Anion Capture and Exchange by Functional Coatings: New Routes to Mitigate Steel Corrosion in Concrete Infrastructure

Chloride-induced corrosion is a major cause of degradation of reinforced concrete infrastructure. While the binding of chloride ions (Cl^-) by cementitious phases is known to delay corrosion, this approach has not been systematically exploited as a mechanism to increase structural service life. Recently, Falzone et al. [Cement and Concrete Research72, 54-68 (2015)] proposed calcium aluminate cement (CAC) formulations containing NO₃-AFm to serve as anion exchange coatings that are capable of binding large quantities of Cl^- ions, while simultaneously releasing corrosion-inhibiting NO₃^- species. To examine the viability of this concept, Cl^- binding isotherms and ion-diffusion coefficients of a series of hydrated CAC formulations containing admixed Ca(NO₃)₂ (CN) are quantified. This data is input into a multi-species Nernst-Planck (NP) formulation, which is solved for a typical bridge-deck geometry using the finite element method (FEM). For exposure conditions corresponding to seawater, the results indicate that Cl^- scavenging CAC coatings (i.e., top-layers) can significantly delay the time to corrosion (e.g., 5 ≤ d_f ≤ 10, where d_f is the steel corrosion initiation delay factor [unitless]) as compared to traditional OPC-based systems for the same cover thickness; as identified by thresholds of Cl^-/OH^- or Cl^-/NO₃^- (molar) ratios in solution. The roles of hindered ionic diffusion, and the passivation of the reinforcing steel rendered by NO₃^- are also discussed.