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.

Mechanisms of passivation and chloride-induced corrosion of mild steel in sulfide-containing alkaline solutions

“…However, in the mildly reducing binders, the onset of pitting is much likely to occur due to the imperfect passivation capability of the binder, as well as the ability of the system to sustain the cathodic reaction of oxygen reduction, leading to the formation of a macro-cell. Similar observations have been reported by various authors [14,26,64], indicating a higher susceptibility of steel to chloride-induced corrosion for binders with mildly reducing pore solutions when compared to binders with highly oxidizing or highly reducing pore solutions. However, as the onset of chloride-induced corrosion is highly dependent on the availability of dissolved oxygen at the steel-concrete interface, the transport of oxygen through the binder is also highly influential in defining the actual resistance to corrosion initiation, which is likely to account for the welldocumented excellent durability of PC-slag blends, as these cements are known to yield concretes with high mass transport resistance [65,66].…”

Formation of Iron Sulfides on Carbon Steel in a Specific Cement Grout Designed for Radioactive Waste Repository and Associated Corrosion Mechanisms

“…Because the solution used to saturate the cement grout did not contain sulfide species, these species necessarily come from the grout. This result is consistent with those observed in previous study [15].…”

“…Weight loss measurements performed on the three coupons confirmed that the degradation (if any) was negligible. The steel coupons more likely remained in a passive state during the 8 weeks of setting at RT, as observed in a previous study [15] achieved in aerated conditions.…”

“…In this work, carbon steel electrodes covered by a 2-cm-thick layer of the grout were immersed in a 0.01 M NaCl + 0.01 M NaHCO 3 solution (pH 7 measured at 20 • C) at 80 • C in aerated conditions. A passive to active transition was observed in a few days, indicating that in these experimental conditions the metal could not remain passive for a long time [15]. It was also shown that iron sulfides could form as corrosion products on the steel surface.…”

“…Only one preliminary study dealing with the corrosion of carbon steel in the specific cement grout envisaged for radioactive waste disposal was achieved [15]. In this work, carbon steel electrodes covered by a 2-cm-thick layer of the grout were immersed in a 0.01 M NaCl + 0.01 M NaHCO 3 solution (pH 7 measured at 20 • C) at 80 • C in aerated conditions.…”