NiandCrare
1.3,1.6and2.0respectivelyandconsequentlytherateofoxidationof
CrislessascomparedtoAlandNi.
f.Solubilityofcorrosionproductformed:-Inelectrochemicalcorrosion,
thesolubilityofthecorrosionproductinthecorrodingmediumisa
decidingfactorfortheextentandrateofcorrosion.Ifthecorrosion
productissolubleinthecorrodingmedium,corrosionofthemetal
takesplaceatahigherrate.Butifthecorrosionproductisinsolublein
thecorrodingmedium,itformsaprotectivelayeronthemetal
surfaceandinhibitsfurthercorrosionofthemetal(PbSO
4formedis
insolubleinH
2SO
4actsasabarrierwhenPbisexposedtoH
2SO
4)
Cont ………
1. Nature of the Metal

9. g.RelativeareasofCathode&anode:-Whentwodissimilar
metalsareelectricallyconnectedinthepresenceofan
electrolyticmedium,thecorrosionoftheanodicpartisdirectly
proportionaltotheratiooftheareasofthecathodicpartand
theanodicpart.Corrosionismorerapid,severeandhighly
localized,iftheanodicareaissmallbecausethecurrentdensity
atasmalleranodicareaismuchgreaterandthedemandfor
electronscanbemetbysmalleranodicareaonlybyundergoing
corrosionmorerapidly.
h.Passivecharacterofthemetal:-Higherthepassivecharacterof
themetalsuchasAl,Cr,Pb,Sn,Ti,Ni,Co,Mnetc,lesseristhe
rateofcorrosionastheyexhibitoutstandingcorrosion
resistance.
Cont ………
1. Nature of the Metal

10. a. Temperature:-Rise in temperature increases the rate of corrosion
due to increase in the rate of diffusion of ions.
b.Humidityinair:-Therateofcorrosionwillbemorewhentherelative
humidityoftheenvironmentishigh.Themoistureactsasasolvent
foroxygen,CO
2,SO
2etcintheairtoproducetheelectrolytewhichis
requiredforsettingupofanelectrochemicalcell.
c.Effect of pH:-Lesser the pH of the corroding medium, greater is the
extent of corrosion i.eacidic medium is more corrosive in nature.
d.Presenceofimpuritiesinatmosphere:-Atmosphereinindustrial
areascontainscorrosivegaseslikeCO
2,SO
2,H
2SandvapoursofHCl,
H
2SO
4etcandinthepresenceofthesegasestheacidityaswellas
electricalconductivityoftheliquidadjacenttothemetalsurface
increasestherebyincreasingtherateofcorrosion.
Cont ………
2. Nature of Corroding Environment

11. e.Conductanceoftheelectrolyticmedium:-Highertheelectrical
conductivityoftheliquidincontactwiththemetalsurface,greateris
therateofcorrosion.Salineenvironmentismorecorrosivethannormal
environmentduetothepresenceofhigherconcentrationofdissolved
saltsintheliquidduetowhichmetalsnearbyseaareaismorecorroded.
f.FormationofOxygenconcentrationcell:-Formationofoxygen
concentrationcellduetodifferenceinoxygenconcentrationleadsto
higherextentofcorrosion.Therateofcorrosionincreaseswiththe
increaseinoxygenconcentration.Insuchcase,lessoxygenatedpartof
themetalactsasanodewhilemoreoxygenatedpartactsascathode
leadingtotheformationofanelectrochemicalcell.
g.PresenceofSuspendedparticlesinatmosphere:-Ifthesuspended
particlesarechemicallyactiveinnaturelikeNaCl,(NH
4)
2SO
4etc,they
absorbmoistureandactsasstrongelectrolyte,therebyincreasingthe
rateofcorrosion.Similarly,ifthesuspendedparticlesarechemically
inactiveinnaturelikeCharcoal,theyalsoabsorbbothsulphurgasesand
moistureansslowlyenhancestherateofcorrosion.
2. Nature of Corroding Environment

12. Protective Measures Against Corrosion

13. There are many methods of protecting metals against corrosion.
•Barrierprotection
•Sacrificialprotection
•Cathodicprotection
•Alloyformation
Barrierprotection:
Inthiscase,athinbarrierisdevelopedbetweenthesurfaceofironand
atmospherebyoneofthefollowingmethods:
a)Paintingofthemetallicsurfaceuniformly.
b)Coatingthebasemetalsurfacewithathinfilmofsomenon
corrosive/passivemetalslikenickel,chromium,tin,copperetc.

