Metal Finishing Guide Book


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Page 171 of 903

Zinc die castings may be treated in a specially blended acid solution, commonly referred to as immersion chemical polishing. This process facilitates surface preparation by deburring, smoothening, leveling, and brightening. Common base metal defects, such as nodules and pores, are effectively worked out. A typical solution consists of: 42�� Be` nitric acid (20-30%), 66�� Be` sulfuric acid (20-25%), ammonium bifluoride (20-40%), and nonionic or amphoteric surfactant (>0.5%). Application: 65-115��F (18-46��C). Maintaining temperature is critical to avoid etching or dulling the surface. Immersion time depends on particular surface requirements. Organic soils (grease, oils, buffing compound, mold release, etc.) should be removed in a suitable soak or ultrasonic cleaner before the chemical polishing step. Thorough rinsing is understood between steps. Copper strike as per formulas given for zincated aluminum. Castings should be sealed with at least 0.03 to 0.05 mil. Additional copper as plated to 0.08 to 0.14 mil before application of nickel plating. 168

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