Mechanisms Of Localized Corrosion Of Carbon Steel Associated With Magnetite/Mackinawite Layers In A Cement Grout

Carbon steel electrodes covered with a specific low-pH cement grout (pH similar to 10.7 at 20 degrees 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 degrees C), in aerated conditions, at 80 degrees C. The corrosion processes were studied by voltammetry and linear polarization resistance measurements while the corrosion product layers were analyzed by mu-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.