Investigation of the effect of CrO 42− and MoO 42− on carbon steel corrosion using AFM and electrochemical techniques

The effect of chromate (CrO 2- 4 ) and molybdate (MoO 2- 4 ) ions on the corrosion of carbon steel in 0.5 M NaCl solution has been studied using electrochemical measurements and atomic force microscopy (AFM) technique. Potentiodynamic polarization data suggest that both CrO 2- 4 and MoO 2- 4 have inhibition effect on carbon steel corrosion, and the inhibition efficiency increases with increase in concentrations of CrO 2- 4 and MoO 2- 4 ; at the same concentration, the inhibition efficiency of CrO 2- 4 is higher than that of MoO 2- 4. . The increase in concentrations of CrO 2- 4 and MoO 2- 4 anions causes a shift of the breakdown potential (Eb) in the positive direction, indicating the inhibitive effect of the added anions on the pitting attack. At the same concentrations, the breakdown potential of CrO 2- 4 is higher than that of MoO 2- 4 . Electrochemical impedance spectroscopy (EIS) tests reveal that the charge transfer resistance and passive film resistance increase with increase in concentrations of CrO 2- 4 or MoO 2- 4 ; at the same concentrations, as for CrO 2- 4 the charge transfer resistance and passive film resistance were bigger than those of MoO 2- 4 . AFM imaging technique shows that local corrosion was inhibited obviously after the addtion of CrO 2- 4 or MoO 2- 4 , and passive film of CrO 2- 4 was much more compact than that of MoO 2- 4 . AFM force-distance curves indicate that the passive film of CrO 2- 4 is much stiffer than that of MoO 2- 4 .