Q. Our company reconditions and repairs parts that have been previously brush-plated with nickel. We find that the stripping process sometimes does not remove all of the nickel plate. When re-plated, the areas that still have “old” nickel on the surface tend to flake. Do you have any suggestions for solving this problem? R.M.
A. Nickel forms oxides quickly when exposed to the environment. If the stripping step does not remove all of the previously plated nickel, the nickel areas will most likely have oxides on the surface and will give you the flaking that you are observing. As most platers recognize, stripping of defective old deposits is both a science and an art. Over-stripping will damage the base material and will consign the part to the scrap heap, and not removing all of the old deposit prior to replating will often generate scrap.
Plating nickel on nickel is not the easiest process. Some platers recommend a cathodic activation in alkaline cyanide solution prior to plating on the old nickel surface. To many people cyanide is essentially a dirty word, and if you don’t have cyanide in your plant this is not a reasonable solution.
Another approach is to make the part cathodic in a good grade of sulfuric acid. Bagged carbon anodes are used and a small amount of high-quality ammonium bifluoride is added to the sulfuric acid. The problem with this method is that if you have surfaces that are already nickel-free, you may be adding metal impurities to your process tanks.
Before embarking on this process you should test it on some sample pieces to determine if it works with your particular parts.