Transcript Folie 1 - ARCCHIP

Monitoring the conservation of
metal objects:
evaluation of a new approach
Annemie ADRIAENS1 and Mark DOWSETT2
1 Ghent University, Belgium
2 University of Warwick, UK
New approach
Simultaneous electrochemistry and surface analysis giving
Time-resolved chemical monitoring of a conservation treatment
to determine whether
A surface is chemically stable
A coating is free from pinholes
A surface protection is effective
…
The role of electrochemistry in metal conservation

Analytical method
– Identification
• Qualitative or quantitative analysis
of a component in an
electrochemical cell through
measurement of an electrical
parameter (current, voltage,
resistance)

Treatment method
– Cleaning
– Stabilization and consolidation
– Coating
Standard three electrode electrochemical cell
The role of surface analysis
Processes take place on the surface
 In-situ analytical techniques, e.g. XRD, XRF, Raman, ….
– Electrochemical treatment methods can be evaluated and
optimized
Applications in context
 Lead
– Corrosion passivation and application of coatings
Lead tobacco jar
Photos © K. Stemann Petersen
Applications in context
 Copper
– Monitoring the stability of marine artefacts
Photos © Dowsett and Adriaens
Photo © J.B. Memet
Monitoring the stability of marine copper objects
archaeological copper artefacts recovered from wet saline
environments corrode at accelerated rate in oxygen-rich air
storage in a solution
tap water
sodium
sesquicarbonate
solution
The artefacts often show a certain instability
(e.g. chemical transformation of the natural patina
and development of active corrosion)
monitoring the treatment remains necessary
present monitoring method
 analysis of the chloride concentration in solution
– change of solution when
predetermined value is
exceeded
– repetition until value low enough
 disadvantages
– time consuming
– indirect monitoring method
– no idea of potential side reactions
Objective
 investigate the use of corrosion potential measurements
(Ecorr) to monitor the behaviour of copper based alloys
during their storage and stabilization
 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
In collaboration with C. Degrigny, Heritage Malta
Ecorr measurements
 Measure the corrosion potential of the metal object
against a stable reference electrode using a potentiostat
 Potential depends on:
– the solution (known)
– the metal
• composition
• corrosion products
• coatings
Stable Ecorr data would imply
stable surface chemistry
Nantokite (CuCl) protocol



Immersing pure copper
samples for one hour in a
saturated CuCl2.2H2O
solution
Rinsing with deionised water
Exposure to atmosphere
over night
10 mm
Electrochemical cell for in-situ X-ray analysis
Outer windowX-rays (IR …)
Working
®
(8 µm Kapton )
Electrode
Inner window
(125 µm PET)
Window
~120
µm
Working
Fluid pocket
adjustment
Encapsulation
Working Electrode
Counter
Electrode
Counter Electrode
1 cm
M.G. Dowsett and A. Adriaens, Analytical Chemistry 78 (2006) 3360.
electrode
height
Electrolyte
… on SRS station 6.2
M.G. Dowsett and A. Adriaens, Analytical Chemistry 78 (2006) 3360.
Cuprite
Cuprite
Copper
Nantokite
XRD data
XRD
Ecorr data
-0.04
100
80
-0.08
-0.10
60
-0.12
Nantokite (29.6
(29.6oo))
Nantokite
Cuprite (37.7
(37.7oo))
Cuprite
40
Cuprite (43.8o)
Copper
Copper
Nantokite (49.3oo)
Nantokite (49.3
)
Cuprite (43.8oo) corrected for Cu
Cuprite (43.8 ) corrected for Cu
Corrosion Potential (Ecorr)
20
-0.14
-0.16
-0.18
-0.20
0
0
50
100
Elapsed time / minutes
150
-0.22
200
Ecorr/V
Relative peak area / percent
-0.06
Conclusions
 Need for surface analysis in-situ with electrochemical
techniques to provide real time chemistry for
– Process developments (e.g. reduction, passivation and
coating
– Evaluation of sensor principles (e.g. for simple process
monitoring)
 Work so far has used simulants but in future the validated
monitoring methods can be used on authentic and largescale objects
Acknowledgements
 Christian Degrigny, Heritage Malta
 Manolis Pantos, Daresbury Laboratories
 Station scientists: Tony Bell, Chris Martin, Steve Fiddy,
Nigel Poolton, Sergey Nikitenko, Laurence Bouchenoire
 Karen Leyssens, Bart Schotte, Ghent University
 Gareth Jones, Ingrid Oloff, Warwick University
 Pieter Van Hoe, Derrick Richards, Adrian Lovejoy
 COST Action G8
 Cameca GmbH