Evaluation of Corrosion Potential Measurements
as a Means To Monitor the Storage and
Stabilization Processes of Archaeological
Copper-Based Artifacts
A. Adriaens1, K. Leyssens1, M. Dowsett2 and C. Degrigny3
1 Ghent
University, Belgium
2 University of Warwick, UK
3 Chateau Germolles, France
CSA, EAS06, Somerset, November 13-14, 2006
Background
archaeological copper artefacts recovered from wet saline
environments corrode at accelerated rate in oxygen-rich air
storage and stabilization in a solution
CSA, EAS06, Somerset, November 13-14, 2006
Photo © Western Australia's Maritime Museum"
Background
archaeological copper artefacts recovered from wet saline
environments corrode at accelerated rate in oxygen-rich air
storage and stabilization in a solution
tap water
sodium
sesquicarbonate
solution
CSA, EAS06, Somerset, November 13-14, 2006
W.A. Oddy and M.J. Hughes, 1970; I.D. MacLeod 1987; D. Scott, 2002.
Present monitoring method
ƒ Analysis of the chloride concentration in solution
– Change of solution when
predetermined value is
exceeded
– Repetition until value low enough
ƒ Disadvantages
– Time consuming
– Indirect monitoring method
– No idea of potential side reactions
CSA, EAS06, Somerset, November 13-14, 2006
Objective
ƒ Investigate the use of corrosion potential measurements
(Ecorr) to monitor the behaviour of copper based alloys
during their stabilization
ƒ Benefits
–
–
–
–
Simple tool
Inexpensive to conservators
Direct monitoring method of the metal surface
More complete reaction profile when combined with the
analysis of the solution
CSA, EAS06, Somerset, November 13-14, 2006
Corrosion potential
ƒ
= potential difference between metal object and reference
electrode
reference electrode
Photo: EVTEK
ƒ
Depends on
– Metal composition
– Solution
– Interface metal - solution
object (porthole)
Hypothesis
Ecorr
ƒ Surface composition is stable when the corrosion
potential measurements do not change as a function of
time
time
Strategy of the study
ƒ Corrosion simulation
ƒ Corrosion potential meaurements
ƒ Surface characterization before and after the immersion
in sodium sesquicarbonate using XRD
Corrosion simulation
electrical connection
ƒ Electrodes
– reference materials with known composition
• copper
• copper-tin alloy
• copper-tin-lead alloy
epoxy resin
metal surface
(12 mm diameter)
ƒ Corrosion products
cuprite
nantokite
atacamite
CSA, EAS06, Somerset, November 13-14, 2006
atacamite and
paratacamite
chalcocite
Corrosion simulation
corrosion product
chemical formula
protocol
cuprite
Cu2O
electrochemical *
nantokite
CuCl
chemical °
atacamite
Cu2(OH)3Cl
chemical °
atacamite and paratacamite
Cu2(OH)3Cl
chemical °
chalcocite
Cu2S
chemical °
*T. Beldjoudi, 1999.
°R. Hughes and M. Rower, 1997.
CSA, EAS06, Somerset, November 13-14, 2006
Nantokite (CuCl) protocol
ƒ
ƒ
ƒ
Immersing pure copper
samples for one hour in a
saturated CuCl2.2H2O
solution
Rinsing with deionised water
Exposure to atmosphere
over night
1 mm
CSA, EAS06, Somerset, November 13-14, 2006
Corrosion potential measurements
ƒ Copper covered with nantokite
Ecorr (mV vs MSE)
-425
-450
-475
-500
-525
0
2
4
6
8
time (days)
CSA, EAS06, Somerset, November 13-14, 2006
10
12
14
XRD results
nantokite in sodium sesquicarbonate
nantokite
cuprite
paratacamite
malachite
relative intensity
relative
intensity
not immersed
n o t im m e rs e d
1 day
o n e d immersed
a y im m e rs e d
fo u rte e n d a ys
14 days immersed
im m e rs e d
1 .5
2 .5
3 .5
4 .5
5 .5
d -s p a c e (a n g s trö m )
d-space (angstrom)
CSA, EAS06, Somerset, November 13-14, 2006
6 .5
7 .5
8 .5
Overview XRD spectra nantokite samples
sodium sesquicarbonate
corrosion product
cuprite
(Cu2O)
pure copper
before
after
copper-tin
alloy
copper-tin-lead
alloy
x
paratacamite
(Cu2(OH)3Cl)
malachite
(CuCO3.Cu(OH)2)
x
x
x
x
before
after
atacamite
(Cu2(OH)3Cl)
x
before
after
nantokite
(CuCl)
