Passive corrosion of steel in concrete in context of nuclear waste disposal

A large experimental campaign has been developed in order to study the passivity of steel embedded in mortar. Mortar samples, containing steel pill (initially polished or precorroded), have been placed in various conditions [relative humidities of 80, 90 and 95%; solution simulating the clay rock underground water of the French candidate site of Bure at two temperatures (25 and 50 degrees C) and in aerated and anoxic conditions]. The passive corrosion behaviour of steel has been assessed after 6 months and after 1 year for each experiment. Gravimetric measurements (average corrosion rate evaluation), X-ray photoelectron spectroscopy and Raman spectroscopy characterisations (identification of the passive film and corrosion product nature) as well as optical and electronic microscopy analyses (corrosion layer thickness and steel/mortar interface elementary analysis) have been performed. Such analyses have been also carried out after 15 days and 1 and 3 months in order to consider the first stages of passivity in mortars. The results show that average corrosion rates determinate for polished steels are typical of passive conditions (<1 mu m/year) and vary from 0.4 to 5 mu m/year for the precorroded pills. Moreover, iron is detected within the cementitious material, up to 5-50 mu m from the metal, indicating the transport of iron species.