Comparison between Lacunary and Saturated Keggin Polyoxometalates as Steel Corrosion Inhibitors in Chloride Solution: Contribution of the Lacuna in the Inhibition Mechanism

The effect of two lacunary Keggin polyoxometalates (K 7 [PW 11 O 39 ].14H 2 O and K 9 [AlW 11 O 39 ].15H 2 O) on the corrosion inhibition of XC38 steel in 0.1 M NaCl was investigated. Electrochemical methods (polarization curves and electrochemical impedance spectroscopy) showed that the presence of controlled amounts of these compounds improved protection of the steel against corrosion. These results were supported by surface analysis, where only a minor amount of rust was produced on metal surface in the presence of polyoxometalates (POMs). The lacunary POMs were more efficient compared to their saturated species, highlighting the key role played by the lacuna on the inhibition process. Efficiency of POMs (both saturated and lacunary) was explained by their ability to increase the Fe(II)/Fe(III) molar ratio in greenish protective oxide layer formed upon steel oxidation. The mechanism proposed clearly explains the higher efficiency observed with lacunary POMs, in relation to their ability to react with free Fe(III) to yield a substituted structure.