14. Coatingthemetal:
Inordertopreventcorrosion,resistantcoatingismadebetweenmetal
andenvironment.Differenttypesofmetalliccoatingsare
•Galvanizing(thincoatingofZnoveriron)
•Electroplating(coatingofCu,NiorCroverironwiththeaidofdirect
current.
•TinplatingorTinning(coatingoftinoveriron)
•Sheradizing(itconsistsofdustingofZnpowderoverironsurface
followedbyheating)
•Cladding(sandwichingthebasemetalwithcoatingmetal)

15. Alloying the metal:
Metalhasbetterresistancetocorrosionafterformingalloywithother
metalse.g.Stainlesssteel,inwhichordinarysteelisalloyedwith
chromium,nickel,cobalt,manganeseetc.
Alloysarehomogeneoussolidsolutionsinwhichthecomponentsare
completelysolubleinoneanother,e.gFe–Cr–Ni(Stainlesssteel),Cu
–Nialloy(Monelmetal),Au–Agalloy,Ag–Cualloy(Sterlingsilver),Cu
–Znalloy(Brass),Cu–Snalloy(Bronze),Pb–Snalloy(Solder),Cu–Sn
–Zn(Gunmetal),Al–Cu–Mn–Mgalloy(Duralumin),Fe–Al–Ni–Co
(Alnico)etc.
Solidsolutionalloysaremorecorrosionresistant.

16. SacrificialAnodicProtection:
Inthiscase,thesurfaceofthebasemetal(sayIron)iscoveredwithamore
electropositivecoatingmetallikeMg,ZnorAletc.Sincethiscoatingmetallooses
electronsmorereadilythaniron,rustingisprevented.Aslongasthecoatingmetal
ispresent,irondoesnotgetrusted.Thistypeofprotectioniscalled‘Sacrificial
AnodicProtection’inwhichthecoatingmetalactsasanodew.r.t.thebasemetal
andsacrificesitselftoprotectthebasemetal.
CathodicProtection:
Itistheprotectionoftheparentmetalfromcorrosionbyconnectingwithamore
activemetallikeMg,Al,Znetc.Themoreelectropositive(active)metalactslike
anode(supplieselectrons)andparentmetalactslikecathode(receiveselectrons).
Thus,connectedmetalundergoescorrosiontherebyprotectingtheparentmetal
fromcorrosionbyforcingittoactasacathode.Hence,themethodiscalled
‘CathodicProtection’.

17. Protective Measures Against Corrosion
Metallic Coatings:
•Electroplating:Acoatingmetalisdepositedonthebasemetalbypassing
directcurrentthroughanelectrolyticsolutionthroughelectrolysis.Itiswidely
adoptedtocoatthebasemetalwithaprotectivemetalliccoatingofZn,Au,Ag,
Cu,Ni,Pb,Snetc.
•MetalCladding:Thebasemetaltobeprotectedandcoatingmetalare
sandwichedbypressingthroughtherollersundertheactionofheat&pressure
(Alcladsheetingobtainedbycladdingasheetofduraluminwithaluminium).
•HotDipping:Thebasemetaltobecoatedisimmersedinabathofthe
moltencoatingmetal(suchasGalvanizingandTinning).
•Cementation:Auniformsurfacecoatingisobtainedbyheatingthebase
metalinapowderofcoatingmetal.Thismethodislimitedtothecoatingoflow
meltingmetalslikeZn,Pb,Snetc.Thiscanbeappliedtofabricatedstructure
andthereisnopossibilityofdamage.
•Metalspraying:Thecoatingmetalinmoltenstateissprayedonbasemetal
bymeansofsprayinggun.Thebasemetalsurfacemustberough.Thismethod
islimitedtothecoatingoflowmeltingmetalslikeZn,Pb,Snetc.

