Study On The Effect Of Chloride Ions On The Corrosion Mechanism Of Reinforcing Steel

The corrosion of reinforcing steel is the main reason for the premature degradation of reinforced concrete structures, which concerns the national economy and the people's livelihood. Whether or not the reinforcing steel in concrete maintains its passivity depends on the physical and chemical conditions at the steel/concrete interface. In general, the two major factors which cause the depassivation of reinforcing steel in concrete are carbonation and penetration of chloride ions. So far, many investigations have been carried out to explore the corrosion mechanism of reinforcing steel. However, there are still a lot of problems needing great effort to solve.In this work, many research methods including in-situ, semi-situ and ex-situ measurements were utilized to study on the corrosion behavior of reinforcing steel in simulated concrete pore solutions and the effects of chloride ions and pH of the solution on the steel depassivation. The progress of this work and the main results are as follows:(1) The scanning micro-reference electrode technique (SMRE) has been developed to study the mechanism of the steel corrosion induced by chloride ions and measure the chloride threshold concentration for the steel corrosion initiation in simulated concrete pore solutions. The potential distribution on the surface of the reinforcing steel in the simulated concrete pore solution reveals that there are a number of unstable micro-pitting nuclei on the steel surface in the solution with chloride ions. These active pits compete with each other and only a few of them can develop into real pitting corrosion. The chloride threshold concentration measured by SMRE is lower than the result obtained by the potentiodynamic anodic polarization, which shows that SMRE has higher sensitivity.(2) The semiconductor properties and relevant corrosion resistance of the passive film on the reinforcing steel surface in simulated concrete pore solutions were explored. The effects of pH of the solution and different film formation potentials on the semiconductor properties were also studied. The result shows that the films behave as heavily doped amorphous n-type semiconductors in which oxygen vacancies play the main role of donors. Along with the reduction of pH, the oxide film covering on the steel partly dissolves and the structure of the oxide crystal lattice changes, which result in higher oxygen vacancy concentration and more negative equilibrium flat potential. Accordingly the steel shows a higher tendency to corrode. Film formation potentials can change the component and structure of the film and reduce the oxygen vacancy concentration, thus strengthen the protection function of the film.(3) The surface physical and chemical properties of the reinforcing steel immersed in different simulated pore solutions and the effects of carbonation and chloride ions were investigated using XRD, XPS and SEM. Carbonation can make the film surface dissolve equably with indissolvable corrosion products depositing. However, chloride ions can penetrate the film through the surface defects and corrode the steel base, which turn off dissolvable corrosion products.