plating, anodizing, & finishing Q&As since 1989
Black marks on gold plated items(nose pins)
March 1, 2010
I am Arun. We manufacture light weight (0.060g to 0.250g) gold nose pins of 9k to 22k.
Most of the items have to be fully finished (which includes gold plating) and delivered. The items though appear good initially, get black marks over the surface (it begins to show up after a week or so) which worsens with time. This problem is present only in the recent months. The local platers have no clue. They use gold cyanide bath with inert anode.
After their consultation these changes were made but in vain.
1. The composition of gold alloy was changed.
2. The surface finishing was made more even and shiny.
3. The thickness of gold plating was increased (as said by them!)
When we insisted on increasing the dipping time the articles started to turn dull & brownish.
One could compare this to rusting of iron over nickel coating.
I want to know the possible reasons for this problem.
Your valuable suggestions will be appreciated by many goldsmiths here.
employer - Trichy, Tamilnad, India
March 4, 2010
Black streaks can be caused by a few things. From what you say about the consultation with your plater, I think they are missing the mark with troubleshooting this problem. Increasing the gold dwell time and changing the alloy won't get rid of the black streaks. These are the preventative steps I recommend.
1) Make sure the nickel coating is thick enough. You will want to rule out migration of the base metal through the nickel and eventually through the gold.
2) Have them inspect the nickel plate before going into the gold. This problem could very well stem from hydrogen gassing. They may have to add a wetting agent, or increase it. If they see black streaks after nickel, it is most likely hydrogen gassing.
3) Make sure the nickel and gold bath is not contaminated with iron,lead or copper. If they have a decent lab, they can check for common metallic contaminants. If not, their nickel and gold bath supplier can do it for them.
4) Have the plater run a analysis on the gold bath to check for gold and cyanide concentrations.
5) Make sure the current density is not too high for nickel and gold.
6) Review cleaning and rinsing steps.
Because the gold coating is probably so thin (usually a flash coating in jewelry applications), imperfections in the nickel are visible after a short period of time. I would concentrate on suggestions # 2 and 4 to start with. Good Luck to you.
process engineer - Malone, New York
March 8, 2010
Maybe something has changed in your manufacturing process. You should expose some of your gold items that have not been plated to the same conditions as the plated ones and see if you still get those black marks.Neil Bell
Red Sky Plating
Albuquerque, New Mexico