Problem with Kovar in AR

[DesertDave]

I encountered an issue with Kovar, at least I'm pretty sure it was Kovar, when I first attempted recovery with plated and capped transistors similar to T05 type. My partner and I first tried to process them by putting them directly into aqua regia but they quickly reacted to form a thick white muck. At that point, we set it aside not knowing what to do with it and changed our approach, using just nitric to dissolve the base metal enough to separate the gold foils.

I have since read through much information including Hoke's books as recommended along with countless threads on this forum as well as sciencemadness.com. I've been convinced that incineration is a very important process and have used it in more than one situation. We have decided to go this route with the thick white muck so it has been set out to evaporate naturally. The dry weather here in Arizona should dry it out soon enough. Then incineration and re-processing the remaining solids according to Harold's guidelines should produce results.

My question is this: Does anyone know if this is actually Kovar used as the base metal (see pics below) and is this the expected reaction to AR? And to the chemists out there, what is the cause of this reaction to white muck? I am always wanting to know why things are they way they are and would appreciate any insights.

More info regarding the parts: There were no numbers on the parts and the caps did not contain any substance inside. The small ones have an inert material sealing them on the bottom of the cap which do not seem to be affected by the acids. Thanks for reading.

 

[lazersteve]

Have you tested the packages with any other straight mineral acids (HNO3, H2SO4, or HCl) to see if they give the same muck?

Steve

 

[goldsilverpro]

If I recall, some TO5s and TO18s are filled with some sort of organic gel, foam, etc. that reacts with the acids. I would bet that is the source of the white muck. Are the caps magnetic? If so, they are likely Kovar. I have seen aluminum caps on the real cheap ones but the ones in the photo are probably Kovar, since the header is gold plated.

 

[DesertDave]

Thank you for your quick replies! Yes, I have tested some of these transistors in just HCl with no reaction, then others in H2SO4 without reaction (had success stripping some in a sulfuric cell), soaked some for 5+ days in AP without any noticeable effect, and then finally in just HNO3. Only in AR did they make the white paste.

In 68% HNO3, they would exhibit no reaction until the caps (which are readily attracted to a magnet as is the whole piece) released. They would come off after 15-20 minutes of soaking and would then begin a rapid dissolution releasing red NOx gases then were consumed in 5-10 mins. The remaining transistors, plated uniformly even under the cap, would then soak quietly at room temp for a day or more to finally peel off the gold foils. The wires took 2-3 days to lose the rigid base metals. I have successfully processed these parts using the latter method, finishing the foils in AR without issue.

My best results were to clip off the wires and process them separately from the tops. The wires took seemingly forever to become soft and free of base metals. I would use heated dilute HNO3 (50% H2O) for about an hour then turn heat off and leave for many hours/overnight. I would then pour off acid and refresh with new and repeat the process. Each time I used fresh acid, it would boil up a little in a chemical reaction for maybe a minute and then settle down again. I never understood exactly why that was.

The tops never boiled up. After the caps were gone, I used the two-bucket method demonstrated by Lazersteve in his AP videos to separate the foils. (Thank you, Steve for such well done videos!) Also, I have removed a number of the caps with pliers/wire cutters and did not find any substance inside. In contrast, I have also seen a few of the type that do contain gel under a thinner cap so I'm familiar and I don't believe that was a factor.

I appreciate the help in trying to understand what happened. From everything I've read about Kovar, it seems that others have had no problems with it. I will follow up with results from incineration when that happens. Thanks again.

 

[lazersteve]

My experience with Kovar is that it's nearly immune to nitric acid. I've also never seen a thick white muck when dissolving it with other acids (sulfuric or muriatic). I've never tried to dissolve Kovar with AR.

The muck almost reminds me of copper I chloride which starts out gray to whitish and changes to an olive green color when exposed to air for a short length of time. It also resembles silver chloride which changes to a purple gray color when exposed to bright light. Silver metal (along with other metals) is also soluble in dilute nitric. Perhaps the magnetism is from a barrier layer of nickel on the items under the gold.

Has the muck changed color since it was generated?

Steve

 

[glondor]

I am curious to see how this goes as well as I have some of these myself I need to process. I do not have nitric and was curious as to an alternate method. I hope you have good success.

 

[DesertDave]

No change in color so far and it's been sitting in open air for at least 2 weeks now, but not in bright light except for the pictures taken a few days ago.
With my successful batches processed in HNO3 to separate the foils, the base metal sure was resistant to dissolve.