Corrosion products transformations during subcritical stabilization of archaeological iron artefacts

摘要

Stabilization of archaeological iron artefacts is essential to avoid the loss of the information contained on the original surface due to the transformation of the corrosion product microstructure. The nature of the corrosion phases is determined by the environmental conditions such as the oxygen rate, alkalinity and presence of chlorides during periods of burial or immersion. After excavation, these conditions are disturbed and the corrosion layers transform rapidly. Severe reactivation of the corrosion processes may also occur. Therefore, restorers carry out stabilization treatments in order to remove elements responsible of this phenomenon such as chlorides. These processes, mainly based on immersing artefacts in chemical baths also induce microstructural transformations . One of these processes recently developed in the conservation field consists of pressurizing and heating an alkaline fluid (NaOH, 35 bar, 180℃) to the subcritical liquid phase. In our study we try to understand the impact of the process on the archaeological corrosion products .rnA-CORROS Expertise has established a prototype chamber to perform several experiments on synthetic and archaeological samples. Phases commonly found on corroded archaeological artefacts have been synthetized, treated at several temperatures and pressures and then analyzed with Raman spectroscopy, X-ray diffraction and SEM-EDX. The crystalline structure of some ferric oxyhydroxides such as akaganeite (chlorinated, β-FeO_(1-x)(OH)_(1+x)Cl_x), lepidocrocite (γ-FeOOH) or oxides such as ferrihydrite have transformed into hematite (α-Fe_2O_3). For others, the nature of the phases remains the same (magnetite Fe_3O_4, hematite, goethite α-FeOOH) but the process modifies the crystals’ morphology. The archaeological phases’ reactivity during the process is compared to synthetic phases’ behavior. Furthermore, cross-sections of representative sets of artefacts from several archaeological environments are analyzed before and after treatment. They are treated in the experimental chamber at various durations to follow the corrosion profile transformation steps with a specific attention to the crystals’ morphology evolution during the subcritical treatment. The understanding of chemical and physical transformations of iron corrosion layers inside the sub-critical chamber could optimize the stabilization of archaeological artefacts.