On Wed, 08 Dec 2010 19:02:38 -0500, Tony Miklos
wrote:

On 12/8/2010 5:47 PM, wrote:
On Wed, 08 Dec 2010 11:44:23 -0500, Tony Miklos
wrote:



A few weeks ago I installed a couple of 35 amp bridge rectifiers. The
leads were, as they often are, blackish colored and do not tin very
well. Seems they always come that way, even from different
distributors. They always get hand cleaned with the same brass wool I
use for the soldering iron before being installed.

It sounds like these are contaminated. Any abrasive cleaning can damage the
plating and could *easily* cause problems down the road.

No problem with them 30 years later. I'm not too worried.

Black coloured component leads??
Sure sounds like silver plating. Sulphur in the air tarnishes silver,
making a very black coating. Removing the tarnish by abrading should
make a reliable solder joint.

Actually it does look like silver... oxide? I've been told that it is
a great electrical conductor, but not great to solder to. Do you know
if the black stuff does really make a better conductor for something
like a switch?
No, it does not - and if my hunch is right they were not black when
they left the factory. It is NOT silver Oxide - it is more like siver
sulphide - Ag2S, or silver chloride.

Both sulphur and chlorine will tarnish (blacken) silver very quickly.


Sulphur will often also blacken tin - so it is possible you just have
"tarnished" tin coating.
Blackened tin is neither a good conductor or a suitable substrate for
soldering.
Tin can also de-zinc brass, which can cause blackening as well if tin
coating is used on brass terminals without a copper or nickel strike
plating as a barrier.

I know Smitty is going to dissagree with me, but so be it.