This work investigates the Corrosion Layer Accumulation Rate (CLAR) of iron carbonate (FeCO3) onto X65 carbon steel in carbon dioxide containing environments using the direct method of corrosion layer mass gain measurement. Glass cell experiments were performed at 80 degrees C and pH 6.3 or 6.8 over a range of bulk FeCO3 saturation ratios using both actively corroding carbon steel and steel pre-filmed with FeCO3. The CLARs obtained from experiments using actively corroding samples displayed strong agreement with the most recently developed precipitation model by Sun and Nesic at high supersaturation for pH 6.3 and 6.8, but a disparity at low supersaturation for the solution at pH 6.8. The observed discrepancy was attributed to the significant difference in surface saturation ratio between the two conditions when the steel is actively corroding. CLARs determined for pre-FeCO3 filmed carbon steel show that the kinetics of FeCO3 formation reduce significantly once the film establishes a protective barrier at lower values of bulk supersaturation. The results highlight the contrast between surface layer accumulation kinetics in the early stages of growth and those encountered in the long-term after the development of a protective film.