corrosion & its prevention gc-08

8/6/2019 Corrosion & Its Prevention GC-08

1/65

CORROSION AND ITS PREVENTION

G. Chowdhury Prof.(MET)


2/65

INTRODUCTIONINTRODUCTION

CorrosionCorrosion isis anan inevitableinevitable phenomenonphenomenon..

InIn factfact ourour economyeconomy would would bebe directlydirectlychangedchanged ifif therethere werewere nono corrosioncorrosion..

TheThe approximateapproximate estimateestimate ofof lossloss ofof metalmetalduedue toto corrosion,corrosion, 22 toto 22..55 billionbillion dollarsdollars perper

annumannum allall-- overover thethe worldworld..

InIn IndiaIndia itit isis RsRs..600600..0000 crorescrores..


3/65

INTRODUCTIONINTRODUCTION

ThoughThough corrosioncorrosion isis inevitable,inevitable, itsits costcost cancanbebe considerablyconsiderably reducedreduced..

An An inexpensiveinexpensive MagnesiumMagnesium anodeanode couldcould

doubledouble thethe lifelife ofof aa domesticdomestic hothot waterwater tanktank.. Washing Washing aa carcar toto removeremove roadroad--saltsalt isis

helpfulhelpful..

ProperProper selectionselection ofof materialsmaterials andand goodgood

designdesign reducesreduces costcost ofof corrosioncorrosion.. AA goodgood maintenancemaintenance--paintingpainting programmeprogramme

payspays forfor itselfitself manymany timestimes


4/65

Corrosion TheoryCorrosion Theory

AllAll mattersmatters wantwant toto remainremain inin thethe lowestlowest energyenergy

state,state, wherewhere itit isis thermodynamicallythermodynamically moremore stablestable

BarringBarring aa fewfew exceptions,exceptions, metallicmetallic oxidesoxides andand

saltssalts areare moremore stablestable thanthan thethe metalmetal itselfitself

Hence,Hence, NaturalNatural OccurrenceOccurrence isis inin thethe formform ofof

Ores,Ores, whichwhich areare nothingnothing butbut oxidesoxides andand saltssalts

MetalsMetals areare extractedextracted fromfrom OresOres throughthrough thetheapplicationapplication ofof considerableconsiderable amountamount ofof energyenergy

An An energyenergy humphump preventsprevents spontaneousspontaneous

reversalreversal


5/65

Corrosion TheoryCorrosion Theory

AsAs aa resultresult theythey areare forcedforced toto remainremain inin

HighHigh EnergyEnergy StateState

IfIf somesome energyenergy isis providedprovided toto crosscross thethehump,hump, spontaneousspontaneous reactionreaction cancan occuroccur

CertainCertain environmentsenvironments offeroffer opportunitiesopportunities

toto thesethese metalsmetals toto combinecombine chemicallychemically withwith

otherother elementselements toto formform compoundscompounds andand

returnreturn toto lowerlower energyenergy levelslevels


6/65

Corrosion TheoryCorrosion Theory ContdContd

CorrosionCorrosion isis thethe primaryprimary meansmeans by by whichwhich metalsmetalsdeterioratedeteriorate..

MostMost metalsmetals corrodecorrode inin contactcontact withwith waterwater (and(andmoisturemoisture inin thethe air),air), acids,acids, bases,bases, salts,salts, oils,oils, aggressiveaggressivemetalmetal polishes,polishes, andand otherother solidsolid andand liquidliquid chemicalschemicals..

MetalsMetals alsoalso corrodecorrode whenwhen exposedexposed toto gaseousgaseous materialsmaterialslikelike acidacid vapors,vapors, formaldehydeformaldehyde gas,gas, ammoniaammonia gas,gas, andandsulfursulfur containingcontaining gasesgases..


7/65

Corrosion TheoryCorrosion Theory ContdContd

AlmostAlmost allall metalsmetals exhibitexhibit aa tendencytendency totobebe oxidizedoxidized,, somesome moremore easilyeasily thanthanothersothers..

AA tabulationtabulation ofof thethe relativerelative affinityaffinity ofof thisthistendencytendency isis calledcalled thethe GalvanicGalvanic SeriesSeries..

