Doménech, A. y Doménech, T. Layer-by-layer identification bronze alteration. 2010

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Layer-by-layer identification of brass and bronzealteration products using Tafel analysis of

frequency-dependent square wave voltammetricresponses

Antonio Doménech Carbó

Departament de Química Analítica. Universitat deValència. Dr. Moliner, 50, 46100 Burjassot

(València) Spain. E-mail: [email protected]

María Teresa Doménech-Carbó

Institut de Restauració del Patrimoni, UniversitatPolitècnica de València. Camí de Vera s/n. 46022

València, Spain.



• The determination of the structure and chemicalcomposition of natural patinas grown on ancient andhistorical bronzes is an important target for archaeometry,

conservation and restoration:

• Alteration layers are usually multicomponent systems

Alteration of copper materials

• Such products are frequently distributed in different layersover the basal metallic piece

• Mutual interference of analytes is possible

• Matrix effects due to organic products, dust and complexing

species in the electrolyte can eventually occur



• The long-term exposition of bronzes to atmosphere, wateror sea water and soils gives rise to different corrosionpatterns

• The corrosion deposits are in general complex wherecorroded layers are often stratified and intergranaular or


transgranu ar corros on occurs

• Localized corrosion phenomena and/or generalized attackshaving high dissolution rate produce areas on which

corrosion deposits (crusts, ‘limpets’, ‘buboes’) hide theoriginal metallic surface and regions where the originalsurface is clearly destroyed with loss of matter (pits,crevices, lamellar plates)



• Typical alteration products:


Corrosion of

Atmospheres

Minerals of thebrochantite

(Cu(OH)6SO4) group

Minerals of the

copper alloysdepending ontheenvironment

Waters

Soils

atacamite(Cu(OH)3Cl) group

Minerals of themalachite(CuCO3·Cu(OH)2)group




Corrosionof copperand copperallo s

‘Noble patinas’

- Bilayer structure

- 5-50 µµµµm thick

(Robbiolaet al. Stud.Conservat.

1988, 33,205-214.

‘Coarse structures’

-Trilayer structure

- 200 µµµµm – 2 mm thick




Cuprite (Cu2O) layer

Idealized bi- and trilayer

corrosion structures oncopper-based materials

Bilayer structure Trilayer structure

Metal



• Primary aim:

• Identification of alteration products in copper and copperalloy artifacts existing in different layers on the metallicpiece

Electrochemical study of copper alteration products

• Additionally:

• Obtaining quantitative information on the different materials

• Obtaining information on the distribution of such species:the layer-by-layer identification



Copper corrosion in aq. NaClElectrochemical study of copper alteration products

SEM image of a Cu

coupon submitted to

corrosion of a Cu

coupon in aq. Na2SO4



(a)


SEM image of a Cu

coupon submitted to

corrosion of a Cu

coupon in aq. Na2SO4



SEM image of a Cu

coupon submitted to

corrosion of a Cu

coupon in aq. NaHCO3




Copper corrosion in aq. NaClElectrochemical study of copper alteration products

SEM image of aCu couponsubmitted to test

corrosion in 0.10M NaCl



(aSEM imagesof a Cu

couponsubmitted totest corrosionin 0.10 MNaCl

Arrowsindicatepoints where

(1)

(2)

(bEDX analysiswasperformed



O

(a


SEM/EDXanalysis ofpoints marked byarrows in theprecedent figure

(b



• Primary aim:

• Identification of alteration products in copper and copperalloy artifacts existing in different layers on the metallicpiece


• Additionally:

• Obtaining quantitative information on the different materials

• Obtaining information on the distribution of such species:the layer-by-layer identification



Identification ofcomponents

‘Mean’


Solid stateElectrochemistry

Analyticaldemands

composition of thesample

Layer-by-layeridentification andquantification ofcomponents



Identification of specific components

Identification of individual components inmixtures

Mineralogical

Voltammetry of microparticles

Voltammetry ofmicroparticles(VMP)

Speciation

Quantification

Relative

Absolute

Oxidation state




VMP is an electrochemical technique developed by Scholz et al.; see:

-Scholz F, Meyer B (1998) Voltammetry of solid microparticles immobilized on

electrode surfaces in Electroanalytical Chemistry, A Series of Advances. 20: 1-86.-Scholz F, Schröder U, Gulabowski R (2005) Electrochemistry of ImmobilizedParticles and Droplets. Springer, Berlin-Heidelberg.

- ,

Electroactive Micro- and Nano-Particles in a Liquid Electrolyte Environment , inHandbook of Solid State Electrochemistry, V.V. Kharton (ed), Wiley.

Application of VMP to the analysis of samples from works of art was

initiated by Doménech et al.; see:

Doménech A, Doménech MT, Costa V (2009) Electrochemical Methods inArcheometry, Conservation and Restoration, in Monographs inElectrochemistry, F. Scholz (ed), Springer, Berlin.




