Electrochemical Characteristics And Monitoring Sensor System Of The Corrosion Of The Steel Bar In Concrete Structure

Corrosion monitoring of reinforcing steel in concrete with chloride iron environment has been studied in this paper. Electrochemcial methods have been used to explore the corrosion mechanics. Some corrosion monitoring methods for practical engineering have been discussed.Also, five-electrode corrosion sensor and all solid state-current confined corrosion snesor has been developed and the corrosion monitoring system has been setup. The corrosion monitoring system has been practiced in the RC beams. The results show that the research in the paper has explored a new way to realize the corrosion monitoring and detecting.The main works of this paper are as follows:1. The impedance character of the specimens of Q235 carbon steel embedded in cement mortar at different temperature and in different concentration of NaCl solution have been studied by electrochemical impdedance spectrum technique (EIS).The results show that the impedance charater of the specimen includes three aspects.1) There are two parts of capacitive arcs at the high frequencies and medium frequencies of the Nyquist diagram. This shows that the concrete has some capacitive character besides the interface between the steel bar and the concrete.2) The capcitive arc at the medium frequencies has been flattened in some degree.This phenomen is cuased by the constant phase element (CPE).CPE is also named dispersion effect.The coarse interface of the steel/concrete, isolated reaction region and the heterogeneous nature of concrete are the most important factors causing the dispersion effect.3) The phenomen of smearing effect has appeared at low frequencies.The Warburg impedance which is used to explain the diffusion effect can cause the smearning phenomen. But the plots in the Nyqusit diagram are deviated from the line with the coeff. of 1. There are two kinds of factors which can cause such phenomen. Firstly, the diffusion process from the concrete to the carbon steel can be treated as spheric electrode diffusion in some degree for the coarse surface of the specimen.Secondly, the corrosion pit causes that the character of the Nyqusit plots at low frequencies embodies some capacitive character.2. The electrochemical noise (EN) technique has been applied to study the pitting character of the carbon steel embedded in cement mortar. The concentration of the NaCl solution used to immersed the cement specimen are 0% and 3.5%, respectively.The results show that the EN of the passivated steel is white nosie. There is a large trend in the EPN when the specimen is immersed in 3.5%NaCl solution. The apparent transient peaks have appeared in EPN and ECN. Decreasing (or increasing) suddenly and recovery slowly is the typical shape of the transient peak. The typical three-segment plots have apperared in the PSD diagrams of EN.When the EN is detrended, the Rf→0 calculated by PSD is equal approximatly to the impedance Z measured by EIS.Concering the larger amplitude of the trend in EN in practical engineering, Sym4 wavelet has been used to analyze the EN. The results show that the energy distribution plot (EDP) can be used to confirm the occurrence of the corrosion pit.The EDP method is more effectively than PSD to treat time variance system.3. The reaction equation of the electrode which is controlled by mixed potential theory has been deduced and the simplified methods which adapt to practical engineering in steady state and transient state measurement have been studied.The influence of diffusion and CPE effects to the results by transitent measurement have been discussed. As the extition of the system is a current step, the time constant of the potential response is a more effective parameter to identify the corrosion status of the steel than that of the half-potential method.The time constant can identify the corrosion status correctly when the oxygen is rare or the humidity is very high in the concrete. Many specimens have been used in the experiments to certify the reability of the simplified methods.The data during the charging process has been analyzed by segmented method.4. A novel all solid state reference electrodes have been prepared and the performance of the reference electrode is studied. The reference electrode is immune to the irons in the concrete and the long time stability is perfect. Also, the anti-polarizetion character of the reference electrode is perfect. The coefficient of temperature response is from -0.48mv/℃to -5.2 mv/℃. Secondly, applying the all solid state reference electrode, the five-electrode corrosion sensor and all solid state-current confined corrosion sensor have been developed. The general electrochemical measurements and EN measurement can be realized on five-electrode corrosion sensor.According to the analysis results of the finite element method, the area of the working electrode(WE) can be identified exactly by the all solid state-current confined corrosion sensor, so the corrosion rate of the steel bar can be measured accurately by the corrosion sensor.5. The developed corrosion sensors have been used to monitor the corrison statues of the concrete beam.The basical mechanical property of the corroded beam has also been studied. After the accelerated corrosion, the corrosion current degree of the steel bar is almost 10+1μA/cm2.But the current of the steel bar in fresh concrete is 10-1μA/cm2.According to the change of the EDP, we can see that the pitting corrosion has occurred on the steel surface.By the experinments of the cross-section bearing capacity of the beams, we can find that the failure mode of the beams change from tipical adequacy beam to the insufficient beam.As the corrosion degree increases, the ulitimate bearing capacity, the ductility and the stiffness of the beams decrease gradually.