An iron-carbon alloy initially containing 0.258 wt% C is exposed to an oxygen-rich and virtually carbon-free atmosphere at 1120°C. Under these circumstances the carbon diffuses from the alloy and reacts at the surface with the oxygen in the atmosphere; that is, the carbon concentration at the surface position is maintained essentially at 0.0 wt% C. At what position will the carbon concentration be 0.194 wt% after a 6 h treatment? The value of D at 1120°C is 6.9 × 10-11 m2/s.