Wet corrosion and its types with mechanisms

Types of Wet or Electrochemical Corrosion, Differential aeration corrosion, Galvanic corrosion, Pitting corrosion, Waterline corrosion, Crevice corrosion, Stress corrosion and their mechanisms and suitable examples.

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

3. TypesofElectrochemicalCorrosion:
•GalvanicCorrosion
•Differentialaerationcorrosion
•Waterlinecorrosion
•Pittingcorrosion
•Crevicecorrosion
•Stresscorrosion.

4. Types of Wet or Electrochemical Corrosion
Wet or Electrochemical Corrosion processes are of the following
types as given below.
Galvanic or Bimetallic Corrosion
Concentration Cell or Differential aeration Corrosion
Waterline Corrosion
Pitting Corrosion
Crevice Corrosion
Stress Corrosion
(a) Season Cracking
(b) Caustic Embrittlement

5. Galvanic or Bimetallic corrosion
SuchtypeofCorrosionoccurswhentwodifferentordissimilarmetalshavingdifferent
electrodepotentials(Reductionpotential)areelectricallyincontactwitheachotherin
thepresenceofaconductingmediumorwhentwodifferentmetalsareelectrically
connectedwitheachotherinthepresenceoftheirrespectivesaltsolutions.
Themetalplacedhigherinpositioninelectrochemicalseriesbecomesmoreactivemetal
andactsasanodewhiletheothermetalplacedlowerinpositioninelectrochemical
seriesbecomesthemorenoble/passivemetalandactsascathode.
Hence,themoreactivemetalsufferscorrosionwhilethemorenoblemetalissaidtobe
protected.
Hence,Galvaniccouplesarerequiredtobeavoidedindifferentapplications.
Examples:-Nutsandboltsofthesamemetalispreffered.
Asmallsteelboltconnectedtoacopperequipment.
Asmallsteelpipeconnectedtoalargecoppertank.
Znsufferscorrosion,whenelectricallycoupledwithCuinthepresenceofan
electrolyticmedium.

6. Mechanism of Galvanic corrosion
InGalvaniccorrosion,moreactivemetalactsasanodeandundergoesoxidationwhile
morenoble/passivemetalactsascathodeandundergoesreduction.Hence,themore
activemetalsufferscorrosionwhilethemorenoblemetalissaidtobeprotected.
Dependingonthenatureoftheconductingmediumorelectrolyticmedium,
thecathodicreactiononthemorenoblemetalmayinvolveEvolutionof
Hydrogentype(H
2)reactionorAbsorptionofOxygentypeofreaction(O
2).
InAcidicsolution:-EvolutionofHydrogentype(H
2)reaction
Inneutralorslightlyalkalinesolution:-AbsorptionofOxygentypeofreaction(O
2)
ExampleofGalvanicCouples:-
i)Zn(-0.76V)&Cu(+0.34V)→Znundergoescorrosion(moreactivemetal)
ii)Fe(-0.44V)&Cu(+0.34V)→Feundergoescorrosion(moreactivemetal)
iii)Ni(-0.24V)&Cu(+0.34V)→Niundergoescorrosion(moreactivemetal)
iv)Zn(-0.76V)&Fe(-0.44V)→Znundergoescorrosion(moreactivemetal)
v)Zn(-0.76V)&Al(-1.66V)→Alundergoescorrosion(moreactivemetal)

7. Image of Galvanic corrosion

8. Minimization of Galvanic corrosion
GalvanicorBimetalliccorrosioncanbeminimizedbyadoptingthefollowing
methods.
i)ByavoidingtheuseofGalvaniccoupleinpresenceofconductingmedium.
ii)WhentheuseofGalvaniccoupleisunavoidable,metalsmustbeasmuch
closeaspossibletoeachotherinelectrochemicalseries.
iii)Whenthedirectjoiningofdissimilarmetalsisunavoidable,aninsulating
materiallikewood,glass,rubberetcmaybeusedtoavoiddirectelectrical
contactbetweenthemetals.
iv)Whentwodissimilarmetalsaretobeusedincontactwithanelectrolytic
medium,theanodicmetalshouldhaveaslargeareaaspossibleand
cathodicmetalshouldhaveassmallareaaspossiblesincesmallanode
andlargecathodeleadstoexcessivecorrosion.
v)Theanodicmetalshouldnotbepaintedorcoatedwhenincontactwith
cathodicmetalsinceanybreakinthecoatingleadstoseverelocalized
galvaniccorrosion.

