I checked another Firestone Challenger keg I have, and the interior looks the same. To my knowledge it has never had StarSan in it. It is a "planter keg" that Northern Brewer or Midwest sold a few years ago guaranteed NOT to hold pressure, and suitable only for planter use. I wrap my CFC around it. Just from looking at the interiors, I cannot tell it apart from my StarSan keg, which has held various batches of StarSan for about 3 years essentially without a break. I do push the StarSan with CO2, not air, so atmospheric oxygen exposure is pretty limited. Just my observations; I'm not a metallurgist. YMMV.