Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 128 of 707

Table XVII. Die Cast Alloy Compositions Element Magnesium Zinc Manganese Silicon Copper Iron Nickel Titanium Chromium Tin Lead Cadmium Other ALUMINUM Aluminum is in a class by itself. It requires special handling, using some unique steps and considerations. Because of its light weight, heat capacity, dura- bility, and corrosion resistance, aluminum is the metal of choice for many appli- cations. A surface preparation cycle for electroplating or electroless plating gen- erally consists of soak clean, etch, desmut, zincate, optional double zincate, strike plate, and plate. It may seem easy but aluminum demands we invest in a quality effort to obtain a quality finish. Knowing the alloy designation is critical to selecting the optimum bath chemistries for each step in the surface preparation cycle. (See Table XII.) Soak cleaning denotes no etching or attack of the base metal. (See Table XIII.) The cleaner bath pH ranges from 8 to 9.5. Ultrasonic soak cleaners also have a similar chemistry profile. They differ in containing higher detergency levels along with selected solvents. Etching is accomplished using acidic or highly alkaline solutions. (See Tables XIV and XV.) This is the primary method of removing the outer, passive aluminum oxide skin. Etching also cleans the surface by undercutting soils and lifting them off. Etchants and preferences: • Alkaline—aluminum alloy extrusions, and stampings. • Acidic—castings, polished parts, and prior to electroless nickel. When etched, some alloys (in the 5000, 6000 series, and castings) tend to generate heavy smuts. This can lead to incomplete desmutting, detrimentally 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 127 Series 380 Assay 0.10 max. 3.0 max. 0.50 max. 7.5-9.5 3.0-4.0 1.0 max. 0.50 max. — 0.35 max. — — — 0.50 max. Series 413 Assay 0.10 max. 0.50 max. 0.35 max. 11.0-13.0 0.60 max. 0.80-1.1 0.50 max. — 0.35 max. 0.15 max. — — 0.20 max.

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