Pitting of carbon steel in the synthetic concrete pore solution

Abstract

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.

Point defect model for passivity breakdown on hyper-duplex stainless steel 2707 in solutions containing bromide at different temperatures

“…For a potential scan rate of zero, i.e., V c (v = 0), the breakdown potentials of HDSS 2707 in 1 M NaBr solution at 70 ℃, 80 ℃ and 90 ℃ are calculated as 0.604 VSCE , 0.575 V SCE and 0.500 V SCE , respectively. Thus, the critical breakdown potential, V c (v = 0), decreases with increasing temperature, implying that during an immersion test, where the OCP gradually increases with time (as predicted theoretically by the PDM [42] confirmed by the 343 days of measured OCP values for P355 QL2 carbon steel in Ca (OH) 2 + NaOH solution [35]), the corrosion potential (ECP) ultimately may become greater than the critical breakdown potential, thereby ensuring spontaneous pitting [43,44] is the PDM postulates that the cation vacancies are apt to accumulate at the site of inclusions (e.g., MnS [45]), dislocations [46] and precipitates (e.g., M 23 C 6 [47]); these being sites of high lattice disorder at their intersection with the barrier layer that are characterized by high cation vacancy diffusivity in the layer. Accordingly, these are the preferred sites for pit nucleation.…”

“…The PDM expresses the dependence of the E b (or V c ) on temperature, pH, and anion activity as given by Eqs. ( 10) to (12), which show that V c decreases linearly with increasing log(a x ) and with decreasing pH, where a x is the activity of the aggressive anion (e.g., Br -), [35].…”

“…Thus, at higher bromide concentrations, more cation vacancies are produced, and the excess vacancies are envisioned to condense either on the cation sublattice or on the lattice of substrate metal at the m/bl interface, resulting in the separation of the bl from the substrate metal. The passive film eventually ruptures under the dissolution at the bl/s interface, augmented by stress, and passivity breakdown and subsequently pitting occurs [35]. The inflection point at which the current density suddenly increases is defined as the breakdown potential (E b ), and the stable pitting morphology is inspected in Fig.…”

Mixed potential model for passivity characters of hyper-duplex stainless steel 2707 in ammonium carbonate solution containing chloride

“…According to the PDM, the passive film on the steel surface is sensitive to the kinetics of the point defects, which are generated and annihilated at the metal/film (m/f) and film/ solution (f/s) interfaces, as shown in Fig. 1 [37,38]. Therefore, the passive film is postulated to comprise a bilayer structure of a highly-defective barrier layer that grows into the metal, and a precipitated, porous layer that grows into the solution [8,39,40].…”

“…The bl provides a transmission path for the point defects including cation vacancies (V χ' M ), oxygen vacancies (V ⋅⋅ o ), and cation interstitials (M χ+ i ) that are generated and annihilated at the bl interfaces [23,24]. Because of a high electric field (ԑ) that exists within the bl (1 ~ 3 MV/cm), these defects are transported across the bl by migration rather than by diffusion [25].…”