Corrosion product transport in cementitious media

Abstract

Predicting the longevity of concrete structures exposed to corrosive conditions remains to a considerable challenge of engineering science. Steel corrosion in concrete can damage the concrete structure through cracking of the concrete, which is generally agreed on to be due to larger volumes of precipitating iron (hydr)oxides compared to Fe(0), generating expansive stresses in the concrete matrix. However, the fundamental processes leading to concrete cracking still need to be elucidated. The complex, interrelated processes of the release of ions from the oxidation of ferrous iron at the metal surface, transport, and chemical reactions such as oxidation, complexation, and precipitation are still far from being fully understood. Reactive transport models simulating the process of steel corrosion can solve the governing equations of energy and species transfer within the framework of interest. To capture changes in the chemical composition at the corrosion interface and within the pore solution, these models rely on a variety of experimentally determined physical and kinetic constants. The intricacy of iron oxidation and corrosion product transport in cementitious media is governed by the precipitation and dissolution of more than 15 characterized solid iron oxides, oxide-hydroxides and hydroxides and more than 40 aqueous phase reactions. Whilst the thermodynamic feasibility of various reactions can be comprehensively tabulated, the influence of chlorides on the overall iron solubility is not yet well described. Furthermore, studies in acidic and neutral environments suggest that the precipitation of corrosion products may occur via the formation of higher order, transitional, mixed Fe(II) - Fe(III) chloride complexes and solids, however, the structure and behavior of these compounds are not well understood in high pH environments. This study investigates the reactive transport processes of iron species considering the kinetic and thermodynamic processes that govern their compositional and phase states in concrete. We present in detail the co-existence of iron in solid and liquid phases, their sensitivity to changes in chloride and oxygen concentrations, and the implications for the transport of iron species into regions distant from the steel – concrete interface.