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99.93% Ag 0.07% Cu alloy: Can I use HF to remove organic contaminants?

February 12, 2008

I am investigating a cosmetic problem that occurs slowly over months to years in spots on the surface of a fine silver alloy, 99.93% Ag 0.07% Cu. Microscopy shows surface contamination, and an FTIR report on three spots turned up a very long list of possible organic contaminants in a very thin film. The analysis was made difficult by surface roughness. If the problem is entirely surface contamination, and the metal alloy has undergone no change, I would like to attempt to remove these unknown contaminants to see if the cosmetic problem goes away. Of course, if the cosmetic problem remains, perhaps not all contaminant was removed, or perhaps they were and the defect is in the alloy. I am looking for one or more candidates for chemical surface cleaning. Mechanical surface cleaning is (for now) not a possibility.

I am not from this field, though I worked as a technical generalist in semiconductors and hard disk drives, so I do have some experience in speaking with people in this area. And by the way, I know that HF is extremely dangerous and requires great precautions.

In the book "Corrosion Basics" by Pierre Roberge, in the section on silver, it states "Applications include stills, heating coils and condensers for pure HF acid, ..." Perhaps pure silver is inert when exposed to some forms of HF? Would the small amount of copper cause problems?

I also found this web page: http://people.bu.edu/jtien/Xia%20Langmuir%201998%2014%20363-371.pdf which had this sentence: "The patterns of electroless silver, however, could be used as masks in the isotropic etching of SiO2 in a dilute aqueous HF solution." This seems to lend some additional weight to the possibility.

If the correct precautions and safety procedures were used, would placing the silver alloy in some type of HF bath be a reasonable way to attempt to remove the contaminants without disturbing the alloy? If so, what can be said more specifically?

By the way, the same text contained: "It [silver] is highly resistant to organic acids." Is there a reasonable choice of organic acid for the same purpose?

Thank you very kindly for your time, interest, and effort.

Karl Lofgren
Private Consultant; client company has the contaminant problem - Newport Beach, California, USA

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I would stay away from HF acid, even though it may work. You could make up a solution of 5% thiourea ⇦ this on eBay or Amazon [affil links] and 1% non- ionic detergent in distilled water. Dwell time would depend on the level of "contamination". This should do the trick for you.

Mark Baker
Process Engineer - Syracuse, New York
February 19, 2008