Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 164 of 707

Figure 1: Magnesium alloy "AZ91D" before (top) and after (bottom) plating with electroless nickel. ing takes place. The deoxidizer generally used on these metals and their alloys is a mineral acid (nitric, sulfuric, hydrofluoric, and/or hydrochloric) of low or high con- centration, depending on the met- al or alloy in question, followed once again by a rinse in high-qual- ity demineralized water. In the past, these metals were in many cases deoxidized with chromic acid solutions or given chromate-based conversion coatings as a way of preventing the growth of metal oxides, and, thus, assisting in the application of the zincate and/or stannate solutions. Environmental concerns have now served to stop many of these procedures. MAGNESIUM Figure 2: Cast aluminum alloy "383" before (bottom) and after (top) plating with electroless nickel. Due to the extreme reactivity of magnesium and its alloys, very lit- tle plating of the pure metal or its alloys takes place. Quite often the plating is over a copper strike applied to mitigate any oxidation that may have taken place on the surface of the metal during the cleaning and deoxidation process- es or that will take place on the metal after it is plated. In addi- tion, the copper strike acts to smooth over any scratches and/or pits on the surface of the metal.5 A com- monly used procedure for the application of electroless nickel to a magnesium surface would involve the steps listed in Table 1.5 The new process being proposed would involve steps noted in Table 2. To accomplish the newly proposed process, make the magnesium or magne- sium alloy the cathode of an electrolytic cell containing a conductive emulsion of various polyamines and/or polyamides at about 15 amps per square foot for a few seconds, rinse in demineralized water, and go directly into the electroless nickel plating bath to the desired plating thickness (see Fig. 1). Subsequently rinse and then dry. The bonding surface generated by this process is at most about 400- nm thick and is self limiting in that leaving the part in the processing solution for longer periods will not cause any harm. Any excess polymer will simply go back into the processing solution to be used over again or be rinsed off. Adhesion was excellent (baked at 375°F for one-half hour and quenched in cold water) using this process and subsequent plating of copper on the part was not a problem. In addition, it should be noted that rinsing, drying, and storing the 163

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