Corso di Laurea magistrale in
Scienze e Tecnologie Chimiche per la
Conservazione ed il restauro
Tesi di Laurea
ORMOCER
®s as protective coating materials
for outdoor bronze objects:
Evaluation after 17 years of natural exposure
Relatore
Prof. Emilio Francesco ORSEGA
Corelatore
Dr. Paul Bellendorf
Laureando
Cristian mazzon
Matricola 831226
Anno Accademico
2012 / 2013
C
ONTENTS
1.
I
NTRODUCTION 11.1. Phase of the study and objectives 2
1.2. Aim of the thesis 3
1.3. Bronze 4
1.3.1. The bronze alloys 5
1.3.2. The casting process 6
2.
B
ACKGROUND 82.1. A new coating system 8
2.2. The test substrate 10
2.3. The coating application 11
2.4. Initial testing 11
2.5. Extended test program 12
2.6. Long-‐term testing by outdoor weathering 16
3.
C
ORROSION 193.1. Mechanisms of corrosion 20
3.1.1. Electrochemistry of corrosion 22
3.1.2. The potential – pH diagram 24
3.2. Atmosphere and corrosion 26
3.2.1. Mechanisms of environmental corrosion of metals 28
3.2.2. Patina components 29
4.
C
OATINGS 334.1. Coatings 33
4.1.1. Traditional protection methods 34
4.1.2. New protection methods 36
4.2. The sol – gel reaction 36
4.2.1. Hybrid materials by the sol – gel process 38
4.2.2. ORMOCER®s 40
5.
M
ATERIALS AND METHODS 425.1. The samples 42
5.1.1. The bronze substrate 42
5.1.2. Selected ORMOCER coating variations and application 43
5.2. The techniques 45
5.2.1. Visual overview 45
5.2.2. Light Optical Microscope 45
5.2.3. Coatings thickness measurement instrument 46
5.2.4. FT-‐IR spectroscopy in Attenuated Total Reflectance (FT-‐IR ATR) 47 5.2.5. Scanning Electron Microscopy (SEM-‐EDX) 48
5.2.6. X-‐ray Diffraction (XRD) 49 6.
R
ESULTS AND DISCUSSION 506.1. Visual overview 50
6.2. Light Optical Microscope 54
6.2.1. Rolled bronze samples – NC 54 6.2.2. Cast flat bronze samples – NM 59 6.2.3. Cast shaped bronze samples – GGNM 63 6.3. Coatings thickness measurement instrument 68 6.4. FT-‐IR spectroscopy in Attenuated Total Reflectance (FT-‐IR ATR) 70 6.5. Scanning Electron Microscopy (SEM-‐EDX) 76
6.6. X-‐Ray Diffraction (XRD) 80
7.
C
ONCLUSIONS 82References 83
Annex
1.
I
NTRODUCTIONWorks of art, like bronze sculptures, are heavily affected by degradation through corrosion. They are mostly exposed outdoors and unsheltered against weathering and air pollution. Corrosion is a natural process and can be followed by observing the formation of a corrosive layer – patina1 on the metal’s surface. Different environmental conditions (presence and concentration of corrosive or aggressive agents) will determine colour, chemical composition and amorphous character of the patina (Römich, 1995)
A natural patina is a very thin conversion coat on the surface of the bronze. An unprotected bronze surface exposed to natural weathering conditions loses its original appearance and its natural (or artificial) protective patina. This patina may slowly become green (most compounds: brochantite2) and in many cases this coat layer has a protective action against any further corrosion process.
Atmospheric corrosion is becoming harder and aggressive and results from an increasing production of corrodants. These corrodants affect various materials including bronze (G. Bierwagen et. al., 2003).
Generally pollutants (sulphur compounds), acid rain, dirt deposit and moisture (condensation) cause a rapid corrosion and a consequently changing of the original artistic intention and aesthetic expression (loss of original patina and sculptural detail). This process produces a mottled, streaked, pitted or powdery green/black surface and during the rain the corrosion products are easily washed away and leave behind a new surface where the corrosions process can start again (G. Bierwagen, 2003; Römich, 1995].
Protection from bronze corrosion became so very important. Different polymer lacquers, waxes and natural resins are used for the protection against corrosion. Unfortunately over time most coating systems do not provide adequate protection
1Patination is the name for the process of colouring metals. These colours arise from chemical
interaction between elements in the metals and various chemicals or natural interaction between the elements in the metals and the environment.
