Nano-scale TEM studies of the passive film and the chloride-induced depassivation of carbon steel in concrete

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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 increased rates of rebar corrosion which may have considerable safety implications. 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. Since the passive films are typically measured in nano scale, a detailed compositional and morphological study cannot be carried out by traditional electrochemical (e.g. EIS) or microscopic techniques (e.g. SEM). In this study, highly specialized nano-scale TEM microscopic techniques (i.e., bright-field and dark-field TEM imaging, selected area electron diffraction – SAD, convergent beam electron diffraction – CBED, energy-dispersive X-ray spectrometry – EDS, and electron energy loss spectroscopy – EELS) have been used to characterize the passive films and to identify their depassivation mechanisms under different exposure conditions. The TEM investigations were carried out on 100-nm-thick electron-transparent samples obtained with focused ion beam (FIB) method. The unique way of preparing the TEM samples from the surface of the rebar using FIB method allowed us to make important observations on the compositional and morphological characteristics of the passive film, as well as on its depassivation pattern after it comes in contact with chlorides.

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Journal: TechConnect Briefs
Volume: 1, Nanotechnology 2010: Advanced Materials, CNTs, Particles, Films and Composites
Published: June 21, 2010
Pages: 49 - 52
Industry sector: Advanced Materials & Manufacturing
Topic: Materials Characterization & Imaging
ISBN: 978-1-4398-3401-5