A combination of an in-situ microzone injection test and ab-initio DFT calculations was employed to investigate the effect of H+ and Cl- ions on the degradation of the FeCO3 corrosion products. The experimental results show that an acidic environment promoted the dissolution kinetics of the FeCO3 scales while the presence of Cl- ions induced FeCO3 surface reconstruction. The DFT calculations validate the experimental results and indicate that both H+ and Cl- corroded FeCO3 surface through weakening of the Fe-O bond and increased the corrosion active sites.