Corrosion & Mechanism of Dry corrosion

Corrosion, types of corrosion, examples and mechanism of Dry or Chemical Corrosion, Oxidation Corrosion, Nature of Oxide layer formed, Pilling - Bedworth rule, Passivity or Passivation

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

3. •MetallicCorrosion:Definition,typesandexamples.
•IntroductiontoDryorChemicalcorrosion
•MechanismofDryorChemicalcorrosion.
•Pilling–BedworthRule
•ConceptofPassivityorPassivation

4. Corrosioncanbedefinedasadestructivechemicalandelectrochemicalreactionofa
metalwithitsenvironment(likeO
2,moisture,CO
2etc.)whichdisfiguresmetallic
productsleadingtoreductionintheirthickness,strengthandalsocauseslossof
usefulpropertiessuchasmalleability,ductility,electricalconductivityandoptical
refractivity.
Or
Corrosioncanbedefinedas“Anyprocessofdeteriorationordestructionand
consequentlossofasolidmetallicmaterialthroughanunwantedorunintentional
chemicalorelectrochemicalattackbyitsenvironmentatitssurfaceiscalled
corrosion”.
Thuscorrosionisareverseprocessofextractionofmetals.
ExceptafewmetalssuchasGold,SilverandPlatinum(callednoblemetal),other
metalsarepronetocorrosion.
TypicalexamplesofCorrosionare
•Rustingofironduetoformationofhydratedferricoxide(Fe
2O
3.3H
2O).
•TarnishingofsilverwaresinH
2Sladenairduetoformationofsilversulphide.
•Formationofgreenfilmofbasiccarbonate-[CuCO
3+Cu(OH)
2]onthesurfaceof
copperwhenexposedtomoistaircontainingCO
2.

5. Formation of Iron oxide:
Fe →Fe
2+
+ 2e
–
(oxidation)
1/2O
2+ 2e
–
→O
2–
(reduction)
Overall reaction:-Fe + 1/2O
2→Fe
2+
+ O
2–
Or, 2Fe + O
2→2Fe
2+
+ 2O
2–
→2FeO
Similarly, in excess supply of oxygen, 4Fe + 3O
2→2Fe
2O
3
Rusting of Iron in presence of Electrolyte:-
Fe →Fe
2+
+ 2e
–
(oxidation)
1/2O
2+ H
2O + 2e
–
→2OH
–
(reduction)
Overall: Fe + 1/2O
2+ H
2O →Fe
2+
+ 2OH
–
→Fe(OH)
2
Or, 2Fe + O
2+ 2H
2O →2Fe
2+
+ 4OH
–
→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
General formula of Rust: Fe
2O
3. xH
2O, x = 2 to 5
Corrosion of Copper when exposed to moist air containing CO
2
2Cu + O
2+ H
2O + CO
2→CuCO
3.Cu(OH)
2
(Green protective coating)

6. Metalsareelectropositiveinnature.ExceptafewmetalslikeGold,Silverand
Platinum(noblemetal),othermetalsarefoundinnatureastheircompounds
(suchasoxides,hydroxides,carbonates,bicarbonates,chlorides,nitrates,
sulphates,sulphides,phosphates,silicatesetc.)whicharecalledastheirore.
Metalsarethusobtainedbyextractionfromtheiroresbyreductionprocess.
Innature,whenmetalsexistsastheircompounds(orore)theyarestable
andtheyareinthelowenergystates.
However,duringextractionofmetalsfromtheirores,freemetalsarebecomeless
stableandareinthehigherenergystatethanintheionicstateorore.
So,metalshaveatendencytoconvertedbacktotheionicstate(ore)andhence
metalatomsarepronetogetattackedbyenvironment.Thisisthemainreasonfor
corrosionofmetals.
ThemechanismofCorrosionofmetalsinvolvetheconceptofRedoxreaction.Metal
(M)
Mineral
or
ore (M
n+
)
Corrosion
product
(M
n+
)
Extraction
by reduction
+ ne
Corrosion
-ne

7. Types of Corrosion
Dry or Chemical Corrosion
Wet or Electrochemical
Corrosion

8. DryorChemicalCorrosion:Itinvolvesdirectchemical
attackofatmosphericgaseslikeCO
2,O
2,H
2S,SO
2,halogens,
andinorganicacidvapoursonexposedmetallicsurface.
Example:-TarnishingofsilverwareinH
2Sladenair.
WetorElectrochemicalCorrosion:Itoccursduetosetting
upofalargenumberoftinygalvaniccellsinmetalsin
presenceofanimpurityaswellasinpresenceofmoistureor
anelectrolyticmedium.Generally,impurity(moreactive
metal)actsasanodeandoriginalmetalactsascathode.So
anodeistheareawherecorrosionoccurs.
Example:-Rustingofironinmoistatmosphere.

