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Research Opportunities in Corrosion Science and Engineering
Scanning Probe Microscopy
Scanning probe microscopy (SPM) makes it possible to form images of surfaces using a physical probe that scans the specimen. An image of the surface is obtained by mechanically moving the probe in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a function of position. SPM began with the invention of the scanning tunneling microscope in 1981. Many scanning probe microscopy techniques can image several interactions simultaneously. The manner of using these interactions to obtain an image is generally called a mode. The resolution varies somewhat from technique to technique, but some probe techniques reach a rather impressive atomic resolution, owing largely to the ability of piezoelectric actuators to execute motions with a precision and accuracy at the atomic level or better on electronic command. One could rightly call this family of techniques “piezoelectric techniques.” The other common denominator is that the data are typically obtained as a two-dimensional grid of data points, visualized in false color as a computer image.77
A number of methods have applications in studies of corrosion, such as Kelvin probe atomic force and chemical force and scanning tunneling methods. Of these techniques, atomic force microscopy and scanning tunneling microscopy are the most commonly used for roughness measurements. NSOM and SNOM are scanning probe methods used to obtain optical imaging or some form of contrast.
Summary Observations on Instrumentation
Some of the techniques and tools outlined above have been used in corrosion research over the past 20 years. The future direction for these techniques is extension to heterogeneous corrosion processes on smaller and smaller length scales, and combination with other spectroscopies such as local atom and molecular spectroscopies in ways such that spatial and temporal information is given from the same surfaces at once. Hidden corrosion remains a challenge, as do sensor interpretation and subsequent decision-making algorithms. Pan and Leygraf78 have combined atomic force microscopy with scanning electrochemical methods to obtain a coordinated x-y surface view of metrology and electrochemical reactivity. The future for instrumentation is to expand the number of channels of information and to ensure sufficient dynamic range to sample large enough areas so as to obtain a clear picture about corrosion in complex materials.