Corrosion of metal reinforcements within concrete and localisation of supporting reactions under natural conditions

TL;DR

This work develops a first-principles computational model for natural corrosion of steel reinforcements in concrete, enforcing current conservation so anodic pit currents are sustained by nonlocal cathodic reactions without external sources. By coupling Nernst–Planck transport with Butler–Volmer surface kinetics and a time-evolving metal potential, the model captures how pits acidify their surroundings and how oxygen diffusion, porosity, water saturation, and chloride content shape local reaction currents. Key findings show hydrogen evolution is highly localized near pits while oxygen evolution can occur over the full rebar surface; porosity dominates corrosion rate via oxygen supply, whereas conductivity (chloride) plays a secondary role, especially under saturated conditions. Under partial saturation, diffusion is hindered, corrosion products are retained, and hydrogen-driven cathodic support becomes more influential, highlighting the limitations of using simple resistivity as a predictor and emphasizing the importance of decoupling porosity and ion concentration in assessing corrosion risk. The model thus provides insight into the spatial distribution of reactions, pit dynamics, and the sensitivity of natural corrosion to environmental and material properties, with potential to inform more realistic assessments of reinforced concrete durability.

Abstract

Corrosion in concrete prevents in-situ observation, necessitating models to provide insight into the local reaction currents. We present a computational method for predicting corrosion rates of reinforcements within concrete under natural conditions, i.e. requiring the corrosion current to be supported by equal cathodic currents. In contrast to typical corrosion models, where these two counteracting currents are required to be co-located, we allow these currents to be separated such that pitting corrosion can be supported by cathodic reactions over a much larger area. Pitting corrosion is investigated, elucidating the effects of the concrete porosity and water saturation, the presence of dissolved oxygen, and chlorine concentration within the pore solution. The presented model is capable of capturing the dynamic growth of acidic regions around corrosion pits, showing the limited region over which the hydrogen evolution reaction occurs and how this region evolves over time. The ability of oxygen to diffuse towards the metal surface due to increased porosity is seen to have a major effect on the corrosion rate, whereas changes in the chlorine concentration (and thus changes in the conductivity of the pore solution) play a secondary role. Furthermore, external oxygen is seen to enhance corrosion but is not required to initialise and sustain acidic corrosion pits.

Figures (16)

• Figure 1: Schematic overview of the electro-chemical system modelled, consisting of porous concrete saturated with an electrolyte and metal reinforcements. The electrolyte contains ions and dissolved oxygen, causing electro-chemical reactions with the metal restricted by the charge conservation across reactions.
• Figure 2: Overview of the simulated domain, including boundary conditions.
• Figure 3: Reaction currents and area over which the reactions occur for the case with external oxygen, using $\phi=1\%$ and $C_\text{Cl}=500\;\mathrm{mol}/\mathrm{m}^3$.
• Figure 4: pH and locations of reaction currents at steady-state ($28\;\text{days}$) for the case with external oxygen, using $\phi=1\%$ and $C_\text{Cl}=500\;\mathrm{mol}/\mathrm{m}^3$.
• Figure 5: Reaction currents and area over which the reactions occur for the case without external oxygen, using $\phi=1\%$ and $C_\text{Cl}=500\;\mathrm{mol}/\mathrm{m}^3$.