9. Conc. Cell or Differential Aeration Corrosion
•Concentrationcellcorrosionoccursduetodifferenceinconcentrationof
theliquidmediumarounddifferentpartsofthemetal.
•DifferentialaerationcorrosionisatypeofConcentrationcellcorrosion
whichOccursduetodifferenceinpotentialbetweendifferentlyaerated
partsofametalinthepresenceofanelectrolyticmedium.
•Partofthemetalexposedtohigherconcentrationofairisthemore
oxygenatedpart&actsasCathode.
•Partofthemetalimmersedinsidethegreaterdepthoftheelectrolyteis
thepoorlyoxygenatedpart&actsasAnode.
•Electronflowsfromtheanode(pooroxygenatedpart)tothecathode(more
oxygenatedpart)throughthemetalwhileionsmigratetoeachother
throughtheelectrolyticmediumproducingthecorrosionproduct.
•Attheanodicarea,dissolutionofmetaloccursduetooxidation.
•Atthecathodicarea,absorptionofoxygentypeofreductionreaction
occursproducinghydroxideions.

10. Mechanism of Differential Aeration Corrosion
In case of Iron metal
Reaction at anode:-Fe(s) →Fe
+2
+ 2e
-
(Oxidation)
Reaction at cathode:-1/2O
2+ H
2O + 2e
–
→ 2OH
–
(Reduction)
Overall Reaction:-Fe + 1/2O
2+ H
2O → Fe(OH)
2Or 2Fe + O
2+ 2H
2O → 2Fe(OH)
2
In the presence of excess Oxygen:-
4Fe(OH)
2+ O
2+ 2H
2O → 4 Fe(OH)
3 or 2Fe
2O
3. 3H
2O
Similarly in case of Zn metal
Reaction at anode:-Zn(s) →Zn
+2
+ 2e
-
(Oxidation)
Reaction at cathode:-1/2O
2+ H
2O + 2e
–
→ 2OH
–
(Reduction)
Overall Reaction:-Zn + 1/2O
2+ H
2O → Zn(OH)
2Or 2Zn + O
2+ 2H
2O → 2Zn(OH)
2

11. Image of Differential Aeration Corrosion
A metal (say Zn or Fe) partially immersed in a neutral salt solution undergoes
differential aeration corrosion due to potential difference between less oxygenated
part (anode) and more oxygenated part (cathode).

12. Image of Differential Aeration Corrosion
A metal (say Fe) partly covered with few drops of water undergoes differential aeration
corrosion as the portion of the surface covered with water becomes less oxygenated part
(anode) than the uncovered more oxygenated part of the metal (cathode).

13. Image of Differential Aeration Corrosion
A metal (say Fe) partly covered with drops of salt solution undergoes differential aeration
corrosion as the part of the surface covered with salt solution becomes less oxygenated
part (anode) than the uncovered more oxygenated part of the metal (cathode).

14. Image of Differential Aeration Corrosion
A part of metal surface (say Fe) covered with extraneous matter such as dust, dirt, sand,
clay etc undergoes differential aeration corrosion as the part of the surface covered with
the extraneous matter becomes less oxygenated Part (anode) than the uncovered more
oxygenated part of the metal (cathode).

15. Waterline Corrosion
Asteeltankcontainingwaterkeptstagnantforalongtime,undergoesdifferentialaeration
corrosionjustbelowthewaterlineorwaterlevelsincetheconcentrationofoxygeninthe
greaterdepthbecomeslessoxygenatedPart(anode)thantheportionofthetankjust
abovethewaterlineandbecomemoreoxygenatedpartofthemetal(cathode).

16. Pitting Corrosion
Pittingcorrosionisaspecialtypeofdifferentialaerationcorrosionwhichinvolveslocalized
acceleratedattackresultingintheformationofcavitiesaroundthemetal.Pittingcorrosion
arisesduetotheformationofcracks,pinholes,pitsandcavitiesonthemetalsurfacedue
towhichsmallanodic(pitorcavity)andlargecathodicareas(restpart)arecreated.

17. Pitting Corrosion
Pittingcorrosionisaspecialtypeofdifferentialaerationcorrosionwhichinvolveslocalized
acceleratedattackresultingintheformationofcavitiesaroundthemetal.Pittingcorrosion
arisesduetotheformationofcracks,pinholes,pitsandcavitiesonthemetalsurfacedue
towhichsmallanodic(pitorcavity)andlargecathodicareas(restpart)arecreated.

18. Crevice Corrosion
Crevicecorrosionisatypeofdifferentialaerationcorrosionwhichinvolveslocalizedattack
resultingintheformationofinaccessibleareasatthejunctionoftwometals.Crevicearea
hasalackofoxygenandactsasanodeandbecomespronetocorrosionwhilethewell
exposedpartsbecomemoreoxygenatedandactascathoderesultinginintensecorrosion.

