Nano-scale Imaging of Corrosion: Application of Scanning Polarization Force Microscopy
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tPermanent address: Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-201, Shanghai 201800, PRC ABSTRACT Scanning Polarization Force Microscopy, a recently developed non-contact imaging technique, is employed for the study of sulfuric acid induced aluminum corrosion. The morphological changes occurring during the corrosion of aluminum by sulfuric acid droplets were imaged in situ with nanometer resolution using Scanning Polarization Force Microscopy. The results of these experiments not only demonstrate the potential applications of SPFM in imaging liquid surfaces and weakly adsorbed species, they also offer new insights into the mechanisms of atmospheric corrosion. INTRODUC(TION Recently we have developed a new type of scanning probe microscopy, Scanning Polarization Force Microscopy (5,6). In the SPFM technique, the electrically charged tip of an atomic force microscope scans the surface at a distance of a few hundred angstroms. The electric field is strongly concentrated in the vicinity of the tip apex and induces a polarization charge distribution on the substrate surface. It is the attractive force due to this polarization that is measured and used to produce an image. Due to the long range nature of the electric field, the attractive electrostatic force can be measured with the tip held a few hundreds of Angstrom away above the surface. This unique imaging feature allows the imaging of delicate surfaces such as liquids or other weakly adsorbed species with SPFM. In this present study, we demonstrate the potential application of this technique in the investigation of liquid-solid interfacial interactions
through the study of sulfuric acid induced aluminum corrosion. Atmospheric corrosion reactions involve submicron thick liquid films and droplets from rain or dew deposition on metal or metal oxide surfaces. The corrosion kinetics are governed by both the solid-liquid and liquid-air interfacial interactions (1-4). Standard contact-mode imaging techniques, which are unable to image liquids, cannot supply a complete picture of the corrosion process. In this context, SPFM is an excellent technique for studying corrosion processes since it allows for nanometer- scale imaging of liquid surfaces and liquid-solid interfacial phenomena such as capillarity and wetting. By operating the SPFM instrument in the non-contact mode, the morphological changes of the liquid droplets as a result of corrosion reaction can be monitored in situ.. In addition, the SPFM can also be operated in the contact mode where the SPFM functions as a normal Atomic Force Microscopy (AFM). In this mode, only the solid substrate is imaged while any liquid deposit is locally displaced by the tip. In our experiments, contact imaging mode was also used to confirm the formation of solid precipitates from the corrosion reactions. EXPERIMENT In an earlier paper, we reported the results of in situ infrared reflection absorption
spectroscopy (IRAS) experiments on the sulfuric acid-induced corrosion of thin Al films (7).
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