Mechanisms of Localized Corrosion of Carbon Steel Associated With Magnetite/Mackinawite Layers in a Cement Grout

Abstract

Carbon steel electrodes covered with a specific low-pH cement grout (pH ~10.7 at 20°C), designed for nuclear waste management applications, were immersed for 30 days in a 0.01-M NaCl + 0.01-M NaHCO₃ solution (pH 7 measured at 20°C), in aerated conditions, at 80°C. The corrosion processes were studied by voltammetry and linear polarization resistance measurements while the corrosion product layers were analyzed by µ-Raman spectroscopy and X-ray diffraction. Most of the electrodes (75%) suffered from localized corrosion, a phenomenon associated with the formation of a heterogeneous Fe₃O₄/FeS layer. It is proposed that the mechanisms of the particular corrosion process observed here are associated with galvanic effects, the large magnetite-covered zone acting as cathode and the locally mackinawite-covered zones being anodic regions.