This review traces the development of X-ray mapping from its beginning 50 years ago through current analysis procedures that can reveal otherwise obscure elemental distributions and associations. X-ray mapping or compositional imaging of elemental distributions is one of the major capabilities of electron beam microanalysis because it frees the operator from the necessity of making decisions about which image features contain elements of interest. Elements in unexpected locations, or in unexpected association with other elements, may be found easily without operator bias as to where to locate the electron probe for data collection. X-ray mapping in the SEM or EPMA may be applied to bulk specimens at a spatial resolution of about 1 mm. X-ray mapping of thin specimens in the TEM or STEM may be accomplished at a spatial resolution ranging from 2 to 100 nm, depending on specimen thickness and the microscope. Although mapping has traditionally been considered a qualitative technique, recent developments demonstrate the quantitative capabilities of X-ray mapping techniques. Moreover, the long-desired ability to collect and store an entire spectrum at every pixel is now a reality, and methods for mining these data are rapidly being developed.
Because the atmospheric corrosion products that form on the surface of Al, Zn, and AIZn hotdip coatings on sheet steel are difficult to characterize, they have not been extensively studied In the past. In this study, the composition and structure of products formed in industrial and marine environments were identified, and the effects of time, side exposed, and surface chromate treatment were described. Amorphous aluminum sulfate hydrate is the most abundant corrosion product on Al and AIZn coatings in both environments; it is well formed after only three years of exposure and is changed little after nine years. Al hydroxides were also present on aluminum-containing coatings exposed to the marine atmosphere, but not on those in the industrial atmosphère. The groundward side of the AIZn-coated panel exposed 250 m from the Atlantic Ocean had roughly as much AIZn sulfate hydrate as the skyward side of a panel exposed 25 m from the ocean. This finding discounts sea spray as the source of sulfur and indicates an atmospheric source. The effect of chromate treatment was to reduce the overall corrosion rate and, particularly, to reduce the loss of zinc. In conclusion, it is evident that sulfur is a significant factor in the atmospheric corrosion of metallic coatings, even in a marine environment; however, further work is necessary to explore the properties of the sulfate and to ascertain whether it might even be protective in some cases.
The rapid advance and falling costs of computing power and data storage during the last 40 years have greatly enhanced the data reduction speed and analytical capacity of energy dispersive spectroscopy (EDS) systems. At the same time, the solid state X-ray detector [until recently, the Si(Li) diode] has seen performance increases due to the use of advanced materials and processing techniques. In addition, the performance of the electronic components (field effect transistor, preamp, and pulse processor) of an EDS system has been constantly improved. These technology advances have resulted in improved spectral quality, excellent light element detection, and increased count rate performance. The results have been truly remarkable and driven by the needs of the analyst. This article will summarize the progress made in these areas in the last 40 years and make a brief reference to prospects for future development in EDS system performance.
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