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features and not just what one observes from the topographic features. Although SEM can be performed on as-received or rough surfaces, EDS is best performed on polished or flat surfaces. The analysis by SEM/EDS in combination with XRD and optical microscopy is a powerful tool in the evaluation of corrosion. See Fig. 5.3, which shows optical, SEM/EDS, and XRD data for the corrosion of a mullite refractory, and the corresponding text for an example of the use of EDS in phase identification.
Transmission Electron Microscopy
Transmission electron microscopy (TEM) can be used to evaluate the corrosion effects upon grain boundary phases. Transmission electron microscopy used in this way can be very useful; however, it is a very time consuming method, and, quite often, the samples are not representative due to their small size (several millimeters or less) and the thinning process. Transmission electron microscopy does not lend itself to the observation of porous samples and thus is confined to observation of dense regions of corroded samples.
3.5.2 Chemical Analysis
Bulk Analysis
The bulk chemical analysis of a corroded material is also a widely used tool in the evaluation of corrosion. In most cases, it is the minor constituents that will be most important. It may even be necessary to examine the trace element chemistry. When corrosion has taken place through reaction with a liquid, it is important to analyze the chemistry of the liquid. In this way, it is possible to establish whether it is the bulk or the bonding phases that are being corroded. Schmidt and Rickers [3.13] determined the concentration of chemical species in corroding fluids and melts by synchrotron radiation x-ray fluorescence (SR-XRF). The studies of Schmidt and Rickers are quite interesting since they were performed in situ at pressures up to 1.1 GPa and temperatures up to 800°C in a hydrothermal diamond-anvil cell.
Copyright © 2004 by Marcel Dekker, Inc.
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A chemical analysis that is normally not done is that of the gaseous phases produced during corrosion. This is not an easy task for large-scale experiments but can be accomplished on the microscale, such as that done with the aid of a thermobalance (TG) connected to a gas chromatograph (GS), mass spectrometer (MS), or infrared absorption spectrometer (FTIR).
Surface Analysis
Since corrosion takes place through reaction with the surface of a material, it is easier to determine mechanisms when the chemistry of the surfaces involved is analyzed. In this way, one may no longer be confronted with evaluation of minor constituents and trace elements since the corrosive reactants and products are more concentrated at the surface. The only drawback to surface analysis is that of the cost of the equipment and the necessity of a skilled technician. Secondary ion mass spectroscopy (SIMS) is a technique that currently receives wide use since it provides element detection limits in the subparts per million range and very good spatial resolution. Profiling of the various elements, another form of surface analysis, in question can be a very enlightening experiment. In this way, the depth of penetration can be determined and the elements that are the more serious actors can be evaluated. Lodding [3.14] has provided an excellent review of the use of SIMS to the characterization of corroded glasses and superconductors.
Determination of surface structures of ceramics for corrosion studies is most likely best accomplished by techniques such as Auger photoelectron diffraction (APD), x-ray photoelectron diffraction (XPD), or atomic force microscopy (AFM). Other techniques are available (e.g., LEED*), but they are better suited to other materials or suffer from various limitations. Gibson and LaFemina [3.15] offer an excellent discussion of how the various surface analysis techniques are used to characterize mineral surface dissolution.
* LEED is the acronym for low-energy electron diffractometry.
Copyright © 2004 by Marcel Dekker, Inc.