1-forms of corrosion

8/12/2019 1-Forms of Corrosion

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Forms of CorrosionForms of Corrosion

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Uniform CorrosionUniform Corrosion

Most commonMost common

Corrosive environment evenly distributed over entire metalCorrosive environment evenly distributed over entire metalsurfacesurface

Metal:Metal: metallurgicallymetallurgically and compositionally uniformand compositionally uniform

On the basis of tonnage waste, the most important forms ofOn the basis of tonnage waste, the most important forms ofcorrosioncorrosion

AtmosphericAtmospheric corrosion (rusting)corrosion (rusting)

Corrosion in acid solutionsCorrosion in acid solutions

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Localized Corrosion (Attack)Localized Corrosion (Attack)

Sources of localized corrosionSources of localized corrosion

Oxygen concentrationOxygen concentration

Chloride ion concentrationChloride ion concentration pHpH ow ra eow ra e

MaterialMaterial Se re ationSe re ation InclusionsInclusions Different phasesDifferent phases Grain boundariesGrain boundaries

MechanicalMechanical Static stressStatic stress Fluctuating stressFluctuating stress

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Galvanic CorrosionGalvanic CorrosionDissimilar Metal CorrosionDissimilar Metal Corrosion

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a current will flow between thema current will flow between them corrosion rate of less resistance metalcorrosion rate of less resistance metal(anodic)(anodic) ..


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GalvanicGalvanic CorrosionCorrosion

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Galvanic Corrosion (cont.)Galvanic Corrosion (cont.)

TheThe driving force for galvanic corrosion isdriving force for galvanic corrosion is a potential differencea potential differencebetween the difference metalsbetween the difference metals. In a bimetallic couple, the less noble. In a bimetallic couple, the less noble

to corrode more compared with the uncoupled condition.to corrode more compared with the uncoupled condition.

TheThe more noble metal will act as the cathode in the corrosion cellmore noble metal will act as the cathode in the corrosion celland corrode less.and corrode less.

Important factors in galvanic corrosionImportant factors in galvanic corrosion Relative areas of anode and cathodeRelative areas of anode and cathode Difference in potential between anode and cathodeDifference in potential between anode and cathode

Effect of anodic polarization onEffect of anodic polarization on anodeanode

he relative nobility of a material can be predicted by measuring itshe relative nobility of a material can be predicted by measuring itscorrosion potential.corrosion potential.

The well knownThe well known galvanic seriesgalvanic series lists the relative nobility of certainlists the relative nobility of certain

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r v v r . ., w rr v v r . ., w r ..


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Galvanic Corrosion (cont.)Galvanic Corrosion (cont.)TABLE. Galvanic Series in Seawater (or Fig. 6.1)

The galvanic seriesThe galvanic series

Ordered by observed behavior inservice

Allows for corrosion products

Ordered by observed behavior inservice

Allows for corrosion products

Can be defined for alloys

Different for every environment

Can be defined for alloys

Different for every environment

o e a some a oys can appeartwice according to whether they areactive or passive.

o e a some a oys can appeartwice according to whether they areactive or passive.

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A more detailed alvanicA more detailed alvanic

series in waterseries in water

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ExampleExample

QUESTIONQUESTION

with leadwith lead--tin solder. Will the brass or the solder corrode?tin solder. Will the brass or the solder corrode?

From the galvanic series, we find that all of the copperFrom the galvanic series, we find that all of the copper--based alloysbased alloys

are more cathodic than a 50%are more cathodic than a 50% PbPb -- 5050%% SnSn solder.solder. Therefore,Therefore, thethe..

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Galvanic Corrosion (cont.)Galvanic Corrosion (cont.)

Galvanic corrosion can occur between different phases in an alloy.Galvanic corrosion can occur between different phases in an alloy.