Typical Faradaic processes in conventional‘solution electrochemistry’:

Fe(CN)64-

Pb2+

Fe(CN)63-

e

-

Pb

e

-



MX-componentHydrated layer


Schematics for a reduction process of the type: MX + H+ + e- → M + X-

(Described by Hasse and Scholz, Electrochem. Commun. 3, 429-434 (2001)for: PbO + 2H+ + 2e- → Pb + H2O

e-

M deposit




External X-component

H+

Hydrated layer

Internal Y-component

Schematics fora two-component,

stratifiedsystem

e-



• Depending on the electrochemical conditions, the

reduction of metal compounds to metal gives rise to‘external’ metal deposits:


Mn+ (aq)M (s)

H+

e-




AFM image during the

electrochemicalreduction of minium(Pb3O4) in contact witha ueous sodium

acetate buffer (pH 4.75)

Gross mineral particlesare accompanied byfine grains of Pb metaldeposited during

electrochemicalturnovers




Graphite barPt wire auxiliaryelectrode

Pt disk pseudo-reference electrode

In situ

voltammetricmeasurements: Costa’s

Metal lamina

Electrolyte solution

Teflon covergraphite pencilmethodology




Working electrode

(graphite bar)

e-H+ (aq)

Corrosion layers

Hydrated layer plus newmetal deposit

Metal substrate




e-

H+ (aq)

Corrosion layers

First hydrated layer plusmetal deposit

Second hydratedlayer plus metaldeposit

Metal substrate



• Two general methodologies for layer-by-layer analysis:

• 1) Successive reductive potential scans maintaining the

‘pencil’ electrode on the probe


• 2) Alternating reductive potential steps at a ‘cathodic

potential’ and potential scans for determination

• In both cases, the aim is to stepwise exhaust thedifferent corrosion layers



Tafel analysis

Tafel analysis of the rising portionof voltammetric peaks provides

provides a method for discerningbetween different materials withalmost identical composition. Thisis the case of copper pigments

,

whose square wavevoltammograms in contact with0.50 M phosphate buffer, pH 7.4are presented.

• a) azurite

• b) cuprite

• c) verdigris

• d) atacamite



Tafel analysis

Normalized Tafel plots of ln(i/ip) vs. E forcopper pigments and alteration products

Tafel analysis of the rising portion ofvoltammetralyic curves facilitatesidentification of different compounds,namely, cuprite (square), atacamite(triangles) and a mixture of both

components (rhomb)

0,021

0,014

0,015

0,016

0,017

0,018

0,019

0,02

1 1,5 2 2,5

Tafel OO

Tafel SL (mV-1

atacamite

cupriteazurite

malachite



• In several cases, the rising portion of voltammetric peaks can beapproached by a Tafel-type linear variation of log(i) vs. potential (E)

• This applies for irreversible electron transfer processes betweenspecies in solution phase (Reinmuth, W.H. Anal. Chem. 1960, 32 ,1891-1892; Buck, R.P. Anal. Chem. 1964, 36 , 947-949)

Tafel analysis

(2)exp21

−−=

RT

)E F(E αnDck )Gn(αni ia

o

/

a



For reversible and irreversible electron transfer processes involving speciesattached to the electrode surface (Bard, A. J.; Faulkner, L. R.Electrochemical methods . John Wiley & Sons, New York, 1980, pp. 521-525)

)3()'º(

exp)/(1

)'º(exp)/(

2

22

−−+

−−Γ

=

E E nF bbT

RT

E E nF bbvAF n

i

rd ox

rd ox

Tafel analysis

)4()/)'(exp(exp)/)'(exp(

−−−−Γ = RT E E F n

Fvn

RTk RT E E F nenFAk i o

a

a

oo

ao α

α

α



Square wave voltammetry for reversible electron transferprocesses involving species in solution phase (Ramaley, L.;Krause, M.,S. Jr. Anal. Chem . 1969, 41, 1362-1365; Krause,

M.S. Jr.; Ramaley, L. Anal. Chem. 1969, 41, 1365-1369):

Tafel analysis

[ ])5(

)/)'º(exp(1

)/)'º(exp(22/1

2/12/122

RT E E nF

RT E E nF

RT

f cE ADF nC i SW

dif −+

−=

π



To remark:

For several solid-state electrochemical processes well-defined Tafel plots

are obtained (in particular for Cu-based minerals) from LSVs andSQWVs

Generalized Tafel slope (SL) and ordinate at the origin (OO) recorded for

Tafel analysis

solid state processes using voltammetry of microparticles are phase-characteristic, enabling for mineral identification

Using the above parameters, relative quantification of components X andY in homogeneous mixtures of particulate deposits can be obtained

Doménech, A. et al. Fresenius Journal of Analytical Chemistry . 2001, 369, 576-581.Microchimica Acta , 2008, 162, 351-359; Analytical Chemistry , 2008, 80,

2704-2716.



