Metal Finishing Guide Book


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Q: I'm doing an acid-copper plating process on zinc die cast material ( MAZAK ). The finish obtained from acid-copper plating is excellent, but after that, when I apply lacquer to it, the material tarnishes in few days. What's the problem? A: Copper and copper alloys are prone to tarnishing on exposure to the atmosphere. It does require a suitable lacquer with sufficient thickness to prevent exposure to oxidation. A neutralizing step is recommended to remove the acid from the surface, which in itself can cause issues when coming directly from acid copper into a lacquer. A 1% ammonia dip or a heavily silicated brass cleaner followed by a thorough, clean rinse would be suitable. Extra care should be taken here to ensure adequate neutralization and rinsing of porous or rough surfaces. This will result in a "bleed out" type of tarnish pattern. Proper lacquers for your application are specifically designed for copper alloys. For extra protection, a hex chrome passivation or a non-chrome passivate for copper is available to provide additional protection as the lacquer cures. Q: I m working with cyanide copper barrel plating. Parts are zip sliders made of zinc die casting. The problem I'm facing is controlling this solution to get bright copper parts. Can I use any other alloy in this solution? If yes, then which metal and how much? A: There are various brightener systems used in cyanide copper. Some are metallic, some are organic, and some are both. I would suggest you contact a local finishing supplier to get information on common additives available in your area. Something to consider regarding cyanide material sources: Sulfur is a common contaminant found in certain sources of sodium and potassium cyanide. This can cause a dark/dull red low current density area. This is easily fixed by a small addition of zinc cyanide, (1-2 g/l) as the zinc reacts with the sulfur compound. The small amount of zinc will give a very low co-deposition of zinc, which is not a problem when plating zippers. Q: I am plating semi-brilliant nickel bath over steel; my customer is heating parts (after nickel plating) up to 1250 degress celsius, and they are having blistering problems. What can I do to solve this problem? A: This situation may be caused by either base metal preparation or conditions (i.e., stress) in your nickel plate. The problem needs to be isolated in order to solve it. I would recommend that you plate a zinc-coated steel hull cell panel in the tank using the same semi-bright nickel solution. Strip the zinc with fresh hydrochloric acid, then remove the panel from the acid immediately after stripping is complete to avoid over pickling. Ensure that there is no water break film before you plate in your nickel. Heat treat the plated panel and check for blistering. If you see blistering, it would appear you have an issue with the semi-bright nickel. If you do not see blistering, chances are your problem is related to surface preparation. You can repeat the plating test for confirmation in the lab by running hull cell panels of your nickel and a newly made nickel, and heat treating both panels. If you confirm that the existing solution blisters and the new solution does not, you will then need to investigate several potential factors, using the hull cell to confirm appropriate corrective action. 1. Excess semi-bright brightener additive. Semi-bright nickel does use leveling agents. Excess class 2 nickel brightener will impact stress levels. This can be removed by electrolysis. 574

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