On Sun, 03 Jul 2016 16:28:52 +0200, Emanuel Berg
wrote:

John B. writes:

But I suggest the real question may be, how
long have these things been in use"?

How do I find out? Carbon-14?

Why, I assume "just look at it" :-)
Or, more seriously, identify the components and ask Andrew Muzi who
can probably tell you the period in which the components were used in
bicycle manufactured.


No, when I see the rim again, I'll examine it
closer for some brand or other clue...

Does corrosion typically take a long time to
happen? With batteries the salt seems to be
around pretty frequently...

Think of the battery terminal as a worst case (although it may not be)
but un insulated aluminum to stainless joinings do, in the presence of
salt water, usually show corrosion in a matter of months.

The speed seems to be related by the distance the between the metals
on as listed on a "galvanic chart".

From: http://www.engineersedge.com/galvanic_capatability.htm

"For harsh environments, such as outdoors, high humidity, and salt
environments fall into this category. Typically there should be not
more than 0.15 V difference in the "Anodic Index". For example; gold
silver would have a difference of 0.15V being acceptable.

For normal environments, such as storage in warehouses or
non-temperature and humidity controlled environments. Typically there
should not be more than 0.25 V difference in the "Anodic Index"."

Aluminum - ~0.60, Aluminum - ~0.90

But note that for galvanic corrosion to occur there must be an
electrical conducting path between the two. Pure, distilled, water is
essentially an insulator while water with impurities (such as salt)
are a conductor.
--
cheers,

John B.