Corrosion product transformations in alkaline baths under pressure and high temperature: The sub‐critical stabilisation of marine iron artefacts stored under atmospheric conditions

Abstract

International audienceIn the context of conservation of ferrous cultural heritage, dechlorination processes of archaeological iron artefacts are studied in subcritical conditions which consist of immerging an artefact inside an alkaline bath under pressure (30 bar) and temperature (160 °C). Submarine archaeological environments lead to the presence of chlorides inside the corrosion layers. After exposure to the atmosphere following excavation, initial corrosion phases evolve. Chlorides remain trapped inside the structure of corrosion products particularly during the crystallisation of akaganeite. Stabilisation treatment objectives are to remove chlorides from the corrosion layers and to obtain more stable phases. A Roman archaeological bar has been studied on cross-section before and after treatment in sub-critical conditions. Less stable phases, such as ferrihydrite and akaganeite, transformed into hematite. That was confirmed by the use of synthetic and pure phases treated in the same conditions. The presence of goethite before and after the treatment in the corrosion profile raises the question of its reactivity during dechlorination process. Akaganeite has been synthesised and compared to archaeological powder sampled on an artefact. The results highlight reactivity differences indicating that sub-critical transformation processes are not only dependent on the nature of phases but also on intrinsic physical characteristic

FTIR spectroscopic semi-quantification of iron phases: A new method to evaluate the protection ability index (PAI) of archaeological artefacts corrosion systems

“…The quantitative analysis of iron corrosion can also find reliable applications in the stages following the conservation works. In the short term, it can be used to control whether conservation treatments were able to stabilize the reactive corrosion phases [11]. In the long term, it can also be used to identify any new corrosion processes that may be entailed by the interaction between the object and the storage/exhibition environmental conditions [12,13].…”

ZVI (Fe0) Desalination: Stability of Product Water

“…Mineralogical studies [40,41] and museum conservation studies [30][31][32][33][34][35][36] have established that both Na + and Cl´ions can be removed from the ZVI corrosion products by washing. Displacement of Na + and Cl´ions concentrated in the ZVI may occur when a batch of product water is replaced by a new batch of feed water [28][29][30][31][32][33][34]38,39].…”

An Investigation of Electrochemical Dechlorination of Wrought Iron Specimens from the Marine Environment

“…The usual conservation methods aim for the extraction of chloride ions from the objects. Most of these treatments are based on the immersion of iron artifacts in high pH alkaline solutions [10][11][12]. The most common treatments of dechlorination and the stabilization of metallic objects include electrolysis, hydrogen plasma, and subcritical fluid, and the most common of all are simply the immersion in alkaline solutions and the alkaline reduction method [13,14].…”