2Brochantite is a sulphate mineral. Its chemical name an formula is cupric sulphates -‐
and don’t fulfil the requirement of long-‐term stability (About 5 to 10 years) (G. Bierwagen, 2003).
During the last decades transparent protective coatings were used for conservation and to slow down corrosion processes of outdoor sculptures. A requirement for a coating system is the effective protection from further corrosion of the substrate, good adhesion on the surface and good penetration of the pores of the metal and the patina. An efficient coating must have high barrier properties against water vapour, sun light (UV) and air pollutants (Römich, 1995).
Another important factor is the aspect of reversibility of the treatment. “Every treatment must be reversible”, this is a challenge presented by the conservation community because for the conservator a good adhesion means irreversibility. The applied coatings need to be removable without damaging or changing the visible appearance of the sculpture (G. Bierwagen, 2003).
The coating layer should be removable using organic solvents. In contrary mechanical methods for removal bear the risk to remove not only the coating but also the patina (natural or artificial) (Römich, 1995).
1.1. Phase of the study and objective (Römich, 1995)
Within a two year European research project (New Conservation Methods for Outdoor Bronze Conservation), from 1993 to 1995, some chemical variations of a new organic-‐inorganic copolymer (ORMOCER®) were developed as protective coatings for bronze objects. These coatings are tested in the laboratory of the Fraunhofer-‐Institute for Silicate Research (ISC), member of the Fraunhofer Society for the advancement of applied research. These organic-‐inorganic copolymers were supposed to have a great potential because they can be adapted to any substrate situation and can be chemically modified to satisfy the needs of conservators.
At the end of the project, in 1995, a series of different testing substrates were coated with the most promising six ORMOCER® variations, according to the results obtained in the laboratory testing phase.
The six variations classificated as optimistic candidates are different concerning the following parameters: diluition rate, kind of polymeric additives and in their protective concept. Mono-‐layers consists of one coating material of the ORMOCER® lack and for only one variation the coating was applied with several applications in order to increase the coating thickness of the protective layer. Bi-‐layers consists of an additional organic top coat (Paraloid® B72) applied after drying the mono-‐layer.
Each test series of bronze substrates (able to simulate a different porosity of the surfaces) consists of ten samples: seven coated with different ORMOCER coating variations; two coated with Paraloid® B72 and Incralac® (common used products in the bronze conservation) chosen as reference coating materials and one reference sample of each test series remained untreated.
In order to study the protective effect of this conservation method under natural weathering conditions, an extended and detailed examination program during a long-‐term exposure was started. All investigated samples had been exposed at different sites in Europe and on the roof of the logistic cottage of the Fraunhofer Institute for Silicate Research, in the rural environment of Bronnbach.
After a 17 years natural weathering, a laboratory investigation started to analyse and evaluate these ORMOCER® -‐ coatings in comparison to commercial products on different test specimen.
1.2. Aim of the thesis
The aim of this master thesis is a second evaluation and interim ranking of the effectiveness protection of the same coatings variations on the bronze sample used for the laboratory testing after a 17 years natural weathering (14 years after the first interim ranking).
The detailed laboratory investigation starts first with an optical inspection and a photo documentation (macro photography and light optical microscopy) to obtain an overview of the state of preservation of the samples after the exposure: information about general visual appearance of the coating quality, evaluation of the weathering resistance.
According to the first interim ranking, the analytical methods applied on the coated substrates are:
1) Coating thickness: to obtain information about the thickness variation after the action of the weathering.
2) IR spectroscopy in Attenuated Total Reflectance (IR – ATR): to obtain information about the presence of the coating after 17 years of exposure and after the reversibility test.
3) Scanning Electron Microscopy (SEM – EDX): to obtain information about the corrosion products developed on the substrate.
4) X – ray Diffraction (XRD): to obtain information about the crystal structure and chemical composition of the different corrosion crystal.
5) Reversibility testing.
The evaluation is done on the samples exposed at Bronnbach, a rural environment, on the roof of the logistic cottage of the Fraunhofer – Institute for Silicate Research (ISC). The samples exposed at different sites in Europe are to evaluate because they’re missing.
1.3. Bronze
Bronze is a generic name for all copper based alloys with other alloying elements except zinc (P. Skočovský, 2000 – 2006). There are many different kinds of bronze for different applications (O. Duhamel, 2009 – 2011). Bronze is an alloy consisting primarily of copper and the name of bronzes is defined according to the element used as main additive. Elements that can be added to the bronze alloy are: zinc, lead, iron, phosphor, aluminium, nickel, silicon, etc (R. F. Schmidt, D. G. Schmidt & Sahoo, 1988).
