22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma chemical treatment for metal artefacts: conservation approach
H. Grossmannová1 and F. Krcma2
1
Methodical Centre for Conservation, Technical Museum in Brno, Purkynova 105, CZ-61200 Brno, Czech Republic
2
Brno University of Technology, Faculty of Chemistry, Purkynova 118, CZ-61200 Brno, Czech Republic
Abstract: Article describes the possibilities and conditions of the hydrogen plasma
treatment application for the iron artefacts conservation from the conservation-restoration
approach. Specific groups of samples were treated to describe the whole process – pure
artificial minerals, model samples, real archaeological samples. Electron microscopy
SEM-EDS and XRD techniques were used to understand the transformation of compounds.
Keywords: low pressure plasma, metal restoration, conservation-restoration
1. Introduction - technology and conservation
This work is dealing with the possibilities and
conditions of the plasma treatment application for the iron
artefacts conservation. Generally, it was proved that the
application of hydrogen/argon plasma allows reduction of
chlorinated products as well as oxides from the corrosion
layers of archaeological objects [1]. Plasma chemical
treatment has some advantages compare to the other
methods, as abbreviation of the conservation procedures
and relative regardful of the artefacts. The corrosion
removal process is very complex and it is very important
to understand well the treatment mechanisms and to find
the optimal conditions for the whole procedure.
This technology itself is well defined and verified [2 –
4], but still some important tasks are related to the
conservation issues. For these reasons, we cooperated
with conservator specialists, to understand fully the
process details and its specific parameters. Our apparatus,
usable for the large amount of the treated objects (kind of
"mass conservation techniques" could be applied) or for
the big archaeological object such as swords or hatchets,
needs to be studied for the plasma homogeneity (i.e.
homogeneity of the process, represented by the sample
temperature). Next important factor of this technique is
how to combine effectively plasma with the conservation
pre-treatment and/or after treatment. An important part of
the experiments is performed also to understand the
changes in material composition and migration of
corrosion activators (chlorides, sulphates).
Fig. 1. Plasma reactor, model samples treated in plasma.
The plasma is produced in cylindrical reactor (0.75 m3)
by 13.56 MHz generator at operating pressure of 30 Pa,
P-III-6-21
total gas flow of 400 ml/min (200 ml/min H 2 , 200 ml/min
Ar). Reactor is equipped with gas inlets and mass flow
controllers for hydrogen and argon and a pumping
system. Whole technology is fully automated, supplied by
Programmable Logic Controller unit allowing a digital
recording of all treatment parameters.
2. Experimental
Model iron samples or newly excavated originals used
for all the presented experiments were always pre-dried
by molecular sieve only, treated in plasma reactor and
cooled down in inert atmosphere. Optical emission
spectrometer (Ocean Optics HR4000) was used to
monitor the process during some measurements. Optical
fibre was focused axially to the central part of the reactor.
The OH radicals are generated during the plasma
chemical treatment and can be used for the process
monitoring [5].
We also tried to understand the desalination process
induced by plasma. As the original artefacts are mostly
contaminated
by
chlorides
and
sulphates,
is
quite
challenging
to
determine
and
analyse the decrease of chlorides in metal samples
(solution is to use higher volume of the samples or to use
model samples with defined chloride concentration).
Despite this, migration of chloride ions is confirmed by
XRD analysis. Sample temperature during the plasma
treatment is one of the most important parameters,
because it is necessary to avoid the transformation of the
metal phases during the eutectoid transformation.
Moreover, the higher temperature corresponds to the
activity of the chemical species and it also helps to the
migration of contaminants. The temperature strongly
depends on the supplied power, its mode (continuous or
pulsed) and on the sample position in the reactor.
Generally, it would basically be taken into account as the
maximum allowed process temperature of the eutectoid
transformation for a particular metal - from the phase
diagrams (Fe approx. 700 °C.). However, due to the
object manufacturing technology (turbidity, carburising)
specific phases are formed in the metal, which are very
important for the authenticity of the object. This means it
1
is very difficult to determine one exact value of the
maximum allowed temperature. Because of various
impurities etc., the most sensitive material, which should
be the part of the object (verified at least by X-rays
analysis before the treatment) must be taken into account.
Thus contemporary practically used maximal temperature
for iron objects should not exceed about 150 °C, only.
Fig. 2. Sample temperature during 2 h treatment. C –
continuous, P – pulsed (duty cycle 50 %).
Fig. 3. Sample temperature during 90 min treatment, 400
W, continuous. Coordinates present the sample position in
plasma.
