Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 353 of 707

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 suc- cessful use of mechanical plating. The electric current used in electroplating, for example, acts to increase the potential of this condition because the process gen- erates hydrogen at the cathode and because the negative charge acts to pull hydrogen into the part. Hydrogen embrittlement can cause unexpected devel- opment 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-car- bon 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 solu- tion. 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 reduc- ing the embrittlement risk caused by the plating process itself. There is a hydro- gen-producing reaction that occurs in mechanical plating, but this reaction happens mostly on the surface of the powdered zinc (or other plating metal) par- ticles, 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 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 amount of each depends com- pletely on the total surface area of the parts to be plated and, therefore, it is impor- tant 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 pre- cleaning heavy oils or scale, all of the steps are performed in the same tum- bling barrel, normally without rinsing or stopping the rotation. A typical process cycle includes a series of surface preparation chemical additions, designed to mild- ly acid clean and activate the substrate and then to apply a copper strike. The preparation chemicals normally contain sulfuric acid, surfactants, inhibitors, dis- persing 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 "acceler- ator" 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 352

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