1.3.1. The bronze alloys
Initially bronze was made out of copper mixed with arsenic because the alloy became more stronger and malleable. Arsenic was than replaced by tin to obtain a superior alloy composition. The process itself can more easily be controlled, the alloy is stronger and easier to cast. Tin on the contrary of arsenic is not toxic (G. Thomas, 1996)
The main metal in the bronze is always copper. Tin is added in variable ratio, usually from 5% to 10% (over 25% the alloy became overly fragile). Modern statuary bronze is 90% copper and 10% tin. The result of alloying copper with tin is:
• An improved hardness;
• A high resistance against corrosion;
• A more fusibility and an easier casting of the bronze. The more complex alloys contain besides copper:
• Lead: decrease the melting point of the metal, improved fusibility, an easier casting and improved the plasticity of the alloy
• Zinc: the alloy copper – zinc is also called brasses. The addition of zinc increases mechanical properties and corrosion resistance. Small amount of other elements like lead, tin, aluminium, silicon or nickel improves the materials workability, resistance against corrosion and ductility.
• Phosphor: the addition of a small amount of this metal (1 – 2%) improved the bronze in hardness and strength. Is added as a deoxidizing agent during melting.
• Aluminium bronze: contains from 4% to 10% aluminium and other alloying agents such as iron, nickel, manganese and silicon are also sometimes added to the bronze. These kinds of bronzes are important for their higher strength, higher corrosion resistance and low melting point compared to the ordinary alloys.
• Silicon bronze: contains about 3% of silicon and these alloys are very resistant against corrosion. It has mechanic quality just like steel and high fusibility. In the 20th century, the introduction of silicon as primary alloying
element creates a bronze with wide application in the industry and in contemporary sculpture. Due to, the high resistance against acids, some alkalis and their good mechanical, chemical and wear properties the tin bronze has been replaced from the silicon bronze.
• Nickel: improves strength, the resistance against corrosion, stress corrosion and wear of the alloy. The addition of iron and manganese improve markedly the corrosion resistance of the alloy in seawater.
1.3.2. The casting process
Generally, casting is the process of pouring a liquid material into an empty mould, letting it set, cure, freeze or otherwise solidify and then opening or breaking the mould to reveal a solid reproduction. In the case of metal, it must first be melted in its liquid form before it can be poured into a mould (O. Duhamel 2009 – 2011).
To obtain good quality product results the casting processes are the most important factor (P. Skočovský, 2000 – 2006). There are many types of copper and its alloys casting (G. Thomas, 1996), such as:
1) The main technique for casting bronze, since ancient times, is the lost – wax
casting. Today this process is call investment method but carries on the same
technique.
To resume the process in few words, a sculpture made in wax is first encased in clay. During the baking process, the wax melts away and the clay becomes the mould where the melted bronze can be poured into and fill the empty cavity. After the bronze cools down, the clay mould is broken to reveal the solid bronze cast. The bronze then has to be worked on to remove any casting defects.
The investment casting, based on lost – wax casting, is an industrial process where the use of high – technology (such as waxes, refractory materials and alloy) allow the production of components with accuracy in a variety of metals and high – performance alloys.
These types of casting processes can be performed in two ways:
• The indirect methods, where a wax copy of a model made of clay, wood, plastic, steel or another material is used. This method, contrariwise , allows a mass production.
2) The ceramic shell method consists in a process of dipping the wax sculpture into a ceramic mixture to build up a thin layer or shell. During the baking in the kiln the ceramic coating becomes very hard. The wax is again burnt out and lost (like the investment method) and the empty cavity is filled with melted bronze.
3) The sand casting is a process characterized by using sand as the mould material. In addition, clay is mixed to the sand and moistened with water to obtain a plasticity mixture good for moulding. The sand is contained in a mould box divided in two halves.
The process is to bury an object, made of wood or metal, half – way in one half of the box and freeze the sand with CO2 gas. The second part of the box is attached and the sand packed into the second half and frozen. The following split of the two halves and the removal of the object leave an empty cavity in the sand. This cavity can now be filled with molten bronze.
This technique allows a large production of exemplars, it is cheaper than the lost – wax casting and is the quickest method for casting bronze, but it only suits geometrical forms.
2. BACKGROUND (Römich, 1995)
2.1. A new coating systemThe main task of the Fraunhofer-‐Institute for Silicate research (ISC) within the project team3 was to develop an ORMOCER® based material for a new protective coating system for outdoor bronze sculptures. As mentioned before there is a need for alternative conservation materials because conventional materials, like waxes and Incralac®, have only a short lifetime, which is not very satisfying.