9. Dry or Chemical Corrosion
Occurs
•Due to direct chemical reaction of atmospheric gases.
•It involves the process of adsorption.
•Corrosion occurs uniformly on the entire exposed metallic surface.
•It occurs on both homo and heterogeneous metallic surface.
•Both oxidation and reduction occurs at the exposed metallic
surface without the formation of anodic and cathodicarea.
•Extent of dry corrosion depends on the nature of the layer formed
on the metallic surface and also on the attraction or affinity
between the gas and the exposed metal.
Types
•Oxidation corrosion due to Oxygen
•Corrosion by other atmospheric gases
•Liquid Metal Corrosion

10. Types of dry corrosion
Oxidation corrosion
Occurs
Due to direct chemical reaction of atm. O2 with metal surface forming metal oxide.
It involves the process of adsorption of Oxygen on metallic surface.
It occurs in the absence of moisture or any electrolytic medium.
Increases with increase in temperature and may take place at low or high
temperature.
Extent of Oxidation corrosion depends on the nature of oxide layer formed on the
metallic surface.
Mechanism
On exposure to atm., metal gets oxidized to form metal ions.
(i)
2M(s) → 2M
+n
+ 2ne
-
(Oxidation)
Electrons lost by the metal are taken up by oxygen to forms oxide ions.
(ii)
n/2O
2(g) + 2ne
-
→nO
2-
(Reduction)
Overall reaction:
2M + n/2O
2→ 2M
+n
+ nO
2-
→M2On
Metal Oxide

11. Mechanism of Dry or Chemical Corrosion

12. Mechanism of Dry or Chemical Corrosion

13. Nature of metal oxide layer
Types of Oxide Layers formed
Stable layer: -Al, Pb, Cu, Cr, Ni, Mn, Snetc. (Negligible corrosion)
Unstable layer:-Ag, Au, Pt etc. (No oxidation corrosion)
Volatile layer:-Mo (as MoO
3). (Excess corrosion)
Porous layer:-Alkali metals & Alkaline earth metals. (Excess corrosion)
Pilling –BedworthRule
A protective and Non-Porous metal oxide layer has volume equal to or greater
than the volume of metal from which it is formed.
A Non-Protective and Porous metal oxide layer has volume lesser than the
volume of metal from which it is formed.
Specific Volume Ratio (R) = Volume of oxide layer formed
Volume of parent metal exposed
If, R ≥ 1, the oxide layer is said to be protective and non porous.
If, R < 1, the oxide layer is said to be non -protective and porous.

14. Types of dry corrosion
Corrosion by Other Gases
2Ag + Cl2 → 2AgCl
(Non-Porous layer)
Fe + H2S → FeS+ H2
(Porous Layer)
Sn+ 2Cl2 → SnCl4
(Volatile Layer)
Liquid Metal Corrosion
•Occurs due to the action of flowing liquid metal at high temp on solid metals
or alloys and the solid metal or alloy usually gets weakened.
•Observed in nuclear reactors where Na metal used as a coolant leads to
corrosion of Cd.
•In such type of corrosion, either the liquid metal dissolves the solid metal
surface or penetrates into the solid surface and weakens the bond.

15. Passivity or Passivation
PassivityorPassivationisthephenomenoninwhichametaloran
alloyexhibitsmuchhighercorrosionresistancethanexpectedfrom
itspositionintheelectrochemicalseries.Passivityistheresultofthe
formationofahighlyprotectivebutverythin(about0.0004mm
thick)andquiteinvisiblefilmonthesurfaceofmetaloranalloy
whichmakeitmorenoble.Thisthinfilmisinvisible,nonporousand
ofsuchaSelfhealingnaturethatwhenbroken,itrepairsitselfon
exposuretotheoxidisingenvironment.
Examplesofsuchpassivemetalsandalloysare:Ti,Al,Cr,Ni,Co,Mn,
Pb,CuandstainlesssteelalloyscontainingCr(about13–25%)etc.
Outstandingcorrosionresistanceexhibitedbyvarioussurgical
instrumentsisduetothepresenceofCr(about13%)init.
Similarly,AlisnotattackedbyConc.HNO
3duetopassivitywhileFeis
easilyattackedbyevendiluteHNO
3.

16. 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.