The kinetic competition between transport and oxidation of ferrous ions governs precipitation of corrosion products in carbonated concrete

Corrosion products, originating from steel generalized (carbonation induced) corrosion and precipitating in the concrete pore system, can lead to concrete cracking and to spalling of the concrete cover. Related premature structural repair causes high costs; therefore, reliable quantitative models are needed. Here, we present a new conceptual model to describe the fate of ferrous ions that are released at the steel surface during the corrosion process. The key novelty of our approach can be found in explicitly considering the kinetics of oxidation and transport of Fe2+ in the pore solution. These two processes constantly dilute the Fe2+ concentration and are in competition with the supply of Fe2+ from the anodic iron dissolution reaction. We use a numerical model to elucidate which of the described processes is the fastest. The results find good agreement with experimental data and reveal that under natural corrosion conditions, Fe2+ hardly reaches the saturation level, which permits the diffusion of corrosion products up to millimeters away from the steel without necessarily leading to expansive stresses. Under accelerated corrosion conditions, however, in a chloride-free environment, precipitation is also accelerated immediately at the steel surface. This influences the stresses build-up mechanism and suggests careful evaluation of accelerated tests and related models.