The sol-‐gel technique provides a method suitable for preparing hybrid coating materials with properties of organic and inorganic polymers (Pilz M. and Romich H., 1997). So, it is possible to prepare coating materials with properties varying between these two extremes. The crosslinking of these copolymers can be controlled by modifying the starting compounds and the reaction conditions (Brinker C.J. and Scherer G.W., 1990).
Three kinds of starting compound can be used for the preparation of ORMOCER®: 1. Reactive organometallic precursors like alkoxides, Si(OR)4 (where R is an
alkyl group, CxH2x + 1) build up the inorganic backbone of the polymer by a sequence of hydrolysis and condensation reactions.
2. The organic-‐polymeric network derives from crosslinkable functional groups (R´) of organo-‐functionalized alkoxides of the general formula R´nSi(OR)4-‐n were R´ is a vinyl or epoxy group (these group can build up an additional organic network).
3. Alkoxides with non-‐reactive organic groups, R´nSi(RO)4-‐n, with R´ like phenyl or alkyl, that gives the polymer specific properties like hydrophobicity, elasticity, etc.
3 The members of the team are experts in surface coating technology, metal corrosion and
bronze conservation from research institutes: Danish Technological Institute (DTI), INCERTRANS (INC), Institute of Inorganic Chemistry (IIC); museums: National Museum of Denmark (NM),
The staring compounds that were tested are listed in the table below.
Table 1: Starting components for sol-‐gel derived lacquers used for bronze conservation (Pilz M. and
Romich H., 1997) Network formers (no functionalities)
Network formers
(with crosslinkable organic groups)
Network modifiers (no crosslinkable organic
groups) Tetraalkoxysilanes 3-‐Glycidoxypropyltrimethoxysilane
(epoxy functionalized silane, Glymo)
Propyl-‐ Methyl Phenyl-‐ Diphenyl-‐ alkoxy-‐ or hydroxysilanes Tetraalkoxyzirconates γ-‐Methacryloxyprpyltrimethoxysilane (methacrylate functionalized, Memo)
The epoxy functionalized silane (Glymo) or the methacrylate functionalized silane (Memo) were always used as the main component; further characterization and testing in the laboratory summarized in these paper proved that the two component lacquer GDiphenyl based on Glymo modified with Diphenylsilandiol (an hydroxysilane) are the most promising one.
A total number of 13 lacquers had been synthesized and each of these different multi-‐component systems were used as base material for several types of
monolayer coating systems. Variations can be made concerning the hardener, the
organic solvent, the dilution rate, the curing conditions and the possibility to use organic oligomer as additives. The organic oligomer was also applied as a second layer on the top of the first layer based on ORMOCER® to create a bilayer coating
system.
The ORMOCER® lacquers were characterized by measurements viscosity4, water and epoxy contents5 after a storage period of 30 days to determine their stability. Infrared spectroscopy (IR) and gel permeation chromatography (GPC) gave additional information about the molecular structure of the precondensate before the application of the test substrate.
4 According to Ubbelohde
2.2. The Test Substrate (Holm et. al., 1995)
The development of new conservation coatings needs an extensive tests program on test specimen or model samples that are easy to evaluate and comparable with original substrates. Therefore, the new coatings were first tested on very simple bronze substrates with reproducible surface conditions to investigate the general qualities of the lacquers.
Non-‐corroded rolled bronze sheets – NC (Sn bronze 90/10) were used as test samples for the evaluation of the general performance of the new conservation materials. Approximately 1300 samples of 10% tin rolled bronze (nomenclature: 90-‐
10) were prepared and distributed by the private company Naylor Conservation in
order to evaluate the protective effect of the coatings.
After this first phase, the most promising coating systems were tested on another different test samples like cast bronzes to simulate the porous surface structure, the alloys and the patina of the metal sculptures. The samples were prepared by specialists from the National Museum of Denmark. Different alloys corrode differently and often the addition of alloying elements to a pure metal will increase the corrosion rate, cause the formation of local galvanic cells. Two different alloys
90-‐10 and RG9 according to Swedish standards6 were used and two different cast forms, shaped cast bronze – GGNM and flat cast bronze – NM are prepared for the first test series (Table 2).