KnowledgeKnowledge ofof aa metal'smetal's locationlocation inin thetheseriesseries isis anan importantimportant piecepiece ofof

informationinformation toto havehave inin makingmaking decisionsdecisionsaboutabout itsits potentialpotential usefulnessusefulness forforstructuralstructural andand otherother applicationsapplications


8/65

FORMSOF CORROSIONFORMSOF CORROSION

CORROSION CAN BE OF TWO TYPES:CORROSION CAN BE OF TWO TYPES:

UNIFORMUNIFORM

NONNON--UNIFORM OR SELECTIVEUNIFORM OR SELECTIVE


9/65

UNIFORM CORROSIONUNIFORM CORROSION

IDEALLY, UNIFORM CORROSION TAKESIDEALLY, UNIFORM CORROSION TAKESPLACE BETWEEN A PURE UNSTRESSEDPLACE BETWEEN A PURE UNSTRESSEDMETAL & AN UNIFORM ENVIRONMENTMETAL & AN UNIFORM ENVIRONMENT

-- DRY AIR OR CORROSIVE GASDRY AIR OR CORROSIVE GAS

-- WET AIR OR CORROSIVE GASWET AIR OR CORROSIVE GAS

-- AERATED ACID OR CORROSIVEAERATED ACID OR CORROSIVE

MEDIUMMEDIUM-- NONNON--AERATED ACID OR CORROSIVEAERATED ACID OR CORROSIVEMEDIUMMEDIUM


10/65

SELECTIVE CORROSIONSELECTIVE CORROSION

SELECTIVESELECTIVE OR OR NONNON UNIFORMUNIFORM

CORROSIONCORROSION TAKESTAKES PLACEPLACE WHENWHEN

EITHEREITHER TWOTWO DISSIMILARDISSIMILAR METALMETAL OROR

ENVIRONMENTENVIRONMENT ISIS PRESENTPRESENT

ITIT ISIS ESSENTIALLYESSENTIALLY ANAN ELECTROELECTRO--

CHEMICALCHEMICAL REACTIONREACTION LIKELIKE AA

BATTERYBATTERY OROR GALVANICGALVANIC CELLCELL


11/65


DISSIMILARITY CAN BE DUE TODISSIMILARITY CAN BE DUE TO

1.1. DIFFERENT METALS OR PHASESDIFFERENT METALS OR PHASES

2.2. DIFFERENT STRESS LEVEL DUE TO COLDDIFFERENT STRESS LEVEL DUE TO COLDWORKING OR GRAIN BOUNDARYWORKING OR GRAIN BOUNDARY

3.3. DIFFERENT MICROSTRUCTUREDIFFERENT MICROSTRUCTURE

4.4. DIFFERENT CONCENTRATION OFDIFFERENT CONCENTRATION OF

CONDUCTING MEDIUMCONDUCTING MEDIUM5.5. DIFFERENT AVAILABILITY OF OXYGENDIFFERENT AVAILABILITY OF OXYGEN


12/65


BASED ON THE FIELD CONDITION ITBASED ON THE FIELD CONDITION IT

IS NAMED ASIS NAMED AS

1.1.

GALVANIC OR TWOGALVANIC OR TWO--METAL CORROSIONMETAL CORROSION2.2. CREVICE CORROSIONCREVICE CORROSION

3.3. PITTING CORROSIONPITTING CORROSION

4.4. STRESS CORROSIONSTRESS CORROSION

5.5. INTERINTER--GRANULAR CORROSION ETC.GRANULAR CORROSION ETC.


13/65

PRODUCTSOF CORROSIONPRODUCTSOF CORROSION

ESSENTIALLY, THE CORROSIONESSENTIALLY, THE CORROSION

PRODUCTS ARE OXIDE, HYDRATEDPRODUCTS ARE OXIDE, HYDRATED

OXIDE OR SOME SALT WITH ACIDOXIDE OR SOME SALT WITH ACID

RADICALS.RADICALS.

WATER NORMALLY DOES NOT REACTWATER NORMALLY DOES NOT REACT

WITH METAL DIRECTLY, BUT TAKES AWITH METAL DIRECTLY, BUT TAKES A

LEADING ROLE IN PROPAGATING THELEADING ROLE IN PROPAGATING THEPROCESS.PROCESS.