This is especially important when one of the phases is much moreThis is especially important when one of the phases is much more

active as a cathode, e.g.,active as a cathode, e.g., CuCu--containing precipitates in aluminum alloys initiate pitting corrosioncontaining precipitates in aluminum alloys initiate pitting corrosion

Fe and Cu impurities in commercial Zn cause a large increase inFe and Cu impurities in commercial Zn cause a large increase incorrosion rate compared to pure zinccorrosion rate compared to pure zinc

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Local electrode cellLocal electrode cell

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Avoiding Galvanic CorrosionAvoiding Galvanic Corrosion

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Beneficial Application of Galvanic CorrosionBeneficial Application of Galvanic Corrosion

Cathodic protection (CP), sacrificial anode protectionCathodic protection (CP), sacrificial anode protection

Galvanized steels; Zn coating is anodic to steel, act as a sacrificialGalvanized steels; Zn coating is anodic to steel, act as a sacrificialmetalmetal

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Differential Aeration (cont.)Differential Aeration (cont.)

cathode cathode

anode

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water-line corrosion

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CreviceCrevice Corrosion (Corrosion ())

AA localized form of corrosion usually associated with a stagnant solution onlocalized form of corrosion usually associated with a stagnant solution onthe microthe micro--environmental level.environmental level.

ucuc s agnan m croenv ronmen s en o occur n crev ces s e e areass agnan m croenv ronmen s en o occur n crev ces s e e areassuch as those formed under gaskets, washers, insulation material, fastenersuch as those formed under gaskets, washers, insulation material, fastenerheads, surface deposits, disbonded coatings, threads, lap joints andheads, surface deposits, disbonded coatings, threads, lap joints and clampsclampsusuall mismatch of com onents at interface .usuall mismatch of com onents at interface .

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Initiation of Crevice CorrosionInitiation of Crevice Corrosion

Crevice corrosion is initiated by changes in local chemistry withinCrevice corrosion is initiated by changes in local chemistry withinthe crevicethe crevice

Depletion of corrosion inhibitor in the creviceDepletion of corrosion inhibitor in the crevice

Depletion of oxygen in the creviceDepletion of oxygen in the crevice A shift to acid condition in the creviceA shift to acid condition in the crevice

BuildBuild--up of aggressive ion species (e.g., chloride) in the creviceup of aggressive ion species (e.g., chloride) in the crevice

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Chronology of Crevice CorrosionChronology of Crevice CorrosionM n+

O2 OH-

M

Cathodic

n+ O - Mn+

2 -

MM

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1 anodic and cathodic reactions occur over the entire surface


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Chronology of Crevice CorrosionChronology of Crevice CorrosionM n+

O2 OH-

M

Cathodic

O -M n+

O - Mn+ M n+

M M M

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2 Depletion of oxygen inside the crevice3 - Metal dissolution takes place inside the crevice


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Chronology of Crevice CorrosionChronology of Crevice Corrosion

( ) nHClOHMOnHnClM n2n +++ +

Cl-

O2 OH-M n+

O2 OH-

Mn+

M n+

M

MM

-

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5 pH decreases inside the crevice (more acid) enhance dissolution


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Chronology ofChronology of CreviceCrevice CorrosionCorrosion

-

Cl-

O2 OH-M n+

O2 OH-

Mn+

M n+ +

M

MM

-

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Crevice CorrosionCrevice Corrosion -- SummarySummary

AutoAuto--catalyticcatalytic processprocess

A small area anodic inside the creviceA small area anodic inside the crevice

a large cathodica large cathodic aereatedaereated area outside.area outside.

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Crevice CorrosionCrevice Corrosion -- ExampleExample

rev ce corros on o ange n o seawa er rev ce corros on o

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Crevice Corrosion at Pipe Socket JointCrevice Corrosion at Pipe Socket Joint

Crevice corrosion o socket weld at a ormed between t e 304L i e

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And type 316L valve


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Pitting Corrosion (Pitting Corrosion ())

PittingPitting corrosioncorrosion is a localized form of corrosion by which cavities is a localized form of corrosion by which cavitiesor holes are produced in the material.or holes are produced in the material.

Pitting is considered to be more dangerous than uniform corrosionPitting is considered to be more dangerous than uniform corrosion

damage because it is more difficult to detect, predict and designdamage because it is more difficult to detect, predict and designagainst.against.

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Cause of PittingCause of Pitting

Localized chemical or mechanical damage to the protective oxideLocalized chemical or mechanical damage to the protective oxidefilm.film.

Low dissolved oxygen concentrations (which tends to render aLow dissolved oxygen concentrations (which tends to render a

protective oxide film less stable) and high concentrations of chlorideprotective oxide film less stable) and high concentrations of chloride(as in seawater)(as in seawater)

Localized damage to, or poor application of, a protective coatingLocalized damage to, or poor application of, a protective coating

The presence of nonThe presence of non--uniformities in the metal structure of theuniformities in the metal structure of the

component, e.g., inclusions.component, e.g., inclusions.

