Investigation of chloride-induced depassivation of iron in alkaline media by reactive force field molecular dynamics

Abstract

The passivity of iron in alkaline media enables the use of carbon steel as reinforcement in concrete, which makes up the majority of modern infrastructure. However, chlorides, mainly from deicing chemicals or marine salts, can break down the iron passive film and cause active corrosion. Despite recent advances in nanoscale characterization of iron passivity, significant gaps exist in our understanding of the dynamic processes that lead to the chloride-induced breakdown of passive films. In this study, chlorideinduced depassivation of iron in pH 13.5 NaOH solution is studied using reactive force field molecular dynamics. The depassivation process initiates by local acidification of the electrolyte near the film surface, followed by iron dissolution into the electrolyte, and iron vacancy formation in the passive film. Chlorides do not penetrate the passive film, but mainly act as a catalyst for the formation of iron vacancies, which diffuse toward the metal/oxide interface, suggesting a depassivation mechanism consistent with the pointdefect model.

Thermodynamic feasibility of the four-stage chloride-induced depassivation mechanism of iron

“…These lower oxidation state sites can adsorb up to three Cl while regular Fe sites (Fe III ) have weaker Cl interactions and can adsorb one Cl at most. This is consistent with the adsorption configuration observed in the Reaxff-MD study, where lower charged Fe atoms interact more readily with multiple Cl or OH adsorbates 7 .…”

“…The iron vacancy, resulting from iron complex dissolution, is represented using dashed circle. Previous studies 7,8 show that these vacancies diffuse through the oxide film toward the film/metal interface to create discontinuities eventually leading to detachment/ breakdown of the film, similar to the mechanism proposed by the point defect model. These stages are not shown in this figure since they were not studied herein.…”

“…Therefore, in the third and fourth stages, only one or none of the adsorbed chloride is replaced, forming Fe(OH)Cl 2 and FeCl 3 respectively. These surface species can dissolve into the solution, releasing chlorides back to the solution, while creating iron vacancies that diffuse toward the oxide-film/metal interface as suggested in previous studies 7,8 . This indicates that chloride can act as catalyst in the depassivation process consistent with previous density functional theory (DFT) studies 8,9 , while also providing support for the point defect model [10][11][12][13] .…”

“…The atomistic depassivation mechanism of iron is still not well understood, but a four-stage chloride-induced depassivation mechanism was proposed recently based on reactive force field molecular dynamics (Reaxff-MD) simulations 7 . In this study, the depassivation process was investigated using an iron passive film created by an earlier Reaxff-MD study 6 and various NaCl concentrations (2 M, 5 M and 10 M) in 0.316 M NaOH solution at a pH ≈ 13.5 with a total simulation time of 500 s at an applied external potential of 30 MeV.…”

Chloride‐induced steel corrosion in alkali‐activated fly ash mortar: Increased propensity for corrosion initiation at defects

“…Chloride‐induced steel corrosion in reinforced concrete structures is one of the most important deterioration mechanisms limiting their service life. [ 1 ] Chloride ions that reach the steel–concrete interface cause, above a certain concentration, depassivation and stable pitting corrosion of the steel, [ 2–5 ] eventually leading to significant loss of reinforcement cross‐sectional area and potentially to a collapse of the structure. The time to the onset of chloride‐induced reinforcement corrosion is thus determined by the rate of chloride ingress into the concrete and the critical chloride content ( c crit ; also referred to as chloride threshold value) of the steel/concrete system.…”