Electrochemistry

as a conservation tool

Virginia Costa

MCSI 2006

• part I

deteriorated artifacts

electrochemistry: thermodynamics and kinetics

polarization curves

• part II

characterization of artifacts

evaluation of conservation conditions: environment and materials

• part III

treatment: cleaning, stabilization and consolidation

metals deterioration …most metals have an inherent tendency to corrode…

Free energy

Reaction co-ordinate

orecorrosion product

metal

electrochemical reaction

Cu2+

+

+

+

+

+

+

Cu

2e-=+ H+

pollutants H+

Cu → Cu 2+ + 2e- oxydation anode

2e- + 2H+ → H2 reduction cathodepile

Corrosion:

degradation of a metal by an electrochemical reaction with its environment

Uniform corrosion: from tarnishing…

Payer 1995

Organ 1967

… to mineralization

Sease 1997

localized corrosion

pitting

filiform

selective corrosion

(Meyer-Roudet 1999)

fragilization: stress and aging

MAE-Brazil

MAE-Brazil

electrochemical reaction

Corrosion:

degradation of a metal by an electrochemical reaction with its environment

charge transfer takes place

Cu → Cu 2+ + 2e-

2e- + 2H+ → H2

⇒ to evaluate and/or modifysuch reactions

tendency to take place (thermodynamics)

reaction rate (kinetics)electrochemistry

thermodynamicsAll spontaneous changes occurs with a release of FREE ENERGY from the system to the surroundings (at constant T and P)

Spontaneous reaction: ∆G < 0

Mg + H2O + ½ 02 → Mg(OH)2 ∆G° = -596 kJ/mol

Cu + H2O + ½ 02 → Cu(OH)2 ∆G° = -119 kJ/mol

Au + 3/2H2O + 3/4 02 → Au(OH)3 ∆G° = +66 kJ/mol

Faraday’s law: work done (free energy change) can be expressed in terms of the potential difference and the charge transported.∆G = - E. nF

standard potential

e- x+

∆G = - E. nF

v

Electrode potential (E°)

standard ox. potentialWE RE

Zn = Zn2+ + 2e- + 0.76

Fe = Fe 2+ + 2e- + 0.44

Pb = Pb 2+ + 2e- + 0.13

H2 = 2H+ + 2e- zero

Cu = Cu2+ + 2e- - 0.34

Ag = Ag+ + e- - 0.80

Au = Au+ + e- - 1.68

∆G < 0

E (mV)zero 242 641250

SHE SCE AgCl MSE

∆G > 0

corrosion potential (Ecor = OCP)

E°= -340mVshe

v v v

standard conditions:

[ ]1M ; T 278K; p 1 atm

stability of corrosion products (E-pH diagrams)

kinetics Faraday’s law of electrolysis:

Q (C) = nF. M (g)

3-electrode cell

polarization curves (LV or CV)

I (A)

M

X+

Y-

e-

-----

V

A

E (VSHE)

Inert electrode

Clean surface

Corroded surface

electrochemistryreactions involving charge transfer at a solid-liquid interface

3-electrodes arrangement

i = f(E)

i (µA)

E (VSHE)

oxidation

reduction

what?

how much?

we

V

re

A

ce

potentiostat

⇒ reactions occur at different energy levels (potentials) producing current flow

ETCE

ER

Potentiostatin the practice…

• Object complete or partially immersed in the electrolyte

• Adaptation of cell size

• Surface products attached to a Paraffin Impregnated Graphite Electrode (PIGE) Voltammetry of Microparticles (F. Scholz)

N2 CE(Pt)WERE

(SHE)

some characteristics… • elemental and structural analysis

- E (V)

-I (µA)< 1 µg

Faraday’s law

96500 C ⇔ eq.g.

100µC• extremely high sensitivity

• local and selective sampling

• really portable

• low cost

oxidation identification of metals

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

I(a.u

.)

E(VMSE)

Zn SnPb Cu

Ag

Au

oxalic acid 0.1M

definition ofstandards

Costa, Madec/ ENSCP 2006

oxidation identification of metals

-1.5 -1.0 -0.5 0.0 0.5 1.0

I (a.

u.)

E(VSCE)

Sn Pb

~350 mV

choice ofadequateelectrolyte

NaOH 0.1M

Sn

Pb75Sn

Pb60Sn

Pb25Sn

Pb10Sn

Pb

-1.5 -1.0 -0.5 0.0 0.5 1.0

I (a.

u.)

E (VSCE)-1.5 -1.0 -0.5 0.0 0.5 1.0

I (a.

u.)

E (VSCE)-1.5 -1.0 -0.5 0.0 0.5 1.0

I (a.

u.)

