Hello Angelo:
Actually, there is not an oxidative process causing the discoloration.
Silver plating does not oxide without exposure to temperatures well above
200 degrees centigrade. That is one of the reasons that the Teflon coated
wire you speak of is rated at 200 degrees C, if it is PTFE insulated.
The standard military thickness of the coating of the hook up wire is 40
microinches minimum thickness, regardless of the AWG size. The wire is
typically electroplated at a larger size, perhaps 16 to 18 AWG, and then
drawn to finish sizes. The ratio of the weight of the silver plating to the
copper substrate remains constant during the drawing process, although the
coating becomes thinner and thinner. At finished size, the wire is then
resistance annealed, to return the elongation and ductility to the hard
drawn silver plated wire.
Larger tank coils, on the other hand, are probably wound of bare unplated
copper or copper tubing, and then are electroplated at finished size. These
parts are barrel plated, in a tank, and are surface treated following the
electroplating, to avoid discoloration. In this process, the work piece,
the coil, is not moving thru the tank. In the plating of the redraw wire,
the wire is moving continuously through the bath.
The discoloration you speak of, is normally sulfurization of the silver
plating. Silver has a great affinity for sulfur ions, often found in the
air. Clean silver plated copper, if left exposed to air containing only
minute amounts of sulfur, even over night, will turn yellow. The compound
on the surface is silver sulfide, which is highly soluble in very dilute
hydrochloric acid solutions, and can be removed by a brief dip in that
acidic solution. The formation of the tarnish, silver sulfide, is not
proportional to the plating thickness, but rather to its cleanliness and
exposure to sulfur ions. The surface treatments following barrel plating
inhibit the formation of this compound, and yellowing.
While you can calculate the depth of current penetration into the silver
plating, for any given frequency, in practice, the coils are probably plated
to about 2 percent by weight of silver to copper. On large tank coils, this
will be hundreds of microinches, rather than the 40 micro inches found on
the mil spec hook up wire, such as Mil W 22759 types. The thickness of the
plating may become a significant factor at VHF and UHF if lower losses are
desirable. By the time we consider frequencies in the several gigahertz
range, we have to use plating thicknesses several times thicker than the
standard coatings to accommodate the depth of penetration of the RF currents
and minimize the RF resistance component related to skin depth.
73 and Happy Dxing,
Mike Schatzberg
WB2AJI
----- Original Message -----
From: "Angelo Karabetsos" <ve3yn@hotmail.com>
To: <amps@contesting.com>
Sent: Wednesday, August 17, 2005 12:09 AM
Subject: [Amps] Silver plating tank hardware
> What is the best way to silver plate tank components/hardware? Is it
> better to use copper or brass as strapping for a given application? Any
> suggested minimum thickness? Some silver plated tank components last for
> years without heavy oxidization yet multi-conductor silver plated teflon
> shielded wire oxidizes in a few days once stripped. I suspect that it is
of
> a much higher silver content. There isn't too much information covering
this
> topic floating around on the net. Hopefully there are some hints, tips or
> tricks to eliminate the snake oil factor.
> 73 VE3YN Angelo
>
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