14/65

CORROSIONMECHANISMCORROSIONMECHANISM

UNIFORMUNIFORM CORROSIONCORROSION (DRY)(DRY)

1.1. SOMESOME METALSMETALS LIKELIKE Ag,Ag, TinTin etcetc..

DIRECTLYDIRECTLY REACTSREACTS WITHWITHATOMOSPHERICATOMOSPHERIC OXYGENOXYGEN OROR SOSO22 TOTO

FORMFORM COMPOUNDCOMPOUND

2.2. THETHE FILMFILM OFOF CORROSIONCORROSION PRODUCTPRODUCT

FORMEDFORMED PREVENTS/SLOWSPREVENTS/SLOWS DOWNDOWN

FURTHERFURTHER REACTIONREACTION

3.3. NOTNOT PREVELANTPREVELANT ININ STEELSTEEL EXCEPTEXCEPT SSSS


15/65


OXIDATION CORROSION:

4Mp4Mn+ + 4ne- (Loss of electron)

Metal ion

nO2 + 4ne-p 2nO2- (Gain of electron)

Oxide ion

Overall

4M + nO2p 4Mn+ + 2nO2-Metal ion Oxide ion

Metal oxide


16/65


UNIFORM CORROSION (WET):UNIFORM CORROSION (WET):

4Mp4Mn+ + 4ne- (loss of electron)

2nH2O + nO2 + 4ne- = 4n(OH)- (gain of electron)Mn+ + n(OH)- = M(OH)n

IF ENOUGH OXYGEN IS PRESENT, IT CANIF ENOUGH OXYGEN IS PRESENT, IT CAN

FORM HIGHER OXIDES.FORM HIGHER OXIDES.

WHEN OXYGEN SUPPLY IS POOR, A MIXUREWHEN OXYGEN SUPPLY IS POOR, A MIXUREOF HIGHER AND LOWER OXIDES MAY BEOF HIGHER AND LOWER OXIDES MAY BEFORMED.FORMED.


17/65


UNIFORM CORROSION IN ACID ORUNIFORM CORROSION IN ACID OR

ELECTROLYTE MEDIUM (NONELECTROLYTE MEDIUM (NON--

AERATED)AERATED)

Fe + HCl = FeClFe + HCl = FeCl22 + H+ H22

Fe + CuSOFe + CuSO44 = FeSO= FeSO44 + Cu+ Cu

HYDROGEN OR COPPER TENDS TOHYDROGEN OR COPPER TENDS TOFORM A PROTECTIVE FILM SLOWINGFORM A PROTECTIVE FILM SLOWING

DOWN FURTHER REACTIONDOWN FURTHER REACTION


18/65

ELECRTOCHEMICAL REACTIONS OF ZINC IN AIR FREE HCI


19/65


UNIFORM CORROSION IN ACID ORUNIFORM CORROSION IN ACID OR

ELECTROLYTE MEDIUM (AERATED)ELECTROLYTE MEDIUM (AERATED)

ASAS ENOUGHENOUGH OXYGENOXYGEN ISIS PRESENTPRESENTHYDROGENHYDROGEN ISIS CONSUMEDCONSUMED ASAS HH22OO ANDANDTHETHE PROTECTIVEPROTECTIVE FILMFILM DISSAPEARSDISSAPEARS..

THATTHAT ISIS WHY WHY ININ PLAINPLAIN WATER WATER

CONTAININGCONTAINING OXYGEN,OXYGEN, THETHE RATERATE OFOFCORROSIONCORROSION ISIS QUITEQUITE FASTFAST ASASCOMPAREDCOMPARED TOTO DISTILLEDDISTILLED WATERWATER..


20/65

Electro-chemical reactions of Zn in aerated HCl acid


21/65


SELECTIVE CORROSION (GALVANIC)SELECTIVE CORROSION (GALVANIC)

TWO METALLIC PARTS, ANODE &TWO METALLIC PARTS, ANODE &

CATHODECATHODETHEY MAY OR MAY NOT BETHEY MAY OR MAY NOT BE

PHYSICALLY DISTINCT AND SEPARATEPHYSICALLY DISTINCT AND SEPARATE

CONDUCTING ELECTROLYTECONDUCTING ELECTROLYTEANODE DISSOLVES SELECTIVELY INANODE DISSOLVES SELECTIVELY IN

PREFERENCE TO CATHODEPREFERENCE TO CATHODE


22/65


AT ANODE:-M Mn+ + ne- M n+ p Dissolved in solution.

