Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 460 of 707

instrumental methods. Chrome plating: Chromic acid in the presence of sulfuric acid and additives is employed in high-speed chrome applications. Trivalent and hexavalent chromi- um are determined by ion chromatography and titrations. A hydrometer may also be employed for chromium estimation after correcting for sulfate content. Iron and iron alloys: Iron alloys such as Fe-C, Fe-B, Fe-P, Fe-Cr-Ni, Fe-Co, Fe-Zn, and Fe-Ni are used in special applications. Metals can be determined by AA or ICP. Iron (II) is estimated by titrations, spectrophotometry, and Fe(III) reduced to Fe(II) for titrations. Imbalance in plating processes is manifested in solutions due to varying anode or cathode efficiencies, depending on the mechanism. For example, at high anode efficiencies (soluble anodes), a high pH is encountered, resulting in high- er metal ion concentrations. On a micro-scale, high variations in pH are observed in the immediate vicinity of the electrodes. Conversely, in the case of high cath- ode efficiency, pH is lowered. In cases of high deposition rates, quick depletion of metal ions in the bath necessitates frequent addition of metal salts. In the case of gold and silver baths, high-free cyanide may be due to low anode efficiency and low-free cyanide may trigger high anode efficiency. CONCLUSIONS Metals may be determined by wet titrations, UV-Vis, colorimetry, AA, ICP, and XRF. Acids and alkalis are determined by acid–base and electrometric titrations. Additives are quantitatively analyzed by chromatographic methods (GC, HPLC, IC), CVS, and hull cells. Surface tension (Sensadyne), solution baume, and refractive index may be used, where applicable, for indirect estimation of a par- ticular parameter. The difficulty is that no single instrument can analyze all the components. A plating facility should develop a logical approach to control solution composition and follow the schedule. Poor control of bath constituents is a major problem and can lead to decreased reliability of parts. Substandard, poor-quality parts produced using an imbalanced process leads to customer complaints and rework, negatively impacting the rep- utation of the plating shop. The primary reason is due to limited analytical capabilities for the analysis of additives (which are usually a mixture of two or more chemicals). For troubleshooting, adequate resources are very important, and these analytical methods are the backbone of successful plating operations. The analysis of baths should not be discounted, and job shops should conduct frequent analyses of baths to ensure robust SPC process control. Author's note: This article provides an overview of popular analytical methods used in process control but does not claim its completeness. Special applications may require amended methods or simple tests, such as specific gravity, baume, refractive index, conductivity, etc., to suit a shop's needs in day-to-day operations. Additionally, the author does not endorse any commercial products mentioned in this paper. Plating shops should evaluate baths, analyt- ical techniques, and ancillary procedures for best results in their operations. 459

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