The Irreversibility of the Passive State of Carbon Steel in the Alkaline Concrete Pore Solution under Simulated Anoxic Conditions

Abstract

The commonly observed irreversibility of the passive state on carbon steel in alkaline solutions was examined in simulated concrete pore solution under anoxic conditions replicated by applying relatively negative film formation potentials, at which the cathodic process dominates the passive system. The fundamental source of this irreversibility was investigated by describing the kinetics of the passive system via a previously developed mixed potential model that combines the point defect model accounting for the anodic process and the Butler-Volmer equation accounting for the cathodic process. Electrochemical impedance spectroscopy, Mott-Schottky analysis and model optimization were performed at each potential when the potential was first stepped in the anodic direction and then in the cathodic direction. The experiment and optimization results demonstrate that the irreversibility of the passive state is closely associated with the discrepancy in the defect structure of the passive film upon opposite stepping directions of the film formation potential, and is essentially caused by the slower film formation and slower film dissolution during the cathodic potential stepping than those during the anodic potential stepping.The deep geological repositories for the storage of high level nuclear waste in Belgium have been proposed to adopt the "supercontainer" concept. 1-3 Corrosion of the carbon steel overpack in concrete pore water (pH of 13.5 at 25 • C 4 ) and under anoxic conditions characterized by very negative corrosion potential due to continuous oxygen consumption and following hydrogen buildup 5 has been determined to be the main threat to the integrity of the supercontainer structure. A huge amount of work 6-27 has been done on the corrosion of iron and steels in highly alkaline environment, including concrete pore solution, where the phenomenon of passivity breakdown that is usually induced by chloride, carbonate, and/or sulfide ions, et al. [6][7][8][9][10][11][12][13][14][15] and the kinetics and composition of the formed passive film 16-27 are the subjects being primarily focused on. However, very few studies employed sufficiently negative potentials that are comparable to the corrosion potential inside the supercontainer, which could reach down to −0.8 V SHE . 28 At such negative potentials, the cathodic reaction dominates the corrosion system and thus the information regarding to the partial anodic process which is required to study the passivity electrochemically, such as the anodic corrosion current density and the impedance associated with the anodic reactions, cannot be obtained directly, a difficulty that, however, has been solved previously by using a mixed potential model proposed by the authors. 29 Additionally, the passive carbon steel also exhibits apparent corrosion irreversibility when the film formation potential shifts in opposite anodic and cathodic directions; specifically, a strong resistance to cathodic potential reduction of the passive state is evident if a pre-exiting passive film h...

Studies on Pitting Corrosion of Al–Cu–Li Alloys Part III: Passivation Kinetics of AA2098–T851 Based on the Point Defect Model

“…Figure 5 compares the EIS spectrum scanned from high-to-low frequency with that obtained immediately afterwards from low-to-high frequency after 24 h polarization at 300 mV SCE during anodic potential stepping in 0.1 M NaHCO 3 in a CO 2 atmosphere. Since, in a steady state system, the current density and thickness of the passive layer are both time-independent [21], the good agreement of the EIS results in Figure 5 validates the stability of the system.…”

“…It is important to note that the observed current density comprises both anodic and cathodic components. Although the partial anodic current density is predicted to be independent of E app , there exists a competition between partial anodic and partial cathodic currents in determining the total (observed) current [21]. This observation has been made by other authors [21,28] and shows the importance of considering the cathodic current produced by hydrogen evolution due to the water reduction in deaerated condition when considering the electrochemistry of reactive metals, such as Al, Li, Mg, and Zn.…”

“…According to the PDM, the passive film formation is controlled by the kinetics of the point defects generation and annihilation at the metal/barrier layer (m/bl) and the barrier layer/outer layer (bl/ol) interfaces. A newly modified PDM, the mixed potential model (MPM), considers the influence of the partial cathodic reaction on the passive layer properties in addition to the partial anodic process [21].…”

“…It has been reported that the kinetics of passivation (i.e., the rate with which the passive state is formed) depend on the potential stepping direction [21]. This inequality of certain kinetic processes leads to the irreversibility of the metallic passivation in opposite potential scanning directions.…”

Numerical Investigation of the Role of Mill Scale Crevices on the Corrosion Initiation of Carbon Steel Reinforcement in Concrete

“…[1][2][3][4][5][6][7] Whether they are represented in terms of total or free chloride concentrations, or chloride-to-hydroxide concentration ratio, Cl − /OH − , reported values of chloride thresholds for carbon steel reinforcement in concrete cover a wide range and have a large degree of variability. 9,10 The surface conditions of rebar, in particular, the presence of mill scale on the steel surface, is one of the potential causes of the reported uncertainty and variability in chloride thresholds.…”