AT CATHOD

E:-2nH2O + nO2 + 4ne- = 4n(OH)

-

(dissolved constituents in the conducting medium

accepts the electron to form some ions, (OH)-,

O2- etc.) The metallic & non-metallic ions reacts to form

corrosion products


23/65

MECHANISM OF WET CORROSION BY HYDROGEN EVOLUTION


24/65


RUST FORMATION:RUST FORMATION:

At Anode, FeAt Anode, Fe2+2+ or Feor Fe3+3+ ions are formedions are formed

At Cathode, (OH)At Cathode, (OH)

--

ions are formed in presenceions are formed in presenceof water and oxygenof water and oxygen

Both combine to form Fe(OH)Both combine to form Fe(OH)22 or Fe(OH)or Fe(OH)33

This product, called Yellow Rust, corresponds to

Fe2O3 3H2O If the supply of oxygen and water is limited, the

corrosion product may even be black anhydrous

oxide, Fe3O4, known as Magnetite


25/65

MECHANISM OF WET CORROSION BY OXYGEN ABSORPTION.


26/65

FOUR TYPES OF ELECTROCHEMICAL CELL

* GALVANIC CELL OR DISSIMILAR METAL CELL

* CONCENTRATION CELL

* DIFFERENTIAL STRESS CELL

* DIFFERENTIAL TEMPERATURE CELL


27/65

GALVANIC CELLGALVANIC CELL

GalvanicGalvanic cellscells (also(also knownknown asas compositioncomposition

cellscells)) arisearise whenwhen twotwo metalsmetals withwith dissimilardissimilar

compositionscompositions oror microstructuresmicrostructures comecome intointo

contactcontact inin presencepresence ofof anan electrolyteelectrolyte..

THETHE TWOTWO MOSTMOST COMMONCOMMON EXAMPLESEXAMPLES ::

DISSIMILARDISSIMILAR METALSMETALS

MULTIMULTI--PHASEPHASE ALLOYALLOY


28/65

Dissimilar metals:Dissimilar metals:

FormedFormed byby twotwo singlesingle--phasephase metalsmetals inin

contact,contact, suchsuch asas ironiron andand zinc,zinc, oror nickelnickel

andand goldgold..

TheThe metalmetal thatthat isis higherhigher onon thetheElectrochemicalElectrochemical SeriesSeries willwill bebe thethe

cathodecathode..

TheThe otherother metalmetal willwill suffersuffer anodicanodic

reactionsreactions andand willwill corrodecorrode..


29/65

MultiMulti--phase alloy:phase alloy:

FormedFormed byby aa metalmetal alloyalloy composedcomposed ofof

multiplemultiple phases,phases, suchsuch asas aa stainlessstainless steel,steel,

aa castcast iron,iron, oror anan aluminumaluminum alloyalloy..

TheThe individualindividual phasesphases possesspossess differentdifferent

electrodeelectrode potentials,potentials, resultingresulting inin oneonephasephase actingacting asas anan anodeanode andand subjectsubject totocorrosioncorrosion..


30/65

Stress CellStress Cell

StressStress cellscells cancan existexist inin aa singlesingle piecepiece ofofmetalmetal wherewhere aa portionportion ofof thethe metalmetal

possessespossesses moremore storedstored strainstrain energyenergy thanthan

thethe restrest ofof thethe metalmetal..

TheThe threethree mostmost commoncommon examplesexamples::

GrainGrain boundariesboundaries

HighHigh localizedlocalized stressstress ColdCold workedworked


31/65

Grain boundaries:Grain boundaries:

ByBy definition,definition, metalmetal atomsatoms situatedsituated alongalonggraingrain boundariesboundaries areare notnot locatedlocated inin aaregularregular crystalcrystal arrayarray (i(i..ee.. aa grain)grain)..

TheirTheir increasedincreased strainstrain energyenergy translatestranslatesintointo anan electrodeelectrode potentialpotential thatthat isis anodicanodic totothethe metalmetal inin thethe grainsgrains properproper..

