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pfonline.com/experts 42 NOVEMBER 2015 — pfonline.com p f o n l i n e . c o m / e x p e r t s PARTS CLEANING C L I N I C DAVID S. PETERSON / Consultant partscleaning@pfonline.com Ultrasonic Cleaning for Plating Q. We are in the plating-on-plastics business. We plate only silver on ABS. We mold these little "buttons," tumble them with water and pumice to scratch the surface, then spray rinse the pumice off the parts, dip them in a chrome/ sulfuric solution, neutralize with sodium sulfite, rinse, and then put them into an electroless coating of silver using tin for the "catalyst" and formaldehyde as the reducer. My question to you is related to cleaning after the pumice step. How critical is it to have the surface free of any loose particulate to limit plating problems? I have found that, when I ultrasonically clean the parts, there is a considerable amount of "powder" that comes off the parts, making the solution cloudy. My point to management is that we need to add an ultrasonic cleaning step. What are your thoughts? A. It sounds like you have a well-established process, so one of my first questions would be: what are you targeting for improvement? From the basis of your question, it would appear that that the addition of ultrasonic cleaning to your process would be a no-brainer. I would not expect the chemical processes you describe to completely dissolve the pumice, so the burden of removal would primarily fall to the spray rinse immedi- ately following the abrasive. But you are completely reliant on this step to remove all the residual pumice, otherwise it will lead to coating defects at the plating step. In an ideal- ized improvement pattern, the addition of the ultrasonic cleaning following the abrasive step would be a natural evolution of the process. Your management may be exam- ining the short-term payoff, which is usually what it is their job to do. If the current process does not result in a signifi- cant reject rate (whatever that may be), then they may be reluctant to spend the capital for the ultrasonic instal- lation. You may be able to review some of the intangible benefits along with the increase in process robustness to help your view. Necessity for Passivation Q. I work for an organization that makes equipment for the beer, wine and liquor industries. All equipment is made from stainless steel. We passivate all equipment with citric acid. What is the importance of this? Is it even necessary? A. The passivation of stainless steel is a common industrial practice intended to mitigate the effects of processing and restore the stainless steel's corrosion resistant properties to optimal condition. All stainless steels have one element in common: chromium, which is responsible for corrosion resistance associated with stainless steels and works by forming a very thin chromium oxide layer on the surface. Since this corrosion resistant layer is very thin, it can be subject to disruption and degradation by many industrial methods such as welding, grinding and other handling and fabrication processes. Generally, when a stainless steel assembly is completed (after fabricating, welding, grinding and so on), the entire assembly is citric acid passivated, particularly the parts expected to need and retain the most corrosion resis- tance. The two typical methods of doing this are with nitric or citric acid. Since both are oxidizing acids, they restore the chromium oxide layer by acceler- ating oxide growth in the depleted regions. Nitric acid has been the historical choice for passiv- ating stainless steel, but citric acid has seen significant growth due to similar or better effectiveness, less hazard to workers and fewer disposal issues when spent. It is very common to passivate stainless equipment used in any of the food and beverage industries since the need for cleanliness and product safety for consumption is extremely high. Without it, it is likely the heat affected zone of welds, weld seams and ground areas would start to rust and contaminate the end product. Eventually it would lead to localized corrosion that could perforate the assembly; however, in the case of the food and beverage industry, it would never last long. Such equipment would need to be taken out of service prior to further use. Oil and Water Separation Q. We are planning to install a coalescer as the first stage of filtration in our parts washing system. How do we choose the capacity of the coalescer? Does the capacity have to be the same as the discharge of the jet pump flow rate? A. I am glad to see you considering coalescers as an option for oil removal from an aqueous parts washer system. Alone or combined with an oil skimmer, this technology can greatly extend the life of the cleaning stage. They are also gener- ally cost effective when accounting for the extension of the cleaner life, improved quality due to a more consistent cleaning bath and a reduction in waste disposal. I can only give you general guidance regarding system design based on experience. I know you would not need to size a coalescing system to handle 100 percent of your tank turnover if using a recirculation system for turbulence and agitation. The coalescer needs residence time to enable the lubricant feed through the coalescing media, split from the cleaner and float. A lamellar flow is needed for this to occur. If a highly turbulent flow is sent through a coalescer, it would be too disruptive to enable the system to separate