Table 2: Element composition in % of the two different alloys (Holm et. al., 1995)
Sn Zn Pb Cu
RG9 9 2 3 86
90-‐10 10 0 0 90
Later was decided to use only the RG9 alloy for further specimen for tests of the ORMOCER® coatings development. Part of the cast flat samples was artificially patinated, using traditional patination methods or experimenting with new ones.
Moreover, Danish and Swedish copper roofs with natural patina were included in the test programme. In addition, some special samples – RO for outdoor long-‐term exposition were prepared by metal spraying with an electric arc from INCERTRANS (INC). Metallization is a technical procedure, which makes it possible to deposit metal coatings on a support by melting the spraying material on the surface by means of compressed air. The samples for testing ORMOCER®s consisted of a steel support covered with Bz Al 10 (10 – 11% Al, 0.5 – 1.5% Mn, 0.5 – 1% Fe, rest Cu), deposited with the procedure described above.
2.3. Application methods
The multi-‐component ORMOCER® lacquer systems were used in high dilution with organic solvents (up to a ratio of 1:6 wt. % with butoxyethanol) to reduce the viscosity of the lacquer and to improve the impregnation behaviour on porous bronze surfaces, both on patinated and none-‐patinated one.
In the beginning of the project the coatings were applied by a brush technique. Later, the application of the coatings was carried out using spray equipment (spray gun “sata jet”). Multilayers were applied after allowing the previous layer to dry for several hours at ambient temperatures. To speed up the curing process a moderately warming up to 50°C or IR treatments were used.
2.4. Initial testing
During the initial test phase about 400 coating variations (basing on the initial 13 multi-‐component ORMOCER® lacquers) were synthesized and tested by using different:
• hardeners, • solvents, • additives,
• curing conditions or • application techniques.
The performance of the applications on rolled bronze sheets, were investigated by methods like:
• coating thickness (an inductive method), • adhesion properties (crosscut test7) and
• resistance against weathering: at the ISC the stability of the coating materials was studied by a short-‐term exposition (48 hours up to 14 days) to accelerated weathering conditions. The corrosive test atmosphere in the climatic chamber included changes in humidity (from 30% to 98%), in temperature (-‐20°C to +40°C) and a high concentration of polluting gas (5 ppm SO2, 15 times higher than the concentration in an industrial city). At the DTI a different weathering procedure was carried out for an extended test programme (the total time of exposure varied between 30 and 64 days). This cycle included resistance to humidity 8 (changes in humidity and temperature), freeze/thaw (cycle of temperature/humidity changes between -‐20°C and 40°C/30% and 98% r.h.), UV-‐light/water spray (fluorescent UV-‐light cycle and heat at 60°C, interval water-‐spray and condensation at 50°C in QUV-‐apparatus9) and a corrosive test (surface treatment with a kaoline slurry of copper(II)nitrate, iron(III)chloride, ammonium chloride and water containing 5% sulphur dioxide10).
2.5. Extended test programme
A total number of 16 most promising monolayer or bilayer coatings were selected for the second extensive testing phase. These ORMOCER® coatings are listed and described in the table below.
Table 3: Overview on selected ORMOCER® coatings (Römich, 1995)
Coating
System Starting components Additives Curing Application
OR 1 GDiphenyl (Glymo + Diphenylsilanediol) Paraloid B72® 20%, hardener room temperature monolayer (sprayed)
7 According to DIN 53151 8 According to DIN 50017 KFW 9 According to ASTM G 53-‐88 modified
Coating
System Starting components Additives Curing Application OR 2 GDiphenylT (Glymo + Diphenylsilanediol + Tetraethoxysilane) Paraloid B72® 20%, hardener room temperature bilayer11 1. sprayed 2. brushed OR 3 MDiphenyl (Memo + Diphenylsilanediol)
-‐
room temperature monolayer (sprayed) OR 4 GDiphenylDT (Glymo + Diphenylsilanediol + Dimethyldiethoxysilane)-‐
room temperature monolayer (sprayed) OR 5 GDiphenyl (Glymo + Diphenylsilanediol) Paraloid B72® 20%, hardener room temperature bilayer 1. sprayed 2. brushed OR 6 GDiphenyl (Glymo + Diphenylsilanediol) Paraloid B72® 20%, hardener room temperature monolayer (brushed) OR 7 GDiphenyl (Glymo + Diphenylsilanediol) Paraloid B72® 20%, hardener one day dried at 50°C monolayer (sprayed) OR 8 GDiphenyl(Glymo + Diphenylsilanediol) hardener
room temperature bilayer 1. sprayed 2. brushed OR 9 MDiphenylZr (Memo + Diphenylsilanediol + Zirconiumtetraisopropylate) Paraloid B72® 10% two hour dried at 100°C monolayer (sprayed) OR 10 GDiphenylTH
