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Corrosion of Archaeological Metals

Professor Michael NotisArchaeometallurgy Laboratory

Lehigh University442 Whitaker Laboratory

Bethlehem, PA 10815 USAmrn1@lehigh.edu

Kojindani, Sendai, Japan

Identical pieces in immediate contact but Identical pieces in immediate contact but totallytotally

different corrosion productsdifferent corrosion products

CorrosionCorrosion--11EverythingEverything corrodes(oxidizes, corrodes(oxidizes, sulfidizessulfidizes, , rusts, tarnishes)rusts, tarnishes)--corrosion is inevitable, even corrosion is inevitable, even for organic and inorganic materials such as for organic and inorganic materials such as glass or slag inclusionsglass or slag inclusions

CorrosionCorrosion--22

EvenEven precious metalsprecious metals--Au,Pt,Pd (restricted)Au,Pt,Pd (restricted)Economic impact is hugeEconomic impact is huge--$296 billion $296 billion ((BattelleBattelle/NIST Report, April 1995)/NIST Report, April 1995)Coating protection (Galvanizing,Coating protection (Galvanizing, GalvalumeGalvalume, , Anodizing) significantly slows corrosion with Anodizing) significantly slows corrosion with resulting enormous reduction in costresulting enormous reduction in cost

CorrosionCorrosion--33Emphasis on coatings research in the 1950’s Emphasis on coatings research in the 1950’s shifted research focus away from basic shifted research focus away from basic corrosion/microstructure studies (important corrosion/microstructure studies (important for archaeological/conservation work)for archaeological/conservation work)Types: Types: -- dry vs. aqueousdry vs. aqueous

-- low temperature (room) vs. highlow temperature (room) vs. highKinetics Kinetics -- linear, logarithmic, paraboliclinear, logarithmic, parabolic

FeFe--FeFe33C C Phase diagramPhase diagram

Phase diagrams vs. Predominance diagrams

R.T.DeHoff, Thermodynamics in Materials Science (1993)

Predominance vs. Phase DiagramsPredominance vs. Phase Diagrams--11

Predominance diagrams take the form of a cell structure with Predominance diagrams take the form of a cell structure with areas that represent areas that represent domains of predominancedomains of predominance of a particular of a particular component separated by lines that are the limits of component separated by lines that are the limits of predominance of competing components. predominance of competing components. While a predominance diagram (such as aWhile a predominance diagram (such as a Pourbaix Pourbaix diagramdiagram) may provide an approximate representation of a ) may provide an approximate representation of a phase diagram they are not identical to phase diagrams. phase diagram they are not identical to phase diagrams. Domains of predominance displayed in these diagrams are not Domains of predominance displayed in these diagrams are not identical with the domains of stability normally presented in identical with the domains of stability normally presented in phase diagrams. phase diagrams. Boundary lines on a predominance diagram represent 50% lines Boundary lines on a predominance diagram represent 50% lines for the bounding reactions. Phase boundaries on phase for the bounding reactions. Phase boundaries on phase diagrams represent bounding conditions for the equilibrium diagrams represent bounding conditions for the equilibrium presence of (n) phases (1,2,3,etc.) with any point inside a presence of (n) phases (1,2,3,etc.) with any point inside a multiple phase region indicative of the quantitative amount of tmultiple phase region indicative of the quantitative amount of the he phases.phases.

Predominance vs. Phase DiagramsPredominance vs. Phase Diagrams--22

Predominance diagrams have only one component labeledPredominance diagrams have only one component labeledin each fieldin each field

Phase diagrams have different phases identified in each fieldPhase diagrams have different phases identified in each fieldsuch that if a field is labeled as (n) phases, then the adjacentsuch that if a field is labeled as (n) phases, then the adjacentfields have (n+1) or (nfields have (n+1) or (n--1) phases1) phasesThe location within a component field in a predominance The location within a component field in a predominance diagram does not indicate the quantitative relative amount ofdiagram does not indicate the quantitative relative amount ofthat component other than that it is the major speciesthat component other than that it is the major species

The location within a multiphase (n>1) region in a phase The location within a multiphase (n>1) region in a phase diagram gives a quantitative indication of the relative amount diagram gives a quantitative indication of the relative amount of each phase within that phase field. For example, in a binaryof each phase within that phase field. For example, in a binaryphase diagram the position along horizontal tie lines in phase diagram the position along horizontal tie lines in twotwo--phase fields indicates the relative percentage of each ofphase fields indicates the relative percentage of each ofthe phases indicated by the ends of the tie line.the phases indicated by the ends of the tie line.

“Electron Number Diagrams”“Electron Number Diagrams”J.C.Angus & M.J.J.C.Angus & M.J.ZappiaZappia, J., J.ElectrochemElectrochem.Soc..Soc.134134[6]1374[6]1374--1383(1987)1383(1987)

Cu single crystal sphere oxidizes at different rates inDifferent crystallographic directions

100

110

111

High Temperature Corrosion Kinetics

High Temperature Oxidation Kinetics(Simple Models)

The parabolic rate law assumes that the diffusion of metal cations or oxygenanions is the rate controlling step.

The rate constant, kp, changes with temperature according to an Arrhenius type relationship.Experimentally, there is also a cubic law similar to this case (ex.,Cu)

The logarithmic rate law is an empirical relationship with no fundamentalunderlying mechanism.

