Same sample, same 60-minute hold — two different experiments. In air the oxide scale thickens parabolically; under inert purge the same thermal history leaves the surface almost untouched.
Schematic parabolic oxidation Δm² = kpt with an Arrhenius kp(T) and a strongly suppressed (not zero — real purges carry ppm O₂) inert-side constant. Teaching parameters throughout; real kinetics are material-specific and can follow other rate laws.