The computer-based Decision Support System (DSS) MiCorr is a new non-invasive diagnostic tool for metal objects. It is based on the comparison between corrosion forms observed on an artefact under investigation and those from case studies / corrosion forms available in a database constructed by the project team and its partners. Two search engines have been developed: the first one uses interlinked keywords and the second digital schematic representations of corrosion forms. The latter, based on Bertholon’s schematic description of metal corrosion, is the most innovative tool of this application. Corrosion forms are described as structures of strata (metal, corroded metal, corrosion layers…), each stratum having its specific characteristics (morphology, microstructure, texture and other properties). A modelling program allows the construction of a corrosion structure made from encoded building blocks (strata containing up to 30 characteristics).
The same set of analytical techniques with similar operating conditions have been used to characterize the corrosion forms / case studies of the database. They required the destructive sampling of artefacts to determine the precise nature, organization and composition of the corresponding strata. As they are characterized with the same digital construction model which is used for the corrosion forms under investigation a comparison becomes possible.
By comparing the corrosion form of an artefact under observation or a sample from the same artefact observed on cross-section with the database through the DSS, a MiCorr user is able to find case studies / corrosion forms of objects showing similar corrosion phenomena. In case this visitor is a conservation professional, this should help him / her to implement an appropriate conservation protocol.
The search engines allow the visitor to question the database but if registered he / she has also the possibility to enrich the database with new corrosion forms and/or case studies, thus becoming an active contributor. The basic format of the corrosion forms can be further improved with any additional analytical data and the operating conditions used.
In a nutshell, MiCorr aims at:
The didactic, self-learning and open-source MiCorr tool should enable the diverse scientific communities involved in metal analysis and conservation to better apprehend existing knowledge and to contribute to research problems not solved yet.
A thorough understanding of alteration processes developed on metals enables us to slow down, limit and / or stop existing corrosion. In the industrial field, sampling of metals to investigate the amount of alteration observed is not unusual and the information gained provide knowledge on the corrosion developed such as stress corrosion cracking, fatigue corrosion, corrosion due to exposure to high temperature or pressure which are rarely encountered in the heritage domain. If this type of alteration does appear on historic or archaeological artefacts, it will not necessarily be similar due to the long periods of stress or weathering involved. Therefore, the examination of contemporary metals cannot solely be used to predict the long-term preservation of materials (Neff 2006).
When conserving metal artefacts, conservation professionals try to carry out the most precise diagnosis in order to find an appropriate conservation treatment. This diagnosis requires a thorough description of the surface condition of the object under investigation. Based on experience and consultation of corrosion models published in the literature (Corrosion Science, Studies in Conservation…), conservation professionals try to find analogies with the observed corrosion forms. On the basis of this knowledge conservation strategies can be proposed.
But the matching between the corrosion forms
investigated and those that are used for comparison is difficult to achieve.
This is due to the lack of standardization in the description of the object
surfaces (according to his/her experience, each conservator perceives the
object in a different way) and the heterogeneity of heritage metal surfaces. The
use of different analytical tools can bring further bias in the description of
Professional associations of corrosion experts (NACE - the Professional society for corrosion engineers since 1943 and the European Corrosion Federation since 1955) have extensively published on the corrosion of metals. Even though corrosion mechanisms in particular environmental conditions (as well as the means to prevent them) are known, there are no online diagnostic support tools that link observed alterations with corrosion causes. Websites such as http://corrosion-doctors.org/Contents.htm provide illustrations of corrosion forms. Unlike heritage metals that display extensive and complex alteration forms, such as active or reactivated corrosion, they only show the result of the reactivity of the clean metal in a corrosive environment. In the conservation domain, only one database (www.materialspathology.com) used to exist but is not anymore available. It offered descriptions of corrosion forms found on heritage metals, but without providing a diagnosis.
Over the years much research on the diagnosis of corroded heritage metals has been carried out and regularly published: Dillmann and his team (Dillmann 2005) have been investigating the alterations of archaeological and historic iron ; Robbiola (Robbiola 1998) has focussed on the study of natural patinas on copper-based alloys and how they undergo changes when buried in a chlorinated soil that favours active corrosion ; Turgoose (Turgoose 1985) has investigated alterations of lead artefacts in organic acid-rich environments, as found in museums showcases and storage cabinets. The outcome of all this research is dispersed in the literature due to the different backgrounds of the researchers involved (corrosion scientists, archaeometers, conservation scientists) as well as the audience to which the publications are addressed to (industry, engineering, conservation of cultural heritage).
Most of this research is based on the study of samples taken from the core of the metal under investigation. Until the research of Bertholon on the visual description of corrosion products and their structure (Bertholon 20011 ), it was impossible to adequately describe corrosion phenomena in a standardized way and refer then to existing models. Some conservation training programmes such as at HE-Arc CR, are trying to implement this method to diagnose alterations observed on both studied and unstudied heritage metals. MiCorr is the logical continuation of these efforts that will allow a broader community to adopt this methodology and improve their diagnosis on heritage metals.
The research methodology of MiCorr follows the steps of a conservation project. It starts with the visual observation of the altered object, continues with the diagnosis and ends with the proposal of conservation treatments. Figure 1 illustrates these 3 steps:
Fig. 1: Schematic representation of the research methodology.
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