19. Stress Corrosion
Stresscorrosionisatypeofelectrochemicalcorrosionwhichishighlyspecific
andoccurswhenametalissubjectedtothecombinedeffectofstressand
exposedtoaspecificelectrolyticmedium.Generally,thepartofthemetal
understress(suchassharpcorners,bends,rivets,pits,joints,crevicesetc)acts
asanodewhilethewellmetalpartactsascathode.
PuremetalsaregenerallyresistanttoStresscorrosionbutfabricatedarticles
orfabricatedmetallicstructureslikealloysofsteel,brassetcundergoesstress
corrosionwhenexposedtosomespecificelectrolyticmediumandunder
stress.
Examples:-
Brassalloy(Cu–ZnorCu–Ni)whenexposedtoammonia.
Stainlesssteel(containing0.1–0.4%carbon)exposedtoacidchlorides(HCl).
Mildsteelinboilersexposedtocausticalkali(Na
2CO
3)duringsofteningprocessof
waterbylime–sodaprocess.

20. Types of Stress Corrosion
Stresscorrosioncanbeoftwotypes.
i)SeasonCracking–OccursinBrassexposedtoammonia.
ii)CausticEmbrittlement–OccursinmildSteelinwatersofteningboilers
exposedtocausticalkalilike(Na
2CO
3)usedinLime–Sodaprocess.
SeasonCracking:-
Thistypeofcorrosiongenerallyreferstothecorrosionofcopperalloyssuchas
Brass(Cu–Zn)whenexposedtotheactionofammonia.Whenbrassisexposedto
ammoniasolution,bothcopperandzincformcomplexesbylosingelectronsinthe
solutionduetowhichdissolutionofbrassoccursattheboundariesandforms
cracksforstresscorrosion.Seasoncrackingischaracterizedbydeepbrittlecracks
whichpenetrateintotheaffectedcomponents.Iftheconcentrationofammoniais
veryhigh,thenattackismuchmoresevere.
Reactions:-
Zn→Zn
+2
+2e
-
& Cu→Cu
+2
+2e
-
Zn
+2
+4NH
3→[Zn(NH
3)
4]
+2
&Cu
+2
+4NH
3→[Cu(NH
3)
4]
+2

21. Images of Season Cracking Corrosion

22. Caustic Embrittlement
Causticembrittlementisthephenomenonduringwhichtheboilermaterialbecomes
brittleduetotheaccumulationofcausticsubstances.Thistypeofboilercorrosionis
causedbytheuseofhighlyalkalinewaterinthehighpressureboilerandalsodueto
stress.WatersoftenedbylimesodaprocessmaycontainNaOHwhichisformedbythe
hydrolysisofNa
2CO
3.
ThistypeofcorrosiongenerallyOccursinmildSteelmaterialofwatersofteningboilers
exposedtocausticalkali(Na
2CO
3)usedinlime–sodaprocessathightemperatureand
pressure.Whenwaterofhighalkalinityattackthemildsteelnearthestressedpartslike
bends,sharpcorners,joints,rivets,hair–cracksetc.,ironoftheboilermaterialsuffers
corrosionduetotheformationofFe
3O
4.
WatercontainingNaOHflowsthroughthesmallporesinthestressedareaslikebends,
joints,sharpcorners,hair-cracksetcbycapillaryactionandwhenwaterintheseplaces
evaporatesathightemperature,theconcentrationofNaOHincreasesascomparedto
theunstressedpartsanditcorrodestheironofthesurroundingareabyformingSodium
ferroate(Na
2FeO
2).Thiscausesembrittlementofboilermaterialsparticularlyatstressed
partscausingevenfailureoftheboiler.

23. Reactions during Caustic Embrittlement
Athightemperatureandpressureinsteelboilerplants
Na
2CO
3+H
2O→2NaOH+CO
2
Fe+2NaOH→Na
2FeO
2+H
2
(Sodiumferroate)
3Na
2FeO
2+4H
2O→Fe
3O
4+6NaOH+H
2
or6Na
2FeO
2+6H
2O+O
2→2Fe
3O
4+12NaOH
(Sodiumferroate)
ConcentrationCellPresentationofCausticembrittlement:-
Causticembrittlementarisesduetothesettingupofaconcentrationcell.WiththeIron
surroundedbydil.NaOHactingastheCathode,whiletheironsurroundedbyconc.NaOH
actingastheanode.Theironintheanodicpartgetsdissolvedorcorroded.
Fe(understress)│ConcentratedNaOH‖DiluteNaOH│Fe(stressfree)
(inaccessiblearea) (mainbody)
Anode Cathode
PreventionofCausticembrittlement:-
1.ByusingNa
2SO
4andNa
3PO
4inplaceofNa
2CO
3thatblockstheminutehaircracks.
2.Bymaintaininguniformflowrateofboilerwaterbyavoidinginaccessibleareas.
3.Usingtanninorligninasadditiveinboilerwaterwhichblocksthehaircracks.

24. Images of Caustic Embrittlement

25. Text books references
1. Jain P C and Jain M: Engineering Chemistry (15th Edition) 2006
DhanpatRaiPublishing Company, NewDelhi.
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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.