(Glymo + Diphenylsilanediol) hardener
room temperature bilayer 1. sprayed 2. brushed OR 11 MDiphenylV (Memo + Diphenylsilanediol + Vinyltrimethoxysilane) Paraloid B72® 10% dried at sunlight monolayer (sprayed) OR 12 GDiphenyl (Glymo + Diphenylsilanediol) epoxy oligomer 10%, hardener room temperature monolayer (sprayed) OR 13 GDiphenyl (Glymo + Diphenylsilanediol) epoxy oligomer 20%, solvent mixture, hardener one day dried at 50°C monolayer (sprayed)
11 In the bilayer system an acrylic resin coating (Paraloid B72®) is applied as a second layer on
Coating
System Starting components Additives Curing Application
OR 14 GDiphenyl (Glymo + Diphenylsilanediol) polyacrylate 20%, lower dilution, hardener one day dried at 50°C monolayer (sprayed) OR 15 GDiphenylT (Glymo + Diphenylsilanediol + Tetraethoxysilane) epoxy oligomer 10%, hardener room temperature bilayer 1. sprayed 2. sprayed OR 16 GDiphenyl (Glymo + Diphenylsilanediol) lower dilution, hardener room temperature bilayer 1. sprayed 2. sprayed
In addition and for comparison issues microcrystalline wax, Incralac® or Paraloid B72® coating are applied as commercial references used as references to evaluate the protective effectiveness of the ORMOCER® coating system OR1 to OR16.
The following analytical methods had been applied for the evaluation of the coated substrates:
1. Coating thickness: to obtain information about influence of solvent, dilution rate, storage time and application method of the lacquer.
2. IR spectroscopy: to obtain information about reversibility and the impregnation depth of the coating on patinated copper surface.
3. Adhesion crosscut (coating quality): to obtain information about influence of hardener, additives, curing conditions, influence of layer and influence of corrosion inhibitor BTA.
4. Water, SO2 and Radon permeation rate: to obtain information about
barrier properties against water, SO2 and water/SO2.
5. Contact angle measurement: to obtain information about hydrophobicity (water repellent effect).
6. Electron microscopy (SEM): to obtain information about coating thickness and impregnation of patina.
7. Radiometric emanation method: to obtain information about process of hardening and sequence of layers.
8. Microscopy and visual inspection: to obtain information about general visual appearance of the coating quality, evaluation of the weathering
resistance after the ISC-‐ and DTI-‐cycle, condensed moisture and the drop test with acids or H2O2.
To summarize the important results of the extended test program it can be said that:
• The thickness of the coatings is decisive for their protective effect. The spray application gives coating thickness of about 4 – 8 µm for monolayer and 10 – 12 µm for bilayer. The thickness is influenced by the solvent, the dilution rate, the degree of condensation of the lacquer and the application technique. A thicker coating have a better barrier proprieties bat a worse adhesion qualities [3].
• Test showed that the reversibility of the coatings is good when the lacquers contain minor amounts of organic fillers. Coating systems like OR
1 can easily be removed with organic solvents. The coatings without these
additives are extremely crosslinked and they can be removed only through stronger measure treatments like aggressive solvents and mechanical methods. It needs to hold in consideration that for patinated surfaces the reversibility is limited.
• The sulphur dioxide permeability is one of the important properties for the protective effect of a coating. The selected ORMOCER® system OR 14, with a polyacrylate as additive, show the better sulphur dioxide permeability then the monolayer ORMOCER® OR1, with Paraloid B72® as additive, and the bilayer ORMOCER® OR16, with Paraloid B72 on top. With Paraloid B72®, Incralac® has also a relatively low resistance to the permeability of sulphur dioxide. The OR10 coating has the best resistance against H2O2 [3].
• Contact angle measurements indicated that these new protective coatings reduce the wetting of the surface compared with Incralac® and uncoated surface. The properties of monolayer coatings changed during weathering – water drops are absorbed on the surface [3].
• The accelerated weathering program give the best valuable basis for evaluate the protective effect of the coatings. On unpatinated bronze
sheets the bilayer systems give in general a better protection than monolayer. The coatings OR15 and OR16 give the best results amongst all bilayer and OR1 amongst all monolayer.
• After exposure of coated samples to condensed moisture it was possible to appreciate the adhesion qualities to the bronze surface of both ORMOCER® coating system, mono-‐ and belayer.