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Morphology of PittingMorphology of Pitting

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Underdeposit CorrosionUnderdeposit Corrosion

Underdeposit pitting of carbon steel sprinkler pipes

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Pitting on Stainless Steel in ClPitting on Stainless Steel in Cl-- EnvironmentsEnvironments

3+3+down the passive film. The cathodic oxygen reduction reaction continuesdown the passive film. The cathodic oxygen reduction reaction continuesoutside the pitoutside the pit..

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The presence of chloride is important, as it allows a pH of about 1 to beThe presence of chloride is important, as it allows a pH of about 1 to beachieved (achieved (HClHCl is a strong acid, and does not associate) and the metalis a strong acid, and does not associate) and the metalchloride are very solublechloride are very soluble..

Pitting Corrosion: ExamplesPitting Corrosion: Examples


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Pitting Corrosion: ExamplesPitting Corrosion: Examples

Chloride pitting of

stainless steel

Pitting of exterior of lead cable sheathdue to stray current

due to soldering flux

contamination

Carbon steel

Pitting

underground

Pitting of Aluminum air duct

due to salt contamination

33Pitting of Inconel in Brine


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Management of PittingManagement of Pitting

Stainless steelsStainless steels Pitting breakdown potential (that potential above which the anodicPitting breakdown potential (that potential above which the anodic

greater susceptibility to pitting.greater susceptibility to pitting.

Critical pitting temperature (CPT, temperature at which pitting isCritical pitting temperature (CPT, temperature at which pitting isobserved in aggressive conditions); lower CPT implies greaterobserved in aggressive conditions); lower CPT implies greatersuscep y.suscep y.

Pitting resistance equivalent (formula based on composition (e.g.,Pitting resistance equivalent (formula based on composition (e.g.,

Cr+3Mo+30N) gives ranking of alloysCr+3Mo+30N) gives ranking of alloys

Pit growth tends to follow a law of the formPit growth tends to follow a law of the formDepth = constantDepth = constant timetimenn

wherewhere nn is between 0.3 and 0.5is between 0.3 and 0.5

Pitting is a statistical process, and can be modeled by extreme valuePitting is a statistical process, and can be modeled by extreme valuestatistics.statistics.

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Dealloying (Selective Leaching)Dealloying (Selective Leaching)

Alloy containing one noble element and one base element (e.g.,Alloy containing one noble element and one base element (e.g.,brass, Cu + Zn)brass, Cu + Zn)

Base element can dissolve leaving a matrix of noble elementBase element can dissolve leaving a matrix of noble element Dezincification of brassDezincification of brass

Graphitization of grey cast ironGraphitization of grey cast iron

Causes a loss of mechanical stren th without obvious chan e inCauses a loss of mechanical stren th without obvious chan e inshapeshape

Color changeColor change

Drastic reduction in stren thDrastic reduction in stren th Minimal volume changeMinimal volume change

Special evaluation techniqueSpecial evaluation technique

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Dezincification of a BrassDezincification of a Brass

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De-zincification

ug ype o

De-zincification


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Dezincification of a BrassDezincification of a Brass

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Denickelification


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Dealuminification of a C95800 Ni-Al bonze pump

impeller in service in a wastewater plant

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Dezincification of a admiralty brass


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Intergranular Corrosion (IGC;Intergranular Corrosion (IGC;))

IGC is a localized attack along the grain boundaries, or immediatelyIGC is a localized attack along the grain boundaries, or immediatelyadjacent to grain boundaries, while the bulk of the grains remainadjacent to grain boundaries, while the bulk of the grains remainarge y una ec e .arge y una ec e .

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Cause of Intergranular CorrosionCause of Intergranular Corrosion

Grain boundaries are somewhat more reactive than the matrix of anGrain boundaries are somewhat more reactive than the matrix of analloy, but since the difference in reactivity is slight, grain boundaryalloy, but since the difference in reactivity is slight, grain boundarye ec s are usua y o e consequence.e ec s are usua y o e consequence.

Chemical segregation effects (Impurities have a tendency to beChemical segregation effects (Impurities have a tendency to beenriched at grain boundaries.)enriched at grain boundaries.)

Specific phases precipitated on the grain boundaries.Specific phases precipitated on the grain boundaries.

SuchSuch precipitation can produce zones of reduced corrosionprecipitation can produce zones of reduced corrosionresistance in the immediate vicinity.resistance in the immediate vicinity.