The sample temperature contact monitoring system was
developed and successfully tested. It allows measurement
only at one spot, but simultaneously with plasma
operation. The results give the information about typical
temperature profile – growing up during about 2 hours of
the process. The maximum temperature achieved under
the different electrical parameters (RF discharge
generation at 200–700 W in continuous and pulsed
regime) was varied between 71 and 194 °C. The
application of pulsed discharge regime could maintain the
corrosion removal process at lower mean power and thus
lower heating stress affected the treated samples. The use
of pulsed regime helps to regulate the temperature but
when the mean supplied power is similar (200 W/C
versus 400 W/P) the achieved temperature is almost the
same. The plasma reactor is 1.5 m long and
outer electrodes surround the cylinder in the length 1 m
2
around its centre. Active plasma can be visually
observed particularly in the area between the electrodes,
but its intensity is not homogenous. Due to this fact, we
tried to determine this non-homogeneity on the samples
treatment determined via their temperature. The samples
located in several positions. Point [-20,0] presented the
openings of the reactor, point [0,0] – X (start of the
electrodes), Y - centre of the reactor diameter, point
[100,0] – X (end of the electrodes), Y - centre of
the reactor diameter (see the immersed sketch in Fig. 3).
The highest maximum temperature was achieved at the
reactor centre as it was expected. Based on the results
shown in Fig. 2, it can be concluded that only the part of
the reactor between the cylindrical electrodes can be used
as the working space for the objects treatment
3. Results and conclusions
Treatment of pure minerals
To verify the processes leading to the corrosion
removal, the pure corrosion model samples were used.
Pure lepidocrocite (γ-FeOOH) was formed in laboratory,
and treated together with the whole real samples (pieces
of the agricultural tools), to understand the process of the
Cl- migration and mineral reduction. Lepidocrocite is
an orange iron corrosion product that is close to tone of
the mineral akaganeite. These two types of
corrosion products are generally less stable compared
to
goethite
(α-FeO(OH))
or
magnetite
Lepidocrocite
creates
a
secondary
(Fe 3 O 4 ).
corrosion coating in the neutral water during conservation
(desalination process). These types of corrosion
products are commonly found on the surface at
points or sockets, and they are the typical products of the
so-called chloride corrosion. Its colour allows
approximately distinguish it from the black-brown
corrosion
products
oxides
with
hydrated
forms Fe 2 O 3 .H 2 O and goethite. The plasma induced
reduction of lepidocrocite and particularly of akaganeite,
too, leads to the mechanical stabilization (upper layers of
the
treated
object
destabilized
by
plasma and are easily removable). Experimental
observation showed that treated lepidocrocite (amount of
1 g) is visually transformed to goethite on the powder
sample surface; original lepidocrocite orange products
remained unchanged in the powder bulk. Significant
amount of chlorine containing mineral akaganeite is
formed. This supports the theory of chloride ions trapping
(from other real samples) at the lattice of akaganeite [6].
Table. 1. Results of XRD analysis before and after
plasma treatment of pure lepidocrocite. L – lepidocrocite,
L h – lepidocrocite (homogenized L after treatment).
Sample
Treatment
Lepidocrocite
γ-FeO(OH)
Pure L
No
100%
Akaganeite
β-FeO(OH,Cl)
-
Goethite
α-FeO(OH)
-
P-III-6-21
Pure L h
6h cycle
51.4 %
17.3 %
31.29 %
Treatment of model and archaeological samples
Two groups of samples were used for other series of
experiments. At the first, the effect of the plasma
treatment of artificial corrosion layer (without the core,
chlorinated) was tested. Secondary, the effect of the
plasma and separately of the temperature, only (at the
same time profile as in plasma), was studied under 6
h treatment of model corrosion layers without any
metallic core and corrosion on archaeological samples
with metallic core. Archaeological samples were
determined as parts of the sickles, excavated in BrnoŽebětín, track "U Újezda" extinct medieval village
from 15th century. Layered artificial corrosion was
powdered
and
analysed
by
XRD
on
a
Bruker D8 Advance apparatus with Cu anode
(λ Kα = 1.54184 Å) and variable divergence screens at ΘΘ Bragg-Brentano. The qualitative phase analysis was
done to determine the phase (mineral) composition
changes due to the plasma treatment. Simultaneously, part
of the samples was analysed using a scanning electron
microscope
with
an
energy-dispersive
micro
analyser (SEM-EDX). Analytical work was carried out on
electron microscope PHILIPS XL 30th. Surface micro
analyses
were
performed
on
the analytical complex PHILIPS-EDAX. On the real
samples, detailed microscopy of the chloride and sulphate
nests was performed.
