Metal Finishing Guide Book


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amounts of energy; can be used to deposit a wide variety of metals in a broad range of coating thicknesses; does not use toxic chemicals; simplifies waste treatment; does not require baking of parts after plating in most cases; and provides greater uniformity and control of coatings when used for galvanizing. HYDROGEN EMBRITTLEMENT AND MECHANICAL PLATING A significant concern in electroplating and other metal-finishing processes is the embrittling effects of hydrogen absorbed by the part. The critical need to prevent hydrogen embrittlement was one of the major reasons for the creation and successful use of mechanical plating. The electric current used in electroplating, for example, acts to increase the potential of this condition because the process generates hydrogen at the cathode and because the negative charge acts to pull hydrogen into the part. Hydrogen embrittlement can cause unexpected development of cracks or weak regions in highly stressed areas, with subsequent total failure of the part or assembly. The risk increases for items that have elevated hardness from heat treating or cold working, especially parts made of high-carbon steels. In electroplating and other metal-finishing operations, a major source of hydrogen gas is the reaction between acids and metals present in the plating solution. The hydrogen transfers through the metal part substrate and concentrates at high stress points and grain boundaries. The trapped hydrogen generates internal pressures that can reduce the tolerance to stresses applied in actual use. Hazardous failures in critical applications can result. The mechanical process plates metals while eliminating or at least greatly 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 ��m in diameter. The reaction proceeds at a very slow rate and within a microscopically more porous, less oriented grain structure deposit than produced by electroplating. It is for this reason 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 447

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