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.
