Pitting of carbon steel in the synthetic concrete pore solution

• Published in: Materials and corrosion Vol. 72; no. 1-2; pp. 166 - 193
• Main Authors: Macdonald, Digby D., Qiu, Jie, Sharifi‐Asl, Samin, Yang, Jie, Engelhardt, George R., Xu, Yi, Ghanbari, Elmira, Xu, Aoni, Saatchi, Alireza, Kovalov, Danyil
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.01.2021
• Subjects: Aeration; Aqueous solutions; Barrier layers; Blistering; Breakdown; Carbon steel; Carbon steels; concrete pore solution; Failure; failure probability; high‐level nuclear waste (HLNW); Nucleation; Passivity; pitting corrosion; Radioactive wastes; Vacancies
• ISSN: 0947-5117; 1521-4176
• DOI: 10.1002/maco.202011875

Pitting corrosion is a possible mode of failure of the carbon steel overpack of the Belgian supercontainer concept for the isolation of high‐level nuclear waste (HLNW). However, no firm experimental data are currently available to estimate the probability of failure over the extended storage time (100,000 years). Extensive work shows that passivity breakdown results from the condensation of cation vacancies (CVs) at the metal/barrier layer (m/bl) interface, in response to the absorption of Cl− into oxygen vacancies at the surface of the barrier oxide layer. The CVs migrate across the bl to the m/bl interface where they condense, leading to the separation of the bl from the metal. The resulting blister prevents the growth of bl into the metal and dissolution results in blister rupture, marking a passivity breakdown event. Stabilization via differential aeration produces a potentially damaging, stable pit. We review our work on passivity breakdown and the nucleation of pits on P355 QL2 carbon steel in high‐pH aqueous media typical of concrete pore solution, with emphasis on the mechanistic aspects. We conclude that failure of the carbon steel overpack containing the HLNW over a storage horizon of 100,000 years is improbable. Our research reveals the dependence of the mean value of the critical breakdown potential (Vc) for P355 QL2 carbon steel in deaerated, sat. Ca(OH)2/borate buffers + 0.1 M NaCl on pH. The breakdown voltage shifts strongly in the positive direction with increasing pH because hydroxide (OH−) effectively competes with chloride [Cl−] for absorption into surface oxygen vacancies ( V O 2 • •)