The Synergistic Inhibitions of Tungstate and Molybdate Anions on Pitting Corrosion Initiation for Q235 Carbon Steel in Chloride Solution

Abstract

In this work, the synergistic inhibitions of tungstate (WO42−) and molybdate (MoO42−) anions, including role and mechanism, on the initiation of pitting corrosion (PC) for Q235 carbon steel in chloride (Cl−) solution were investigated with electrochemical and surface techniques. The pitting potential (Ep) of the Q235 carbon steel in WO42− + MoO42- + Cl− solution was more positive than that in WO42− + Cl− or MoO42− + Cl− solution; at each Ep, both peak potential and affected region of active pitting sites in WO42− + MoO42− + Cl− solution were smaller than those in WO42− + Cl− or MoO42− + Cl− solution. WO42− and MoO42− showed a synergistic role to inhibit the PC initiation of the Q235 carbon steel in Cl− solution, whose mechanism was mainly attributed to the influences of two anions on passive film. Besides iron oxides and iron hydroxides, the passive film of the Q235 carbon steel formed in WO42− + Cl−, MoO42− + Cl−, or WO42− + MoO42− + Cl− solution was also composed of FeWO4 plus Fe2(WO4)3, Fe2(MoO4)3, or Fe2(WO4)3 plus Fe2(MoO4)3, respectively. The film resistance and the defect quantity for Fe2(WO4)3 plus Fe2(MoO4)3 film were larger and smaller than those for FeWO4 plus Fe2(WO4)3 film and Fe2(MoO4)3 film, respectively; for the inhibition of PC initiation, Fe2(WO4)3 plus Fe2(MoO4)3 film provided better corrosion resistance to Q235 carbon steel than FeWO4 plus Fe2(WO4)3 film and Fe2(MoO4)3 film did.