Metal Finishing Guide Book

2013

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A: The oldest answer to your question is the use of a chromic acid passivation step after nickel plating. Use 20-40 g/l of chromic acid, preferably hot, 35-40 degrees C. This has the added benefit of removing any flash rusting that may have occurred in any of the process steps, such as acid rinse or nickel rinse. There are options on the market for similar processes that are free of hexavalent chrome. There are also many water-based lacquers or topcoats that can be used over nickel plating to supplement corrosion protection in thin coverage areas. Q: Is Alkaline Non Cyanide Copper effective & successful on Zinc Die Casted (Zamak 5) components? A: When plating over zinc diecast, there are generally two copper processes involved. I will discuss both old and new methods here: 1. A copper strike is traditionally used directly over diecast to provide optimal adhesion. When using cyanide processes, the proper chemistry is needed in terms of pH, free cyanide, and copper content to ensure optimal adhesion. To replace this without cyanide, there are a few commercial processes available. If you google "non cyanide copper" you will find them near the top. These require more attention than the standard cyanide processes and have a higher operating cost due to the use of insoluble anodes. There are recommended conditions that should be followed to optimize adhesion over zinc diecast. 2. The copper strike is followed by a cyanide copper plate that is typically optimized for higher efficiency in order to provide a sufficient barrier layer prior to nickel plating. This helps improve the corrosion resistance of the final coating system as well as minimize contamination of the nickel processes. The same non-cyanide processes as mentioned above can also be used as copper plate. Pyrophosphate copper is also a time-tested process that is fully bright, and works as a suitable replacement for a heavy cyanide copper. It does use copper anodes, which keeps operating cost down. It also is superior to acid copper for use over diecast due to the mildly alkaline operating pH. This prevents attack of the zinc base metal that is seen in acid copper in areas where the copper strike is thin or in unplated internal areas. In general, non-cyanide copper is a more common choice when a facility has little or no other sources of cyanide in the facility. This makes the increased operating cost easily justified. Q: We own and operate a small nickel plating facility. We only have a nickel sulfate bath. Would it behoove us to also have a nickel "strike" or copper "strike?" Would there be any benefits in this? Also, do you see manufacturers that honestly want to go green, even if it costs more? A: The need for a strike bath is dictated by the type of substrate you are plating. If you are plating brass, copper, or steel, then a strike bath is not necessary. If you are plating zinc die cast or tin alloys, then a strike bath is considered necessary.  It is usually a copper strike. There is one benefit of a strike that is worth considering for substrates that do not necessarily require it. That is, it does isolate your main bath from contaminants, especially those that are cleaning related. This could mean the oils and soils themselves, or even the cleaners, can become contaminants when dealing with difficult-to-rinse part configurations, such as hollow interiors or cup-shaped areas that cause high drag-out. 576

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