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, chloride-induced 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 point-defect model.
Highlights
In highly alkaline electrolytes, typically those with pH greater than 12, a protective passive film forms on iron that reduces metal dissolution rates from the substrate to the electrolyte to levels that can be considered practically insignificant.[1,2] The passivity of iron in alkaline media allows the use of carbon steel in reinforced concrete, which makes up the majority of the modern infrastructure
NaOH was chosen to match the pH of typical concrete pore solutions, the authors acknowledge that real concrete pore solutions are complex and contain several other ions, such as Ca+2, K+, (SO4)−2 9,16. These ions are known to affect the passivation process and the properties of the passive film, as shown in an earlier work of one of the co-authors[9,16]; here we study a simplified electrolyte to gain fundamental understanding into the dynamic processes that lead to a chloride-induced depassivation process in a simple high pH electrolyte
It should be noted that FeO is not a stable oxide on its own at room temperature, but it can exist as a transitional phase at the interface between the metal and the passive film, where the iron ions from the base metal is initially oxidized.[20]
Summary
Typically those with pH greater than 12, a protective passive film forms on iron that reduces metal dissolution rates from the substrate to the electrolyte to levels that can be considered practically insignificant.[1,2] The passivity of iron in alkaline media allows the use of carbon steel in reinforced concrete (pH > 13), which makes up the majority of the modern infrastructure. All studies were performed on pure (and in the case of ReaxFF-MD simulations, defect-free) iron, instead of carbon steel, in order to eliminate the effects of possible confounding variables that might hinder the answering of fundamental questions related to the passivation and chloride-induced depassivation processes in alkaline electrolytes. It should be noted that FeO is not a stable oxide on its own at room temperature, but it can exist as a transitional phase at the interface between the metal and the passive film, where the iron ions from the base metal is initially oxidized.[20] These results are in agreement with the existing experimental data, which indicate that passive films of iron in highly alkaline electrolytes consist of Fe2+-rich inner oxide layers and Fe3+-rich outer oxide layers.[14,15,16,17,18,19,20] Similar supporting evidence was obtained in our XPS investigation, which was performed on a passive film
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