Protection of cultural heritage by real-time corrosion monitoring
|Background||Principle||Prototype||Monitoring in cultural heritage||Project objectives||Application||Partnership||References|
It is generally agreed that control of the air quality is vital to the protection of the valuable, culturally-significant objects in museums, expositions, depositories, and archives. The main factors affecting air corrosivity are temperature, relative humidity, the concentration of pollutants such as SO2, NOx, O3, NH3, HCl, H2S, dispersed chlorides, organic acids, other volatile compounds and dust particles. However, it is only the relative humidity and temperature that are usually controlled and monitored. Additional anti-corrosion measures may be applied after valuable and often irreplaceable historical objects have already been affected. Since control of the air without the application of any monitoring technique to give rapid feedback on the air quality might be either inadequate or excessive, and thus too costly, information on the actual corrosivity of the atmosphere is crucial for effective corrosion protection.
In a project financed within FP6, a prototype of an electronic logger for continuous measurement of air corrosivity was developed. The main goal of the present project is to fine-tune the monitoring system for application in the cultural heritage sphere.
The concept of the measuring device is simple and yet highly effective: the electronic unit measures and registers the change over time in the electrical resistance of a thin metal track applied on an insulating substrate. If the metal corrodes, the cross-sectional area of the track decreases and the electrical resistance increases. A part of the metal track is protected by an organic coating and, thus, serves as a reference to compensate for resistivity changes due to varying temperature.
The prototype monitoring unit is comprised principally of two parts, i.e., corrosion sensor and electronic logger.
Corrosion sensor. The corrosion sensors consist of metallic tracks of different thicknesses deposited on a non-conductive substrate using either the printed circuit board (PCB) technique or a combination of electroless and electrolytic deposition (ED). The width of the measuring track is 1 mm and the length is 110 mm. This geometry ensures high sensitivity to changes in the electrical resistance due to metal corrosion. The sensitivity and service life of the sensors depend mainly on the thickness of the metallic track: the lower the thickness, the higher the sensitivity. On the other hand, low thickness leads to a shorter service life. Therefore, sensors with several track thicknesses are available for application in environments with different aggressivenesses.
The corrosion depth (CD) of the metallic sensor is calculated according to the equation:
where tinit is the initial thickness of the reference portion of the track (assumed to be equal to the sensing track thickness at the beginning of exposure); Rsens and Rref are the actual resistances of the sensor and reference tracks, respectively; and Rsens,init and Rref,init are the initial resistances of the sensor and reference tracks. The calculation is based on the initial and actual electrical resistances of the sensing and reference tracks measured as a potential drop along the track through which defined currents pass . Protection of the reference track against corrosion is provided by a thin organic coating; the coating is transparent to avoid a difference in the temperature of the reference and sensing tracks when irradiated by the sun.
Electronic logger. Corrosion sensors cannot work without a device that measures the electrical resistance of both the sensor and reference tracks, while also saving data for further processing. The basic version of the electronic logger is encased in a small polycarbonate box that measures 100×65×37 mm. The logger lid provides a connection to the sensor, which can easily be replaced when necessary. Furthermore, if the corrosion depth threshold defined by the user is passed, an LED signals that the limit has been exceeded.
The frequency of the measurement can be adjusted from minutes to hours in order to match the sensitivity of the measurement with the anticipated corrosion rate. If a very low or constant corrosivity is expected, several measurements a day are fully sufficient for a precise classification of the environment. Conversely, if the corrosivity is high or fluctuation in the corrosion rate is expected, it may be useful to collect data often, e.g., with a frequency of minutes. In principle, the unit allows for a frequency setting from seconds to days.
The loggers are designed to be autonomous for two to three years. Collected and stored data can be transferred to a computer via a non-contact inductive data reader, even through a showcase glass. The logger with a data reader is shown in Figure 1. A GPRS/GSM access unit is optionally available and allows for remote data access and control with automatic data delivery via e-mail. The logger runs on a replaceable battery and is therefore fully independent from any power supply, any external control in fact, and can thus be placed practically anywhere. More details are available in references [1–6].