2.6. Long-‐term testing by outdoor weathering
The outdoor exposition program of coated samples was the next step after the two years development and evaluation of a new material for outdoor bronze sculptures. Samples were exposed on four different places under different natural weathering conditions in Europe12. It was expected that a different environment must have a different impact on the bronze surface and on the protective effect of the chosen variations coating.
A series of different test substrates were coated with the most promising ORMOCER® variations in according to the results obtained from the laboratory test phase (Table 4). Finally, six ORMOCER® variations were chosen for these more extended and detailed examination during a long-‐term exposure.
Table 4: Promising ORMOCER® coating variations selected for the outdoor exposure program on bronze test substrates (Römich, 1995).
ORMOCER® coatings variations ORMOCER® systems Protective concept ORMOCER®,
dilution with solvent Additives
uncoated reference
OR1, usual13
GDiphenyl, 1:6 in BE14 20% Paraloid B72® mono-‐layer
OR1, thick mono-‐layer
12 The coated test substrates were sanded and exposed at the project partner’s sites: Telford
(Great Britain), Copenhagen (Denmark), Bucharest (Romania) and in Bronnbach (Germany) as well.
13 In order to increase the coating thickness of the protective layer the surface was coated with a
ORMOCER® coatings variations ORMOCER® systems Protective concept ORMOCER®,
dilution with solvent Additives
OR5 GDiphenyl, 1:6 in BE 20% Paraloid B72® bi-‐layer15
OR14 GDiphenyl, 1:4 in BE 20% Poly acrylate mono-‐layer
OR15 GDiphenylT, 1:6 in BE 10% Araldite GY260 bi-‐layer
OR16 GDiphenyl, 1:4 in BE
-‐
bi-‐layerParaloid B72® Acrylic resin (co-‐polymer) reference
Incralac® Acrylic resin (co-‐polymer) with BTA16 (corrosion inhibitor) reference
OR8 GDiphenyl, 1:6 in BE
-‐
bi-‐layer
The different kinds of substrates with different porosity were selected: • rolled bronze sheets (NC),
• cast flat bronzes (NM) and in addition
• cast shaped bronze samples (GGNM) and
• bronze samples prepared by metallization (RO), coated as a separate test series.
The size of the samples is 5x5 cm2 (NC), 6x10 cm2 (NM and GGNM) and 3x5 cm2 (RO); each test series consists of ten samples coated with the six different coatings variation (see previously), the two reference coatings (Paraloid and Incralac) and one reference sample untreated.
The natural weathering programme started in 1995 and the first interim evaluation of the performance of the new protective treatments under natural condition was made after about three years. The evaluation had as objective to exanimate the differences between the weathered coatings on the samples and the
15 The bi-‐layer protection system consist of an additional organic top coat (applied after drying
of the mono-‐layers) to increase the protective effect and as victim coat (serve as protective coating for the basic ORMOCER® system).
16 Benzotriazole: a heterocyclic compound with the chemical formula C6H5N3. Is an effective
original objects17 to ranking it. The detailed laboratory investigations were performed on the whole variety of coated samples exposed at Bronnbach and on a part of the test series (return temporary to the ISC) exposed in Telford (Great Britain), Copenhagen (Denmark) and Bucharest (Romania).
Table 5: Overview of the evaluated coatings and test substrates exposed at different sites during the first interim
evaluation (Vogel and Pilz, 1999).
ORMOCER®
coatings variations
Germany
Bronnbach Denmark Copenhagen Great Britain Telford Romania Bucharest
NC NM GGNM RO GGNM GGNM RO
uncoated
x
x
x
x
X
missingx
OR1, usual
x
x
x
x
X
missingx
OR1, thick
x
x
x
x
X
missingx
OR5
x
x
x
x
X
missingx
OR14
x
x
x
x
X
missingx
OR15
x
x
x
x
X
missing missingOR16
x
x
x
x
X
missing missingParaloid B72®
x
x
x
x
X
missingx
Incralac®
x
x
x
x
X
missing missingOR8
x
x
x
x
X
missingx
3. CORROSION
Metals are present in the earth’s crust in form of compounds (called minerals) such as oxides, sulphates, sulphides, carbonates, nitrates, chlorides with a complex and stable structure. However, after several metal-‐ extraction processes from these abovementioned compounds, the final products have an unstable form. Consequently, a corrosion process “encourages” metals to reduce their energy via spontaneous reactions with a formation of compounds having a greater thermodynamic stability (Bradford, 1993).
