On Tuesday, September 8, 2020 at 12:03:32 AM UTC-7, Clifford Heath wrote:
... Can anyone explain the metallurgy or
physics here? Very interesting! I mean it looks like they might have
been cracked for some time - is that possible? Does spring steel work
with cracks in it normally? Or has the acid just aged freshly-cracked
surfaces?

Clifford Heath

Stress corrosion is well studied (used to be a problem in auto springs, was solved
by putting a soft metal surface on the hardened spring material). The basic
principle, is that stress in a flexed spring chemically activates a bit of the
metal, which bonds to an H+ ion. That little hydrogen bond, unlike the
metallic bond, does NOT link adjacent atoms, so it is a zero-strength
bit of the steel. Now that the stress cannot use that atom's binding
force, the high-stress chemically activated region moves to another atom...
and a corrosion-accelerated crack zooms through the metal.

There's lots of hydrogen ions in acid, of course, by definition.

If there weren't any hydrogen bonding going on, the metal under stress could
deform plastically, and metallic ductility would ensure that the metal/metal
bonding stayed intact; spring steel, however, is NOT high ductility.