Sect 745 detectors the aperture has a diameter of 06

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A Fischerscope XDAL with a PIN diode as detector has been used (cf. Sect. 7.4.5 Detectors). The aperture has a diameter of 0.6 mm. Additional instrument details: X-ray tube with W-anode, 50 kV, primary filter 10 µ m Ni, measuring time 30 s become more difficult. Alloy elements may be Ni (6–15%), Fe (about 1%) or Co (about 1%). In the case of Ni, the composition can be measured well using instruments with proportional counter tubes. For Fe and Co they cannot be used because of the severe peak overlap caused by the poor energy resolution of the detector. Of course, it is still possible to measure the thickness for those alloy coatings. Instruments with semiconductor detectors are well capable of measuring the other alloy elements, except for Fe due to the substrate material of steel. Table 7.13 shows an example. This application measures simultaneously the Cr-containing passivation layer. These typically very thin layers additionally improve the corrosion resistance, such that they are very common in the automotive and other industries. Many types of passivations are available [250]. For XRFA, it is important that only the Cr mass per unit area can be measured and not the oxidation state of the Cr. For effective passivation a minimum layer of Cr is necessary, although the Cr mass per unit area by itself is not sufficient [251]. The instrument must be sensitive enough due to the small amount of Cr; therefore only instruments with semiconductor detectors can be used. On the other hand, the chromatization does not influence the “normal” measurement of the zinc layer [252]. In addition to the galvanic Zn layers, Zinc flake coatings have found accep- tance in certain applications [253]. They are applied as an aqueous suspension and cured when heat-treated. For XRFA, they present a mixture of Zn and Al, often dye and other organic additives are added as well. In principle, X-ray fluorescence can only determine the zinc amounts. However, since the sig- nal of the Fe substrate material is also available as information, such a layer may be measured as a two-component “alloy layer”AlZn. If TiO 2 is added as pigment, the model AlTiZn/Fe will provide reproducible results. The den- sity must be determined with a different layer thickness measuring method,
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588 V. R¨oßiger and B. Nensel e.g., by cross-section or by the magnetic induction method. Typically, these corrosion protection layers are combined with a functional top layer. These top layers also consist of light elements, and to a degree again of Zn and Ti. This poses a difficult if not hopeless situation: both layers consist of the same elements. However, they have a different composition. The problem could in- deed be solved if the concentrations in the individual layers were fixed or at were least within very narrow limits. After all, the top layer is low in Zn and the layer below rich in Zn. If samples of the individual layers, deposited on Fe, are available, their compositions can be measured using the alloy layer measurement application AlTiZn/Fe, the density of the individual coatings can be measured using one of the above mentioned methods. A combination of such top and interim layers can then be measured reliably. Unfortunately,
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  • Spring '14
  • MichaelDudley

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