Abstract
This work used the unique capability of synchrotron x-rays to probe a buried polymer/metal interface with high energy resolution and spatial resolution on the micron scale. X-rays of sufficient energy can penetrate the organic polymer coating and interrogate heavier elements found at the interface. X-ray fluorescence (XRF) spectra can be collected provided the element specific energy of the fluoresced photon is sufficient to penetrate through the polymer coating. We characterized the interface beneath several polymer coatings on steel reinforcing bar after exposure to alkaline potash brine using the Soft X-ray Microcharacterization Beamline (SXRMB) at the Canadian Light Source. Characterization included correlated XRF maps for Fe, Mn, K and Cl as well as micro x-ray absorption near edge spectroscopy (µ-XANES) to analyze Fe corrosion products at certain areas of interest. By collecting Fe XRF maps with an incident beam energy of 7115 eV, we exploited the differences in absorption cross section between Fe metal and oxidized Fe species at this energy. The result is a map with excellent contrast between corroded and non-corroded regions of the interface. This provides a relatively quick means to locate localized anodic zones beneath opaque polymer coatings without a visual indicator (i.e. a blistered region of polymer). XRF maps for K and Cl showed these elements were segregated at the interface with K hotspots providing a surrogate measurement for cathodic activity at the interface. µ-XANES analysis showed evidence of Fe corrosion products that varied somewhat from those reported in previous studies, owing to our efforts to perform sample handling in an inert atmosphere after the exposure to potash brine. The speciation of Fe combined with the spatial distribution of Mn, K, Cl provided evidence to support previous literature models pertaining to mechanisms of coating delamination and the spread of corrosion at the polymer/metal interface.
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