Electrochemical Corrosion Characteristics Of Low-Carbon Alloy Steel In NaCl Solution

In the marine environments, seawater as a kind of strong electrolyte solution can acceleratethe corrosion process of low-carbon alloy steel. Moreover, the electrochemical corrosioncharacteristics of low-carbon alloy steel are ascribed to different marine environments. In thispaper, the electrochemical corrosion characteristics of low-carbon alloy steel in different marineenvironments were studied by using polarization curve technique and electrochemicalimpedance spectroscopy (EIS), the morphologies of corroded regions were observed andanalyzed by using of canning electron microscope (SEM), and phase structure of corrosionproducts were measured by X-Ray Powder Diffraction (XRD) and Fourier TransformationInfrared Spectrum (FITR). Major conclusions are as follows:The electrochemical corrosion characteristics of the three low-carbon alloy steels (Q345B,Q325HY and NSB) immerged in3.5%NaCl solution at different time were relation with thephase structure and microstructure of rust layer. The corrosion rate increase gradually with theprolonging of dunking time. SEM observation reveals that the rust layer of Q325HY steel wasbonded tightly to matrix and their microstructure was more compacted, as comparison withthose of Q345B and NSB steels. Moreover, the corrosion resistance of Q325HY steel is the bestamong the three low-carbon alloy steels. The rust layer is composed of γ-FeOOH, α-FeOOHand Fe3O4phases, and the phase abundance of γ-FeOOH in the rust layer is about50%for thethree low-carbon alloy steel immerged in3.5%NaCl solution at different time, and this paperfind that γ-FeOOH and α-FeOOH phases can translate into Fe3O4phases.The electrochemical characteristics of the three low-carbon alloy steels with and withoutrust layer immerged in3.5%NaCl solution at different temperature were compared andanalyzed. It is found that the protective ability of the rust layer increased and corrosion ratedecreased with increasing of temperature, and the sensibility to temperature decreased in theelectrochemical corrosion process of the three low-carbon alloy steels with temperature rising.The phase abundance of γ-FeOOH is the highest in the rust layer. This phenomenon is the sameas the three low-carbon alloy steel immerged in3.5%NaCl solution at different time. The phaseabundance of Fe3O4increased with the temperature rising, and the phase β-FeOOH of rust layerwas found at the lower temperature. Furthermore, under the lower temperature, the nucleationrate of phases was more slowly.During the wet-dry cycles, the results of electrochemical measurements show that thecorrosion rate of three low-carbon alloy steels is distinct enhanced, as compared with those ofimmerged samples. The anode electrochemical reaction is the process of iron electrodissolution which was controlled by the rate of electron transfer. The cathode electrochemical reaction isthe process of dissoluble oxygen reduction which was controlled by diffusion process. It isfound that the three low-carbon alloy steels in3.5%NaCl solution shows the poorest corrosionresistant among all test NaCl solution. Their rust layers are mainly composed of γ-FeOOH andFe3O4phases, the formation of the phase α-FeOOH is benefited from low concentration ofNaCl solution.