Insight into the Corrosion Inhibition Mechanism of Sodium Silicate on the Magnesium Alloy Surface: Experimental and Theoretical Calculations

The corrosion inhibition mechanism of sodium silicate (SS) for WE43 magnesium alloy in NaCl solution was investigated by in situ observation, electron probe microanalysis (EPMA), electrochemical test, and theoretical calculations. In situ observation showed that local corrosion was markedly inhibited after the addition of the SS inhibitor. Electrochemical data revealed that a protective layer was formed on the metal/solution interface and an optimum inhibition efficiency of 98.3% in the case of 2.5 g/L SS. A uniform magnesium silicate layer with a thickness of ∼2 μm formed on the uncorroded area was confirmed. Quantum chemical calculations revealed that SiO32- could absorb on the MgO (1 0 0) surface in a parallel orientation through the coordinate bonds between the O and Mg atoms. The distances of Mg-O bonds are 2.052 and 2.249 Å, suggesting that they are coordinated. The adsorption/interaction mechanisms of SiO32- were also analyzed through charge density distribution and atomic densities. Molecular dynamics simulations further confirmed that a uniform SiO32- layer was absorbed on the MgO surface. In the local corroded area, free SiO32- would react with Mg2+ and OH- produced by corrosion to form the insoluble magnesium silicate compound, which hindered the spread of corrosion.