Porous Gold 1

[Yobyor]

Hi ALL:
I am working with gold plating, over Ni, on Cu... standard stuff. But, I ran into an engineer who insists that gold plating is porous, or more exactly, gold itself is porous. As far as I know, that is not the case. But, he insists that since Hg gets absorbed by gold: the Hg moves into the pores in the gold (his explanation), and actually can move through the gold. He also insists that since Hg is a large atom (true) that other atoms like Ni and Iron can move through the pores in the gold, or gold plating, since a large atom, like Hg can.
On thin gold plating, without a Ni substrate (or suitable substrate, and barriers), you can see the copper, "migrate", over extended time, through the gold plating. He says, the copper atoms travel through the pores in the gold, to the surface of the gold plating, which is wrong. Maybe someone can comment, because he doesn't believe me... I'm not a materials guy.
Regards

 

[IRstuff]

Perhaps it's a question of the definition of "pore," as indicated in

https://www.copper.org/applications/industrial/DesignGuide/failure/porosity04.html

where the "porosity" is really more like a plating defect

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[Yobyor]

Hi IRstuff:
Thanks for the reply.
No, it is not a plating defect he is referring to. That's the first thing I pointed out... so called "pin holes"… or pits, in the plating... when there are defects. No.. he is insisting that there are actual tiny pores in bulk gold. In fact he thinks they are the interatomic spaces, due to atomic packing, and that somehow Hg, or Ni, or Fe, can migrate through them. He further claims that gold disks were used in gaseous diffusion... not sintered, or fused, but pure thin gold disks... because gold is porous, and the gases can migrate through them.

 

[MintJulep]

Seems plausible, although the mechanism might be more subtle than "pores"

https://ieeexplore.ieee.org/document/382669

 

[ornerynorsk]

Passive diffusion of metal atoms through other metal atoms? at room temperature?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[EdStainless]

Hg and gold is LME, I can do the same with many low melting point metals, such as Cu on SS or Zn on SS.
The only difference is that Hg is liquid at RT and the others I have to heat.
If you combine Hg and gold at LN2 temps there is no reaction.
The word is diffusion.
If there is some degree of solubility almost all metals will diffuse in each other.
But the diffusion coefficients very by a few orders of magnitude.
Uranium gaseous diffusion was done with sintered Ni filters made from Ni powder.
Gold is far less permeable than most metals.

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P.E. Metallurgy, Plymouth Tube

 

gold itself is porous

Your engineer advisor just forfeited all credibility.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[racookpe1978]

Elemental Hydrogen will diffuse through the walls of the liquid hydrogen piping, but at much lower rates than through a pinhole or connection leak. But even oxygen or Nitrogen at high pressure does not diffuse through walls. (Absence other leaks of course.)

To go "through" a metallic coating, seems like you would have to near-saturate the entire coating thickness first by "forcing" the diffused Hg into the surface of the coating, then thru the coating, then through the base metal (Cu in this case.) Not realistically going to happen in this case.