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Corrosionofmetals

MBJASS09FlaviusDeleanu

KathrinLorenz

ChristinaEngelhardt

Outline

corrosion of metals in aqueous solutions

corrosion processes

(material, biomaterial, current density potential diagram)

-

different types of corrosion

surface corrosion

pitting corrosion

corrosion of metals

titanium

ion

316L

magnesium

Stents

ion

magnesium

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Corrosion

Whatiscorrosion?

Definition(DIN50900):

Reaction ofamaterial withtheenvironment

Measurablechange (properties,behavior)

Probablydamage ofafunctionorsystem

Appearance:

Basicallyelectrochemical

Corrosion

Genericchemicalformulaforanodic metalloss:

M M+ + e (Oxidation)

Theproducedelectronsareconsumedatthecathodic side:2possibilitiesinaqueoussolution:

I) 2H+ +2e H2 (Reduction)II) O2+2H2O+4e

4OH (Reduction)

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Corrosion

Corrosionofiron(Fe):

Corrosion

Anode: Fe Fe2+ + 2e

Cathode: O2+4e +2H2O 4OH

Formationofrust: Fe2+ +2OH Fe(OH)2

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Corrosion

Onlynoblemetals arestable inaqueousenvironment

Oxidationpotential > Reductionpotential

(noblemetal) (environmentalspecies)

Otherreasonforstability:

PASSIVITY:stablenondissolvableoxidelayeron thesurface

primarilytitanium!

Corrosion

:noblemetals,cathode

:basemetals,anode

U=Eanode Ecathode

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Corrosion

Correlation: CurrentFlowandWeightLoss

Metallossisproportionaltotheproducedelectrons

Metallossisproportionaltothecurrentflow

Corrosionratescanbedeterminedbyapplying

Corrosion

Measurementofj/Ecurve:

Voltagesettingbetween

referenceelectrode

Currentbetween

corrodingelectrodeand

counterelectrode

Thereferenceelectrode

keepsitspotential

(highohmicinput)

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Corrosion Pourbaix-diagram

EFe /Fe

EFe /Fe 0

0,059

2 lgc

Fe

Fe Fe

2e

E

0,62Volt

NopHvaluedepandance:horizontallineI

Corrosion Pourbaix-diagram

2Fe 3H O Fe O 6H 2e

EFe /Fe2O3 EFe / Fe2O30

0,0592

lg cH

6

cFe 2

EFe /Fe2O3 1,09 0,18pH

pHvaluedepandance:lineII

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Corrosion Pourbaix-diagram

2Fe 3H2O Fe2O3 6H

cH6

cFe 2

K 101,45

pH 1,76

purechemicalreaction:vertical lineIII

Corrosion Pourbaix-diagram

Yellow:inertarea

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Corrosion Pourbaix-diagram

Yellow:corrosionarea

Corrosion Pourbaix-diagram

Yellow:areaswithstableoxides

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Corrosion

Corrosionofimplantsinhumanbody:

Blood=complexelectrolyte

*pH=7,4(0,05) generatedwithabuffersystem

*bloodconsistsofcellsandplasma (proteinsand

electrolyteslikeCl orPO43 dissolvedinwater)

moreaggressivethanseawater!Corrosionincreasedatareaswithmechanicalstress

Canprovokeaninflammation

Corrosion

Howdometallicionsdamagethetissue?

I) Phvalueandtheoxygenpartialpressurecandiversifyand

thiscan

change

the

chemical

environment(metalionsformheavilysolvablecompounds)

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Different types of corrosion

corrosion surface corrosion

atmospheric corrosion

galvanic (contact) corrosion

types of loc l corrosion pitting corrosion

crevice corrosion

stress-cracking corrosion intercrystalline corrosion

Surface corrosion

Mechanismusmechanism:

2H 2e H2

O2 2H2O 4e 4OH

acid medium

alcaline medium

preconditions:

- conductive surrounding (even a very thin water film is enough)

- homogeneity of surrounding & material

! anodic and cathodic reactions are parallel !

