[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

63 solid specimens 631 metallic specimens xrf is a

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tube system or synchrotron radiation source [65]. 6.3 Solid Specimens 6.3.1 Metallic Specimens XRF is a widely used technique in metal production. This industry stresses the need for a simple and fast method; therefore, the preparation of a specimen has to be fast, simple and reproducible. The most universal specimen prepara- tion procedures are cutting, milling and casting. XRF analysis of metals allows to determine the composition of a relatively thin surface layer of the investi- gated specimen. The surface must therefore be representative of the bulk sam- ple. The major surface preparation methods are machining (milling, turning), mechanical grinding, as well as polishing and etching. In any surface prepara- tion technique special care must be taken to prevent any surface contamination during specimen preparation. Special care is very important in the case of rela- tively soft metals such as aluminium, where particles of the grinding agent may penetrate into the sample surface during surface finishing. The selection of a suitable abrasive is difficult, particularly when elements of interest are silicon, aluminium or iron. For instance, SiC and Al 2 O 3 are very effective abrasives, but both of them contain elements of interest to the metals industry. In such cases electrolytic polishing or etching is recommended. After each grinding or polishing step, it is important to make sure that the surfaces are kept clean. Hence, all traces of lubricant, cutting fluid or finger marks must be removed by cleaning with isopropyl alcohol prior to examination. Occasionally, the
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420 J. Injuk et al. surface of several metals is affected by corrosion which progresses with time. Therefore, the best results are always obtained when analysing a specimen without much delay after surface preparation [1]. When the surface of the specimen is rough and common methods such as polishing and milling are not effective, the samples can be prepared by obtain- ing turnings of the samples and then pressing the turnings into pellets with a hydraulic press at several hundreds of mega Pascal. When this procedure is done properly, the turnings are pressed to produce a surface that is smooth enough for analysis. Specimen preparation by pressing is discussed in more detail in Sect. 6.3.2. Selection of the most suitable surface preparation tech- nique depends on the characteristics of an alloy or group of alloys. For ex- ample, hard alloys resistant to cutting and abrasion, such as cast iron, can be ground and pelletized prior to analysis. On the other hand, if a sample is composed of soft, malleable, multiphase alloys, smearing effects of the softer components, like lead and aluminum, cause serious problems. These soft ele- ments will concentrate on the surface. That is why the fluorescence intensities of these soft metals are too high, while those of the harder metals, and metals with lower Z, are too low. In such cases, when the sample surface does not contain a representative concentration of elements in the bulk, electrolytic polishing or etching should be employed. For some archaeological materials,
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  • Spring '14
  • MichaelDudley

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