E (VSCE)-1.5 -1.0 -0.5 0.0 0.5 1.0

I (a.

u.)

E (VSCE)-1.5 -1.0 -0.5 0.0 0.5 1.0

I (a.

u.)

E (VSCE)-1.5 -1.0 -0.5 0.0 0.5 1.0

I (a.

u.)

E (VSCE)

Sn-Pb

NaOH 0.1M

identification of alloysoxidation

oxidation

STSM UGhent

presence of decoration?

N2 CE(Pt)WERE

(SHE)

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

I (a.u.)

E (VMSE)

Sn / Pb CuCu

⇒ Costa, Leyssens, Adriaens, Richard -Archaeometry / ISA06, Quebec May 2006

oxidationAg or Sn ?

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

I(a.u

.)

E(VMSE)

Cu Ag

-1,0 -0,5 0,0 0,5 1,0

0

3

5

8

10

i (m

A/cm

2)

E (Vsse)

FN / extract pH 5-6 polyester resin plaster

‘ extract ’ ASTM C871-84

inox

-1,5 -1,0 -0,5 0,0 0,5 1,0 1,5

0

2

4

6

i (m

A/cm

²)

E (Vsse)

SS / extract pH 5-6 polyester resin plaster

oxidation ⇒ corrosivity of sealing products

Fe-Ni

identification of compoundsreduction

- i (A/cm²)

- E (VSHE)

Ag2S AgCl

-2.0 -1.5 -1.0 -0.5

-3

-2

-1

0

i mA

/cm

²E Vsse

alloy 80Ag-20Cu

Ag2S

Ag2SCu2S

pure silver

establishing standardsreduction

X–0,36–1 et –1,38(vague)

–1 et –1,38

–1 et –1,38(vague)

Covellite

XXXXXParatacamite

-0,25 ; -0,55 ; -0,89 et -1,1X–0,03–0,070,10 et –1,18Acétate de

cuivre

-0,25 et -0,90-0,08 et –0,20X–0,06XChalcanthite

X–0,20XXXMalachite

X–0,20XXXAzurite

-0,90XXX–0,90Cuprite

Sesquicarbonatede sodium

Acideoxalique

Nitrate de sodium

Acétate de sodium

Chlorure de potassium

identification of compoundsreduction

IRPA / Brussels

-2000 -1500 -1000 -500 0-4

-3

-2

-1

0

1

i m

A/c

m²

E mVsce

inner side outer side

St. Domitien reliquary (Huy)

12th century

identification of compoundsreduction

KHM -Wien

-1600 -1400 -1200 -1000 -800 -600 -400 -200 0-30

-20

-10

0

i (uA

/mm

²)

E (mVsse)

03KHM1948 03KHM1948f

evaluation of environmental conditionsreduction

⇒ Costa, Texier, De Reyer / Heritage Weather Conservation 2006 / Madrid, June 2006

Palais du Tau / Reims

Musée de la Photographie -Bièvres

Musée le Secq des Tournelles

Trésor des cathédrales de Nantes et Angers

Trésor de Notre Dame de Paris

Palais du Tau -Reims

Musée de la Musique

Unité Archéologique de la Ville de Saint Denis

Musée du Petit Palais

Musée du Louvre (antiquités gréco-romaines)

Musée du Louvre (pavillon des Sessions)

Union Central des Arts Décoratifs (UCAD)

Musée-Château de Dieppe

L.Olanier

R. Flammang

100mm

40mm

Evaluation of the

environmental impact on

the conservation of

metallic cultural artifacts

6 m 12 m 18 m• metallic coupons – synergetic effect of all variables: dust, RH, pollutants…

• electrochemistry - identify and quantify (rate of tarnishing)

• examination and analysis of objects

reduction

reduction Musée-Château de Dieppe

Salon Saint Saens Cours du Chateau

oxide?

-2.0 -1.5 -1.0 -0.5 0.0

-0.0008

-0.0005

-0.0003

0.0000

i (m

A/c

m2 )

E (VMSE)

Dieppe 2B Dieppe 2H Dieppe 6H

AgCl

Ag2S

Cu2S

reduction Musée-Château de Dieppe

0

2000

4000

6000

8000

10000

12000

emplacement

char

ge (u

C/cm

²)

hautbas

1 1ext 4 5 6

electrolysis?

⇒ to supply electrons the metal lost due to corrosion…

Ag• to transform the corrosion layer

into the metal

• to replace the expanded and porouscrusts of corrosion products by rigid metallic layers that retain the shape

• to accelerate the elimination of harmful compounds that might catalyse further corrosion.