Monitoring of the corrosiveness of climates and microclimates in museums and depositories is an invaluable tool for protecting deposited objects, not only metallic ones but also wooden, paper, textile, leather, ceramic, glass, plastic etc. There are many parameters that should be monitored in order to prevent corrosion attack of the mentioned materials and to eliminate as much as possible the objects’ degradation. The most significant parameters are temperature, relative humidity, concentration of pollutants (SO2, NOx, O3, NH3, HCl, H2S, organic acids and volatile compounds) and dust particles. One way to assess corrosion aggressiveness is to monitor all the mentioned parameters and to calculate a risk of corrosion using an empirical model. The other way is to expose a piece of the same material from which a particular object is made for a certain length of time and then evaluate the degree of corrosion attack. The former way requires several assumptions that generalize the impact of climatic factors on materials without precise specification to the object’s material properties (such as composition, structure or surface treatment). It, therefore, provides an indirect tool for corrosion monitoring. On the other hand, the result can be obtained immediately after collection of the necessary data. The latter method provides a more direct measure, however the results are affected by the assumption that the coupon behaves identically to the object material. In addition, it can take some time to obtain measurable results.
Because monitoring of all of the climatic parameters that can affect the corrosivity of air is not technically or economically feasible, direct coupon techniques are generally considered the best alternative. At present, there are several standards and recommendations on the application of metal coupons for monitoring of air corrosivity. ISO 9223 recommends exposure of copper sensors for 1 year. Mass loss is then evaluated after removal of corrosion products by pickling. The corrosivity is ranked in 5 corrosivity classes, C1 to C5. ISO/DIS 11844-1 uses a similar approach, but the corrosivity ranges IC1 to IC5 are different. This standard also includes an application of silver as a coupon. The ISA S71-04 standard is focused on indoor conditions and is based on shorter, 1 month, exposures of copper coupons. It characterizes atmospheres into 4 groups, G1, G2, G3, and GX. Recently, a recommendation focused directly on cultural heritage was published by De Santoli et al. . According to this method, copper or silver coupons are exposed for 1 month and the air quality is classified in 5 ranges.
Three main drawbacks can be identified in these methods. First, the results often do not correlate to each other. Second, the measurement takes a relatively long time and only information on the cumulative corrosion is available; it is impossible identify any particular event affecting the actual corrosivity. Third, a laboratory is necessary for the evaluation of the corrosion loss of the coupons and the handling of metal coupons is, therefore, expensive.
Thus, there is a strong need for a simple, reliable, and real-time technique for air-quality monitoring. The electrical resistance technique is the most suitable because it is the only technique that gives direct information on metal loss at reasonable costs. If the technique proves to provide reliable results in an extensive end-user testing program, a standard will be proposed enabling application of the technique as a principal means of air-corrosion monitoring in cultural heritage institutions.
For use in the highly-demanding cultural heritage sphere, further logger development is necessary to create a marketable product. The main innovative objectives are as follows:
In addition, major efforts are planned to raise awareness of the new technology through presentations to cultural heritage professionals, training of staff in cultural institutions, and promotion of the product. A standard on the monitoring of indoor air corrosivity using the electrical resistance technique will be proposed through European interest groups.
Besides cultural heritage, the logger can be applied in various other fields where monitoring of corrosion is vital:
To develop the corrosion-monitoring system, expertise in metal deposition, electronics, logging technology, corrosion science, materials science, and protection of cultural heritage is needed. The consortium brings together institutes and companies with the required expertise. There is a large research and education organization, two non-profit research institutes, an SME that will manufacture the electronic device, and three cultural heritage institutions with specific areas of interest.
Institut de la Corrosion / French Corrosion Institute (ICO), the co-ordinator of this project, is established in Brest, France since April 2002. The core business of ICO is corrosion testing and research and development work in the field of protection against corrosion. Its activities also include consultancy and advisory service, failure analyses, and information and education. There are 30 engineers and technicians working in ICO and its daughter company CORREX. The main fields of activities are Transport industry; Construction, building and infrastructure; Offshore, harbour and seawater; and Sensors and modelling. ICO is financed mainly by the industry through specifically focused cost shared programs and through ARCOR, which is a member organization of companies with strong interest in corrosion research. ICO is a subsidiary of Swerea KIMAB AB (KIMAB) with offices in Stockholm, Sweden. It is a part of Swerea Group of RTD institutes owned jointly by Swedish industry and the government of Sweden. The corrosion part of KIMAB was founded in 1965. With a team of 65 engineers and technicians, about 100 RTD projects, and 500 consultancy works conducted every year in the field of corrosion and corrosion protection, Swerea KIMAB is the largest institute in Europe in this field.
