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Corrosion of Archaeological Metals
Professor Michael NotisArchaeometallurgy Laboratory
Lehigh University442 Whitaker Laboratory
Bethlehem, PA 10815 [email protected]
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
Pot
entia
l
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
Pot
entia
l
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
Pot
entia
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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.