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Pittingcorrosion

Initiation:

Oxidelayer

rea up

Processesin

[23] [24][23] [24]

pittingcorrosiononsurfaceofcastironbathtub

FeCl2+H2O>FeClOH+H+ +Cl

Influences on corrosion

- pH Porbaix-diagram

- m crostructure ga van c e ements

- temperature diffusion constant

O2-conc. in open systems

- O2-concentration drop-model

- velocity of flow

- sa t-concentrat on

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Corrosion of metals

Examples:

titanium

iron

316Lmagnesium

Titanium

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Titanium

CorrosionResistance?

Titanium

Ti:creationofatitaniumoxidelayer

goodcorrosionresistance

Ti Ti2+ +2e 1.75V

Ti3+ +3H2O TiO2+ +2H3O

+ +e +0.10V

TiO2 ntypesemiconductor,Egap3.1eV

amorphous

anodisation:crystalline(athigherpotentialsortemperatures)=underspecificelectrochemicalconditions:specificsurfacemorphology

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Passivation of Titanium

titaniumoxidelayerTiO2 highlystablesurfaceoxidelayer provides

excellentcorrosionresistance

Titanium - applications

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Iron

Iron (Fe)

4,7wt.%ofearthcrust

Resistantindryair,indryCl,

concen ra e su or cac an

inalkalinemedium(except

NaOH)with

pH>9

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Iron (Fe)

The more active metal inthe galvanic couple

becomes the anode andgets oxidized. The noble

metal is the cathode.

316L

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316 L

Surgicalstainlesssteelisanspecifictypeofstainlesssteel,usedinmedicalapplications

ElementalcompositionWt.%

Low content of C

High addition of Cr and Ni

316 L

Corrosion Resistance ?

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316 L

FormationFormationofof aacompactcompact oxideoxide layerlayer(Cr(Cr22OO33))

protectionprotection againstagainst corrosioncorrosion

but

Thermaltreatments (welding,annealing)lead to Cr23C6

precipitationsinthe grain boundaries

23Cr +6C Cr23C6

Insidethe grain decreased Crconcentration

316 L

Cr23C6

precipitations

Crprecipitationsatthegrainboundaries

Crcontentbefore

treatment

LessCrnearthegrainboundaries

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316 L

Cr23C6precipitations

Afterthermaltreatment:

highCrcontentatgrain

boundaries

LessCrnearthegrainboundaries

Critical assivation currentdensit increases

Crcontentbefore

treatment

Aftertreatment

Possibilitytolosepassivation

316 L

Cr23C6precipitations

Afterthermaltreatment:

highCrcontentatgrain

boundaries

Differentconcentrations nearthe rainboundariesand

Crcontentbefore

treatment

Aftertreatment

inthegrainboundaries

Dangerofcontactcorrosion

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Current density potential diagram

evolutionofoxygen

evolution of ox en

currentdensityj

jcrit

active

potentialE

jpass

dissolution

counter

electrode

reference

electrode

cellwith3electrodesworking

electrode

j/Ecurves

316L

Ti

Electrolyte:0,14MNaCl,pretreatment:grinding(1200)

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316L

mainlyprotection

Microscopicpictureof316Lafter

pittingcausedby

chlorideions

Titanium

MicroscopicimageofTiafter

Leve ng o gr n ng

grooves becauseof

grownoxidelayer

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Magnesium

Implants

Eigenschaften Knochen Magnesium Ti Leg. Co-Cr Leg. EdelstahlEigenschaften Knochen Magnesium Ti Leg. Co-Cr Leg. Edelstahlproperties bone magnesium Tialloy CoCralloy 316L

Dichte (g/cm3) 1,8-2,1 1,74-2,0 4,4-4,5 8,3-9,2 7,9-8,1

E-Modul (GPa) 3-20 41-45 110-117 230 73-117

Bruchzhigkeit

Dichte (g/cm3) 1,8-2,1 1,74-2,0 4,4-4,5 8,3-9,2 7,9-8,1

E-Modul (GPa) 3-20 41-45 110-117 230 73-117

Bruchzhigkeit

density(g/cm3)

Youngsmodulus

(GPa)190

(MPam1/2) - - - ,

(MPam1/2) - - - ,

(MPam1/2)

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Magnesium

Hexagonalclosedpacked

magnesium

1.74g/cm3 atomicnumberatomicmass

nameboilingtemperature

meltingpoint

Magnesium

CorrosionResistance?

