7 - Long-term behaviour of iron embedded in concrete: from the characterisation of archaeological analogues to the verification of the oxygen reduction as the limiting step for corrosion rate

Abstract

This chapter discusses the characterization of archaeological ferrous artefacts corroded in binders to better understand the corrosion system and to give some significant parameters, such as the apparent diffusion coefficient of oxygen in the water of the pores of the Dense Product Layers (DPL) for phenomenological modeling of long-term iron corrosion. It presents the first stages of this modeling developed in the French Alternative Energies and Atomic Energy Commission (CEA) in the framework of the CIMETAL project. It presents a study of ferrous artefacts from archaeological sites to understand the corrosion system that can form in binders after long periods. In all the studied samples, the same phases were found in the DPL: mainly goethite containing magnetite and/or maghemite marblings. Other oxyhydroxides were noted on some samples (lepidocrocite and akaganeite). The chapter proposes a mathematical model, as demonstrated by the archaeological analogues analyses, assuming that the corrosion product layer is not conductive and using the apparent diffusion coefficient rate that was measured in the first part of the chapter. The initial results presented here show that the corrosion rate depends strongly on whether the binder on the one hand and the DPL on the other is saturated with water. When only the DPL is saturated with water, the calculated corrosion rates are in good agreement with the average corrosion rates measured on archaeological artefacts.