Structural characterization of corrosion products on archaeological iron: an integrated analytical approach to establish corrosion forms

Abstract

The description and identification of corrosion products formed on archaeological iron artefacts need various approaches at different observation scales. For this study, samples from five sites were prepared using two techniques. The first consists in cutting cross-sections perpendicular to corrosion layers. This allows local observations and analysis of the corrosion layer stratigraphy at different levels. The second consists in performing manual grinding or abrading of the corrosion layers starting from the current surface of the excavated artefact to the metal core. It allows the description of the successive layers and is well adapted for the analysis on a larger scale. In addition to these two observation scales, the identification of the iron oxides formed needs the coupling of several complementary techniques. Elementary compositions were determined by scanning electron microscopy-energy-dispersive x-ray (SEM-EDX) analysis and electron probe microanalysis (EPMA). Structural identification was performed by x-ray micro-diffraction under synchrotron radiation (µXRD) and micro-Raman spectroscopy. These analyses were performed on the same samples with both x-ray diffraction and Raman spectroscopy in order to ensure a reliable characterization. In some cases there are some ambiguities or overlapping between signatures of different phases by µXRD (such as maghaemite-magnetite) or Raman spectroscopy (such as goethite-magnetite) which can be overcome by the association of the two methods. The final aim is to set up an analytical methodology that will be optimal for the study of ancient iron corrosion products. It is the first step in the study of long-term mechanisms of iron in soil.

A Micro-Raman Spectroscopic Study of Marine Atmospheric Corrosion of Carbon Steel: The Effect of Akaganeite

“…Akaganeite is formed only in the presence of salinity 7 and was widely found in marine environments 12,13 and in chloride-rich solutions [14][15][16] and soils. [17][18][19][20] Raman spectroscopy, as a powerful non-destructive spectroscopic technique, has been successfully used for the identification of pure iron oxides [21][22][23][24] and steel corrosion products. [8][9][10]12,[14][15][16][17][18][19][20][24][25][26][27][28][29][30] Its ability to distinguish between the various phases of iron oxides and oxyhydroxides, which are normally found in atmospheric corrosion products, is extremely helpful in the identification of corrosion products.…”

The efficiency of chloride extraction from archaeological iron objects using deoxygenated alkaline solutions

“…Data from collection surveys must be viewed with caution, as the consistent measurement of existing condition in comparison with original condition is difficult and subjective, whereas other variables such as environmental parameters may not be well controlled and a reliable control sample is not always available. Nevertheless, current knowledge with regard to the form, location, and action of chloride ions in iron supports this qualitative assessment that treatment imparts enhanced stability (Neff et al, 2004(Neff et al, , 2005Réguer et al, 2005Réguer et al, , 2006Réguer et al, , 2007aRéguer et al, , 2007bRéguer et al, , 2009Guilminot et al, 2008); quantitative data are required to prove this.…”

Raman spectroscopy in art and archaeology

“…Within Raman spectroscopic research on archaeological and artistic objects, different disciplines can be distinguished. Some groups focus on ceramics, 12 -14 others on glasses, 15,16 mural paintings, 17 -19 works on paper, 20,21 geological materials, 22,23 corrosion products 24,25 and biomaterials. 26 -28 Pigments and binding media in painted artworks are often examined using Raman spectrosocpy.…”