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ExampleExample ofof IGC (Sensitization)IGC (Sensitization)

Sensitization ofSensitization ofStainless Steel or WeldStainless Steel or WeldDecaDeca

When grain boundary CrWhen grain boundary Crcarbide precipitation hascarbide precipitation has

,,said to besaid to be sensitizedsensitized..

Sensitization frequently occursSensitization frequently occursin the heatin the heat--affected zone HAZaffected zone HAZduring welding, and theduring welding, and theresultant corrosion is calledresultant corrosion is calledWeld decayWeld decay..

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Solving Welding DecaySolving Welding Decay

Use a low carbon grade of stainless steel (304L, 316L SS)Use a low carbon grade of stainless steel (304L, 316L SS)

Use a stabilized grade of steel, which includes a strong carbideUse a stabilized grade of steel, which includes a strong carbide--

forming element (Ti orforming element (Ti or NbNb) to lock up the carbon in the grain.) to lock up the carbon in the grain.

For existing components that have been sensitized, heat treat to reFor existing components that have been sensitized, heat treat to re--dissolve the carbides. (but this is very difficult in practice)dissolve the carbides. (but this is very difficult in practice)

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Flow EffectsFlow Effects

Flow canFlow can

passivatepassivate))

Increase the rate of dissolution of corrosion product filmsIncrease the rate of dissolution of corrosion product films

Mechanicall remove oxidesMechanicall remove oxides

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Erosion CorrosionErosion Corrosion

Corrosion accelerated by impact by solid particlesCorrosion accelerated by impact by solid particles

These may remove metal, or they may just remove oxide and allowThese may remove metal, or they may just remove oxide and allowmetal to corrode more quickly.metal to corrode more quickly.

They are unexpectedThey are unexpected mostlymostly because evaluation corrosion tests runbecause evaluation corrosion tests rununder static conditions or because the erosion effects were notunder static conditions or because the erosion effects were notconsidered.considered.

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Schematic of erosion corrosion of heat exchanger handling water


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Erosion Corrosion (cont.)Erosion Corrosion (cont.)

Vulnerable equipment includes:Vulnerable equipment includes:

- pipes (bends, elbows, tees);

- valves;- pumps;-

- flow-measuring orifices, venturies;

- turbine blades;- nozzles;-

- propellers, impellers;- stirrers;

- stirred vessels;-

- metal-working equipment (scrapers, cutters,grinders, mills);

- spray impingement components;- , .

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Example of Erosion CorrosionExample of Erosion Corrosion Seawater pumpSeawater pump

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Cavitation CorrosionCavitation Corrosion

Cavitation occurs when a fluids operational pressure drops below the vapor pressureCavitation occurs when a fluids operational pressure drops below the vapor pressurecausing gas pockets and bubbles to form and collapse.causing gas pockets and bubbles to form and collapse.

HighHigh--speed pressure oscillations (pumps, etc.) can createspeed pressure oscillations (pumps, etc.) can create shockwaves > 60shockwaves > 60 ksiksi..

--

av a on corros on o a eaera or

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Plastic deformation of metal: the presence of slip lines in equipments subject to cavitationsPlastic deformation of metal: the presence of slip lines in equipments subject to cavitations


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Fretting CorrosionFretting Corrosion

Wear process due to relative motions in highly loaded devicesWear process due to relative motions in highly loaded devicesexaggerated by corrosive environmentexaggerated by corrosive environment

RoughnessRoughness of contacting surfacesof contacting surfaces

DeviceDevice micromotionsmicromotions

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Fretting CorrosionFretting Corrosion

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Environmental CrackingEnvironmental Cracking

Corrosion cracking caused by a combination of conditions that canCorrosion cracking caused by a combination of conditions that canspecifically result in one of the following form of corrosion damage.specifically result in one of the following form of corrosion damage.

