Corrosion inhibitors.ppt

Engineering chemistry

1. Thesubstancewhichwhenaddedinsmallquantitiesto
theaqueouscorrosiveenvironment,effectivelydecrease
thecorrosionofametaliscalledcorrosioninhibitor.
Inhibitorsareoftwotypes
Anodicinhibitors
Cathodicinhibitors
CORROSION INHIBITORS

2. ANODIC INHIBITORS
These inhibitorsavoidthecorrosion reactions
occurringattheanodebyformingasparinglysoluble
compound withthenewlyproducedmetalion.
Theyareadsorbedonthemetalsurface,forminga
protectivefilm,therebyreducingthecorrosionrate.
Ifcertainareasareleftunprotectedseverecorrosion
occurstothemetal.
Ex:Somecommon anodiccorrosioninhibitorsare
chromates,phosphates,tungstatesandotherionsof
transitionmetalswithhighoxygencontent.

3. CATHODIC INHIBITORS
Inacidicenvironment,thecorrosionreactiontakes
placewithevolutionofhydrogen.
2H
+
+ 2e
-
H
2
Corrosionmaybereducedby
SlowingdownthediffusionofhydratedH
+
ionstothe
cathode(or)
Byincreasingtheovervoltageofhydrogenevolution.

4. CATHODIC INHIBITORS
ThediffusionofH
+
isdecreasedbytheadditionof
inorganic inhibitorslikeamines, mercaptans,
heterocyclicnitrogencompounds, substitutedureas
andthioureaswhicharecapableofbeingadsorbedon
themetalsurface.
Antimonyandarsenicoxidesareusedasinhibitors
because theyproduce adherentfilmofmetallic
arsenicorantimonyatthecathodicarea,considerably
increasingthehydrogenovervoltage.

5. CATHODIC INHIBITORS
Inneutralenvironment,thecathodicreactionis
H
2O +1/2 O
2+ 2e
-
2OH
-
Hence,corrosioncanbecontrolledby
Eliminatingoxygenfromthecorrodingmediumbythe
additionofNa
2SO
3.
Retardingthediffusiontothecathodicareabythe
inhibitorslikeMg,ZnorNisaltstotheenvironment
which reactwithhydroxyl ionsforming the
corresponding insolublehydroxides which form
impermeable selfbarriersbetween themetaland
corrosionenvironment.

6. PASSIVITY
Passivityorpassivationisthe“phenomenon inwhichametalor
analloyexhibitsamuchhighercorrosion-resistancethan
expectedfromitspositionintheelectrochemicalseries.
Itistheresultoftheformationofahighlyprotective,butvery
thinandquiteinvisiblefilmonthesurfaceofmetaloranalloy,
whichmakesitmorenoble.
Thisfilmisinsoluble,non-porousand“self-healing”innature
thatwhenbrokenitwillrepairitselfonre-exposuretooxidizing
conditions.
Ex:Ti,Al,Cr,andawidevarietyofstainlesssteelalloys
containingCr.

7. PASSIVITY
These exhibitoutstanding corrosionresistance in
oxidizingenvironments, butinreducingenvironments,
theybecomechemicallyactive.
Basedonexperimentconductedinaerated0.5MNaCl
solution,thepassivityofcertainmetalsfallsinthe
followingorder:
TlAlCrBeMoMgNiCoFeMnZnCdSnPbCu
Passivationisnotaconstantstate,butexistsonlyin
certainenvironments whichtendtomaintainthin
protectiveoxidefilmsontheirsurfaces.

8. PASSIVITY
Inthepresence ofoxygen,theoxidefilmis
automaticallyrepaired,wheneveranydamageoccurs,
butinoxygenabsence,thepassivemetalsandalloys
becomechemicallyactiveandarerapidlycorroded.
Ex:Austenticstainlesssteelsarequitegoodresistant
totheactionofaerateddiluteH
2SO
4,butshowlow
resistanceinair-freeacid.

9. COMPARISON OF CHEMICAL CORROSION WITH ELECTROCHEMICAL CORROSION
Dry or chemical corrosion
Thiscorrosionoccursindryconditions.
Corrosionisuniform.
Itisaslowprocess.
Itinvolvesdirectchemicalattackof
themetalsbyenvironment.
Itisexplained by absorption
mechanism.
Itoccursbothinhomogeneous and
heterogeneous solutions.
Corrosionproductsareproducedatthe
placewherecorrosiontakesplace.
Wet or electrochemical corrosion
Thiscorrosionoccursinwetconditions
inpresenceofanelectrolytemedium.
Corrosionisnotuniform.Iftheanodic
areaissmall,pittingismorefrequent.
Itisarapidprocess.
Itinvolvestheformationoflarge
numberofelectrochemicalcells.
Itisexplainedbythemechanism of
electrochemicalreactions.
Ittakesplaceonlyinheterogeneous
processes.
Corrosionoccursattheanodeand
corrosionproductoccursatcathode.