Study On The Corrosion Behavior Of Mild Steel And Inhibition Effect Of Inhibitors Under Wet-dry Cyclic Conditions

The corrosion of mild steel at splash zone is serious because of the high salt content, shock waves, wet-dry alternate,sufficient oxygenand plenty of ultraviolet irradiation and so on. The most prominent feature of splash zone is wet-dry alternate of the electrolyte, so it is of important significance to study the corrosion behavior of mild steel under wet-dry cyclic conditions, so that to ascertain the influence factors of mild steel at splash zone and take effective measures to prevent the corrosion.In this paper, electrochemical impedance spectroscopy(EIS), Potentiodynamic polarization test, weight loss test, scanning electron microscopy(SEM) andX-ray diffraction(XRD) were used to investigate the corrosion behavior of Q235 steel and the inhibition effect of inhibitors at wet-dry alternate conditions. Besides, mechanism of corrosion inhibition was discussed and proposed. The inhibitors we studied were two kinds of combined inhibitor, that was the compound of sodium tungstate(Na2WO4·2H2O) along with zinc sulfate(ZnSO4·7H2O) and the compound of sodium gluconate(C6H11NaO7) along with zinc sulfate(ZnSO4·7H2O). The results turned out that both of these two compound inhibitors were excellent.The compound of sodium tungstate along with zinc sulfate(the initials of these words are selected and constitute the abbreviation of the compound inhibitor, that is STZS) showed good inhibition to Q235 steel in seawater, and the inhibitive efficiency was excellent when the concentration are 1g/L and 0.03g/L respectively. These two substances were synergistic with each other. The selected concentrations were used for the wet-dry cyclic experiments. Compared to that in seawater, the corrosion of mild steel after 10 wet-dry cycles was weakened obviously after adding the compound inhibitor, which indicated that the compound inhibitor STZS showed good inhibition effect to steel at wet-dry cyclic conditions. Potentiodynamic polarization test results showed that the corrosion current density icorr decreased immensely with the compound inhibitors in, and the inhibition efficiencywas86.79%, which is in agreement with that obtained by weight loss test(80.51%). EIS test showed that the value of the charge transfer resistance Rct increased a lot after adding STZS. With time going on, Rct decreased a little at first, then increased and remained stable after about 5 cycles, revealing the durability of the inhibitor. The corrosion of mild steel in seawater wet-dry condition was the continuous formation of Fe3O4 and FeOOH, and the transition between the two substances. The rust itself acted as catalysts and took part in the electrochemical reaction, which accelerated the corrosion process. While in the system that contained STZS, Zn2+could combined with Cl- and formed clathrate [Zn2+Cl-OH-], which made it difficult for the transportation of Cl-, thus the anodic dissolution was suppressed. Besides, the products Zn(OH)2 and Fe2WO6 formed a precipitation film and attached to the electrode surface and held back the corrosion reaction immensely.The compound of sodium gluconate along with zinc sulfate(SGZS) also showed good inhibition to Q235 steel in seawater. The concentration of 1g/L and 0.03g/L for these two substances were selected respectively for wet-dry cyclic experiment. The inhibition efficiency obtained by potentiodynamic polarization test was 89.78%,which is in agreement with that obtained by weight loss test(83.04%). EIS test revealed the similar result with that of STZS. Compared with the blank experiment, Rct increased a lot after adding SGZS. With the time going on, Rct decreased a little at first, then increased and remained stable after about 5 cycles, revealing the durability of the inhibitor. In the system that contained SGZS, C6H11O7-combined with Fe2+and formed chelate which attached to the electrode surface, thus restrained the anodic dissolution. Meanwhile, the formation of insoluble Zn(OH)2 could also slow down the cathodic reaction rate, besides, C6H11O7- combined with Fe2+/Fe3+ and formed chelate, which retarded the anode dissolution.