ICO is very well-equipped for laboratory, accelerated, and outdoor corrosion testing, as well as for evaluation and interpretation of the results. The main domains of excellence within the current project are the corrosion testing in atmospheric conditions. The results of the corrosion tests will be used as a feedback in the sensor and logger development process. Because of its wide web of contacts, ICO will also be involved in the dissemination and exploitation process.
Vysoka skola chemicko-technologicka v Praze / Institute of Chemical Technology, Prague (ICT), is a university institution providing education and scientific, research, development and implementation activities. It is the largest institution of higher education in Central Europe focused on applied chemistry. Two departments of ICT will be involved in the current project: Department of Metals and Corrosion Engineering and Department of Chemical Technology of Monument Conservation.
Among the priorities of the Department of Metals and Corrosion Engineering research activities, development of metallic materials with required properties and methods for corrosion monitoring can be highlighted. Personnel of the department will be involved mainly in development of corrosion sensors for aggressive outdoor environments and reference part masking, in design of the logger-sensor connection in collaboration with NKE and in laboratory corrosion testing. Experiences of the Department of Chemical Technology of Monument Conservation and partners from cultural heritage institutions will be utilized in reliability evaluation of the product in museums and cultural heritage.
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V., Institut für Elektronenstrahl- und Plasmatechnik / Fraunhofer Institute for Electron Beam and Plasma Technology (FhG) is devoted to the development of innovative solutions using Electron Beam and Plasma Technology in industrial processes. FhG is established in Dresden and it is one of 58 institutes belonging to the Fraunhofer-Gesellschaft. Following the German reunification, it was established by working groups of the former Manfred von Ardenne R&D institute in Dresden. Today, about 98 permanent employees, including 57 scientists and 31 technical staff as well as another 64 doctoral students, graduates and students are employed. Electron beam technology, pulsed magnetron sputtering and plasma-activated high rate deposition are the core areas of expertise on which the work is based. One main field of activity of FhG is the thin film technology. This includes coating of sheets, strips and components made of diverse materials with various layers or layer systems. The second main field of activity of the FhG is electron beam technology. The electron beam is used for the welding and evaporation of metals as well as for the modification in edge layers. Further applications are the curing of lacquers, the improvement of plastics properties, the sterilization of medical devices and the germ reduction of seed and grain. Many of those developments are closely correlated with the fields of electrical engineering, electronics as well as microelectronics. Numerous innovative products for example thin film solar cells, sensors, microelectronic components or data media are being produced by means of technologies developed by FhG.
FhG has developed a strong know-how in PVD processes for high quality coating on different substrates, including glass, metal, plastics, and ceramics. FhG is well-equipped with vacuum process devices and well-prepared for PVD processes onto materials of different scale. FhG possesses a variety of analytical equipment for characterizing surfaces and layers. This equipment and the expertise of the employees are available for development of ultra-thin corrosion sensors.
nke SA (NKE), formerly Micrel, was created in 1984 with specialization in electronics. The company employs 71 people. NKE is ISO 9001-V2000 certified. NKE is focused on the application of high-tech electronic measuring devices for new and special purposes. There are four fields of activity: Instrumentation, Marine electronics, Regulation of domestic heating, and Custom design, dealing with development and fabrication of specialty electronic products for individual customers. Since 1993, NKE has been developing autonomous loggers for measurement of physical and chemical parameters of seawater. These loggers work independently over several years without maintenance. Several products for corrosion monitoring in aqueous environments were developed in recent years that are produced and marketed by the company. NKE has also developed prototypes of loggers for atmospheric corrosion monitoring.
