| The corrosion behavior of ASTM A416 steel in alkaline simulated pore solution under active aeration was studied by cyclic voltammetry, electrochemical impedance spectroscopy, and measurement of open-circuit potential and polarization curves. Scanning electron microscopy (SEM), X-ray photonelectron spectroscopy (XPS), high-resolution cross-sectional transmission electron microscopy (HR-XTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADS-TEM) were used to study the oxide film's composition and thickness. Preliminary experiments were conducted to investigate the influence of chloride ions and dissolved oxygen on corrosion of steel in simulated pore solution. The impedance measurements at open-circuit potential did not show a significant difference between ASTM A416 steel immersed in a solution of 40g/L NaCl and a chloride-free solution. A significant difference was also not found between ASTM A416 steel immersed in aerated and deaerated solutions, suggesting the cause for the corrosion of ASTM A416 steel in concrete can be attributed to other factors. The open-circuit potential of ASTM A416 required about 300 ks (85 h) to reach steady-state. At other potentials, the steady-state current was generally obtained after 50ks. At +0.22 V(Hg/HgO), an elapsed time of 250 ks was required to reach the steady-state.;The low-magnification SEM images for corroded and passive ASTM A416 steel in Sika Grout 300PT suggested that corroded ASTM A416 steel has a rougher surface than passive steel, and that the size of corrosion products is varied, but generally within 1 to 20mum. The XPS spectra for ASTM A416 steel after 8 cycles of cyclic voltammetry scanning suggested that calcium plays a role in oxide film composition, which may be attributed to the deposition of Ca(OH) 2 from the cathodic reaction. The images from HR-XTEM cannot provide clear information about the oxide film thickness; however, HAADF-STEM showed the oxide film thickness for steel in simulated pore solution at open-circuit potential was about 3-7nm. The essential parameter to estimate oxide film thickness, rhodelta, is about 10000 Ocm. The oxide film thickness increased over time. The variation of film thickness with applied potential could be attributed to the greater immersion time required to perform steady-state measurements at the applied potential.;The corrosion of ASTM A416 steel in simulated pore solution could only be observed before steady-state was reached and when positive potentials were applied. Chloride ion contamination and the lack of dissolved oxygen in the simulated pore solution were not the main factors contributing towards corrosion. The chemical composition and oxide film thickness, as observed from surface analysis, were not consistent for all ASTM A416 steels.;Since the impedance data could become corrupted by various errors, the measurement model analysis was applied to determine the portion of the frequency range that could be used for subsequent regression. The fitting strategies employed in this present dissertation were downhill simplex and Levenberg-Marquardt methods. Before steady-state was reached, the electrochemical impedance spectra were interpreted by use of a model that accounted for porous electrode behavior and for the contributions of both anodic and cathodic reactions. After steady-state was reached, two different electrochemical impedance responses could be observed: one still shows porous electrode behavior and another showed non-porous electrode behavior. The open-circuit corrosion rate for a stationary working electrode immersed in the actively-aerated electrolyte was 64 mum/ year shortly after the cathodic pre-treatment was completed, but it was too small to be detected after steady-state was reached. |
