On Thu, 27 Oct 2011 07:51:16 -0400, "Jim Wilkins"
wrote:
wrote in message
.. .
http://www.flickr.com/photos/2768312...in/photostream
Previously when I saw pits like these I thought it was caused by pin
holes in the resist. There was some discussion about its here. I
firmly believed that it was the hydrogen bubbles cavitating causing
this. However, in this case there is *no* resist!
Michael Koblic,
Campbell River, BC
What I remember from chemistry lectures long ago is that the components of
an alloy separate as the cast ingot cools, giving crystals of the more
stable composition surrounded by a less organized mass of somewhat different
proportions. Rolling the metal into sheets spreads out the crystals and
depending on their size (cooling rate) may make the alloy vary with location
at visible scales, like granite rock.
The junction of two different alloys has an electrical potential that is the
difference between their attraction for electrons, like a thermocouple or
battery. In a corrosive environment these slightly different alloys act like
weak batteries, so the more active material will dissolve faster than the
less active one. This is part of why rust forms isolated deep pits instead
of cutting in evenly.
Notice the uncertainty:
http://en.wikipedia.org/wiki/Pitting_corrosion
Another part:
http://en.wikipedia.org/wiki/Crevice_corrosion
I read both several times. Interesting. I can sort of see the pitting
type of corrosion more than the crevice type. I am not sure that it is
the whole explanation. Some of the considerations that puzzle me a
1) Why would a weak local battery not be swamped by a 2.5A current
passing through the whole system?
2) Why are the pits, if they develop as described in the article, so
deep and apparently so uniform in shape (I enlarged the photo - most
of them seem comma shaped)?
3) Why does one not see similar phenomenon with brass? There I am
removing copper from zinc. One would have thought that local batteries
would if anything be "stronger".
There are other considerations derived from the previous experience of
similar pits developing under a resist. However, I am not even sure
this is the same phenomenon. The pits sure look similar.
I am going to run a few test and see what gives.
Michael Koblic,
Campbell River, BC