18. Protective Measures Against Corrosion
Organic Coating:
Applyonmetallicsurfaceforprotectionfromcorrosion&alsoto
impartdecorativevaluesuchaspaints,enamel,Varnishesetc.
Corrosion Inhibitors:
Substanceswhichwhenaddedinasmallamountinanenvironment
reducestherateofcorrosionofametalexposedtothat
environment.TheseareoftwotypesofinhibitorssuchasCathodic
&anodicinhibitors.
Using Pure Metal:
Impuritiesinametalcauseheterogeneity,whichdecreasecorrosion
resistanceofthemetal.Hencecorrosionresistanceofanymetalis
improvedbyincreasingitspurity.
Using Alloys:
Corrosionresistanceofmostmetalsisbestincreasedbyalloying
themwithsuitableelements.Formaximumcorrosionresistance,
thealloyshouldbecompletelyhomogeneous.

19. Protective Measures Against Corrosion
Organic Coating & its constituents:
Organiccoatingsareinertbarriersappliedonmetallicsurfacesandotherconstruction
materialforbothcorrosionprotectionanddecoration.Themostimportantorganic
surfacecoatingispaint.
Paintisamechanicaldispersionofmixtureofoneormorepigmentsinavehicle.This
vehicleisaliquidconsistingofnon-volatilefilmformingmaterialandavolatilesolvent
calledasthinner.
Pigment:-Itisasolidsubstance,whichprovidecolourtothepaint.Itisalsousedto
improvethestrengthandadhesionofthepaint,protectagainstcorrosion.Itimparts
impermeabilitytomoistureandincreasesweatherresistance.
Common Pigment Colour
White lead, Zinc oxide, lithopone White
Red lead, ferric oxide, Chrome redRed
Chromium oxide Green
Prussian blue Blue
Carbon black Black
Umber Brown Brown

20. Protective Measures Against Corrosion
Organic Coating & its constituents:
Vehicle(or)dryingoil:-Itisafilmformingconstituentofpaint.Thesearethe
glycerylestersofhighmolecularweightfattyacids.Thisvehicleorbinder
providesdesiredchemicalandphysicalproperties.Itdeterminestheadhesion,
cohesionandflexibilityofthepaint.Themostwidelyuseddryingoilsarelinseed
oil,soybeanoil,tungoilanddehydratedcastoroil.
Thinner:-It reduces the viscosity of the paint to a suitable consistency, suspends
the pigments, dissolves the vehicle and other additives. It increases the
penetration power of vehicle and elasticity of the paint film. It also helps in drying
of the paint as it evaporates easily. Common thinners used are turpentine,
mineral spirits, acetone, benzene, naphtha, toluene, xylene, kerosene, methyl
Ethyl ketone, dimethylformamide, methylatednaphthalene etc.
Driers:-These are the oxygen carrier catalysts. They accelerate the drying of the
oil film through oxidation, polymerization and condensation. The main function of
the drier is to improve the drying quality of the oil film. Common driers used are
resinates, linoleates, tungstatesand naphthenates of Co, Cu, Fe, Mn, Pband Zn.

21. Protective Measures Against Corrosion
Organic Coating & its constituents:
Filler(or)Extender:-Theseareoftencolorlessinorganicsubstanceslikegypsum
aluminiumsilicate,bariumcarbonate,bariumsulphate,asbestos,clay,mica,
calciumcarbonate,magnesiumsilicateetcaddedtothepaintswiththeaimto
improvetheStrength,toughness,abrasionresistanceandadhesionofthepaint
andalsotoreducethecost.Italsoactsascarriersforthepigmentcolorandfillthe
voidsinthepaintfilm,reducethecrackingofthepaintfilmandimprovethe
durabilityofthefilm.
Plasticizers:-Theyremainpermanentlyinpaintsandimprovestheelasticityofthe
paintfilmwhichpreventscrackingofthefilm.Commonlyusedplasticizersare
tricresylphosphate,triphenylphosphate,dibutylphthalateetc.
Anti–SkinningAgent:-Anti–skinningagentslikeoximes,quinones,naphthols,
aromaticamines,polyhydricphenolsetcareaddedtothepaintsothatskinning
ofpaintcanbepreventedwhenstoredforasignificantperiodoftimeinthe
containerandcanbeusedforalongperiodoftime.