Thus,Thus, corrosioncorrosion cancan selectivelyselectively occuroccuralongalong graingrain boundariesboundaries


32/65

High localized stress:High localized stress:

LocalisedLocalised highhigh--stressstress regionsregions willwill bebe

anodicanodic toto lowlow--stressstress regionsregions andand cancan

corrodecorrode selectivelyselectively..

ForFor example,example, boltsbolts underunder loadload areare subjectsubject

toto moremore corrosioncorrosion thanthan similarsimilar boltsbolts thatthat

areare unloadedunloaded..

AA goodgood rulerule ofof thumbthumb isis toto selectselect fastenersfasteners

thatthat areare cathodiccathodic toto thethe metalmetal beingbeing

fastenedfastened inin orderorder toto preventprevent fastenerfastener

corrosioncorrosion..


33/65

Cold worked:Cold worked:

RegionsRegions withinwithin aa metalmetal subjectedsubjected toto coldcold--

workwork containcontain aa higherhigher concentrationconcentration ofof

dislocations,dislocations, andand asas aa resultresult willwill bebe anodicanodic

toto nonnon--coldcold--workedworked regionsregions..

Thus,Thus, coldcold--workedworked sectionssections ofof aa metalmetal willwill

corrodecorrode fasterfaster..

ForFor example,example, nailsnails thatthat areare bentbent willwill oftenoften

corrodecorrode atat thethe bend,bend, oror atat theirtheir headhead wherewhere

theythey werewere workedworked byby thethe hammerhammer


34/65

Concentration CellConcentration Cell

ConcentrationConcentration cellscells cancan arisearise whenwhen thethe

concentrationconcentration ofof oneone ofof thethe speciesspecies

participatingparticipating inin aa corrosioncorrosion reactionreaction variesvaries

withinwithin thethe electrolyteelectrolyte..

TheThe twotwo mostmost commoncommon examplesexamples

ElectrolyteElectrolyte concentrationconcentration

OxygenOxygen concentrationconcentration


35/65

Electrolyte concentrationElectrolyte concentration::

IfIf aa regionregion ofof thethe electrolyteelectrolyte (adjacent(adjacent toto thethemetal)metal) exhibitsexhibits aa decreaseddecreased concentrationconcentrationofof metalmetal ions,ions, thisthis regionregion wouldwould becomebecomeanodicanodic toto thethe otherother portionsportions ofof thethe metalmetalsurfacesurface..

AsAs aa result,result, inin thisthis portion,portion, thethe metalmetal wouldwouldcorrodecorrode fasterfaster inin orderorder toto increaseincrease thethe locallocal

ionion concentrationconcentration toto homogenizehomogenize.. TheThe situationsituation maymay occuroccur inin anan anchoredanchored

shipship wherewhere thethe immersionimmersion lineline getsgets washedwashedbyby waveswaves..


36/65

Oxygen concentrationOxygen concentration

PerhapsPerhaps thethe mostmost commoncommon concentrationconcentrationcellcell affectingaffecting engineeredengineered structuresstructures isis thatthatformedformed duedue toto dissolveddissolved oxygenoxygen..

WhenWhen oxygenoxygen hashas accessaccess toto aa moistmoist metalmetalsurface,surface, corrosioncorrosion isis promotedpromoted andand oxygenoxygengetget depleteddepleted..

IfIf thethe depletiondepletion isis notnot replenished,replenished, itit formsformsanan anodicanodic cellcell inin comparisoncomparison toto highhigh

oxygenoxygen areaarea..


37/65

Oxygen concentrationOxygen concentration

As As aa result,result, sectionssections coveredcovered byby dirtdirt oror

scalescale willwill oftenoften corrodecorrode faster,faster, sincesince thethe

flowflow ofof oxygenoxygen toto thesethese sectionssections isis

restrictedrestricted..

An An increasedincreased corrosioncorrosion raterate willwill leadlead toto

increasedincreased residue,residue, furtherfurther restrictingrestricting thethe

oxygenoxygen flowflow toto worsenworsen thethe situationsituation.. PittingPittingoftenoften resultsresults fromfrom thisthis Runaway"Runaway" reactionreaction


38/65

FACTS ABOUT CREVICE CORROSION:

Corrosion is accelerated in apparently

inaccessible areas.

