Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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methods involve comparing the color produced by a standard containing a known quantity. Errors in this method may be due to turbidity, sensitivity to the eye, color blindness, dilutions, photometer filters, chemical interferences, and temperature variations. 8. Atomic absorption (AA): Metals in plating baths and wastewater effluents are readily determined by AA spectrophotometry. Optimum ranges, detection limits, and sensitivities of metals vary with different instruments. The solution is directly aspirated in the flame (air-acetylene or nitrous oxide-acetylene), which absorbs radiations from a hollow cathode lamp of a given metal. The difference between flame photometry and AA is that flame photometry measures the amount of emitted light, whereas AA measures the absorbed light. Graphite furnace-AA is utilized for ppb levels of metal ions. ANALYTICAL METHODS FOR ADDITIVES AND SURFACTANTS Plating baths, in addition to electrolytes and metal salts, also contain certain pro- prietary organic chemicals, such as wetting agents, grain refiners, and brighteners. Additives play a significant role in controlling the material properties of deposits. Additive concentrations may be determined by cyclic stripping voltam- metry (CSV), polarography, spectrophotometry, HPLC, and IC. Voltammetric tech- niques are employed to study solution composition (oxidizable or reducible) and valence states using the current–potential relationship in an electrochemical cell. Current–time response of a microelectrode provides useful information about a system. The common electrodes used include dropping mercury electrodes (DME), hanging mercury drop electrodes (HMDE), thin-film mercury electrode, and solid electrodes, such as gold, platinum, and glassy carbon. Additives are indi- rectly determined by CSV by studying their influence on deposition of metal from the plating bath using a rotating platinum disk electrode.13 Anionic, cationic, and non-ionic surfactants may be present in plating baths. After extraction in hexane or another suitable solvent, color is developed using a specific reagent. The intensity of absorbed light at a given wavelength provides an estimate of surfactant concentration. Anionic surfactants are measured as methylene blue active substances (MBAS), but the method is not specific and is prone to interferences. Non-ionic surfactants are measured as cobalt thiocyanate active substances (CTAS). The methods are not specific and may be used as an empirical correlation. Brighteners may be empirically estimated by plating brass panels for 1 or 2 min- utes at 2 or 5 amperes in a hull cell. Plated panels indicate the useful current den- sity range for bright deposits, and additions to plating tanks are made based on these results. OPERATIONS CARRIED OUT IN COMMON PLATING PROCESSES Substrate preparation using chemical etchants: In order to achieve a robust deposit, it is extremely important to prepare the substrate to be free of soils, scales, rust, and oily residues. The parts may be mechanically polished, buffed with varying degrees of abrasive compounds, belt polished, undergo simple tumbling, and/or impact blasting in the presence of various types of media. It is imperative to remove any residual contaminants from these operations prior to starting the plat- 456

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