Pouria Ghods

Kinetics of Passivation and Chloride-Induced Depassivation of Iron in Simulated Concrete Pore Solutions Using Electrochemical Quartz Crystal Nanobalance

Kinetics of passivity and chloride-induced depassivation of iron exposed to simulated concrete pore solutions were studied using electrochemical quartz crystal nanobalance (EQCN), electrochemical impedance spectroscopy (EIS), and open circuit potential (OCP) monitoring. Passivation followed a two-stage logarithmic film formation process: protective film mostly formed within the first 10 min to 20 min of exposure to the passivating solutions as indicated by a sharp mass increase accompanied by impedance and phase angle data showing trends toward passivation. After this initial passivation period, mass continued to increase, albeit at a significantly slower rate. Electrochemical indicators during this period remained relatively constant and stable, suggesting that the iron remained passive. The mass increase during the post-passivation period was indicative of the formation of additional oxides, while relative stability of the OCP, impedance and phase angle measurements suggested that these oxides were like...

Numerical Study of Pore Solution Chemistry in Surface Crevices of Carbon Steel Rebar

The variability and uncertainty associated with chloride thresholds can be partly explained by the surface conditions of carbon steel rebar, in particular, by the presence of crevices on the steel surface. It has been suggested in the literature that pore solution in the crevices on the steel surface may be different from that of the bulk pore solution, and this difference may create the necessary conditions for the breakdown of the passive film. To test this hypothesis, a numerical investigation was carried out using a non-linear transient finite element algorithm, which involved the solution of coupled extended Nernst-Planck and Poisson’s equations in a domain that represented typical surface crevices on carbon steel rebar. The numerical simulations showed that the chemistry of the pore solution, in particular pH and Cl-/OH-, within crevices provided more favourable conditions for depassivation than the bulk concrete pore solution. Local acidification and increase in Cl-/OH- within the crevice were observed in all simulations, albeit to different degrees. Simulations supported the hypothesis that the chemical composition of the pore solution within the crevices differs from that of the bulk solution through a process similar to the suggested mechanism of typical crevice corrosion.

Numerical Study on the Effect of Cracking on Surface Resistivity of Plain and Reinforced Concrete Elements

AbstractThe effect of discrete cracks on the Wenner probe measurements for surface resistivity of plain and reinforced concrete elements is investigated. A numerical modeling approach is utilized to study the effect of a wide range of parameters that are related to discrete cracks (type, depth, width, location, and orientation with respect to the probe), reinforcement (cover thickness, bar spacing, and bar orientation with respect to the crack), and the Wenner probe (electrode spacing). Depending on the type of the crack and its location, the results indicate that measurements might have errors as much as ∼200% higher or ∼50% lower than the actual resistivity of concrete. Rebar mesh reduces measured resistivity and causes additional error in measurements—the denser the reinforcement, the higher the error. Whereas in most scenarios, crack and rebar mesh affect the accuracy of measurements independently, the effect of deep cracks that surpass the depth of rebars is limited by the presence of rebar mesh. A d...

Practical Model for Three-Stage Corrosion Behavior of Galvanized Steel Reinforcement in Soil

Failure of mechanically stabilized earth (MSE) walls attributed to the corrosion of galvanized steel in soil has raised concern on the adequacy of current design requirements. In this work, a numer...

Nano-scale Investigation of Interactions of Chlorides with Oxides That Form on Carbon Steel in Concrete Pore Solutions

Passivation and chloride-induced depassivation of carbon steel in concrete are processes that take place in nano scale. Spectroscopic and microscopic studies in this scale demonstrate that the thickness of the oxide films that form on carbon steel in the passivating environment of concrete pore solutions is typically in the range of 3–15 nm. Nano-scale analytical studies show that oxide films that form on carbon steel exposed to simulated concrete pore solutions consist of two layers separated with an indistinct border. The thinner FeII-rich inner oxide film is protective but unstable in the presence of chlorides. The thicker FeIII-rich outer oxide film is unprotective and likely porous. Chloride exposure decreases the thickness of the oxide films and changes their stoichiometry such that near the film/substrate interface FeIII/FeII ratio increases. Electrochemical quartz crystal nano-balance studies coupled with electrochemical observations provide kinetic evidence of how chlorides interact with these two layers and transform protective oxides into unprotective ones.

Parametric Studies and Application of a Practical Model for Corrosion of Galvanized Steel in Soil

The ability to influence decision at the design stage is one of the main advantages that corrosion prediction has over corrosion monitoring and proper design mitigation for corrosion, and the economical burden associated with them. In this work, a previously developed model for the corrosion rate of galvanized steel in soil at three different stages of corrosion, which considers the key soil corrosion parameters, is briefly introduced, some parametric and validation case studies are performed, and a few examples of some important applications are provided. Some modifications were also applied on the numerical model to extend the application of the models by including the effect of environmental conditions on the corrosion rate of galvanized steel in the soil. The improved model is an easy-to-use and flexible tool that can accurately estimate corrosion damage evolution of galvanized steel in the soil.