| Carbonization of concrete and chloride invasion in the environment can cause the corrosion of the steel in concrete. One of the most effective methods to extend the service life of concrete structure is to adopt stainless steel instead of carbon steel. Therefore, comparative study on the corrosion behaviors of different reinforcing steels in concrete simulated environment is very important for reforced conctete structure. In this thesis, the electrochemical measurement technologies such as potentiodynamic polarization, potentiostatic polarization, Mott-Schottky plots and Electrochemical impedance spectroscopy (EIS), combined with X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) methods were used to study the electrochemical corrosion behaviors of Q235 carbon steel and 304 stainless steel (304SS) in five kinds of simulated concrete pore (SCP) solutions. The effect of Cl-concentration on the metastable pitting corrosion of carbon steel in two kinds of carbonization simulated concrete pore solutions were analyzed. The transformation of metastable pits to stable pits were discussed.The main conclusions are as follows:(1)The corrosion behaviors of Q235 carbon steel and 304 SS were studied and compared in five kinds of SCP solutions (pH 9.7～13.3) without and with 0.05 mol/L Cl-. Both steels can keep good passivation states in five SCP solutions without Cl-, demonstrating similar corrosion resistance. Adding 0.05 mol/L Cl* have no influence on the corrosion resistance of 304SS in five solutions. But Q235 carbon steel only remained passivation in high alkaline solution, while in pH 11.5, pH 10.3 and pH 9.7 solutions its corrosion resistance decreased obviously as apparent metastable current fluctuations and pitting corrosion occurs. Mott-Schottky analysis results indicated that doping densities of passive film formed on 304SS surface increase slightly with pH decreasing, with no big difference under conditions with Cl* or not. For Q235 steel the doping densities of passive films show no apparent change with pH under the condition without Cl*, while after Cl* was added the doping densities of the films increase distinctly as pH decreases, resulting in the decrease in the corrosion resistances of carbon steel. XPS results demonstrated that the ratio of Fe3+/Fe2+ of the film on Q235 carbon steel has no apparent change as pH decreases without Cl-, and in solutions with Cl* the ratio decreases evidently, leading to the decrease in the film stability. The compositions of 304 SS passive film formed in different SCP solutions show little variations, so the corrosion resistance of 304 SS has no apparent change.(2) In two kinds of simulated carbonated concrete solutions (pH 10.3 and 9.7), the metastable pitting corrosion potential Em and stable pitting corrosion potential Eb of Q235 carbon steel decrease as the Cl- content increases, and Ebhas good liner relationship with ln[Cl-]. Both Em and Eb increase linearly with ln[Cl-]/[OH-] and lnCagg/Cinh decreasing. As the Cl-content increasing, the donor concentration Nd and flat band potential Efb of passive film increase, the vacancy in the film increases, and the electrical resistance of passive film decreases while the capacitance increase, demonstrating the decrease in the density of passive film and the pitting susceptibility of Q235 carbon steel improved.(3) As the Cl- content increasing, the initiation frequency, lifetime of metastable pits and the diameters of calculated metastable pits increase, and the potential at which the pits initiation rate reaches to maximum decreases. Therefore, Cl- could promote both the nucleation and growth of metastable pittings. The potential at which the metastable pit diameter reaches to maximum is close to the potential at which the pits initiation rate to maximum. These two potentials are slightly lower than the stable pitting potential Eb, which means that the bigger the metastable pit size and the larger the number of metastable pits, the more chance the metastable pits turning into stable pits. After an explosion of large number of metastable pits occurs, the metastable pits would soon transformed to stable pits.(4) When Q235 carbon steel is under potentiostatically polarization at the potential a little higher than Em, the size of metastable pit formed on the steel surface is tiny (less than 1 μm) with open status. When the polatization potential rises (while far below Eb), the size of pit increases (about 1 μm) and many metastable pits have the tendency to nucleate and grow together. New pits can be observed growing at the bottom of the former pits. There are residual oxide films around the pits mouth, and the metastable pits continuing to develop under the reserved film is confirmed by the calculation of the current fluctuations integration on the polarization curve. When the polatization potential is close to Eb, there are stable pits formed on the surface of Q235 carbon steel with the size around several hundred micrometers, partial surface covered by corrosion products. The stable pits are gathered by many small pits. Cl- enriched corrosion products film covered on the steel matrix could induce a lot of new metastable pits nucleation. The metastable pits mainly nucleate and grow around the stable pits which causes the pits develop in a planar direction on the surface. |
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