Electrochemical Behavior from Early Corrosion of Stainless Steel Bars in Seawater Mixed Concrete

With the shortage of freshwater and river sand resources, the application of seawater and sea sand concrete in marine structures has important practical significance. Seawater and sea sand concrete faces serious corrosion problems of steel bars. Previous studies have shown that using stainless steel bars can effectively solve the corrosion problem of steel bars in seawater and sea sand concrete. However, there is a lack of research on the passivation and depassivation mechanisms of stainless steel bars in seawater and sea sand concrete, and relevant theoretical results cannot support their application in practical engineering. The work aims to study the electrochemical behavior from early corrosion of stainless steel bars in seawater mixed concrete. The steel bars used were 304 stainless steel bars and 316L stainless steel bars with a diameter of 1 cm. Copper wires were welded at one end of the steel bars, and epoxy resin was used for sealing treatment. Sand paper was used to grind the steel bars step by step, and finally polished to a mirror surface to ensure a uniform surface state of the steel bars. The surface was cleaned with deionized water and ethanol, then blown to be dry with N2, and placed in a dryer for standby. A simulated solution was used to simulate the mixing of cement paste with seawater. The simulated seawater was deionized water composed of NaCl, MgCl2, Na2SO4, CaCl2, KCl, and NaHCO3. In the experimental group, simulated seawater was used to mix cement paste (SW), while in the control group, deionized water was used to mix cement paste (DW), with a water cement ratio (w/c) of 0.4. The sample size of cement paste was 40 mm × 40 mm× 40 mm, and a protective layer thickness of 1 cm was used. Firstly, the pressure filtration method was used to extract the pore solution from the clean slurry samples at different ages. A pH meter was used to test the pH value of the pore solution at different times, and a Mettler Easyplus chloride ion potentiometric titrator was used to test the chloride ion concentration. The electrochemical behavior was monitored by the Swiss Vantone Autolab M 204 electrochemical workstation. The electrochemical impedance spectrum of the sample was tested regularly, with a sinusoidal disturbance amplitude of ± 10 mV and a frequency range of 100 kHz-10 mHz. Zsimpwin software was used to fit the AC impedance spectrum data and the corrosion rate of the reinforcement was calculated based on the fitting results. After the sample was split, the surface morphology and composition of the reinforcement were characterized by scanning electron microscopy (VEGA 3 TESCAN) combined with BSE and EDS. The results showed that although the potential of 304 and 316L stainless steel bars decreased to a relatively negative value at an extremely early stage, below –0.35 V, and the chloride ion concentration reached 0.7 mol/L, there was no biological corrosion, and normal passivation could occur. Compared with the control group, the polarization resistance of 316L stainless steel in seawater mixed cement paste was improved, and the corrosion rate was lower. With the development of cement hydration process, the corrosion current density gradually decreased to a lower level, and the corrosion current density of 304 stainless steel was about 0.014 μA/cm2 and that of 316L stainless steel was below 0.006 μA/cm2, exhibiting high corrosion resistance. The resistivity of cement paste mixed with seawater was somewhat different from that of the control group, but the overall difference was small. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.