Electrochemical Investigation of the Synergistic Effect Between Molybdate and Tungstate on Surface Passivation of Carbon Steel

The purpose of this paper was to understand the synergistic effect between molybdate and tungstate for the surface passivation of carbon steel, particularly to find the optimum ratio of molybdate and tungstate. For Q235 carbon steel in mixed Na2MoO4 and Na2WO4 solutions with different ratios of MoO42- and WO42-, the electrochemical methods, including potentiodynamic polarization, Mott-Schottky and electrochemical impedance spectroscopy (EIS), were applied. Under the total concentration of 0.2 mmol/L, the ratio variation of MoO(4)(2-)and WO42- did not affect the corrosion behavior of Q235 carbon steel, and the main corrosion parameters, including corrosion potential (E.), corrosion current density (i.), flat band potential (Uf) and inner-layer passive film resistance (Ri) were independent of the ratio variation; however, the ratio variation of MoO42- and WO42- affected the passivation behavior of Q235 carbon steel obviously, and the main passivation parameters, including transpassive potential (Et), donor density (ND) and outer-layer passive film resistance (R.), presented the optimum level when the ratio of MoO42- and WO42- was one to one. At the same time, the related mechanisms of corrosion and passivation were also discussed.