Nano-scale TEM Studies of the Passive Film and Chloride-induced Depassivation of Carbon Steel in Concrete

摘要

In the highly alkaline environment of concrete, carbon steel rebar is protected against corrosion by a passive iron oxide film. The partial or complete loss of the protective oxide film, known as depassivation, leads to increased rates of rebar corrosion. Understanding the compositional and morphological characteristics of the passive film on rebar, and how it depassivates, is key to mitigate problems associated with corrosion of rebar in concrete. In this study, nano-scale TEM techniques have been used to characterize the passive films on carbon steel in surrogate concrete pore solutions. In chloride-free solutions, oxide films on the samples had a relatively uniform thickness that ranged from 5 to 8 run, however, after exposure to chloride in amounts greater than typical depassivation thresholds, the thickness of the films became more variable and thinner on average. TEM dark-field images showed the oxide to be comprised of crystalline nano-particles with widths less than 8 nm.