Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Mg, Ni, Pd, Pt, K, Rh, Ru, Ag, Na, Sr, Sn, and Zn. 3. Emission spectrometry (ES): ES involves exciting a cast metal or a solution by electric discharge by AC or DC in a graphite cavity. Graphite electrodes are favored due to the least spectral interference and a temperature of 4,000–6,000 K. At these higher temperatures, elements emit a higher number of spectral line characteristics of each element. The method is mostly qualitative, and density of a given line provides semi-quantitative results. ES is sparsely used, mainly for trace analysis. 4. X-ray fluorescence (XRF): XRF is based on the excitation of samples by an X-ray source of high energy, resulting in the emission of fluorescence radiation. The concentration of elements being determined is proportional to the intensity of its characteristic wavelength. XRF is a non-destructive technique and can be applied in measuring the constituents of plating baths, such as Cd, Cr, Co, Au, Ni, Ag, Sn, and Zn. The method is less sensitive compared with ES, and for XRF, proper calibration standards with similar matrices should be employed. Applications have been developed by Spectra-Asoma Instruments. Coating thickness and its composition can also be determined using XRF after calibrations. 5. Mass spectrometry (MS): This technique utilizes gas or vapors derived from liquids or solids that are bombarded by a beam of electrons in an ionization chamber, causing ionization and resulting in the rupture of chemical bonds. Charged moieties are formed and may contain elements, molecules, and fragments and are separated by electric and magnetic fields based on mass to charge (m/e) ratio. Based on the mass spectrum, best possible fit from the software library is suggested. MS is only applicable to substances that have sufficient vapor pressure and is good for compounds with a boiling point <450ºC. This technique is used in speciation and molecular weight determination. 6. Inductively coupled plasma (ICP): A liquid sample is aspirated in a stream of argon gas and ionized by an applied radio-frequency field. The field is inductively coupled to the atomized gas by a coil surrounding a quartz torch that supports and encloses the plasma. The sample aerosol is heated in plasma, molecules become almost completely dissociated, and then, atoms emit light at their characteristics frequencies. A high temperature of 7,000 K of argon plasma produces efficient atomic emissions and provides low detection limits for many elements. ICP allows simultaneous analysis of many elements in a short time with sensitivity to parts per billion (ppb) levels. A comparison of both techniques is given in Table 1. ICP instrumentation is not practical for small plating operations due to the initial cost and its prohibitive operating expenses. 7. Photometric methods: Photometry is based on the absorption of UV light (200–400 nm) or visible radiant energy (400–1,000 nm) by species in solution. The absorption is proportional to the concentration of absorbing species in solution and valid up to 2% concentration. Colorimetric 455

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