Darkroom chemicals versus natural environment? (Though maybe it's pollution or trace elements in organic materials causing the rust / pitting on the CT?).
Darkroom chemicals have varying pH, both acid and alkaline, there’s various salts involved, and there’s a decent amount of metal-on-metal contact in development tanks (you load the film onto a metal reel, and the metal reel goes into a metal tank, and they rub against each other).
Fair, 90% of my darkroom experience was a stat cam and we used plastic tubs and rubber(ized?) rollers. They always stained but since I wasn't using metal...