Ag2S

1900

Rathgen

‘ Die Konservierung vonAltertumsfunden ’, 1924

‘ ...A treatment used for many years for silver coins: they are kept in lemon juicein contact with an iron needle…’

galvanic or contact method (electrochemical)

G. Eggert

1900

Rathgen

1950-70’s

Organ, Werner, Lane

G. Eggert

galvanostatic (electrolytic)

OrganPower supply

Inert counter electrode

Choice of solution‘Consolidative reduction’

1900

Rathgen

1950-70’s

Organ, Lane, Werner

1980’s

EDF

marine wreckelectrophoresis

1900

Rathgen

1950-70’s

Organ, Lane, Werner

1980’s

Grenn, EDF

1990’s

Campbell, Degrigny, EH

Potentiostat: selectivity, control and monitoring

- i (A/cm²)

- E (VSHE)

Ag2S AgClArc Antique

cathodic protectionpotentiostatic

0 100 200 300 400 500 600 700

0

50

100

150

200

250

potential

pote

ntia

l (V ES

S)

Cl- m

ass

(mg)

time (h)

-1,4

-1,2

-1,0

-0,8

-0,6

-0,4 Cl- mass

Bertholon

english heritage

stabilization

1900

Rathgen

1950-70’s

Organ, Lane, Werner

1980’s

Grenn, Campbell, EDF

1990’s

Degrigny

Bertholon,

EH

todayArc’Antique, IRRAP

ICOM-CC metal sub-Working Group:Use of Electrochemical Techniques in metal Conservation (march 06)

• Evaluation of corrosion potential measurements as a means to monitor the storage and stabilization processes of archaeological copper based artefacts - Karen Leyssens (B) • Some remarks on the monitoring of the storage / treatment of iron based alloys artefacts & Introduction to the Ecorr vs time Droplet technique as a spot test for metal artefacts - Christian Degrigny (F)• Remote monitoring of the polarisation of large scale artefacts recovered from the sea: first in situ results - Jean-Bernard Memet (F)•In-situ characterisation of patina and coatings on outdoor bronze with electrochemical methods - Paola Letardi (I)• ECell: a new electrochemical cell and data system for real time corrosion studies using synchrotron XAS/XRD, Raman spectroscopy, and other techniques - Mark Dowsett (UK) and Annemie Adriaens (B)•Impact of the environment on cultural metal artefacts: evaluation by electrochemical method - Virginia Costa (F)• Electrochemistry for monitoring effects of environments on silver, lead and steels and protective coating evaluation - David Thickett (UK) • A fundamental study of the electrochemical reduction treatment applied to corroded lead artefacts - Bart Schotte(B)• Reduction treatment of a corroded lead tobacco jar - Karen Stemann Peterson (DK)• Conductive polymers in the protection against metal corrosion with interest in conservation area - Isabel Tissot and Luisa Maria Abrantes (P)

…to do… experimental solutions

surface modifications

structural modifications

-2,5 -2,0 -1,5 -1,0 -0,5 0,0

-50

-40

-30

-20

-10

0

E mV MSE

i (m

A/cm

²)

nitrate de sodium 0,1M acide formique 5%

ρ AgCl = 5,5 g/cm3

ρ Ag = 10,5 g/cm3

…in the practice…

1st principle:

what do you want?

2nd principle:

grime removal…

⇒ to see what is bellow

⇒ to ‘wet’ the surface

…in the practice…3rd:

identifying surface compounds

- E (V)

-i (A/cm²)Ag+ → Ag

H+ → H2

what?

how much?

…in the practice…4th:

choosing adequate conditions

- E (V)

-i (A/cm²)

H+ → H2

Ag+ → Ag

i(A/cm²)

temps (s)

E = - 1000 mV

…in the practice…

5th: obtaining wanted finish

…in the practice…

‘ pencil ’

comparative overview

- don’t scratch;- hydrogen evolution, if strong, can leave

traces;- can stain or polish

- high reactivity;- don’t scratch;

- can scratch;- can stain- can polish

aspect after treatment

- leave a powdery reduced silver

- need very efficient rinse

- all abrasives remain- CaCO3 is very difficult toremove, specially when thoroughly rinse is not possible

residues

- selective: possible to differentiate corrosion products and metal;

- removes of old lacquers

- difficult to control to notremove metal;

- choice of solution as function of the alloy;

- risk of selective or localised corrosion

- indiscriminate

- corrosion products;- can be controlled to not remove metal

removal of material

- quick;- not necessarily toxic;- can need equipment

- quick;- can be toxic

- slow- needs skill and hand ability

- applicable to composites

process

electrochemicalchemicalmechanicalmethod

parameter