Tensile stresses that cause environmental cracking arise fromTensile stresses that cause environmental cracking arise from Residual cold workResidual cold work

WeldingWelding

GrindingGrinding

Thermal treatmentThermal treatment

Service conditionsService conditions

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Types of Environmental CrackingTypes of Environmental Cracking

Stress corrosion cracking (SCC)Stress corrosion cracking (SCC)

Sulfide stress corrosion crackingSulfide stress corrosion cracking

Chloride induced SCCChloride induced SCC

Caustic induced SCCCaustic induced SCC

CorrosionCorrosion fatigue (CF)fatigue (CF)

HydrogenHydrogen embrittlement (HE)embrittlement (HE)

Li uid metal embrittlement LMELi uid metal embrittlement LME

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Stress Corrosion Cracking (SCC)Stress Corrosion Cracking (SCC)

SCC is the cracking is the cracking induced from theSCC is the cracking is the cracking induced from the combinedcombinedinfluence of tensile strength and a corrosive environmentsinfluence of tensile strength and a corrosive environments

Usually most of the surface remainsUsually most of the surface remains unattackedunattacked, with fine cracks, with fine crackspenetrating into the metalpenetrating into the metal

These cracks can have anThese cracks can have an intergranularintergranular (IG) or(IG) or transgranulartransgranular (TG)(TG)morphology in the microstructure.morphology in the microstructure.

SCCSCC is classified as a catastrophic form of corrosion, as theis classified as a catastrophic form of corrosion, as thedetection of such fine cracks can be very difficult and the failure notdetection of such fine cracks can be very difficult and the failure not

easil redicted.easil redicted.

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SCCSCC -- ExamplesExamples

IGSCC of an Inconel heat exchanger tube (500X) Chloride SCC of S31603 (316L) chemical processing piping

system. It can be characterized by the multi-branched

lightning bolt TG crack pattern (300X)

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Caustic SCC in a 304L SS pipe in methanol

reforming service oxalic acid etch (50X)


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Hydrogen EmbrittlementHydrogen Embrittlement

Also called: hydrogen induced corrosion (HIC)Also called: hydrogen induced corrosion (HIC)

Involves the ingress of hydrogen into the metal causingInvolves the ingress of hydrogen into the metal causing

Reduced ductility and loadReduced ductility and load--bearing capacitybearing capacity

Subsequent cracking andSubsequent cracking and Catastrophic brittle failures at stresses below the yield stress ofCatastrophic brittle failures at stresses below the yield stress of

susceptible materialssusceptible materials

Most vulnerable are highMost vulnerable are high--strength steels, titanium alloys andstrength steels, titanium alloys andaluminum alloysaluminum alloys

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Source of HydrogenSource of Hydrogen

In the metal making processIn the metal making process In metal processing (In metal processing (phosphatingphosphating, pickling), pickling) WeldingWelding

In storage or containment of hydrogen gasIn storage or containment of hydrogen gas As a byAs a by--product of a general corrosionproduct of a general corrosion

In electroplatingIn electroplating

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Corrosion Fatigue (Cracking)Corrosion Fatigue (Cracking)

The fatigue fracture of a metal aggravated by a corrosive environment or the SCC ofThe fatigue fracture of a metal aggravated by a corrosive environment or the SCC ofa metal aggravated by cyclic stress to decrease the number of cycles to failure.a metal aggravated by cyclic stress to decrease the number of cycles to failure.

Stress

Range

Ferrous

Ferrous with

corrosion

Non ferrous

log. number of cycles

to failure (Nf)

with corrosion

107

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Corrosion FatigueCorrosion Fatigue

58Corrosion fatigue cracks on the ID of a Admiralty Brass exchanger tube


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ReferencesReferences

CorrosionCorrosion: Understanding the: Understanding the Basics, ASMBasics, ASM International, 2000International, 2000

L.L.L.L. ShreirShreir, Corrosion, 3, Corrosion, 3rdrd ed., Butterworthed., Butterworth--Heinemann, UK, 2000.Heinemann, UK, 2000.

Basic Corrosion Course, NACE International, 2003.Basic Corrosion Course, NACE International, 2003.

D.R.D.R. AskelandAskeland, The Science and Engineering of Materials, 4, The Science and Engineering of Materials, 4

thth

ed., Ch.22:ed., Ch.22:Corrosion and Wear.Corrosion and Wear.

. . on ana, orros on ng neer ng,. . on ana, orros on ng neer ng, e ., .e ., .

Corrosion Doctors, www.corrosionCorrosion Doctors, www.corrosion--doctors.org.doctors.org., www.s ee, www.s ee --n.com.n.com.

KennedyKennedy Space Center, NASA,Space Center, NASA, corrosion.ksc.nasa.gov.corrosion.ksc.nasa.gov.

CorrosionCorrosion Testing Laboratories, Inc., www.corrosionlab.comTesting Laboratories, Inc., www.corrosionlab.com

eta urg caeta urg ca ec no og es, nc., www.metec no og es, nc., www.met--tec .comtec .com

1-forms of corrosion

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