22. Protective Measures Against Corrosion
Corrosion Inhibitors:
Substanceswhichwhenaddedinasmallamountinanenvironmentreducestherateofcorrosionofa
metalexposedtothatenvironment.Therearetwotypesofinhibitorsasanodicandcathodic
inhibitors.
Anodicinhibitors:-Anodicinhibitorsstopthecorrosionreaction,occurringatanode,byforminga
precipitatewithanewlyproducedmetalion.Theseareadsorbedonthemetalsurfaceintheformofa
protectivefilmorbarrier.Examples:-Chromates,Phosphates,Silicates,Tungstatesandothertransition
metalionswithhighoxygencontent.
Cathodicinhibitors:-a) In acidic solutions, the cathodicreaction is evolution of hydrogen.
2H
+
+ 2e
-
→ H
2(g)
CorrosionmaybereducedeitherbyslowingdownthediffusionofhydratedH
+
ionstothecathode.The
diffusionofH
+
ionsisconsiderablydecreasedbyorganicinhibitorslikeamines,mercaptansorthiol,
heterocyclicnitrogencompounds,substitutedureaandthioureaetc.
b) In neutral solutions, the cathodicreaction is the absorption of Oxygen.
1/2O
2+ H
2O + 2e
–
→ 2OH
–
Corrosioniscontrolledeitherbyeliminatingoxygenfromthecorrodingmediumorbyretardingits
diffusiontothecathodicarea.TheoxygeniseliminatedeitherbyreducingagentslikeNa
2SO
3,Na
2S,
NH
2NH
2orbydeaeration.TheinhibitorslikeMg,ZnorNisaltstendtoretardthediffusionofOH
-
ionsto
cathodicareas.

23. Protective Measures Against Corrosion
By Modifying Environment:
•By lowering the temperature.
•By reducing the moisture content.
•By reducing the acidity of the corroding medium or environment.
Electrochemical Protection or CathodicProtection:
•SacrificialAnodicProtection:-Metaltobeprotectedisconnectedtomore
anodicmetaltoavoidcorrosion.
•ImpressedCurrentCathodicProtection:-Thisprocessconsistsofconnecting
thematerialtobeprotectedto(–)veterminalofDCsource&(+)veterminal
ofDCsourceisconnectedtoaninsolubleanode.Thecurrentsuppliedisin
oppositedirectiontothecorrosioncurrent.Thus,themetaltobeprotected
actascathode&getprotected.

24. Protective Measures Against Corrosion
Electroplating:
Theprocessofdepositingorcoatingametalonthesurfaceofbasemetal/
nonmetalbyelectrolysisiscalledelectroplating.Itiswidelyadoptedtocoat
basemetalswithprotectivemetalliccoatingsofZn,Au,Ag,Cu,Ni,Pb,Snetc.
Process:-Thebasemetalsurfaceiscleanedthoroughly.Thearticletobe
electroplatedismadeascathode.Theanodeismadeofpuremetal,whichis
tobecoatedonthearticle.Theelectrolyteisthesaltofthemetaltobe
coatedonthearticle.Adirectcurrentispassedthroughtheelectrolyte.The
anodedissolves,depositingthemetalionsfromthesolutiononthearticleat
cathodeintheformofafinethinmetalliccoating.
Ex:ElectroplatingofSilveronIron:
Cathode:-Articletobeelectroplated(spoonofFe)
Anode:- AblockofSilver(Ag)metal
Electrolyte:-AqueoussolutionofAgNO
3

25. Image of ElectroplaingProcess

26. Protective Measures Against Corrosion
Proper Designing:
The design of the material should be such that corrosion, even if it occurs, is
uniform and does not result in intense and localized corrosion”.
•Avoid the direct contact of dissimilar metals in presence of a liquid medium.
•Whentwodissimilarmetalsaretobeincontact,theanodicmaterialshould
haveaslargeareaaspossible;whereasthecathodicmetalshouldhaveas
muchsmallerareaaspossible.
•Iftwodissimilarmetalsincontacthavetobeused,theyshouldbeascloseas
possibletoeachotherintheelectrochemicalseries.
•Wheneverthedirectjoiningofdissimilarmetalsisunavoidable,aninsulating
fittinglikewood,glass,rubber,plasticetcmaybeappliedinbetweenthemto
avoidthedirectmetaltometalcontact.
•Theanodicmetalshouldnotbepaintedorcoated,whenincontactwitha
dissimilarcathodicmetalinthepresenceofaliquidmedium.
•Uniformflowofthecorrodingliquidisdesirable.

