Metal finishing Q&As since 1989
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Gold plating on Bronze
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September 30, 2014 Q. I am making a range of lamps for house decoration in 20th century french style. Gold plating on bronze. To achieve the required colour and texture we gold plated directly on the bronze body. We decided not to copper plate first, because we preferred a more yellow colour. Once the items were finished we had some discolouration on the gold plate. The platers and the casters tell me that this is due to seepage of the acid used in the plating bath through pores in the bronze. They suggest first plating with nickel (copper gave us a red gold colour which the client didn't want) then seeing if there was discolouration. If there was discolouration they suggest spot welding those spots and trying again. Once we had no further discolouration we would do the gold plating. product designer - Beijing, China. |
A. Hello Allan,
If you want a long lasting finish on your gold plating without colour changes you will need a diffusion barrier between the bronze and or copper plate and the gold. The reason being is that the bronze base metal or copper plate will migrate through the gold layer and cause discoloration of the gold. You will also sacrifice the good properties that gold presents, such as corrosion resistance. It is recommended that a copper plate be employed on the bronze. It will help close up any pores in the bronze. The time it takes for migration to occur depends on the type of gold you are plating (soft or hard) and the deposit thickness of the gold. The advice you received regarding using Nickel as a diffusion barrier is correct. You can still get your nice yellow colour with the nickel underplate. Good luck!
Process Engineer - Phoenix, Arizona USA
A. I have been plating gold onto brass and bronze base materials for years. I have tried to find quick flash methods and shortcuts around a diffusion barrier in order to save time or just make a quick repair to jewelry. Trust me there is no acceptable way to lay gold down over bronze or brass without one. Either palladium or nickel will best do the trick (nickel is of coarse more cost effective). All the advise mark left was spot on. The only reason the copper was having such an effect on your final color was because of migration. A proper barrier that won't fight the gold shouldn't effect your gold color unless the gold is being flashed at under 7.0 micro inches. Under the FTC (U.S. federal trades commission) anything being commercially sold as "gold electroplate" must comply with the 7.0 µm or over.
Depending on the gold bath being used there is typically a number of ways to adjust the color of the gold you are plating with your gold bath. From increasing or decreasing the acid or cyanide value, or if it is an alkaline bath increasing hardener. simple temperature adjustments or ASF adjustments can also help in combination to achieve the color you desire. If you are using a proprietary gold bath contact your chemical supplier, they can usually give you the exact factor you'll need to increase or decrease to achieve what you are looking for. Best of luck.
Chance Dunstan
Electroplating/Forming/Coating Manager
Placerville, California USA
January 30, 2015
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