For a homogeneous mixture of particulate deposits of two components Xand Y, relative quantification can be obtained:

)/)((exp

)(

)(

+

+−

+

+≈

oY oY oX oX

aY Y oY oY aX X oX oX

oY aY Y Y oX aX X X

oY oY oX oX

p k qk q

RT FE nk qnk q

nFvqnH qnH

RT k qk q

i

i α α

α α

Tafel analysis

Doménech, A. et al. Analytical Chemistry , 2008, 80, 2704-2716.

e-

H+



In ‘stratified’ systems:

Tafel parameters can be used for a layer-by-layer identification of thecomponents using successive potential scans. Here, the advance of the

reaction layer through the system involves the successive exhaustion ofthe different layers

Phase-characteristic generalized Tafel slope (SL) and ordinate at the

Tafel analysis

layer to those for the components of successive layers

e- e-

H+H+

Initial response: X-component

Final response: Y-component



Test systems

• Generalized Tafel plots forsuccessive potential scans on

a copper coupon submitted toelectrochemical oxidation in0.05 M electrolytes. FromSQWVs subsequently -2

-1,5

-1

-0,5

ip)

NaHCO3

recorded in contact with 0,50

M potassium phosphate buffer,pH 7.4. Potential stepincrement 4 mV; square waveamplitude 15 mV; frequency 5

Hz.-4

-3,5

-3

-2,5

-180 -130 -80 -30 20

E (mV)

ln (i/

NaCl

Na2

SO4

·10H2

O



Test systems

• Tafel ln(i/ip) vs. E plotsfor cuprite (Cu2O)

attached to graphite incontact with 0.50 Mpotassium phosphatebuffer.

-1,6

-1,4

-1,2

-1

)

43

2

• Successive scans inLSVs at v = 100 mV/s

-2,8

-2,6

-2,4

-2,2

-2

-1,8

-140 -120 -100 -80 -60 -40

E (mV)

ln

(i/ip

Scan number



Test systems

• Plots of E p vs. the scannumber for copper

coupons submitted toaggression in 0.05 M.From successive SQWVsinitiated at +0.65 V in the -300

-280

-260

-240

l (m

V)

NaHCO3

.

Potential step increment 4mV; square waveamplitude 15 mV;

frequency 5 Hz.

-400

-380

-360

-340

-320

0 2 4 6 8 10

Scan number

Peak pote

ntia

NaCl

Na2SO4·10H2O



Test systems

• Variation of Tafel SL onTafel OO , for successive

SQWVs performed oncopper couponssubmitted to aggressionin 0.05 M electrolytes. -0,012

-0,011

-0,01

V-1)

NaHCO3

1st scan

initiated at +0.65 V in thenegative direction.Potential step increment 4mV; square waveamplitude 15 mV;

frequency 5 Hz

-0,016

-0,015

-0,014

-0,013

-5 -4,5 -4 -3,5 -3 -2,5 -2

Tafel OO

Tafel SL

(m

NaCl

Na2SO4·10H2O1st scan

1st scan



Sample analysis

Analysis of alteration products in

the sculpture: “Hábitat en órbita baja de la Tierra ” (Elvira Alfageme,1981), currently exposed in theMuseo Popular de Arte

"Vicente Aguilera Cerni" (Spain).



Sample analysis

• Variation of Tafel (OO)with the scan number

in successive LSVs formicroparticulatedeposits attached tora hite in contact -4

-3,5

-3

Cuprite

Sam le

with 0.50 M potassiumphosphate buffer.LSVs at 100 mV/s

-5,5

-5

-4,5

1 2 3 4 5

Scan number

Tafe

l OO

Atacamite

Sample from Hábitat en órbita baja de la Tierra ”sculpture

Malachite



Sample analysis

• Variation of Eonset withthe scan number in

successive LSVs formicroparticulatedeposits attached tora hite in contact

Atacamite

-90

-80

-70

-60

-50

)

Cuprite

with 0.50 M potassiumphosphate buffer.LSVs at 100 mV/s

Sample from Hábitat en órbita baja de la Tierra ”sculpture

Cuprite

Malachite

-150

-140

-130

-120

-110

-100

1 2 3 4 5

Scan numbe r

Eons

et (m

acam e

Malachite



Sample analysis

• Variation of the Tafelparameters for

samples S1, S2 andS3 from the first tothe fourth scan. FromSQWVs initiated at -0,0115

-0,011

-0,0105

-0,01

V-1)

MalachitegroupCuprite

.

negative direction.Potential stepincrement 4 mV;square waveamplitude 15 mV;

frequency 5 Hz-0,0145

-0,014

-0,0135

-0,013

-0,0125

-0,012

-5 -4,5 -4 -3,5 -3 -2,5 -2

Tafel OO

Tafel SL (

Atacamite groupBrochantitegroup

1st scan



Final considerations

The use of both ‘graphite pencil’ and conventionalvoltammetry of microparticles methodologies permits the

layer-by-layer identification of corrosion products in copper-based works of art and archaeological artifacts

The use of repetitive voltammetry and/or reductive potentialsteps at a constant potential provides an unambiguousidentification of components in stratified corrosion layers of

bronze and brass archaeological and/or artistic artifacts

Documents

Doménech, A. y Doménech, T. Layer-by-layer identification bronze alteration. 2010