x
x
x
x
x
x
K. Leyssens et al., Analytical Chemistry 78 (2006) 2794.
CSA, EAS06, Somerset, November 13-14, 2006
Overview corrosion potential data
sodium sesquicarbonate
ƒ
corrosion products: three groups
ƒ
substrate: Ecorr (Cu-Sn-Pb) < Ecorr (Cu-Sn) < Ecorr (Cu)
-420
Ecorr (mV vs MSE)
-440
Type 2
-460
-480
Type 3
-500
-520
Type 1
-540
-560
0.0
0.2
2
4
6
time (days)
K. Leyssens et al., Analytical Chemistry 78 (2006) 2794.
CSA, EAS06, Somerset, November 13-14, 2006
8
10
12
14
-420
Type 2:
• atacamite
• chalcocite
Ecorr (mV vs MSE)
-440
formation of cuprite, tin
oxide, lead carbonate
-460
-480
Type 3:
• nantokite
• mixture at- & paratacamite
nantokite disappears,
formation of cuprite,
atacamite, paratacamite and
malachite
-500
-520
-540
-560
0.0
Type 1:
• uncorroded
• cuprite
0.2
2
stable surface
composition (qualitative)
4
6
time (days)
K. Leyssens et al., Analytical Chemistry 78 (2006) 2794.
8
10
12
14
First set of conclusions
ƒ Variations of the corrosion potential as a function of time
mainly depend on the composition of the corrosion layer
ƒ Transformation of aggressive copper chloride species
seems well reflected in the behaviour of the corrosion
potential
ƒ What on a quantitative level?
a more precise method for correlating the corrosion potential with
changes in the corrosion products
K. Leyssens et al., Analytical Chemistry 78 (2006) 2794.
Prevous experiments
ƒ
Destructive XRD analyses
–
ƒ
ƒ
Several samples used for analysing one corrosion product on one
substrate
–
Sample 1: XRD before treatment
–
Sample 2: 1 day immersed + XRD
–
Sample 3: 14 days immersed + XRD
–
...
Every sample does not have exactly the same amount of corrosion
product
–
ƒ
Corrosion layer scraped from the surface and measured as a powder sample
Reaction rates can differ
Contact with the air between immersion and XRD measurement
spectroelectrochemistry
ƒ
simultaneous spectral and electrochemical information on electrode
surfaces and/or the electrolyte solution
ƒ
spectral observation of electrochemical induced processes
X-rays
Working electrode
(spectral position)
Detector
Window
(Kapton©)
Reference
electrode
Counter electrode
(platinum)
Electrolyte
Working electrode
(electrochemical
position)
© M. Dowsett
Working electrode
(X-ray position)
Inner/outer
windows
50 mm
Reference
electrode
Counter
electrode
Window
curvature
adjust
Working electrode
adjust
M.G. Dowsett and A. Adriaens, Analytical Chemistry 78 (2006) 3360.
On SRS station MPW 6.2
Possibilities
ƒ Time-correlated spectral and electrochemical data
ƒ Positive identification of compounds and mixtures
ƒ Observe reduction, passivation and coating in real time
ƒ Obtain spectral information from the electrolyte as well
as the surface
M.G. Dowsett and A. Adriaens, Analytical Chemistry 78 (2006) 3360.
CSA, EAS06, Somerset, November 13-14, 2006
Cuprite
Cuprite
Nantokite
Copper
XRD data
M.G. Dowsett and A. Adriaens, Analytical Chemistry 78 (2006) 3360.
CSA, EAS06, Somerset, November 13-14, 2006
Ecorr and XRD data
-0.04
100
80
-0.08
-0.10
-0.12
Nantokite (29.6o)
Cuprite (37.7o)
-0.14
Ecorr/V
60
40
Copper
Nantokite (49.3o)
Cuprite (43.8o) corrected for Cu
Corrosion Potential (Ecorr)
20
-0.16
100
-0.18
-0.20
0
0
50
100
150
-0.22
200
Elapsed time / minutes
Relative peak area / percent
Relative peak area / percent
-0.06
80
60
Nantokite (29.6o)
Cuprite (37.7o)
Cuprite (43.8o)
Copper
Nantokite (49.3o)
Cuprite (43.8o) corrected for Cu
40
20
M.G. Dowsett and A. Adriaens, Analytical Chemistry 78 (2006) 3360.
0
0
50
100
Elapsed time / minutes
CSA, EAS06, Somerset, November 13-14, 2006
150
200
Second set of conclusions
ƒ Corrosion potential measurements need to be treated with
caution
ƒ Further investigations needed
• which reactions take place?
– CuCl -> Cu2O
– CuCl -> Cu -> Cu2O
• influence of other corrosion products (SnO2, PbO, ...)
• multilayered structures (real artefacts)
Acknowledgments
ƒ Bart Schotte and Gareth Jones
ƒ Pieter Van Hoe, Derrick Richards, Adrian Lovejoy
ƒ
ƒ
ƒ
ƒ
Dr. Manolis Pantos, SRS
Dr. Tony Bell, SRS
Dr. Chris Martin, SRS
Dr. Laurence Bouchenoire, ESRF
ƒ COST Action G8
http://srs.dl.ac.uk/arch/cost-g8/
CSA, EAS06, Somerset, November 13-14, 2006