27. Protective Measures Against Corrosion
Proper Designing:
•Aproperdesignshouldavoidthepresenceofcrevicesbetweenadjacent
partsofstructure,evenincaseofthesamemetal,sincecrevicespermit
concentrationdifferences.
•Sharpcorners,bendsandrecessesshouldbeavoided,astheyarefavorable
fortheformationofstagnantareasandaccumulationofsolids.
•Theequipmentshouldbesupportedonlegstoallowfreecirculationofair
andpreventtheformationofstagnantpoolsordampareas.

28. Protective Measures Against Corrosion
Proper Designing:

29. Metallic Coatings
A metal (say Zn) is coated on the base metal (say Fe) so as to prevent corrosion.
Metallic Coatings are of two types.
Anodiccoatings:-Theseareproducedfromcoatingmetals,whichare“anodic”
tothebasemetal.Thisprovidesthecompleteprotectiontotheunderlying
basemetalaslongasthecoatingremainincontactwiththebasemetal.
However,ifanycorrosionoccurs,itwillbeconcentratedonthecoatingmetal
asitisanodictothebasemetaltobeprotected(becomescathodic)dueto
settingupofagalvaniccell.
Example:-Incaseofgalvanizedsteel,Zinc,thecoatingmetalbeinganodicis
attacked;leavingthebehindthecathodicmetal(iron)unattackedand
protected.
Cathodiccoatings:-Theseareobtainedbycoatingamorenoblemetalhaving
higherelectrodepotentialthanthebasemetaltobecoated.Thecathodic
coatingprovideseffectiveprotectiontothebasemetalonlywhentheyare
completelycontinuousandfreefromanypores,breaksordiscontinuities.
Example:-CoatingofTinonIronisacathodiccoating.

30. Methods of applying metallic coatings
Hot dipping:
ItisusedforproducingacoatingoflowmeltingmetalsuchasZn,Sn,
Pb,Aletc.oniron,steelandcopper,whichhaverelativelyhigher
meltingpoints.
Meltingpoint:-Zn–419
0
C,Sn–232
0
C,Pb–327
0
C,Al–659
0
C
Meltingpoint:-Cu–1084
0
C,Fe–1540
0
C
Theprocessconsistsofimmersingthebasemetalinabathofthe
moltencoatingmetal,coveredbyamoltenfluxlayer(usuallyZnCl
2or
NH
4Cl).Thefluxcleansthebasemetalsurfaceandpreventsthe
oxidationofthemoltencoatingmetal.
Forgoodadhesion,thebasemetalsurfacemustbeveryclean;
otherwiseitcannotbeproperlywettedbythemoltenmetal.
The two most widely applied hot dipping methods are:
Galvanizing and Tinning

31. Methods of applying metallic coatings
Galvanizing:
IttheprocessofcoatingironorsteelsheetswithathincoatofZincto
preventthesheetsfromrusting.
Thebasemetalsheetofironorsteeliscleanedbyacidpicklingmethod
withdiluteSulphuricacidfor15–20minutesat60–90
0
Cwhich
removesanyscale,rustorimpuritiesandthenwashedanddried.Itis
thendippedinabathofmoltenzincmaintainedatatemperatureof
425–430
0
Candaftertakingoutofbathitispassedbetweenhotrollers
toremoveexcesszinctoproduceauniformcoatingandthenannealed
(slowcooling).Duringtheprocessthesurfaceofthebathiscovered
withflux(NH
4Cl)topreventanyoxideformation.

32. Image of Galvanizing Process

33. Methods of applying metallic coatings
Tinning:
Theprocessofcoatingmetallictinovertheironorsteelarticlesis
calledtinning.Thesurfacethebasemetali.e.,ironsheetiscleanedby
acidpicklingwithdiluteSulphuricacidandpassedthroughabathof
ZnCl
2fluxwhichpreventsanyoxideformationandhelpsthemolten
metaltoadheretotheironmetalsheetsurface.Thenthesheetis
passedthroughthemoltentinbathmaintainedat250–290
0
Cand
pressedbetweentworollerswithalayerofpalmoil.Theoilwillhelpto
protectthetincoatedlayerfromanyoxidation.Therollersalsoremove
excesstinandproduceauniformcoating.
Tinmetalpossessesgoodresistanceagainstatmosphericcorrosionand
isnontoxic.Hencesuchcontainerscanbesafelyusedforstoringfood
material.