Corrosion is accelerated under accumulation ofdirt, sand, or other contaminants.

Contacts between metals & non metallic

surfaces (Rubber gaskets, woods, plastics, glass,asbestos etc.) are prone to Crevice Corrosion


39/65

MECHANISM OF DIFFERENTIAL ATTACK CAUSED BY

THE PRESENCE OF DROP OF SALT SOLUTION ON THE

IRON SURFACE.


40/65

ANODE IS FORMED AT INACCESSIBLE

LOCATION,WHERE OXYGEN IS LESS AVAILABLE.


41/65

EMF SERIES

Potential difference between metals exposed

to solution containing unit activity aremeasured at constant temperature.

It is presented in tabular form known as EMF

series.

All potentials are referred against Hydrogen

electrode.

Potential between metals are determined by

taking the absolute differences between their

standard EMF potentials.


42/65

Table 10Table 10--1 Standard EMF Series for Metals1 Standard EMF Series for Metals

MetalMetal--metal ionmetal ion

equilibriumequilibrium

Electrode potential vs.Electrode potential vs.

normal hydrogrennormal hydrogren

electrode at 25electrode at 2500C (volts)C (volts)

Noble or cathodicNoble or cathodic AuAu--AuAu+3+3 +1.498+1.498

PtPt--PtPt+2+2 +1.2+1.2

PdPd--PdPd+2+2 +0.987+0.987

AgAg--AgAg+1+1 +0.799+0.799

HgHg--HgHg33+2+2 +0.788+0.788

CuCu--CuCu+2+2 +0.337+0.337

HH22--HH++ 0.0000.000

PbPb--PbPb+2+2 --0.1260.126

SnSn--SnSn+2+2 --0.1360.136

NiNi--NiNi+2+2 --0.2500.250

CoCo--CoCo+2+2 --0.2770.277

CdCd--CdCd+2+2 --0.4030.403

FeFe--FeFe+2+2 --0.4400.440

CrCr--CrCr+2+2 --0.7440.744

ZnZn--ZnZn+2+2 --0.7630.763

AlAl--AlAl+3+3 --1.6621.662

MgMg--MgMg+2+2 --2.3632.363

NaNa--NaNa+1+1 --2.7142.714

+1+1


43/65

GALVANIC SERIES

Covers most engineering metals and alloys.

Gives more practical prediction of galvanic

relationship than the EMF series.

The series is based on potential measurements

& galvanic corrosion tests in un-polluted sea

water.

TheThe relativerelative nobilitynobility ofof aa materialmaterial cancan bebe

predictedpredicted byby measuringmeasuring itsits corrosioncorrosion potentialpotential..


44/65

GALVANIC SERIESGALVANIC SERIES ContdContd

GalvanicGalvanic seriesseries relationshipsrelationships areare usefulusefulasas aa guideguide forfor selectingselecting metalsmetals toto bebejoinedjoined

HelpsHelps inin thethe selectionselection ofof metalsmetals havinghavingminimumminimum tendencytendency toto interactinteractgalvanicallygalvanically

IndicatesIndicates thethe needneed ofof protectionprotection toto bebeappliedapplied toto lessenlessen interactionsinteractions..


45/65

Table 10 2 Galvanic Series of Some Commercial Metals and Alloys in Seawater


46/65

Table 10-2 Galvanic Series of Some Commercial Metals and Alloys in Seawater

Noble or cathodic Platinum

Gold

Graphite

Titanium

Silver

18-8 stainless steel (passive)

Nickel (passive)

Silver solder

Cupronickels (60-90Cu, 40-10Ni)

Bronzes (Cu-Sn)

Copper

Brasses (Cu-Zn)

Nickel (active)

Tin

Lead

Lead-T

in solders18-8 stainless stel (active)

Steel or iron

2024 Aluminum (4.5Cu 1.5Mg 0.6Mn)

Cadmium

Commercially pure aluminum

Zinc

Active or anodic Ma nesium and ma nesium allo s


47/65

GALVANIC SERIESGALVANIC SERIES

A small anode/cathode area ratio is highlyundesirable.

