Ross Herbert wrote:
On Sat, 08 Mar 2008 11:42:24 +0000, "Ron(UK)" wrote:

:
:Why is it always the negative lead that corrodes?
:
:Ron(UK)

I am not a chemical engineer so I don't have any practical knowledge of galvanic
corrosion protection. There is a so-called Galvanic Table from which the degree
of corrosion can be predicted and it basically says that the higher the order of
one metal (on the table) relevant to another metal in close proximity, the the
higher order metal will corrode when an electrolyte covers them both.

Copper wire connected to the negative terminal is fairly high on the galvanic
table and assuming the casing of the solar lamp is made of stainless steel, this
metal is lower on the galvanic table. Moisture, complete with impurities will
condense across the surfaces of both metals during the night and set up
electrolytic action, thus the higher order metal will corrode.

Interesting stuff, and there is a correlation between disimilar metals
corroding - sacrificial anodes and all that, but I`m not sure that is
what happens in the case of nicad battery packs

It`s not only in metal cased fixtures tho, most of the garden lights
here are plastic - they only cost a few pounds, not worth repairing really.

I spent a great part of my youth and early adulthood messing about with
radio controlled models, and whenever a Nicad batterpack failure
occured, it was always the negative wire which had turned black and
literally rotted away. I don't think moisture has much of a role in the
matter, as many battery packs are quite well sealed.

I wonder if the same problem occurs with types of battery technology
other than Nicad, NiMH for example?

Even in cars, I believe that it`s usually the Negative battery
connection which gives trouble, either at the battery termnal or the
chassis ground connection.


Ron(UK)