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Magnesium

redoxseries

Mg:highlynegativeelectrochemicalpotentialof2.38V lowcorrosionresistance

MgO:noformationofprotectionlayerWHY? differenceinmolarvolumeofMg&MgO

flakingoff causedbytensile&compressionstresses

Mg corrosioneveninwaterMginacids highestcorrosion

increaseofcorrosionrateby impuritiesinMg especiallyFe,Ni,Cu

galvaniccorrosion

Magnesium

corrosionreactionsofmagnesium

M.P.Staigeretal./Biomaterials 27(2006)17281734

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Magnesium-corrosion

local corrosion of passive

layer by Cl-

rapid increase of corrosion rate

pitting corrosion

Na+ K+ Mg++ Cl-

Blut [mmol/l] 142,0 3,6-5,5 1,0 95,0-107,0

Pourbaix-diagram of Mg

Passivation

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Magnesium

Mgcorrosioninthebody?

Magnesium corrosion in the body?

Simulatedbodyfluid

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Magnesium corrosion in the body

EDXanalysis:

degradedimplant:replaced

byaconversionlayer

containingmainlyCa&P

rareearthelementswere

distributedhomogeneously

inthecorrosionlayer&in

theremainingimplant

surroundingbone

Biomaterials26(2005)35573563

Magnesium corrosion in the body

Subcutaneousgasbubbles

observedonpostoperative

radiographsfor4weeks

duringmagnesiumimplantdegradation

Biomaterials26(2005)35573563

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Oxidelayers specificvolume

MgAl

PBRatio2

nM+mO2=MnO2m

Stents

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Bioabsorbable stents

PossibleMaterials??

Bioabsorbableironstents

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Bioabsorbable Iron Stent

Ironisanessentialelement(dailyneed:0,55mg)

stentweighsabout40mg(coronary)or250mg

(peripheral)

lowtoxicity,becauseofthelowrateofcorrosion

systemictoxicityisnottobeanticipatedeven

afterimplantationofmultiplestents

Bioabsorbable Iron Stent

Studieswithpureiron(lessthan1%contamination)foils

(diameter26mm,thickness0,91mm)inelectrolyte:

Lossof15%massafter1weeks

incubationinhumanserum

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Bioabsorbable Iron Stent

Invivostudies

ironstent(left)and316Lstent(right)inthedescendingaortoofaminipig

Bioabsorbable Iron Stent

Macroscopicaspectofcorrosionprocess3(a),6(b)and12(c)

monthsafterimplantation

(a) Strutsofthestentsareclearlyvisible,corrosionplagueslimitedto

theintersections

(b) Progressivelossofstructure,accumulationofcorrosionproducts

(c) Lossofintegrity

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Biodegradablemagnesiumstents

ReasonsforfavoringMgasstentmaterial

Hypothromboticproperties

Predictabletissuetolerance

Mechanicalproperties:Outstandingstabilitytomassratio

Therapeuticallyused

Vasodilatingproperties:

[][]

,

magnesiuminhibitsthemusclecontraction

andthusthetoneofvessels.

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Bioabsorbable magnesium-stent

0.7ldrinkingwater: 110mgmagnesium1stent(1.1mm 10mm): 4 mgmagnesium

Mg essentialmineral:

350mgrecommendedingestionperday

Mg relaxationofmuscels

Mg verylowallergenreactions

DegradationofMgstents

remainingMgalloy

Mgalloy

after14days

conversionlayer

after56days

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Thank you for your attention!