In this case, the galvanic current is

concentrated onto a small anodic area. Rapid thickness loss of the dissolving

anode tends to occur under these

conditions.

That is why a tin plating gets perforated

much faster than Zn plating.

FACTORS INFLUENCING CORROSION


48/65

FACTORS INFLUENCING CORROSION:-

NATURE OF METALS ( POSITION IN GALVANIC SERIES)

RELATIVE AREAS OF ANODIC & CATHODIC PARTS.

PURITY OF METALS.

PHYSICAL STATE OF METALS.

SOLUBILITY OF CORROSION PRODUCTS.

VOLATILITY OF CORROSION PRODUCTS.

NATURE OF THE CORRODING ENVIRONMENT


49/65

NATURE OF THE CORRODING ENVIRONMENT.

TEMPARATURE.

HUMIDITY OF THE AIR.

PRESENCE OF IMPURITES IN ATMOSPHERE.

PRESENCE OF SUSPENDED PARTICLES IN AIR.

FORMATION OF OXYGEN CONCENTRATION CELL.


50/65

PREVENTION OF CORROSION:-

- MATERIAL SELECTION.

- DESIGN & FABRICATION PROCEDURE.

- ALTERATON OF ENVIRONMENT.

- CATHODIC PROTECTION.

* SACRIFICIAL ANODE METHOD.

* EXTERNAL POWER SUPPLY.

APPLICATION OF PROTECTIVE COATING:


51/65

-APPLICATION OF PROTECTIVE COATING:-

* ELECTRO PLATING.

* FLAME SPRAYING.

* CLADDING.

* HOT DIPPING.

* VAPOUR DEPOSITION.

* DIFFUSION.

*CHEMICAL CONVERSION.(ANODISING.

PHOSPHATING, CHROMATIZING.)

*ORGANIC COATING. (PAINTS, VARNISHES,

ENAMELS, LACQUERS, etc.)


52/65

Corrosion Performance of MetalsCorrosion Performance of MetalsThe following numbers refer to the corrosion rates, expressed in terms of depth of metalThe following numbers refer to the corrosion rates, expressed in terms of depth of metal

removed in unit time, for mild steel and various stainless steels inremoved in unit time, for mild steel and various stainless steels in

different acids. (different acids. (referencereference))

Penetration ratesPenetration rates (mm/year) for various steels in different acids(mm/year) for various steels in different acids

SteelSteel 70% HNO70% HNO33(60(60 ooC)C)

20% HCl20% HCl

(20(20 ooC)C)

80% H80% H22SOSO44(20(20 ooC)C)

Mild SteelMild Steel Very highVery high 3838 0.40.4

13% Chromium13% Chromium 0.150.15 120120 4.54.5

12% Chromium12% Chromium

12% Nickel12% Nickel

0.050.05 55 0.50.5

17% Chromium17% Chromium 0.10.1 3535 0.70.7

18% Chromium18% Chromium

8% Nickel8% Nickel

NilNil 2525 1.51.5


53/65

Corrosion Performance of Metals contd..Corrosion Performance of Metals contd..

StainlessStainless steelssteels wouldwould provide provide betterbetter lifelifethanthan mildmild steelsteel inin hothot concentratedconcentrated nitricnitricacidacid

InIn hydrochlorichydrochloric acidacid onlyonly thethe 1212 perper centcentChromiumChromium && 88 per per centcent NickelNickel steelsteel isissignificantlysignificantly betterbetter thanthan thethe mildmild steelsteel

MildMild SteelSteel inin thethe sulfuricsulfuric acidacid isis betterbetter

thanthan anyany ofof thethe stainlessstainless steels,steels, costingcostingaboutabout anan orderorder ofof magnitudemagnitude lessless perper unitunit

weightweight..


54/65


TheseThese differencesdifferences areare largelylargely relatedrelated toto

DifferingDiffering oxidizingoxidizing characteristicscharacteristics ofof thethe acidsacids..

TheThe corrosioncorrosion resistanceresistance ofof stainlessstainless steelssteels

mainlymainly dependsdepends uponupon thethe formationformation ofof aaprotectiveprotective chromiumchromium oxideoxide filmfilm uponupon theirtheir

surfacessurfaces..

