From my days working with Neon signs..
I'm sure this will apply to metal parts that have been worked with a lot of friction or heat.
I found out the hard way about the adage " Hot glass looks EXACTLY like cold glass".
Bent a nice star shape, set it down on the bench, then proceeded to pick it up...
As a point of interest, Nitric acid and Titanium do not play together well.
Nitric and Titanium will explode.
We had a customer change their deposition chemistry to include titanium without letting us know, our tech lowered the stainless parts with what was supposed to be only copper deposition...