Corrosion mechanism and kinetic model of steel in alkali-activated slag

Abstract

Corrosion of steel is the most frequent cause of structural premature degradation of reinforced concrete infrastructure serving in various complex and harsh environments, e.g., atmosphere with high temperature and relative humidity, wet-dry cycle in the splash zone, and chloride-rich seaside environment, which eventually leads to significant maintenance and life-cycle costs. As one of the clinker-less alternatives to traditionally ordinary Portland cement (OPC) materials, alkali-activated slag (AAS) material shows potential advantages in durability performance against chloride-induced steel corrosion; however, there is a lack of comprehensive investigation in related fields. This thesis aims to investigate the corrosion behavior, mechanism, and kinetics of steel in alkali-activated slag, especially in partially saturated conditions. In the first part of this thesis, the impact of chloride content, activator type, alkali dosage, and carbonation on the corrosion behavior of steel in AAS equilibrated at different relative humidity conditions is comprehensively investigated. Based on these systematical experiments, the linear relationship between concrete electrical resistivity and steel corrosion rate (R-C relationship) of AAS material is established and compared with that of reference OPC material, aiming at determining the key parameters and uncovering underlying mechanisms that cause the discrepancy in R-C relationships of various binder chemistry and corrosion type (chloride- versus carbonation-induced corrosion). The results show that the hydroxide ion concentration (pH value) of pore solution has a significant and decisive effect on R-C relationships. With the decrease of pH, the material electrical resistivity increases because of decreased pore solution conductivity whilst the corrosion rate of steel increases due to the drop of [Cl-]/[OH-] ratio, leading to a shift of R-C relationship lines. The newly proposed mechanism for explaining the differences in R-C relationships is further verified by the experimental results in the published literature. In the second part, this thesis provides new experimental evidence for the diffusion control mechanism dominating the anodic reaction of steel and further establishes a new kinetic model for corrosion of steel in unsaturated cementitious porous materials. It is found that the relative ion diffusion coefficient of cementitious porous media is a decisive factor controlling corrosion rate in unsaturated conditions by limiting the diffusion of ferrous ions around steel surface. A new corrosion kinetic model is proposed and developed based on the chloride-induced steel corrosion results in small-size mortar specimens, which can also reasonably predict the chloride-induced or/and carbonation-induced steel corrosion in full- size reinforced concrete elements. Moreover, the proposed kinetic model can be applied to explain some relevant unclarified corrosion phenomena, e.g., the high dependence of critical [Cl-]/[OH-] on degree of saturation, classifying corrosive conditions, and predicting the R-C relationships in various environments. This thesis contributes to a better understanding of steel corrosion mechanisms in AAS material and further development of general and reliable kinetic models for corrosion rate prediction. The degradation mechanism and model of reinforced concrete considering steel corrosion, especially for partially saturated concrete, are meaningful for the performance assessments of existing marine structures.