Surface & interface investigation of electrochemically induced corrosion on bronze

Surface and Interface Investigation Surface and Interface Investigation of Electrochemically induced of Electrochemically induced

Corrosion on a quaternary bronzeCorrosion on a quaternary bronze

O.Papadopoulou, M.Delagrammatikas, O.Papadopoulou, M.Delagrammatikas, P.VassilliouP.Vassilliou, S.Grassini, E.Angelini, V.Gouda, S.Grassini, E.Angelini, V.Gouda

OutlineOutline

Aim of the study

Brief introduction general corrosion patterns of ancient copper alloys & chloride attack in marine environments

Experimental Description of electrochemical setup & Analytical methods

Results Chemical characterization of produced patina at surface & alloy interface Surface observations on corroded metal substrate

Evaluation / Conclusions

Future Work

Aim of this studyAim of this study

Understanding complex & heterogeneous corrosion processes that threaten Cu-based cultural heritage objectsCu-based cultural heritage objects

More focused & effective conservation methods

Our research rationaleOur research rationale

The simulation of severe electrochemical corrosion in the electrochemical corrosion in the presence of chloridespresence of chlorides (marine/ coastal environment or burial in Cl- enriched soils) in an ancient-like roman bronze → production of synthetic patina

Interpretation of the role of alloying elementsalloying elements and dendritic dendritic segregationsegregation to the corrosion evolution

INTRODUCTION INTRODUCTION –– Selective corrosion of Selective corrosion of phases in ancient Cu-based alloysphases in ancient Cu-based alloys

Cu – Sn Alloys

Cu – Zn Alloys

The ancient bronzes manufactured by casting are characterized by chemical heterogeneity (dendritic macro-segregation), impurities and other metallurgical features (pores , cracks etc. )

Selective Cu dissolution ((decuprificationdecuprification))

Selective Zn dissolution ((dezincificationdezincification)) – for compositions of Zn>14%w/w

Immiscible with alloy phasesIntense local corrosion & formation of passive compounds

Addition of Pb



Cu – Sn Alloys Selective Cu dissolution ((decuprificationdecuprification))

Cu – Sn Alloys

Cu – Zn Alloys Selective Zn dissolution ((dezincificationdezincification)) – for compositions of Zn>14%w/w

Cu – Zn Alloys

Addition of Pb Immiscible with alloy phasesIntense local corrosion & formation of passive compounds

Addition of Pb




Cu – Sn Alloys Selective Cu dissolution ((decuprificationdecuprification))

Cu – Sn Alloys

Cu – Zn Alloys


Cu – Sn Alloys

Cu – Zn Alloys


Cu – Sn Alloys


Cu – Zn Alloys


Cu – Sn Alloys

Addition of Pb


Cu – Zn Alloys


Cu – Sn Alloys

Addition of Pb


Cu – Zn Alloys


Cu – Sn Alloys


Addition of Pb


Cu – Zn Alloys


Cu – Sn Alloys


Addition of Pb


Cu – Zn Alloys


Cu – Sn Alloys



Addition of Pb


Cu – Zn Alloys


Cu – Sn Alloys

INTRODUCTION INTRODUCTION –– The behaviour of a Cu- The behaviour of a Cu-Zn-Sn-Pb alloy in NaCl solutionZn-Sn-Pb alloy in NaCl solution

A quaternary cast bronze is expected to exhibit all these corrosion patterns with local variations according to the composition of the each phase and as a result of the metallurgical heterogeneity.

Like all Cu-based alloys it is vulnerable to chloride attack and pitting corrosion (artefacts are mainly endangered by ‘bronze ‘bronze disease’disease’ ) while the phase segregation induces local local galvanic cellsgalvanic cells.

The interaction of metal cations with Cl and O species in a NaCl solution involves both charge charge and mass transfermass transfer.

EXPERIMENTALEXPERIMENTAL–– Reference Alloy Reference Alloy Chemical Chemical Composition & Specimen PreparationComposition & Specimen Preparation

ConcentrationConcentration CuCu SnSn PbPb ZnZn% w/w 82.3 3 0.5 14

% at 84.1 1.8 0.2 13.9

The alloy was produced within the EFESTUS project by casting that simulates ancient metallurgy techniques.

Dimensions : 26mm & 3mm height / coin-resembling shape

For the manufacture of the WE the specimen was embedded in epoxy resin

Surface treatment : Polishing with SiC abrasive papers from 400 up to 2000 gritt

EXPERIMENTALEXPERIMENTAL– – Electrochemical Setup & Electrochemical Setup & ParametersParameters

Three-electrode aerated electrochemical cell : saturated calomel electrode(SCE) / reference

Pt wire /counter

static working electrode alloy ZB with 5cm2 area

electrolyte volume (aqueous NaCl 0.1M): 2L

absence of stirring/no buffer solution added(non-constant pH and diffusion-controlled concentration of ionic species in the electrolyte

• Initial pH = 6.3

• 30 min at open circuit (OC) conditions before polarization sweep

• Anodic polarization up to +1500mV/Eoc

• Scan rate = 0.25mV/s → slow

ParametersParameters

EXPERIMENTALEXPERIMENTAL–– Instrumentation & Instrumentation & Analytical MethodsAnalytical Methods

Electrochemical polarization device GAMRY CMSGAMRY CMS 100 100 was used for the production of the chloride patina.