NKE is a leading partner in the development of the corrosion loggers. The new loggers will include new sensitivity ranges for the specific sensors developed for cultural heritage applications, new watertight sensor connection, inductive communication interface for a USB port, and user-friendly PC software for corrosion measurement and data evaluation and interpretation.
Centre de recherche et de restauration des musées de France, Département Conservation preventive (C2RMF), is one of the most experienced institutions in Europe in research and scientific applications in the field of protection of cultural heritage artefacts. Notably, it joins basic knowledge of analytical and spectroscopic techniques together with experience in the materials of cultural heritage. C2RMF is closely connected to Centre national de la recherche scientifique (CNRS). C2RMF was founded by the French Ministry of Culture in 1999 by merging the research laboratory of the Museums of France (LRMF), founded in 1931, and the Service of Restoration (SRMF), created in 1968.
The research laboratory is installed in premises of 5000 square metres under the Carrousel gardens in central Paris, with an underground connection to the Louvre Museum. The staff of the research department includes about 60 curators, scientists, engineers, and technicians. Its mission is to carry out research on the creation, life and ageing of the cultural heritage kept in museums. Its tasks include (i) diagnostic work at the museum curators' request for the authentication of objects before their acquisition or in view of their conservation and restoration and (ii) research projects relevant to the materials science aspect of art history and archaeology as well as to the development of new non-destructive analytical techniques well fitted to the study of cultural heritage.
Within this project, C2RMF will be active in the internal and external cultural heritage testing. In the first step, a group of museums and exhibition halls will be selected, the personnel will be trained, equipped with the corrosion monitoring devices and data evaluated after the end of exposure. C2RMF will lead activities in standardization of the technique, which is seen as a very important point in the product exploitation. C2RMF has good contacts to European interest groups where a new standard on the electrical resistance monitoring in atmosphere can be proposed.
Schweizerisches Nationalmuseum / Swiss National Museum, Collections Centre, Department of Conservation Research (SNM). The Collections Centre of the Swiss National Museums, constructed in 2007 in the township of Affoltern-am-Albis, is a state-of-the-art facility which contains the newly designed conservation treatment laboratories, the laboratory for conservation research as well as the museum’s storage facility. Housed are the nearly one million objects that comprise the museum’s entire collections. For the first time, a very close collaboration has been made possible by the new location at the new Collections Centre. The Department of Conservation Research with its 50 years of experience is doing research in collaboration with the conservators in order to answer specific questions about corrosion products and restoration materials and with the archaeologists about characterisation and examination of different materials. The scientists are giving advice concerning exhibition materials based on research of the compatibility between museum objects and exhibition materials such as “Oddy tests”. Moreover, they are performing extensive research projects in collaboration with other museums and research institutes, e.g. ETH Zurich.
SNM will participate in cultural heritage testing since a large variety of different storage and exhibition conditions in the Collections Centre and the museum’s main building are available. Conservators and specialized technicians will be involved in the project by answering questions about conservation materials such as woods, textiles etc. and support the project from the conservation point of view. SNM will be responsible for preparation of a workshop on monitoring in cultural heritage, which will be organized in the end of the project.
Nationalmuseet / The National Museum of Denmark (NATMUS). NATMUS collects, studies, exhibits and preserves materials from Denmark’s earliest history to the present day. The collection contains well over two million objects. Several hundred thousand of these objects are metal and many more are composite objects with metal parts. The objects are stored and exhibited in a variety of conditions, including indoors (with and without climate control) and outdoors.
The Department of Conservation of the National Museum comprises around 60 conservators, craftsmen, scientific researchers and support staff who have expertise in conservation and restoration of metals, building materials and paint, furniture, textiles, leather, ceramics and glass, wall paintings, easel paintings, paper, photographs, waterlogged materials and modern materials. The conservation laboratory (Research, Analysis and Consulting) develops and applies analytical techniques to research the composition, manufacture and degradation of cultural heritage objects and buildings.
The role of NATMUS will be to lead preparation of the Survey on requirements on corrosion monitoring in culture heritage. Contacts with other cultural institutions will be used to establish a network of partners interested in collaboration in the survey and later on in end-users testing. NATMUS will also be very active in cultural heritage testing, focusing particularly on outdoor and uncontrolled indoor environments.