Raman imaging of ancient rust scales on archaeological iron artefacts for long‐term atmospheric corrosion mechanisms study

Abstract

Iron archaeological artefacts were studied to understand long-term corrosion by the atmosphere. Indeed, these samples collected on the construction elements of ancient monuments present ancient rust layers formed during their exposure over centuries to the indoor atmosphere. Thanks to Raman spectroscopy and the acquisition of hyperspectral images of the corrosion scales, several zones of the samples observed on cross sections could be characterised. It has been shown on six dated samples that the main phase is goethite (a-FeOOH). Lepidocrocite and akaganeite (g-FeOOH and b-FeOOH) occur locally in the corroded products, often correlated with cracks. A less crystallised phase, a hydrated oxy-hydroxide, has been identified abundantly in more or less extended zones inside the layer. This phase could play an important role in atmospheric corrosion mechanisms.

Atmospheric corrosion of mild steel

“…(a) In atmospheres with low Cl -pollution (3 < Cl -≤ 60 mg/m 2 /day) and low SO 2 pollution (10 < SO 2 ≤ 35 mg/m 2 /day) the atmospheres showed corrosivity categories C2-C3 according to ISO 9223 [81] and presented relatively compact and rounded structures characteristic of the presence of SO 2 , especially composed of lepidocrocite and goethite.…”

“…(b) Higher Cl -pollution (60 < Cl -≤ 300 mg/m 2 /day) of the atmosphere meant higher steel corrosion rates of to C3 to ≥ C5 [81] . The incorporation of chloride ions in the open steel corrosion product layer structures seems to have a detrimental effect on atmospheric corrosion resistance.…”

“…(c) A higher SO 2 concentration (35 < SO 2 ≤ 80 mg/m 2 /day) led to significant stabilisation of the steel corrosion layers with exposure time and consequently a significantly lower steel corrosion rate (C4) [81] . showing a region rich in chlorides (dark area).…”

Comparing the effectiveness of hyperspectral imaging and Raman spectroscopy: a case study on Armenian manuscripts

“…Raman mapping has been used to examine areas on the micrometre scale over several hours. For example, one archaeological study examining rust took 21 h to image a 52.2 × 46.2 µm rectangle [28], another looking at cross sections of a sixteenth century painting took 4.5 h to image a 60 × 45 µm rectangle [29]. An area can be mapped simultaneously by defocussing the laser beam, but this greatly reduces the intensity and as such is limited to a field of view on the order of micrometres [30].…”

Técnicas Arqueométricas Combinadas Aplicadas Al Análisis De Diseños De Alfarería "Tricolor" De Quebrada De Humahuaca, Jujuy, Argentina

“…Para conocer la composición de los pigmentos empleados para la obtención de los diferentes colores se empleó espectroscopia Raman, técnica adecuada para caracterizar pigmentos (Magistro et al 2001;Neff et al 2006;Vandenabeele et al 2007). La misma se aplicó sobre superficies cerámicas y sobre micromuestras tomadas para el estudio de secciones transversales.…”