Passive corrosion of steel in concrete in context of nuclear waste disposal
Résumé
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 pre-corroded), 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 μm/year) and vary from 0.4 to 5 μm/year for the pre-corroded pills. Moreover, iron is detected within the cementitious material, up to 5-50 μm from the metal, indicating the transport of iron species.