The linear rate law is also an empirical relationship that is applicable to theformation and build-up of a non-protective oxide layer.

Cubic law

Cubic law Rate constantRate Constant

Red-Ox reactions

The Pourbaix (E-pH) DiagramThe Pourbaix (E-pH) Diagram

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7 14

pH = - log [H+]pH = - log [H+]

2H+ + 2e- = H2Equilibrium

potential falls as pH increases

2H+ + 2e- = H2Equilibrium

potential falls as pH increases

2.01.6

0.81.2

-0.4

0.40.0

-1.6

-0.8-1.2

0

2H2O = O2 + 4H+ + 4e-

Equilibrium potential fallsas pH increases

2H2O = O2 + 4H+ + 4e-

Equilibrium potential fallsas pH increases

The Pourbaix (E-pH) DiagramThe Pourbaix (E-pH) Diagram

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H2O is stable

H2 is stable

7 14

2.01.6

0.81.2

-0.4

0.40.0

-1.6

-0.8-1.2

0

O2 is stable

Pourbaix diagrams include information on solvent stability,and information about oxidizing vs. reducing and

acidic vs. alkaline conditions

R.T.DeHoff, Thermodynamics in Materials Science (1993)

“corrosion” defined as amount of permissible metal dissolution[log(a-species)=-6 = “no corrosion for all practical purposes”]

Pourbaix Diagrams - Alloys

Pourbaix diagrams for systems with only a single metallic speciessuch as Ag, Cu, Fe, Sn or Zn can be easily determined and tabulated.

Pourbaix diagrams for systems with more than one metal species aredifficult (or nearly impossible) to obtain and requires computer calculation.

For example, even the Cu-Ni system which has complete solid solubilityis difficult because of the varying thermodynamic activity across the Cu-Nisystem.

The Cu-Sn (bronze) system is difficult because of the intermediate phases that occur.

Pourbaix Pourbaix diagram diagram forCuforCu--SnSn--HH22O (F*A*C*T) O (F*A*C*T) w/o w/o εε--CuCu33Sn phaseSn phase

Pourbaix Pourbaix diagramdiagram forCuforCu--SnSn--HH22O (F*A*C*T) O (F*A*C*T) withwith εε--CuCu33Sn phaseSn phase

Pourbaix Diagram for Iron

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7 14

2.01.6

0.81.2

-0.4

0.40.0

-1.6

-0.8-1.2

0

Fe metal stable

Fe3+

Fe oxidesstable

Fe2+ stable

Electrolytic Reduction

Conservation of an Iron Cannon from Finland

Mechanical cleaning and removal of graphitized layer Lifting the cannon

Electrolytic reduction

EndeavorEndeavor iron cannon after iron cannon after electrolytic reductionelectrolytic reduction

Chinese mirror with almost no corrosion

Chinese vessel showing both Azurite and Malachite

Corrosion along grain boundaries & mechanical twinsCorrosion along grain boundaries & mechanical twinsCuCu--8 w/o 8 w/o Sn Sn bronze (Scott)bronze (Scott)

Diffusion is more rapid along grain boundaries & surfaces

Corrosion along dendrites in a Chinese bronze arrowhead

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Interdendritic corrosion

Cuprite Malachite

Copper

Roman Coin with Bronze Disease

Bronze diseaseBronze disease

Tel Tel Miqne Miqne KnifeKnifean example of changing corrosion conditionsan example of changing corrosion conditions

Baqah Baqah Iron ArtifactsIron Artifacts

Light microscope

Corroded Fe object with Residual spheroidized Fe3C

Spheroidized pearlite andvery large carbides

SEM

Sword from Sword from Vered Vered JerichoJericho

SEM (SE) Residual SEM (SE) Residual PearlitePearlite

EPMA EDS Survey mapEPMA EDS Survey map

EPMA WDS mapsEPMA WDS maps

Fe Si O C

Progressing corrosion outlines microstructureProgressing corrosion outlines microstructure

without etchingwithout etching

Corrosion and Corrosion and AuthenticationAuthentication

1. Single phase alloy (ex., Cu, bronze, 1. Single phase alloy (ex., Cu, bronze, lowlow C iron)C iron)High angle grain boundaries, mechanical twin/slip interfaces (usHigh angle grain boundaries, mechanical twin/slip interfaces (usually, ually, notnot annealing twins)annealing twins)Dendrites, especially those due to segregation during solidificaDendrites, especially those due to segregation during solidificationtionVersusVersus uniform corrosion front typical of more rapid high temperature uniform corrosion front typical of more rapid high temperature processes or in very aggressive solutions where rate of defect iprocesses or in very aggressive solutions where rate of defect interface nterface attack similar to rate of bulk attackattack similar to rate of bulk attack

2. Multiphase alloy (ex., High 2. Multiphase alloy (ex., High SnSn--bronze, Febronze, Fe--C steel)C steel)Ghost structuresGhost structures

AbstractAbstract

This talk will present an introductory view of the very complicatedand diverse field of corrosion of metallic materials. The presentation will first cover the fundamentals of oxidation of a single metallicspecies and highlight the kinetics and morphology of growth. We will demonstrate the relationship between Pourbaix Diagrams,predominance diagrams, phase diagrams and a new concept termed“electron number diagrams”. We will finally focus on thecomplicated corrosion processes that take place in typicalarchaeological iron objects.