34. Image of TInningProcess

35. Metal Cladding
Itistheprocessbywhichadense,homogeneouslayerofcoating
metalisbonded(cladded)firmlyandpermanentlytothebasemetal
ononeorbothsides.Thechoiceofthecladdingmetaldependson
thecorrosionresistancerequiredforanyparticularenvironment.
Here,themetaltobeprotectedissandwichedbetweenthetwo
layersoftheprotectingmetal.Thewholecombinationispressedby
rollersundertheactionofheatandpressure.
Thecladdingmaterialsgenerallyusedarecorrosionresistantsuchas
Al,Ni,Cu,Pb,Pt,Agetc.Thebasematerialsonwhichcladdingis
doneareFe,Al,mildsteel,Niandtheiralloysetc.Thismethodis
widelyadoptedinaircraftandautomobileindustryforthe
manufactureofouterbodyparts.

36. Image of Metal Cladding Process

37. Image of Metal Cladding Process

38. Anodic & CathodicCoating Comparison
SlNo Anodic Coating Cathodic Coating
1
It protects the base metal sacrificially due
to more electropositive character of the
coating metal.
It protects the base metal due to high
corrosion resistance & noble behavior of the
coating metal.
2
Coating metal is at lower potential than the
base metal.
Coating metal is at higher potential than the
base metal.
3
Corrosion of base metal does not increase
even on breaking of the coating as it heals
its film.
Corrosion of base metal increases, if there is a
break in the coating.
4
Galvanizing i.e. coating of Zn coating
iron/steel is an example of anodic coating.
Tinning i.e. coating of Sncoating iron/steel is
an example of cathodiccoating.

39. Comparison: Galvanizing & Tinning
SlNo Galvanizing Tinning
1
Process of covering iron or steel, with a thin coating
of Zinc to prevent it from corrosion.
Process of covering iron or steel with a thin coating of
Tin to prevent it from corrosion.
2
Zinc protects iron sacrificially Since it is more electro
positive than iron and does not permit iron to pass
into the solution.
Tin protects the base metal iron from corrosion due to
its noble or passive nature and higher corrosion
resistance.
3
In galvanized articles, Zinc continues to protect the
underlying iron by galvanic cell action, even if the
coating of Zinc is broken at any place still Zinc will
undergo corrosion and protects iron.
[Fe (-0.44V & Zn (-0.76)]
Tin protects underlying iron till the coat is intact and
continuous. Any break in coating causes rapid
corrosion of iron due to lower reduction potential.
[Fe (-0.44V & Sn(-0.14)]
4
Galvanized containers cannot be used for storing
acidic food stuffs as zinc reacts with acidic food
forming poisonous compounds.
Tin coated containers and utensils can be used for
storing any food stuff as tin is non-toxic and protects
metal from corrosion.
5
Ideal temperature of operation is around 450
0
C.
(Melting point of Zinc is 419
0
C)
Ideal temperature of operation is around 250
0
C.
(Melting point of Zinc is 232
0
C)
6
Ammonium chloride is used as flux which helps in
prevention of formation of any oxide on the parent
metal surface and help the molten coating metal to
adhere to the parent metal.
Zinc chloride is used as flux which helps in prevention
of formation of any oxide on the parent metal surface
and help the molten coating metal to adhere to the
parent metal.

40. Text books references
1. Jain P C and Jain M: Engineering Chemistry (15th Edition) 2006
DhanpatRaiPublishing Company, NewDelhi.
2. DaraS.S. & UmareS.S. A Text Book of Engineering Chemistry(12th
Edition ) 2008 S.ChandPublishing Company, New Delhi
3. ChawlaShashi: A text book of Engineering Chemistry (3rd Edition)
2010 DhanpatRaiPublishing Company, New Delhi.
4. PalannaO G : A text book of Engineering Chemistry(4th Reprint)
2012 McGraw Hill, New Delhi
5. Sharma BK, Industrial Chemistry (16th Edition), 2014, Krishna
PrakashanMedia (P) ltd. Meerut.