During the measurement :• Digital photography (1 pic/ min) • pH measurements

ex situ characterization of corroded surfaces & interfaces:• indicative SEM/EDS on detached patina samples (both sides)• ΟΜ observations – in bright field and polarized light – on patina samples and revealed bulk metal

RESULTS RESULTS –– Anodic curve & surface reactions Anodic curve & surface reactions

0

0,01

0,02

0,03

0,04

0,05

0,06

-0,2 0 0,2 0,4 0,6 0,8 1 1,2 1,4

Potential /SCE (V)

Cu

rre

nt

De

ns

ity (

mA

/cm

^2

)

1

2

3

4

5

6

78 9

1 2 3 4 5

2

3

4

RESULTS RESULTS –– Anodic curve & surface reactions Anodic curve & surface reactions

0

0,01

0,02

0,03

0,04

0,05

0,06

-0,2 0 0,2 0,4 0,6 0,8 1 1,2 1,4

Potential /SCE (V)

Cu

rre

nt

De

ns

ity (

mA

/cm

^2

)

1

2

3

4

5

6

78 9

5 6 7 8 9

5

6

7

1. Limited O concentration

2. Other alloying elements : Zn and Sn present in traces/ Pb not detected

1. Significant O concentration

2. High Cl concentration

3. Cu and Cl predominate – atomic percentages approximate stoichiometric proportions(1:1) of nantokite (CuCl)

Element

%% Atomic Concentration on Atomic Concentration on corrosion layer sample - EDScorrosion layer sample - EDS

External surface (patina/ electrolyte

interface)

Internal surface of detached patina (alloy

interface)

O 7.4 23.4

Cl 43.0 35.7

Sn 0.5 3

Cu 48.3 37.9

Zn 0.8 -

Pb - -

RESULTS RESULTS –– Corrosion layer analysis & Corrosion layer analysis & CharacterizationCharacterization

Element

%% Atomic Concentration of Alloying Atomic Concentration of Alloying Elements / EDSElements / EDS

External corrosion

layer surface

Inner side of corrosion

layer (alloy interface)

Alloy

Sn 1 7.3 1.8

Cu 97.4 92.7 84.1

Zn 1.6 - 13.9

Pb - - 0.2

Absence of Zn at the interface : dezincification during electrodissolution → Enrichment in Sn compared to the initial bulk alloy composition

RESULTS RESULTS –– Corrosion layer analysis & Corrosion layer analysis & CharacterizationCharacterization

Zn encountered only at the surface together with some Sn compounds that have not been dissolved and contribute to passivation.

Zn is the first alloying element which dissolves

The increasing Cu content from alloy substrate towards the surface is the evidence of intense decuprification process after an initial dezincification.

RESULTS RESULTS –– Internal surface of detached Internal surface of detached Patina / OMPatina / OM

Polarized lightBright fieldx100

Epitaxial growth of the corrosion products on the dendritic structure

Corrosion products accumulated around a pit : CuCl –nantokite (yellow) surrounded by a mixture of Sn hydrated oxyhydroxides

Cu2O – cuprite (dark red) in a corroded Cu-enriched interdendritic region

RESULTS RESULTS –– Bulk metal (under corrosion Bulk metal (under corrosion layer)/ OM observationslayer)/ OM observations Bright field Polarized light

x100

pittings

dendritic structure enriched in Zn & Sn

corrosion layer

Cl containing compounds (green, blue)

Cu2O –cuprite (orange)

Local attacks

Cu-rich region around a pit attacked by Cl. CuCl at the edges of the ‘flower’

Analyzed area O Cl Cu Sn Zn Pb

Electrochemical corrosion

External surface 7.4 43.0 48.3 0.5 0.8 -

Internal surface 23.4 35.7 37.9 3.0 - -

Chemical corrosion Immersion in aq. CuCl2

1M/ 72 h (1)

External surface 16.0 41.0 40.4 1.0 - -

Semi-accelerated corrosion in Attica soil enriched with 3.5% w/w NaCl solution (5 years)

External surface 74.1 5.8 17.2 0.2 2.7 -

RESULTS RESULTS –– Cl induced corrosion cases / Cl induced corrosion cases / review of EDS results for the same alloyreview of EDS results for the same alloy

(1) O.Papadopoulou, J.Novakovic, P.Vassilliou, E.Filippaki, Y.Bassiakos, Applied Physics A, DOI 10.1007/s00339-013-7726-z

CONCLUSIONSCONCLUSIONS• The observation of surface reactions during the anodic polarization of

an ancient-like quaternary bronze in a 0.1M NaCl solution as well as the ex situ characterization of the created patina and corroded metal substrate underlined the importance of the alloying elements and metallurgical features in the corrosion evolution.

• The initial epitaxial formation of stable Cu and Sn oxides at the surface and the chemical heterogeneity attributed to dendritic structures define decisively the corrosion rate and the nature of the corrosion products.

• A dezincification and a subsequent decuprification process are observed, while the metal dissolution and the invasion of Cl- and O2- anions takes place through a Sn-enriched layer located at the alloy interface.

• Although at the beginning of the polarization the charge transfer rules the system reactions, in the end the produced patina has very similar chemical composition with the one created by chemical corrosion. This proves the importance of diffusion at high potentials and pH.

Future WorkFuture Work Further investigation of the electrochemical & mass transport

mechanisms during corrosion in Cl- environment

Investigation of the oxidation states of the alloying elements with regard to their migration and the formation of corrosion products at the surface and the metal – patina interfaces

Identification of complex corrosion species with combination of spectroscopic and microscopic methods

Comparison of the results of different artificial corrosion methods (both chemical and electrochemical) with naturally corroded metal artefacts in order to understand the complex corrosion mechanisms of copper alloys by Cl- and design tailored conservation treatment protocols

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Surface & interface investigation of electrochemically induced corrosion on bronze