Metal Finishing Guide Book


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ly reducing the embrittlement risk caused by the plating process itself. There is a hydrogen-producing reaction that occurs in mechanical plating, but this reaction happens mostly on the surface of the powdered zinc (or other plating metal) particles, which are approximately 5 to 10 μm in diameter. The reaction proceeds at a very slow rate and within a microscopically more porous, less oriented grain structure deposit than produced Fig. 1. Specially lined by electroplating. It is for this reason variable-speed tumbling/plating barrel. that the hydrogen gas is not likely to be trapped within or under the metal particles in the coating. The escape of the hydrogen through the deposit and away from the part substrate is more likely than absorption into the base metal. PROCESS DESCRIPTION The mechanical plating process requires a sequence of chemical additions added to the rotating tumbling/plating barrel. The amount of each depends completely on the total surface area of the parts to be plated and, therefore, it is important to calculate this number prior to each cycle. The variable-speed plating barrels rotate at a surface speed of 43 to 75 m/min (140-250 ft/min), depending on part type and at a tilt angle of about 30° from horizontal. Except for precleaning heavy oils or scale, all of the steps are performed in the same tumbling barrel, normally without rinsing or stopping the rotation. A typical process cycle includes a series of surface preparation chemical additions, designed to mildly acid clean and activate the substrate and then to apply a copper strike. The preparation chemicals normally contain sulfuric acid, surfactants, inhibitors, dispersing agents, and copper in solution. This step results in a clean, galvanically receptive part surface. The next step is the addition of a "promoter" or "accelerator" chemical, which acts as a catalyst as well as an agent that controls the rate of deposition and subsequent uniform bonding of the plating metals. A defoamer is used to control foaming caused by the surfactant additives, so loss of plating solution is avoided and operator visual monitoring is maintained. A series of plating metal (usually zinc) additions added as a powder or water slurry is introduced in a number of equal additions totaling an amount proportional to the plating thickness desired. Table I represents a typical sequence. Table I. Typical Process Sequence for Mechanical Plating Process Stop Alkaline or acid preclean (if necessary) Time, min 5 Rinse Surface preparation 5 Copper strike or "flash" 5 Accelerator/promoter Plating metal additions (series of small equal adds) Water polish 394 3 15-20 5

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