Mechanisms of localized corrosion of carbon steel associated with magnetite/mackinawite layers in a cement grout

• Published in: Materials and corrosion Vol. 72; no. 1-2; pp. 194 - 210
• Main Authors: Robineau, Mathieu, Sabot, René, Jeannin, Marc, Deydier, Valérie, Crusset, Didier, Refait, Philippe
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01-01-2021
• Subjects: Aeration; Carbon steel; Carbon steels; Corrosion; Corrosion effects; Corrosion products; Corrosion resistance; Electrode polarization; Electrodes; Grout; Iron oxides; Linear polarization; Localized corrosion; Magnetite; Radioactive waste disposal; Radioactive wastes; Raman spectroscopy; Sodium bicarbonate; voltammetry; Waste management; µ‐Raman spectroscopy
• ISSN: 0947-5117; 1521-4176
• DOI: 10.1002/maco.202011696

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.
Depassivation of steel occurs rapidly after immersion in the aggressive solution. A corrosion product layer mainly composed of magnetite then grows on the steel surface. It is, however, heterogeneous in most cases, with mackinawite forming locally. A localized corrosion process is associated with these magnetite/mackinawite layers that are detrimental in the aerated conditions considered here.