In situ structural characterisation of nonstable phases involved in atmospheric corrosion of ferrous heritage artefacts

The prediction of very long term corrosion of iron and low alloy steel in atmospheric conditions or in hydraulic binder media is a crucial issue for the conservation and restoration of heritage artefacts. For both media, the typical iron corrosion product layers (CPL) can be described as a matrix of goethite (alpha-FeOOH) crossed by marblings of reactive phases: maghemite (gamma-Fe2O3), ferrihydrite (Fe5HO8 center dot 4H(2)O), feroxyhyte (delta-FeOOH), etc. The aim of the experiments presented here is to bring new insights on the role that the maghemite could potentially play in the mechanisms of corrosion. For that purpose, electrochemical reductions have been coupled with in situ Raman microspectroscopy. These experiments enable the authors to propose a hypothesis of local mechanisms in the specific case of marblings of maghemite connected to the metallic substrate. These local mechanisms could drastically influence the global corrosion rate.