Nano-scale Investigation of Interactions of Chlorides with Oxides That Form on Carbon Steel in Concrete Pore Solutions
Passivation and chloride-induced depassivation of carbon steel in concrete are processes that take place in nano scale. Spectroscopic and microscopic studies in this scale demonstrate that the thickness of the oxide films that form on carbon steel in the passivating environment of concrete pore solutions is typically in the range of 3–15 nm. Nano-scale analytical studies show that oxide films that form on carbon steel exposed to simulated concrete pore solutions consist of two layers separated with an indistinct border. The thinner FeII-rich inner oxide film is protective but unstable in the presence of chlorides. The thicker FeIII-rich outer oxide film is unprotective and likely porous. Chloride exposure decreases the thickness of the oxide films and changes their stoichiometry such that near the film/substrate interface FeIII/FeII ratio increases. Electrochemical quartz crystal nano-balance studies coupled with electrochemical observations provide kinetic evidence of how chlorides interact with these two layers and transform protective oxides into unprotective ones.
KeywordsCorrosion Passive film Rebar Chloride Concrete
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