On Saturday, March 4, 2017 at 10:17:05 PM UTC-5, Phil Allison wrote:
John Robertson wrote:

** That should be "Silver Sulphide" = Ag2S


Do you know what "black leg" corrosion is ?

Commonly seen on the leads of small signal transistors made in Japan in the 70s. It often results in the device going noisy or failing completely. Marantz stereo amps were notorious for it.


Can you provide a link to more in depth discussion on your black leg
corrosion? I did a quick search and could only find one reference, and
it merely said watch out for 'black leg corrosion'?


** Not much to be found on the net, but something most service techs have seen.

Bit like the "yellow glue" problem that has plagued countless items and still is.


I have run into a lot of Namco branded ICs from Japan, made in the late
70s that have this problem...


** It's got be due to silver plating the leads.



.... Phil

No, Phil, it is not *DUE* to the silver plating on the leads, nor is the silver plating on the leads even contributory to the primary failure, though it does contribute to the visual results.

Let me start over as you clearly need the little-steps-for-little-feet approach.

a) Silver electroplating does not adhere well to anything but copper, low-tin bronze and very low zinc brass.
b) So, most materials to be silver-plated are first flash-plated with pure copper. Otherwise the failure of the silver-plate due to simple peeling would be quick and fast.
c) Over time, the different coefficients of expansion, simple age, heat/cool cycles and other environmental factors cause the copper plating to crack, microscopically, initially.
d) At which point, electrolysis takes place between the copper and base-metal of the lead/pin/whatever.
e) The first product of failure is CuO - because typically the substrate (base metal) would be sacrificial to the copper, in this case, the copper becomes sacrificial to the silver.
f) CuO is a black semi-conductor, still (very rarely) used in very heavy duty rectifiers.
g) Silver reactions with sulphur do not produce a grainy material, Even extensive reactions.
h) Copper oxides, with the fine silver still attached, are grainy. The fine silver will be entirely covered by the copper oxide, but will not actually react itself.

So, that you state something *MUST* be due to something else does not make it so. Jewelers and metalsmiths have been exploiting this phenomenon for many years and it is very well understood. You need to learn that there is an entire world outside your cave.

Peter Wieck
Melrose Park, PA