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Why are gold plating baths turning green?




I am a chemical technician working for a company that manufactures small electronic parts for disk drives and we use a Ni/Au plating process. My problem is that we have some of our gold plate baths turning slightly green in color. I am trying to identify the source of contamination, but I am having difficulty. I ran a sample on the UV/vis, but no peaks were in the visible region. I ran samples on the AA, but there was no significant increase in Cu, Ni, Cr, Pb, Fe. Total metal content = 74 ppm with Ni being 45. Ran bacteria test, no growth. I think it is organic. Carbon treated 250 ml sample with 0.25 g act. carbon and no color change. Hull cells looked brighter after carbon treatment in the lab. Treated a 500 L tank with carbon filter for 4 hrs and no change. Has anyone else had this problem before?

Carol Anderson
Plating - Eau Claire, Wisconsin
2004



Dear Carol,

Generally no organic additives as brightener is employed in gold plating baths. The alloy component as nickel, cobalt, indium etc. behave as brightener. Organic additives as citric acid /salt, EDTA or phosphate salts acts as buffering. Therefore the reason of becoming greenish can not be organic additives in the baths. The main reason is contamination due to drag in which is most probable nickel plating bath. The organic additives may come from the nickel bath can not be green because they are not green (some of them colorless some of them yellowish). But nickel salts are always green. Therefore most probable reason is nickel salts.

You ran sample in UV and AA. I recommend you to check your instruments. First contaminate your sample with nickel salt (as chloride or sulphate) and then compare with the original sample using AA. I don't recommend to use UV because the sample is coloured.

You mentioned the total metallic quantity as 74 ppm. It is impossible because only metallic gold is about 1000 - 5000 ppm according to your bath type. I hope this information is helpful to you.

Fethi Dirim
- Istanbul, Turkey
2004




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