"jim rozen" wrote in message
...
In article , Harold and Susan Vordos says...

You actually can see the distortion from that.

That's an interesting observation. If you don't know, the reason the
nickel is plated to begin with is to prevent migration of the gold to the
base metal. In my years of refining I discovered it was pretty much
common
practice to find nickel under gold plating when stripping. Sir T.K.
Rose
concluded long ago that gold and silver have an affinity for certain
metals
and will migrate to them when in close proximity, and they need not be
molten or even hot. Ingots of pure silver and pure gold end up
transferring to one another when in contact, for example. Nickel
prevents
the migration. If what you say is correct, unless gold is plated quite
thick (I have no idea what that may mean) , there is likely no benefit in
using it at all, because, over time, and it's not long, it simply
vanishes.
One problem gets replaced with another. Comments?

Transition metals are commonly used as both diffusion barriers and
as adhesion layers. (Ti, Ni, Mo, W, etc)

The first prevents the plated, or deposited, layer from diffussing into
the the substrate. This is a real issue in semiconductor processing,
where it's important to keep layers of metal (wiring) on chips from
being contaminated by the layers adjacent. This is especially important
because some of the processing is carried out at elevated temperatures.
A layer of, say, Ti, that is about 200 atomic layers thick will
suffice to keep the layers of the cake from smearing out.

Adhesion layers are another issue. This is why nickel strikes are
used under gold plating as a rule.

Adhesion is a peculiar thing - what makes one thing 'stick' to another?
At atomic levels this question is a) of considerable practical interest
and b) not entirely well understood. There's a lot of research being
done on the fundamentals.

I can recall desperately trying to figure out some way to deposit
copper on sapphire stubstrates - but it would *not* stick. The
common trick is to put down a Ni adhesion layer, but nickel is magnetic
and for our work that was unacceptable.

That company I mentioned (trompeter) makes rf connectors for low
level, high freqency applications. All of the other connectors
I investigated were magnetic because of the ni strike in the plating
process. Because they build for the small-signal rf market - mostly
satallite communications gear - they understand that distortion of
the rf signal can occur from any magnetic materials in the connector,
so they use something they call a "pulse plating" process to
eliminate the Ni strike beforehand. This is specifically to
replace an *adhesion* layer, not a diffusion barrier.

Does the gold diffuse through into the brass metal underneath?

Yeah, and I've seen a few examples of it through the years, but just a few,
obviously something that was done rarely. Almost everything I processed
with cyanide had nickel underneath. Without the nickel strike, the gold
layer would start oxidizing, but only because it had been absorbed by the
base metal. As you well know, it's not lost, but is no longer providing a
barriers because it has diffused too much, leaving the base metal
unprotected, and it's the base metal oxidizing, not the gold. The gold is
recovered in refining, assuming you dissolve the base metal, not strip it,
so long as nothing is discarded. I found that by filtering with a well
clogged filter, or allowing the solution to settle well before decanting, I
would end up with the gold, although it certainly didn't look like gold at
that point. When the base metal is dissolved, the gold, for all practical
purposes is down to what might be considered a colloid, (go easy on me here,
Jim, I'm not a chemist) perhaps? Very finely divided particles, clusters
of atoms, nothing you can really see, but you do see the affect, a purple
color. Various processes before dissolving the gold allow complete
recovery.

I honestly don't know. They may use some other kind of diffusion
barrier, but I've never seen them degrade over the 20 or so years
I've been using them. I will look up in their catalog at work
and find their spec for plating thickness.

That varies according to need, and one pays accordingly. I supplied a
modified connector to Univac on several occasions, where four were made from
one, a Cinch wire wrap connector. This was prior to May of '83, when I
closed the doors on my shop permanently. I purchased them directly from
the mfg. and modified them to Univac's specs. They specified a given
depth of plating, under a tenth, 80 millionths as I recall, but one had
options as need required. At least that's the way I remember it. It was
interesting to see the pricing schedule and gold adder, considering I was
machining at the same time I was refining and by then had a firm
understanding of the amount of gold in question. Need I tell you the
price of the gold adder was quite lucrative for the manufacturer?

I did have one very interesting experience, the opportunity to strip some
micro-wave gear that hailed from what must have been WW II equipment. The
plating on it was so heavy that it was in thousandths, not millionths.
There was no barrier under it, which made stripping with cyanide very
difficult because there was nothing to prevent the base metal from
dissolving along with the gold (done electrolytically). Nickel does just
that. Some of this gear was one of the rare times I recall seeing gold
oxidize, but only one part in particular. It was a brass casting (wave
guide? dunno!) that was heavily plated, but still managed, some areas much
worse than others. The gold had obviously been absorbed. A sulfuric
process would have been far better suited to stripping that stuff. It
doesn't attack the copper base, but removes the gold quickly. I would have
to wonder if the diffused gold would have been recovered, though.

Harold