Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 449 of 707

control, analysis, and testing CONTROL AND CHEMICAL ANALYSIS OF PLATING SOLUTIONS BY SUDARSHAN LAL, MECHANICSBURG, PA. The quality of plated deposits primarily depends on factors such as current density, solution composition, temperature, effective bath pH, additives con- centration, speed (rpm for barrel plating or line speed for reel to reel), and solu- tion agitation in the tank. Apart from mechanical factors, tight control of main ingredient and additive concentrations in a plating bath is extremely important to achieve successful plating operations. Plating solutions must be maintained at the recommended limits, as suggested by the manufacturers. Sometimes, the limits may not be so rigid, and bath parameters need to be optimized by a given job shop based on the type and layout of the plating line. The status of a given bath is dependent on the rate of its depletion due to plating operations and proper replenishment. The operator should monitor bath components frequently in order to maintain chemistries within a con- trolled window. The depletion of primary bath components mainly depends on the following factors: 1. Drag-out into rinse tanks causing loss of useful chemicals depending on mode of draining. 2. Evaporation rate of plating baths depending on temperature, air flow, and mechanical agitation in the tank. 3. Imbalance in anode and cathode efficiencies. Soluble anodes may increase or decrease metal content due to current efficiencies. The current efficiency issues may be ascribed to side reactions occurring at the anode and cathode. For insoluble anodes, metal replenishments are frequently required. 4. Depletion of additives due to co-deposition in electrodeposits and breakdown products. 5. Drastic pH changes in the bath, which may cause precipitation or turbidity. 6. Impurities introduced due to tech-grade chemical additions and leaching of impurities from extraneous objects that have fallen into the tanks. Wet chemical methods have been routinely employed in monitoring major bath components. Advanced automatic instrumentation is also available for analysis of inorganic and organic species. Plating baths are usually analyzed offline after harvesting samples from various tanks. Metal ions are monitored using atomic absorption spectrophotometry (AA), inductively coupled plasma (ICP), wet titra- tions, colorimetry, polarography, and ion selective electrodes, depending on laboratory facilities. The analytical methods for analysis of plating solutions should be simple, direct, and operator friendly. In order to facilitate this, a standard operating pro- cedure should be documented, and adequate records with tank ID, date and time 448

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