Table. 2. Comparison of XRD analysis according
treatment procedure. (MCL - model corrosion layer, AS archaeological sample, NT - no treatment, P - plasma, T temperature).
MCL P
AS NT
AS P
AS T
3.5 %
-
1,5 %
-
-
3.7 %
4.2 %
-
-
-
50.3 %
44.2 %
58.9 %
69.3 %
66.0 %
42.5 %
51.6 %
9.5 %
21.8 %
13.6 %
-
-
17.4 %
6.0 %
11.9 %
-
-
7.3 %
2.9 %
6.6 %
-
-
5.4 %
-
-
MCL
Lepidocrocite
γ-FeO(OH)
Akaganeite
β-FeO(OH,Cl)
Goethite
α-FeO(OH)
Magnetite
Fe 3 O 4
Silica
SiO 2
Albite
NaAlSi 3 O 8
Microcline
KAlSi 3 O 8
NT
Analysing XRD data (see Table 2), it was
confirmed that the influence of the plasma results in the
transformation
of
corrosion
products.
The
significant increase of magnetite concentration, both in
model and real samples, was observed. For model
samples, lepidocrocite and goethite are transformed in to
magnetite. The results for original archaeological
objects are affected by a high content of soil silicate
P-III-6-21
minerals and thus it is difficult to precede the correct
methodology of the sampling for the XRD analysis. Data
are distorted by different percentage of soil minerals, but
calculating the ratio of ferrous compounds without the
soil minerals confirms the formation of magnetite also for
these samples. Decreasing lepidocrocite concentration
confirms the results obtained during the treatment of the
pure mineral. To better understanding these chemical
processes, new experimental series using samples of other
pure minerals will be prepared.
As it is shown on the Fig. 4, 5 and 6, SEM-EDX
(TESCAN MIRA3) was experimentally used to
understand the changes in morphology and in distribution
of chemical elements. At these first experiments, we
focused on the analyzes of chloride and sulphate nests on
the
surface
of
real
archaeological
samples.
Macroscopically, nest is a hollow space in the corrosion
layer filled with tiny crystals shining brownish coloration.
Fig. 4. Left - untreated corr. nest, right - treated corr.
nest.
Fig. 5. Detailed SEM pictures (edge of the nest) with
pointed areas for EDX microanalyzes, untreated nest.
Comparison
of
treated
and
untreated corrosive nest visually confirm a morphological
changes - elimination of flat oriented crystals. EDX
3
surface microanalyses (spots spectrum 3 - 6) indicate the
presence of chlorides and sulfur (sulphate crystals are
presented, not only chloride FeCl 3 crystals). However, the
analysis compares two different corrosion nests with
different representation stimulators corrosion, therefore
no loss in weight of sulfur and chlorine is clearly
proved. Morphological changes - the absence of similar
looking nests on the corrosion of untreated subjects verify
the changes in samples composition. Changes in surface
morphology can significantly influence the analysis with
respect to the disclosure of the surface of the electron
beam, (less diffraction and reflection phenomena
of characteristic X-rays at crystalline surface of corrosion
products).
[4] S. Veprek, Ch. Eckmann and J Th Elmer. Plasma
Chemistry and Plasma Processing, 445, 4 (1998)
[5] Z. Rašková, F. Krčma, M. Klíma and J. Kousal.
Czechoslovak Journal of Physics, 927, 52 (2002)
[6] I. Kotzamanidi, P.Vassiliou, Em. Sarris, A.
Anastassiadis, L. Filippakis, S. E. Filippakis. AntiCorrosion Methods and Materials 256, 49 (2002)
Fig. 6. Detailed SEM pictures (edge of the nest) with
pointed areas for EDX microanalyzes, treated nest.
In future experiments, we would like to focus on
the effect of plasma applications on the sulphate
concentration by analyzing corrosion by EDX before and
after the treatment. An important research would be also
performed on the artificially prepared pure corrosion
minerals to understand well the effect of the
transformation and migration process.
4. Acknowledgement
The work was supported by the Ministry of Culture of
the Czech Republic, project No. DF11P01OVV004.
5. References
[1] J. Patscheider and S. Vepřek. Studies in Conservation,
31 ,29 (1986)
[2] V. Sázavská, F. Krčma, T. Šimšová and N. Zemánek.
Journal of Physics: Conference Series, 207, 1742 (2010)
[3] K. Schmidt-Ott and V. Boissonnas. Studies in
Conservation, 81, 47 (2002)
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