# Gold plating thickness, nitric acid resistance



## snoman701 (Aug 19, 2017)

I cannot find this for some reason. In my head, I want to say that if a plated object is resistant to nitric it's at least 25-30 micro inches thick, but I can't figure out the right search terms. 

Have a couple pounds of pins that do nothing when put in nitric. When I cut them up in to tiny pieces, the gold foils cleaned up and held together, then took forever to get picked up by the AR. I'm waiting on settling right now.


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## nickvc (Aug 19, 2017)

Did you try a sample in Hcl ? 
It's always worth a try.


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## snoman701 (Aug 19, 2017)

Why?


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## anachronism (Aug 19, 2017)

snoman701 said:


> I
> 
> Have a couple pounds of pins that do nothing when put in nitric. When I cut them up in to tiny pieces, the gold foils cleaned up and held together, then took forever to get picked up by the AR. I'm waiting on settling right now.



Do you mean that you cut them up and put them in AR?


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## snoman701 (Aug 19, 2017)

No, I cut them up and digested the base metals in nitric to determine if the gold disintegrated or held it's shape. 

Following three distilled water rinses, I added a few mL HCl (just enough to cover in 20 mL beaker) and nitric dropwise. It was the digestion of the clean gold foils in AR that seemed to take quite a while.

On Edit: Change DI to distilled water


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## nickvc (Aug 19, 2017)

snoman701 said:


> Why?




Not all pins are created equal, some are made of copper or of copper alloys others are not, it's always worth trying a small sample in Hcl to see if you get a better dissolution if the nitric seems slow or resistant.


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## snoman701 (Aug 19, 2017)

nickvc said:


> snoman701 said:
> 
> 
> > Why?
> ...



I see...in this case it was just due to the thickness of the gold plating. Once I cut them in to pieces they cleaned right up. These are goofy pins...I'll post a picture later.

Ended up with a little under 1% by weight gold (.015 grams), so not bad at all. But, i only cut 1.8 grams of pins up. 

Need to find my centrifuge so I can settle gold out faster.


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## anachronism (Aug 19, 2017)

A centrifuge? Although I've never seen the need for one I'd be honestly interested in why one would be of regular benefit?


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## snoman701 (Aug 19, 2017)

No more waiting for the fines to settle before pouring off supernatant. Pop it in the centrifuge, spin it, gold settles at the bottom of the tube in a nice little hard blob. Pour off, rinse & repeat.

A majority of the time it takes to run an assay is waiting for the fines to settle.


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## goldsilverpro (Aug 20, 2017)

snoman701 said:


> I cannot find this for some reason. In my head, I want to say that if a plated object is resistant to nitric it's at least 25-30 micro inches thick, but I can't figure out the right search terms.
> 
> Have a couple pounds of pins that do nothing when put in nitric. When I cut them up in to tiny pieces, the gold foils cleaned up and held together, then took forever to get picked up by the AR. I'm waiting on settling right now.



For the hard, bright, Ni or Co alloyed, stressed, acid gold used on pins and fingers, the figure is closer to 100 micro". It takes that much to seal up all of the porosity inherent in all gold plating. That's for a brand new plating bath. On parts that need to be heated to mount a chip or lid, very pure gold plating is used and this thickness figure is usually less, maybe 40-50 micro".

As a plating bath ages, it is nearly impossible to keep all contaminants out of the bath. Many contaminants, whether organic or metallic, create stress. This creates micro-cracking which can allow penetration of the acid, thus raising the thickness it takes to prevent this penetration. For hard acid golds, we used to get an indication of stress by plating 100 micro" on a polished brass panel and then putting it in nitric. If the plating fell apart, it was stressed.

Due to the great differences in the plating characteristics, from part to part and from bath to bath, it is impossible to determine the thickness when the nitric doesn't penetrate - there are too many factors involved. Also, non-penetration is very rare when the solution is heated. About all you can say, when the hot nitric doesn't penetrate, is that the plating is pretty damned thick.


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## snoman70 (Aug 22, 2017)

Thank you gentleman....I am finding that there is a lot of working knowledge that is difficult to remember. However I like "pretty darn thick" as compared to multiple numbers! 

In the end it's not what it's supposed to be but what's in the bottom of the beaker that counts!


Sent from my iPhone using Tapatalk


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