Metal Finishing Guide Book

2013

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being processed. Thus the need for very clean rinse solutions and numerous processing tanks that need constant attention to avoid the expense of having to reprocess parts. Once the parts have been processed and rinsed, they need to be immediately placed in the plating bath and quickly plated while the surface is still active and ready to accept the metal being deposited on them. The larger the parts, the greater the degree of difficulty involved. Table 1 illustrates one of the more commonly used methods for processing wrought aluminum with "zincates" or "stannates"[v]. Table two illustrates one of the more commonly used methods in the processing of aluminum castings[vi]. Castings are much more difficult to clean in that the silicon and other elements in a given casting are generally present in much higher percentages. Silicon, for example, will range from about 6 percent to as high as 25 percent and it is rather difficult to remove. 1. 2. 3. 4. 5. 6. 7. 8. 9. Clean in a sulfuric acid or phosphoric acid-based cleaner at about 120 degrees F. for two to three minutes Mineral-free water rinse. Strong potassium hydroxide etch for one to three minutes at ambient temperatures. Rinse in mineral-free water. Acid etch in 15% - 25% sulfuric acid containing 1% - 2% fluoride at ambient temperatures. Double rinse in mineral-free water. Process in "Zincate" or "Stannate" solution. Double rinse in mineral-free water. Immediately place the parts in the plating bath. Table 1. Commonly used procedure for processing wrought aluminum alloy parts. 1. 2. 3. 4. 5. 6. 7. Strong alkaline (pH of 12 or higher) cleaner at about 150 – 170 degrees F.for two or three minutes. Double rinse in mineral-free water. 1% – 3% fluoride salt added to 20% - 25% sulfuric acid with the balance being nitric acid at ambient temperatures for one to three minutes. Double rinse in mineral-free water. "Zincate" or "Stannate" processing of the casting. Double rinse in mineral-free water. Go directly to plating bath. Table 2. Commonly used procedure for the processing of cast aluminum alloy parts. Table 3 is similar to the process outlined in the "ASTM B253 specification" "Standard Guide for the Preparation of Aluminum Alloys for Electroplating" and now in use as an alternative to the standard "zincate"and/or "stannate" process for the preparation of wrought alloys to be plated. Once the organic-based "Plating Catalyst" is applied, the surface of the metal being processed is more or less sealed from the outside elements and may, if so desired, be dried and stored in a clean/dry area to be processed at a later date by first of all reactivating the surface by soaking the parts in warm (100 – 120 degrees 242

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