Use of X-ray and other techniques to analyse the phase transformation induced in archaeological cast iron after its stabilisation by the electrolytic method

Abstract

Electrolytic treatment at low currents intensities has been applied to a piece of cast iron, with the object of optimising a suitable methodology for the conservation of archaeological objects of iron. This technique has proved to be effective in the extraction of chloride from the structure of akaganeite (β-FeOOH(Cl x )), the principal corrosion product of iron in the marine medium. The effectiveness of the electrolytic treatment has been proven by applying the Rietveld method to the patterns of X-ray powder diffraction of samples extracted from the corroded surface before and after treatment. This method has permitted the unequivocal determination of akaganeite by means of the deconvolution of the diffraction peaks overlapping those of other components of the sample. At the same time, the chemical composition of this non-stoichiometric iron oxyhydroxide has been determined. The presence or absence of chloride has been corroborated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy. After the electrolytic treatment, akaganeite was not present in the sample. Thus, Rietveld analysis has been shown to be a very useful tool for monitoring the chloride extraction process, and for determining the presence of this oxyhydroxide of iron.