As far as I know, pot metal was made up from whatever materials were
lying around.  It was cheap and easy to brew up.  The manufacturers
of this stuff probably didn't realize that it would all fall apart 50
years later.  The intergranular corrosion is caused by the mixing of
metals that are not compatible; in time, they disassociate themselves
with their forced brethren.  Hot and cold cycling helps move this
process along.

Bob Hodge