Abstract
The atomistic mechanism of chloride-induced depassivation of iron is still debated. A recent study suggests a four-stage depassivation mechanism, in general agreement with the point defect model. The proposed four-stage mechanism is based on reactive force field molecular dynamics simulations and is rather complex but here we use density functional theory to confirm the thermodynamic feasibility of the proposed mechanism. We find that the four surface species, formed in the four stages, have decreasing surface stability, which is consistent with the order of species formed in the depassivation process proposed in the reactive force field molecular dynamics study. The Fe vacancy formation energy, that is the energy needed to form a surface Fe vacancy by removing different surface species, indicates that surface species with more chlorides dissolve more easily from the surface, suggesting that chloride acts as catalyst in the iron dissolution process. The results are consistent with the suggested four-stage reaction mechanism and the point defect model.
Highlights
Concrete is a highly alkaline media in which a passive film forms on the steel surface, protecting it from active corrosion
The passive film used in the Reaxff-MD study is a mixed oxide film build through simulations of the oxidation process in an alkaline media[6]
The mixed oxide film is too complex for periodic density functional theory (DFT) calculations, but the critical Fe site, with lower charged Fe atom, is well represented by a step edge in a periodic crystal structure
Summary
Concrete is a highly alkaline media (pH > 13) in which a passive film forms on the steel surface, protecting it from active corrosion. An intermediate layer between the inner and outer layers has been reported[1,6] Aggressive ions such as chloride can cause depassivation and activate corrosion of iron, which is of great importance for the durability of the reinforced concrete structures that are exposed to the chloride-containing salts. In the third and fourth stages, only one or none of the adsorbed chloride is replaced, forming Fe(OH)Cl2 and FeCl3 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]. The Reaxff-MD simulations show that a local acidification followed by iron dissolution and vacancy formation can only occur if there is a sufficient number of chloride ions in the electrolyte, in general agreement with the critical chloride threshold concept
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
