We want to plate aluminum with electroless nickel. We require excellent adhesion. How should we do this? D.H.
Plating aluminum with electroless nickel is not difficult. The aluminum parts must be cleaned to remove greases, oils and other soils. After this step the parts are treated with an alkaline dip and/or an acid dip, depending on the type of aluminum alloy being used. For example, a common process is to use a 6.5 oz/gal sodium hydroxide solution for 30 sec at a temperature of 120–130F. This step is followed by desmutting in a solution made of 75% nitric acid and 16 oz/gal of ammonium bifluoride. This solution is used at 65–75F for 30–45 sec. Other cleaning/conditioning procedures can be found in the Electroplating Engineering Handbook, 4th Edition, edited by Lawrence J. Durney, Metal Finishing Publications, 914-333-2578.
The next step is critical in the process and involves the zincating of the aluminum surface. Zincating solutions can be formulated from various combinations of zinc oxides, sodium hydroxide and ferric oxides, but I recommend a commercial formulation over a "home brew." Most people use a double zincate process. After the first zincating step the zinc is stripped by using a 50% nitric acid dip and then zincated a second time.
After the zincating step, a cyanide or alkaline copper strike is applied followed by the specified metal deposit. In the case of electroless nickel you can skip the copper strike and apply the electroless nickel directly to the zincated part. For maximum adhesion the parts should be baked at a low temperature.