Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 166 of 707

Step Degreasing if necessary Strong alkaline soak cleaner Rinse in demineralized water Temperature 180–200°F Table 3: Non-toxic Pretreatment Process for Plating on Titanium Figure 3: Titanium bolts before (left) and after (right) plating with electroless nickel. Due to the unusually high silicon con- tent of most aluminum castings, they present a rather difficult surface to prop- erly plate. These castings are generally deoxidized in fluoride-containing con- centrated nitric acid solutions because nitric acid will prevent attack on the alu- minum and allow the surface silicon to be removed by the fluoride. In the newly proposed process, a cleaned and deoxi- dized "383" cast alloy was cleaned and deoxidized, rinsed in demineralized water, and then made the anode in the polyamide and/or polyamine emulsion at 15 amps per square foot for a few sec- onds to generate the plating surface (in the case of aluminum it is sometimes more desirable to generate the plating surface by making the metal the anode). The casting was then rinsed and placed in the electroless nickel bath to the desired plating thickness, removed, rinsed, and then dried. The casting showed no signs of adhesion failure when heated to 375°F for half an hour and then quenched in cold water. A freshly formed electroless nick- el–plated surface could easily be plated with any one of a number of different met- als. As in the case of magnesium, the parts may—if so desired—be rinsed, dried, and stored to be plated or painted at a later time. However, it is generally rec- ommended that the parts be plated shortly after forming the bonding surface (see Fig. 2). TITANIUM Titanium is a rather noble metal that quickly forms a thin film of titanium diox- ide when exposed to oxidizing conditions, making it very difficult to plate or paint. Quite a number of patents have been issued on the plating of titanium, most of which require extremely toxic and/or corrosive cleaning conditions,7 and in some cases a zincate solution will be used. The new process uses the non-toxic and relatively simple process noted in Table 3. To accomplish this process, make the titanium the cathode in a electrolytic cell containing a conductive emulsion of various polyamines and/or polyamides at about 15 amps per square foot for a few seconds, rinse, and then go into your elec- troless nickel bath to the desired plating thickness (see Fig. 3). Once again, the 165 Duration 5–8 minutes

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