Jim Lesurf wrote:

In article , Dave Plowman (News)
wrote:
In article , John R Leddy
wrote:

Java Jive;94051 Wrote:
On Sat, 13 Jun 2015 15:35:44 +0100, RJH wrote:
Unlike gold, silver corrodes quite easily. That's why your family
silver needs constant cleaning by the ball of your butler's thumb
wrapped in a suitable cloth.
I'm under the impression silver tarnishes; that's to say, there's a
layer of staining sitting on top of the surface of the silver, which
is easily wiped away with lime juice. I've been thinking copper
corrodes; that's to say, there's a chemical reaction within the copper
itself, which is destructive and similar to rust in iron. Anybody have
a more precise explanation? Just curious.

As regards switch contacts, silver is certainly a better choice than
copper in terms of staying low resistance. But there may well be a
better 'plating' than silver which is also cheaper than gold.

IIRC There are a number of different treatings which can generate a 'hard'
(i.e. resistant to being scraped away) don't tarnish, etc. Even 'gold'
tends to be a series of layers of alloys, etc. By itelf, gold tends to wipe
off metal surfaces very easily. I think the coatings involve layers /
alloys using things like rhodium, tec.

I recall on a particular project some years ago, a make of relay
that had been fine previously was suddenly giving great problems
- they appeared to be physically closing the contacts, but not
passing current.

After much head scratching the story emerged that the supplier's
moulds were wearing, but they didn't have time to replace them
and meet our large order schedule. Instead, to get the plastic
components down to size, they had been linished. After assembly
and test they were then individually bagged and shipped, and we
duly built them into our kit, tested as OK and delivered the
finished equipment.

The dust from linishing, trapped in the plastic bag, formed a
thin film over the contacts, and once they had opened under load
a few times (this was a 110 V dc system) the arcing was
sufficient to fuse a little of the material into an insulating
layer. Routine testing alone was sadly insufficient to trigger
this.

No butlers were required, but the simple solution was the service
engineer's thumb, which was sufficiently abrasive to clean the
contacts without damaging the material.

Chris
--
Chris J Dixon Nottingham UK


Plant amazing Acers.