Investigation of the effect of CrO 4 2− and MoO 4 2− on carbon steel corrosion using AFM and electrochemical techniques

The effect of chromate ( ${\rm CrO}_4^{2 - }$ ) and molybdate ( ${\rm MoO}_4^{2 - }$ ) 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 ${\rm CrO}_4^{2 - }$ and ${\rm MoO}_4^{2 - }$ have inhibition effect on carbon steel corrosion, and the inhibition efficiency increases with increase in concentrations of ${\rm CrO}_4^{2 - }$ and ${\rm MoO}_4^{2 - }$ ; at the same concentration, the inhibition efficiency of ${\rm CrO}_4^{2 - }$ is higher than that of ${\rm MoO}_4^{2 - }$ . The increase in concentrations of ${\rm CrO}_4^{2 - }$ and ${\rm MoO}_4^{2 - }$ anions causes a shift of the breakdown potential ( E b ) in the positive direction, indicating the inhibitive effect of the added anions on the pitting attack. At the same concentrations, the breakdown potential of ${\rm CrO}_4^{2 - }$ is higher than that of ${\rm MoO}_4^{2 - }$ . Electrochemical impedance spectroscopy (EIS) tests reveal that the charge transfer resistance and passive film resistance increase with increase in concentrations of ${\rm CrO}_4^{2 - }$ or ${\rm MoO}_4^{2 - }$ ; at the same concentrations, as for ${\rm CrO}_4^{2 - }$ the charge transfer resistance and passive film resistance were bigger than those of ${\rm MoO}_4^{2 - }$ . AFM imaging technique shows that local corrosion was inhibited obviously after the addtion of ${\rm CrO}_4^{2 - }$ or ${\rm MoO}_4^{2 - }$ , and passive film of ${\rm CrO}_4^{2 - }$ was much more compact than that of ${\rm MoO}_4^{2 - }$ . AFM force–distance curves indicate that the passive film of ${\rm CrO}_4^{2 - }$ is much stiffer than that of ${\rm MoO}_4^{2 - }$ .