Metal Finishing Guide Book


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Table XVIII. Suggested Copper Strike Formulations Component Concentration (oz/gal) Copper cyanide 3.50 Sodium cyanide 4.25 Sodium carbonate 4.00 Rochelle salts Copper cyanide 4.00 1.5-3.0 Free sodium cyanide 0.2-0.4 « Copper cyanide conc. Potassium hydroxide 0.1-0.5 affecting the zincate treatment. Acidic etchants, being less aggressive, raise less smut. Typical desmutters are given in Table XVI. Other desmutter baths for consideration: • 50-100% v/v nitric acid • 15-25% v/v nitric acid + 10-20% v/v sulfuric acid • Iron salts (ferric sulfate 3-4 oz/gal + 5-10% v/v sulfuric acid • Universal tri-acid. Mixture of 50% v/v nitric acid + 20-25% v/v sulfuric acid + 1-2 lb/gal ammonium bifluoride, balance water to 100%. Aluminum die cast alloys (see Table XVII) are based on six major elements: silicon, copper, magnesium, iron, manganese, and zinc. An example of applying the preferred desmutting bath can be illustrated by the following casting comparisons. Tips: • The universal tri-acid is best suited to desmut both of these castings; however, the formula containing 2 lb/gal of ammonium bifluoride is recommended for the series 413 casting. That's because of its greater silicon content (41% more). • Usually, the aluminum part will exit the desmut bath white and smut free. Close inspection may also indicate a very fine surface etch, which is actually beneficial for zincating or chromating. If the part fails a white paper towel wipe (smutty) chances are slim that subsequent processing will be successful. • If the part gasses while immersed in the zincate there is a good possibility it hasn't been properly desmutted. • If the desmut bath contains nitric acid be certain that good operating, compliant exhaust is in use to safely vent off nitric oxide fumes. Zincating This is an immersion treatment where a coating of zinc or zinc alloy is deposited over cleaned and activated aluminum. It is over this tightly surface-adherent film that plating can occur. There are three common zincating solutions: 1. Conventional zincate. This solution contains one metal, zinc, which is immersion deposited over aluminum. It also contains an oxidizer, such as sodium nitrate, conditioning the aluminum surface by mildly etching it. Tartrates are included as complexors. The viscous working solution is concentrated in sodium hydroxide (forming the chemical zincate). Baths prepared from powdered concentrates must be cooled for several hours before they can be used. 11-13 oz/gal sodium hydroxide, 2-3 oz/gal zinc oxide, 0.6-0.8 oz/gal sodium nitrate, 75-85°F (24-29°C), 0.5-2 minutes. 149

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