I've been cleaning up a lot of older steel tools using citric acid, which does a nice job.
Sadly these calipers emerged from an overnight dunking with the spring broken. It looks like it's probably been cracked for some time.
What are my options? Is it possible to get or make a replacement?
Clifford Heath.
Buy a cheep one at harbor fright and swap out the spring like I did, spring steel does not respond well to acid derusting.
Thanks, I learned something. 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
Just a WAG with absolutely no backup knowledge - I think it might have something to do with hydrogen and surface micro cracks.
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.
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