金属在流动氯化物体系中流动腐蚀的EIS研究

EIS RESEARCH ON FLOW-INDUCED CORROSION OF METAL IN FLOWING CHLORIDE MEDIA

流动氯化物体系中碳钢、双相钢电极的电化学阻抗谱(EIS)明显不同.随着流速增大,电极表面受到的切应力增大,反应阻抗减小,电化学腐蚀加速,协同效应增强,结果使电极流动腐蚀速度增加.与碳钢相比,双相钢电极在较高流速下其电化学阻抗谱才在低频区发生特征变化.证明了曹楚南提出的不可逆电极阻抗谱理沦同样适用于流动腐蚀过程中电极过程的研究,揭示了在流动体系中,当电极表面受到足够大的切应力作用时,在低频区电极电化学阻抗谱会发生特征改变.其中,电化学阻抗谱的低频感抗弧特征预示着在流动腐蚀过程中,受到强力流体力学作用的电极表面会改变其常规的溶解速度和溶解机制,同时表面伴随有坑、点局部腐蚀的发生.在此情况下,钝态金属电极的电化学阻抗谱进一步反映了电极表面受到固体颗粒摩擦或撞击的阻抗谱特征.

The electrochemical impedance spectroscopies （EIS） of electrodes made of carbon steel and duplex stainless steel were obviously different in single-phase or two-phase chloride media with different flow volocities. With increasing flow velocity the shear stress on the metal electrode surface and the electrochemical corrosion rate increased, and the electrochemical reaction impedance decreased. Finally the flow induced corrosion rate of metal electrode was increased by the synergism effect. Compared with EIS of carbon steel electrode, it was at higher flowing velocity that the feature of Nyquist plot in low frequency region of duplex stainless steel electrode was changed. All above these proved that Cao＇s EIS theory for irreversible electrode can be used for studying electrode process under flow-induced corrosion. The experimental results showed that it was in low frequency region that the EIS feature of electrode in flowing media was changed when the shear stress on the electrode surface was high enough and that the inductive impedance feature of Nyquist plot in low frequency region meant that the normally dissolving mechanism and rate of metal electrode surface were changed by strong hydrodynamics. At the same time, the local corrosion, pits and holes, occurred on the electrode surface. Under this condition, the EIS of passive electrode further showed that the metal electrode surface was subject to scraping and stroke of solid particles in flowing solid / liquid two-phase media.