| Corrosion of steel rebar in concrete becomes a main reason for the premature degradation of reinforced concrete, which is by far the most widely used construction material in the modem society. Due to the complexity of the steel/concrete system, great effort is still in need to explore the mechanism of the interfacial reaction and to develop non-destructive detection technique.Electrochemical impedance spectroscopy (EIS) was employed to study the corrosion behavior of reinforcing steel in concrete and the equivalent circuit analysis was adapted to analyze its electrochemical reaction model. The result indicates that reinforced concrete system contains two time constants, one derived from the electrical double layer of reinforcing steel and the other from concrete layer. With the increase of immersion in NaCl solution, steel rebar turned from passive to active corrosion and the overall reaction became mass transportation controlling. Because of the inhomogeneity of concrete and roughness of steel surface, the charging/discharging behavior of the electrical double layer deviated from that of a pure capacitor.The micro chemical environment, especially Cl" and pH value at steel/concrete plays a key role in the corrosion process of reinforcing steel in concrete. The Ag/AgCl electrode serving as Cl" sensor was fabricated by Galvanostatic anodic oxidation, the Cl" sensor exhibited good selectivity to Cl" and long-term stability in simulated concrete pore solution (SPS) containing Cl". The Ir oxide pH sensor was prepared by thermal oxidation in carbonate, after ageing, potential drift and super-Nernst response were eliminated. The variation of interfacial Cl" and pH were followed in situ by the Cl" and pH sensors embedded at steel/concrete interface. The results verify that the breakdown of the passivity is preceded by a drop in pH and aggregation of Cl" at the steel/concrete interface.In situ STM was successfully applied to monitor the surface morphology of steel rebar in a series of SPS based solution. In SPS with pH = 12.40 and no externally added Cl", the growth of passive film was observed. In the supercritical solution, where a severe corrosion for steel may happen, the surface morphology in the scanning area almost kept intact except for the appearance and growth of dunes during the measurements. It is evidenced that in this media, the localized corrosion featured by a large cathode/anode ratio took place. Home-made LD-2 compound inhibitor eliminates structural defects which is sensitive to initiate localized corrosion on steel surface, thus greatly improvedthe corrosion resistance of steel in concrete. The experiment carried out heretofore proves that in situ STM can provide clear real time microscopic images of the metal surface in solution and is a promising technique to deepen the understanding on the dynamic process of growth and breakdown of passive film.Electrochemical noise (ECN) measurement can provide information about both corrosion rate and corrosion type while inflicting no perturbation on the study system. The ECN data of steel rebar in SPS with Cl" were collected and analyzed in time domain at the first step. Then statistical, spectral and wavelet analysis were carried out to compare the results from the different methods. As a window-adjustable method, wavelet analysis showed an advantage over other methods in processing the non-stationary ECN data, abruption and contribution from component processes were extracted from overlapped overall signals. In SPS with the addition of 0.0001 mol/L NaCl, the transients with time constant between 16-32 s were found to be predominant over other processes, and in SPS with higher Cl" concentration, a trend toward active corrosion became the main process. The result of discrete wavelet transformation provided a criterion to determine the threshold value determining the corrosion state of steel in concrete .
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