Oxidation kinetics and diffusion behaviour of ferrous ions in cementitious media

Background

Corrosion of the steel reinforcement proceeds via the anodic dissolution of metallic Fe into ferrous cations, that can oxidise, diffuse and precipitate, simultaneously, in the cementitious matrix. Majority of the service-life models assume the development of corrosion products in the vicinity of the reinforcement at the steel-concrete interface. This leads to the prediction of expansive stresses generating from the steel-concrete interface, thereby, resulting in extremely conservative design codes and service-life models.

Recent works by several authors have shown to be the case otherwise, where precipitation of corrosion products was evidenced a few hundred microns away from the steel-concrete interface. These findings pose significant challenges to the efficacy and applicability of most of the existing service-life models. However, there is scarcity in the experimental data regarding the oxidation kinetics and diffusion kinetics of ferrous ions to be fed into service-life models, to accurately estimate the duration of the propagation phase of corrosion.

Aims and objectives

This project aims to quantify the oxidation kinetics of ferrous ion oxidation in simulated solutions with varying pH, representative of concretes exposed to carbonation and chloride-contaminated environments. The project also aims to quantify the diffusion coefficient of ferrous ions in carbonated and chloride-contaminated cementitious media. This data would then feed into a reactive transport model to assess the propagation phase of corrosion.

Methodology

The stability of ferrous ions in mildly alkaline and highly alkaline solutions will be assessed with a UV-VIS Spectrophotometer. Owing to the low solubility of ferrous ions, the concentration of aqueous ferrous species as a function of time and dissolved oxygen concentration will be quantified using a liquid waveguide capillary flow cell. This data will then be used to derive a kinetic rate law.

Custom-built diffusion cells will be used to study the diffusion behaviour of ferrous ions in cementitious media. As the timescales of interest are in the orders of tens of years, these measurements will be done using in-diffusion setups rather than through-diffusion setups, where the concentration of ferrous ions will be measured spatially within the cementitious media at different points in times. Given, the oxidation of ferrous ions is extremely quick and depends strongly on the pH, a surrogate divalent Mn species will be considered to simulate the diffusion of ferrous ions.