Investigation of atmospheric corrosion layers on historic iron nails by micro-Raman spectroscopy

In this study, some Ottoman low carbon steel nails from a 16th century brickmasonry dome of Kilic Ali Pasha Bath in Istanbul and from a 19th century stone dwelling in Phocaea Izmir were studied for their long term corrosion characteristics. Their corrosion layers were examined with a Senterra Bruker micro-Raman spectrometer and Tescan Vega model Scanning Electron Microscopy (SEM) with Inca X-Stream 2 Energy-Dispersive X-ray spectroscopy (EDX). Powdered samples scraped from the corrosion layers were studied by X-ray powder diffraction and Fourier transform infrared spectroscopy. The corrosion layer consisted of two main sub-layers, namely an 'inner corrosion layer' including a 'reaction front' and a 'transformed medium' which included the depositions from the surrounding media. Peak intensities of X-ray powder diffraction traces were not proportional to the abundance of oxides and oxyhydroxides in the corrosion layer, most probably because of their partly amorphous and nano-size nature. Inner corrosion layer and transformed medium layers were studied in detail by micro Raman spectroscopy at 532 nm, 0.2 mW, 200 s. The Protective Ability Index (PAI) of the corrosion layers were expressed as the ratio of isotropic goethite band thickness to the thickness of reaction front composed of other oxides and oxyhydroxides being magnetite, maghemite, hematite, lepidocrocite, distinguished by micro-Raman which is a unique method for phase identification. PAI was then estimated to be similar to 6-10 for 16th century nails and similar to 3 for 19th century nails. Corrosion mechanisms and PAI were concluded to be affected by compositional characteristics of artefacts and their atmospheric conditions. Copyright (C) 2016 John Wiley & Sons, Ltd