A. Hi Yogesh.
Please read on for more appropriate tests, but if copper sulfate reacted with the finish I believe your gold plating is porous and you would have failed any and all porosity tests. How thick are the nickel and gold plating layers?
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Yezdi. The nitric acid porosity test sounds like what you're looking for. Start with ASTM B735 or ISO 14647. Good luck!

A. There is surely a more easy test; the so-called Dimethylglyoxim test.

You have to make a solution of:
980 ml Ethanol,
7 g dimethylglyoxim
20 ml De-ionised water
1 g Ammonium chloride

Dissolve the components in this sequence. Stir until all components are dissolved

Use a voltage stabilized rectifier at 1.5 V
Make the test sample the anode; use SS-cathodes at 5 - 10 cm distance
Place the sample 1.5 minutes in this solution and rinse it with clear ethanol.
Pores will be visible as very intense red discolored area's.

Use the normal care while handling all chemicals and equipment

Success and let me hear if you were satisfied with this; as far as I know this is an official DIN-ISO -test method.

Bye,

Harry

A. Thanks Harry. I don't know the spec number that covers that test, but maybe ISO 10308, "Metallic coatings - Review of porosity tests" would be a good starting point.

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Nitric acid porosity test maybe?

A. The gold needs to be thick enough to be non-porous. One way this is tested is by exposing the parts to nitric acid fumes. Gold is resistant to nitric acid so if the gold is pore-free the fumes will have no effect; if the fumes do reach the nickel, nickel nitrate crystals will grow at the pores. See ASTM B735 or ISO 14647 for further info on this test.

A. The cleaning and activation of the music wire has to be pretty stringent as the delayed reaction [i.e., corrosion at a later date], are causing you problems. Do not be fooled as it may not be the gold that is bad but rather the nickel to the substrate. If that's bad, then so is the gold.

I used to use nitric acid, rather than the fumes. I would plate, then subject the parts to a 50% v/v nitric acid at room temp for up to 15 minutes. Then would rinse, dry, then tape test and look under magnification. If they passed that test, especially the tape, then they should all be good. I used a percentage of parts in the lot plated against the total of parts in the lot. Ours were small parts so I could take 1 part for each 50 thousand parts (50 parts) and do the test.

If one failed, then I figured one in a thousand would fail. I got it down to zero defect as I had zero in my test results. That came from realizing the cleaning and number of turns on the EN bath was critical. Oh, and the gold adhesion to the nickel ... and the gold purity.

A. Have you tried using an acid gold strike?

A. Is your gold suffering from porosity? Try a mild steel panel and test with a ferricyanide / sodium chloride solution for porosity. This could by why the nitric is eating the nickel.

A. Have you tried using an electroless nickel process with a high phosphorous content. These processes can withstand nitric acid better than nickel on its own.

A. As recommended before, a high Phosphorous coating is a good resolution, The amount of gold you require is significant -- if I am reading correctly, shouldn't the gold be enough? Anyway there is more than one solution, duplex coatings, etc.

A. Because of the cost of gold there is enormous and continuous pressure to keep reducing the specified thickness. If you look at gold plating specs from 40 years ago you would find that gold plating thickness was typically 10x what it is today. The chemistry has improved greatly but today's thicknesses are often marginal at best because pragmatism dictates that we just keep pushing the envelope.

Maybe a nitric acid porosity test could demonstrate that the gold is too thin, maybe not. Yes, there are other possible explanations for the black growth, but try doubling the gold thickness on some sample parts before investing months of study on esoterica :-)

A. What is under the gold? Silver and copper will migrate right through the gold and show tarnish on top. Give us more information.

A. Hi Anil. ASTM B735 "Standard Test Method for Porosity in Gold Coatings on Metal Substrates by Nitric Acid Vapor" isn't exactly the paper you are looking for, but it covers that technology.

"Gold Plating in the Electronics Industry" by Alain Blair, link.springer.com/content/pdf/10.1007/BF03216562.pdf, is a summary of the session in which that paper was presented; so you could look for the CD-ROM from Sur/Fin 1981 as another source for the paper as well.

A few libraries around the world, often personal ones, would have this. You might inquire of the Indian Association for Surface Finishing if anyone in your country is maintaining such a library.

"Finishing Publications Ltd & Metal Finishing Information Services" abstracted the article to their CD-ROM and presumably have it, but they obviously must charge for their work so I don't know under what terms they might offer it to you.

Luck & Regards,