As far as the present work is concerned, bronze is the most common alloy used for outdoor sculpture and consists of a mixture of three different metals, such as copper, tin and lead (American Institute for Conservation of Historic and Artistic Works, 1993). However, alteration of copper properties due to the formation of this alloy can occur and the well known anti–corrosion copper features can dramatically decrease (Xiao et. al., 2012).
Copper and its alloys exposed to the atmosphere form a typical thin layer known as “patina”, as a consequence of the alteration products of the metal surface corrosion (Gradel et. al., 1987). However, a better explanation of these alteration products needs to be carried out. Patina consists in a thin compact layer that protects the sculptural shape and details. Corrosion is a kind of mineral deposit that attacks this layer, with a subsequent aggression process on the metal surface (Scott, 2002).
Bronze patinas are chemically complex structures and their compositions are well known and related to the species present in the atmosphere (Gradel et. al., 1987).
The degradation speed is increasing along the years following the raise of aggressive chemical agents in the atmosphere. Chemical agents present in the rain (mostly acid), sulphate radicals originating from the burning of fossil fuel and nitrate radicals from combustion engines are the main causes of degradation process of cultural heritage, especially in the case of bronze. The corrosion depletes the bronze surface to completely destroy it (Riederer, 1995).
The protection of the bronze surface is an important aim for the conservation of the bronze handworks. Nevertheless, the protective materials available today didn’t give always satisfactory results. A better protection could be obtained through the application of new materials and processes (Riederer, 1995).
3.1. Mechanisms of corrosion
Before discussing about corrosion products, it is necessary introduce several principles of corrosion and minerals composition.
The limited studies about corrosion processes are based on models not always applicable to a single and particular event (Scott, 2002). Metallic materials are subjected to deterioration through a series of physical and chemical interactions and biological activity. In addition, to the environmental impact of the atmospheric pollutants accelerate the damage and increase the weathering effect (Moncmanová, 2007).
Further studies on the concepts included on Pourbaix diagrams (Sheir et. al., 2000) kinetic and thermodynamic principles and chemical background knowledge are essential for a better comprehension of the final corrosion products. The corroded material is chemically altered or completely dissolved to form a new material very different from the original one (Scott, 2002).
Mainly chemical and electrochemical reactions are involved in the formation of corrosion products. The first one it is a reaction that doesn’t involve water or, in general, aqueous solutions (dry corrosion). It involves an electric charge transfer that takes place only locally, e.g. between metal (iron, copper, etc.) and oxygen (Shreir et. al., 1994). Corrosion in an aqueous environment (wet corrosion) and in an atmospheric environment (which involves also thin aqueous layer or moisture) is an electrochemical process with transfer of electrons between a metal surface and an aqueous electrolyte solution. Most of the corrosion processes are primarily electrochemical because they involve an anodic and a cathodic site. The reaction is controlled by a potential gap. This leads to a flow of ionic species from one region or surface to another one (Scott, 2002).
Corrosion can affect metal in a variety of ways, which depend on the metal type and on the particular environmental conditions. A broad classification of the various forms of corrosion, in which five main types have been identified, are show in Table 6.
Table 6: Types of corrosion (Shreir et. al., 1994)
Corrosion pattern Features Examples
Uniform (or almost
uniform)
All metal areas corrode at the same (or similar) rate
• Oxidation and tarnishing: • Active dissolution in acids; • anodic oxidation and passivity; • chemical and electrochemical
polishing;
• atmospheric and immersed corrosion (in some cases)
Localised Some areas of the metal surface corrode at higher rates than others, due to heterogeneities of metal or environmental interaction, or to the geometry of the whole structure. Attack can range from slight
localisation to pitting
• Crevice corrosion; • filiform corrosion; • deposit attack; • bimetallic corrosion; • intergranular corrosion; • weld decay
Pitting Highly localised attacks at specific areas cause small pits that penetrate into the metal and may lead to perforation
• Pitting of passive metals such as the stainless steels, alluminium alloy, copper alloys, etc., in the presence of specific ions, e. g. Cl-‐ ions
Selective dissolution One component of an alloy (usually the most active) is selectively removed from it
• denzincification; • dealuminification; • detinification
Joint action of corrosion and a mechanical factor
Localised attack or fracture due tot a synergistic action of a mechanical factor and corrosion
• Erosion – corrosion, fretting corrosion, impingment attack, cavitation damage;
• stress corrosion cracking, hydrogen cracking, corrosion fatigue