So,So, inin thethe absenceabsence ofof anan adequateadequate supplysupply ofof

oxygen,oxygen, asas withwith thethe hydrochlorichydrochloric acids,acids, thethe

chromiumchromium additionaddition isis ofof littlelittle benefitbenefit..


55/65


TheThe samesame pointpoint cancan bebe mademade inin relationrelation toto otherotherformsforms ofof corrosioncorrosion

MolybdenumMolybdenum additionsadditions toto ferriticferritic steelssteels arearebeneficialbeneficial withwith respectrespect toto stressstress corrosioncorrosion

crackingcracking resistanceresistance inin carbonatecarbonate environments,environments,butbut areare detrimentaldetrimental inin hydroxidehydroxide solutionssolutions..

Stainless steels represents a class of highlycorrosion resistance steel.

BUT REMEMBERBUT REMEMBER


56/65


Stainless steel is not stainless IN ALL

ENVIRONMENT.

It is not most corrosion- resistant.

It is not a specific alloy. A group ofnearly 30 different alloys ( 11.5-30%Cr,

0-22% Ni).


57/65


Stainless steels are less resistant alloys in

chloride-containing mediums & stressed

structures than ordinary structural steel.

Stainless steel are most susceptible to localisedcorrosion- inter-granular corrosion, stress

corrosion & pitting than ordinary steels.

A large number of corrosion failure can be

directly attributed to the indiscriminate selection

of stainless steel.


58/65


TheThe importantimportant pointpoint toto bebe graspedgrasped isis thatthat ininspecifyingspecifying thethe operatingoperating conditionsconditions forfor aacomponentcomponent oror structurestructure itit isis vital vital thatthat thethe

environmentalenvironmental conditionsconditions bebe defineddefined withwith asasmuchmuch precision precision asas anyany otherother parameters,parameters,suchsuch asas stress,stress, temperaturetemperature etcetc..

OnlyOnly byby soso doingdoing cancan thethe selectionselection ofof aamaterialmaterial fromfrom thethe corrosioncorrosion resistanceresistance

viewpointviewpoint bebe mademade satisfactorilysatisfactorily

COMBINATIONS OF METAL & CORROSIVE ENVIRONMENT


59/65

COMBINATIONS OF METAL & CORROSIVE ENVIRONMENT

THE MAXMIMUM AMOUNT OF CORROSION RESISTANCE

FOR THE LEAST AMOUNT OF MONEY.

1. NITRIC ACID - STAINLESS STEEL

2. CAUSTIC - NICKEL & ICKEL ALLOYS

3. HYDROFLUORIC ACID - MONEL

4. HOT HYDROCHLORIC ACID - HASTELLOYS ( CHORIMETS)

5. DILUTE SULFURIC ACID - LEAD

6. NONSTAINING ATOMSPHERIC EXPOSURE - ALUMINUM

7. DISTILLED WATER - TIN

8. HOT STRONG OXIDIZING SOLUTIONS - TITANIUM.

9. CONCENTRATED SULFURIC ACID MILD STEEL

GUIDELINE FOR DESIGN & FABRICATION PROCESS:-


60/65

GUIDELINE FOR DESIGN & FABRICATION PROCESS:-

Weld rather than rivet tanks & other

containers. Bottom of tank & containers should be

sloped towards drain holes for easy cleaning.

Easy replacement systems of corrosionprone components.

Avoid excessive mechanical stresses in

components exposed to corrosive medium.

Avoid electrical contact between dissimilar

materials to prevent galvanic corrosion.

Avoid sharp bend in piping system


61/65

Avoid sharp bend in piping system.

Avoid hot spot during heat transfer system.

Design to exclude air.

Avoid heterogeneity, dissimilar metals, vapourspaces, uneven heat, & stress distribution.


62/65

CATHODIC PROTECTION OF AN UNDERGROUND TANK

USING IMPRESED CURRENT.


63/65

PROTECTION OF AN UNDERGROUND PIPELINE

WITH A MAGNESIUM ANODE.


64/65

CATHODIC PROTECTION OF A DOMESTIC HOT-WATER

TANK USING A SACRIFICIAL ANODE.


65/65

THANK U

corrosion & its prevention gc-08

Documents